The Origin Of Species
1859
THE ORIGIN OF SPECIES
by Charles Darwin
1859
INTRODUCTION
INTRODUCTION
WHEN on board H.M.S. Beagle as naturalist, I was much struck with
certain facts in the distribution of the organic beings inhabiting
South America, and in the geological relations of the present to the
past inhabitants of that continent. These facts, as will be seen in
the latter chapters of this volume, seemed to throw some light on
the origin of species- that mystery of mysteries, as it has been
called by one of our greatest philosophers. On my return home, it
occurred to me, in 1837, that something might perhaps be made out on
this question by patiently accumulating and reflecting on all sorts of
facts which could possibly have any bearing on it. After five years'
work I allowed myself to speculate on the subject, and drew up some
short notes; these I enlarged in 1844 into a sketch of the
conclusions, which then seemed to me probable: from that period to the
present day I have steadily pursued the same object. I hope that I may
be excused for entering on these personal details, as I give them to
show that I have not been hasty in coming to a decision.
My work is now (1859) nearly finished; but as it will take me many
more years to complete it, and as my health is far from strong, I have
been urged to publish this abstract. I have more especially been
induced to do this, as Mr. Wallace, who is now studying the natural
history of the Malay Archipelago, has arrived at almost exactly the
same general conclusions that I have on the origin of species. In 1858
he sent me a memoir on this subject, with a request that I would
forward it to Sir Charles Lyell, who sent it to the Linnean Society,
and it is published in the third volume of the Journal of that
society. Sir C. Lyell and Dr. Hooker, who both knew of my work- the
latter having read my sketch of 1844- honoured me by thinking it
advisable to publish, with Mr. Wallace's excellent memoir, some
brief extracts from my manuscripts.
This abstract, which I now publish, must necessarily be imperfect.
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my
accuracy. No doubt errors will have crept in, though I hope I have
always been cautious in trusting to good authorities alone. I can here
give only the general conclusions at which I have arrived, with a
few facts in illustration, but which, I hope, in most cases will
suffice. No one can feel more sensible than I do of the necessity of
hereafter publishing in detail all the facts, with references, on
which my conclusions have been grounded; and I hope in a future work
to do this. For I am well aware that scarcely a single point is
discussed in this volume on which facts cannot be adduced, often
apparently leading to conclusions directly opposite to those at
which I have arrived. A fair result can be obtained only by fully
stating and balancing the facts and arguments on both sides of each
question; and this is here impossible.
I much regret that want of space prevents my having the satisfaction
of acknowledging the generous assistance which I have received from
very many naturalists, some of them personally unknown to me. I
cannot, however, let this opportunity pass without expressing my
deep obligations to Dr. Hooker, who, for the last fifteen years, has
aided me in every possible way by his large stores of knowledge and
his excellent judgment.
In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings,
on their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that species had not been independently created, but had
descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world
have been modified, so as to acquire that perfection of structure
and coadaptation which justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food,
&c., as the only possible cause of variation. In one limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
mistletoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.
It is, therefore, of the highest importance to gain a clear
insight into the means of modification and coadaptation. At the
commencement of my observations it seemed to me probable that a
careful study of domesticated animals and of cultivated plants would
offer the best chance of making out this obscure problem. Nor have I
been disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I
may venture to express my conviction of the high value of such
studies, although they have been very commonly neglected by
naturalists.
From these considerations, I shall devote the first chapter of
this Abstract to Variation under Domestication. We shall thus see that
a large amount of hereditary modification is at least possible; and,
what is equally or more important, we shall see how great is the power
of man in accumulating by his Selection successive slight
variations. I will then pass on to the variability of species in a
state of nature; but I shall, unfortunately, be compelled to treat
this subject far too briefly, as it can be treated properly only by
giving long catalogues of facts. We shall, however, be enabled to
discuss what circumstances are most favourable to variation. In the
next chapter the Struggle for Existence amongst all organic beings
throughout the world, which inevitably follows from the high
geometrical ratio of their increase, will be considered. This is the
doctrine of Malthus, applied to the whole animal and vegetable
kingdoms. As many more individuals of each species are born than can
possibly survive; and as, consequently, there is a frequently
recurring struggle for existence, it follows that any being, if it
vary however slightly in any manner profitable to itself, under the
complex and sometimes varying conditions of life, will have a better
chance of surviving, and thus be naturally selected. From the strong
principle of inheritance, any selected variety will tend to
propagate its new and modified form.
This fundamental subject of Natural Selection will be treated at
some length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less
improved forms of life, and leads to what I have called Divergence
of Character. In the next chapter I shall discuss the complex and
little known laws of variation. In the five succeeding chapters, the
most apparent and gravest difficulties in accepting the theory will be
given: namely, first, the difficulties of transitions, or how a simple
being or a simple organ can be changed and perfected into a highly
developed being or into an elaborately constructed organ; secondly,
the subject of Instinct, or the mental powers of animals; thirdly,
Hybridism, or the infertility of species and the fertility of
varieties when intercrossed; and fourthly, the imperfection of the
Geological Record. In the next chapter I shall consider the geological
succession of organic beings throughout time; in the twelfth and
thirteenth, their geographical distribution throughout space; in the
fourteenth, their classification or mutual affinities, both when
mature and in an embryonic condition. In the last chapter I shall give
a brief recapitulation of the whole work, and a few concluding
remarks.
No one ought to feel surprise at much remaining as yet unexplained
in regard to the origin of species and varieties, if he make due
allowance for our profound ignorance in regard to the mutual relations
of the many beings which live around us. Who can explain why one
species ranges widely and is very numerous, and why another allied
species has a narrow range and is rare? Yet these relations are of the
highest importance, for they determine the present welfare and, as I
believe, the future success and modification of every inhabitant of
this world. Still less do we know of the mutual relations of the
innumerable inhabitants of the world during the many past geological
epochs in its history. Although much remains obscure, and will long
remain obscure, I can entertain no doubt, after the most deliberate
study and dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained, and which I
formerly entertained- namely, that each species has been independently
created- is erroneous. I am fully convinced that species are not
immutable; but that those belonging to what are called the same genera
are lineal descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one species are
the descendants of that species. Furthermore, I am convinced that
Natural Selection has been the most important, but not the
exclusive, means of modification.
CHAPTER I
VARIATION UNDER DOMESTICATION
Causes of Variability
WHEN we compare the individuals of the same variety or sub-variety
of our older cultivated plants and animals, one of the first points
which strikes us is, that they generally differ more from each other
than do the individuals of any one species or variety in a state of
nature. And if we reflect on the vast diversity of the plants and
animals which have been cultivated, and which have varied during all
ages under the most different climates and treatment, we are driven to
conclude that this great variability is due to our domestic
productions having been raised under conditions of life not so uniform
as, and somewhat different from, those to which the parent species had
been exposed under nature. There is, also, some probability in the
view propounded by Andrew Knight, that this variability may be
partly connected with excess of food. It seems clear that organic
beings must be exposed during several generations to new conditions to
cause any great amount of variation; and that, when the organisation
has once begun to vary, it generally continues varying for many
generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat,
still yield new varieties: our oldest, domesticated animals are
still capable of rapid improvement or modification.
As far as I am able to judge, after long attending to the subject,
the conditions of life appear to act in two ways,- directly on the
whole organisation or on certain parts alone, and indirectly by
affecting the reproductive system. With respect to the direct
action, we must bear in mind that in every case, as Professor Weismann
has lately insisted, and as I have incidentally shown in my work on
Variation under Domestication, there are two factors: namely, the
nature of the organism, and the nature of the conditions. The former
seems to be much the more important; for nearly similar variations
sometimes arise under, as far as we can judge, dissimilar
conditions; and, on the other hand, dissimilar variations arise
under conditions which appear to be nearly uniform. The effects on the
offspring are either definite or indefinite. They may be considered as
definite when all or nearly all the offspring of individuals exposed
to certain conditions during several generations are modified in the
same manner. It is extremely difficult to come to any conclusion in
regard to the extent of the changes which have been thus definitely
induced. There can, however, be little doubt about many slight
changes,- such as size from the amount of food, colour from the nature
of the food, thickness of the skin and hair from climate, &c. Each
of the endless variations which we see in the plumage of our fowls
must have had some efficient cause; and if the same cause were to
act uniformly during a long series of generations on. many
individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out-growths which variably
follow from the insertion of a minute drop of poison by a
gall-producing insect, show us what singular modifications might
result in the case of plants from a chemical change in the nature of
the sap.
Indefinite variability is a much more common result of changed
conditions than definite variability, and has probably played a more
important part in the formation of our domestic races. We see
indefinite variability in the endless slight peculiarities which
distinguish the individuals of the same species, and which cannot be
accounted for by inheritance from either parent or from some more
remote ancestor. Even strongly marked differences occasionally
appear in the young of the same litter, and in seedlings from the same
seed-capsule. At long intervals of time, out of millions of
individuals reared in the same country and fed on nearly the same
food, deviations of structure so strongly pronounced as to deserve
to be called monstrosities arise; but monstrosities cannot be
separated by any distinct line from slighter variations. All such
changes of structure, whether extremely slight or strongly marked,
which appear amongst many individuals living together, may be
considered as the indefinite effects of the conditions of life on each
individual organism, in nearly the same manner as the chill affects
different men in an indefinite manner, according to their state of
body or constitution, causing coughs or colds, rheumatism, or
inflammation of various organs.
With respect to what I have called the indirect action of changed
conditions, namely, through the reproductive system of being affected,
we may infer that variability is thus induced, partly from the fact of
this system being extremely sensitive to any change in the conditions,
and partly from the similarity, as Kreuter and others have remarked,
between the variability which follows from the crossing of distinct
species, and that which may be observed with plants and animals when
reared under new or unnatural conditions. Many facts clearly show
how eminently susceptible the reproductive system is to very slight
changes in the surrounding conditions. Nothing is more easy than to
tame an animal, and few things more difficult than to get it to
breed freely under confinement, even when the male and female unite.
How many animals there are which will not breed, though kept in an
almost free state in their native country! This is generally, but
erroneously, attributed to vitiated instincts. Many cultivated
plants display the utmost vigour, and yet rarely or never seed! In
some few cases it has been discovered that a very trifling change,
such as a little more or less water at some particular period of
growth, will determine whether or not a plant will produce seeds. I
cannot here give the details which I have collected and elsewhere
published on this curious subject; but to show how singular the laws
are which determine the reproduction of animals under confinement, I
may mention that carnivorous animals, even from the tropics, breed
in this country pretty freely under confinement, with the exception of
the plantigrades or bear family, which seldom produce young; whereas
carnivorous birds, with the rarest exceptions, hardly ever lay fertile
eggs. Many exotic plants have pollen utterly worthless, in the same
condition as in the most sterile hybrids. When, on the one hand, we
see domesticated animals and plants, though often weak and sickly,
breeding freely under confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature perfectly
tamed, long-lived and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail to act, we need not be surprised at
this system, when it does act under confinement, acting irregularly,
and producing offspring somewhat unlike their parents. I may add, that
as some organisms breed freely under the most unnatural conditions
(for instance, rabbits and ferrets kept in hutches), showing that
their reproductive organs are not easily affected; so will some
animals and plants withstand domestication or cultivation, and vary
very slightly- perhaps hardly more than in a state of nature.
Some naturalists have maintained that all variations are connected
with the act of sexual reproduction; but this is certainly an error;
for I have given in another work a long list of "sporting plants,"
as they are called by gardeners;- that is, of plants which have
suddenly produced a single bud with a new and sometimes widely
different character from that of the other buds on the same plant.
These bud variations, as they may be named, can be propagated by
grafts, offsets, &c., and sometimes by seed. They occur rarely under
nature, but are far from rare under culture. As a single bud out of
the many thousands, produced year after year on the same tree under
uniform conditions, has been known suddenly to assume a new character;
and as buds on distinct trees, growing under different conditions,
have sometimes yielded nearly the same variety- for instance, buds
on peach-trees producing nectarines, and buds on common roses
producing moss-roses- we clearly see that the nature of the conditions
is of subordinate importance in comparison with the nature of the
organism in determining each particular form of variation;- perhaps of
not more importance than the nature of the spark, by which a mass of
combustible matter is ignited, has in determining the nature of the
flames.
Effects of Habit and of the Use or Disuse of Parts; Correlated
Variation; Inheritance
Changed habits produce an inherited effect, as in the period of
the flowering of plants when transported from one climate to
another. With animals the increased use or disuse of parts has had a
more marked influence; thus I find in the domestic duck that the bones
of the wing weigh less and the bones of the leg more, in proportion to
the whole skeleton, than do the same bones in the wild-duck; and
this change may be safely attributed to the domestic duck flying
much less, and walking more, than its wild parents. The great and
inherited development of the udders in cows and goats in countries
where they are habitually milked, in comparison with these organs in
other countries, is probably another instance of the effects of use.
Not one of our domestic animals can be named which has not in some
country drooping ears; and the view which has been suggested that
the drooping is due to disuse of the muscles of the ear, from the
animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly seen,
and will hereafter be briefly discussed. I will here only allude to
what may be called correlated variation. Important changes in the
embryo or larva will probably entail changes in the mature animal.
In monstrosities, the correlations between quite distinct parts are
very curious; and many instances are given in Isidore Geoffroy
St-Hilaire's great work on this subject. Breeders believe that long
limbs are almost always accompanied by an elongated head. Some
instances of correlation are quite whimsical: thus cats which are
entirely white and have blue eyes are generally deaf; but it has
been lately stated by Mr. Tait that this is confined to the males.
Colour and constitutional peculiarities go together, of which many
remarkable cases could be given amongst animals and plants. From facts
collected by Heusinger, it appears that white sheep and pigs are
injured by certain plants, whilst dark-coloured individuals escape:
Professor Wyman has recently communicated to me a good illustration of
this fact; on asking some farmers in Virginia how it was that all
their pigs were black, they informed him that the pigs ate the
paint-root (Lachnanthes), which coloured their bones pink, and which
caused the hoofs of all but the black varieties to drop off; and one
of the "crackers" (i.e. Virginia squatters) added, "we select the
black members of a litter for raising, as they alone have a good
chance of living." Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many
horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly modify unintentionally other
parts of the structure, owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly understood laws of
variation are infinitely complex and diversified. It is well worth
while carefully to study the several treatises on some of our old
cultivated plants, as on the hyacinth, potato, even the dahlia, &c.;
and it is really surprising to note the endless points of structure
and constitution in which the varieties and sub-varieties differ
slightly from each other. The whole organisation seems to have
become plastic, and departs in a slight degree from that of the
parental type.
Any variation which is not inherited is unimportant for us. But
the number and diversity of inheritable deviations of structure,
both those of slight and those of considerable physiological
importance, are endless. Dr. Prosper Lucas's treatise, in two large
volumes, is the fullest and the best on this subject. No breeder
doubts how strong is the tendency to inheritance; that like produces
like is his fundamental belief: doubts have been thrown on this
principle only by theoretical writers. When any deviation of structure
often appears, and we see it in the father and child, we cannot tell
whether it may not be due to the same cause having acted on both;
but when amongst individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary
combination of circumstances, appears in the parent- say, once amongst
several million individuals- and it reappears in the child, the mere
doctrine of chances almost compels us to attribute its reappearance to
inheritance. Every one must have heard of cases of albinism, prickly
skin, hairy bodies, &c., appearing in several members of the same
family. If strange and rare deviations of structure are really
inherited, less strange and commoner deviations may be freely admitted
to be inheritable. Perhaps the correct way of viewing the whole
subject would be, to look at the inheritance of every character
whatever as the rule, and non-inheritance as the anomaly?
The laws governing inheritance are for the most part unknown. No one
can say why the same peculiarity in different individuals of the
same species, or in different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes, or to one
sex alone, more commonly but not exclusively to the like sex. It is
a fact of some importance to us, that peculiarities appearing in the
males of our domestic breeds are often transmitted, either exclusively
or in a much greater degree, to the males alone. A much more important
rule, which I think may be trusted, is that, at whatever period of
life a peculiarity first appears, it tends to reappear in the
offspring at a corresponding age, though sometimes earlier. In many
cases this could not be otherwise; thus the inherited peculiarities in
the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider
extension, and that, when there is no apparent reason why a
peculiarity should appear at any particular age, yet that it does tend
to appear in the offspring at the same period at which it first
appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are
of course confined to the first appearance of the peculiarity, and not
to the primary cause which may have acted on the ovules or on the male
element; in nearly the same manner as the increased length of the
horns in the offspring from a short-horned cow by a long-horned
bull, though appearing late in life, is clearly due to the male
element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists- namely, that our domestic
varieties, when run wild, gradually but invariably revert in character
to their aboriginal stocks. Hence it has been argued that no
deductions can be drawn from domestic races to species in a state of
nature. I have in vain endeavoured to discover on what decisive
facts the above statement has so often and so boldly been made.
There would be great difficulty in proving its truth: we may safely
conclude that very many of the most strongly marked domestic varieties
could not possibly live in a wild state. In many cases, we do not know
what the aboriginal stock was, and so could not tell whether or not
nearly perfect reversion had ensued. It would be necessary, in order
to prevent the effects of intercrossing, that only a single variety
should have been turned loose in its new home. Nevertheless, as our
varieties certainly do occasionally revert in some of their characters
to ancestral forms, it seems to me not improbable that if we could
succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in
very poor soil (in which case, however, some effect would have to be
attributed to the definite action of the poor soil), that they
would, to a large extent, or even wholly, revert to the wild
aboriginal stock. Whether or not the experiment would succeed, is
not of great importance for our line of argument; for by the
experiment itself the conditions of life are changed. If it could be
shown that our domestic varieties manifested a strong tendency to
reversion,- that is, to lose their acquired characters, whilst kept
under the same conditions, and whilst kept in a considerable body,
so that free intercrossing might check, by blending together, any
slight deviations in their structure, in such case, I grant that we
could deduce nothing from domestic varieties in regard to species. But
there is not a shadow of evidence in favour of this view: to assert
that we could not breed our cart- and race-horses, long and
short-horned cattle, and poultry of various breeds, and esculent
vegetables, for an unlimited number of generations, would be opposed
to all experience.
Character of Domestic Varieties; Difficulty of distinguishing
between Varieties and Species; Origin of Domestic Varieties from one
or more Species
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often
have a somewhat monstrous character; by which I mean, that, although
differing from each other, and from other species of the same genus,
in several trifling respects, they often differ in an extreme degree
in some one part, both when compared one with another, and more
especially when compared with the species under nature to which they
are nearest allied. With these exceptions (and with that of the
perfect fertility of varieties when crossed,- a subject hereafter to
be discussed), domestic races of the same species differ from each
other in the same manner as do the closely-allied species of the
same genus in a state of nature, but the differences in most cases are
less in degree. This must be admitted as true, for the domestic
races of many animals and plants have been ranked by some competent
judges as the descendants of aboriginally distinct species, and by
other competent judges as mere varieties. If any well marked
distinction existed between a domestic race and a species, this source
of doubt would not so perpetually recur. It has often been stated that
domestic races do not differ from each other in character of generic
value. It can be shown that this statement is not correct; but
naturalists differ much in determining what characters are of
generic value; all such valuations being at present empirical. When it
is explained how genera originate under nature, it will be seen that
we have no right to expect often to find a generic amount of
difference in our domesticated races.
In attempting to estimate the amount of structural difference
between allied domestic races, we are soon involved in doubt, from not
knowing whether they are descended from one or several parent species.
This point, if it could be cleared up, would be interesting; if, for
instance, it could be shown that the greyhound, bloodhound, terrier,
spaniel, and bull-dog, which we all know propagate their kind truly,
were the offspring of any single species, then such facts would have
great weight in making us doubt about the immutability of the many
closely allied natural species- for instance, of the many foxes-
inhabiting different quarters of the world. I do not believe, as we
shall presently see, that the whole amount of difference between the
several breeds of the dog has been produced under domestication; I
believe that a small part of the difference is due to their being
descended from distinct species. In the case of strongly marked
races of some other domesticated species, there is presumptive or even
strong evidence, that all are descended from a single wild stock.
It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to
vary, and likewise to withstand diverse climates. I do not dispute
that these capacities have added largely to the value of most of our
domesticated productions: but how could a savage possibly know, when
he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the
little variability of the ass and goose, or the small power of
endurance of warmth by the reindeer, or of cold by the common camel,
prevented their domestication? I cannot doubt that if other animals
and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from
a state of nature, and could be made to breed for an equal number of
generations under domestication, they would on an average vary as
largely as the parent species of our existing domesticated productions
have varied.
In the case of most of our anciently domesticated animals and
plants, it is not possible to come to any definite conclusion, whether
they are descended from one or several wild species. The argument
mainly relied on by those who believe in the multiple origin of our
domestic animals is, that we find in the most ancient times, on the
monuments of Egypt, and in the lake-habitations of Switzerland, much
diversity in the breeds; and that some of these ancient breeds closely
resemble, or are even identical with, those still existing. But this
only throws far backwards the history of civilisation, and shows
that animals were domesticated at a much earlier period than has
hitherto been supposed. The lake-inhabitants of Switzerland cultivated
several kinds of wheat and barley, the pea, the poppy for oil, and
flax; and they possessed several domesticated animals. They also
carried on commerce with other nations. All this clearly shows, as
Reer has remarked, that they had at this early age progressed
considerably in civilisation; and this again implies a long
continued previous period of less advanced civilisation, during
which the domesticated animals, kept by different tribes in
different districts, might have varied and given rise to distinct
races. Since the discovery of flint tools in the superficial
formations of many parts of the world, all geologists believe that
barbarian man existed at an enormously remote period; and we know that
at the present day there is hardly a tribe so barbarous, as not to
have domesticated at least the dog.
The origin of most of our domestic animals will probably for ever
remain vague. But I may here state, that, looking to the domestic dogs
of the whole world, I have, after a laborious collection of all
known facts, come to the conclusion that several wild species of
Canidae have been tamed, and that their blood, in some cases mingled
together, flows in the veins of our domestic breeds. In regard to
sheep and goats I can form no decided opinion. From facts communicated
to me by Mr. Blyth, on the habits, voice, constitution, and
structure of the humped Indian cattle, it is almost certain that
they are descended from a different aboriginal stock from our European
cattle; and some competent judges believe that these latter have had
two or three wild progenitors,- whether or not these deserve to be
called species. This conclusion, as well as that of the specific
distinction between the humped and common cattle, may, indeed, be
looked upon as established by the admirable researches of Professor
Rutimeyer. With respect to horses, from reasons which I cannot here
give, I am doubtfully inclined to believe, in opposition to several
authors, that all the races belong to the same species. Having kept
nearly all the English breeds of the fowl alive, having bred and
crossed them, and examined their skeletons, it appears to me almost
certain that all are the descendants of the wild Indian fowl, Gallus
bankiva; and this is the conclusion of Mr. Blyth, and of others who
have studied this bird in India. In regard to ducks and rabbits,
some breeds of which differ much from each other, the evidence is
clear that they are all descended from the common wild duck and
rabbit.
The doctrine of the origin of our several domestic races from
several aboriginal stocks, has been carried to an absurd extreme by
some authors. They believe that every race which breeds true, let
the distinctive characters be ever so slight, has had its wild
prototype. At this rate there must have existed at least a score of
species of wild cattle, as many sheep, and several goats, in Europe
alone, and several even within Great Britain. One author believes that
there formerly existed eleven wild species of sheep peculiar to
Great Britain! When we bear in mind that Britain has now not one
peculiar mammal, and France but few distinct from those of Germany,
and so with Hungary, Spain, &c., but that each of these kingdoms
possesses several peculiar breeds of cattle, sheep, &c., we must admit
that many domestic breeds must have originated in Europe; for whence
otherwise could they have been derived? So it is in India. Even in the
case of the breeds of the domestic dog throughout the world, which I
admit are descended from several wild species, it cannot be doubted
that there has been an immense amount of inherited variation; for
who will believe that animals closely resembling the Italian
greyhound, the bloodhound, the bull-dog, pug-dog, or Blenheim spaniel,
&c.- so unlike all wild Canidae- ever existed in a state of nature? It
has often been loosely said that all our races of dogs have been
produced by the crossing of a few aboriginal species; but by
crossing we can only get forms in some degree intermediate between
their parents; and if we account for our several domestic races by
this process, we must admit the former existence of the most extreme
forms, as the Italian greyhound, bloodhound, bulldog, &c., in the wild
state. Moreover, the possibility of making distinct races by
crossing has been greatly exaggerated. Many cases are on record,
showing that a race may be modified by occasional crosses, if aided by
the careful selection of the individuals which present the desired
character; but to obtain a race intermediate between two quite
distinct races, would be very difficult. Sir J. Sebright expressly
experimented with this object and failed. The offspring from the first
cross between two pure breeds is tolerably and sometimes (as I have
found with pigeons) quite uniform in character, and everything seems
simple enough; but when these mongrels are crossed one with another
for several generations, hardly two of them are alike and then the
difficulty of the task becomes manifest.
Breeds of the Domestic Pigeon, their Differences and Origin
Believing that it is always best to study some special group, I
have, after deliberation, taken up domestic pigeons. I have kept every
breed which I could purchase or obtain, and have been most kindly
favoured with skins from several quarters of the world, more
especially by the Hon. W. Elliot from India, and by the Hon. C. Murray
from Persia. Many treatises in different languages have been published
on pigeons, and some of them are very important, as being of
considerable antiquity. I have associated with several eminent
fanciers, and have been permitted to join two of the London Pigeon
Clubs. The diversity of the breeds is something astonishing. Compare
the English carrier and the short-faced tumbler, and see the wonderful
difference in their beaks, entailing corresponding differences in
their skulls. The carrier, more especially the male bird, is also
remarkable from the wonderful development of the carunculated skin
about the head; and this is accompanied by greatly elongated
eyelids, very large external orifices to the nostrils, and a wide gape
of mouth. The short-faced tumbler has a beak in outline almost like
that of a finch; and the common tumbler has the singular inherited
habit of flying at a great height in a compact flock, and tumbling
in the air head over heels. The runt is a bird of great size, with
long massive beak and large feet; some of the sub-breeds of runts have
very long necks, others very long wings and tails, others singularly
short tails. The barb is allied to the carrier, but, instead of a long
beak has a very short and broad one. The pouter has a much elongated
body, wings, and legs; and its enormously developed crop, which it
glories in inflating, may well excite astonishment and even
laughter. The turbit has a short and conical beak, with a line of
reversed feathers down the breast; and it has the habit of continually
expanding slightly, the upper part of the oesophagus. The Jacobin
has the feathers so much reversed along the back of the neck that they
form a hood; and it has, proportionally to its size, elongated wing
and tail feathers. The trumpeter and laugher, as their names
express, utter a very different coo from the other breeds. The fantail
has thirty or even forty tailfeathers, instead of twelve or
fourteen- the normal number in all the members of the great pigeon
family: these feathers are kept expanded, and are carried so erect,
that in good birds the head and tail touch: the oil-gland is quite
aborted. Several other less distinct breeds might be specified.
In the skeletons of the several breeds, the development of the bones
of the face in length and breadth and curvature differs enormously.
The shape, as well as the breadth and length of the ramus of the lower
jaw, varies in a highly remarkable manner. The caudal and sacral
vertebrae vary in number; as does the number of the ribs, together
with their relative breadth and the presence of processes. The size
and shape of the apertures in the sternum are highly variable; so is
the degree of divergence and relative size of the two arms of the
furcula. The proportional width of the gape of mouth, the proportional
length of the eyelids, of the orifice of the nostrils, of the tongue
(not always in strict correlation with the length of beak), the size
of the crop and of the upper part of the oesophagus; the development
and abortion of the oil-gland; the number of the primary wing and
caudal feathers; the relative length of the wing and tail to each
other and to the body; the relative length of the leg and foot; the
number of scutellae on the toes, the development of skin between the
toes, are all points of structure which are variable. The period at
which the perfect plumage is acquired varies, as does the state of the
down with which the nestling birds are clothed when hatched. The shape
and size of the eggs vary. The manner of flight, and in some breeds
the voice and disposition, differ remarkably. Lastly, in certain
breeds, the males and females have come to differ in a slight degree
from each other.
Altogether at least a score of pigeons might be chosen, which, if
shown to an ornithologist, and he were told that they were wild birds,
would certainly be ranked by him as well-defined species. Moreover,
I do not believe that any ornithologist would in this case place the
English carrier, the short-faced tumbler, the runt, the barb,
pouter, and fantail in the same genus; more especially as in each of
these breeds several truly-inherited sub-breeds, or species, as he
would call them, could be shown him.
Great as are the differences between the breeds of the pigeon, I
am fully convinced that the common opinion of naturalists is
correct, namely, that all are descended from the rock-pigeon
(Columba livia), including under this term several geographical
races or sub-species, which differ from each other in the most
trifling respects. As several of the reasons which have led me to this
belief are in some degree applicable in other cases, I will here
briefly give them. If the several breeds are not varieties, and have
not proceeded from the rock-pigeon, they must have descended from at
least seven or eight aboriginal stocks; for it is impossible to make
the present domestic breeds by the crossing of any lesser number: how,
for instance, could a pouter be produced by crossing two breeds unless
one of the parent-stocks possessed the characteristic enormous crop?
The supposed aboriginal stocks must all have been rock-pigeons, that
is, they did not breed or willingly perch on trees. But besides C.
livia, with its geographical sub-species, only two or three other
species of rock-pigeons are known; and these have not any of the
characters of the domestic breeds. Hence the supposed aboriginal
stocks must either still exist in the countries where they were
originally domesticated, and yet be unknown to ornithologists; and
this, considering their size, habits, and remarkable characters, seems
improbable; or they must have become extinct in the wild state. But
birds breeding on precipices, and good fliers, are unlikely to be
exterminated; and the common rock-pigeon, which has the same habits
with the domestic breeds, has not been exterminated even on several of
the smaller British islets, or on the shores of the Mediterranean.
Hence the supposed extermination of so many species having similar
habits with the rock-pigeon seems a very rash assumption. Moreover,
the several above-named domesticated breeds have been transported to
all parts of the world, and, therefore, some of them must have been
carried back again into their native country; but not one has become
wild or feral, though the dovecot-pigeon, which is the rock-pigeon
in very slightly altered state, has become feral in several places.
Again, all recent experience shows that it is difficult to get wild
animals to breed freely under domestication, yet on the hypothesis
of the multiple origin of our pigeons, it must be assumed that at
least seven or eight species were so thoroughly domesticated in
ancient times by half-civilised man, as to be quite prolific under
confinement.
An argument of great weight, and applicable in several other
cases, is, that the above-specified breeds, though agreeing
generally with the wild rock-pigeon in constitution, habits, voice,
colouring, and in most parts of their structure, yet are certainly
highly abnormal in other parts; we may look in vain through the
whole great family of Columbidae for a beak like that of the English
carrier, or that of the short-faced tumbler, or barb; for reversed
feathers like those of the Jacobin; for a crop like that of the
pouter; for tail-feathers like those of the fantail. Hence it must
be assumed not only that half-civilised man succeeded in thoroughly
domesticating several species, but that he intentionally or by
chance picked out extraordinarily abnormal species; and further,
that these very species have since all become extinct or unknown. So
many strange contingencies are improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, with white loins;
but the Indian sub-species, C. intermedia of Strickland, has this part
bluish. The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black
bars. Some semi-domestic breeds, and some truly wild breeds, have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the
whole family. Now, in every one of the domestic breeds, taking
thoroughly well-bred birds, all the above marks, even to the white
edging of the outer tail-feathers, sometimes concur perfectly
developed. Moreover, when birds belonging to two or more distinct
breeds are crossed, none of which are blue or have any of the
above-specified marks, the mongrel offspring are very apt suddenly
to acquire these characters. To give one instance out of several which
I have observed:- I crossed some white fantails, which breed very
true, with some black barbs- and it so happens that blue varieties
of barbs are so rare that I never heard of an instance in England; and
the mongrels were black, brown, and mottled. I also crossed a barb
with a spot, which is a white bird with a red tail and red spot on the
forehead, and which notoriously breeds very true; the mongrels were
dusky and mottled. I then crossed one of the mongrel barb-fantails
with a mongrel barb-spot, and they produced a bird of as beautiful a
blue colour, with the white loins, double black wing-bar, and barred
and white-edged tail-feathers, as any wild-rock pigeon! We can
understand these facts, on the well-known principle of reversion to
ancestral characters, if all the domestic breeds are descended from
the rock-pigeon. But if we deny this, we must make one of the two
following highly improbable suppositions. Either, first, that all
the several imagined aboriginal stocks were coloured and marked like
the rock-pigeon, although no other existing species is thus coloured
and marked, so that in each separate breed there might be a tendency
to revert to the very same colours and markings. Or, secondly, that
each breed, even the purest, has within a dozen, or at most within a
score, of generations, been crossed by the rock-pigeon: I say within
dozen or twenty generations, for no instance is known of crossed
descendants reverting to an ancestor of foreign blood, removed by a
greater number of generations. In a breed which has been crossed
only once, the tendency to revert to any character derived from such a
cross will naturally become less and less, as in each succeeding
generation there will be less of the foreign blood; but when there has
been no cross, and there is a tendency in the breed to revert to a
character which was lost during some former generation, this tendency,
for all that we can see to the contrary, may be transmitted
undiminished for an indefinite number of generations. These two
distinct cases of reversion are often confounded together by those who
have written on inheritance.
Lastly, the hybrids or mongrels from between all the breeds of the
pigeon are perfectly fertile, as I can state from my own observations,
purposely made, on the most distinct breeds. Now, hardly any cases
have been ascertained with certainty of hybrids from two quite
distinct species of animals being perfectly fertile. Some authors
believe that long-continued domestication eliminates this strong
tendency to sterility in species. From the history of the dog, and
of some other domestic animals, this conclusion is probably quite
correct, if applied to species closely related to each other. But to
extend it so far as to suppose that species, aboriginally as
distinct as carriers, tumblers, pouters, and fantails now are,
should yield offspring perfectly fertile inter se, would be rash in
the extreme.
From these several reasons, namely,- the improbability of man having
formerly made seven or eight supposed species of pigeons to breed
freely under domestication;- these supposed species being quite
unknown in a wild state, and their not having become anywhere
feral;- these species presenting certain very abnormal characters,
as compared with all other Columbidae, though so like the
rock-pigeon in most respects;- the occasional reappearance of the blue
colour and various black marks in all the breeds, both when kept
pure and when crossed;- and lastly, the mongrel offspring being
perfectly fertile;- from these several reasons taken together, we
may safely conclude that all our domestic breeds are descended from
the rock-pigeon or Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that the wild C. livia
has been found capable of domestication in Europe and in India; and
that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, that, although an
English carrier or a short-faced tumbler differs immensely in
certain characters from the rock-pigeon, yet that, by comparing the
several sub-breeds of these two races, more especially those brought
from distant countries, we can make, between them and the rock-pigeon,
an almost perfect series; so we can in some other cases, but not
with all the breeds. Thirdly, those characters which are mainly
distinctive of each breed are in each eminently variable, for instance
the wattle and length of beak of the carrier, the shortness of that of
the tumbler, and the number of tailfeathers in the fantail; and the
explanation of this fact will be obvious when we treat of Selection.
Fourthly, pigeons have been watched and tended with the utmost care,
and loved by many people. They have been domesticated for thousands of
years in several quarters of the world; the earliest known record of
pigeons is in the fifth AEgyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr. Birch informs me
that pigeons are given in a bill of fare in the previous dynasty. In
the time of the Romans, as we hear from Pliny, immense prices were
given for pigeons; "nay, they are come to this pass, that they can
reckon up their pedigree and race." Pigeons were much valued by
Akber Khan in India, about the year 1600; never less than 90,000
pigeons were taken with the court. "The monarchs of Iran and Turan
sent him some very rare birds"; and continues the courtly historian,
"His Majesty by crossing the breeds, which method was never
practised before, has improved them astonishingly." About this same
period the Dutch were as eager about pigeons as were the old Romans.
The paramount importance of these considerations in explaining the
immense amount of variation which pigeons have undergone, will
likewise be obvious when we treat of Selection. We shall then, also,
see how it is that the several breeds so often have a somewhat
monstrous character. It is also a most favourable circumstance for the
production of distinct breeds, that male and female pigeons can be
easily mated for life; and thus different breeds can be kept
together in the same aviary.
I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons
and watched the several kinds, well knowing how truly they breed, I
felt fully as much difficulty in believing that since they had been
domesticated they had all proceeded from a common parent, as any
naturalist could in coming to a similar conclusion in regard to the
many species of finches, or other groups of birds, in nature. One
circumstance has struck me much; namely, that nearly all the
breeders of the various domestic animals and the cultivators of
plants, with whom I have conversed, or whose treatises I have read,
are firmly convinced that the several breeds to which each has
attended, are descended from so many aboriginally distinct species.
Ask, as I have asked, a celebrated raiser of Hereford cattle,
whether his cattle might not have descended from long-horns, or both
from a common parent-stock, and he will laugh you to scorn. I have
never met a pigeon, or poultry, or duck, or rabbit fancier, who was
not fully convinced that each main breed was descended from a distinct
species. Van Mons, in his treatise on pears and apples, shows how
utterly he disbelieves that the several sorts, for instance a
Ribston-pippin or Codlin-apple, could ever have proceeded from the
seeds of the same tree. Innumerable other examples could be given. The
explanation, I think, is simple: from long-continued study they are
strongly impressed with the differences between the several races; and
though they well know that each race varies slightly, for they win
their prizes by selecting such slight differences, yet they ignore all
general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May not
those naturalists who, knowing far less of the laws of inheritance
than does the breeder, and knowing no more than he does of the
intermediate links in the long lines of descent, yet admit that many
of our domestic races are descended from the same parents- may they
not learn a lesson of caution, when they deride the idea of species in
a state of nature being lineal descendants of other species?
Principles of Selection anciently followed, and their Effects
Let us now briefly consider the steps by which domestic races have
been produced, either from one or from several allied species. Some
effect may be attributed to the direct and definite action of the
external conditions of life, and some to habit; but he would be a bold
man who would account by such agencies for the differences between a
dray- and race-horse, a greyhound and bloodhound, a carrier and
tumbler pigeon. One of the most remarkable features in our
domesticated races is that we see in them adaptation, not indeed to
the animal's or plant's own good, but to man's use or fancy. Some
variations useful to him have probably arisen suddenly, or by one
step; many botanists, for instance, believe that the fuller's
teasel, with its hooks, which cannot be rivalled by any mechanical
contrivance, is only a variety of the wild Dipsacus; and this amount
of change may have suddenly arisen in a seedling. So it has probably
been with the turnspit dog; and this is known to have been the case
with the ancon sheep. But when we compare the dray-horse and
race-horse, the dromedary and camel, the various breeds of sheep
fitted either for cultivated land or mountain pasture, with the wool
of one breed good for one purpose, and that of another breed for
another purpose; when we compare the many breeds of dogs, each good
for man in different ways; when we compare the game-cock, so
pertinacious in battle, with other breeds so little quarrelsome,
with "everlasting layers" which never desire to sit, and with the
bantam so small and elegant; when we compare the host of agricultural,
culinary, orchard, and flower-garden races of plants, most useful to
man at different seasons and for different purposes, or so beautiful
in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed, in
many cases, we know that this has not been their history. The key is
man's power of accumulative selection: nature gives successive
variations; man adds them up in certain directions useful to him. In
this sense he may be said to have made for himself useful breeds.
The great power of this principle of selection is not
hypothetical. It is certain that several of our eminent breeders have,
even within a single lifetime, modified to a large extent their breeds
of cattle and sheep. In order fully to realise what they have done, it
is almost necessary to read several of the many treatises devoted to
this subject, and to inspect the animals. Breeders habitually speak of
an animal's organisation as something plastic, which they can model as
they please. If I had space I could quote numerous passages to this
effect from highly competent authorities. Youatt, who was probably
better acquainted with the works of agriculturists than almost any
other individual, and who was himself a very good judge of animals,
speaks of the principle of selection as "that which enables the
agriculturist, not only to modify the character of his flock, but to
change it altogether. It is the magician's wand, by means of which
he may summon into life whatever form and mould he pleases." Lord
Somerville, speaking of what breeders have done for sheep, says:-
"It would seem as if they had chalked out upon a wall a form perfect
in itself, and then had given it existence." In Saxony the
importance of the principle of selection in regard to merino sheep
is so fully recognised, that men follow it as a trade: the sheep are
placed on a table and are studied, like a picture by a connoisseur;
this is done three times at intervals of months, and the sheep are
each time marked and classed, so that the very best may ultimately
be selected for breeding.
What English breeders have actually effected is proved by the
enormous prices given for animals with a good pedigree; and these have
been exported to almost every quarter of the world. The improvement is
by no generally due to crossing different breeds; all the best
breeders are strongly opposed to this practice, except sometimes
amongst closely allied sub-breeds. And when a cross has been made, the
closest selection is far more indispensable even than in ordinary
cases. If selection consisted merely in separating some very
distinct variety, and breeding from it, the principle would be so
obvious as hardly to be worth notice; but its importance consists in
the great effect produced by the accumulation in one direction, during
successive generations, of differences absolutely inappreciable by
an uneducated eye- differences which I for one have vainly attempted
to appreciate. Not one man in a thousand has accuracy of eye and
judgment sufficient to become an eminent breeder. If, gifted with
these qualities, he studies his subject for years, and devotes his
lifetime to it with indomitable perseverance, he will succeed, and may
make great improvements; if he wants any of these qualities, he will
assuredly fail. Few would readily believe in the natural capacity
and years of practice requisite to become even a skilful pigeon
fancier.
The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our
choicest productions have been produced by a single variation from the
aboriginal stock. We have proofs that this has not been so in
several cases in which exact records have been kept; thus, to give a
very trifling instance, the steadily-increasing size of the common
gooseberry may be quoted. We see an astonishing improvement in many
florists' flowers, when the flowers of the present day are compared
with drawings made only twenty or thirty years ago. When a race of
plants is once pretty well established, the seed-raisers do not pick
out the best plants, but merely go over their seed-beds, and pull up
the "rogues," as they call the plants that deviate from the proper
standard. With animals this kind of selection is, in fact, likewise
followed; for hardly any one is so careless as to breed from his worst
animals.
In regard to plants, there is another means of observing the
accumulated effects of selection- namely, by comparing the diversity
of flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen garden, in comparison with the
flowers of the same varieties; and the diversity of fruit of the
same species in the orchard, in comparison with the leaves and flowers
of the same set of varieties. See how different the leaves of the
cabbage are, and how extremely alike the flowers; how unlike the
flowers of the heartsease are, and how alike the leaves; how much
the fruit of the different kinds of gooseberries differ in size,
colour, shape, and hairiness, and yet the flowers present very
slight differences. It is not that the varieties which differ
largely in some one point do not differ at all in other points; this
is hardly ever,- I speak after careful observation, perhaps never, the
case. The law of correlated variation, the importance of which
should never be overlooked, will ensure some differences; but, as a
general rule, it cannot be doubted that the continued selection of
slight variations, either in the leaves, the flowers, or the fruit,
will produce races differing from each other chiefly in these
characters.
It may be objected that the principle of selection has been
reduced to methodical practice for scarcely more than three-quarters
of a century; it has certainly been more attended to of late years,
and many treatises have been published on the subject; and the
result has been, in a corresponding degree, rapid and important. But
it is very far from true that the principle is a modern discovery. I
could give several references to works of high antiquity, in which the
full importance of the principle is acknowledged. In rude and
barbarous periods of English history choice animals were often
imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may
be compared to the "roguing" of plants by nurserymen. The principle of
selection I find distinctly given in an ancient Chinese encyclopaedia.
Explicit rules are laid down by some of the Roman classical writers.
From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes
cross their dogs with wild canine animals, to improve the breed, and
they formerly did so, as is attested by passages in Pliny. The savages
in South Africa match their draught cattle by colour, as do some of
the Esquimaux their teams of dogs. Livingstone states that good
domestic breeds are highly valued by the negroes in the interior of
Africa who have not associated with Europeans. Some of these facts
do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is
now attended to by the lowest savages. It would, indeed, have been a
strange fact, had attention not been paid to breeding, for the
inheritance of good and bad qualities is so obvious.
Unconscious Selection
At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything of the kind in the country. But, for our purpose,
a form of Selection, which may be called Unconscious, and which
results from every one trying to possess and breed from the best
individual animals, is more important. Thus, a man who intends keeping
pointers naturally tries to get as good dogs as he can, and afterwards
breeds from his own best dogs, but he has no wish or expectation of
permanently altering the breed. Nevertheless we may infer that this
process, continued during centuries, would improve and modify any
breed, in the same way as Bakewell, Collins, &c., by this very same
process, only carried on more methodically, did greatly modify, even
during their lifetimes, the forms and qualities of their cattle.
Slow and insensible changes of this kind can never be recognised
unless actual measurements or careful drawings of the breeds in
question have been made long ago, which may serve for comparison. In
some cases, however, unchanged, or but little changed individuals of
the same breed exist in less civilised districts, where the breed
has been less improved. There is reason to believe that King Charles's
spaniel has been unconsciously modified to a large extent since the
time of that monarch. Some highly competent authorities are
convinced that the setter is directly derived from the spaniel, and
has probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in
this case the change has, it is believed, been chiefly effected by
crosses with the foxhound; but what concerns us is, that the change
has been effected unconsciously and gradually, and yet so effectually,
that, though the old Spanish pointer certainly came from Spain, Mr.
Borrow has not seen, as I am informed by him, any native dog in
Spain like our pointer.
By a similar process of selection, and by careful training,
English race-horses have come to surpass in fleetness and size the
parent Arabs, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights which they carry. Lord
Spencer and others have shown how the cattle of England have increased
in weight and in early maturity, compared with the stock formerly kept
in this country. By comparing the accounts given in various old
treatises of the former and present state of carrier and tumbler
pigeons in Britain, India, and Persia, we can trace the stages through
which they have insensibly passed, and come to differ so greatly
from the rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconscious, in so far that
the breeders could never have expected, or even wished, to produce the
result which ensued- namely, the production of two distinct strains.
The two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess,
as Mr. Youatt remarks, "have been purely bred from the original
stock of Mr. Bakewell for upwards of fifty years. There is not a
suspicion existing in the mind of any one at all acquainted with the
subject, that the owner of either of them has deviated in any one
instance from the pure blood of Mr. Bakewell's flock, and yet the
difference between the sheep possessed by these two gentlemen is so
great that they have the appearance of being quite different
varieties."
If there exist savages so barbarous as never to think of the
inherited character of the offspring of their domestic animals, yet
any one animal particularly useful to them, for any special purpose,
would be carefully preserved during famines and other accidents, to
which savages are so liable, and such choice animals would thus
generally leave more offspring than the inferior ones; so that in this
case there would be a kind of unconscious selection going on. We see
the value set on animals even by the barbarians of Tierra del Fuego,
by their killing and devouring their old women, in times of dearth, as
of less value than their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked at their first appearance, as
distinct varieties, and whether or not two or more species or races
have become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of the
heartsease, rose, pelargonium, dahlia, and other plants, when compared
with the older varieties or with their parent-stocks. No one would
ever expect to get a first-rate heartsease or dahlia from the seed
of a wild plant. No one would expect to raise a first-rate melting
pear from the seed of the wild pear, though he might succeed from a
poor seedling growing wild, if it had come from a garden-stock. The
pear, though cultivated in classical times, appears, from Pliny's
description, to have been a fruit of very inferior quality. I have
seen great surprise expressed in horticultural works at the
wonderful skill of gardeners, in having produced such splendid results
from such poor materials; but the art has been simple, and, as far
as the final result is concerned, has been followed almost
unconsciously. It has consisted in always cultivating the best-known
variety, sowing its seeds, and, when a slightly better variety chanced
to appear, selecting it, and so onwards. But the gardeners of the
classical period who cultivated the best pears which they could
procure, never thought what splendid fruit we should eat; though we
owe our excellent fruit in some small degree, to their having
naturally chosen and preserved the best varieties they could
anywhere find.
A large amount of change, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a number of cases
we cannot recognise, and therefore do not know, the wild parent-stocks
of the plants which have been longest cultivated in our flower and
kitchen gardens. If it has taken centuries or thousands of years to
improve or modify most of our plants up to their present standard of
usefulness to man, we can understand how it is that neither Australia,
the Cape of Good Hope, nor any other region inhabited by quite
uncivilised man, has afforded us a single plant worth culture. It is
not that these countries, so rich in species, do not by a strange
chance possess the aboriginal stocks of any useful plants, but that
the native plants have not been improved by continued selection up
to a standard of perfection comparable with that acquired by the
plants in countries anciently civilised.
In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure would often succeed
better in the one country than in the other; and thus by a process
of "natural selection," as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains why the
varieties kept by savages, as has been remarked by some authors,
have more of the character of true species than the varieties kept
in civilised countries.
On the view here given of the important part which selection by
man has played, it becomes at once obvious, how it is that our
domestic races show adaptation in their structure or in their habits
to man's wants or fancies. We can, I think, further understand the
frequently abnormal characters of our domestic races, and likewise
their differences being so great in external characters, and
relatively so slight in internal parts or organs. Man can hardly
select, or only with much difficulty, any deviation of structure
excepting such as is externally visible; and indeed he rarely cares
for what is internal. He can never act by selection, excepting on
variations which are first given to him in some slight degree by
nature. No man would ever try to make a fantail till he saw a pigeon
with a tail developed in some slight degree in an unusual manner, or a
pouter till he saw a pigeon with a crop of somewhat unusual size;
and the more abnormal or unusual any character was when it first
appeared, the more likely it would be to catch his attention. But to
use such an expression as trying to make a fantail, is, I have no
doubt, in most cases, utterly incorrect. The man who first selected
a pigeon with a slightly larger tail, never dreamed what the
descendants of that pigeon would become through long-continued, partly
unconscious and partly methodical, selection. Perhaps the
parent-bird of all fantails had only fourteen tail-feathers somewhat
expanded, like the present Java fantail, or like individuals of
other and distinct breeds, in which as many as seventeen tail-feathers
have been counted. Perhaps the first pouter-pigeon did not inflate its
crop much more than the turbit now does the upper part of its
oesophagus,- a habit which is disregarded by all fanciers, as it is
not one of the points of the breed.
Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier's eye: he perceives extremely
small differences, and it is in human nature to value any novelty,
however slight, in one's own possession. Nor must the value which
would formerly have been set on any slight differences in the
individuals of the same species, be judged of by the value which is
now set on them, after several breeds have fairly been established.
I is known that with pigeons many slight variations now occasionally
appear, but these are rejected as faults or deviations from the
standard of perfection in each breed. The common goose has not given
rise to any marked varieties; hence the Toulouse and the common breed,
which differ only in colour, that most fleeting of characters, have
lately been exhibited as distinct at our poultry shows.
These views appear to explain what has sometimes been noticed-
namely, that we know hardly anything about the origin or history of
any of our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have a distinct origin. man
preserves and breeds from an individual with some slight deviation
of structure, or takes more care than usual in matching his best
animals, and thus improves them, and the improved animals slowly
spread in the immediate neighbourhood. But they will as yet hardly
have a distinct name, and from being only slightly valued, their
history will have been disregarded. When further improved by the
same slow and gradual process, they will spread more widely, and
will be recognised as something distinct and valuable, and will then
probably first receive a provincial name. In semi-civilised countries,
with little free communication, the spreading of a new sub-breed would
be a slow process. As soon as the points of value are once
acknowledged, the principle, as I have called it, of unconscious
selection will always tend,- perhaps more at one period than at
another, as the breed rises or falls in fashion,- perhaps more in
one district than in another, according to the state of civilisation
of the inhabitants,- slowly to add to the characteristic features of
the breed, whatever they may be. But the chance will be infinitely
small of any record having been preserved of such slow, varying, and
insensible changes.
Circumstances favourable to Man's Power of Selection
I will now say a few words on the circumstances, favourable, or
the reverse, to man's power of selection. A high degree of variability
is obviously favourable, as freely giving the materials for
selection to work on; not that mere individual differences are not
amply sufficient, with extreme care, to allow of the accumulation of a
large amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept. Hence, number is of the
highest importance for success. On this principle Marshall formerly
remarked, with respect to the sheep of parts of Yorkshire, "as they
generally belong to poor people, and are mostly in small lots, they
never can be improved." On the other hand, nurserymen, from keeping
large stocks of the same plant, are generally far more successful than
amateurs in raising new and valuable varieties. A large number of
individuals of an animal or plant can be reared only where the
conditions for its propagation are favourable. When the individuals
are scanty, all will be allowed to breed, whatever their quality may
be, and this will effectually prevent selection. But probably the most
important element is that the animal or plant should be so highly
valued by man, that the closest attention is paid to even the
slightest deviations in its qualities or structure. Unless such
attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to vary
just when gardeners began to attend to this plant. No doubt the
strawberry had always varied since it was cultivated, but the
slightest varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or
better fruit, and raised seedlings from them, and again picked out the
best seedlings and bred from them, then (with some aid by crossing
distinct species) those many admirable varieties of the strawberry
were raised which have appeared during the last half-century.
With animals, facility in preventing crosses is an important element
in the formation of new races,- at least, in a country which is
already stocked with other races. In this respect enclosure of the
land plays a part. Wandering savages or the inhabitants of open plains
rarely possess more than one breed of the same species. Pigeons can be
mated for life, and this is a great convenience to the fancier, for
thus many races may be improved and kept true, though mingled in the
same aviary; and this circumstance must have largely favoured the
formation of new breeds. Pigeons, I may add, can be propagated in
great numbers and at a very quick rate, and inferior birds may be
freely rejected, as when killed they serve for food. On the other
hand, cats from their nocturnal rambling habits cannot be easily
matched, and, although so much valued by women and children, we rarely
see a distinct breed long kept up; such breeds as we do sometimes
see are almost always imported from some other country. Although I
do not doubt that some domestic animals vary less than others, yet the
rarity or absence of distinct breeds of the cat, the donkey,
peacock, goose, &c., may be attributed in main part to selection not
having been brought into play: in cats, from the difficulty in pairing
them; in donkeys, from only a few being kept by poor people, and
little attention paid to their breeding; for recently in certain parts
of Spain and of the United States this animal has been surprisingly
modified and improved by careful selection: in peacocks, from not
being very easily reared and a large stock not kept: in geese, from
being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of
distinct breeds; but the goose, under the conditions to which it is
exposed when domesticated seems to have a singularly inflexible
organisation, though it has varied to a slight extent, as I have
elsewhere described.
Some authors have maintained that the amount of variation in our
domestic productions is soon reached, and can never afterwards be
exceeded. It would be somewhat rash to assert that the limit has
been attained in any one case; for almost all our animals and plants
have been greatly improved in many ways within a recent period; and
this implies variation. It would be equally rash to assert that
characters now increased to their utmost limit, could not, after
remaining fixed for many centuries, again vary under new conditions of
life. No doubt, as Mr. Wallace has remarked with much truth, a limit
will be at last reached. For instance, there must be a limit to the
fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight of body to be carried, and the
power of contraction in the muscular fibres. But what concerns us is
that the domestic varieties of the same species differ from each other
in almost every character, which man has attended to and selected,
more than do the distinct species of the same genera. Isidore Geoffroy
St-Hilaire has proved this in regard to size, and so it is with colour
and probably with the length of hair. With respect to fleetness, which
depends on many bodily characters, Eclipse was far fleeter, and a
dray-horse is incomparably stronger than any two natural species
belonging to the same genus. So with plants, the seeds of the
different varieties of the bean or maize probably differ more in size,
than do the seeds of the distinct species in any one genus in the same
two families. The same remark holds good in regard to the fruit of the
several varieties of the plum, and still more strongly with the melon,
as well as in many other analogous cases.
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion,
determine whether variations shall endure. Variability is governed
by many unknown laws, of which correlated growth is probably the
most important. Something, but how much we do not know, may be
attributed to the definite action of the conditions of life. Some,
perhaps a great, effect may be attributed to the increased use or
disuse of parts. The final result is thus rendered infinitely complex.
In some cases the intercrossing of aboriginally distinct species
appears to have played an important part in the origin of our
breeds. When several breeds have once been formed in any country,
their occasional intercrossing, with the aid of selection, has, no
doubt, largely aided in the formation of new sub-breeds; but the
importance of crossing has been much exaggerated, both in regard to
animals and to those plants which are propagated by seed. With
plants which are temporarily propagated by cuttings, buds, &c., the
importance of crossing is immense; for the cultivator may here
disregard the extreme variability both of hybrids and of mongrels, and
the sterility of hybrids; but plants not propagated by seed are of
little importance to us, for their endurance is only temporary. Over
all these causes of Change, the accumulative action of Selection,
whether applied methodically and quickly, or unconsciously and
slowly but more efficiently, seems to have been the predominant Power.
CHAPTER II
VARIATION UNDER NATURE
BEFORE applying the principles arrived at in the last chapter to
organic beings in a state of nature, we must briefly discuss whether
these latter are subject to any variation. To treat this subject
properly, a long catalogue of dry facts ought to be given; but these
shall reserve for a future work. Nor shall I here discuss the
various definitions which have been given of the term species. No
one definition has satisfied all naturalists; yet every naturalist
knows vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distant act of creation. The
term "variety" is almost equally difficult to define; but here
community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but they
graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure, generally injurious, or not
useful to the species. Some authors use the term "variation" in a
technical sense, as implying a modification directly due to the
physical conditions of life; and "variations" in this sense are
supposed not to be inherited; but who can say that the dwarfed
condition of shells in the brackish waters of the Baltic, or dwarfed
plants on Alpine summits, or the thicker fur of an animal from far
northwards, would not in some cases be inherited for at least a few
generations? And in this case I presume that the form would be
called a variety.
It may be doubted whether sudden and considerable deviations of
structure such as we occasionally see in our domestic productions,
more especially with plants, are ever permanently propagated in a
state of nature. Almost every part of every organic being is so
beautifully related to its complex conditions of life that it seems as
improbable that any part should have been suddenly produced perfect,
as that a complex machine should have been invented by man in a
perfect state. Under domestication monstrosities sometimes occur which
resemble normal structures in widely different animals. Thus pigs have
occasionally been born with a sort of proboscis, and if any wild
species of the same genus had naturally possessed a proboscis, it
might have been argued that this had appeared as a monstrosity; but
I have as yet failed to find, after diligent search, cases of
monstrosities resembling normal structures in nearly allied forms, and
these alone bear on the question. If monstrous forms of this kind ever
do appear in a state of nature and are capable of reproduction
(which is not always the case), as they occur rarely and singularly,
their preservation would depend on unusually favourable circumstances.
They would, also, during the first and succeeding generations cross
with the ordinary form, and thus their abnormal character would almost
inevitably be lost. But I shall have to return in a future chapter
to the preservation and perpetuation of single or occasional
variations.
Individual Differences
The many slight differences which appear in the offspring from the
same parents, or which it may be presumed have thus arisen, from being
observed in the individuals of the same species inhabiting the same
confined locality, may be called individual differences. No one
supposes that all the individuals of the same species are cast in
the same actual mould. These individual differences are of the highest
importance for us, for they are often inherited, as must be familiar
to every one; and they thus afford materials for natural selection
to act on and accumulate, in the same manner as man accumulates in any
given direction individual differences in his domesticated
productions. These individual differences generally affect what
naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view,
sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at
the number of the cases of variability, even in important parts of
structure, which he could collect on good authority, as I have
collected, during a course of years. It should be remembered that
systematists are far from being pleased at finding variability in
important characters, and that there are not many men who will
laboriously examine internal and important organs, and compare them in
many specimens of the same species. It would never have been
expected that the branching of the main nerves close to the great
central ganglion of an insect would have been variable in the same
species; it might have been thought that changes of this nature
could have been effected only by slow degrees; yet Sir J. Lubbock
has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of a stem of a
tree. This philosophical naturalist, I may add, has also shown that
the muscles in the larvae of certain insects are far from uniform.
Authors sometimes argue in a circle when they state that important
organs never vary; for these same authors practically rank those parts
as important (as some few naturalists have honestly confessed) which
do not vary; and, under this point of view, no instance will ever be
found of an important part varying; but under any other point of
view many instances assuredly can be given.
There is one point connected with individual differences, which is
extremely perplexing: I refer to those genera which have been called
"protean" or "Polymorphic," in which the species present an inordinate
amount of variation. With respect to many of these forms, hardly two
naturalists agree whether to rank them as species or as varieties.
We may instance Rubus, Rosa, and Hieracium amongst plants, several
genera of and of brachiopod shells. In most polymorphic genera some of
the species have fixed and definite characters. Genera which are
polymorphic in one country seem to be, with a few exceptions,
polymorphic in other countries, and likewise, judging from
brachiopod shells, at former periods of time. These facts are very
perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that
we see, at least in some of these polymorphic genera, variations which
are of no service or disservice to the species, and which consequently
have not been seized on and rendered definite by natural selection, as
hereafter to be explained.
Individuals of the same species often present, as is known to
every one, great differences of structure, independently of variation,
as in the two sexes of various animals, in the two or three castes
of sterile females or workers amongst insects, and in the immature and
larval states of many of the lower animals. There are, also, cases
of dimorphism and trimorphism, both with animals and plants. Thus, Mr.
Wallace, who has lately called attention to the subject, has shown
that the females of certain species of butterflies, in the Malayan
archipelago, regularly appear under two or even three conspicuously
distinct forms, not connected by intermediate varieties. Fritz
Muller has described analogous but more extraordinary cases with the
males of certain Brazilian crustaceans: thus, the male of the Tanais
regularly occurs under two distinct forms; one of these has strong and
differently shaped pincers, and the other has antennae much more
abundantly furnished with smelling-hairs. Although in most of these
cases, the two or three forms, both with animals and plants are not
now connected by intermediate gradations, it is probable that they
were once thus connected. Mr. Wallace, for instance, describes a
certain butterfly which presents in the same island a great range of
varieties connected by intermediate links, and the extreme links of
the chain closely resemble the two forms of an allied dimorphic
species inhabiting another part of the Malay Archipelago. Thus also
with ants, the several worker castes are generally quite distinct; but
in some cases, as we shall hereafter see, the castes are connected
together by finely graduated varieties. So it is, as I myself
observed, with some dimorphic plants. It certainly at first appears
a highly remarkable fact that the same female butterfly should have
the power of producing at the same time three distinct female forms
and a male; and that an hermaphrodite plant should produce from the
same seed-capsule three distinct hermaphrodite forms, bearing three
different kinds of females and three or even six different kinds of
males. Nevertheless these cases are only exaggerations of the common
fact that the female produces offspring of two sexes which sometimes
differ from each other in a wonderful manner.
Doubtful Species
The forms which possess in some considerable degree the character of
species, but which are go closely similar to other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of
these doubtful and closely allied forms have permanently retained
their characters for a long time; for as long, as far as we know, as
have good and true species. Practically, when a naturalist can unite
by means of intermediate links any two forms, he treats the one as a
variety of the other; ranking the most common, but sometimes the one
first described, as the species, and the other as the variety. But
cases of great difficulty, which I will not here enumerate,
sometimes arise in deciding whether or not to rank one form as a
variety of another, even when they are closely connected by
intermediate links; nor will the commonly-assumed hybrid nature of the
intermediate forms always remove the difficulty. In very many cases,
however, one form is ranked as a variety of another, not because the
intermediate links have actually been found, but because analogy leads
the observer to suppose either that they do now somewhere exist, or
may formerly have existed; and here a wide door for the entry of doubt
and conjecture is opened.
Hence, in determining whether a form should be ranked as a species
or a variety, the opinion of naturalists having sound judgment and
wide experience seems the only guide to follow. We must, however, in
many cases, decide by a majority of naturalists, for few well-marked
and well-known varieties can be named which have not been ranked as
species by at least some competent judges.
That varieties of this doubtful nature are far from uncommon
cannot be disputed. Compare the several floras of Great Britain, of
France, or of the United States, drawn up by different botanists,
and see what a surprising number of forms have been ranked by one
botanist as good species, and by another as mere varieties. Mr. H.
C. Watson, to whom I lie under deep obligation for assistance of all
kinds, has marked for me 182 British plants, which are generally
considered as varieties, but which have all been ranked by botanists
as species; and, in making this list, he has omitted many trifling
varieties, which nevertheless have been ranked by some botanists as
species, and he has entirely omitted several highly polymorphic
genera. Under genera, including the most polymorphic forms, Mr.
Babington gives 251 species, whereas Mr. Bentham gives only 112,- a
difference of 139 doubtful forms! Amongst animals which unite for each
birth, and which are highly locomotive, doubtful forms, ranked by
one zoologist as a species and by another as a variety, can rarely
be found within the same country, but are common in separated areas.
How many of the birds and insects in North America and Europe, which
differ very slightly from each other, have been ranked by one
eminent naturalist as undoubted species, and by another as
varieties, or, as they are often called, geographical races! Mr.
Wallace, in several valuable papers on the various animals, especially
on the Lepidoptera, inhabiting the islands of the great Malayan
archipelago, shows that they may be classed under four heads,
namely, as variable forms, as local forms, as geographical races or
sub-species, and as true representative species. The first or variable
forms vary much within the limits of the same island. The local
forms are moderately constant and distinct in each separate island;
but when all from the several islands are compared together, the
differences are seen to be so slight and graduated, that it is
impossible to define or describe them, though at the same time the
extreme forms are sufficiently distinct. The geographical races or
sub-species are local forms completely fixed and isolated; but as they
do not differ from each other by strongly marked and important
characters, "there is no possible test but individual opinion to
determine which of them shall be considered as species and which as
varieties." Lastly, representative species fill the same place in
the natural economy of each island as do the local forms and
sub-species; but as they are distinguished from each other by a
greater amount of difference than that between the local forms and
sub-species, they are almost universally ranked by naturalists as true
species. Nevertheless, no certain criterion can possibly be given by
which variable forms, local forms, sub-species, and representative
species can be recognised.
Many years ago, when comparing, and seeing others compare, the birds
from the closely neighbouring islands of the Galapagos Archipelago,
one with another, and with those from the American mainland, I was
much struck how entirely vague and arbitrary is the distinction
between species and varieties. On the islets of the little Madeira
group there are many insects which are characterised as varieties in
Mr. Wollaston's admirable work, but which would certainly be ranked as
distinct species by many entomologists. Even Ireland has a few
animals, now generally regarded as varieties, but which have been
ranked as species by some zoologists. Several experienced
ornithologists consider our British red grouse as only a
strongly-marked race of a Norwegian species, whereas the greater
number rank it as an undoubted species peculiar to Great Britain. A
wide distance between the homes of two doubtful forms leads many
naturalists to rank them as distinct species; but what distance, it
has been well asked, will suffice; if that between America and
Europe is ample, will that between Europe and the Azores, or
Madeira, or the Canaries, or between the several islets of these small
archipelagos, be sufficient?
Mr. B. D. Walsh, a distinguished entomologist of the United
States, has described what he calls phytophagic varieties and
phytophagic species. Most vegetable-feeding insects live on one kind
of plant or on one group of plants; some feed indiscriminately on many
kinds, but do not in consequence vary. In several cases, however,
insects found living on different plants, have been observed by Mr.
Walsh to present in their larval or mature state, or in both states,
slight, though constant differences in colour, size, or in the
nature of their secretions. In some instances the males alone, in
other instances both males and females, have been observed thus to
differ in a slight degree. When the differences are rather more
strongly marked, and when both sexes and all ages are affected, the
forms are ranked by all entomologists as good species. But no observer
can determine for another, even if he can do so for himself, which
of these phytophagic forms ought to be called species and which
varieties. Mr. Walsh ranks the forms which it may be supposed would
freely intercross, as varieties; and those which appear to have lost
this power, as species. As the differences depend on the insects
having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found.
The naturalist thus loses his best guide in determining whether to
rank doubtful forms as varieties or species. This likewise necessarily
occurs with closely allied organisms, which inhabit distinct
continents or islands. When, on the other hand, an animal or plant
ranges over the same continent, or inhabits many islands in the same
archipelago, and presents different forms in the different areas,
there is always a good chance that intermediate forms will be
discovered which will link together the extreme states, and these
are then degraded to the rank of varieties.
Some few naturalists maintain that animals never present
varieties; but then these same naturalists rank the slightest
difference as of specific value; and when the same identical form is
met with in two distant countries, or in two geological formations,
they believe that two distinct species are hidden under the same
dress. The term species thus comes to be a mere useless abstraction,
implying and assuming a separate act of creation. It is certain that
many forms, considered by highly-competent judges to be varieties,
resemble species so completely in character, that they have been
thus ranked by other highly-competent judges. But to discuss whether
they ought to be called species or varieties, before any definition of
these terms has been generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argument,
from geographical distribution, analogical variation, hybridism,
&c., have been brought to bear in the attempt to determine their rank;
but space does not here permit me to discuss them. Close
investigation, in many cases, will no doubt bring naturalists to agree
how to rank doubtful forms. Yet it must be confessed that it is in the
best-known countries that we find the greatest number of them. I
have been struck with the fact, that if any animal or plant in a state
of nature be highly useful to man, or from any cause closely
attracts his attention, varieties of it will almost universally be
found recorded. These varieties, moreover, will often be ranked by
some authors as species. Look at the common oak, how closely it has
been studied; yet a German author makes more than a dozen species
out of forms, which are almost universally considered by other
botanists to be varieties; and in this country the highest botanical
authorities and practical men can be quoted to show that the sessile
and pedunculated oaks are either good and distinct species or mere
varieties.
I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have
worked on them with more zeal and sagacity. He first gives in detail
all the many points of structure which vary in the several species,
and estimates numerically the relative frequency of the variations. He
specifies above a dozen characters which may be found varying even
on the same branch, sometimes according to age or development,
sometimes without any assignable reason. Such characters are not of
course of specific value, but they are, as Asa Gray has remarked in
commenting on this memoir, such as generally enter into specific
definitions. De Candolle then goes on to say that he gives the rank of
species to the forms that differ by characters never varying on the
same tree, and never found connected by intermediate states. After
this discussion, the result of so much labour, he emphatically
remarks: "They are mistaken, who repeat that the greater part of our
species are clearly limited, and that the doubtful species are in a
feeble minority. This seemed to be true, so long as a genus was
imperfectly known, and its species were founded upon a few
specimens, that is to say, were provisional. Just as we come to know
them better, intermediate forms flow in, and doubts as to specific
limits augment." He also adds that it is the best known species
which present the greater number of spontaneous varieties and
sub-varieties. Thus Quercus robur has twenty-eight varieties, all of
which, excepting six, are clustered round three sub-species, namely,
Q. pedunculata, sessiliflora, and pubescens. The forms which connect
these three sub-species are comparatively rare; and, as Asa Gray again
remarks, if these connecting forms which are now rare, were to
become wholly extinct, the three sub-species would hold exactly the
same relation to each other, as do the four or five provisionally
admitted species which closely surround the typical Quercus robur.
Finally, De Candolle admits that out of the 300 species, which will be
enumerated in his Prodromus as belonging to the oak family, at least
two-thirds are provisional species, that is, are not known strictly to
fulfil the definition above given of a true species. It should be
added that De Candolle no longer believes that species are immutable
creations, but concludes that the derivative theory is the most
natural one, "and the most accordant with the known facts in
palaeontology, geographical botany and zoology, of anatomical
structure and classification."
When a young naturalist commences the study of a group of
organisms quite unknown to him, he is at first much perplexed in
determining what differences to consider as specific, and what as
varietal; for he knows nothing of the amount and kind of variation
to which the group is subject; and this shows, at least, how very
generally there is some variation. But if he confine his attention
to one class within one country, he will soon make up his mind how
to rank most of the doubtful forms. His general tendency will be to
make many species, for he will become impressed, just like the
pigeon or poultry fancier before alluded to, with the amount of
difference in the forms which he is continually studying; and he has
little general knowledge of analogical variation in other groups and
in other countries, by which to correct his first impressions. As he
extends the range of his observations, he will meet with more cases of
difficulty; for he will encounter a greater number of closely-allied
forms. But if his observations be widely extended, he will in the
end generally be able to make up his own mind: but he will succeed
in this at the expense of admitting much variation,- and the truth
of this admission will often be disputed by other naturalists. When he
comes to study allied forms brought from countries not now continuous,
in which case he cannot hope to find intermediate links, he will be
compelled to trust almost entirely to analogy, and his difficulties
will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species- that is, the forms which in the opinion of
some naturalists come very near to, but do not quite arrive at, the
rank of species: or, again, between sub-species and well-marked
varieties, or between lesser varieties and individual differences.
These differences blend into each other by an insensible series; and a
series impresses the mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest
to the systematist, as of the highest importance for us, as being
the first steps towards such slight varieties as are barely thought
worth recording in works on natural history. And I look at varieties
which are in any degree more distinct and permanent, as steps
towards more strongly-marked and permanent varieties; and at the
latter, as leading to sub-species, and then to species. The passage
from one stage of difference to another may, in many cases, be the
simple result of the nature of the organism and of the different
physical conditions to which it has long been exposed; but with
respect to the more important and adaptive characters, the passage
from one stage of difference to another may be safely attributed to
the cumulative action of natural selection, hereafter to be explained,
and to the effects of the increased use or disuse of parts. A
well-marked variety may therefore be called an incipient species;
but whether this belief is justifiable must be judged by the weight of
the various facts and considerations to be given throughout this work.
It need not be supposed that all varieties or incipient species
attain the rank of species. They may become extinct, or they may
endure as varieties for very long periods, as has been shown to be the
case by Mr. Wollaston with the varieties of certain fossil
land-shell in Madeira, and with plants by Gaston de Saporta. If a
variety were to flourish so as to exceed in numbers the parent
species, it would then rank as the species, and the species as the
variety; or it might come to supplant and exterminate the parent
species; or both might co-exist, and both rank as independent species.
But we shall hereafter return to this subject.
From these remarks it will be seen that I look at the term species
as one arbitrarily given, for the sake of convenience, to a set of
individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied
arbitrarily, for convenience' sake.
Wide-ranging, much diffused, and common Species vary most
Guided by theoretical consideration, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H.
C. Watson, to whom I am much indebted for valuable advice and
assistance on this subject, soon convinced me that there were many
difficulties, as did subsequently Dr. Hooker, even in stronger
terms. I shall reserve for a future work the discussion of these
difficulties, and the tables of the proportional numbers of the
varying species. Dr. Hooker permits me to add that after having
carefully read my manuscript, and examined the tables, he thinks
that the following statements are fairly well established. The whole
subject, however, treated as it necessarily here is with much brevity,
is rather perplexing, and allusions cannot be avoided to the "struggle
for existence," "divergence of character," and other questions,
hereafter to be discussed.
Alphonse de Candolle and others have shown that plants which have
very wide ranges generally present varieties; and this might have been
expected, as they are exposed to diverse physical conditions, and as
they come into competition (which, as we shall hereafter see, is an
equally or more important circumstance) with different sets of organic
beings. But my tables further show that, in any limited country, the
species which are the most common, that is abound most in individuals,
and the species which are most widely diffused within their own
country (and this is a different consideration from wide range, and to
a certain extent from commonness), oftenest give rise to varieties
sufficiently well marked to have been recorded in botanical works.
Hence it is the most flourishing, or, as they may be called, the
dominant species,- those which range widely, are the most diffused
in their own country, and are the most numerous in individuals,- which
oftenest produce well-marked varieties, or, as I consider them,
incipient species. And this, perhaps, might have been anticipated; for
as varieties, in order to become in any degree permanent,
necessarily have to struggle with the other inhabitants of the
country, the species which are already dominant will be the most
likely to yield offspring, which, though in some slight degree
modified, still inherit those advantages that enabled their parents to
become dominant over their compatriots. In these remarks on
predominance, it should be understood that reference is made only to
the forms which come into competition with each other, and more
especially to the members of the same genus or class having nearly
similar habits of life. With respect to the number of individuals or
commonness of species, the comparison of course relates only to the
members of the same group. One of the higher plants may be said to
be dominant if it be more numerous in individuals and more widely
diffused than the other plants of the same country, which live under
nearly the same conditions. A plant of this kind is not the less
dominant because some conferva inhabiting the water or some
parasitic fungus is infinitely more numerous in individuals and more
widely diffused. But if the conferva or parasitic fungus exceeds its
allies in the above respects, it will then be dominant within its
own class.
Species of the Larger Genera in each Country vary more frequently
than the Species of the Smaller Genera
If the plants inhabiting a country, as described in any Flora, be
divided into two equal masses, all those in the larger genera (i.e.,
those including many species) being placed on one side, and all
those in the smaller genera on the other side, the former will be
found to include a somewhat larger number of the very common and
much diffused or dominant species. This might have been anticipated;
for the mere fact of many species of the same genus inhabiting any
country, shows that there is something in the organic or inorganic
conditions of that country favourable to the genus; and, consequently,
we might have expected to have found in the larger genera or those
including many species, a larger proportional number of dominant
species. But so many causes tend to obscure this result, that I am
surprised that my tables show even a small majority on the side of the
larger genera. I will here allude to only two causes of obscurity.
Fresh-water and salt-loving plants generally have very wide ranges and
are much diffused, but this seems to be connected with the nature of
the stations inhabited by them, and has little or no relation to the
size of the genera to which the species belong. Again, plants low in
the scale of organisation are generally much more widely diffused than
plants higher in the scale; and here again there is no close
relation to the size of the genera. The cause of lowly-organised
plants ranging widely will be discussed in our chapter on Geographical
Distribution.
From looking at species as only strongly marked and well-defined
varieties, I was led to anticipate that the species of the larger
genera in each country would oftener present varieties, than the
species of the smaller genera; for wherever many closely related
species (i.e., species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now
forming. Where many large trees grow, we expect to find saplings.
Where many species of a genus have been formed through variation,
circumstances have been favourable for variation; and hence we might
expect that the circumstances would generally be still favourable to
variation. On the other hand, if we look at each species as a
special act of creation, there is no apparent reason why more
varieties should occur in a group having many species, than in one
having few.
To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts,
into two nearly equal masses, the species of the larger genera on
one side, and those of the smaller genera on the other side, and it
has invariably proved to be the case that a larger proportion of the
species on the side of the larger genera presented varieties, than
on the side of the smaller genera. Moreover, the species of the
large genera which present any varieties, invariably present a
larger average number of varieties than do the species of the small
genera. Both these results follow when another division is made, and
when all the least genera, with from only one to four species, are
altogether excluded from the tables. These facts are of plain
signification on the view that species are only strongly-marked and
permanent varieties; for wherever many species of the same genus
have been formed, or where, if we may use the expression, the
manufactory of species has been active, we ought generally to find the
manufactory still in action, more especially as we have every reason
to believe the process of manufacturing new species to be a slow
one. And this certainly holds true, if varieties be looked at as
incipient species; for my tables clearly show as a general rule
that, wherever many species of a genus have been formed, the species
of that genus present a number of varieties, that is of incipient
species, beyond the average. It is not that all large genera are now
varying much, and are thus increasing in the number of their
species, or that no small genera are now varying and increasing; for
if this had been so, it would have been fatal to my theory; inasmuch
as geology plainly tells us that small genera have in the lapse of
time often increased greatly in size; and that large genera have often
come to their maxima, declined, and disappeared. All that we want to
show is, that when many species of a genus have been formed, on an
average many are still forming; and this certainly holds good.
Many of the Species included within the Larger Genera resemble
Varieties in being very closely, but unequally, related to each other,
and in having restricted ranges
There are other relations between the species of large genera and
their recorded varieties which deserve notice. We have seen that there
is no infallible criterion by which to distinguish species and
well-marked varieties; and when intermediate links have not been found
between doubtful forms, naturalists are compelled to come to a
determination by the amount of difference between them, judging by
analogy whether or not the amount suffices to raise one or both to the
rank of species. Hence the amount of difference is one very
important criterion in settling whether two forms should be ranked
as species or varieties. Now Fries has remarked in regard to plants,
and Westwood in regard to insects, that in large genera the amount
of difference between the species is often exceedingly small. I have
endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted
some sagacious and experienced observers, and, after deliberation,
they concur in this view. In this respect, therefore, the species of
the larger genera resemble varieties, more than do the species of
the smaller genera. Or the case may be put in another way, and it
may be said, that in the larger genera, in which a number of varieties
or incipient species greater than the average are now manufacturing,
many of the species already manufactured still to a certain extent
resemble varieties, for they differ from each other by less than the
usual amount of difference.
Moreover, the species of the larger genera are related to each
other, in the same manner as the varieties of any one species are
related to each other. No naturalist pretends that all the species
of a genus are equally distinct from each other; they may generally be
divided into sub-genera, or sections, or lesser groups. As Fries has
well remarked, little groups of species are generally clustered like
satellites around other species. And what are varieties but groups
of forms, unequally related to each other, and clustered round certain
forms- that is, round their parent-species. Undoubtedly there is one
most important point of difference between varieties and species;
namely, that the amount of difference between varieties, when compared
with each other or with their parent-species, is much less than that
between the species of the same genus. But when we come to discuss the
principle, as I call it, of Divergence of Character, we shall see
how this may be explained, and how the lesser differences between
varieties tend to increase into the greater differences between
species.
There is one other point which is worth notice. Varieties
generally have much restricted ranges: this statement is indeed
scarcely more than a truism, for, if a variety were found to have a
wider range than that of its supposed parent-species, their
denominations would be reversed. But there is reason to believe that
the species which are very closely allied to other species, and in
so far resemble varieties, often have much restricted ranges. For
instance, Mr. H. C. Watson has marked for me in the well-sifted London
Catalogue of Plants (4th edition) 63 plants which are therein ranked
as species, but which he considers as so closely allied to other
species as to be of doubtful value: these 63 reputed species range
on an average over 6.9 of the provinces into which Mr. Watson has
divided Great Britain. Now, in this same Catalogue, 53 acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas,
the species to which these varieties belong range over 14.3 provinces.
So that the acknowledged varieties have nearly the same, restricted
average range, as have the closely allied forms, marked for me by
Mr. Watson as doubtful species, but which are almost universally
ranked by British botanists as good and true species.
Summary
Finally, varieties cannot be distinguished from species,- except,
first, by the discovery of intermediate linking forms; and,
secondly, by a certain indefinite amount of difference between them;
for two forms, if differing very little, are generally ranked as
varieties, notwithstanding that they cannot be closely connected;
but the amount of difference considered necessary to give to any two
forms the rank of species cannot be defined. In genera having more
than the average number of species in any country, the species of
these genera have more than the average number of varieties. In
large genera the species are apt to be closely, but unequally,
allied together, forming little clusters round other species.
Species very closely allied to other species apparently have
restricted ranges. In all these respects the species of large genera
present a strong analogy with varieties. And we can clearly understand
these analogies, if species once existed as varieties, and thus
originated; whereas, these analogies are utterly inexplicable if
species are independent creations.
We have, also, seen that it is the most flourishing or dominant
species of the larger genera within each class which on an average
yield the greatest number of varieties; and varieties, as we shall
hereafter see, tend to become converted into new and distinct species.
Thus the larger genera tend to become larger; and throughout nature
the forms of life which are now dominant tend to become still more
dominant by leaving many modified and dominant descendants. But by
steps hereafter to be explained, the larger genera also tend to
break u into smaller genera. And thus, the forms of life throughout
the universe become divided into groups subordinate to groups.
CHAPTER III
STRUGGLE FOR EXISTENCE
BEFORE entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual
variability: indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be
called species or sub-species or varieties; what rank, for instance,
the two or three hundred doubtful forms of British plants are entitled
to hold, if the existence of any well-marked varieties be admitted.
But the mere existence of individual variability and of some few
well-marked varieties, though necessary as the foundation for the
work, helps us but little in understanding how species arise in
nature. How have all those exquisite adaptations of one part of the
organisation to another part, and to the conditions of life, and of
one organic being to another being, been perfected? We see these
beautiful co-adaptations most plainly in the woodpecker and the
mistletoe; and only a little less plainly in the humblest parasite
which clings to the hairs of a quadruped or feathers of a bird; in the
structure of the beetle which dives through the water; in the plumed
seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic
world.
Again, it may be asked, how is it that varieties, which I have
called incipient species, become ultimately converted into good and
distinct species which in most cases obviously differ from each
other far more than do the varieties of the same species? How do those
groups of species, which constitute what are called distinct genera,
and which differ from each other more than do the species of the
same genus, arise? All these results, as we shall more fully see in
the next chapter, follow from the struggle for life. Owing to this
struggle, variations, however slight and from whatever cause
proceeding, if they be in any degree profitable to the individuals
of a species, in their infinitely complex relations to other organic
beings and to their physical conditions of life, will tend to the
preservation of such individuals, and will generally be inherited by
the offspring. The offspring, also, will thus have a better chance
of surviving, for, of the many individuals of any species which are
periodically born, but a small number can survive. I have called
this principle, by which each slight variation, if useful, is
preserved, by the term Natural Selection, in order to mark its
relation to man's power of selection. But the expression often used by
Mr. Herbert Spencer of the Survival of the Fittest is more accurate,
and is sometimes equally convenient. We have seen that man by
selection can certainly produce great results, and can adapt organic
beings to his own uses, through the accumulation of slight but
useful variations, given to him by the hand of Nature. But Natural
Selection, as we shall hereafter see, is a power incessantly ready for
action, and is as immeasurably superior to man's feeble efforts, as
the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for
existence. In my future work this subject will be treated, as it
well deserves, at greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are
exposed to severe competition. In regard to plants, no one has treated
this subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult- at least I have found it so-
than constantly to bear this conclusion in mind. Yet unless it be
thoroughly engrained in the mind, the whole economy of nature, with
every fact on distribution, rarity, abundance, extinction, and
variation, will be dimly seen or quite misunderstood. We behold the
face of nature bright with gladness, we often see superabundance of
food; we do not see or we forget, that the birds which are idly
singing round us mostly live on insects or seeds, and are thus
constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that, though food
may be now superabundant, it is not so at all seasons of each
recurring year.
The Term, Struggle for Existence, used in a large sense
I should premise that I use this term in a large and metaphorical
sense including dependence of one being on another, and including
(which is more important) not only the life of the individual, but
success in leaving progeny. Two canine animals, in a time of dearth
may be truly said to struggle with each other which shall get food and
live. But a plant on the edge of a desert is said to struggle for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a
thousand seeds, of which only one of an average comes to maturity, may
be more truly said to struggle with the plants of the same and other
kinds which already clothe the ground. The mistletoe is dependent on
the apple and a few other trees, but can only in a far-fetched sense
be said to struggle with these trees, for, if too many of these
parasites grow on the same tree, it languishes and dies. But several
seedling mistletoes, growing close together on the same branch, may
more truly be said to struggle with each other. As the mistletoe is
disseminated by birds, its existence depends on them; and it may
methodically be said to struggle with other fruit-bearing plants, in
tempting the birds to devour and thus disseminate its seeds. In
these several senses, which pass into each other, I use for
convenience' sake the general term of Struggle for Existence.
Geometrical Ratio of Increase
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season
or occasional year, otherwise, on the principle of geometrical
increase, its numbers would quickly become so inordinately great
that no country could support the product. Hence, as more
individuals are produced than can possibly survive, there must in
every case be a struggle for existence, either one individual with
another of the same species, or with the individuals of distinct
species, or with the physical conditions of life. It is the doctrine
of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial
increase of food, and no prudential restraint from marriage.
Although some species may be now increasing, more or less rapidly,
in numbers, all cannot do so, for the world would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in less than a
thousand years, there would literally not be standing-room for his
progeny. Linnaeus has calculated that if an annual plant produced only
two seeds- and there is no plant so unproductive as this- and their
seedlings next year produced two, and so on, then in twenty years
there should be a million plants. The elephant is reckoned the slowest
breeder of all known animals, and I have taken some pains to
estimate its probable minimum rate of natural increase; it will be
safest to assume that it begins breeding when thirty years old, and
goes on breeding till ninety years old, bringing forth six young in
the interval, and surviving till one hundred years old; if this be so,
after a period of from 740 to 750 years there would be nearly nineteen
million elephants alive, descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world; if the statements of the rate of increase of
slow-breeding cattle and horses in South America, and latterly in
Australia, had not been well authenticated, they would have been
incredible. So it is with plants; cases could be given of introduced
plants which have become common throughout whole islands in a period
of less than ten years. Several of the plants, such as the cardoon and
a tall thistle, which are now the commonest over the whole plains of
La Plata, clothing square leagues of surface almost to the exclusion
of every other plant, have been introduced from Europe; and there
are plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from America
since its discovery. In such cases, and endless others could be given,
no one supposes that the fertility of the animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been highly
favourable, and that there has consequently been less destruction of
the old and young, and that nearly all the young have been enabled
to breed. Their geometrical ratio of increase, the result of which
never fails to be surprising, simply explains their extraordinarily
rapid increase and wide diffusion in their new homes.
In a state of nature almost every full-grown plant annually produces
seed, and amongst animals there are very few which do not annually
pair. Hence we may confidently assert, that all plants and animals are
tending to increase at a geometrical ratio,- that all would rapidly
stock every station in which they could anyhow exist,- and that this
geometrical tendency to increase must. be checked by destruction at
some period of life. Our familiarity with the larger domestic
animals tends, I think, to mislead us: we see no great destruction
falling on them, but we do not keep in mind that thousands are
annually slaughtered for food, and that in a state of nature an
equal number would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large.
The condor lays a couple of eggs and the ostrich a score, and yet in
the same country the condor may be the more numerous of the two; the
Fulmar petrel lays but one egg, yet it is believed to be the most
numerous bird in the world. One fly deposits hundreds of eggs, and
another, like the hippobosca, a single one; but this difference does
not determine how many individuals of the two species can be supported
in a district. A large number of eggs is of some importance to those
species which depend on a fluctuating amount of food, for it allows
them rapidly to increase in number. But the real importance of a large
number of eggs or seeds is to make up for much destruction at some
period of life; and this period in the great majority of cases is an
early one. If an animal can in any way protect its own eggs or
young, a small number may be produced, and yet the average stock be
fully kept up; but if many eggs or young are destroyed, many must be
produced, or the species will become extinct. It would suffice to keep
up the full number of a tree, which lived on an average for a thousand
years, if a single seed were produced once in a thousand years,
supposing that this seed were never destroyed, and could be ensured to
germinate in a fitting place. So that, in all cases, the average
number of any animal or plant depends only indirectly on the number of
its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind- never to forget that every single
organic being may be said to be striving to the utmost to increase
in numbers; that each lives by a struggle at some period of its
life; that heavy destruction inevitably falls either on the young or
old, during each generation or at recurrent intervals. Lighten any
cheek, mitigate the destruction ever so little, and the number of
the species will almost instantaneously increase to any amount.
Nature of the Checks to Increase
The causes which cheek the natural tendency of each species to
increase are most obscure. Look at the most vigorous species; by as
much as it swarms in numbers, by so much will it tend to increase
still further. We know not exactly what the checks are even in a
single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, although so
incomparably better known than any other animal. This subject of the
checks to increase has been ably treated by several authors, and I
hope in a future work to discuss it at considerable length, more
especially in regard to the feral animals of South America. Here I
will make only a few remarks, just to recall to the reader's mind some
of the chief points. Eggs or very young animals seem generally to
suffer most, but this is not invariably the case. With plants there is
a vast destruction of seeds, but, from some observations which I
have made, it appears that the seedlings suffer most from
germinating in ground already thickly stocked with other plants.
Seedlings, also, are destroyed in vast numbers by various enemies; for
instance, on a piece of ground three feet long and two wide, dug and
cleared, and where there could be no choking from other plants, I
marked all the seedlings of our native weeds as they came up, and
out of 357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown
plants; thus out of twenty species growing on a little plot of mown
turf (three feet by four) nine species perished, from the other
species being allowed to grow up freely.
The amount of food for each species of course gives the extreme
limit to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to be
little doubt that the stock of partridges, grouse, and hares on any
large estate depends chiefly on the destruction of vermin. If not
one head of game were shot during the next twenty years in England,
and, at the same time, if no vermin were destroyed, there would, in
all probability, be less game than at present, although hundreds of
thousands of game animals are now annually shot. On the other hand, in
some cases, as with the elephant, none are destroyed by beasts of
prey; for even the tiger in India most rarely dares to attack a
young elephant protected by its dam.
Climate plays an important part in determining the average number of
a species, and periodical seasons of extreme cold or drought seem to
be the most effective of all checks. I estimated (chiefly from the
greatly reduced numbers of nests in the spring) that the winter of
1854-5 destroyed four-fifths of the birds in my own grounds; and
this is a tremendous destruction, when we remember that ten per cent
is an extraordinarily severe mortality from epidemics with man. The
action of climate seems at first sight to be quite independent of
the struggle for existence; but in so far as climate chiefly acts in
reducing food, it brings on the most severe struggle between the
individuals, whether of the same or of distinct species, which subsist
on the same kind of food. Even when climate, for instance, extreme
cold, acts directly, it will be the least vigorous individuals, or
those which have got least food through the advancing winter, which
will suffer most. When we travel from south to north, or from a damp
region to a dry, we invariably see some species gradually getting
rarer and rarer, and finally disappearing; and the change of climate
being conspicuous, we are tempted to attribute the whole effect to its
direct action. But this is a false view; we forget that each
species, even where it most abounds, is constantly suffering
enormous destruction at some period of its life, from enemies or
from competitors for the same place and food; and if these enemies
or competitors be in the least degree favoured by any slight change of
climate, they will increase in numbers; and as each area is already
fully stocked with inhabitants, the other species must decrease.
When we travel southward and see a species decreasing in numbers, we
may feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel
northward, but in a somewhat lesser degree, for the number of
species of all kinds, and therefore of competitors, decreases
northwards; hence in going northwards, or in ascending a mountain,
we far oftener meet with stunted forms, due to the directly
injurious action of climate, than we do in proceeding southwards or in
descending a mountain. When we reach the arctic regions, or snowcapped
summits, or absolute deserts, the struggle for life is almost
exclusively with the elements.
That climate acts in main part indirectly by favouring other
species, we clearly see in the prodigious number of plants which in
our gardens can perfectly well endure our climate, but which never
become naturalised, for they cannot compete with our native plants nor
resist destruction by our native animals.
When a species, owing to highly favourable circumstances,
increases inordinately in numbers in a small tract, epidemics- at
least, this seems generally to occur with our game animals- often
ensue; and here we have a limiting check independent of the struggle
for life. But even some of these so-called epidemics appear to be
due to parasitic worms, which have from some cause, possibly in part
through facility of diffusion amongst the crowded animals, been
disproportionally favoured: and here comes in a sort of struggle
between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of
the same species, relatively to the numbers of its enemies, is
absolutely necessary for its preservation. Thus we can easily raise
plenty of corn and rape-seed, &c., in our fields, because the seeds
are in great excess compared with the number of birds which feed on
them; nor can the birds, though having a super-abundance of food at
this one season, increase in number proportionally to the supply of
seed, as their numbers are checked during the winter; but any one
who has tried, knows how troublesome it is to get seed from a few
wheat or other such plants in a garden: I have in this case lost every
single seed. This view of the necessity of a large stock of the same
species for its preservation, explains, I believe, some singular facts
in nature, such as that of very rare plants being sometimes
extremely abundant, in the few spots where they do exist; and that
of some social plants being social, that is abounding in
individuals, even on the extreme verge of their range. For in such
cases, we may believe, that a plant could exist only where the
conditions of its life were so favourable that many could exist
together, and thus save the species from utter destruction. I should
add that the good effects of intercrossing, and the ill effects of
close interbreeding, no doubt come into play in many of these cases;
but I will not here enlarge on this subject.
Complex Relations of all Animals and Plants to each other in the
Struggle for Existence
Many cases are on record showing how complex and unexpected are
the checks and relations between organic beings, which have to
struggle together in the same country. I will give only a single
instance, which, though a simple one, interested me. In Staffordshire,
on the estate of a relation, where I had ample means of investigation,
there was a large and extremely barren heath, which had never been
touched by the hand of man; but several hundred acres of exactly the
same nature had been enclosed twenty-five years previously and planted
with Scotch fir. The change in the native vegetation of the planted
part of the heath was most remarkable, more than is generally seen
in passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished
in the plantations, which could not be found on the heath. The
effect on the insects must have been still greater, for six
insectivorous birds were very common in the plantations, which were
not to be seen on the heath; and the heath was frequented by two or
three distinct insectivorous birds. Here we see how potent has been
the effect of the introduction of a single tree, nothing whatever else
having been done, with the exception of the land having been enclosed,
so that cattle could not enter. But how important an element enclosure
is, I plainly saw near Farnham, in Surrey. Here there are extensive
heaths, with a few clumps of old Scotch firs on the distant
hilltops: within the last ten years large spaces have been enclosed,
and self-sown firs are now springing up in multitudes, so close
together that all cannot live. When I ascertained that these young
trees had not been sown or planted, I was so much surprised at their
numbers that I went to several points of view, whence I could
examine hundreds of acres of the unenclosed heath, and literally I
could not see a single Scotch fir, except the old planted clumps.
But on looking closely between the stems of the heath, I found a
multitude of seedlings and little trees which had been perpetually
browsed down by the cattle. In one square yard, at a point some
hundred yards distant from one of the old clumps, I counted thirty-two
little trees; and one of them, with twenty-six rings of growth, had,
during many years, tried to raise its head above the stems of the
heath, and had failed. No wonder that, as soon as the land was
enclosed, it became thickly clothed with vigorously growing young
firs. Yet the heath was so extremely barren and so extensive that no
one would ever have imagined that cattle would have so closely and
effectually searched it for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state;
and Azara and Rengger have shown that this is caused by the greater
number in Paraguay of a certain fly, which lays its eggs in the navels
of these animals when first born. The increase of these flies,
numerous as they are, must be habitually checked by some means,
probably by other parasitic insects. Hence, if certain insectivorous
birds were to decrease in Paraguay, the parasitic insects would
probably increase; and this would lessen the number of the
navel-frequenting flies- then cattle and horses would become feral,
and this would certainly greatly alter (as indeed I have observed in
parts of South America) the vegetation: this again would largely
affect the insects; and this, as we have just seen in Staffordshire,
the insectivorous birds, and so onwards in ever-increasing circles
of complexity. Not that under nature the relations will ever be as
simple as this. Battle within battle must be continually recurring
with varying success; and yet in the long run the forces are so nicely
balanced, that the face of nature remains for long periods of time
uniform, though assuredly the merest trifle would give the victory
to one organic being over another. Nevertheless, so profound is our
ignorance, and so high our presumption, that we marvel when we hear of
the extinction of an organic being; and as we do not see the cause, we
invoke cataclysms to desolate the world, or invent laws on the
duration of the forms of life!
I am tempted to give one more instance showing how plants and
animals remote in the scale of nature, are bound together by a web
of complex relations. I shall hereafter have occasion to show that the
exotic Lobelia fulgens is never visited in my garden by insects, and
consequently, from its peculiar structure, never sets a seed. Nearly
all our orchidaceous plants absolutely require the visits of insects
to remove their pollen-masses and thus to fertilise them. I find
from experiments that humble-bees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do
not visit this flower. I have also found that the visits of bees are
necessary for the fertilisation of some kinds of clover; for instance,
90 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but
20 other heads protected from bees produced not one. Again, 100
heads of red clover (T. pratense) produced 2,700 seeds, but the same
number of protected heads produced not a single seed. Humble-bees
alone visit red clover, as other bees cannot reach the nectar. It
has been suggested that moths may fertilise the clovers; but I doubt
whether they could do so in the case of the red clover, from their
weight not being sufficient to depress the wing petals. Hence we may
infer as highly probable that, if the whole genus of humble-bees
became extinct or very rare in England, the heartsease and red
clover would become very rare, or wholly disappear. The number of
humble-bees in any district depends in a great measure upon the number
of field-mice, which destroy their combs and nests; and Col. Newman,
who has long attended to the habits of humble-bees, believes that
"more than two-thirds of them are thus destroyed all over England."
Now the number of mice is largely dependent, as every one knows, on
the number of cats; and Col. Newman says, "Near villages and small
towns I have found the nests of humble-bees more numerous than
elsewhere, which I attribute to the number of cats that destroy the
mice." Hence it is quite credible that the presence of a feline animal
in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of
certain flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally
the most potent; but all will concur in determining the average number
or even the existence of the species. In some cases it can be shown
that widely-different checks act on the same species in different
districts. When we look at the plants and bushes clothing an entangled
bank, we are tempted to attribute their proportional numbers and kinds
to what we call chance. But how false a view is this! Every one has
heard that when an American forest is cut down a very different
vegetation springs up; but it has been observed that ancient Indian
ruins in the southern United States, which must formerly have been
cleared of trees, now display the same beautiful diversity and
proportion of kinds as in the surrounding virgin forest. What a
struggle must have gone on during long centuries between the several
kinds of trees each annually scattering its seeds by the thousand;
what war between insect and insect- between insects, snails, and other
animals with birds and beasts of prey- all striving to increase, all
feeding on each other, or on the trees, their seeds and seedlings,
or on the other plants which first clothed the ground and thus checked
the growth of the trees! Throw up a handful of feathers, and all
fall to the ground according to definite laws; but how simple is the
problem where each shall fall compared to that of the action and
reaction of the innumerable plants and animals which have
determined, in the course of centuries, the proportional numbers and
kinds of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite
on its prey, lies generally between beings remote in the scale of
nature. This is likewise sometimes the case with those which may be
strictly said to struggle with each other for existence, as in the
case of locusts and grass-feeding quadrupeds. But the struggle will
almost invariably be most severe between the individuals of the same
species, for they frequent the same districts, require the same
food, and are exposed to the same dangers. In the case of varieties of
the same species, the struggle will generally be almost equally
severe, and we sometimes see the contest soon decided: for instance,
if several varieties of wheat be sown together, and the mixed seed
be resown, some of the varieties which best suit the soil or
climate, or are naturally the most fertile, will beat the others and
so yield more seed, and will consequently in a few years supplant
the other varieties. To keep up a mixed stock of even such extremely
close varieties as the variously-coloured sweet peas, they must be
each year harvested separately, and the seed then mixed in due
proportion, otherwise the weaker kinds will steadily decrease in
number and disappear. So again with the varieties of sheep; it has
been asserted that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same
result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any
of our domestic plants or animals have so exactly the same strength,
habits, and constitution, that the original proportions of a mixed
stock (crossing being prevented) could be kept up for half-a-dozen
generations, if they were allowed to struggle together, in the same
manner as beings in a state of nature, and if the seed or young were
not annually preserved in due proportion.
Struggle for Life most severe between Individuals and Varieties of
the same Species
As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always
in structure, the struggle will generally be more severe between them,
if they come into competition with each other, than between the
species of distinct genera. We see this in the recent extension over
parts of the United States of one species of swallow having caused the
decrease of another species. The recent increase of the
missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In
Russia the small Asiatic cockroach has everywhere driven before it its
great congener. In Australia the imported hive-bee is rapidly
exterminating the small, stingless native bee. One species of charlock
has been known to supplant another species; and so in other cases.
We can dimly see why the competition should be most severe between
allied forms, which fill nearly the same place in the economy of
nature; but probably in no one case could we precisely say why one
species has been victorious over another in the great battle of life.
A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being
is related, in the most essential yet often hidden manner, to that
of all the other organic beings, with which it comes into
competition for food or residence, or from which it has to escape,
or on which it preys. This is obvious in the structure of the teeth
and talons of the tiger; and in that of the legs and claws of the
parasite which clings to the hair on the tiger's body. But in the
beautifully plumed seed of the dandelion, and in the flattened and
fringed legs of the water-beetle, the relation seems at first confined
to the elements of air and water. Yet the advantage of plumed seeds no
doubt stands in the closest relation to the land being already thickly
clothed with other plants; so that the seeds may be widely distributed
and fall on unoccupied ground. In the water-beetle, the structure of
its legs, so well adapted for diving, allows it to compete with
other aquatic insects, to hunt for its own prey, and to escape serving
as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems
at first to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and
beans, when sown in the midst of long grass, it may be suspected
that the chief use of the nutriment in the seed is to favour the
growth of the seedlings, whilst struggling with other plants growing
vigorously all around.
Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand
a little more heat or cold, dampness or dryness, for elsewhere it
ranges into slightly hotter or colder, damper or drier districts. In
this case we can clearly see that if we wish in imagination to give
the plant the power of increasing in number, we should have to give it
some advantage over its competitors, or over the animals which prey on
it. On the confines of its geographical range, a change of
constitution with respect to climate would clearly be an advantage
to our plant; but we have reason to believe that only a few plants
or animals range so far, that they are destroyed exclusively by the
rigour of the climate. Not until we reach the extreme confines of
life, in the Arctic regions or on the borders of an utter desert, will
competition cease. The land may be extremely cold or dry, yet there
will be competition between some few species, or between the
individuals of the same species, for the warmest or dampest spots.
Hence we can see that when a plant or animal is placed in a new
country amongst new competitors, the conditions of its life will
generally be changed in an essential manner, although the climate
may be exactly the same as in its former home. If its average
numbers are to increase in its new home, we should have to modify it
in a different way to what we should have had to do in its native
country; for we should have to give it some advantage over a different
set of competitors or enemies.
It is good thus to try in imagination to give to any one species
an advantage over another. Probably in no single instance should we
know what to do. This ought to convince us of our ignorance on the
mutual relations of all organic beings; a conviction as necessary as
it is difficult to acquire. All that we can do, is to keep steadily in
mind that each organic being is striving to increase in a
geometrical ratio; that each at some period of its life, during some
season of the year, during each generation or at intervals, has to
struggle for life and to suffer great destruction. When we reflect
on this struggle, we may console ourselves with the full belief,
that the war of nature is not incessant, that no fear is felt, that
death is generally prompt, and that the vigorous, the healthy, and the
happy survive and multiply.
CHAPTER IV
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST
How will the struggle for existence, briefly discussed in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply under
nature? I think we shall see that it can act most efficiently. Let the
endless number of slight variations and individual differences
occurring in our domestic productions, and, in a lesser degree, in
those under nature, be borne in mind; as well as the strength of the
hereditary tendency. Under domestication, it may be truly said that
the whole organisation becomes in some degree plastic. But the
variability, which we almost universally meet with in our domestic
productions, is not directly produced, as Hooker and Asa Gray have
well remarked, by man; he can neither originate varieties, nor prevent
their occurrence; he can preserve and accumulate such as do occur.
Unintentionally he exposes organic beings to new and changing
conditions of life, and variability ensues; but similar changes of
conditions might and do occur under nature. Let it also be borne in
mind how infinitely complex and close-fitting are the mutual relations
of all organic beings to each other and to their physical conditions
of life; and consequently what infinitely varied diversities of
structure might be of use to each being under changing conditions of
life. Can it, then, be thought improbable, seeing that variations
useful to man have undoubtedly occurred, that other variations
useful in some way to each being in the great and complex battle of
life, should occur in the course of many successive generations? If
such do occur, can we doubt (remembering that many more individuals
are born than can possibly survive) that individuals having any
advantage, however slight, over others, would have the best chance
of surviving and of procreating their kind? On the other hand, we
may feel sure that any variation in the least degree injurious would
be rigidly destroyed. This preservation of favourable individual
differences and variations, and the destruction of those which are
injurious, I have called Natural Selection, or the Survival of the
Fittest. Variations neither useful nor injurious would not be affected
by natural selection, and would be left either a fluctuating
element, as perhaps we see in certain polymorphic species, or would
ultimately become fixed, owing to the nature of the organism and the
nature of the conditions.
Several writers have misapprehended or objected to the term
Natural Selection. Some have even imagined that natural selection
induces variability, whereas it implies only the preservation of
such variations as arise and are beneficial to the being under its
conditions of life. No one objects to agriculturists speaking of the
potent effects of man's selection; and in this case the individual
differences given by nature, which man for some object selects, must
of necessity first occur. Others have objected that the term selection
implies conscious choice in the animals which become modified; and
it has even been urged that, as plants have no volition, natural
selection is not applicable to them! In the literal sense of the word,
no doubt, natural selection is a false term; but who ever objected
to chemists speaking of the elective affinities of the various
elements?- and yet an acid cannot strictly be said to elect the base
with which it in preference combines. It has been said that I speak of
natural selection as an active power or Deity; but who objects to an
author speaking of the attraction of gravity as ruling the movements
of the planets? Every one knows what is meant and is implied by such
metaphorical expressions; and they are almost necessary for brevity.
So again it is difficult to avoid personifying the word Nature; but
I mean by Nature, only the aggregate action and product of many
natural laws, and by laws the sequence of events as ascertained by us.
With a little familiarity such superficial objections will be
forgotten.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some slight physical change,
for instance, of climate. The proportional numbers of its
inhabitants will almost immediately undergo a change, and some species
will probably become extinct. We may conclude, from what we have
seen of the intimate and complex manner in which the inhabitants of
each country are bound together, that any change in the numerical
proportions of the inhabitants, independently of the change of climate
itself, would seriously affect the others. If the country were open on
its borders, new forms would certainly immigrate, and this would
likewise seriously disturb the relations of some of the former
inhabitants. let it be remembered how powerful the influence of a
single introduced tree or mammal has been shown to be. But in the case
of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should
then have places in the economy of nature which would assuredly be
better filled up, if some of the original inhabitants were in some
manner modified; for, had the area been open to immigration, these
same places would have been seized on by intruders. In such cases,
slight modifications, which in any way favoured the individuals of any
species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would have free scope
for the work of improvement.
We have good reason to believe, as shown in the first chapter,
that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions have changed,
and this would manifestly be favourable to natural selection, by
affording a better chance of the occurrence of profitable
variations. Unless such occur, natural selection can do nothing. Under
the term of "variations," it must never be forgotten that mere
individual differences are included. As man can produce a great result
with his domestic animals and plants by adding up in any given
direction individual differences, so could natural selection, but
far more easily from having incomparably longer time for action. Nor
do I believe that any great physical change, as of climate, or any
unusual degree of isolation to check immigration, is necessary in
order that new and unoccupied places should be left, for natural
selection to fill up by improving some of the varying inhabitants. For
as all the inhabitants of each country are struggling together with
nicely balanced forces, extremely slight modifications in the
structure or habits of one species would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence. No country can be named in which all
the native inhabitants are now so perfectly adapted to each other
and to the physical conditions under which they live, that none of
them could be still better adapted or improved; for in all
countries, the natives have been so far conquered by naturalised
productions, that they have allowed some foreigners to take firm
possession of the land. And as foreigners have thus in every country
beaten some of the natives, we may safely conclude that the natives
might have been modified with advantage, so as to have better resisted
the intruders.
As man can produce, and certainly has produced, a great result by
his methodical and unconscious means of selection, what may not
natural selection effect? Man can act only on external and visible
characters: Nature, if I may be allowed to personify the natural
preservation or survival of the fittest, cares nothing for
appearances, except in so far as they are useful to any being. She can
act on every internal organ, on every shade of constitutional
difference, on the whole machinery of life. Man selects only for his
own good: Nature only for that of the being which she tends. Every
selected character is fully exercised by her, as is implied by the
fact of their selection. Man keeps the natives of many climates in the
same country; he seldom exercises each selected character in some
peculiar and fitting manner; he feeds a long and a short beaked pigeon
on the same food; he does not exercise a long-backed or long-legged
quadruped in any peculiar manner; he exposes sheep with long and short
wool to the same climate. He does not allow the most vigorous males to
struggle for the females. He does not rigidly destroy all inferior
animals, but protects during each varying season, as far as lies in
his power, all his productions. He often begins his selection by
some half-monstrous form; or at least by some modification prominent
enough to catch the eye or to be plainly useful to him. Under
nature, the slightest differences of structure or constitution may
well turn the nicely balanced scale in the struggle for life, and so
be preserved. How fleeting are the wishes and efforts of man! how
short his time! and consequently how poor will be his results,
compared with those accumulated by Nature during whole geological
periods! Can we wonder, then, that Nature's productions should be
far "truer" in character than man's productions; that they should be
infinitely better adapted to the most complex conditions of life,
and should plainly bear the stamp of far higher workmanship?
It may metaphorically be said that natural selection is daily and
hourly scrutinising, throughout the world, the slightest variations;
rejecting those that are bad, preserving and adding up all that are
good; silently and insensibly working, whenever and wherever
opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time
has marked the lapse of ages, and then so imperfect is our view into
long-past geological ages, that we see only that the forms of life are
now different from what they formerly were.
In order that any great amount of modification should be effected in
a species, a variety when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must be again preserved, and so
onwards step by step. Seeing that individual differences of the same
kind perpetually recur, this can hardly be considered as an
unwarrantable assumption. But whether it is true, we can judge only by
seeing how far the hypothesis accords with and explains the general
phenomena of nature. On the other hand, the ordinary belief that the
amount of possible variation is a strictly limited quantity is
likewise a simple assumption.
Although natural selection can act only through and for the good
of each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When we
see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red grouse the colour of
heather, we must believe that these tints are of service to these
birds and insects in preserving them from danger. Grouse, if not
destroyed at some period of their lives, would increase in countless
numbers; they are known to suffer largely from birds of prey; and
hawks are guided by eyesight to their prey- so much so, that on
parts of the Continent persons are warned not to keep white pigeons,
as being the most liable to destruction. Hence natural selection might
be effective in giving the proper colour to each kind of grouse, and
in keeping that colour, when once acquired, true and constant. Nor
ought we to think that the occasional destruction of an animal of
any particular colour would produce little effect: we should
remember how essential it is in a flock of white sheep to destroy a
lamb with the faintest trace of black. We have seen how the colour
of the hogs, which feed on the "paint-root" in Virginia, determines
whether they shall live or die. In plants, the down on the fruit and
the colour of the flesh are considered by botanists as characters of
the most trifling importance: yet we hear from an excellent
horticulturist, Downing, that in the United States, smooth-skinned
fruits suffer far more from a beetle, a Curculio, than those with
down; that purple plums suffer far more from a certain disease than
yellow plums; whereas another disease attacks yellow-fleshed peaches
far more than those with other coloured flesh. If, with all the aids
of art, these slight differences make a great difference in
cultivating the several varieties, assuredly, in a state of nature,
where the trees would have to struggle with other trees, and with a
host of enemies, such differences would effectually settle which
variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed.
In looking at many small points of difference between species,
which, as far as our ignorance permits us to judge, seem quite
unimportant, we must not forget that climate, food, &c., have no doubt
produced some direct effect. It is also necessary to bear in mind
that, owing to the law of correlation, when one part varies, and the
variations are accumulated through natural selection, other
modifications, often of the most unexpected nature, will ensue.
As we see that those variations which, under domestication, appear
at any particular period of life, tend to reappear in the offspring at
the same period;- for instance, in the shape, size, and flavour of the
seeds of the many varieties of our culinary and agricultural plants;
in the caterpillar and cocoon stages of the varieties of the
silk-worm; in the eggs of poultry, and in the colour of the down of
their chickens; in the horns of our sheep and cattle when nearly
adult;- so in a state of nature natural selection will be enabled to
act on and modify organic beings at any age, by the accumulation of
variations profitable at that age, and by their inheritance at a
corresponding age. If it profit a plant to have its seeds more and
more widely disseminated by the wind, I can see no greater
difficulty in this being effected through natural selection, than in
the cotton-planter increasing and improving by selection the down in
the pods on his cotton-trees. Natural selection may modify and adapt
the larva of an insect to a score of contingencies, wholly different
from those which concern the mature insect; and these modifications
may affect, through correlation, the structure of the adult. So,
conversely, modifications in the adult may affect the structure of the
larva; but in all cases natural selection will ensure that they
shall not be injurious: for if they were so, the species would
become extinct.
Natural selection will modify the structure of the young in relation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the whole community, if the community profits by the selected
change. What natural selection cannot do, is to modify the structure
of one species, without giving it any advantage, for the good Of
another species; and though statements to this effect may be found
in works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal's life, if of
high importance to it, might be modified to any extent by natural
selection; for instance, the great jaws possessed by certain
insects, used exclusively for opening the cocoon- or the hard tip to
the beak of unhatched birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons a greater
number perish in the egg than are able to get out of it; so that
fanciers assist in the act of hatching. Now if nature had to make
the beak of a full-grown pigeon very short for the bird's own
advantage, the process of modification would be very slow, and there
would be simultaneously the most rigorous selection of all the young
birds within the egg, which had the most powerful and hardest beaks,
for all with weak beaks would inevitably perish; or, more delicate and
more easily broken shells might be selected, the thickness of the
shell being known to vary like every other structure.
It may be well here to remark that with all beings there must be
much fortuitous destruction, which can have little or no influence
on the course of natural selection. For instance a vast number of eggs
or seeds are annually devoured, and these could be modified through
natural selection only if they varied in some manner which protected
them from their enemies. Yet many of these eggs or seeds would
perhaps, if not destroyed, have yielded individuals better adapted
to their conditions of life than any of these which happened to
survive. So again a vast number of mature animals and plants,
whether or not they be the best adapted to their conditions, must be
annually destroyed by accidental causes, which would not be in the
least degree mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the
destruction of the adults be ever so heavy, if the number which can
exist in any district be not wholly kept down by such causes,- or
again let the destruction of eggs or seeds be so great that only a
hundredth or a thousandth part are developed,- yet of those which do
survive, the best adapted individuals, supposing that there is any
variability in favourable direction, will tend to propagate their kind
in larger numbers than the less well adapted. If the numbers be wholly
kept down by the causes just indicated, as will often have been the
case, natural selection will be powerless in certain beneficial
directions; but this is no valid objection to its efficiency at
other times and in other ways; for we are far from having any reason
to suppose that many species ever undergo modification and improvement
at the same time in the same area.
Sexual Selection
Inasmuch as peculiarities often appear under domestication in one
sex and become hereditarily attached to that sex, so no doubt it
will be under nature. Thus it is rendered possible for the two sexes
to be modified through natural selection in relation to different
habits of life, as is sometimes the case; or for one sex to be
modified in relation to the other sex, as commonly occurs. This
leads me to say a few words on what I have called Sexual Selection.
This form of selection depends, not on a struggle for existence in
relation to other organic beings or to external conditions, but on a
struggle between the individuals of one sex, generally the males,
for the possession of the other sex. The result is not death to the
unsuccessful competitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally, the most
vigorous males, those which are best fitted for their places in
nature, will leave most progeny. But in many cases, victory depends
not so much on general vigor, as on having special weapons, confined
to the male sex. A hornless stag or spurless cock would have a poor
chance of leaving numerous offspring. Sexual selection, by always
allowing the victor to breed, might surely give indomitable courage,
length to the spur, and strength to the wing to strike in the
spurred leg, in nearly the same manner as does the brutal
cockfighter by the careful selection of his best cocks. How low in the
scale of nature the law of battle descends, I know not; male
alligators have been described as fighting, bellowing, and whirling
round, like Indians in a war-dance, for the possession of the females;
male salmons have been observed fighting all day long; male
stagbeetles sometimes bear wounds from the huge mandibles of other
males; the males of certain hymenopterous insects have been frequently
seen by that inimitable observer M. Fabre, fighting for a particular
female who sits by, an apparently unconcerned beholder of the
struggle, and then retires with the conqueror. The war is, perhaps,
severest between the males of polygamous animals, and these seem
oftenest provided with special weapons. The males of carnivorous
animals are already well armed; though to them and to others,
special means of defence may be given through means of sexual
selection, as the mane of the lion, and the hooked jaw to the male
salmon; for the shield may be as important for victory, as the sword
or spear.
Amongst birds, the contest is often of a more peaceful character.
All those who have attended to the subject, believe that there is
the severest rivalry between the males of many species to attract,
by singing, the females. The rock-thrush of Guiana, birds of paradise,
and some others, congregate; and successive males display with the
most elaborate care, and show off in the best manner, their gorgeous
plumage; they likewise perform strange antics before the females,
which, standing by as spectators, at last choose the most attractive
partner. Those who have closely attended to birds in confinement
well know that they often take individual preferences and dislikes:
thus Sir R. Heron has described how a pied peacock was eminently
attractive to all his hen birds. I cannot here enter on the
necessary details; but if man can in a short time give beauty and an
elegant carriage to his bantams, according to his standard of
beauty, I can see no good reason to doubt that female birds, by
selecting, during thousands of generations, the most melodious or
beautiful males, according to their standard of beauty, might
produce a marked effect. Some well-known laws, with respect to the
plumage of male and female birds, in comparison with the plumage of
the young, can partly be explained through the action of sexual
selection on variations occurring at different ages, and transmitted
to the males alone or to both sexes at corresponding ages; but I
have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in
structure, colour, or ornament, such differences have been mainly
caused by sexual selection: that is, by individual males having had,
in successive generations, some slight advantage over other males,
in their weapons, means of defence, or charms, which they have
transmitted to their male offspring alone. Yet, I would not wish to
attribute all sexual differences to this agency: for we see in our
domestic animals peculiarities arising and becoming attached to the
male sex, which apparently have not been augmented through selection
by man. The tuft of hair on the breast of the wild turkey-cock
cannot be of any use, and it is doubtful whether it can be
ornamental in the eyes of the female bird; indeed, had the tuft
appeared under domestication, it would have been called a monstrosity.
Illustrations of the Action of Natural Selection, or the Survival of
the Fittest
In order to make it clear how, as I believe, natural selection acts,
I must beg permission to give one or two imaginary illustrations.
Let us take the case of a wolf, which preys on various animals,
securing some by craft, some by strength, and some by fleetness; and
let us suppose that the fleetest prey, a deer for instance, had from
any change in the country increased in numbers, or that other prey had
decreased in numbers, during that season of the year when the wolf was
hardest pressed for food. Under such circumstances the swiftest and
slimmest wolves would have the best chance of surviving and so be
preserved or selected,- provided always that they retained strength to
master their prey at this or some other period of the year, when
they were compelled to prey on other animals. I can see no more reason
to doubt that this would be the result, than that man should be able
to improve the fleetness of his greyhounds by careful and methodical
selection, or by that kind of unconscious selection which follows from
each man trying to keep the best dogs without any thought of modifying
the breed. I may add, that, according to Mr. Pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains, in the United
States, one with a light greyhound-like form, which pursues deer,
and the other more bulky, with shorter legs, which more frequently
attacks the shepherd's flocks.
It should be observed that, in the above illustration, I speak of
the slimmest individual wolves, and not of any single
strongly-marked variation having been preserved. In former editions of
this work I sometimes spoke as if this latter alternative had
frequently occurred. I saw the great importance of individual
differences, and this led me fully to discuss the results of
unconscious selection by man, which depends on the preservation of all
the more or less valuable individuals, and on the destruction of the
worst. I saw, also, that the preservation in a state of nature of
any occasional deviation of structure, such as a monstrosity, would be
a rare event; and that, if at first preserved, it would generally be
lost by subsequent intercrossing with ordinary individuals.
Nevertheless, until reading an able and valuable article in the
North British Review (1867), I did not appreciate how rarely single
variations, whether slight or strongly-marked, could be.
perpetuated. The author takes the case of a pair of animals, producing
during their lifetime two hundred offspring, of which, from various
causes of destruction, only two on an average survive to procreate
their kind. This is rather an extreme estimate for most of the
higher animals, but by no means so for many of the lower organisms. He
then shows that if a single individual were born, which varied in some
manner, giving it twice as good a chance of life as that of the
other individuals, yet the chances would be strongly against its
survival. Supposing it to survive and to breed, and that half its
young inherited the favourable variation; still, as the reviewer
goes on to show, the young would have only a slightly better chance of
surviving and breeding; and this chance would go on decreasing in
the succeeding generations. The justice of these remarks cannot, I
think, be disputed. If, for instance, a bird of some kind could
procure its food more easily by having its beak curved, and if one
were born with its beak strongly curved, and which consequently
flourished, nevertheless there would be a very poor chance of this one
individual perpetuating its kind to the exclusion of the common
form; but there can hardly be a doubt, judging by what we see taking
place under domestication, that this result would follow from the
preservation during many generations of a large number of
individuals with more or less strongly curved beaks, and from the
destruction of a still larger number with the straightest beaks.
It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organisation being
similarly acted on- of which fact numerous instances could be given
with our domestic productions. In such cases, if the varying
individual did not actually transmit to its offspring its
newly-acquired character, it would undoubtedly transmit to them, as
long as the existing conditions remained the same, a still stronger
tendency to vary in the same manner. There can also be little doubt
that the tendency to vary in the same manner has often been so
strong that all the individuals of the same species have been
similarly modified without the aid of any form of selection. Or only a
third, fifth, or tenth part of the individuals may have been thus
affected, of which fact several instances could be given. Thus Graba
estimates that about one-fifth of the guillemots in the Faroe
Islands consist of a variety so well marked, that it was formerly
ranked as a distinct species under the name of Uria lacrymans. In
cases of this kind, if the variation were of a beneficial nature,
the original form would soon be supplanted by the modified form,
through the survival of the fittest.
To the effects of intercrossing in eliminating variations of all
kinds, I shall have to recur; but it may be here remarked that most
animals and plants keep to their proper homes, and do not needlessly
wander about; we see this even with migratory birds, which almost
always return to the same spot. Consequently each newly-formed variety
would generally be at first local, as seems to be the common rule with
varieties in a state of nature; so that similarly modified individuals
would soon exist in a small body together, and would often breed
together. If the new variety were successful in its battle for life,
it would slowly spread from a central district, competing with and
conquering the unchanged individuals on the margins of an
ever-increasing circle.
It may be worth while to give another and more complex
illustration of the action of natural selection. Certain plants
excrete sweet juice, apparently for the sake of eliminating
something injurious from the sap: this is effected, for instance, by
glands at the base of the stipules in some Leguminosae and at the
backs of the leaves of the common laurel. This juice, though small
in quantity, is greedily sought by insects; but their visits do not in
any way benefit the plant. Now, let us suppose that the juice or
nectar was excreted from the inside of the flowers of a certain number
of plants of any species. Insects in seeking the nectar would get
dusted with pollen, and would often transport it from one flower to
another. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, as can be fully
proved, gives rise to vigorous seedlings which consequently would have
the best chance of flourishing and surviving The plants which produced
flowers with the largest glands or nectaries, excreting most nectar,
would oftenest be visited by insects, and would oftenest be crossed;
and so in the long run would gain the upper hand and form a local
variety. The flowers, also, which had their stamens and pistils
placed, in relation to the size and habits of the particular insects
which visited them, so as to favour in any degree the transportal of
the pollen, would likewise be favoured. We might have taken the case
of insects visiting flowers for the sake of collecting pollen
instead of nectar; and as pollen is formed for the sole purpose of
fertilisation, its destruction appears to be a simple loss to the
plant; yet if a little pollen were carried, at first occasionally
and then habitually, by the pollen-devouring insects from flower to
flower, and a cross thus effected, although nine-tenths of the
pollen were destroyed it might still be a great gain to the plant to
be thus robbed; and the individuals which produced more and more
pollen, and had larger anthers, would be selected.
When our plant, by the above process long continued, had been
rendered highly attractive to insects, they would, unintentionally
on their part, regularly carry pollen from flower to flower; and
that they do this effectually, I could easily show by many striking
facts. I will give only one, as likewise illustrating one step in
the separation of the sexes of plants. Some holly-trees bear only male
flowers, which have four stamens producing a rather small quantity
of pollen, and a rudimentary pistil; other holly-trees bear only
female flowers; these have a full-sized pistil, and four stamens
with shrivelled anthers, in which not a grain of pollen can be
detected. Having found a female tree exactly sixty yards from a male
tree, I put the stigmas of twenty flowers, taken from different
branches, under the microscope, and on all, without exception, there
were a few pollen grains, and on some a profusion. As the wind had set
for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and
boisterous, and therefore not favourable to bees, nevertheless every
female flower which I examined had been effectually fertilised by
the bees, which had flown from tree to tree in search of nectar. But
to return to our imaginary case: as soon as the plant had been
rendered so highly attractive to insects that pollen was regularly
carried from flower to flower, another process might commence. No
naturalist doubts the advantage of what has been called the
"physiological division of labour"; hence we may believe that it would
be advantageous to a plant to produce stamens alone in one flower or
on one whole plant, and pistils alone in another flower or on
another plant. In plants under culture and placed under new conditions
of life, sometimes the male organs and sometimes the female organs
become more or less impotent; now if we suppose this to occur in
ever so slight a degree under nature, then, as pollen is already
carried regularly from flower to flower, and as a more complete
separation of the sexes of our plant would be advantageous on the
principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes might be effected. It
would take up too much space to show the various steps, through
dimorphism and other means, by which the separation of the sexes in
plants of various kinds is apparently now in progress; but I may add
that some of the species of holly in North America, are, according
to Asa Gray, in an exactly intermediate condition, or, as he expresses
it, are more or less dioeciously polygamous.
Let us now turn to the nectar-feeding insects; we may suppose the
plant, of which we have been slowly increasing the nectar by continued
selection, to be a common plant; and that certain insects depended
in main part on its nectar for food. I could give many facts showing
how anxious bees are to save time: for instance, their habit of
cutting holes and sucking the nectar at the bases of certain
flowers, which, with a very little more trouble, they can enter by the
mouth. Bearing such facts in mind, it may be believed that under
certain circumstances individual differences in the curvature or
length of the proboscis, &c., too slight to be appreciated by us,
might profit a bee or other insect, so that certain individuals
would be able to obtain their food more quickly than others; and
thus the communities to which they belonged would flourish and throw
off many swarms inheriting the same peculiarities. The tubes of the
corolla of the common red and incarnate clovers (Trifolium pratense
and incarnatum) do not on a hasty glance appear to differ in length;
yet the hive-bee can easily suck the nectar out of the incarnate
clover, but not out of the common red clover, which is visited by
humble-bees alone; so that whole fields of red clover offer in vain an
abundant supply of precious nectar to the hive-bee. That this nectar
is much liked by the hive-bee is certain; for I have repeatedly
seen, but only in the autumn, many hive-bees sucking the flowers
through holes bitten in the base of the tube by humble-bees. The
difference in the length of the corolla in the two kinds of clover,
which determines the visits of the hive-bee, must be very trifling;
for I have been assured that when red clover has been mown, the
flowers of the second crop are somewhat smaller, and that these are
visited by many hive-bees. I do not know whether this statement is
accurate; nor whether another published statement can be trusted,
namely, that the Ligurian bee which is generally considered a mere
variety of the common hive-bee, and which freely crosses with it, is
able to reach and suck the nectar of the red clover. Thus, in a
country where this kind of clover abounded, it might be a great
advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this
clover absolutely depends on bees visiting the flowers, if humble-bees
were to become rare in any country, it might be a great advantage to
the plant to have a, shorter or more deeply divided corolla, so that
the hive-bees should be enabled to suck its flowers. Thus I can
understand how a flower and a bee might slowly become, either
simultaneously or one after the other, modified and adapted to each
other in the most perfect manner, by the continued preservation of all
the individuals which presented slight deviations of structure
mutually favourable to each other.
I am well aware that this doctrine of natural selection, exemplified
in the above imaginary instances, is open to the same objections which
were first urged against Sir Charles Lyell's noble views on "the
modern changes of the earth, as illustrative of geology"; but we now
seldom hear the agencies which we see still at work, spoken of as
trifling or insignificant, when used in explaining the excavation of
the deepest valleys or the formation of long lines of inland cliffs.
Natural selection acts only by the preservation and accumulation of
small inherited modifications, each profitable to the preserved being;
and as modern geology has almost banished such views as the excavation
of a great valley by a single diluvial wave, so will natural selection
banish the belief of the continued creation of new organic beings,
or of any great and sudden modification in their structure.
On the Intercrossing of Individuals
I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two
individuals must always (with the exception of the curious and not
well-understood cases of parthenogenesis) unite for each birth; but in
the case of hermaphrodites this is far from obvious. Nevertheless
there is reason to believe that with all hermaphrodites two
individuals, either occasionally or habitually, concur for the
reproduction of their kind. This view was long ago doubtfully
suggested by Sprengel, Knight and Kolreuter. We shall presently see
its importance; but I must here treat the subject with extreme
brevity, though I have the materials prepared for an ample discussion.
All vertebrate animals, all insects, and some other large groups of
animals, pair for each birth. Modern research has much diminished
the number of supposed hermaphrodites, and of real hermaphrodites a
large number pair; that is, two individuals regularly unite for
reproduction, which is all that concerns us. But still there are
many hermaphrodite animals which certainly do not habitually pair, and
a vast majority of plants are hermaphrodites. What reason, it may be
asked, is there for supposing in these cases that two individuals ever
concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts, and
made so many experiments, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross
between different varieties, or between individuals of the same
variety but of another strain, gives vigour and fertility to the
offspring; and on the other hand, that close interbreeding
diminishes vigour and fertility; that these facts alone incline me
to believe that it is a general law of nature that no organic being
fertilises itself for a perpetuity of generations; but that a cross
with another individual is occasionally- perhaps at long intervals
of time- indispensable.
On the belief that this is a law of nature, we can, I think,
understand several large classes of facts, such as the following,
which on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower,
yet what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! If an occasional cross be indispensable,
notwithstanding that the plant's own anthers and pistil stand so
near each other as almost to insure self-fertilisation, the fullest
freedom for the entrance of pollen from another individual will
explain the above state of exposure of the organs. Many flowers, on
the other hand, have their organs of fructification closely
enclosed, as in the great papilionaceous or pea-family; but these
almost invariably present beautiful and curious adaptations in
relation to the visits of insects. So necessary are the visits of bees
to many papilionaceous flowers, that their fertility is greatly
diminished if these visits be prevented. Now, it is scarcely
possible for insects to fly from flower and flower, and not to carry
pollen from one to the other, to the great good of the plant.
Insects act like a camel-hair pencil, and it is sufficient to ensure
fertilisation, just to touch with the same brush the anthers of one
flower and then the stigma of another; but it must not be supposed
that bees would thus produce a multitude of hybrids between distinct
species; for if a plant's own pollen and that from another species are
placed on the same stigma, the former is so prepotent that it
invariably and completely destroys, as has been shown by Gartner,
the influence of the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil,
or slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but the agency of insects is often required to cause the
stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and in this very genus, which seems to have a
special contrivance for self-fertilisation, it is well known that,
if closely allied forms or varieties are planted near each other, it
is hardly possible to raise pure seedlings, so largely do they
naturally cross. In numerous other cases, far from
self-fertilisation being favoured, there are special contrivances
which effectually prevent the stigma receiving pollen from its own
flower, as I could show from the works of Sprengel and others, as well
as from my own observations: for instance, in Lobelia fulgens, there
is a really beautiful and elaborate contrivance by which all the
infinitely numerous pollen-granules are swept out of the conjoined
anthers of each flower, before the stigma of that individual flower is
ready to receive them; and as this flower is never visited, at least
in my garden, by insects, it never sets a seed, though by placing
pollen from one flower on the stigma of another, I raise plenty of
seedlings. Another species of Lobelia which is visited by bees,
seeds freely in my garden. In very many other cases, though there is
no special mechanical contrivance to prevent the stigma receiving
pollen from the same flower, yet, as Sprengel, and more recently
Hildebrand, and others, have shown, and as I can confirm, either the
anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that flower is ready, so that
these so-named dichogamous plants have in fact separated sexes, and
must habitually be crossed. So it is with the reciprocally dimorphic
and trimorphic plants previously alluded to. How strange are these
facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should be in so many cases mutually useless to
each other! How simply are these facts explained on the view of an
occasional cross with a distinct individual being advantageous or
indispensable!
If several varieties of the cabbage, radish, onion, and of some
other plants, be allowed to seed near each other, a large majority
of the seedlings thus raised turn out, as I have found, mongrels:
for instance, I raised 233 seedling cabbages from some plants of
different varieties growing near each other, and of these only 78 were
true to their kind, and some even of these were not perfectly true.
Yet the pistil of each cabbage-flower is surrounded not only by its
own six stamens but by those of the many other flowers on the same
plant; and the pollen of each flower readily gets on its own stigma
without insect agency; for I have found that plants carefully
protected from insects produce the full number of pods. How, then,
comes it that such a vast number of the seedlings are mongrelized?
It must arise from the pollen of a distinct variety having a prepotent
effect over the flower's own pollen; and that this is part of the
general law of good being derived from the intercrossing of distinct
individuals of the same species. When distinct species are crossed the
case is reversed, for a plant's own pollen is almost always
prepotent over foreign pollen; but to this subject we shall return
in a future chapter.
In the case of a large tree covered with innumerable flowers, it may
be objected that pollen could seldom be carried from tree to tree, and
at most only from flower to flower on the same tree; and flowers on
the same tree can be considered as distinct individuals only in a
limited sense. I believe this objection to be valid, but that nature
has largely provided against it by giving to trees a strong tendency
to bear flowers with separated sexes. When the sexes are separated,
although the male and female flowers may be produced on the same tree,
pollen must be regularly carried from flower to flower; and this
will give a better chance of pollen being occasionally carried from
tree to tree. That trees belonging to all Orders have their sexes more
often separated than other plants, I find to be the case in this
country; and at my request Dr. Hooker tabulated the trees of New
Zealand, and Dr. Asa Gray those of the United States, and the result
was as I anticipated. On the other hand, Dr. Hooker informs me that
the rule does not hold good in Australia but if most of the Australian
trees are dichogamous, the same result would follow as if they bore
flowers with separated sexes. I have made these few remarks on trees
simply to call attention to the subject.
Turning for a brief space to animals: various terrestrial species
are hermaphrodites, such as the land-mollusca and earth-worms; but
these all pair. As yet I have not found a single terrestrial animal
which can fertilise itself. This remarkable fact, which offers so
strong a contrast with terrestrial plants, is intelligible on the view
of an occasional cross being indispensable; for owing to the nature of
the fertilising element there are no means, analogous to the action of
insects and of the wind with plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence
of two individuals. Of aquatic animals, there are many
self-fertilizing hermaphrodites; but here the currents of water
offer an obvious means for an occasional cross. As in the case of
flowers, I have as yet failed, after consultation with one of the
highest authorities, namely, Professor Huxley, to discover a single
hermaphrodite animal with the organs of reproduction so perfectly
enclosed that access from without, and the occasional influence of a
distinct individual, can be shown to be physically impossible.
Cirripedes long appeared to me to present, under this point of view, a
case of great difficulty; but I have been enabled, by a fortunate
chance, to prove that two individuals, though both are
self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, both
with animals and plants, some species of the same family and even of
the same genus, though agreeing closely with each other in their whole
organisation, are hermaphrodites, and some unisexual. But if, in fact,
all hermaphrodites do occasionally intercross, the difference
between them and unisexual species is, as far as function is
concerned, very small.
From these several considerations and from the many special facts
which I have collected, but which I am unable here to give, it appears
that with animals and plants an occasional intercross between distinct
individuals is a very general, if not universal, law of nature.
Circumstances favourable for the production of new forms through
Natural Selection
This is an extremely intricate subject. A great amount of
variability, under which term individual differences are always
included, will evidently be favourable. A large number of individuals,
by giving a better chance within any given period for the appearance
of profitable variations, will compensate for a lesser amount of
variability in each individual, and is, I believe, a highly
important element of success. Though Nature grants long periods of
time for the work of natural selection, she does not grant an
indefinite period; for as all organic beings are striving to seize
on each place in the economy of nature, if any one species does not
become modified and improved in a corresponding degree with its
competitors, it will be exterminated. Unless favourable variations
be inherited by some at least of the offspring, nothing can be
effected by natural selection. The tendency to reversion may often
check or prevent the work; but as this tendency has not prevented
man from forming by selection numerous domestic races, why should it
prevail against natural selection?
In the case of methodical selection, a breeder selects for some
definite object, and if the individuals be allowed freely to
intercross, his work will completely fail. But when many men,
without intending to alter the breed, have a nearly common standard of
perfection, and all try to procure and breed from the best animals,
improvement surely but slowly follows from this unconscious process of
selection, notwithstanding that there is no separation of selected
individuals. Thus it will be under nature; for within a confined area,
with some place in the natural polity not perfectly occupied, all
the individuals varying in the right direction, though in different
degrees, will tend to be preserved. But if the area be large, its
several districts will almost certainly present different conditions
of life; and then, if the same species undergoes modification in
different districts, the newly-formed varieties will intercross on the
confines of each. But we shall see in the sixth chapter that
intermediate varieties, inhabiting intermediate districts, will in the
long run generally be supplanted by one of the adjoining varieties.
Intercrossing will chiefly affect those animals which unite for each
birth and wander much, and which do not breed at a very quick rate.
Hence with animals of this nature, for instance, birds, varieties will
generally be confined to separated countries; and this I find to be
the case. With hermaphrodite organisms which cross only
occasionally, and likewise with animals which unite for each birth,
but which wander little and can increase at a rapid rate, a new and
improved variety might be quickly formed on any one spot, and might
there maintain itself in a body and afterwards spread, so that the
individuals of the new variety would chiefly cross together. On this
principle, nurserymen always prefer saving seed from a large body of
plants, as the chance of intercrossing is thus lessened.
Even with animals which unite for each birth, and which do not
propagate rapidly, we must not assume that free intercrossing would
always eliminate the effects of natural selection; for I can bring
forward a considerable body of facts showing that within the same
area, two varieties of the same animal may long remain distinct,
from haunting different stations, from breeding at slightly
different seasons, or from the individuals of each variety
preferring to pair together.
Intercrossing plays a very important part in nature by keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more
efficiently with those animals which unite for each birth; but, as
already stated, we have reason to believe that occasional intercrosses
take place with all animals and plants. Even if these take place
only at long intervals of time, the young thus produced will gain so
much in vigour and fertility over the offspring from long-continued
self-fertilisation, that they will have a better chance of surviving
and propagating their kind; and thus in the long run the influence
of crosses, even at rare intervals, will be great. With respect to
organic beings extremely low in the scale, which do not propagate
sexually, nor conjugate, and which cannot possibly intercross,
uniformity of character can be retained by them under the same
conditions of life, only through the principle of inheritance, and
through natural selection which will destroy any individuals departing
from the proper type. If the conditions of life change and the form
undergoes modification, uniformity of character can be given to the
modified offspring, solely by natural selection preserving similar
favourable variations.
Isolation, also, is an important element in the modification of
species through natural selection. In a confined or isolated area,
if not very large, the organic and inorganic conditions of life will
generally be almost uniform; so that natural selection will tend to
modify all the varying individuals of the same species in the same
manner. Intercrossing with the inhabitants of the surrounding
districts will, also, be thus prevented. Moritz Wagner has lately
published an interesting essay on this subject, and has shown that the
service rendered by isolation in preventing crosses between
newly-formed varieties is probably greater even than I supposed. But
from reasons already assigned I can by no means agree with this
naturalist, that migration and isolation are necessary elements for
the formation of new species. The importance of isolation is
likewise great in preventing, after any physical change in the
conditions, such as of climate, elevation of the land, &c., the
immigration of better adapted organisms; and thus new places in the
natural economy of the district will be left open to be filled up by
the modification of the old inhabitants. Lastly, isolation will give
time for a new variety to be improved at a slow rate; and this may
sometimes be of much importance. If, however, an isolated area be very
small, either from being surrounded by barriers, or from having very
peculiar physical conditions, the total number of the inhabitants will
be small; and this will retard the production of new species through
natural selection, by decreasing the chances of favourable
variations arising.
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously
asserted that the element of time has been assumed by me to play an
all-important part in modifying species, as if all the forms of life
were necessarily undergoing change through some innate law. Lapse of
time is only so far important, and its importance in this respect is
great, that it gives a better chance of beneficial variations
arising and of their being selected, accumulated, and fixed. It
likewise tends to increase the direct action of the physical
conditions of life, in relation to the constitution of each organism.
If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the
number of species inhabiting it is small, as we shall see in our
chapter on Geographical Distribution; yet of these species a very
large proportion are endemic,- that is, have been produced there and
nowhere else in the world. Hence an oceanic island at first sight
seems to have been highly favourable for the production of new
species. But we may thus deceive ourselves, for to ascertain whether
small isolated area, or a large open area like a continent has been
most favourable for the production of new organic forms, we ought to
make the comparison within equal times; and this we are incapable of
doing.
Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of
area is still more important, especially for the production of species
which shall prove capable of enduring for a long period, and of
spreading widely. Throughout a great and open area, not only will
there be a better chance of favourable variations, arising from the
large number of individuals of the same species there supported, but
the conditions of life are much more complex from the large number
of already existing species; and if some of these many species
become modified and improved, others will have to be improved in a
corresponding degree, or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into competition
with many other forms. Moreover, great areas, though now continuous,
will often, owing to former oscillations of level, have existed in a
broken condition; so that the good effects of isolation will
generally, to a certain extent, have concurred. Finally, I conclude
that, although small isolated areas have been in some respects
highly favourable for the production of new species, yet that the
course of modification will generally have been more rapid on large
areas; and what is more important, that the new forms produced on
large areas, which already have been victorious over many competitors,
will be those that will spread most widely, and will give rise to
the greatest number of new varieties and species. They will thus
play a more important part in the changing history of the organic
world.
In accordance with this view, we can, perhaps, understand some facts
which will be again alluded to in our chapter on Geographical
Distribution; for instance, the fact of the productions of the smaller
continent of Australia now yielding before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental
productions have everywhere become so largely naturalised on
islands. On a small island, the race for life will have been less
severe, and there will have been less modification and less
extermination. Hence, we can understand how it is that the flora of
Madeira, according to Oswald Heer, resembles to a certain extent the
extinct tertiary flora of Europe. All fresh-water basins, taken
together, make a small area compared with that of the sea or of the
land. Consequently, the competition between fresh-water productions
will have been less severe than elsewhere; new forms will have been
then more slowly produced, and old forms more slowly exterminated. And
it is in fresh-water basins that we find seven genera of Ganoid
fishes, remnants of a once preponderant order: and in fresh water we
find some of the most anomalous forms now known in the world as the
Ornithorhynchus and Lepidosiren which, like fossils, connect to a
certain extent orders at present widely sundered in the natural scale.
These anomalous forms may be called living fossils; they have
endured to the present day, from having inhabited a confined area, and
from having been exposed to less varied, and therefore less severe,
competition.
To sum up, as far as the extreme intricacy of the subject permits,
the circumstances favourable and unfavourable for the reduction of new
species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many
oscillations of level, will have been the most favourable for the
production of many new forms of life, fitted to endure for a long time
and to spread widely. Whilst the area existed as a continent, the
inhabitants will have been numerous in individuals and kinds, and will
have been subjected to severe competition. When converted by
subsidence into large separate islands, there will still have
existed many individuals of the same species on each island:
intercrossing on the confines of the range of each new species will
have been checked: after physical changes of any kind, immigration
will have been prevented, so that new places in the polity of each
island will have had to be filled up by the modification of the old
inhabitants; and time will have been allowed for the varieties in each
to become well modified and perfected. When, by renewed elevation, the
islands were reconverted into a continental area, there will again
have been very severe competition: the most favoured or improved
varieties will have been enabled to spread: there will have been
much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the reunited
continent will again have been changed; and again there will have been
a fair field for natural selection to improve still further the
inhabitants, and thus to produce new species.
That natural selection generally acts with extreme slowness I
fully admit. It can act only when there are places in the natural
polity of a district which can be better occupied by the
modification of some of its existing inhabitants. The occurrence of
such places will often depend on physical changes, which generally
take place very slowly, and on the immigration of better adapted forms
being prevented. As some few of the old inhabitants become modified,
the mutual relations of others will often be disturbed; and this
will create new places, ready to be filled up by better adapted forms,
but all this will take place very slowly. Although the individuals
of the same species differ in some slight degree from each other, it
would often be long before differences of the right nature in
various parts of the organisation might occur. The result would
often be greatly retarded by free intercrossing. Many will exclaim
that these several causes are amply sufficient to neutralise the power
of natural selection. I do not believe so. But I do believe that
natural selection will generally act very slowly, only at long
intervals of time, and only on a few of the inhabitants of the same
region. I further believe that these slow, intermittent results accord
well with what geology tells us of the rate and manner at which the
inhabitants of the world have changed.
Slow though the process of selection may be, if feeble man can do
much by artificial selection, I can see no limit to the amount of
change, to the beauty and complexity of the coadaptations between
all organic beings, one with another and with their physical
conditions of life, which may have been effected in the long course of
time through nature's power of selection, that is by the survival of
the fittest.
Extinction caused by Natural Selection
This subject will he more fully discussed in our chapter on Geology;
but it must here be alluded to from being intimately connected with
natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which
consequently endure. Owing to the high geometrical rate of increase of
all organic beings, each area is already fully stocked with
inhabitants; and it follows from this, that as the favoured forms
increase in number, so, generally, will the less favoured decrease and
become rare. Rarity, as geology tells us, is the precursor to
extinction. We can see that any form which is represented by few
individuals will run a good chance of utter extinction, during great
fluctuations in the nature of the seasons, or from a temporary
increase in the number of its enemies. But we may go further than
this; for, as new forms are produced, unless we admit that specific
forms can go on indefinitely increasing in number, many old forms must
become extinct. That the number of specific forms has not indefinitely
increased, geology plainly tells us; and we shall presently attempt to
show why it is that the number of species throughout the world has not
become immeasurably great.
We have seen that the species which are most numerous in individuals
have the best chance of producing favourable variations within any
given period. We have evidence of this, in the facts stated in the
second chapter showing that it is the common and diffused or
dominant species which offer the greatest number of recorded
varieties. Hence, rare species will be less quickly modified or
improved within any given period; they will consequently be beaten
in the race for life by the modified and improved descendants of the
commoner species.
From these several considerations I think it inevitably follows,
that as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the
most closely-allied forms,- varieties of the same species, and species
of the same genus or of related genera,- which, from having nearly the
same structure, constitution, and habits, generally come into the
severest competition with each other; consequently, each new variety
or species, during the progress of its formation, will generally press
hardest on its nearest kindred, and tend to exterminate them. We see
the same process of extermination amongst our domesticated
productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds of
cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the
long-horns, and that these "were swept away by the shorthorns" (I
quote the words of an agricultural writer) "as if by some murderous
pestilence."
Divergence of Character
The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In
the first place, varieties, even strongly-marked ones, though having
somewhat of the character of species- as is shown by the hopeless
doubts in many cases how to rank them- yet certainly differ far less
from each other than do good and distinct species. Nevertheless,
according to my view, varieties are species in the process of
formation, or are, as I have called them, incipient species. How,
then, does the lesser difference between varieties become augmented
into the greater difference between species? That this does habitually
happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas
varieties, the supposed prototypes and parents of future well-marked
species, present slight and ill-defined differences. Mere chance, as
we may call it, might cause one variety to differ in some character
from its parents, and the offspring of this variety again to differ
from its parent in the very same character and in a greater degree;
but this alone would never account for so habitual and large a
degree of difference as that between the species of the same genus.
As has always been my practice, I have sought light on this head
from our domestic productions. We shall here find something analogous.
It will be admitted that the production of races so different as
short-horn and Hereford cattle, race and cart horses, the several
breeds of pigeons, &c., could never have been effected by the mere
chance accumulation of similar variations during many successive
generations. In practice, a fancier is, for instance, struck by a
pigeon having a slightly shorter beak; another fancier is struck by
a pigeon having a rather longer beak; and on the acknowledged
principle that "fanciers do not and will not admire a medium standard,
but like extremes," they both go on (as has actually occurred with the
sub-breeds of the tumbler-pigeon) choosing and breeding from birds
with longer and longer beaks, or with shorter and shorter beaks.
Again, we may suppose that at an early period of history, the men of
one nation or district required swifter horses, whilst those of
another required stronger and bulkier horses. The early differences
would be very slight; but, in the course of time from the continued
selection of swifter horses in the one case, and of stronger ones in
the other, the differences would become greater, and would be noted as
forming two sub-breeds. Ultimately, after the lapse of centuries,
these sub-breeds would become converted into two well-established
and distinct breeds. As the differences became greater, the inferior
animals with intermediate characters, being neither swift nor very
strong, would not have been used for, breeding, and will thus have
tended to disappear. Here, then, we see in man's productions the
action of what may be called the principle of divergence, causing
differences, at first barely appreciable, steadily to increase, and
the breeds to diverge in character, both from each other and from
their common parent.
But how, it may be asked, can any analogous principle apply in
nature? I believe it can and does apply most efficiently (though it
was a long time before I saw how), from the simple circumstance that
the more diversified the descendants from any one species become in
structure, constitution, and habits, by so much will they be better
enabled to seize on many and widely diversified places in the polity
of nature, and so be enabled to increase in numbers.
We can clearly discern this in the case of animals with simple
habits. Take the case of a carnivorous quadruped, of which the
number that can be supported in any country has long ago arrived at
its full average. If its natural power of increase be allowed to
act, it can succeed in increasing (the country not undergoing any
change in conditions) only by its varying descendants seizing on
places at present occupied by other animals: some of them, for
instance, being enabled to feed on new kinds of prey, either dead or
alive; some inhabiting new stations, climbing trees, frequenting
water, and some perhaps becoming less carnivorous. The more
diversified in habits and structure the descendants of our carnivorous
animals become, the more places they will be enabled to occupy. What
applies to one animal will apply throughout all time to all animals-
that is, if they vary- for otherwise natural selection can effect
nothing. So it will be with plants. It has been experimentally proved,
that if a plot of ground be sown with one species of grass, and a
similar plot be sown with several distinct genera of grasses, a
greater number of plants and a greater weight of dry herbage can be
raised in the latter than in the former case. The same has been
found to hold good when one variety and several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one
species of grass were to go on varying, and the varieties were
continually selected which differed from each other in the same
manner, though in a very slight degree, as do the distinct species and
genera of grasses, a greater number of individual plants of this
species, including its modified descendants, would succeed in living
on the same piece of ground. And we know that each species and each
variety of grass is annually sowing almost countless seeds; and is
thus striving, as it may be said, to the utmost to increase in number.
Consequently, in the course of many thousand generations, the most
distinct varieties of any one species of grass would have the best
chance of succeeding and of increasing in numbers, and thus of
supplanting the less distinct varieties; and varieties, when
rendered very distinct from each other, take the rank of species.
The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if
freely open to immigration, and where the contest between individual
and individual must be very severe, we always find great diversity
in its inhabitants. For instance, I found that a piece of turf,
three feet by four in size, which had been exposed for many years to
exactly the same conditions, supported twenty species of plants, and
these belonged to eighteen genera and to eight orders, which shows how
much these plants differed from each other. So it is with the plants
and insects on small and uniform islets: also in small ponds of
fresh water. Farmers find that they can raise most food by a
rotation of plants belonging to the most different orders: nature
follows what may be called a simultaneous rotation. Most of the
animals and plants which live close round any small piece of ground,
could live on it (supposing its nature not to be in any way peculiar),
and may be said to be striving to the utmost to live there; but, it is
seen, that where they come into the closest competition, the
advantages of diversification of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general rule,
belong to what we call different genera and orders.
The same principle is seen in the naturalisation of plants through
man's agency in foreign lands. It might have been expected that the
plants which would succeed in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country. It might also, perhaps, have been expected that naturalised
plants would have belonged to a few groups more especially adapted
to certain stations in their new homes. But the case is very
different; and Alph. de Candolle has well remarked, in his great and
admirable work, that floras gain by naturalisation, proportionally
with the number of the native genera and species far more in new
genera than in new species. To give a single instance: in the last
edition of Dr. Asa Gray's Manual of the Flora of the Northern United
States, 260 naturalized plants are enumerated, and these belong to 162
genera. We thus see that these naturalised plants are of a highly
diversified nature. They differ, moreover, to a large extent, from the
indigenes, for out of the 162 naturalised genera, no less than 100
genera are not there indigenous, and thus a large proportional
addition is made to the genera now living in the United States.
By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes and have there
become naturalised, we may gain some crude idea in what manner some of
the natives would have to be modified, in order to gain an advantage
over their compatriots; and we may at least infer that diversification
of structure, amounting to new generic differences, would be
profitable to them.
The advantage of diversification of structure in the inhabitants
of the same region is, in fact, the same as that of the
physiological division of labour in the organs of the same
individual body- a subject so well elucidated by Milne Edwards. No
physiologist doubts that a stomach adapted to digest vegetable
matter alone, or flesh alone, draws most nutriment from these
substances. So in the general economy of any land, the more widely and
perfectly the animals and plants are diversified for different
habits of life, so will a greater number of individuals be capable
of there supporting themselves. A set of animals, with their
organisation but little diversified, could hardly compete with a set
more perfectly diversified in structure. It may be doubted, for
instance, whether the Australian marsupials, which are divided into
groups differing but little from each other, and feebly
representing, as Mr. Waterhouse and others have remarked, our
carnivorous, ruminant, and rodent mammals, could successfully
compete with these well-developed orders. In the Australian mammals,
we see the process of diversification in an early and incomplete stage
of development.
The Probable Effects of the Action of Natural Selection through
Divergence of Character and Extinction, on the Descendants of a Common
Ancestor
After the foregoing discussion, which has been much compressed, we
may assume that the modified descendants of any one species will
succeed so much the better as they become more diversified in
structure, and are thus enabled to encroach on places occupied by
other beings. Now let us see how this principle of benefit being
derived from divergence of character, combined with the principles
of natural selection and of extinction, tends to act.
The accompanying diagram (See diagram) will aid us in understanding
this rather perplexing subject. Let A to L represent the species of a
genus large in its own country; these species are supposed to resemble
each other in unequal degrees, as is so generally the case in nature,
and as is represented in the diagram by the letters standing at
unequal distances. I have said a large genus, because as we saw in the
second chapter, on an average more species vary in large genera than
in small genera; and the varying species of the large genera present a
greater number of varieties. We have, also, seen that the species,
which are the commonest and the most widely diffused, vary more than
do the rare and restricted species. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in
its own country. The branching and diverging lines of unequal
lengths proceeding from (A), may represent its varying offspring.
The variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear
simultaneously, but often after long intervals of time, nor are they
an supposed to endure for equal periods. Only those variations which
are in some way profitable will be preserved or naturally selected.
And here the importance of the principle of benefit derived from
divergence of character comes in; for this will generally lead to
the most different or divergent variations (represented by the outer
lines) being preserved and accumulated by natural selection. When a
line reaches one of the horizontal lines, and is there marked by a
small numbered letter, a sufficient amount of variation is supposed to
have been accumulated to form it into a fairly well-marked variety,
such as would be thought worthy of record in a systematic work.
The intervals between the horizontal lines in the diagram, may
represent each a thousand or more generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a1 and m1. These two varieties will
generally still be exposed to the same conditions which made their
parents variable, and the tendency to variability is in itself
hereditary; consequently they will likewise tend to vary, and commonly
in nearly the same manner as did their parents. Moreover, these two
varieties, being only slightly modified forms, will tend to inherit
those advantages which made their parent (A) more numerous than most
of the other inhabitants of the same country; they will also partake
of those more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And all
these circumstances are favourable to the production of new varieties.
If, then, these two varieties be variable, the most divergent of
their variations will generally be preserved during the next
thousand generations. And after this interval, variety a1 is
supposed in the diagram to have produced variety a2, which will, owing
to the principle of divergence, differ more from (A) than did variety
a1. Variety m1 is supposed to have produced two varieties, namely m2
and s2, differing from each other, and more considerably from their
common parent (A). We may continue the process by similar steps for
any length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants of
the common parent (A), will generally go on increasing in number and
diverging in character. In the diagram the process is represented up
to the ten-thousandth generation, and under a condensed and simplified
form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in
itself made somewhat irregular, nor that it goes on continuously; it
is far more probable that each form remains for long periods
unaltered, and then again undergoes modification. Nor do I suppose
that the most divergent varieties are invariably preserved: a medium
form may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not
perfectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will increase. In our diagram the line of succession
is broken at regular intervals by small numbered letters marking the
successive forms which have become sufficiently distinct to be
recorded as varieties. But these breaks are imaginary, and might have
been inserted anywhere, after intervals long enough to allow the
accumulation of a considerable amount of divergent variation.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the
same advantages which made their parent successful in life, they
will generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later
and more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and
less improved branches: this is represented in the diagram by some of
the lower branches not reaching to the upper horizontal lines. In some
cases no doubt the process of modification will be confined to a
single line of descent and the number of modified descendants will not
be increased; although the amount of divergent modification may have
been augmented. This case would be represented in the diagram, if all
the lines proceeding from (A) were removed, excepting that from a1 to
a10. In the same way the English race-horse and English pointer have
apparently both gone on slowly diverging in character from their
original stocks, without either having given off any fresh branches or
races.
After ten thousand generations, species (A) is supposed to have
produced three forms, a10, f10, and m10 which, from having diverged in
character during the successive generations, will have come to
differ largely, but perhaps unequally, from each other and from
their common parent. If we suppose the amount of change between each
horizontal line in our diagram to be excessively small, these three
forms may still be only well-marked varieties; but we have only to
suppose the steps in the process of modification to be more numerous
or greater in amount, to convert these three forms into well-defined
or at least into doubtful species. Thus the diagram illustrates the
steps by which the small differences distinguishing varieties are
increased into the larger differences distinguishing species. By
continuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we get
eight species, marked by the letters between a14 and m14, all
descended from (A). Thus, as I believe, species are multiplied and
genera are formed.
In a large genus it is probable that more than one species would
vary. In the diagram I have assumed that a second species (I) has
produced, by analogous steps, after ten thousand generations, either
two well-marked varieties (w10 and z10) or two species, according to
the amount of change supposed to be represented between the horizontal
lines. After fourteen thousand generations, six new species, marked by
the letters n14 to z14, are supposed to have. been produced. In any
genus, the species which are already very different in character
from each other, will generally tend to produce the greatest number of
modified descendants; for these will have the best chance of seizing
on new and widely different places in the polity of nature: hence in
the diagram I have chosen the extreme species (A), and the nearly
extreme species (I), as those which have largely varied, and have
given rise to new varieties and species. The other nine species
(marked by capital letters) of our original genus, may for long but
unequal periods continue to transmit unaltered descendants; and this
is shown in the diagram by the dotted lines unequally prolonged
upwards.
But during the process of modification, represented in the
diagram, another of our principles, namely that of extinction, will
have played an important part. As in each fully stocked country
natural selection necessarily acts by the selected form having some
advantage in the struggle for life over other forms, there will be a
constant tendency in the improved descendants of any one species to
supplant and exterminate in each stage of descent their predecessors
and their original progenitor. For it should be remembered that the
competition will generally be most severe between those forms which
are most nearly related to each other in habits, constitution, and
structure. Hence all the intermediate forms between the earlier and
later states, that is between the less and more improved states of the
same species, as well as the original parent-species itself, will
generally tend to become extinct. So it probably will be with many
whole collateral lines of descent, which will be conquered by later
and improved lines. If, however, the modified offspring of a species
get into some distinct country, or become quickly adapted to some
quite new station, in which offspring and progenitor do not come
into competition, both may continue to exist.
If, then, our diagram be assumed to represent a considerable amount
of modification, species (A) and all the earlier varieties will have
become extinct, being replaced by eight new species (a14 to m14); and
species (I) will be replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genus
were supposed to resemble each other in unequal degrees, as is so
generally the case in nature; species (A) being more nearly related to
B, C, and D, than to the other species; and species (I) more to G,
H, K, L, than to the others. These two species (A) and (I) were also
supposed to be very common and widely diffused species, so that they
must originally have had some advantage over most of the other species
of the genus. Their modified descendants, fourteen in number at the
fourteen-thousandth generation will probably have inherited some of
the same advantages: they have also been modified and improved in a
diversified manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of their
country. It seems, therefore, extremely probable that they will have
taken the places of, and thus exterminated not only their parents
(A) and (I), but likewise some of the original species which were most
nearly related to their parents. Hence very few of the original
species will have transmitted offspring to the fourteen-thousandth
generation. We may suppose that only one, (F), of the two species (E
and F) which were least closely related to the other nine original
species, has transmitted descendants to this late stage of descent.
The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent
tendency of natural selection, the extreme amount of difference in
character between species a14 and z14 will be much greater than that
between the most distinct of the original eleven species. The new
species, moreover, will be allied to each other in a widely different
manner. Of the eight descendants from (A) the three marked a14, q14,
p14, will be nearly related from having recently branched off from
a10; b14, and f14, from having diverged at an earlier period from a1,
will be in some degree distinct from the three first-named species;
and lastly, o14, e14, and m14, will be nearly related one to the
other, but, from having diverged at the first commencement of the
process of modification, will be widely different from the other five
species, and may constitute a sub-genus or a distinct genus.
The six descendants from (I) will form two sub-genera or genera. But
as the original species (I) differed largely from (A), standing nearly
at the extreme end of the original genus, the six descendants from (I)
will, owing to inheritance alone, differ considerably from the eight
descendants from (A); the two groups, moreover, are supposed to have
gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct,
and have left no descendants. Hence the six new species descended from
(I), and the eight descendants from (A), will have to be ranked as
very distinct genera, or even as distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent with modification, from two or more species of the same genus.
And the two or more parent-species are supposed to be descended from
some one species of an earlier genus. In our diagram, this is
indicated by the broken lines, beneath the capital letters, converging
in sub-branches downwards towards a single point; this point
represents a species, the supposed progenitor of our several new
sub-genera and genera.
It is worth while to reflect for a moment on the character of the
new species F14, which is supposed not to have diverged much in
character, but to have retained the form of (F), either unaltered or
altered only in a slight degree. In this case, its affinities to the
other fourteen new species will be of a curious and circuitous nature.
Being descended from a form which stood between the parent-species (A)
and (I), now supposed to be extinct and unknown, it will be in some
degree intermediate in character between the two groups descended from
these two species. But as these two groups have gone on diverging in
character from the type of their parents, the new species (F14) will
not be directly intermediate between them, but rather between types of
the two groups; and every naturalist will be able to call such cases
before his mind.
In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
more generations; it may also represent a section of the successive
strata of the earth's crust including extinct remains. We shall,
when we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light
on the affinities of extinct beings, which, though generally belonging
to the same orders, families, or genera, with those now living, yet
are often, in some degree, intermediate in character between
existing groups; and we can understand this fact, for the extinct
species lived at various remote epochs when the branching lines of
descent had diverged less.
I see no reason to limit the process of modification, as now
explained, to the formation of genera alone. If, in the diagram, we
suppose the amount of change, represented by each successive group of
diverging lines to be great, the forms marked a14 to p14, those marked
b14 and f14, and those marked o14 to m14, will form three very
distinct genera. We shall also have two very distinct genera descended
from (I), differing widely from the descendants of (A). These two
groups of genera will thus form two distinct families, or orders,
according to the amount of divergent modification supposed to be
represented in the diagram. And the two new families, or orders, are
descended from two species of the original genus, and these are
supposed to be descended from some still more ancient and unknown
form.
We have seen that in each country it is the species belonging to the
larger genera which oftenest present varieties or incipient species.
This, indeed, might have been expected; for, as natural selection acts
through one form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which already
have some advantage; and the largeness of any group shows that its
species have inherited from a common ancestor some advantage in
common. Hence, the struggle for the production of new and modified
descendants will mainly lie between the larger groups which are all
trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the
later and more highly perfected sub-groups, from branching out and
seizing on many new places in the polity of Nature, will constantly
tend to supplant and destroy the earlier and less improved sub-groups.
Small and broken groups and sub-groups will finally disappear. Looking
to the future, we can predict that the groups of organic beings
which are now large and triumphant, and which are least broken up,
that is, which have as yet suffered least extinction, will, for a long
period, continue to increase. But which groups will ultimately
prevail, no man can predict; for we know that many groups formerly
most extensively developed, have now become extinct. Looking still
more remotely to the future, we may predict that, owing to the
continued and steady increase of the larger groups, a multitude of
smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that, of the species living at any one
period, extremely few will transmit descendants to a remote
futurity. I shall have to return to this subject in the chapter on
Classification, but I may add that as, according to this view,
extremely few of the more ancient species have transmitted descendants
to the present day, and, as all the descendants of the same species
form a class, we can understand how it is that there exist so few
classes in each main division of the animal and vegetable kingdoms.
Although few of the most ancient species have left modified
descendants' yet, at remote geological periods, the earth may have
been almost as well peopled with species of many genera, families,
orders, and classes, as at the present time.
On the Degree to which Organisation tends to advance
Natural Selection acts exclusively by the preservation and
accumulation of variations, which are beneficial under the organic and
inorganic conditions to which each creature is exposed at all
periods of life. The ultimate result is that each creature tends to
become more and more improved in relation to its conditions. This
improvement inevitable leads to the gradual advancement of the
organisation of the greater number of living beings throughout the
world. But here we enter on a very intricate subject, for
naturalists have not defined to each other's satisfaction what is
meant by an advance in organisation. Amongst the vertebrata the degree
of intellect and an approach in structure to man clearly come into
play. It might be thought that the amount of change which the
various parts and organs pass through in their development from the
embryo to maturity would suffice as a standard of comparison; but
there are cases, as with certain parasitic crustaceans, in which
several parts of the structure become less perfect, so that the mature
animal cannot be called higher than its larva. Von Baer's standard
seems the most widely applicable and the best, namely, the amount of
differentiation of the parts of the same organic being, in the adult
state as I should be inclined to add, and their specialisation for
different functions; or, as Milne Edwards would express it, the
completeness of the division of physiological labour. But we shall see
how obscure this subject is if we look, for instance, to fishes,
amongst which some naturalists rank those as highest which, like the
sharks, approach nearest to amphibians; whilst other naturalists
rank the common bony or teleostean fishes as the highest, inasmuch
as they are most strictly fish-like and differ most from the other
vertebrate classes. We see still more plainly the obscurity of the
subject by turning to plants, amongst which the standard of
intellect is of course quite excluded; and here some botanists rank
those plants as highest which have every organ, as sepals, petals,
stamens, and pistils, fully developed in each flower; whereas other
botanists, probably with more truth, look at the plants which have
their several organs much modified and reduced in number as the
highest.
If we take as the standard of high organisation, the amount of
differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard: for all physiologists admit that the specialisation of
organs, inasmuch as in this state they perform their functions better,
is an advantage to each being; and hence the accumulation of
variations tending towards specialisation is within the scope of
natural selection. On the other hand, we can see, bearing in mind that
all organic beings are striving to increase at a high ratio and to
seize on every unoccupied or less well occupied place in the economy
of nature, that it is quite possible for natural selection gradually
to fit a being to a situation in which several organs would be
superfluous or useless: in such cases there would be retrogression
in the scale of organisation. Whether organisation on the whole has
actually advanced from the remotest geological periods to the
present day will be more conveniently discussed in our chapter on
Geological Succession.
But it may be objected that if all organic beings thus tend to
rise in the scale, how is it that throughout the world a multitude
of the lowest forms still exist; and how is it that in each great
class some forms are far more highly developed than others? Why have
not the more highly developed forms everywhere supplanted and
exterminated the lower? Lamarck, who believed in an innate and
inevitable tendency towards perfection in all organic beings, seems to
have felt this difficulty so strongly, that he was led to suppose that
new and simple forms are continually being produced by spontaneous
generation. Science has not as yet proved the truth of this belief,
whatever the future may reveal. On our theory the continued
existence of lowly organisms offers no difficulty; for natural
selection, or the survival of the fittest, does not necessarily
include progressive development- it only takes advantage of such
variations as arise and are beneficial to each creature under its
complex relations of life. And it may be asked what advantage, as
far as we can see, would it be to an infusorian animalcule- to an
intestinal worm- or even to an earthworm, to be highly organised. If
it were no advantage, these forms would be left, by natural selection,
unimproved or but little improved, and might remain for indefinite
ages in their present lowly condition. And geology tells us that
some of the lowest forms, as the infusoria and rhizopods, have
remained for an enormous period in nearly their present state. But
to suppose that most of the many now existing low forms have not in
the least advanced since the first dawn of life would be extremely
rash; for every naturalist who has dissected some of the beings now
ranked as very low in the scale, must have been struck with their
really wondrous and beautiful organisation.
Nearly the same remarks are applicable if we look to the different
grades of organisation within the same great group; for instance, in
the vertebrata, to the co-existence of mammals and fish- amongst
mammalia, to the coexistence of man and the Ornithorhynchus- amongst
fishes, to the co-existence of the shark and the lancelet (Amphioxus),
which latter fish in the extreme simplicity of its structure
approaches the invertebrate classes. But mammals and fish hardly
come into competition with each other; the advancement of the whole
class of mammals, or of certain members in this class, to the
highest grade would not lead to their taking the place of fishes.
Physiologists believe that the brain must be bathed by warm blood to
be highly active, and this requires aerial respiration; so that
warm-blooded mammals when inhabiting the water lie under a
disadvantage in having to come continually to the surface to
breathe. With fishes, members of the shark family would not tend to
supplant the lancelet; for the lancelet, as I hear from Fritz
Muller, has as sole companion and competitor on the barren sandy shore
of South Brazil, an anomalous annelid. The three lowest orders of
mammals, namely, marsupials, edentata, and rodents, co-exist in
South America in the same region with numerous monkeys, and probably
interfere little with each other. Although organisation, on the whole,
may have advanced and be still advancing throughout the world, yet the
scale will always present many degrees of perfection; for the high
advancement of certain whole classes, or of certain members of each
class, does not at all necessarily lead to the extinction of those
groups with which they do not enter into close competition. In some
cases, as we shall hereafter see, lowly organised forms appear to have
been preserved to the present day, from inhabiting confined or
peculiar stations, where they have been subjected to less severe
competition, and where their scanty numbers have retarded the chance
of favourable variations arising.
Finally, I believe that many lowly organised forms now exist
throughout the world, from various causes. In some cases variations or
individual differences of a favourable nature may never have arisen
for natural selection to act on and accumulate. In no case,
probably, has time sufficed for the utmost possible amount of
development. In some few cases there has been what we must call
retrogression of organisation. But the main cause lies in the fact
that under very simple conditions of life a high organisation would be
of no service,- possibly would be of actual disservice, as being of
a more delicate nature, and more liable to be put out of order and
injured.
Looking to the first dawn of life, when all organic beings, as we
may believe, presented the simplest structure, how, it has been asked,
could the first steps in the advancement or differentiation of parts
have arisen? Mr. Herbert Spencer would probably answer that, as soon
as simple unicellular organism came by growth or division to be
compounded of several cells, or became attached to any supporting
surface, his law "that homologous units of any order become
differentiated in proportion as their relations to incident forces"
would come into action. But as we have no facts to guide us,
speculation on the subject is almost useless. It is, however, an error
to suppose that there would be no struggle for existence, and,
consequently, no natural selection, until many forms had been
produced: variations in a single species inhabiting an isolated
station might be beneficial, and thus the whole mass of individuals
might be modified, or two distinct forms might arise. But, as I
remarked towards the close of the Introduction, no one ought to feel
surprise at much remaining as yet unexplained on the origin of
species, if we make due allowance for our profound ignorance on the
mutual relations of the inhabitants of the world at the present
time, and still more so during past ages.
Convergence of Character
Mr. H. C. Watson thinks that I have overrated the importance of
divergence of character (in which, however, he apparently believes)
and that convergence, as it may be called, has likewise played a part.
If two species, belonging to two distinct though allied genera, had
both produced a large number of new and divergent forms, it is
conceivable that these might approach each other so closely that
they would have all to be classed under the same genus; and thus the
descendants of two distinct genera would converge into one. But it
would in most cases be extremely rash to attribute to convergence a
close and general similarity of structure in the modified
descendants of widely distinct forms. The shape of a crystal is
determined solely by the molecular forces, and it is not surprising
that dissimilar substances should sometimes assume the same form;
but with organic beings we should bear in mind that the form of each
depends on an infinitude of complex relations, namely on the
variations which have arisen, these being due to causes far too
intricate to be followed out,- on the nature of the variations which
have been preserved or selected, and this depends on the surrounding
physical conditions, and in a still higher degree on the surrounding
organisms with which each being has come into competition,- and
lastly, on inheritance (in itself a fluctuating element) from
innumerable progenitors, all of which have had their forms
determined through equally complex relations. It is incredible that
the descendants of two organisms, which had originally differed in a
marked manner, should ever afterwards converge so closely as to lead
to a near approach to identity throughout their whole organisation. If
this had occurred, we should meet with the same form, independently of
genetic connection, recurring in widely separated geological
formations; and the balance of evidence is opposed to any such an
admission.
Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make
an indefinite number of specific forms. As far as mere inorganic
conditions are concerned, it seems probable that a sufficient number
of species would soon become adapted to all considerable diversities
of heat, moisture, &c.; but I fully admit that the mutual relations of
organic beings are more important; and as the number of species in any
country goes on increasing, the organic conditions of life must become
more and more complex. Consequently there seems at first sight no
limit to the amount of profitable diversification of structure, and
therefore no limit to the number of species which might be produced.
We do not know that even the most prolific area is fully stocked
with specific forms: at the Cape of Good Hope and in Australia,
which support such an astonishing number of species, many European
plants have become naturalised. But geology shows us, that from an
early part of the tertiary period the number of species of shells, and
that from the middle part of this same period the number of mammals,
has not greatly or at all increased. What then checks an indefinite
increase in the number of species? The amount of life (I do not mean
the number of specific forms) supported on an area must have a
limit, depending so largely as it does on physical conditions;
therefore, if an area be inhabited by very many species, each or
nearly each species will be represented by few individuals; and such
species will be liable to extermination from accidental fluctuations
in the nature of the seasons or in the number of their enemies. The
process of extermination in such cases would be rapid, whereas the
production of new species must always be slow. Imagine the extreme
case of as many species as individuals in England, and the first
severe winter or very dry summer would exterminate thousands on
thousands of species. Rare species, and each species will become
rare if the number of species in any country becomes indefinitely
increased, will, on the principle often explained, present within a
given period few favourable variations; consequently, the process of
giving birth to new specific forms would thus be retarded. When any
species becomes very rare, close interbreeding will help to
exterminate it; authors have thought that this comes into play in
accounting for the deterioration of the aurochs in Lithuania, of red
deer in Scotland, and of bears in Norway, &e. Lastly, and this I am
inclined to think is the most important element, a dominant species,
which has already beaten many competitors in its own home, will tend
to spread and supplant many others. Alph. de Candolle has shown that
those species which spread widely, tend generally to spread very
widely; consequently, they will tend to supplant and exterminate
several species in several areas, and thus cheek the inordinate
increase of specific forms throughout the world. Dr. Hooker has
recently shown that in the S.E. corner of Australia, where,
apparently, there are many invaders from different quarters of the
globe, the endemic Australian species have been greatly reduced in
number. How much weight to attribute to these several considerations I
will not pretend to say; but conjointly they must limit in each
country the tendency to an indefinite augmentation of specific forms.
Summary of Chapter
If under changing conditions of life organic beings present
individual differences in almost every part of their structure, and
this cannot be disputed; if there be, owing to their geometrical
rate of increase, a severe struggle for life at some age, season, or
year, and this certainly cannot be disputed; then, considering the
infinite complexity of the relations of all organic beings to each
other and to their conditions of life, causing an infinite diversity
in structure, constitution, and habits, to be advantageous to them, it
would be a most extraordinary fact if no variations had ever
occurred useful to each being's own welfare, in the same manner as
so many variations have occurred useful to man. But if variations
useful to any organic being ever do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance, these
will tend to produce offspring similarly characterised. This principle
of preservation, or the survival of the fittest, I have called Natural
Selection. It leads to the improvement of each creature in relation to
its organic and inorganic conditions of life, and consequently, in
most cases, to what must be regarded as an advance in organisation.
Nevertheless, low and simple forms will long endure if well fitted for
their simple conditions of life.
Natural selection, on the principle of qualities being inherited
at corresponding ages, can modify the egg, seed, or young, as easily
as the adult. Amongst many animals, sexual selection will have given
its aid to ordinary selection, by assuring to the most vigorous and
best adapted males the greatest number of offspring. Sexual
selection will also give characters useful to the males alone, in
their struggles or rivalry with other males; and these characters will
be transmitted to one sex or to both sexes, according to the form of
inheritance which prevails.
Whether natural selection has really thus acted in adapting the
various forms of life to their several conditions and stations, must
be judged by the general tenor and balance of evidence given in the
following chapters. But we have already seen how it entails
extinction; and how largely extinction has acted in the world's
history, geology plainly declares. Natural selection also leads to
divergence of character; for the more organic beings diverge in
structure, habits, and constitution, by so much the more can a large
number be supported on the area,- of which we see proof by looking
to the inhabitants of any small spot, and to the productions
naturalised in foreign lands. Therefore, during the modification of
the descendants of any one species, and during the incessant
struggle of all species to increase in numbers, the more diversified
the descendants become, the better will be their chance of success
in the battle for life. Thus the small differences distinguishing
varieties of the same species, steadily tend to increase, till they
equal the greater differences between species of the same genus, or
even of distinct genera.
We have seen that it is the common, the widely-diffused and
widely-ranging species, belonging to the larger genera within each
class, which vary most; and these tend to transmit to their modified
offspring that superiority which now makes them dominant in their
own countries. Natural selection, as has just been remarked, leads
to divergence of character and to much extinction of the less improved
and intermediate forms of life. On these principles, the nature of the
affinities, and the generally well-defined distinctions between the
innumerable organic beings in each class throughout the world, may
be explained. It is a truly wonderful fact- the wonder of which we are
apt to overlook from familiarity- that all animals and all plants
throughout all time and space should be related to each other in
groups, subordinate to groups, in the manner which we everywhere
behold- namely, varieties of the same species most closely related,
species of the same genus less closely and unequally related,
forming sections and sub-genera, species of distinct genera much
less closely related, and genera related in different degrees, forming
sub-families, families, orders, sub-classes and classes. The several
subordinate groups in any class cannot be ranked in a single file, but
seem clustered round points, and these round other points, and so on
in almost endless cycles. If species had been independently created,
no explanation would have been possible of this kind of
classification; but it is explained through inheritance and the
complex action of natural selection, entailing extinction and
divergence of character, as we have seen illustrated in the diagram.
The affinities of all the beings of the same class have sometimes
been represented by a great tree. I believe this simile largely speaks
the truth. The green and budding twigs may represent existing species;
and those produced during former years may represent the long
succession of extinct species. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop
and kill the surrounding twigs and branches, in the same manner as
species and groups of species have at all times overmastered other
species in the great battle for life. The limbs, divided into great
branches, and these into lesser and lesser branches, were themselves
once, when the tree was young, budding twigs, and this connection of
the former and present buds by ramifying branches may well represent
the classification of all extinct and living species in groups
subordinate to groups. Of the many twigs which flourished when the
tree was a mere bush, only two or three, now grown into great
branches, yet survive and bear the other branches; so with the species
which lived during long-past geological periods very few have left
living and modified descendants. From the first growth of the tree,
many a limb and branch has decayed and dropped off; and these fallen
branches of various sizes may represent those whole orders,
families, and genera which have now no living representatives, and
which are known to us only in a fossil state. As we here and there see
a thin straggling branch springing from, a fork low down in a tree,
and which by some chance has been favoured and is still alive on its
summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise
by growth to fresh buds, and these, if vigorous, branch out and
overtop on all sides many a feebler branch, so by generation I believe
it has been with the great Tree of Life, which fills with its dead and
broken branches the crust of the earth, and covers the surface with
its everbranching and beautiful ramifications.
CHAPTER V
LAWS OF VARIATION
I HAVE hitherto sometimes spoken as if the variations- so common and
multiform with organic beings under domestication, and in a lesser
degree with those under nature- were due to chance. This, of course,
is a wholly incorrect expression, but it serves to acknowledge plainly
our ignorance of the cause of each particular variation. Some
authors believe it to be as much the function of the reproductive
system to produce individual differences, or slight deviations of
structure, as to make the child like its parents. But the fact of
variations and monstrosities occurring much more frequently under
domestication than under nature, and the greater variability of
species having wider ranges than of those with restricted ranges, lead
to the conclusion that variability is generally related to the
conditions of life to which each species has been exposed during
several successive generations. In the first chapter I attempted to
show that changed conditions act in two ways, directly on the whole
organisation or on certain parts alone, and indirectly through the
reproductive system. In all cases there are two factors, the nature of
the organism, which is much the most important of the two, and the
nature of the conditions. The direct action of changed conditions
leads to definite or indefinite results. In the latter case the
organisation seems to become plastic, and we have much fluctuating
variability. In the former case the nature of the organism is such
that it yields readily, when subjected to certain conditions, and all,
or nearly all the individuals become modified in the same way.
It is very difficult to decide how far changed conditions, such as
of climate, food, &c., have acted in a definite manner. There is
reason to believe that in the course of time the effects have been
greater than can be proved by clear evidence. But we may safely
conclude that the innumerable complex co-adaptations of structure,
which we see throughout nature between various organic beings,
cannot be attributed simply to such action. In the following cases the
conditions seem to have produced some slight definite effect: E.
Forbes asserts that shells at their southern limit, and when living in
shallow water, are more brightly coloured than those of the same
species from further north or from a greater depth; but this certainly
does not always hold good. Mr. Gould believes that birds of the same
species are more brightly coloured under a clear atmosphere, than when
living near the coast or on islands, and Wollaston is convinced that
residence near the sea affects the colours of insects. Moquin-Tandon
gives a list of plants which, when growing near the sea-shore, have
their leaves in some degree fleshy, though not elsewhere fleshy. These
slightly varying organisms are interesting in as far as they present
characters analogous to those possessed by the species which are
confined to similar conditions.
When a variation is of the slightest use to any being, we cannot
tell how much to attribute to the accumulative action of natural
selection, and how much to the definite action of the conditions of
life. Thus, it is well known to furriers that animals of the same
species have thicker and better fur the further north they live; but
who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during
many generations, and how much to the action of the severe climate?
for it would appear that climate has some direct action on the hair of
our domestic quadrupeds.
Instances could be given of similar varieties being produced from
the same species under external conditions of life as different as can
well be conceived; and, on the other hand, of dissimilar varieties
being produced under apparently the same external conditions. Again,
innumerable instances are known to every naturalist, of species
keeping true, or not varying at all, although living under the most
opposite climates. Such considerations as these incline me to lay less
weight on the direct action of the surrounding conditions, than on a
tendency to vary, due to causes of which we are quite ignorant.
In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include
natural selection, for the conditions determine whether this or that
variety shall survive. But when man is the selecting agent, we clearly
see that the two elements of change are distinct; variability is in
some manner excited, but it is the will of man which accumulates the
variations in certain directions; and it is this latter agency which
answers to the survival of the fittest under nature.
Effects of the increased Use and Disuse of Parts, as controlled by
Natural Selection
From the facts alluded to in the first chapter, I think there can be
no doubt that use in our domestic animals has strengthened and
enlarged certain parts, and disuse diminished them; and that such
modifications are inherited. Under free nature, we have no standard of
comparison, by which to judge of the effects of long-continued use
or disuse, for we know not the parent-forms; but many animals
possess structures which can be best explained by the effects of
disuse. As Professor Owen has remarked, there is no greater anomaly in
nature than a bird that cannot fly; yet there are several in this
state. The logger-headed duck of South America can only flap along the
surface of the water, and has its wings in nearly the same condition
as the domestic Aylesbury duck: it is a remarkable fact that the young
birds, according to Mr. Cunningham, can fly, while the adults have
lost this power. As the larger ground-feeding birds seldom take flight
except to escape danger, it is probable that the nearly wingless
condition of several birds, now inhabiting or which lately inhabited
several oceanic islands, tenanted by no beast of prey, has been caused
by disuse. The ostrich indeed inhabits continents, and is exposed to
danger from which it cannot escape by flight, but it can defend itself
by kicking its enemies, as efficiently as many quadrupeds. We may
believe that the progenitor of the ostrich genus had habits like those
of the bustard, and that, as the size and weight of its body were
increased during successive generations, its legs were used more,
and its wings less, until they became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are often
broken off; he examined seventeen specimens in his own collection, and
not one had even a relic left. In the Onites apelles the tarsi are
so habitually lost, that the insect has been described as not having
them. In some other genera they are present, but in a rudimentary
condition. In the Ateuchus, or sacred beetle of the Egyptians, they
are totally deficient. The evidence that accidental mutilations can be
inherited is at present not decisive; but the remarkable cases
observed by Brown-Sequard in guinea-pigs, of the inherited effects
of operations, should make us cautious in denying this tendency. Hence
it will perhaps be safest to look at the entire absence of the
anterior tarsi in Ateuchus, and their rudimentary condition in some
other genera, not as cases of inherited mutilations, but as due to the
effects of long-continued disuse; for as many dung-feeding beetles are
generally found with their tarsi lost, this must happen early in life;
therefore the tarsi cannot be of much importance or be much used by
these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out
of the 550 species (but more are now known) inhabiting Madeira, are so
far deficient in wings that they cannot fly; and that, of the
twenty-nine endemic genera, no less than twenty-three have all their
species in this condition! Several facts, namely, that beetles in many
parts of the world are frequently blown to sea and perish; that the
beetles in Madeira, as observed by Mr. Wollaston, lie much
concealed, until the wind lulls and the sun shines; that the
proportion of wingless beetles is larger on the exposed Desertas
than in Madeira itself; and especially the extraordinary fact, so
strongly insisted on by Mr. Wollaston, that certain large groups of
beetles, elsewhere excessively numerous, which absolutely require
the use of their wings, are here almost entirely absent;- these
several considerations make me believe that the wingless condition
of so many Madeira beetles is mainly due to the action of natural
selection, combined probably with disuse. For during many successive
generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which
most readily took to flight would oftenest have been blown to sea, and
thus destroyed.
The insects in Madeira which are not ground-feeders, and which, as
certain flower-feeding coleoptera and lepidoptera, must habitually use
their wings to gain their subsistence, have, as Mr. Wollaston
suspects, their wings not at all reduced, but even enlarged. This is
quite compatible with the action of natural selection. For when a
new insect first arrived on the island, the tendency of natural
selection to enlarge or to reduce the wings, would depend on whether a
greater number of individuals were saved by successfully battling with
the winds, or by giving up the attempt and rarely or never flying.
As with mariners shipwrecked near a coast, it would have been better
for the good swimmers if they had been able to swim still further,
whereas it would have been better for the bad swimmers if they had not
been able to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse,
but aided perhaps by natural selection. In South America, a
burrowing rodent, the tucotuco, or Ctenomys, is even more subterranean
in its habits than the mole; and I was assured by a Spaniard, who
had often caught them, that they were frequently blind. One which I
kept alive was certainly in this condition, the cause, as appeared
on dissection, having been inflammation of the nictitating membrane.
As frequent inflammation of the eyes must be injurious to any
animal, and as eyes are certainly not necessary to animals having
subterranean habits, a reduction in their size, with the adhesion of
the eyelids and growth of fur over them, might in such case be an
advantage; and if so, natural selection would aid the effects of
disuse.
It is well known that several animals, belonging to the most
different classes, which inhabit the caves of Carniola and of
Kentucky, are blind. in some of the crabs the foot-stalk for the eye
remains, though the eye is gone;- the stand for the telescope is
there, though the telescope with its glasses has been lost. As it is
difficult to imagine that eyes, though useless, could be in any way
injurious to animals living in darkness, their loss may be
attributed to disuse. In one of the blind animals, namely, the
cave-rat (Noetoma), two of which were captured by Professor Silliman
at above half a mile distance from the mouth of the cave, and
therefore not in the profoundest depths, the eyes were lustrous and of
large size; and these animals, as I am informed by Professor Silliman,
after having been exposed for about a month to a graduated light,
acquired a dim perception of objects.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that, in
accordance with the old view of the blind animals having been
separately created for the American and European caverns, very close
similarity in their organisation and affinities might have been
expected. This is certainly not the case if we look at the two whole
faunas; and with respect to the insects alone, Schiodte has
remarked, "We are accordingly prevented from considering the entire
phenomenon in any other light than something purely local, and the
similarity which is exhibited in a few forms between the Mammoth
cave (in Kentucky) and the caves in Carniola, otherwise than as a very
plain expression of that analogy which subsists generally between
the fauna of Europe and of North America." On my view we must
suppose that American animals, having in most cases ordinary powers of
vision, slowly migrated by successive generations from the outer world
into the deeper and deeper recesses of the Kentucky caves, as did
European animals into the caves of Europe. We have some evidence of
this gradation of habit; for, as Schiodte remarks, "We accordingly
look upon the subterranean faunas as small ramifications which have
penetrated into the earth from the geographically limited faunas of
the adjacent tracts, and which, as they extended themselves into
darkness, have been accommodated to surrounding circumstances. Animals
not far remote from ordinary forms, prepare the transition from
light to darkness. Next follow those that are constructed for
twilight; and, last of all, those destined for total darkness, and
whose formation is quite peculiar." These remarks of Schiodte's it
should be understood, apply not to the same, but to distinct
species. By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have
more or less perfectly obliterated its eyes, and natural selection
will often have effected other changes, such as an increase in the
length of the antennae or palpi, as a compensation for blindness.
Notwithstanding such modifications, we might expect still to see in
the cave-animals of America, affinities to the other inhabitants of
that continent, and in those of Europe to the inhabitants of the
European continent. And this is the case with some of the American
cave-animals, as I hear from Professor Dana; and some, of the European
cave insects are very closely allied to those of the surrounding
country. It would be difficult to give any rational explanation of the
affinities of the blind cave-animals to the other inhabitants of the
two continents on the ordinary view of their independent creation.
That several of the inhabitants of the caves of the Old and New Worlds
should be closely related, we might expect from the well-known
relationship of most of their other productions. As a blind species of
Bathyscia is found in abundance on shady rocks far from caves, the
loss of vision in the cave-species of this one genus has probably
had no relation to its dark habitation; for it is natural that an
insect already deprived of vision should readily become adapted to
dark caverns. Another blind genus (Anophthaimus) offers this
remarkable peculiarity, that the species, as Mr. Murray observes, have
not as yet been found anywhere except in caves; yet those which
inhabit the several eaves of Europe and America are distinct; but it
is possible that the progenitors of these several species, whilst they
were furnished with eyes, may formerly have ranged over both
continents, and then have become extinct, excepting in their present
secluded abodes. Far from feeling surprise that some of the
cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with
blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the scanty inhabitants
of these dark abodes will have been exposed.
Acclimatisation
Habit is hereditary with plants, as in the period of flowering, in
the time of sleep, in the amount of rain requisite for seeds to
germinate, &c., and this leads me to say a few words on
acclimatisation. As it is extremely common for distinct species
belonging to the same genus to inhabit hot and cold countries, if it
be true that all the species of the same genus are descended from a
single parent-form, acclimatisation must be readily effected during
a long course of descent. It is notorious that each species is adapted
to the climate of its own home: species from an arctic or even from
a temperate region cannot endure a tropical climate, or conversely. So
again, many succulent plants cannot endure a damp climate. But the
degree of adaptation of species to the climates under which they
live is often overrated. We may infer this from our frequent inability
to predict whether or not an imported plant will endure our climate,
and from the number of plants and animals brought from different
countries which are here perfectly healthy. We have reason to
believe that species in a state of nature are closely limited in their
ranges by the competition of other organic beings quite as much as, or
more than, by adaptation to particular climates. But whether or not
this adaptation is in most cases very close, we have evidence with
some few plants, of their becoming, to a certain extent, naturally
habituated to different temperatures; that is, they become
acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from the same species growing at different
heights on the Himalaya, were found to possess in this country
different constitutional powers of resisting cold. Mr. Thwaites
informs me that he has observed similar facts in Ceylon; analogous
observations have been made by Mr. H. C. Watson on European species of
plants brought from the Azores to England; and I could give other
cases. In regard to animals, several authentic instances could be
adduced of species having largely extended, within historical times,
their range from warmer to cooler latitudes, and conversely; but we do
not positively know that these animals were strictly adapted to
their native climate, though in all ordinary cases we assume such to
be the case; nor do we know that they have subsequently become
specially acclimatised to their new homes, so as to be better fitted
for them than they were at first.
As we may infer that our domestic animals were originally chosen
by uncivilised man because they were useful and because they bred
readily under confinement, and not because they were subsequently
found capable of far-extended transportation, the common and
extraordinary capacity in our domestic animals of not only
withstanding the most different climates, but of being perfectly
fertile (a far severer test) under them, may be used as an argument
that a large proportion of other animals now in a state of nature
could easily be brought to bear widely different climates. We must
not, however, push the foregoing argument too far, on account of the
probable origin of some of our domestic animals from several wild
stocks; the blood, for instance, of a tropical and arctic wolf may
perhaps be mingled in our domestic breeds. The rat and mouse cannot be
considered as domestic animals, but they have been transported by
man to many parts of the world, and now have a far wider range than
any other rodent; for they live under the cold climate of Faroe in the
north and of the Falklands in the south, and on many an island in
the torrid zones. Hence adaptation to any special climate may be
looked at as a quality readily grafted on an innate wide flexibility
of constitution, common to most animals. On this view, the capacity of
enduring the most different climates by man himself and by his
domestic animals, and the fact of the extinct elephant and
rhinoceros having formerly endured a glacial climate, whereas the
living species are now all tropical or sub-tropical in their habits,
ought not to be looked at as anomalies, but as examples of a very
common flexibility of constitution, brought, under peculiar
circumstances, into action.
How much of the acclimatisation of species to any peculiar climate
is due to mere habit, and how much to the natural selection of
varieties having different innate constitutions, and how much to
both means combined, is an obscure question. That habit or custom
has some influence, I must believe, both from analogy and from the
incessant advice given in agricultural works, even in the ancient
encyclopaedias of China, to be very cautious in transporting animals
from one district to another. And as it is not likely that man
should have succeeded in selecting so many breeds and sub-breeds
with constitutions specially fitted for their own districts, the
result must, I think, be due to habit. On the other hand, natural
selection would inevitably tend to preserve those individuals which
were born with constitutions best adapted to any country which they
inhabited. In treatises on many kinds of cultivated plants, certain
varieties are said to withstand certain climates better than others;
this is strikingly shown in works on fruit-trees published in the
United States, in which certain varieties are habitually recommended
for the northern and others for the southern States; and as most of
these varieties are of recent origin, they cannot owe their
constitutional differences to habit. The case of the Jerusalem
artichoke, which is never propagated in England by seed, and of
which consequently new varieties have not been produced, has even been
advanced, as proving that acclimatisation cannot be effected, for it
is now as tender as ever it was! The case, also, of the kidney-bean
has been often cited for a similar purpose, and with much greater
weight; but until someone will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by
frost, and then collect seed from the few survivors, with care to
prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been tried. Nor let it be supposed that differences in the
constitution of seedling kidney-beans never appear, for an account has
been published how much more hardy some seedlings are than others; and
of this fact I have myself observed striking instances.
On the whole, we may conclude that habit, or use and disuse, have,
in some cases, played a considerable part in the modification of the
constitution and structure; but that the effects have often been
largely combined with, and sometimes overmastered by, the natural
selection of innate variations.
Correlated Variation
I mean by this expression that the whole organisation is so tied
together during its growth and development, that when slight
variations in any one part occur, and are accumulated through
natural selection, other parts become modified. This is a very
important subject, most imperfectly understood, and no doubt wholly
different classes of facts may be here easily confounded together.
We shall presently see that simple inheritance often gives the false
appearance of correlation. One of the most obvious real cases is, that
variations of structure arising in the young or larvae naturally
tend to affect the structure of the mature animal. The several parts
of the body which are homologous, and which, at an early embryonic
period, are identical in structure, and which are necessarily
exposed to similar conditions, seem eminently liable to vary in a like
manner: we see this in the right and left sides of the body varying in
the same manner; in the front and hind legs, and even in the jaws
and limbs, varying together, for the lower jaw is believed by some
anatomists to be homologous with the limbs. These tendencies, I do not
doubt, may be mastered more or less completely by natural selection;
thus a family of stags once existed with an antler only on one side;
and if this had been of any great use to the breed, it might
probably have been rendered permanent by selection.
Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants: and nothing is more
common than the union of homologous parts in normal structures, as
in the union of the petals into a tube. Hard parts seem to affect
the form of adjoining soft parts; it is believed by some authors
that with birds the diversity in the shape of the pelvis causes the
remarkable diversity in the shape of their kidneys. Others believe
that the shape of the pelvis in the human mother influences by
pressure the shape of the head of the child. In snakes, according to
Schlegel, the form of the body and the manner of swallowing
determine the position and form of several of the most important
viscera.
The nature of the bond is frequently quite obscure. Isidore Geoffroy
St-Hilaire has forcibly remarked that certain malconformations
frequently, and that others rarely, co-exist, without our being able
assign any reason. What can be more singular than the relation in cats
between complete whiteness and blue eyes with deafness, or between the
tortoise-shell colour and the female sex; or in pigeons between
their feathered feet and skin betwixt the outer toes, or between the
presence of more or less down on the young pigeon when first
hatched, with the future colour of its plumage; or, again, the
relation between the hair and teeth in the naked Turkish dog, though
here no doubt homology comes into play? With respect to this latter
case of correlation, I think it can hardly be accidental, that the two
orders of mammals which are most abnormal in their dermal covering,
viz., Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters,
&c.,) are likewise on the whole the most abnormal in their teeth;
but there are so many exceptions to this rule, as Mr. Mivart has
remarked, that it has little value.
I know of no case better adapted to show the importance of the
laws of correlation and variation, independently of utility and
therefore of natural selection, than that of the difference between
the outer and inner flowers in some compositous and timbelliferous
plants. Every one is familiar with the difference between the ray
and central florets of, for instance, the daisy, and this difference
is often accompanied with the partial or complete abortion of the
reproductive organs. But in some of these plants, the seeds also
differ in shape and sculpture. These differences have sometimes been
attributed to the pressure of the involuera on the florets, or to
their mutual pressure, and the shape of the seeds in the ray-florets
of some Compositae countenances this idea; but with the
Umbelliferae, it is by no means, as Dr. Hooker informs me, the species
with the densest heads which most frequently differ in their inner and
outer flowers. It might have been thought that the development of
the ray-petals by drawing nourishment from the reproductive organs
causes their abortion; but this can hardly be the sole cause, for in
some Compositae the seeds of the outer and inner florets differ,
without any difference in the corolla. Possibly these several
differences may be connected with the different flow of nutriment
towards the central and external flowers: we know, at least, that with
irregular flowers, those nearest to the axis are most subject to
peloria, that is to become abnormally symmetrical. I may add, as an
instance of this fact, and as a striking case of correlation, that
in many pelargoniums, the two upper petals in the central flower of
the truss often lose their patches of darker colour; and when this
occurs, the adherent nectary is quite aborted; the central flower thus
becoming peloric or regular. When the colour is absent from only one
of the two upper petals, the nectary is not quite aborted but is
much shortened.
With respect to the development of the corolla, Sprengel's idea that
the ray-florets serve to attract insects, whose agency is highly
advantageous or necessary for the fertilisation of these plants, is
highly probable; and if so, natural selection may have come into play.
But with respect to the seeds, it seems impossible that their
differences in shape, which are not always correlated with any
difference in the corolla, can be in any way beneficial: yet in the
Umbelliferae these differences are of such apparent importance- the
seeds being sometimes orthospermous in the exterior flowers and
coelospermous in the central flowers,- that the elder De Candolle
founded his main divisions in the order on such characters. Hence
modifications of structure, viewed by systematists as of high value,
may be wholly due to the laws of variation and correlation, without
being, as far as we can judge, of the slightest service to the
species.
We may often falsely attribute to correlated variation structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and,
after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted
to a whole group of descendants with diverse habits, would naturally
be thought to be in some necessary manner correlated. Some other
correlations are apparently due to the manner in which natural
selection can alone act. For instance, Alph. de Candolle has
remarked that winged seeds are never found in fruits which do not
open; I should explain this rule by the impossibility of seeds
gradually becoming winged through natural selection, unless the
capsules were open; for in this case alone could the seeds, which were
a little better adapted to be wafted by the wind, gain an advantage
over others less well fitted for wide dispersal.
Compensation and Economy of Growth
The elder Geoffroy and Goethe propounded, at about the same time,
their law of compensation or balancement of growth; or, as Goethe
expressed it, "In order to spend on one side, nature is forced to
economise on the other side." I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part
or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to
fatten readily. The same varieties of the cabbage do not yield
abundant and nutritious foliage and a copious supply of oil-bearing
seeds. When the seeds in our fruits become atrophied, the fruit itself
gains largely in size and quality. In our poultry, a large tuft of
feathers on the head is generally accompanied by a diminished comb,
and a large beard by diminished wattles. With species in a state of
nature it can hardly be maintained that the law is of universal
application; but many good observers, more especially botanists,
believe in its truth. I will not, however, here give any instances,
for I see hardly any way of distinguishing between the effects, on the
one hand, of a part being largely developed through natural
selection and another and adjoining part being reduced by this same
process or by disuse, and, on the other hand the actual withdrawal
of nutriment from one part owing to the excess of growth in another
and adjoining part.
I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under a
more general principle, namely, that natural selection is
continually trying to economise every part of the organization. If
under changed conditions of life a structure, before useful, becomes
less useful, its diminution will be favoured, for it will profit the
individual not to have its nutriment wasted in building up an
useless structure. I can only thus understand a fact with which I
was much struck when examining cirripedes, and of which many analogous
instances could be given: namely, that when a cirripede is parasitic
within another cirripede and is thus protected, it loses more or
less completely its own shell or carapace. This is the case with the
male Ibla, and in a truly extraordinary manner with the Proteolepas:
for the carapace in all other cirripedes consists of the three
highly-important anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but in the parasitic
and protected Proteolepas, the whole anterior part of the head is
reduced to the merest rudiment attached to the bases of the prehensile
antennae. Now the saving of a large and complex structure, when
rendered superfluous, would be a decided advantage to each
successive individual of the species; for in the struggle for life
to which every animal is exposed, each would have a better chance of
supporting itself, by less nutriment being wasted.
Thus, as I believe, natural selection will tend in the long run to
reduce any part of the organisation, as soon as it becomes, through
changed habits, superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in
largely developing an organ without requiring as a necessary
compensation the reduction of some adjoining part.
Multiple, Rudimentary, and Lowly-organised Structures are Variable
It seems to be a rule, as remarked by the younger Geoffroy, both
with varieties and species, that when any part or organ is repeated
many times in the same individual (as the vertebrae in snakes, and the
stamens in polyandrous flowers) the number is variable; whereas the
same part or organ, when it occurs in lesser numbers, is constant. The
same author as well as some botanists have further remarked that
multiple parts are extremely liable to vary in structure. As
"vegetable repetition," to use Prof. Owen's expression, is a sign of
low organisation, the foregoing statements accord with the common
opinion of naturalists, that beings which stand low in the scale of
nature are more variable than those which are higher. I presume that
lowness here means that the several parts of the organisation have
been but little specialised for particular functions; and as long as
the same part has to perform diversified work, we can perhaps see
why it should remain variable, that is, why natural selection should
not have preserved or rejected each little deviation of form as
carefully as when the part has to serve for some one special
purpose. In the same way, a knife which has to cut all sorts of things
may be of almost any shape; whilst a tool for some
particular-purpose must be of some particular shape. Natural
selection, it should never be forgotten, can act solely through and
for the advantage of each being.
Rudimentary parts, as it is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only
add that their variability seems to result from their uselessness, and
consequently from natural selection having had no power to check
deviations in their structure.
A Part developed in any Species in an extraordinary degree or
manner, in comparison with the same Part in allied Species, tends to
be highly variable
Several years ago I was much struck by a remark, to the above
effect, made by Mr. Waterhouse. Professor Owen, also, seems to have
come to a nearly similar conclusion. It is hopeless to attempt to
convince any one of the truth of the above proposition without
giving the long array of facts which I have collected, and which
cannot possibly be here introduced. I can only state my conviction
that it is a rule of high generality. I am aware of several causes
of error, but I hope that I have made due allowance for them. It
should be understood that the rule by no means applies to any part,
however unusually developed, unless it be unusually developed in one
species or in a few species in comparison with the same part in many
closely allied species. Thus, the wing of a bat is a most abnormal
structure in the class of mammals, but the rule would not apply
here, because the whole group of bats possesses wings; it would
apply only if some one species had wings developed in a remarkable
manner in comparison with the other species of the same genus. The
rule applies very strongly in the case of secondary sexual characters,
when displayed in any unusual manner. The term, secondary sexual
characters, used by Hunter, relates to characters which are attached
to one sex, but are not directly connected with the act of
reproduction. The rule applies to males and females; but more rarely
to the females, as they seldom offer remarkable secondary sexual
characters. The rule being so plainly applicable in the case of
secondary sexual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual manner- of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case
of hermaphrodite cirripedes; I particularly attended to Mr.
Waterhouse's remark, whilst investigating this Order, and I am fully
convinced that the rule almost always holds good. I shall, in a future
work, give a list of all the more remarkable cases; I will here give
only one, as it illustrates the rule in its largest application. The
opereular valves of sessile cirripedes (rock barnacles) are, in
every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species
of one genus, Pyrgoma, these valves present a marvelous amount of
diversification; the homologous valves in the different species
being sometimes wholly unlike in shape; and the amount of variation in
the individuals of the same species is so great, that it is no
exaggeration to state that the varieties of the same species differ
more from each other in the characters derived from these important
organs, than do the species belonging to other distinct genera.
As with birds the individuals of the same species, inhabiting the
same country, vary extremely little, I have particularly attended to
them; and the rule certainly seems to hold good in this class. I
cannot make out that it applies to plants, and this would have
seriously shaken my belief in its truth, had not the great variability
in plants made it particularly difficult to compare their relative
degrees of variability.
When we see any part or organ developed in a remarkable degree or
manner in a species, the fair presumption is that it is of high
importance to that species: nevertheless it is in this case
eminently liable to variation. Why should this be so? On the view that
each species has been independently created, with all its parts as
we now see them, I can see no explanation. But on the view that groups
of species are descended from some other species, and have been
modified through natural selection, I think we can obtain some
light. First let me make some preliminary remarks. If, in our domestic
animals, any part or the whole animal be neglected, and no selection
be applied, that part (for instance, the comb in the Dorking fowl)
or the whole breed will cease to have a uniform character: and the
breed may be said to be degenerating. In rudimentary organs, and in
those which have been but little specialised for any particular
purpose, and perhaps in polymorphic groups, we see a nearly parallel
case; for in such cases natural selection either has not or cannot
have come into full play, and thus the organisation is left in a
fluctuating condition. But what here more particularly concerns us is,
that those points in our domestic animals, which at the present time
are undergoing rapid change by continued selection, are also eminently
liable to variation. Look at the individuals of the same breed of
the pigeon, and see what a prodigious amount of difference there is in
the beaks of tumblers, in the beaks and wattle of carriers, in the
carriage and tail of fantails, &c., these being the points now
mainly attended to by English fanciers. Even in the same sub-breed, as
in that of the short-faced tumbler, it is notoriously difficult to
breed nearly perfect birds, many departing widely from the standard.
There may truly be said to be a constant struggle going on between, on
the one hand, the tendency to reversion to a less perfect state, as
well as an innate tendency to new variations, and, on the other
hand, the power of steady selection to keep the breed true. In the
long run selection gains the day, and we do not expect to fail so
completely as to breed a bird as coarse as a common tumbler pigeon
from a good short-faced strain. But as long as selection is rapidly
going on, much variability in the parts undergoing modification may
always be expected.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has
undergone an extraordinary amount of modification since the period
when the several species branched off from the common progenitor of
the genus. This period will seldom be remote in any extreme degree, as
species rarely endure for more than one geological period. An
extraordinary amount of modification implies an unusually large and
long-continued amount of variability, which has continually been
accumulated by natural selection for the benefit of the species. But
as the variability of the extraordinarily developed part or organ
has been so great and long-continued within a period not excessively
remote, we might, as a general rule, still expect to find more
variability in such parts than in other parts of the organisation
which have remained for a much longer period nearly constant. And
this, I am convinced, is the case. That the struggle between natural
selection on the one hand, and the tendency to reversion and
variability on the other hand, will in the course of time cease; and
that the most abnormally developed organs may be made constant, I
see no reason to doubt. Hence, when an organ, however abnormal it
may be, has been transmitted in approximately the same condition to
many modified descendants, as in the case of the wing of the bat, it
must have existed, according to our theory, for an immense period in
nearly the same state; and thus it has come not to be more variable
than any other structure. It is only in those cases in which the
modification has been comparatively recent and extraordinarily great
that we ought to find the generative variability, as it may be called,
still present in a high degree. For in this case the variability
will seldom as yet have been fixed by the continued selection of the
individuals varying in the required manner and degree, and by the
continued rejection of those tending to revert to a former and less
modified condition.
Specific Characters more Variable than Generic Characters
The principle discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant:
if in a large genus of plants some species had blue flowers and some
had red, the colour would be only a specific character, and no one
would be surprised at one of the blue species varying into red, or
conversely; but if all the species had blue flowers, the colour
would become a generic character, and its variation would be a more
unusual circumstance. I have chosen this example because the
explanation which most naturalists would advance is not here
applicable, namely, that specific characters are more variable than
generic, because they are taken from parts of less physiological
importance than those commonly used for classing genera. I believe
this explanation is partly, yet only indirectly, true; I shall,
however, have to return to this point in the chapter on
Classification. It would be almost superfluous to adduce evidence in
support of the statement, that ordinary specific characters are more
variable than generic; but with respect to important characters I have
repeatedly noticed in works on natural history, that when an author
remarks with surprise that some important organ or part, which is
generally very constant throughout a large group of species, differs
considerably in closely-allied species, it is often variable in the
individuals of the same species. And this fact shows that a character,
which is generally of generic value, when it sinks in value and
becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Isidore Geoffroy St-Hilaire
apparently entertains no doubt that the more an organ normally differs
in the different species of the same group, the more subject it is
to anomalies in the individuals.
On the ordinary view of each species having been independently
created, why should that part of the structure, which differs from the
same part in other independently-created species of the same genus, be
more variable than those parts which are closely alike in the
several species? I do not see that any explanation can be given. But
on the view that species are only strongly marked and fixed varieties,
we might expect often to find them still continuing to vary in those
parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the
case in another manner:- the points in which all the species of a
genus resemble each other, and in which they differ from allied
genera, are called generic characters; and these characters may be
attributed to inheritance from a common progenitor, for it can
rarely have happened that natural selection will have modified several
distinct species, fitted to more or less widely-different habits, in
exactly the same manner: and as these so-called generic characters
have been inherited from before the period when the several species
first branched off from their common progenitor, and subsequently have
not varied or come to differ in any degree, or only in a slight
degree, it is not probable that they should vary at the present day.
On the other hand, the points in which species differ from other
species of the same genus are called specific characters; and as these
specific characters have varied and come to differ since the period
when the species branched off from a common progenitor, it is probable
that they should still often be in some degree variable,- at least
more variable than those parts of the organisation which have for a
very long period remained constant.
Secondary Sexual Characters Variable.- I think it will be admitted
by naturalists, without my entering on details, that secondary
sexual characters are highly variable. It will also be admitted that
species of the same group differ from each other more widely in
their secondary sexual characters, than in other parts of their
organisation: compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between the females. The cause of the original variability of these
characters is not manifest; but we can see why they should not have
been rendered as constant and uniform as others, for they are
accumulated by sexual selection, which is less rigid in its action
than ordinary selection, as it does not entail death, but only gives
fewer off-spring to the less favoured males. Whatever the cause may be
of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for
action, and may thus have succeeded in giving to the species of the
same group a greater amount of difference in these than in other
respects.
It is a remarkable fact, that the secondary differences between
the two sexes of the same species are generally displayed in the
very same parts of the organisation in which the species of the same
genus differ from each other. Of this fact I will give in illustration
the two first instances which happen to stand on my list; and as the
differences in these cases are of a very unusual nature, the
relation can hardly be accidental. The same number of joints in the
tarsi is a character common to very large groups of beetles, but in
the Engidoe, as Westwood has remarked, the number varies greatly;
and the number likewise differs in the two sexes of the same
species. Again in the fossorial hymenoptera, the neuration of the
wings is a character of the highest importance, because common to
large groups; but in certain genera the neuration differs in the
different species, and likewise in the two sexes of the same
species. Sir J. Lubbock has recently remarked, that several minute
crustaceans offer excellent illustrations of this law. "In Pontella,
for instance, the sexual characters are afforded mainly by the
anterior antennae and by the fifth pair of legs: the specific
differences also are principally given by these organs." This relation
has a clear meaning on my view: I look at all the species of the
same genus as having as certainly descended from a common
progenitor, as have the two sexes of any one species. Consequently,
whatever part of the structure of the common progenitor, or of its
early descendants, became variable, variations of this part would,
it is highly probable, be taken advantage of by natural and sexual
selection, in order to fit the several species to their several places
in the economy of nature, and likewise to fit the two sexes of the
same species to each other, or to fit the males to struggle with other
males for the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the
species;- that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with
the same part in its congeners; and the slight degree of variability
in a part, however extraordinarily it may be developed, if it be
common to a whole group of species;- that the great variability of
secondary sexual characters, and their great difference in closely
allied species;- that secondary sexual and ordinary specific
differences are generally displayed in the same parts of the
organisation,- are all principles closely connected together. All
being mainly due to the species of the same group being the
descendants of common progenitor, from whom they have inherited much
in common,- to parts which have recently and largely varied being more
likely still to go on varying than parts which have long been
inherited and have not varied,- to natural selection having more or
less completely, according to the lapse of time, overmastered the
tendency to reversion and to further variability,- to sexual selection
being less rigid than ordinary selection,- and to variations in the
same parts having been accumulated by natural and sexual selection,
and having been thus adapted for secondary sexual, and for ordinary
purposes.
Distinct Species present analagous Variations, so that a Variety
of one Species often assumes a Character proper to an Allied
Species, or reverts to some of the Characters of an early
Progenitor.- These propositions will be most readily understood by
looking to our domestic races. The most distinct breeds of the pigeon,
in countries widely apart, present sub-varieties with reversed
feathers on the head, and with feathers on the feet,- characters not
possessed by the aboriginal rock-pigeon; these then are analogous
variations in two or more distinct races. The frequent presence of
fourteen or even sixteen tail-feathers in the pouter may be considered
as a variation representing the normal structure of another race,
the fan-tail. I presume that no one will doubt that all such analogous
variations are due to the several races of the pigeon having inherited
from a common parent the same constitution and tendency to
variation, when acted on by similar unknown influences. In the
vegetable kingdom we have a case of analogous variation, in the
enlarged stems, or as commonly called roots, of the Swedish turnip and
Rutabaga, plants which several botanists rank as varieties produced by
cultivation from a common parent: if this be not so, the case will
then be one of analogous variation in two so-called distinct
species; and to these a third may be added, namely, the common turnip.
According to the ordinary view of each species having been
independently created, we should have to attribute this similarity
in the enlarged stems of these three plants, not to the vera causa
of community of descent, and a consequent tendency to vary in a like
manner, but to three separate yet closely related acts of creation.
Many similar cases of analogous variation have been observed by Naudin
in the great gourd-family, and by various authors in our cereals.
Similar cases occurring with insects under natural conditions have
lately been discussed with much ability by Mr. Walsh, who has
grouped them under his law of Equable Variability.
With pigeons, however, we have another case, namely, the
occasional appearance in all the breeds, of slaty-blue birds with
two black bars on the wings, white loins, a bar at the end of the
tail, with the outer feathers externally edged near their basis with
white. As all these marks are characteristic of the parent
rock-pigeon, I presume that no one will doubt that this is a case of
reversion, and not of a new yet analogous variation appearing in the
several breeds. We may, I think, confidently come to this
conclusion, because, as we have seen, these coloured marks are
eminently liable to appear in the crossed offspring of two distinct
and differently coloured breeds; and in this case there is nothing
in the external conditions of life to cause the reappearance of the
slaty-blue, with the several marks, beyond the influence of the mere
act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should
reappear after having been lost for many, probably for hundreds of
generations. But when a breed has been crossed only once by some other
breed, the offspring occasionally show for many generations a tendency
to revert in character to the foreign breed- some say, for a dozen
or even a score of generations. After twelve generations, the
proportion of blood, to use a common expression, from one ancestor, is
only 1 in 2048; and yet, as we see, it is generally believed that a
tendency to reversion is retained by this remnant of foreign blood. In
a breed which has not been crossed, but in which both parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might, as
was formerly remarked, for all that we can see to the contrary, be
transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that one
individual suddenly takes after an ancestor removed by some hundred
generations, but that in each successive generation the character in
question has been lying latent, and at last, under unknown
favourable conditions, is developed. With the barb-pigeon, for
instance, which very rarely produces a blue bird, it is probable
that there is a latent tendency in each generation to produce blue
plumage. The abstract improbability of such a tendency being
transmitted through a vast number of generations, is not greater
than that of quite useless or rudimentary organs being similarly
transmitted. A mere tendency to produce a rudiment is indeed sometimes
thus inherited.
As all the species of the same genus are supposed to be descended
from a common progenitor, it might be expected that they would
occasionally vary in an analogous manner; so that the varieties of two
or more species would resemble each other, or that a variety of one
species would resemble in certain characters another and distinct
species,- this other species being, according to our view, only a well
marked and permanent variety. But characters exclusively due to
analogous variation would probably be of an unimportant nature, for
the preservation of all functionally important characters will have
been determined through natural selection, in accordance with the
different habits of the species. It might further be expected that the
species of the same genus would occasionally exhibit reversions to
long lost characters. As, however, we do not know the common ancestors
of any natural group, we cannot distinguish between reversionary and
analogous characters. If, for instance, we did not know that the
parent rock-pigeon was not feather-footed or turn-crowned, we could
not have told, whether such characters in our domestic breeds were
reversions or only analogous variations; but we might have inferred
that the blue colour was a case of reversion from the number of the
markings, which are correlated with this tint, and which would not
probably have all appeared together from simple variation. More
especially we might have inferred this, from the blue colour and the
several marks so often appearing when differently coloured breeds
are crossed. Hence, although under nature it must generally be left
doubtful, what cases are reversions to formerly existing characters,
and what are new but analogous variations, yet we ought, on our
theory, sometimes to find the varying offspring of a species
assuming characters which are already present in other members of
the same group. And this undoubtedly is the case.
The difficulty in distinguishing variable species is largely due
to the varieties mocking, as it were, other species of the same genus.
A considerable catalogue, also, could be given of forms intermediate
between two other forms, which themselves can only doubtfully be
ranked as species; and this shows, unless all these closely allied
forms be considered as independently created species, that they have
in varying assumed some of the characters of the others. But the
best evidence of analogous variations is afforded by parts or organs
which are generally constant in character, but which occasionally vary
so as to resemble, in some degree, the same part or organ in an allied
species. I have collected a long list of such cases; but here, as
before, I lie under the great disadvantage of not being able to give
them. I can only repeat that such cases certainly occur, and seem to
me very remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case almost certainly of reversion. The ass
sometimes has very distinct transverse bars on its legs, like those on
the legs of the zebra: it has been asserted that these are plainest in
the foal, and, from inquiries which I have made, I believe this to
be true. The stripe on the shoulder is sometimes double, and is very
variable in length and outline. A white ass, but not an albino, has
been described without either spinal or shoulder stripe: and these
stripes are sometimes very obscure, or actually quite lost, in
dark-coloured asses. The koulan of Pallas is said to have been seen
with a double shoulder-stripe. Mr. Blyth has seen a specimen of the
hemionus with a distinct shoulder-stripe, though it properly has none;
and I have been informed by Colonel Poole that the foals of this
species are generally striped on the legs, and faintly on the
shoulder. The quagga, though so plainly barred like a zebra over the
body, is without bars on the legs; but Dr. Gray has figured one
specimen with very distinct zebra-like bars on the hocks.
With respect to the horse, I have collected cases in England of
the spinal stripe in horses of the most distinct breeds, and of all
colours: transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a bay horse.
My son made a careful examination and sketch for me of a dun Belgian
cart-horse with a double stripe on each shoulder and with leg-stripes;
I have myself seen a dun Devonshire pony, and a small dun Welsh pony
has been carefully described to me, both with three parallel stripes
on each shoulder.
In the north-west part of India the kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
this breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are often plainest in the foal; and
sometimes quite disappear in old horses. Colonel Poole has seen both
gray and bay kattywar horses striped when first foaled. I have also
reason to suspect, from information given me by Mr. W. W. Edwards,
that with the English race-horse the spinal stripe is much commoner in
the foal than in the fullgrown animal. I have myself recently bred a
foal from a bay mare (offspring of a Turkoman horse and a Flemish
mare) by a bay English race-horse; this foal when a week old was
marked on its hinder quarters and on its forehead with numerous,
very narrow, dark, zebra-like bars, and its legs were feebly
striped: all the stripes soon disappeared completely. Without here
entering on further details, I may state that I have collected cases
of leg and shoulder stripes in horses of very different breeds in
various countries from Britain to eastern China; and from Norway in
the north to the Malay Archipelago in the south. In all parts of the
world these stripes occur far oftenest in duns and mouse-duns; by
the term dun a large range of colour is included, from one between
brown and black to a close approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse are descended
from several aboriginal species- one of which, the dun, was striped;
and that the above described appearances are an due to ancient crosses
with the dun stock. But this view may be safely rejected; for it is
highly improbable that the heavy Belgian cart-horse, Welsh ponies,
Norwegian cobs, the lanky kattywar race, &c., inhabiting the most
distant parts of the world, should all have been crossed with one
supposed aboriginal stock.
Now let us turn to the effects of crossing the several species of
the horse-genus. Rollin asserts, that the common mule from the ass and
horse is particularly apt to have bars on its legs; according to Mr.
Gosse, in certain parts of the United States about nine out of ten
mules have striped legs. I once saw a mule with its legs so much
striped that any one might have thought that it was a hybrid-zebra;
and Mr. W. C. Martin, in his excellent treatise on the horse, has
given a figure of a similar mule. In four coloured drawings, which I
have seen, of hybrids between the ass and zebra, the legs were much
more plainly barred than the rest of the body; and in one of them
there was a double shoulder-stripe. In Lord Morton's famous hybrid,
from a chestnut mare and male quagga, the hybrid, and even the pure
offspring subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is
even the pure quagga. Lastly, and this is another most remarkable
case, a hybrid has been figured by Dr. Gray (and he informs me that he
knows of a second case) from the ass and the hemionus; and this
hybrid, though the ass only occasionally has stripes on its legs and
the hemionus has none and has not even a shoulder-stripe, nevertheless
had all four legs barred, and had three short shoulder-stripes, like
those on the dun Devonshire and Welsh ponies, and even had some
zebra-like stripes on the sides of its face. With respect to this last
fact, I was so convinced that not even a stripe of colour appears from
what is commonly called chance, that I was led solely from the
occurrence of the face-stripes on this hybrid from the ass and
hemionus to ask Colonel Poole whether such face-stripes ever
occurred in the eminently striped kattywar breed of horses, and was,
as we have seen, answered in the affirmative.
What now are we to say to these several facts? We see several
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like
an ass. In the horse we see this tendency strong whenever a dun tint
appears- a tint which approaches to that of the general colouring of
the other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We
see this tendency to become striped most strongly displayed in hybrids
from between several of the most distinct species. Now observe the
case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races) of
bluish colour, with certain bars and other marks; and when any breed
assumes by simple variation a bluish tint, these bars and other
marks invariably reappear; but without any other change of form or
character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks
to reappear in the mongrels. I have stated that the most probable
hypothesis to account for the reappearance of very ancient characters,
is- that there is a tendency in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen that in
several species of the horse-genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds
of pigeons, some of which have bred true for centuries, species; and
how exactly parallel is the case with that of the species of the
horse-genus! For myself, I venture confidently to look back
thousands on thousands of generations, and I see an animal striped
like a zebra, but perhaps otherwise very differently constructed,
the common parent of our domestic horse (whether or not it be
descended from one or more wild stocks), of the ass, the hemionus,
quagga, and zebra.
He who believes that each equine species was independently
created, will, I presume, assert that each species has been created
with a tendency to vary, both under nature and under domestication, in
this particular manner, so as often to become striped like the other
species of the genus; and that each has been created with a strong
tendency, when crossed with species inhabiting distant quarters of the
world, to produce hybrids resembling in their stripes, not their own
parents, but other species of the genus. To admit this view is, as
it seems to me, to reject a real for an unreal, or at least for an
unknown, cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe, with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created
in stone so as to mock the shells living on the seashore.
Summary.- Our ignorance of the laws of variation is profound. Not in
one case out of a hundred can we pretend to assign any reason why this
or that part has varied. But whenever we have the means of instituting
a comparison, the same laws appear to have acted in producing the
lesser differences between varieties of the same species, and the
greater differences between species of the same genus. Changed
conditions generally induce mere fluctuating variability, but
sometimes they cause direct and definite effects; and these may become
strongly marked in the course of time, though we have not sufficient
evidence on this head. Habit in producing constitutional peculiarities
and use in strengthening and disuse in weakening and diminishing
organs, appear in many cases to have been potent in their effects.
Homologous parts tend to vary in the same manner, and homologous parts
tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining
parts; and every part of the structure which can be saved without
detriment will be saved. Changes of structure at an early age may
affect parts subsequently developed; and many cases of correlated
variation, the nature of which we are unable to understand,
undoubtedly occur. Multiple parts are variable in number and in
structure, perhaps arising from such parts not having been closely
specialised for any particular function, so that their modifications
have not been closely cheeked by natural selection. It follows
probably from this same cause, that organic beings low in the scale
are more variable than those standing higher in the scale, and which
have their whole organisation more specialised. Rudimentary organs,
from being useless, are not regulated by natural selection, and
hence are variable. Specific characters- that is, the characters which
have, come to differ since the several species of the same genus
branched off from a common parent- are more variable than generic
characters, or those which have long been inherited, and have not
differed from this same period. In these remarks we have referred to
special parts or organs being still variable, because they have
recently varied and thus come to differ; but we have also seen in
the second chapter that the same principle applies to the whole
individual; for in a district where many species of a genus are found-
that is, where there has been much former variation and
differentiation, or where the manufactory of new specific forms has
been actively at work- in that district and amongst these species,
we now find, on an average, most varieties. Secondary sexual
characters are highly variable, and such characters differ much in the
species of the same group. Variability in the same parts of the
organisation has generally been taken advantage of in giving secondary
sexual differences to the two sexes of the same species, and
specific differences to the several species of the same genus. Any
part or organ developed to an extraordinary size or in an
extraordinary manner, in comparison with the same part or organ in the
allied species, must have gone through an extraordinary amount of
modification since the genus arose; and thus we can understand why
it should often still be variable in a much higher degree than other
parts; for variation is a long-continued and slow process, and natural
selection will in such cases not as yet have had time to overcome
the tendency to further variability and to reversion to a less
modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants- which on our view must be a very slow process,
requiring long lapse of time- in this case, natural selection has
succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may have been developed. Species
inheriting nearly the same constitution from a common parent, and
exposed to similar influences, naturally tend to present analogous
variations, or these same species may occasionally revert to some of
the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous
variation, such modifications will add to the beautiful and harmonious
diversity of nature.
Whatever the cause may be of each slight difference between the
offspring and their parents- and a cause for each must exist- we
have reason to believe that it is the steady accumulation of
beneficial differences which has given rise to all the more
important modifications of structure in relation to the habits of each
species.
CHAPTER VI
DIFFICULTIES OF THE THEORY
LONG before the reader has arrived at this part of my work, a
crowd of difficulties will have occurred to him. Some of them are so
serious that to this day I can hardly reflect on them without being in
some degree staggered; but, to the best of my judgment, the greater
number are only apparent, and those that are real are not, I think,
fatal to the theory.
These difficulties and objections may be classed under the following
heads:- First, why, if species have descended from other species by
fine gradations, do we not everywhere see innumerable transitional
forms? Why is not all nature in confusion, instead of the species
being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the
modification of some other animal with widely different habits and
structure? Can we believe that natural selection could produce, on the
one hand, an organ of trifling importance, such as the tail of a
giraffe, which serves as a fly-flapper, and, on the other hand, an
organ so wonderful as the eye?
Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to the instinct which leads the bee to
make cells, and which has practically anticipated the discoveries of
profound mathematicians?
Fourthly, how can we account for species, when crossed, being
sterile and producing sterile offspring, whereas, when varieties are
crossed, their fertility is unimpaired?
The two first heads will here be discussed; some miscellaneous
objections in the following chapter; Instinct and Hybridism in the two
succeeding chapters.
On the Absence or Rarity of Transitional Varieties.- As natural
selection acts solely by the preservation of profitable modifications,
each new form will tend in a fully-stocked country to take the place
of, and finally to exterminate, its own less improved parent-form
and other less favoured forms with which it comes into competition.
Thus extinction and natural selection go hand in hand. Hence, if we
look at each species as descended from some unknown form, both the
parent and all the transitional varieties will generally have been
exterminated by the very process of the formation and perfection of
the new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in
the crust of the earth? It will be more convenient to discuss this
question in the chapter on the Imperfection of the Geological
Record; and I will here only state that I believe the answer mainly
lies in the record being incomparably less perfect than is generally
supposed. The crust of the earth is a vast museum; but the natural
connections have been imperfectly made, and only at long intervals
of time.
But it may be urged that when several closely-allied species inhabit
the same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive
intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and as the
one becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from
each other in every detail of structure as are specimens taken from
the metropolis inhabited by each. By my theory these allied species
are descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original
parent-form and all the transitional varieties between its past and
present states. Hence we ought not to expect at the present time to
meet with numerous transitional varieties in each region, though
they must have existed there, and may be embedded there in a fossil
condition. But in the intermediate region, having intermediate
conditions of life, why do we not now find closely-linking
intermediate varieties? This difficulty for a long time quite
confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous
during a long period. Geology would lead us to believe that most
continents have been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been
separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have
been formed on strictly continuous areas; though I do not doubt that
the formerly broken condition of areas now continuous, has played an
important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide
area, we generally find them tolerably numerous over a large
territory, then becoming somewhat abruptly rarer and rarer on the
confines, and finally disappearing. Hence the neutral territory
between two representative species is generally narrow in comparison
with the territory proper to each. We see the same fact in ascending
mountains, and sometimes it is quite remarkable how abruptly, as Alph.
de Candolle has observed, a common alpine species disappears. The same
fact has been noticed by E. Forbes in sounding the depths of the sea
with the dredge. To those who look at climate and the physical
conditions of life as the all-important elements of distribution,
these facts ought to cause surprise, as climate and height or depth
graduate away insensibly. But when we bear in mind that almost every
species, even in its metropolis, would increase immensely in
numbers, were it not for other competing species; that nearly all
either prey on or serve as prey for others; in short, that each
organic being is either directly or indirectly related in the most
important manner to other organic beings,- we see that the range of
the inhabitants of any country by no means exclusively depends on
insensibly changing physical conditions, but in a large part on the
presence of other species, on which it lives, or by which it is
destroyed, or with which it comes into competition; and as these
species are already defined objects, not blending one into another
by insensible gradations, the range of any one species, depending as
does on the range of others, will tend to be sharply defined.
Moreover, each species on the confines of its range, where it exists
in lessened numbers, will, during fluctuations in the number of its
enemies or of its prey, or in the nature of the seasons, be
extremely liable to utter extermination; and thus its geographical
range will come to be still more sharply defined.
As allied or representative species, when inhabiting a continuous
area, are generally distributed in such a manner that each has a
wide range, with a comparatively narrow neutral territory between
them, in which they become rather suddenly rarer and rarer; then, as
varieties do not essentially differ from species, the same rule will
probably apply to both; and if we take a varying species inhabiting
a very large area, we shall have to adapt two varieties to two large
areas, and a third variety to a narrow intermediate zone. The
intermediate variety, consequently, will exist in lesser numbers
from inhabiting a narrow and lesser area; and practically, as far as I
can make out, this rule holds good with varieties in a state of
nature. I have met with striking instances of the rule in the case
of varieties intermediate between well-marked varieties in the genus
Balanus. And it would appear from information given me by Mr.
Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when
varieties intermediate between two other forms occur, they are much
rarer numerically than the forms which they connect. Now, if we may
trust these facts and inferences, and conclude that varieties
linking two other varieties together generally have existed in
lesser numbers than the forms which they connect, then we can
understand why intermediate varieties should not endure for very
long periods:- why, as a general rule, they should be exterminated and
disappear, sooner than the forms which they originally linked
together.
For any form existing in lesser numbers would, as already
remarked, run a greater chance of being exterminated than one existing
in large numbers; and in this particular case the intermediate form
would be eminently liable to the inroads of closely-allied forms
existing on both sides of it. But it is a far more important
consideration, that during the process of further modification, by
which two varieties are supposed to be converted and perfected into
two distinct species, the two which exist in larger numbers, from
inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists in smaller numbers in a narrow
and intermediate zone. For forms existing in larger numbers will
have a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common
forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and improved.
It is the same principle which, as I believe, accounts for the
common species in each country, as shown in the second chapter,
presenting on an average a greater number of well-marked varieties
than do the rarer species. I may illustrate what I mean by supposing
three varieties of sheep to be kept, one adapted to an extensive
mountainous region; a second to a comparatively narrow, hilly tract;
and a third to the wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks
by selection; the chances in this case will be strongly in favour of
the great holders on the mountains or on the plains, improving their
breeds more quickly than the small holders on the intermediate narrow,
hilly tract; and consequently the improved mountain or plain breed
will soon take the place of the less improved hill breed; and thus the
two breeds, which originally existed in greater numbers, will come
into close contact with each other, without the interposition of the
supplanted, intermediate hill variety.
To sum up, I believe that species come to be tolerably
well-defined objects, and do not at any one period present an
inextricable chaos of varying and intermediate links; first, because
new varieties are very slowly formed, for variation is a slow process,
and natural selection can do nothing until favourable individual
differences or variations occur, and until a place in the natural
polity of the country can be better filled by some modification of
some one or more of its inhabitants. And such new places will depend
on slow changes of climate, or on the occasional immigration of new
inhabitants, and, probably, in a still more important degree, on
some of the old inhabitants becoming slowly modified, with the new
forms thus produced, and the old ones acting and reacting on each
other. So that, in any one region and at any one time, we ought to see
only a few species presenting slight modifications of structure in
some degree permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period as isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to
rank as representative species. In this, case, intermediate
varieties between the several representative species and their
common parent, must formerly have existed within each isolated portion
of the land, but these links during the process of natural selection
will have been supplanted and exterminated, so that they will no
longer be found in a living state.
Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these
intermediate varieties will, from reasons already assigned (namely
from what we know of the actual distribution of closely allied or
representative species, and likewise of acknowledged varieties), exist
in the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate varieties
will be liable to accidental extermination; and during the process
of further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they connect;
for these from existing in greater numbers will, in the aggregate,
present more varieties, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory
be true, numberless intermediate varieties, linking closely together
all the species of the same group, must assuredly have existed; but
the very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent-forms and the intermediate
links. Consequently evidence of their former existence could be
found only amongst fossil remains, which are preserved, as we shall
attempt to show in a future chapter, in an extremely imperfect and
intermittent record.
On the Origin and Transitions of Organic Beings with peculiar Habits
and Structure.- It has been asked by the opponents of such views as
I hold, how, for instance, could a land carnivorous animal have been
converted into one with aquatic habits; for how could the animal in
its transitional state have subsisted? It would be easy to show that
there now exist carnivorous animals presenting close intermediate
grades from strictly terrestrial to aquatic habits; and as each exists
by a struggle for life, it is clear that each must be well adapted
to its place in nature. Look at the Mustela vision of North America,
which has webbed feet, and which resembles an otter in its fur,
short legs, and form of tail. During the summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys, like other pole-cats, on mice and land animals.
If a different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a
flying bat, the question would have been far more difficult to answer.
Yet I think such difficulties have little weight.
Here, as on other occasions, I lie under a heavy disadvantage,
for, out of the many striking cases which I have collected, I can only
give one or two instances of transitional habits and structures in
allied species; and of diversified habits, either constant or
occasional, in the same species. And it seems to me that nothing
less than a long list of such cases is sufficient to lessen the
difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from
others, as Sir J. Richardson has remarked, with the posterior part
of their bodies rather wide and with the skin on their flanks rather
full, to the so-called flying squirrels; and flying squirrels have
their limbs and even the base of the tail united by a broad expanse of
skin, which serves as a parachute and allows them to glide through the
air to an astonishing distance from tree to tree. We cannot doubt that
each structure is of use to each kind of squirrel in its own
country, by enabling it to escape birds or beasts of prey, to
collect food more quickly, or, as there is reason to believe, to
lessen the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all possible conditions. Let the climate
and vegetation change, let other competing rodents or new beasts of
prey immigrate, or old ones become modified, and all analogy would
lead us to believe that some at least of the squirrels would
decrease in numbers or become exterminated, unless they also become
modified and improved in structure in a corresponding manner.
Therefore, I can see no difficulty, more especially under changing
conditions of life, in the continued preservation of individuals
with fuller and fuller flank membranes, each modification being,
useful, each being propagated, until, by the accumulated effects of
this process of natural selection, a perfect so-called flying squirrel
was produced.
Now look at the Galeopithecus or so-called flying lemur, which
formerly was ranked amongst bats, but is now believed to belong to the
Insectivora. An extremely wide flank membrane stretches from the
corners of the jaw to the tail, and includes the limbs with the
elongated fingers. This flank-membrane is furnished with an extensor
muscle. Although no graduated links of structure, fitted for gliding
through the air, now connect the Galeopithecus with the other
Insectivora, yet there is no difficulty in supposing that such links
formerly existed, and that each was developed in the same manner as
with the less perfectly gliding squirrels; each grade of structure
having been useful to its possessor. Nor can I see any insuperable
difficulty in further believing that the membrane connected fingers
and fore-arm of the Galeopithecus might have been greatly lengthened
by natural selection; and this, as far as the organs of flight are
concerned, would have converted the animal into a bat. In certain bats
in which the wing-membrane extends from the top of the shoulder to the
tail and includes the hind-legs, we perhaps see traces of an apparatus
originally fitted for gliding through the air rather than for flight.
If about a dozen genera of birds were to become extinct, who would
have ventured to surmise that birds might have existed which used
their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and as front-legs on
the land, like the penguin; as sails, like the ostrich; and
functionally for no purpose, like the Apteryx? Yet the structure of
each of these birds is good for it, under the conditions of life to
which it is exposed, for each has to live by a struggle; but it is not
necessarily the best possible under all possible conditions. It must
not be inferred from these remarks that any of the grades of
wing-structure here alluded to, which perhaps may all be the result of
disuse, indicate the steps by which birds actually acquired their
perfect power of flight; but they serve to show what diversified means
of transition are at least possible.
Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land; and seeing
that we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected, who
would have ever imagined that in an early transitional state they
had been the inhabitants of the open ocean, and had used their
incipient organs of flight exclusively, as far as we know, to escape
being devoured by other fish?
When we see any structure highly perfected for any particular habit,
as the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom have
survived to the present day, for they will have been supplanted by
their successors, which were gradually rendered more perfect through
natural selection. Furthermore, we may conclude that transitional
states between structures fitted for very different habits of life
will rarely have been developed at an early period in great numbers
and under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many
subordinate forms, for taking prey of many kinds in many ways, on
the land and in the water, until their organs of flight had come to
a high stage of perfection, so as to have given them a decided
advantage over other animals in the battle for life. Hence the
chance of discovering species with transitional grades of structure in
a fossil condition will always be less, from their having existed in
lesser numbers, than in the case of species with fully developed
structures.
I will now give two or three instances both of diversified and of
changed habits in the individuals of the same species. In either
case it would be easy for natural selection to adapt the structure
of the animal to its changed habits, or exclusively to one of its
several habits. It is, however, difficult to decide, and immaterial
for us, whether habits generally change first and structure
afterwards; or whether slight modifications of structure lead to
changed habits; both probably often occurring almost simultaneously.
Of cases of changed habits it will suffice merely to allude to that of
the many British insects which now feed on exotic plants, or
exclusively on artificial substances. Of diversified habits
innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over
one spot and then proceeding to another, like a kestrel, and at
other times standing stationary on the margin of water, and then
dashing into it like a kingfisher at a fish. In our own country the
larger titmouse (Parus major) may be seen climbing branches, almost
like a creeper; it sometimes, like a shrike, kills small birds by
blows on the head; and I have many times seen and heard it hammering
the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost
like a whale, insects in the water.
As we sometimes see individuals following habits different from
those proper to their species and to the other species of the same
genus, we might expect that such individuals would occasionally give
rise to new species, having anomalous habits, and with their structure
either slightly or considerably modified from that of their type.
And such instances occur in nature. Can a more striking instance of
adaptation be given than that of a woodpecker for climbing trees and
seizing insects in the chinks of the bark? Yet in North America
there are woodpeckers which feed largely on fruit, and others with
elongated wings which chase insects on the wing. On the plains of La
Plata, where hardly a tree grows, there is a woodpecker (Colaptes
campestris) which has two toes before and two behind, a long pointed
tongue, pointed tail-feathers, sufficiently stiff to support the
bird in a vertical position on a post, but not so stiff as in the
typical woodpeckers, and a straight strong beak. The beak, however, is
not so straight or so strong as in the typical woodpeckers, but it
is strong enough to bore into wood. Hence this Colaptes in all the
essential parts of its structure is a woodpecker. Even in such
trifling characters as the colouring, the harsh tone of the voice, and
undulatory flight, its close blood-relationship to our common
woodpecker is plainly declared; yet, as I can assert, not only from my
own observation, but from those of the accurate Azara, in certain
large districts it does not climb trees, and it makes its nest in
holes in banks! In certain other districts, however, this same
woodpecker, as Mr. Hudson states, frequents trees, and bores holes
in the trunk for its nest. I may mention as another illustration of
the varied habits of this genus, that a Mexican Colaptes has been
described by De Saussure as boring holes into hard wood in order to
lay up a store of acorns.
Petrels are the most aerial and oceanic of birds, but in the quiet
sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, in its manner of
swimming and of flying when made to take flight, would be mistaken
by any one for an auk or a grebe; nevertheless it is essentially a
petrel, but with many parts of its organisation profoundly modified in
relation to its new habits of life; whereas the woodpecker of La Plata
has had its structure only slightly modified. In the case of the
waterouzel, the acutest observer by examining its dead body would
never have suspected its subaquatic habits; yet this bird, which is
allied to the thrush family, subsists by diving- using its wings under
water, and grasping stones with its feet. All the members of the great
order of hymenopterous insects are terrestrial excepting the genus
Proctotrupes, which Sir John Lubbock has discovered to be aquatic in
its habits; it often enters the water and dives about by the use not
of its legs but of its wings, and remains as long as four hours
beneath the surface; yet it exhibits no modification in structure in
accordance with its abnormal habits.
He who believes that each being has been created as we now see it,
must occasionally have felt surprise when he has met with an animal
having habits and structure not in agreement. What can be plainer than
that the webbed feet of ducks and geese are formed for swimming? Yet
there are upland geese with webbed feet which rarely go near the
water; and no one except Audubon has seen the frigate-bird, which
has all its four toes webbed, alight on the surface of the ocean. On
the other hand, grebes and coots are eminently aquatic, although their
toes are only bordered by membrane. What seems plainer than that the
long toes, not furnished with membrane, of the Grallatores are
formed for walking over swamps and floating plants?- the water-hen and
landrail are members of this order, yet the first is nearly as aquatic
as the coot, and the second nearly as terrestrial as the quail or
partridge. In such cases, and many others could be given, habits
have changed without a corresponding change of structure. The webbed
feet of the upland goose may be said to have become almost rudimentary
in function, though not in structure. In the frigate-bird, the
deeply scooped membrane between the toes shows that structure has
begun to change.
He who believes in separate and innumerable acts of creation may
say, that in these cases it has pleased the Creator to cause a being
of one type to take the place of one belonging to another type; but
this seems to me only re-stating the fact in dignified language. He
who believes in the struggle for existence and in the principle of
natural selection, will acknowledge that every organic being is
constantly endeavouring to increase in numbers; and that if any one
being varies ever so little, either in habits or structure, and thus
gains an advantage over some other inhabitant of the same country,
it will seize on the place of that inhabitant, however different
that may be from its own place. Hence it will cause him no surprise
that there should be geese and frigatebirds with webbed feet, living
on the dry land and rarely alighting on the water; that there should
be long-toed corncrakes, living in meadows instead of in swamps;
that there should be woodpeckers where hardly a tree grows; that there
should be diving thrushes and diving Hymenoptera, and petrels with the
habits of auks.
Organs of extreme Perfection and Complication
To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest degree. When it was first said
that the sun stood still and the world turned round, the common
sense of mankind declared the doctrine false; but the old saying of
Vox populi, vox Dei, as every philosopher knows, cannot be trusted
in science. Reason tells me, that if numerous gradations from a simple
and imperfect eye to one complex and perfect can be shown to exist,
each grade being useful to its possessor, as is certainly the case; if
further, the eye ever varies and the variations be inherited, as is
likewise certainly the case and if such variations should be useful to
any animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, should not be
considered as subversive of the theory. How a nerve comes to be
sensitive to light, hardly concerns us more than how life itself
originated; but I may remark that, as some of the lowest organisms, in
which nerves cannot be detected, are capable of perceiving light, it
does not seem impossible that certain sensitive elements in their
sarcode should become aggregated and developed into nerves, endowed
with this special sensibility.
In searching for the gradations through which an orgain in any
species has been perfected, we ought to look exclusively to its lineal
progenitors; but this is scarcely ever possible, and we are forced
to look to other species and genera of the same group, that is to
the collateral descendants from the same parent-form, in order to
see what gradations are possible, and for the chance of some
gradations having been transmitted in an unaltered or little altered
condition. But the state of the same organ in distinct classes may
incidentally throw light on the steps by which it has been perfected.
The simplest organ which can be called an eye consists of an optic
nerve, surrounded by pigment-cells, and covered by translucent skin,
but without any lens or other refractive body. We may, however,
according to M. Jourdain, descend even a step lower and find
aggregates of pigment-cells, apparently serving as organs of vision,
without any nerves, and resting merely on sarcodic tissue. Eyes of the
above simple nature are not capable of distinct vision, and serve only
to distinguish light from darkness. In certain star-fishes, small
depressions in the layer of pigment which surrounds the nerve are
filled, as described by the author just quoted, with transparent
gelatinous matter, projecting with a convex surface, like the cornea
in the higher animals. He suggests that this serves not to form an
image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the
first and by far the most important step towards the formation of a
true, picture-forming eye; for we have only to place the naked
extremity of the optic nerve, which in some of the lower animals
lies deeply buried in the body, and in some near the surface, at the
right distance from the concentrating apparatus, and an image will
be formed on it.
In the great class of the Articulata, we may start from an optic
nerve simply coated with pigment, the latter sometimes forming a
sort of pupil, but destitute of a lens or other optical contrivance.
With insects it is now known that the numerous facets on the cornea of
their great compound eyes form true lenses, and that the cones include
curiously modified nervous filaments. But these organs in the
Articulata are so much diversified that Muller formerly made three
main classes with seven subdivisions, besides a fourth main class of
aggregated simple eyes.
When we reflect on these facts, here given much too briefly, with
respect to the wide, diversified, and graduated range of structure
in the eyes of the lower animals; and when we bear in mind how small
the number of all living forms must be in comparison with those
which have become extinct, the difficulty ceases to be very great in
believing that natural selection may have converted the simple
apparatus of an optic nerve, coated with pigment and invested by
transparent membrane, into an optical instrument as perfect as is
possessed by any member of the articulate class.
He who will go thus far, ought not to hesitate to go one step
further, if he finds on finishing this volume that large bodies of
facts, otherwise inexplicable, can be explained by the theory of
modification through natural selection; he ought to admit that a
structure even as perfect as an eagle's eye might thus be formed,
although in this case he does not know the transitional states. It has
been objected that in order to modify the eye and still preserve it as
a perfect instrument, many changes would have to be effected
simultaneously, which, it is assumed, could not be done through
natural selection; but as I have attempted to show in my work on the
variation of domestic animals, it is not necessary to suppose that the
modifications were all simultaneous, if they were extremely slight and
gradual. Different kinds of modification would, also, serve for the
same general purpose: as Mr. Wallace has remarked, "if a lens has
too short or too long a focus, it may be amended either by an
alteration of curvature, or an alteration of density; if the curvature
be irregular, and the rays do not converge to a point, then any
increased regularity of curvature will be an improvement. So the
contraction of the iris and the muscular movements of the eye are
neither of them essential to vision, but only improvements which might
have been added and perfected at any stage of the construction of
the instrument." Within the highest division of the animal kingdom,
namely, the Vertebrata, we can start from an eye so simple, that it
consists, as in the lancelet, of a little sack of transparent skin,
furnished with a nerve and lined with pigment, but destitute of any
other apparatus. In fishes and reptiles, as Owen has remarked, "the
range of gradations of dioptric structures is very great." It is a
significant fact that even in man, according to the high authority
of Virchow, the beautiful crystalline lens is formed in the embryo
by an accumulation of epidermic cells, lying in a sack-like fold of
the skin; and the vitreous body is formed from embryonic sub-cutaneous
tissue. To arrive, however, at a just conclusion regarding the
formation of the eye, with all its marvellous yet not absolutely
perfect characters, it is indispensable that the reason should conquer
the imagination; but I have felt the difficulty far too keenly to be
surprised at others hesitating to extend the principle of natural
selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope.
We know that this instrument has been perfected by the
long-continued efforts of the highest human intellects; and we
naturally infer that the eye has been formed by a somewhat analogous
process. But may not this inference be presumptuous? Have we any right
to assume that the Creator works by intellectual powers like those
of man? If we must compare the eye to an optical instrument, we
ought in imagination to take a thick layer of transparent tissue, with
spaces filled with fluid, and with a nerve sensitive to light beneath,
and then suppose every part of this layer to be continually changing
slowly in density, so as to separate into layers of different
densities and thicknesses, placed at different distances from each
other, and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power, represented by
natural selection or the survival of the fittest, always intently
watching each slight alteration in the transparent layers; and
carefully preserving each which, under varied circumstances, in any
way or in any degree, tends to produce a distincter image. We must
suppose each new state of the instrument to be multiplied by the
million; each to be preserved until a better one is produced, and then
the old ones to be all destroyed. In living bodies, variation will
cause the slight alterations, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill
each improvement. Let this process go on for millions of years; and
during each year on millions of individuals of many kinds; and may
we not believe that a living optical instrument might thus be formed
as superior to one of glass, as the works of the Creator are to
those of man?
Modes of Transition
If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to the theory, there has been much
extinction. Or again, if we take an organ common to all the members of
a class, for in this latter case the organ must have been originally
formed at a remote period, since which all the many members of the
class have been developed; and in order to discover the early
transitional grades through which the organ has passed, we should have
to look to very ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could
not have been formed by transitional gradations of some kind. Numerous
cases could be given amongst the lower animals of the same organ
performing at the same time wholly distinct functions; thus in the
larva of the dragon-fly and in the fish Cobitis the alimentary canal
respires, digests, and excretes. In the Hydra, the animal may be
turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might specialise,
if any advantage were thus gained, the whole or part of an organ,
which had previously performed two functions, for one function
alone, and thus by insensible steps greatly change its nature. Many
plants are known which regularly produce at the same time
differently constructed flowers; and if such plants were to produce
one kind alone, a great change would be effected with comparative
suddenness in the character of the species. It is, however, probable
that the two sorts of flowers borne by the same plant were
originally differentiated by finely graduated steps, which may still
be followed in some few cases.
Again, two distinct organs, or the same organ under two very
different forms, may simultaneously perform in the same individual the
same function, and this is an extremely important means of transition:
to give one instance,- there are fish with gills or branchiae that
breathe the air dissolved in the water, at the same time that they
breathe free air in their swimbladders, this latter organ being
divided by highly vascular partitions and having a ductus
pneumaticus for the supply of air. To give another instance from the
vegetable kingdom: plants climb by three distinct means, by spirally
twining, by clasping a support with their sensitive tendrils, and by
the emission of aerial rootlets; these three means are usually found
in distinct groups, but some few species exhibit two of the means,
or even all three, combined in the same individual. In all such
cases one of the two organs might readily be modified and perfected so
as to perform all the work, being aided during the progress of
modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be wholly
obliterated.
The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely, flotation, may be converted
into one for a widely different purpose, namely, respiration. The
swimbladder has, also, been worked in as an accessory to the
auditory organs of certain fishes. All physiologists admit that the
swimbladder is homologous, or "ideally similar" in position and
structure with the lungs of the higher vertebrate animals: hence there
is no reason to doubt that the swimbladder has actually been converted
into lungs, or an organ used exclusively for respiration.
According to this view it may be inferred that all vertebrate
animals with true lungs are descended by ordinary generation from an
ancient and unknown prototype, which was furnished with a floating
apparatus or swimbladder. We can thus, as I infer from Owen's
interesting description of these parts, understand the strange fact
that every particle of food and drink & which we swallow has to pass
over the orifice of the trachea, with some risk of falling into the
lungs, notwithstanding the beautiful contrivance by which the
glottis is closed. In the higher Vertebrate the branchiae have
wholly disappeared- but in the embryo the slits on the sides of the
neck and the loop-like course of the arteries still mark their
former position. But it is conceivable that the now utterly lost
branchiae might have been gradually worked in by natural selection for
some distinct purpose: for instance, Landois has shown that the
wings of insects are developed from the tracheae; it is therefore
highly probable that in this great class organs which once served
for respiration have been actually converted into organs for flight.
In considering transitions of organs, it is so important to bear
in mind the probability of conversion from one function to another,
that I will give another instance. Pedunculated cirripedes have two
minute folds of skin, called by me the ovigerous frena, which serve,
through the means of a sticky secretion, to retain the eggs until they
are hatched within the sack. These cirripedes have no branchiae, the
whole surface of the body and of the sack, together with the small
frena, serving for respiration. The Balanidae or sessile cirripedes,
on the other hand, have no ovigerous frena, the eggs lying loose at
the bottom of the sack, within the well-enclosed shell; but they have,
in the same relative position with the frena, large, much-folded
membranes, which freely communicate with the circulatory lacunae of
the sack and body, and which have been considered by all naturalists
to act as branchiae. Now I think no one will dispute that the
ovigerous frena in the one family are strictly homologous with the
branchiae of the other family; indeed, they graduate into each
other. Therefore it need not be doubted that the two little folds of
skin, which originally served as ovigerous frena, but which, likewise,
very slightly aided in the act of respiration, have been gradually
converted by natural selection into branchiae simply through an
increase in their size and the obliteration of their adhesive
glands. If all pedunculated cirripedes had become extinct, and they
have suffered far more extinction than have sessile cirripedes, who
would ever have imagined that the branchiae in this latter family
had originally existed as organs for preventing the ova from being
washed out of the sack?
There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Prof. Cope and others in the United States.
It is now known that some animals are capable of reproduction at a
very early age, before they have acquired their perfect characters;
and if this power became thoroughly well developed in a species, it
seems probable that the adult stage of development would sooner or
later be lost; and in this case, especially if the larva differed much
from the mature form, the character of the species would be greatly
changed and degraded. Again, not a few animals, after arriving at
maturity, go on changing in character during nearly their whole lives.
With mammals, for instance, the form of the skull is often much
altered with age, of which Dr. Murie has given some striking instances
with seals; every one knows how the horns of stags become more and
more branched, and the plumes of some birds become more finely
developed, as they grow older. Prof. Cope states that the teeth of
certain lizards change much in shape with advancing years; with
crustaceans not only many trivial, but some important parts assume a
new character, as recorded by Fritz Muller, after maturity. In all
such cases,- and many could be given,- if the age for reproduction
were retarded, the character of the species, at least in its adult
state, would be modified; nor is it improbable that the previous and
earlier stages of development would in some cases be hurried through
and finally lost. Whether species have often or ever been modified
through this comparatively sudden mode of transition, I can form no
opinion; but if this has occurred, it is probable that the differences
between the young and the mature, and between the mature and the
old, were primordially acquired by graduated steps.
Special Difficulties of the Theory Of Natural Selection
Although we must be extremely cautious in concluding that any
organ could not have been produced by successive, small,
transitional gradations, yet undoubtedly serious cases of difficulty
occur.
One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females;
but this case will be treated of in the next chapter. The electric
organs of fishes offer another case of special difficulty; for it is
impossible to conceive by, what steps these wondrous organs have
been produced. But this is not surprising, for we do not even know
of what use they are. In the Gymnotus and torpedo they no doubt
serve as powerful means of defence, and perhaps for securing prey; yet
in the ray, as observed by Matteucci, an analogous organ in the tail
manifests but little electricity, even when the animal is greatly
irritated; so little, that it can hardly be of any use for the above
purposes. Moreover, in the ray, besides the organ just referred to,
there is, as Dr. R. McDonnell has shown, another organ near the
head, not known to be electrical, but which appears to be the real
homologue of the electric battery in the torpedo. It is generally
admitted that there exists between these organs and ordinary muscle
a close analogy, in intimate structure, in the distribution of the
nerves, and in the manner in which they are acted on by various
reagents. It should, also, be especially observed that muscular
contraction is accompanied by an electrical discharge; and, as Dr.
Radcliffe insists, "in the electrical apparatus of the torpedo
during rest, there would seem be a charge in every respect like that
which is met with in muscle and nerve during rest, and the discharge
of the torpedo, instead of being peculiar, may be only another form of
the discharge which depends upon the action of muscle and motor
nerve." Beyond this we cannot at present go in the way of explanation;
but as we know so little about the uses of these organs, and as we
know nothing about the habits and structure of the progenitors of
the existing electric fishes, it would be extremely bold to maintain
that no serviceable transitions are possible by which these organs
might have been gradually developed.
These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which
several are widely remote in their affinities. When the same organ
is found in several members of the same class, especially if in
members having very different habits of life, we may generally
attribute its presence to inheritance from a common ancestor; and
its absence in some of the members to loss through disuse or natural
selection. So that, if the electric organs had been inherited from
some one ancient progenitor, we might have expected that all
electric fishes would have been specially related to each other; but
this is far from the case. Nor does geology at all lead to the
belief that most fishes formerly possessed electric organs, which
their modified descendants have now lost. But when we look at the
subject more closely, we find in the several fishes provided with
electric organs, that these are situated in different parts of the
body,- that they differ in construction, as in the arrangement of
the plates, and, according to Pacini, in the process or means by which
the electricity is excited- and lastly, in being supplied with
nerves proceeding from different sources, and this is perhaps the most
important of all the differences. Hence in the several fishes
furnished with electric organs, these cannot be considered as
homologous, but only as analogous in function. Consequently there is
no reason to suppose that they have been inherited from a common
progenitor; for had this been the case they would have closely
resembled each other in all respects. Thus the difficulty of an organ,
apparently the same, arising in several remotely allied species,
disappears, leaving only the lesser yet still great difficulty;
namely, by what graduated steps these organs have been developed in
each separate group of fishes.
The luminous organs which occur in a few insects, belonging to
widely different families, and which are situated in different parts
of the body, offer, under our present state of ignorance, a difficulty
almost exactly parallel with that of the electric organs. Other
similar cases could be given; for instance in plants, the very curious
contrivance of a mass of pollen-grains, borne on a foot-stalk with
an adhesive gland, is apparently the same in Orchis and Asclepias,-
genera almost as remote as is possible amongst flowering plants; but
here again the parts are not homologous. In all cases of beings, far
removed from each other in the scale of organisation, which are
furnished with similar and peculiar organs, it will be found that
although the general appearance and function of the organs may be
the same, yet fundamental differences between them can always be
detected. For instance, the eyes of cephalopods or cuttle-fish and
of vertebrate animals appear wonderfully alike; and in such widely
sundered groups no part of this resemblance can be due to
inheritance from a common progenitor. Mr. Mivart has advanced this
case as one of special difficulty, but I am unable to see the force of
his argument. An organ for vision must be formed of transparent
tissue, and must include some sort of lens for throwing an image at
the back of a darkened chamber. Beyond this superficial resemblance,
there is hardly any real similarity between the eyes of cuttle-fish
and vertebrates, as may be seen by consulting Hensen's admirable
memoir on these organs in the Cephalopoda. It is impossible for me
here to enter on details, but I may specify a few of the points of
difference. The crystalline lens in the higher cuttle-fish consists of
two parts, placed one behind the other like two lenses, both having
a very different structure and disposition to what occurs in the
vertebrata. The retina is wholly different, with an actual inversion
of the elemental parts, and with a large nervous ganglion included
within the membranes of the eye. The relations of the muscles are as
different as it is possible to conceive, and so in other points. Hence
it is not a little difficult to decide how far even the same terms
ought to be employed in describing the eyes of the Cephalopoda and
Vertebrata. It is, of course, open to any one to deny that the eye
in either case could have been developed through the natural selection
of successive slight variations; but if this be admitted in the one
case, it is clearly possible in the other; and fundamental differences
of structure in the visual organs of two groups might have been
anticipated, in accordance with this view of their manner of
formation. As two men have sometimes independently hit on the same
invention, so in the several foregoing cases it appears that natural
selection, working for the good of each being, and taking advantage of
all favourable variations, has produced similar organs, as far as
function is concerned, in distinct organic beings, which owe none of
their structure in common to inheritance from a common progenitor.
Fritz Muller, in order to test the conclusions arrived at in this
volume, has followed out with much care a nearly similar line of
argument. Several families of crustaceans include a few species,
possessing an air-breathing apparatus and fitted to live out of the
water. In two of these families, which were more especially examined
by Muller and which are nearly related to each other, the species
agree most closely in all important characters; namely, in their sense
organs, circulating system, in the position of the tufts of hair
within their complex stomachs, and lastly in the whole structure of
the water-breathing branchiae, even to the microscopical hooks by
which they are cleansed. Hence it might have been expected that in the
few species belonging to both families which live on the land, the
equally important air-breathing apparatus would have been the same;
for why should this one apparatus, given for the same purpose, have
been made to differ, whilst all the other important organs were
closely similar or rather identical?
Fritz Muller argues that this close similarity in so many points
of structure must, in accordance with the views advanced by me, be
accounted for by inheritance from a common progenitor. But as the vast
majority of the species in the above two families, as well as most
other crustaceans, are aquatic in their habits, it is improbable in
the highest degree, that their common progenitor should have been
adapted for breathing air was thus led carefully to examine the
apparatus in the air-breathing species; and he found it to differ in
each in several important points, as in the position of the
orifices, in the manner in which they are opened and closed, and in
some accessory details. Now such differences are intelligible, and
might even have been expected, on the supposition that species
belonging to distinct families had slowly become adapted to live
more and more out of water, and to breathe the air. For these species,
from belonging to distinct families, would have differed to a
certain extent, and in accordance with the principle that the nature
of each variation depends on two factors, viz., the nature of the
organism and that of the surrounding conditions, their variability
assuredly would not have been exactly the same. Consequently natural
selection would have had different materials or variations to work on,
in order to arrive at the same functional result; and the structures
thus acquired would almost necessarily have differed. On the
hypothesis of separate acts of creation the whole case remains
unintelligible. This line of argument seems to have had great weight
in leading Fritz Muller to accept the views maintained by me in this
volume.
Another distinguished zoologist, the late Professor Claparide, has
argued in the same manner, and has arrived at the same result. He
shows that there are parasitic mites (Acaridae), belonging to distinct
sub-families and families, which are furnished with hair-claspers.
These organs must have been independently developed, as they could not
have been inherited from a common progenitor; and in the several
groups they are formed by the modification of the fore-legs,- of the
hind-legs,- of the maxillae or lips,- and of appendages on the under
side of the hind part of the body.
In the foregoing cases, we see the same end gained and the same
function performed, in beings not at all or only remotely allied, by
organs in appearance, though not in development, closely similar. On
the other hand, it is a common rule throughout nature that the same
end should be gained, even sometimes in the case of closely-related
beings, by the most diversified means. How differently constructed
is the feathered wing of a bird and the membrane-covered wing of a
bat; and still more so the four wings of a butterfly, the two wings of
a fly, and the two wings with the elytra of a beetle. Bivalve shells
are made to open and shut, but on what a number of patterns is the
hinge constructed,- from the long row of neatly interlocking teeth
in a Nucula to the simple ligament of a Mussel! Seeds are disseminated
by their minuteness,- by their capsule being converted into a light
balloon-like envelope,- by being embedded in pulp or flesh, formed
of the most diverse parts, and rendered nutritious, as well as
conspicuously coloured, so as to attract and be devoured by birds,- by
having hooks and grapnels of many kinds and serrated arms, so as to
adhere to the fur of quadrupeds,- and by being furnished with wings
and plumes, as different in shape as they are elegant in structure, so
as to be wafted by every breeze. I will give one other instance; for
this subject of the same end being gained by the most diversified
means well deserves attention. Some authors maintain that organic
beings have been formed in many ways for the sake of mere variety,
almost like toys in a shop, but such a view of nature is incredible.
With plants having separated sexes, and with those in which, though
hermaphrodites, the pollen does not spontaneously fall on the
stigma, some aid is necessary for their fertilisation. With several
kinds this is effected by the pollen-grains, which are light and
incoherent, being blown by the wind through mere chance on to the
stigma; and this is the simplest plan which can well be conceived.
An almost equally simple, though very different, plan occurs in many
plants in which a symmetrical flower secretes a few drops of nectar,
and is consequently visited by insects; and these carry the pollen
from the anthers to the stigma.
From this simple stage we may pass through an inexhaustible number
of contrivances, all for the same purpose and effected in
essentially the same manner, but entailing changes in every part of
the flower. The nectar may be stored in variously shaped
receptacles, with the stamens and pistils modified in many ways,
sometimes forming trap-like contrivances, and sometimes capable of
neatly adapted movements through irritability or elasticity. From such
structures we may advance till we come to such a case of extraordinary
adaptation as that lately described by Dr. Cruger in the Coryanthes.
This orchid has part of its labellum or lower lip hollowed out into
a great bucket, into which drops of almost pure water continually fall
from two secreting horns which stand above it; and when the bucket
is half full, the water overflows by a spout on one side. The basal
part of the labellum stands over the bucket, and is itself hollowed
out into a sort of chamber with two lateral entrances; within this
chamber there are curious fleshy ridges. The most ingenious man, if he
had not witnessed what takes place, could never have imagined what
purpose all these parts serve. But Dr. Cruger saw crowds of large
humble-bees visiting the gigantic flowers of this orchid, not in order
to suck nectar, but to gnaw off the ridges within the chamber above
the bucket; in doing this they frequently pushed each other into the
bucket, and their wings being thus wetted they could not fly away, but
were compelled to crawl out through the passage formed by the spout or
overflow. Dr. Cruger saw a "continual procession" of bees thus
crawling out of their involuntary bath. The passage is narrow, and
is roofed over by the column, so that a bee, in forcing its way out,
first rubs its back against the viscid stigma and then against the
viscid glands of the pollen-masses. The pollen-masses are thus glued
to the back of the be which first happens to crawl out through the
passage of a lately expanded flower, and are thus carried away. Dr.
Cruger sent me a flower in spirits of wine, with a bee which he had
killed before it had quite crawled out with a pollen-mass still
fastened to its back. When the bee, thus provided, flies to another
flower, or to the same flower a second time, and is pushed by its
comrades into the bucket and then crawls out by the passage, the
pollen-mass necessarily comes first into contact with the viscid
stigma, and adheres to it, and the flower is fertilised. Now at last
we see the full use of every part of the flower, of the
water-secreting horns, of the bucket half full of water, which
prevents the bees from flying away, and forces them to crawl out
through the spout, and rub against the properly placed viscid
pollen-masses and the viscid stigma.
The construction of the flower in another closely allied orchid,
namely the Catasetum, is widely different, though serving the same
end; and is equally curious. Bees visit these flowers, like those of
the Coryanthes, in order to gnaw the labellum; in doing this they
inevitably touch a long, tapering, sensitive projection, or, as I have
called it, the antenna. This antenna, when touched, transmits a
sensation or vibration to a certain membrane which is instantly
ruptured; this sets free a spring by which the pollen-mass is shot
forth, like an arrow, in the right direction, and adheres by its
viscid extremity to the back of the bee. The pollen-mass of the male
plant (for the sexes are separate in this orchid) is thus carried to
the flower of the female plant where it is brought into contact with
the stigma, which is viscid enough to break certain elastic threads,
and retaining the pollen, fertilisation is effected.
How, it may be asked, in the foregoing and in innumerable other
instances, can we understand the graduated scale of complexity and the
multifarious means for gaining the same end. The answer no doubt is,
as already remarked, that when two forms vary, which already differ
from each other in some slight degree, the variability will not be
of the same exact nature, and consequently the results obtained
through natural selection for the same general purpose will not be the
same. We should also bear in mind that every highly developed organism
has passed through many changes; and that each modified structure
tends to be inherited, so that each modification will not readily be
quite lost, but may be again and again further altered. Hence the
structure of each part of each species, for whatever purpose it may
serve, is the sum of many inherited changes, through which the species
has passed during its successive adaptations to changed habits and
conditions of life.
Finally then, although in many cases it is most difficult even to
conjecture by what transitions organs have arrived at their present
state; yet, considering how small the proportion of living and known
forms is to the extinct and unknown, I have been astonished how rarely
an organ can be named, towards which no transitional grade is known to
lead. It certainly is true, that new organs appearing as if created
for some special purpose, rarely or never appear in any being;- as
indeed is shown by that old, but somewhat exaggerated, canon in
natural history of "Natura non facit saltum." We meet with this
admission in the writings of almost every experienced naturalist; or
as Milne Edwards has well expressed it, Nature is prodigal in variety,
but niggard in innovation. Why, on the theory of Creation, should
there be so much variety and so little real novelty? Why should all
the parts and organs of many independent beings, each supposed to have
been separately created for its proper place in nature, be so commonly
linked together by graduated steps? Why should not Nature take a
sudden leap from structure to structure? On the theory of natural
selection, we can clearly understand why she should not; for natural
selection acts only by taking advantage of slight successive
variations; she can never take a great and sudden leap, but must
advance by short and sure, though slow steps.
Organs of little apparent Importance, as affected by Natural
Selection
As natural selection acts by life and death,- by the survival of the
fittest, and by the destruction of the less well-fitted
individuals,- I have sometimes felt great difficulty in
understanding the origin or formation of parts of little importance;
almost as great, though of a very different kind, as in the case of
the most perfect and complex organs.
In the first place, we are much too ignorant in regard to the
whole economy of any one organic being, to say what slight
modifications would be of importance or not. In a former chapter I
have given instances of very trifling characters, such as the down
on fruit and the colour of its flesh, the colour of the skin and
hair of quadrupeds, which, from being correlated with constitutional
differences or from determining the attacks of insects, might
assuredly be acted on by natural selection. The tail of the giraffe
looks like an artificially constructed fly-flapper; and it seems at
first incredible that this could have been adapted for its present
purpose by successive slight modifications, each better and better
fitted, for so trifling an object as to drive away flies; yet we
should pause before being too positive even in this case, for we
know that the distribution and existence of cattle and other animals
in South America absolutely depend on their power of resisting the
attacks of insects: so that individuals which could by any means
defend themselves from these small enemies, would be able to range
into new pastures and thus gain a great advantage. It is not that
the larger quadrupeds are actually destroyed (except in some rare
cases) by flies, but they are incessantly harassed and their
strength reduced, so that they are more subject to disease, or not
so well enabled in a coming dearth to search for food, or to escape
from beasts of prey.
Organs now of trifling importance have probably in some cases been
of high importance to an early progenitor, and, after having been
slowly perfected at a former period, have been transmitted to existing
species in nearly the same state, although now of very slight use; but
any actually injurious deviations in their structure would of course
have been checked by natural selection. Seeing how important an
organ of locomotion the tail is in most aquatic animals, its general
presence and use for many purposes in so many land animals, which in
their lungs or modified swimbladders betray their aquatic origin,
may perhaps be thus accounted for. A well-developed tail having been
formed in an aquatic animal, it might subsequently come to be worked
in for all sorts of purposes,- as a fly-flapper, an organ of
prehension, or as an aid in turning, as in the case of the dog, though
the aid in this latter respect must be slight, for the hare, with
hardly any tail, can double still more quickly.
In the second place, we may easily err in attributing importance
to characters, and in believing that they have been developed
through natural selection. We must by no means overlook the effects of
the definite action of changed conditions of life,- of so-called
spontaneous variations, which seem to depend in a quite subordinate
degree on the nature of the conditions,- of the tendency to
reversion to long-lost characters,- of the complex laws of growth,
such as of correlation, compensation, of the pressure of one part on
another, &c.,- and finally of sexual selection, by which characters of
use to one sex are often gained and then transmitted more or less
perfectly to the other sex, though of no use to this sex. But
structures thus indirectly gained, although at first of no advantage
to a species, may subsequently have been taken advantage of by its
modified descendants, under new conditions of life and newly
acquired habits.
If green woodpeckers alone had existed, and we did not know that
there were many black and pied kinds, I dare say that we should have
thought that the green colour was a beautiful adaptation to conceal
this tree-frequenting bird from its enemies; and consequently that
it was a character of importance, and had been acquired through
natural selection; as it is, the colour is probably in chief part
due to sexual selection. A trailing palm in the Malay Archipelago
climbs the loftiest trees by the aid of exquisitely constructed
hooks clustered around the ends of the branches, and this contrivance,
no doubt, is of the highest service to the plant; but as we see nearly
similar hooks on many trees which are not climbers, and which, as
there is reason to believe from the distribution of the
thorn-bearing species in Africa and South America, serve as a
defence against browsing quadrupeds, so the spikes on the palm may
at first have been developed for this object, and subsequently have
been improved and taken advantage of by the plant, as it underwent
further modification and became a climber. The naked skin on the
head of a vulture is generally considered as a direct adaptation for
wallowing in putridity; and so it may be, or it may possibly be due to
the direct action of putrid matter; but we should be very cautious
in drawing any such inference, when we see that the skin on the head
of the clean-feeding male turkey is likewise naked. The sutures in the
skull? of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of
young birds and reptiles, which have only to escape from a broken egg,
we may infer that this structure has arisen from the laws of growth,
and has been taken advantage of in the parturition of the higher
animals.
We are profoundly ignorant of the cause of each slight variation
or individual difference; and we are immediately made conscious of
this by reflecting on the differences between the breeds of our
domesticated animals in different countries,- more especially in the
less civilised countries where there has been but little methodical
selection. Animals kept by savages in different countries often have
to struggle for their own subsistence, and are exposed to a certain
extent to natural selection, and individuals with slightly different
constitutions would succeed best under different climates. With cattle
susceptibility to the attacks of flies is correlated with colour, as
is the liability to be poisoned by certain plants; so that even colour
would be thus subjected to the action of natural selection. Some
observers are convinced that a damp climate affects the growth of
the hair, and that with the hair the horns are correlated. Mountain
breeds always differ from lowland breeds; and a mountainous country
would probably affect the hind limbs from exercising them more, and
possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and the head would probably be
affected. The shape, also, of the pelvis might affect by pressure
the shape of certain parts of the young in the womb. The laborious
breathing necessary in high regions tends, as we have good reason to
believe, to increase the size of the chest; and again correlation
would come into play. The effects of lessened exercise together with
abundant food on the whole organisation is probably still more
important; and this, as H. von Nathusius has lately shown in his
excellent treatise, is apparently one chief cause of the great
modification which the breeds of swine have undergone. But we are
far too ignorant to speculate on the relative importance of the
several known and unknown causes of variation; and I have made these
remarks only to show that, if we are unable to account for the
characteristic differences of our several domestic breeds, which
nevertheless are generally admitted to have arisen through ordinary
generation from one or a few parent-stocks, we ought not to lay too
much stress on our ignorance of the precise cause of the slight
analogous differences between true species.
Utilitarian Doctrine, how far true: Beauty, how acquired.
The foregoing remarks lead me to say a few words on the protest
lately made by some naturalists, against the utilitarian doctrine that
every detail of structure has been produced for the good of its
possessor. They believe that many structures have been created for the
sake of beauty, to delight man or the Creator (but this latter point
is beyond the scope of scientific discussion), or for the sake of mere
variety, a view already discussed. Such doctrines, if true, would be
absolutely fatal to my theory. I fully admit that many structures
are now of no direct use to their possessors, and may never have
been of any use to their progenitors; but this does not prove that
they were formed solely for beauty or variety. No doubt the definite
action of changed conditions, and the various causes of modifications,
lately specified, have all produced an effect, probably a great
effect, independently of any advantage thus gained. But a still more
important consideration is that the chief part of the organisation
of every living creature is due to inheritance; and consequently,
though each being assuredly is well fitted for its place in nature,
many structures have now no very close and direct relation to
present habits of life. Thus, we can hardly believe that the webbed
feet of the upland goose or of the frigate-bird are of special use
to these birds; we cannot believe that the similar bones in the arm of
the monkey, in the fore-leg of the horse, in the wing of the bat,
and in the flipper of the seal, are of special use to these animals.
We may safely attribute these structures to inheritance. But webbed
feet no doubt were as useful to the progenitor of the upland goose and
of the frigate-bird, as they now are to the most aquatic of living
birds. So we may believe that the progenitor of the seal did not
possess a flipper, but a foot with five toes fitted for walking or
grasping; but we may further venture to believe that the several bones
in the limbs of the monkey, horse, and bat, were originally developed,
on the principle of utility, probably through the reduction of more
numerous bones in the fin of some ancient fish-like progenitor of
the whole class. It is scarcely possible to decide how much
allowance ought to be made for such causes of change, as the
definite action of external conditions, so-called spontaneous
variations, and the complex laws of growth; but with these important
exceptions, we may conclude that the structure of every living
creature either now is, or was formerly, of some direct or indirect
use to its possessor.
With respect to the belief that organic beings have been created
beautiful for the delight of man,- a belief which it has been
pronounced is subversive of my whole theory,- I may first remark
that the sense of beauty obviously depends on the nature of the
mind, irrespective of any real quality in the admired object; and that
the idea of what is beautiful, is not innate or unalterable. We see
this, for instance, in the men of different races admiring an entirely
different standard of beauty in their women. If beautiful objects
had been created solely for man's gratification, it ought to be
shown that before man appeared, there was less beauty on the face of
the earth than since he came on the stage. Were the beautiful volute
and cone shells of the Eocene epoch, and the gracefully sculptured
ammonites of the Secondary period, created that man might ages
afterwards admire them in his cabinet? Few objects are more
beautiful than the minute siliceous cases of the diatomaceae: were
these created that they might be examined and admired under the higher
powers of the microscope? The beauty in this latter case, and in
many others, is apparently wholly due to symmetry of growth. Flowers
rank amongst the most beautiful productions of nature; but they have
been rendered conspicuous in contrast with the green leaves, and in
consequence at the same time beautiful, so that they may be easily
observed by insects. I have come to this conclusion from finding it an
invariable rule that when a flower is fertilised by the wind it
never has a gaily-coloured corolla. Several plants habitually
produce two kinds of flowers; one kind open and coloured so as to
attract insects; the other closed, not coloured, destitute of
nectar, and never visited by insects. Hence we may conclude that, if
insects had not been developed on the face of the earth, our plants
would not have been decked with beautiful flowers, but would have
produced only such poor flowers as we see on our fir, oak, nut and ash
trees, on grasses, spinach, docks, and nettles, which are all
fertilised through the agency of the wind. A similar line of
argument holds good with fruits; that a ripe strawberry or cherry is
as pleasing to the eye as to the palate,- that the gaily-coloured
fruit of the spindle-wood tree and the scarlet berries of the holly
are beautiful objects,- will be admitted by every one. But this beauty
serves merely as a guide to birds and beasts, in order that the
fruit may be devoured and the matured seeds disseminated: I infer that
this is the case from having as yet found no exception to the rule
that seeds are always thus disseminated when embedded within a fruit
of any kind (that is within a fleshy or pulpy envelope), if it be
coloured of any brilliant tint, or rendered conspicuous by being white
or black.
On the other hand, I willingly admit that a great number of male
animals, as all our most gorgeous birds, some fishes, reptiles, and
mammals, and a host of magnificently coloured butterflies, have been
rendered beautiful for beauty's sake; but this has been effected
through sexual selection, that is, by the more beautiful males
having been continually preferred by the females, and not for the
delight of man. So it is with the music of birds. We may infer from
all this that a nearly similar taste for beautiful colours and for
musical sounds runs through a large part of the animal kingdom. When
the female is as beautifully coloured as the male, which is not rarely
the case with birds and butterflies, the cause apparently lies in
the colours acquired through sexual selection having been
transmitted to both sexes, instead of to the males alone. How the
sense of beauty in its simplest form- that is, the reception of a
peculiar kind of pleasure from certain colours, forms, and sounds- was
first developed in the mind of man and of the lower animals, is a very
obscure subject. The same sort of difficulty is presented, if we
enquire how it is that certain flavours and odours give pleasure,
and others displeasure. Habit in all these cases appears to have
come to a certain extent into play; but there must be some fundamental
cause in the constitution of the nervous system in each species.
Natural selection cannot possibly produce any modification in a
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structures of others. But natural selection can and does often produce
structures for the direct injury of other animals, as we see in the
fang of the adder, and in the ovipositor of the ichneumon, by which
its eggs are deposited in the living bodies of other insects. If it
could be proved that any part of the structure of any one species
had been formed for the exclusive good of another species, it would
annihilate my theory, for such could not have been produced through
natural selection. Although many statements may be found in works on
natural history to this effect, I cannot find even one which seems
to me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence, and for the destruction of its
prey; but some authors suppose that at the same time it is furnished
with a rattle for its own injury, namely, to warn its prey. I would
almost as soon believe that the cat curls the end of its tail when
preparing to spring, in order to warn the doomed mouse. It is a much
more probable view that the rattlesnake uses its rattle, the cobra
expands its frill, and the puff-adder swells whilst hissing so
loudly and harshly, in order to alarm the many birds and beasts
which are known to attack even the most venomous species. Snakes act
on the same principle which makes the hen ruffle her feathers and
expand her wings when a dog approaches her chickens; but I have not
space here to enlarge on the many ways by which animals endeavour to
frighten away their enemies.
Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts
solely by and for the good of each. No organ will be formed, as
Paley has remarked, for the purpose of causing pain or for doing an
injury to its possessor. If a fair balance be struck between the
good and evil caused by each part, each will be found on the whole
advantageous. After the lapse of time, under changing conditions of
life, if any part comes to be injurious, it will be modified; or if it
be not so, the being Will become extinct as myriads have become
extinct.
Natural selection tends only to make each organic being as perfect
as, or slightly more perfect than, the other inhabitants of the same
country with which it comes into competition. And we see that this
is the standard of perfection attained under nature. The endemic
productions of New Zealand, for instance, are perfect one compared
with another; but they are now rapidly yielding before the advancing
legions of plants and animals introduced from Europe. Natural
selection will not produce absolute perfection, nor do we always meet,
as far as we can judge, with this high standard under nature. The
correction for the aberration of light is said by Muller not to be
perfect even in that most perfect organ, the human eye. Helmholtz,
whose judgment no one will dispute, after describing in the
strongest terms the wonderful powers of the human eye, adds these
remarkable words: "That which we have discovered in the way of
inexactness and imperfection in the optical machine and in the image
on the retina, is as nothing in comparison with the incongruities
which we have just come across in the domain of the sensations. One
might say that nature has taken delight in accumulating contradictions
in order to remove all foundation from the theory of a pre-existing
harmony between the external and internal worlds." If our reason leads
us to admire with enthusiasm a multitude of inimitable contrivances in
nature, this same reason tells us, though we may easily err on both
sides, that some other contrivances are less perfect. Can we
consider the sting of the bee as perfect, which, when used against
many kinds of enemies, cannot be withdrawn, owing to the backward
serratures, and thus inevitably causes the death of the insect by
tearing out its viscera?
If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so
many members of the same great order, and which has since been
modified but not perfected for its present purpose, with the poison
originally adapted for some other object, such as to produce galls,
since intensified, we can perhaps understand how it is that the use of
the sting should so often cause the insect's own death: for if on
the whole the power of stinging be useful to the social community,
it will fulfil all the requirements of natural selection, though it
may cause the death of some few members. If we admire the truly
wonderful power of scent by which the males of many insects find their
females, can we admire the production for this single purpose of
thousands of drones, which are utterly useless to the community for
any other purpose, and which are ultimately slaughtered by their
industrious and sterile sisters? It may be difficult, but we ought
to admire the savage instinctive hatred of the queen-bee, which
urges her to destroy the young queens, her daughters, as soon as
they are born, or to perish herself in the combat; for undoubtedly
this is for the good of the community; and maternal love or maternal
hatred, though the latter fortunately is most rare, is all the same to
the inexorable principle of natural selection. If we admire the
several ingenious contrivances, by which orchids and many other plants
are fertilised through insect agency, can we consider as equally
perfect the elaboration of dense clouds of pollen by our fir trees, so
that a few granules may be wafted by chance on to the ovules?
Summary: the Law of Unity of Type and of the Conditions of Existence
embraced by the Theory of Natural Selection
We have in this chapter discussed some of the difficulties and
objections which may be urged against the theory. Many of them are
serious; but I think that in the discussion light has been thrown on
several facts, which on the belief of independent acts of creation are
utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural
selection is always very slow, and at any one time acts only on a
few forms; and partly because the very process of natural selection
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a
continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly
graduate away from one part to another. When two varieties are
formed in two districts of a continuous area, an intermediate
variety will often be formed, fitted for an intermediate zone; but
from reasons assigned, the intermediate variety will usually exist
in lesser numbers than the two forms which it connects; consequently
the two latter, during the course of further modification, from
existing in greater numbers, will have a great advantage over the less
numerous intermediate variety, and will thus generally succeed in
supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first only glided through the air.
We have seen that a species under new conditions of life may
change its habits; or it may have diversified habits, with some very
unlike those of its nearest congeners. Hence we can understand,
bearing in mind that each organic being is trying to live wherever
it can live, how it has arisen that there are upland geese with webbed
feet, ground woodpeckers, diving thrushes, and petrels with the habits
of auks.
Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is enough to stagger any one; yet in
the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing
conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural
selection. In the cases in which we know of no intermediate or
transitional states, we should be extremely cautious in concluding
that none can have existed, for the metamorphoses of many organs
show what wonderful changes in function are at least possible. For
instance, a swimbladder has apparently been converted into an
air-breathing lung. The same organ having performed simultaneously
very different functions, and then having been in part or in whole
specialised for one function; and two distinct organs having performed
at the same time the same function, the one having been perfected
whilst aided by the other, must often have largely facilitated
transitions.
We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and
independently formed; but when such organs are closely examined,
essential differences in their structure can almost always be
detected; and this naturally follows from the principle of natural
selection. On the other hand, the common rule throughout nature is
infinite diversity of structure for gaining the same end; and this
again naturally follows from the same great principle.
In many cases we are far too ignorant to be enabled to assert that a
part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly
accumulated by means of natural selection. In many other cases,
modifications are probably the direct result of the laws of
variation or of growth, independently of any good having been thus
gained. But even such structures have often, as we may feel assured,
been subsequently taken advantage of, and still further modified,
for the good of species under new conditions of life. We may, also,
believe that a part formerly of high importance has frequently been
retained (as the tail of an aquatic animal by its terrestrial
descendants), though it has become of such small importance that it
could not, in its present state, have been acquired by means of
natural selection.
Natural selection can produce nothing in one species for the
exclusive good or injury of another; though it may well produce parts,
organs, and excretions highly useful or even indispensable, or again
highly injurious to another species, but in all cases at the same time
useful to the possessor. In each well-stocked country natural
selection acts through the competition of the inhabitants, and
consequently leads to success in the battle for life, only in
accordance with the standard of that particular country. Hence the
inhabitants of one country, generally the smaller one, often yield
to the inhabitants of another and generally the larger country. For in
the larger country there will have existed more individuals and more
diversified forms, and the competition will have been severer, and
thus the standard of perfection will have been rendered higher.
Natural selection will not necessarily lead to absolute perfection;
nor, as far as we can judge by our limited faculties, can absolute
perfection be everywhere predicated.
On the theory of natural selection we can clearly understand the
full meaning of that old canon in natural history, "Natura non facit
saltum." This canon, if we look to the present inhabitants alone of
the world, is not strictly correct; but if we include all those of
past times, whether known or unknown, it must on this theory be
strictly true.
It is generally acknowledged that all organic beings have been
formed on two great laws: Unity of Type, and the Conditions of
Existence. By unity of type is meant that fundamental agreement in
structure which we see in organic beings of the same class, and
which is quite independent of their habits of life. On my theory,
unity of type is explained by unity of descent. The expression of
conditions of existence, so often insisted on by the illustrious
Cuvier, is fully embraced by the principle of natural selection. For
natural selection acts by either now adapting the varying parts of
each being to its organic and inorganic conditions of life; or by
having adapted them during past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being
affected by the direct action of the external conditions of life,
and subjected in all cases to the several laws of growth and
variation. Hence, in fact, the law of the Conditions of Existence is
the higher law; as it includes, through the inheritance of former
variations and adaptations, that of Unity of Type.
CHAPTER VII
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION
I WILL devote this chapter to the consideration of various
miscellaneous objections which have been advanced against my views, as
some of the previous discussions may thus be made clearer; but it
would be useless to discuss all of them, as many have been made by
writers who have not taken the trouble to understand the subject. Thus
a distinguished German naturalist has asserted that the weakest part
of my theory is, that I consider all organic beings as imperfect: what
I have really said is, that all are not as perfect as they might
have been in relation to their conditions; and this is shown to be the
case by so many native forms in many quarters of the world having
yielded their places to intruding foreigners. Nor can organic
beings, even if they were at any one time perfectly adapted to their
conditions of life, have remained so, when their conditions changed,
unless they themselves likewise changed; and no one will dispute
that the physical conditions of each country, as well as the numbers
and kinds of its inhabitants, have undergone many mutations.
A critic has lately insisted, with some parade of mathematical
accuracy, that longevity is a great advantage to all species, so
that he who believes in natural selection "must arrange his
genealogical tree" in such a manner that all the descendants have
longer lives than their progenitors! Cannot our critic conceive that a
biennial plant or one of the lower animals might range into a cold
climate and perish there every winter; and yet, owing to advantages
gained through natural selection, survive from year to year by means
of its seeds or ova? Mr. E. Ray Lankester has recently discussed
this subject, and he concludes, as far as its extreme complexity
allows him to form a judgment, that longevity is generally related
to the standard of each species in the scale of organisation, as
well as to the amount of expenditure in reproduction and in general
activity. And these conditions have, it is probable, been largely
determined through natural selection.
It has been argued that, as none of the animals and plants of Egypt,
of which we know anything, have changed during the last three or
four thousand years, so probably have none in any part of the world.
But, as Mr. G. H. Lewes has remarked, this line of argument proves too
much, for the ancient domestic races figured on the Egyptian
monuments, or embalmed, are closely similar or even identical with
those now living; yet all naturalists admit that such races have
been produced through the modification of their original types. The
many animals which have remained unchanged since the commencement of
the glacial period, would have been an incomparably stronger case, for
these have been exposed to great changes of climate and have
migrated over great distances; whereas, in Egypt, during the last
several thousand years, the conditions of life, as far as we know,
have remained absolutely uniform. The fact of little or no
modification having been effected since the glacial period would
have been of some avail against those who believe in an innate and
necessary law of development, but is powerless against the doctrine of
natural selection or the survival of the fittest, which implies that
when variations or individual differences of a beneficial nature
happen to arise, these will be preserved; but this will be effected
only under certain favourable circumstances.
The celebrated palaeontologist, Bronn, at the close of his German
translation of this work, asks, how, on the principle of natural
selection, can a variety live side by side with the parent species? If
both have become fitted for slightly different habits of life or
conditions, they might live together; and if we lay on one side
polymorphic species, in which the variability seems to be of a
peculiar nature, and all mere temporary variations, such as size,
albinism, &c., the more permanent varieties are generally found, as
far as I can discover, inhabiting distinct stations,- such as high
land or low land, dry or moist districts. Moreover, in the case of
animals which wander much about and cross freely, their varieties seem
to be generally confined to distinct regions.
Bronn also insists that distinct species never differ from each
other in single characters, but in many parts; and he asks, how it
always comes that many parts of the organisation should have been
modified at the same time through variation and natural selection .
" But there is no necessity for supposing that all the parts of any
being have been simultaneously modified. The most striking
modifications, excellently adapted for some purpose, might, as was
formerly remarked, be acquired by successive variations, if slight,
first in one part and then in another; and as they would be
transmitted all together, they would appear to us as if they had
been simultaneously developed. The best answer, however, to the
above objection is afforded by those domestic races which have been
modified, chiefly through man's power of selection, for some special
purpose. Look at the race and dray horse, or at the greyhound and
mastiff. Their whole frames and even their mental characteristics have
been modified; but if we could trace each step in the history of their
transformation,- and the latter steps can be traced,- we should not
see great and simultaneous changes, but first one part and then
another slightly modified and improved. Even when selection has been
applied by man to some one character alone,- of which our cultivated
plants offer the best instances,- it will invariably be found that
although this one part, whether it be the flower, fruit, or leaves,
has been greatly changed, almost all the other parts have been
slightly modified. This may be attributed partly to the principle of
correlated growth, and partly to so-called spontaneous variation.
A much more serious objection has been urged by Bronn, and
recently by Broca, namely, that many characters appear to be of no
service whatever to their possessors, and therefore cannot have been
influenced through natural selection. Bronn adduces the length of
the ears and tails in the different species of hares and mice,- the
complex folds of enamel in the teeth of many animals, and a
multitude of analogous cases. With respect to plants, this subject has
been discussed by Nageli in an admirable essay. He admits that natural
selection has effected much, but he insists that the families of
plants differ chiefly from each other in morphological characters,
which appear to be quite unimportant for the welfare of the species.
He consequently believes in an innate tendency towards progressive and
more perfect development. He specifies the arrangement of the cells in
the tissues, and of the leaves on the axis, as cases in which
natural selection could not have acted. To these may be added the
numerical divisions in the parts of the flower, the position of the
ovules, the shape of the seed, when not of any use for
dissemination, &c.
There is much force in the above objection. Nevertheless, we
ought, in the first place, to be extremely cautious in pretending to
decide what structures now are, or have formerly been, use to each
species. In the second place, it should always be borne in mind that
when part is modified, so will be other parts, through certain dimly
seen causes, such as an increased or diminished flow of nutriment to a
part, mutual pressure, an early developed part affecting one
subsequently developed, and so forth,- as well as through other causes
which lead to the many mysterious cases of correlation, which we do
not in the least understand. These agencies may be all grouped
together, for the sake of brevity, under the expression of the laws of
growth. In the third place, we have to allow for the direct and
definite action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the conditions
apparently plays a quite subordinate part. Bud-variations, such as the
appearance of a moss-rose on a common rose, or of a nectarine on a
peach tree offer good instances of spontaneous variations; but even in
these cases, if we bear in mind the power of a minute drop of poison
in producing complex galls, we ought not to feel too sure that the
above variations are not the effect of some local change in the nature
of the sap, due to some change in the conditions. There must be some
efficient cause for each slight individual difference, as well as
for more strongly marked variations which occasionally arise; and if
the unknown cause were to act persistently, it is almost certain
that all the individuals of the species would be similarly modified.
In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to
spontaneous variability. But it is impossible to attribute to this
cause the innumerable structures which are so well adapted to the
habits of life of each species. I can no more believe in this than
that the well-adapted form of a race-horse or greyhound, which
before the principle of selection by man was well understood,
excited so much surprise in the minds of the older naturalists, can
thus be explained.
It may be worth while to illustrate some of the foregoing remarks.
With respect to the assumed inutility of various parts and organs,
it is hardly necessary to observe that even in the higher and
best-known animals many structures exist, which are so highly
developed that no one doubts that they are of importance, yet their
use has not been, or has only recently been, ascertained. As Bronn
gives the length of the ears and tail in the several species of mice
as instances, though trifling ones, of differences in structure
which can be of no special use, I may mention that, according to Dr.
Schobl, the external ears of the common mouse are supplied in an
extraordinary manner with nerves, so that they no doubt serve as
tactile organs; hence the length of the ears can hardly be quite
unimportant. We shall, also, presently see that the tail is a highly
useful prehensile organ to some of the species; and its use would be
much influenced by its length.
With respect to plants, to which on account of Nageli's essay I
shall confine myself in the following remarks, it will be admitted
that the flowers of orchids present a multitude of curious structures,
which a few years ago would have been considered as mere morphological
differences without any special function; but they are now known to be
of the highest importance for the fertilisation of the species through
the aid of insects, and have probably been gained through natural
selection. No one until lately would have imagined that in dimorphic
and trimorphic plants the different lengths of the stamens and
pistils, and their arrangement, could have been of any service, but
now we know this to be the case.
In certain whole groups of plants the ovules stand erect, and in
others they are suspended; and within the same ovarium of some few
plants, one ovule holds the former and a second ovule the latter
position. These positions seem at first purely morphological, or of no
physiological signification; but Dr. Hooker informs me that within the
same ovarium, the upper ovules alone in some cases, and in other cases
the lower ones alone are fertilised; and he suggests that this
probably depends on the direction in which the pollen-tubes enter
the ovarium. If so, the position of the ovules, even when one is erect
and the other suspended within the same ovarium, would follow from the
selection of any slight deviations in position which favoured their
fertilisation, and the production of seed.
Several plants belonging to distinct orders habitually produce
flowers of two kinds,- the one open of the ordinary structure, the
other closed and imperfect. These two kinds of flowers sometimes
differ wonderfully in structure, yet may be seen to graduate into each
other on the same plant. The ordinary and open flowers can be
intercrossed; and the benefits which certainly are derived from this
process are thus secured. The closed and imperfect flowers are,
however, manifestly of high importance, as they yield with the
utmost safety a large stock of seed, with the expenditure of
wonderfully little pollen. The two kinds of flowers often differ much,
as just stated, in structure. The petals in the imperfect flowers
almost always consist of mere rudiments, and the pollen-grains are
reduced in diameter. In Ononis columnae five of the alternate
stamens are rudimentary; and in some species of Viola three stamens
are in this state, two retaining their proper function, but being of
very small size. In six out of thirty of the closed flowers in an
Indian violet (name unknown, for the plants have never produced with
me perfect flowers), the sepals are reduced from the normal number
of five to three. In one section of the Malpighiaceae the closed
flowers, according to A. de Jussieu, are still further modified, for
the five stamens which stand opposite to the sepals are all aborted,
sixth stamen standing opposite to a petal being alone developed; and
this stamen is not present in the ordinary flowers of these species;
the style is aborted; and the ovaria are reduced from three to two.
Now although natural selection may well have had the power to
prevent some of the flowers from expanding, and to reduce the amount
of pollen, when rendered by the closure of the flowers superfluous,
yet hardly any of the above special modifications can have been thus
determined, but must have followed from the laws of growth,
including the functional inactivity of parts, during the progress of
the reduction of the pollen and the closure of the flowers.
It is so necessary to appreciate the important effects of the laws
of growth, that I will give some additional cases of another kind,
namely of differences in the same part or organ, due to differences in
relative position on the same plant. In the Spanish chestnut, and in
certain fir-trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the upright
branches. In the common rue and some other plants, one flower, usually
the central or terminal one, opens first, and has five sepals and
petals, and five divisions to the ovarium; whilst all the other
flowers on the plant are tetramerous. In the British Adoxa the
uppermost flower generally has two calyx-lobes with the other organs
tetramerous, whilst the surrounding flowers generally have three
calyx-lobes with the other organs pentamerous. In many Compositae
and Umbelliferae (and in some other plants) the circumferential
flowers have their corollas much more developed than those of the
centre; and this seems often connected with the abortion of the
reproductive organs. It is a more curious fact, previously referred
to, that the achenes or seeds of the circumference and centre
sometimes differ greatly in form, colour, and other characters. In
Carthamus and some other Compositae the central achenes alone are
furnished with a pappus; and in Hyoseris the same head yields
achenes of three different forms. In certain Umbelliferae the exterior
seeds, according to Tausch, are orthospermous, and the central one
coelospermous, and this is a character which was considered by De
Candolle to be in other species of the highest systematic
importance. Prof. Braun mentions a Fumariaceous genus, in which the
flowers in the lower part of the spike bear oval, ribbed, one-seeded
nutlets; and in the upper part of the spike, lanceolate, two-valved,
and two-seeded siliques. In these several cases, with the exception of
that of the well developed rayflorets, which are of service in
making the flowers conspicuous to insects, natural selection cannot,
as far as we can judge, have come into play, or only in a quite
subordinate manner. All these modifications follow from the relative
position and inter-action of the parts; and it can hardly be doubted
that if all the flowers and leaves on the same plant had been
subjected to the same external and internal condition, as are the
flowers and leaves in certain positions, all would have been
modified in the same manner.
In numerous other cases we find modifications of structure, which
are considered by botanists to be generally of a highly important
nature, affecting only some of the flowers on the same plant, or
occurring on distinct plants, which grow close together under the same
conditions. As these variations seem of no special use to the
plants, they cannot have been influenced by natural selection. Of
their cause we are quite ignorant; we cannot even attribute them, as
in the last class of cases, to any proximate agency, such as
relative position. I will give only a few instances. It is so common
to observe on the same plant, flowers indifferently tetramerous,
pentamerous, &c., that I need not give examples; but as numerical
variations are comparatively rare when the parts are few, I may
mention that, according to De Candolle, the flowers of Papaver
bracteatum offer either two sepals with four petals (which is the
common type with poppies), or three sepals with six petals. The manner
in which the petals are folded in the bud is in most groups a very
constant morphological character; but Professor Asa Gray states that
with some species of Mimulus, the aestivation is almost as
frequently that of the Rhinanthideae as of the Antirrhinideae, to
which latter tribe the genus belongs. Auguste de Saint-Hilaire gives
the following cases: the genus Zanthoxylon belongs to a division of
the Rutacese with a single ovary, but in some species flowers may be
found on the same plant, and even in the same panicle, with either one
or two ovaries. In Helianthemum the capsule has been described as
unilocular or trilocular; and in H. mutabile, "Une lame, plus ou moins
large, s'etend entre le pericarpe et le placenta." In the flowers of
Saponaria officinalis, Dr. Masters has observed instances of both
marginal and free central placentation. Lastly, Saint-Hilaire found
towards the southern extreme of the range of Gomphia oleaeformis two
forms which he did not at first doubt were distinct species, but he
subsequently saw them growing on the same bush; and he then adds,
"Voila donc dans un meme individu des loges et un style qui se
rattachent tantot a un axe verticale et tantot a un gynobase."
We thus see that with plants many morphological changes may be
attributed to the laws of growth and the inter-action of parts,
independently of natural selection. But with respect to Nageli's
doctrine of an innate tendency towards perfection or progressive
development, can it be said in the case of these strongly pronounced
variations, that the plants have been caught in the act of progressing
towards a higher state of development? On the contrary, I should infer
from the mere fact of the parts in question differing or varying
greatly on the same plant, that such modifications were of extremely
small importance to the plants themselves, of whatever importance they
may generally be to us for our classifications. The acquisition of a
useless part can hardly be said to raise an organism in the natural
scale; and in the case of the imperfect, closed flowers above
described, if any new principle has to be invoked, it must be one of
retrogression rather than of progression; and so it must be with
many parasitic and degraded animals. We are ignorant of the exciting
cause of the above specified modifications; but if the unknown cause
were to act almost uniformly for a length of time, we may infer that
the result would be almost uniform; and in this case all the
individuals of the species would be modified in the same manner.
From the fact of the above characters being unimportant for the
welfare of the species, any slight variations which occurred in them
would not have been accumulated and augmented through natural
selection. A structure which has been developed through long-continued
selection, when it ceases to be of service to a species, generally
becomes variable, as we see with rudimentary organs; for it will no
longer be regulated by this same power of selection. But when, from
the nature of the organism and of the conditions, modifications have
been induced which are unimportant for the welfare of the species,
they may be, and apparently often have been, transmitted in nearly the
same state to numerous, otherwise modified, descendants. It cannot
have been of much importance to the greater number of mammals,
birds, or reptiles, whether they were clothed with hair, feathers,
or scales; yet hair has been transmitted to almost all mammals,
feathers to all birds, and scales to all true reptiles. A structure,
whatever it may be, which is common to many allied forms, is ranked by
us as of high systematic importance, and consequently is often assumed
to be of high vital importance to the species. Thus, as I am
inclined to believe, differences, which we consider as important- such
as the arrangement of the leaves, the divisions of the flower or of
the ovarium, the position of the ovules, &c.- first appeared in many
cases as fluctuating variations, which sooner or later became constant
through the nature of the organism and of the surrounding
conditions, as well as through the intercrossing of distinct
individuals, but not through natural selection; for as these
morphological characters do not affect the welfare of the species, any
slight deviations in them could not have been governed or
accumulated through this latter agency. It is a strange result which
we thus arrive at, namely that characters of slight vital importance
to the species, are the most important to the systematist; but, as
we shall hereafter see when we treat of the genetic principle of
classification, this is by no means so paradoxical as it may at
first appear.
Although we have no good evidence of the existence in organic beings
of an innate tendency towards progressive development, yet this
necessarily follows, as I have attempted to show in the fourth
chapter, through the continued action of natural selection. For the
best definition which has ever been given of a high standard of
organisation, is the degree to which the parts have been specialised
or differentiated; and natural selection tends towards this end,
inasmuch as the parts are thus enabled to perform their functions more
efficiently.
A distinguished zoologist, Mr. St. George Mivart, has recently
collected all the objections which have ever been advanced by myself
and others against the theory of natural selection, as propounded by
Mr. Wallace and myself, and has illustrated them with admirable art
and force. When thus marshalled, they make a formidable array; and
as it forms no part of Mr. Mivart's plan to give the various facts and
considerations opposed to his conclusions, no slight effort of
reason and memory is left to the reader, who may wish to weigh the
evidence on both sides. When discussing special cases, Mr. Mivart
passes over the effects of the increased use and disuse of parts,
which I have always maintained to be highly important, and have
treated in my Variation under Domestication at greater length than, as
I believe, any other writer. He likewise often assumes that I
attribute nothing to variation, independently of natural selection,
whereas in the work just referred to I have collected a greater number
of well-established cases than can be found in any other work known to
me. My judgment may not be trustworthy, but after reading with care
Mr. Mivart's book, and comparing each section with what I have said on
the same head, I never before felt so strongly convinced of the
general truth of the conclusions here arrived at, subject, of
course, in so intricate a subject, to much partial error.
All Mr. Mivart's objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, "that natural selection is incompetent to account for
the incipient stages of useful structures." This subject is intimately
connected with that of the gradation of characters, often
accompanied by a change of function,- for instance, the conversion
of a swimbladder into lungs,- points which were discussed in the
last chapter under two headings. Nevertheless, I will here consider in
some detail several of the cases advanced by Mr. Mivart, selecting
those which are the most illustrative, as want of space prevents me
from considering all.
The giraffe, by its lofty stature, much elongated neck, fore-legs,
head and tongue, has its whole frame beautifully adapted for
browsing on the higher branches of trees. It can thus obtain food
beyond the reach of the other Ungulata or hoofed animals inhabiting
the same country; and this must be a great advantage to it during
dearths. The Niata cattle in S. America show us how small a difference
in structure may make, during such periods, a great difference in
preserving an animal's life. These cattle can browse as well as others
on grass, but from the projection of the lower jaw they cannot, during
the often recurrent droughts, browse on the twigs of trees, reeds,
&c., to which food the common cattle and horses are then driven; so
that at these times the Niatas perish, if not fed by their owners.
Before coming to Mr. Mivart's objections, it may be well to explain
once again how natural selection will act in all ordinary cases. Man
has modified some of his animals, without necessarily having
attended to special points of structure, by simply preserving and
breeding from the fleetest individuals, as with the race-horse and
greyhound, or as with the game-cock, by breeding from the victorious
birds. So under nature with the nascent giraffe the individuals
which were the highest browsers, and were able during dearths to reach
even an inch or two above the others, will often have been
preserved; for they will have roamed over the whole country in
search of food. That the individuals of the same species often
differ slightly in the relative lengths of all their parts may be seen
in many works of natural history, in which careful measurements are
given. These slight proportional differences, due to the laws of
growth and variation, are not of the slightest use or importance to
most species. But it will have been otherwise with the nascent
giraffe, considering its probable habits of life; for those
individuals which had some one part or several parts of their bodies
rather more elongated than usual, would generally have survived. These
will have intercrossed and left offspring, either inheriting the
same bodily peculiarities, or with a tendency to vary again in the
same manner; whilst the individuals, less favoured in the same
respects, will have been the most liable to perish.
We here see that there is no need to separate single pairs, as man
does, when he methodically improves a breed: natural selection will
preserve and thus separate all the superior individuals, allowing them
freely to intercross, and will destroy all the inferior individuals.
By this process long-continued, which exactly corresponds with what
I have called unconscious selection by man, combined no doubt in a
most important manner with the inherited effects of the increased
use of parts, it seems to me almost certain that an ordinary hoofed
quadruped might be converted into a giraffe.
To this conclusion Mr. Mivart brings forward two objections. One
is that the increased size of the body would obviously require an
increased supply of food, and he considers it as "very problematical
whether the disadvantages thence arising would not, in times of
scarcity, more than counterbalance the advantages." But as the giraffe
does actually exist in large numbers in S. Africa, and as some of
the largest antelopes in the world, taller than an ox, abound there,
why should we doubt that, as far as size is concerned, intermediate
gradations could formerly have existed there, subjected as now to
severe dearths. Assuredly the being able to reach, at each stage of
increased size, to a supply of food, left untouched by the other
hoofed quadrupeds of the country, would have been of some advantage to
the nascent giraffe. Nor must we overlook the fact, that increased
bulk would act as a protection against almost all beasts of prey
excepting the lion; and against this animal, its tall neck,- and the
taller the better,- would, as Mr. Chauncey Wright has remarked,
serve as a watch-tower. It is from this cause, as Sir S. Baker
remarks, that no animal is more difficult to stalk than the giraffe.
This animal also uses its long neck as a means of offence or
defence, by violently swinging his head armed with stump-like horns.
The preservation of each species can rarely be determined by any one
advantage, but by the union of all, great and small.
Mr. Mivart then asks (and this is his second objection), if
natural selection be so potent, and if high browsing be so great an
advantage, why has not any other hoofed quadruped acquired a long neck
and lofty stature, besides the giraffe, and, in a lesser degree, the
camel, guanaeo, and macrauchenia? Or, again, why has not any member of
the group acquired a long proboscis? With respect to S. Africa,
which was formerly inhabited by numerous herds of the giraffe, the
answer is not difficult, and can best be given by an illustration.
In every meadow in England in which trees grow, we see the lower
branches trimmed or planed to an exact level by the browsing of the
horses or cattle; and what advantage would it be, for instance, to
sheep, if kept there, to acquire slightly longer necks? In every
district some one kind of animal will almost certainly be able to
browse higher than the others; and it is almost equally certain that
this one kind alone could have its neck elongated for this purpose,
through natural selection and the effects of increased use. In S.
Africa the competition for browsing on the higher branches of the
acacias and other trees must be between giraffe and giraffe, and not
with the other ungulate animals.
Why, in other quarters of the world, various animals belonging to
this same order have not acquired either an elongated neck or a
proboscis, cannot be distinctly answered; but it is as unreasonable to
expect a distinct answer to such a question, as why some event in
the history of mankind did not occur in one country, whilst it did
in another. We are ignorant with respect to the conditions which
determine the numbers and range of each species; and we cannot even
conjecture what changes of structure would be favourable to its
increase in some new country. We can, however, see in a general manner
that various causes might have interfered with the development of a
long neck or proboscis. To reach the foliage at a considerable
height (without climbing, for which hoofed animals are singularly
ill-constructed) implies greatly increased bulk of body; and we know
that some areas support singularly few large quadrupeds, for
instance S. America, though it is so luxuriant; whilst S. Africa
abounds with them to an unparalleled degree. Why this should be so, we
do not know; nor why the later tertiary periods should have been so
much more favourable for their existence than the present time.
Whatever the causes may have been, we can see that certain districts
and times would have been much more favourable than others for the
development of so large a quadruped as the giraffe.
In order that an animal should acquire some structure specially
and largely developed, it is almost indispensable that several other
parts should be modified and co-adapted. Although every part of the
body varies slightly, it does not follow that the necessary parts
should always vary in the right direction and to the right degree.
With the different species of our domesticated animals we know that
the parts vary in a different manner and degree; and that some species
are much more variable than others. Even if the fitting variations did
arise, it does not follow that natural selection would be able to
act on them, and produce a structure which apparently would be
beneficial to the species. For instance, if the number of
individuals existing in a country is determined chiefly through
destruction by beasts of prey,- by external or internal parasites,
&c.,- as seems often to be the case, then natural selection will be
able to do little, or will be greatly retarded, in modifying any
particular structure for obtaining food. Lastly, natural selection
is a slow process, and the same favourable conditions must long endure
in order that any marked effect should thus be produced. Except by
assigning such general and vague reasons, we cannot explain why, in
many quarters of the world, hoofed quadrupeds have not acquired much
elongated necks or other means for browsing on the higher branches
of trees.
Objections of the same nature as the foregoing have been advanced by
man writers. In each case various causes, besides the general ones
just indicated, have probably interfered with the acquisition
through natural selection of structures, which it is thought would
be beneficial to certain species. One writer asks, why has not the
ostrich acquired the power of flight? But a moment's reflection will
show what an enormous supply of food would be necessary to give to
this bird of the desert force to move its huge body through the air.
Oceanic islands are inhabited by bats and seals, but by no terrestrial
mammals; yet as some of these bats are peculiar species, they must
have long inhabited their present homes. Therefore Sir C. Lyell
asks, and assigns certain reasons in answer, why have not seals and
bats given birth on such islands to forms fitted to live on the
land? But seals would necessarily be first converted into
terrestrial carnivorous animals of considerable size, and bats into
terrestrial insectivorous animals; for the former there would be no
prey; for the bats ground-insects would serve as food, but these would
already be largely preyed on by the reptiles or birds, which first
colonise and abound on most oceanic islands. Gradations of
structure, with each stage beneficial to a changing species, will be
favoured only under certain peculiar conditions. A strictly
terrestrial animal, by occasionally hunting for food in shallow water,
then in streams or lakes, might at last be converted into an animal so
thoroughly aquatic as to brave the open ocean. But seals would not
find on oceanic islands the conditions favourable to their gradual
reconversion into a terrestrial form. Bats, as formerly shown,
probably acquired their wings by at first gliding through the air from
tree to tree, like the so-called flying squirrels, for the sake of
escaping from their enemies, or for avoiding falls; but when the power
of true flight had once been acquired, it would never be reconverted
back, at least for the above purposes, into the less efficient power
of gliding through the air. Bats might, indeed, like many birds,
have had their wings greatly reduced in size, or completely lost,
through disuse; but in this case it would be necessary that they
should first have acquired the power of running quickly on the ground,
by the aid of their hind legs alone, so as to compete with birds or
other ground animals; and for such a change a bat seems singularly
ill-fitted. These conjectural remarks have been made merely to show
that a transition of structure, with each step beneficial, is a highly
complex affair; and that there is nothing strange in a transition
not having occurred in any particular case.
Lastly, more than one writer has asked, why have some animals had
their mental powers more highly developed than others, as such
development would be advantageous to an? Why have not apes acquired
the intellectual powers of man? Various causes could be assigned;
but as they are conjectural, and their relative probability cannot
be weighed, it would be useless to give them. A definite answer to the
latter question ought not to be expected, seeing that no one can solve
the simpler problem why, of two races of savages, one has risen higher
in the scale of civilisation than the other; and this apparently
implies increased brain-power.
We will return to Mr. Mivart's other objections. Insects often
resemble for the sake of protection various objects, such as green
or decayed leaves, dead twigs, bits of lichen, flowers, spines,
excrement of birds, and living insects; but to this latter point I
shall hereafter recur. The resemblance is often wonderfully close, and
is not confined to colour, but extends to form, and even to the manner
in which the insects hold themselves. The caterpillars which project
motionless like dead twigs from the bushes on which they feed, offer
an excellent instance of a resemblance of this kind. The cases of
the imitation of such objects as the excrement of birds, are rare
and exceptional. On this head, Mr. Mivart remarks, "As, according to
Mr. Darwin's theory, there is a constant tendency to indefinite
variation, and as the minute incipient variations will be in all
directions, they must tend to neutralise each other, and at first to
form such unstable modifications that it is difficult, if not
impossible, to see how such indefinite oscillations of infinitesimal
beginnings can ever build up a sufficiently appreciable resemblance to
a leaf, bamboo, or other object, for Natural Selection to seize upon
and perpetuate."
But in all the foregoing cases the insects in their original state
no doubt presented some rude and accidental resemblance to an object
commonly found in the stations frequented by them. Nor is this at
all improbable, considering the almost infinite number of
surrounding objects and the diversity in form and colour of the
hosts of insects which exist. As some rude resemblance is necessary
for the first start, we can understand how it is that the larger and
higher animals do not (with the exception, as far as I know, of one
fish) resemble for the sake of protection special objects, but only
the surface which commonly surrounds them, and this chiefly in colour.
Assuming that an insect originally happened to resemble in some degree
a dead twig or a decayed leaf, and that it varied slightly in many
ways, then all the variations which rendered the insect at all more
like any such object, and thus favoured its escape, would be
preserved, whilst other variations would be neglected and ultimately
lost; or, if they rendered the insect at all less like the imitated
object, they would be eliminated. There would indeed be force in Mr.
Mivart's objection, if we were to attempt to account for the above
resemblances, independently of natural selection, through mere
fluctuating variability; but as the case stands there is none.
Nor can I see any force in Mr. Mivart's difficulty with respect to
"the last touches of perfection in the mimicry"; as in the case
given by Mr. Wallace, of a walking-stick insect (Ceroxylus laceratus),
which resembles "a stick grown over by a creeping moss or
jungermannia." So close was this resemblance, that a native Dyak
maintained that the foliaceous excrescences were really moss.
Insects are preyed on by birds and other enemies, whose sight is
probably sharper than ours, and every grade in resemblance which aided
an insect to escape notice or detection, would tend towards its
preservation; and the more perfect the resemblance so much the
better for the insect. Considering the nature of the differences
between the species in the group which includes the above Ceroxylus,
there is nothing improbable in this insect having varied in the
irregularities on its surface, and in these having become more or less
green-coloured; for in every group the characters which differ in
the several species are the most apt to vary, whilst the generic
characters, or those common to all the species, are the most constant.
The Greenland whale is one of the most wonderful animals in the
world, and the baleen, or whale-bone, one of its greatest
peculiarities. The baleen consists of a row, on each side of the upper
jaw, of about 300 plates or laminae, which stand close together
transversely to the longer axis of the mouth. Within the main row
there are some subsidiary rows. The extremities and inner margins of
all the plates are frayed into stiff bristles, which clothe the
whole gigantic palate, and serve to strain or sift the water, and thus
to secure the minute prey on which these great animals subsist. The
middle and longest lamina in the Greenland whale is ten, twelve, or
even fifteen feet in length; but in the different species of cetaceans
there are gradations in length; the middle lamina being in one
species, according to Scoresby, four feet, in another three, in
another eighteen inches, and in the Balaenoptera rostrata only about
nine inches in length. The quality of the whale-bone also differs in
the different species.
With respect to the baleen, Mr. Mivart remarks that if it "had
once attained such a size and development as to be at all useful, then
its preservation and augmentation within serviceable limits would be
promoted by natural selection alone. But how to obtain the beginning
of such useful development?" In answer, it may be asked, why should
not the early progenitors of the whales with baleen have possessed a
mouth constructed something like the lamellated beak of a duck? Ducks,
like whales, subsist by sifting the mud and water; and the family
has sometimes been called Criblatores, or sifters. I hope that I may
not be misconstrued into saying that the progenitors of whales did
actually possess mouths lamellated like the beak of a duck. I wish
only to show that this is not incredible, and that the immense
plates of baleen in the Greenland whale might have been developed from
such lamellae by finely graduated steps, each of service to its
possessor.
The beak of a shoveller-duck (Spatula elypedta) is a more
beautiful and complex structure than the mouth of a whale. The upper
mandible is furnished on each side (in the specimen examined by me)
with a row or comb formed of 188 thin, elastic lamellae, obliquely
bevelled so as to be pointed, and placed transversely to the longer
axis of the mouth. They arise from the palate, and are attached by
flexible membrane to the sides of the mandible. Those standing towards
the middle are the longest, being about one-third of an inch in
length, and they project .14 of an inch beneath the edge. At their
bases there is a short subsidiary row of obliquely transverse
lamellae. In these several respects they resemble the plates of baleen
in the mouth of a whale. But towards the extremity of the beak they
differ much, as they project inwards, instead of straight downwards.
The entire head of the shoveller, though incomparably less bulky, is
about one-eighteenth of the length of the head of a moderately large
Balaenoptera rostrata, in which species the baleen is only nine inches
long; so that if we were to make the head of the shoveller as long
as that of the Balaenoptera, the lamellae would be six inches in
length,- that is, two-thirds of the length of the baleen in this
species of whale. The lower mandible of the shoveller-duck is
furnished with lamellae of equal length with those above, but finer;
and in being thus furnished it differs conspicuously from the lower
jaw of a whale, which is destitute of baleen. On the other hand the
extremities of these lower lamellae are frayed into fine bristly
points, so that they thus curiously resemble the plates of baleen.
In the genus Prion, a member of the distinct family of the petrels,
the upper mandible alone is furnished with lamellae, which are well
developed and project beneath the margin; so that the beak of this
bird resembles in this respect the mouth of a whale.
From the highly developed structure of the shoveller's beak we may
proceed (as I have learnt from information and specimens sent to me by
Mr. Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some
respects through that of the Aix sponsa, to the beak of the common
duck. In this latter species, the lamellae are much coarser than in
the shoveller, and are firmly attached to the sides of the mandible;
they are only about 50 in number on each side, and do not project at
all beneath the margin. They are square-topped, and are edged with
translucent hardish tissue, as if for crushing food. The edges of
the lower mandible are crossed by numerous fine ridges, which
project very little. Although the beak is thus very inferior as a
sifter to that of the shoveller, yet this bird, as every one knows,
constantly uses it for this purpose. There are other species, as I
hear from Mr. Salvin, in which the lamellae are considerably less
developed than in the common duck; but I do not know whether they
use their beaks for sifting the water.
Turning to another group of the same family: in the Egyptian goose
(Chenalopex) the beak closely resembles that of the common ducks;
but the lamellae are not so numerous, nor so distinct from each other,
nor do they project so much inwards; yet this goose, as I am
informed by Mr. E. Bartlett, "uses its bill like a duck by throwing
the water out at the corners." Its chief food, however, is grass,
which it crops like the common goose. In this latter bird, the
lamellae of the upper mandible are much coarser than in the common
duck, almost confluent, about 27 in number on each side, and
terminating upwards in teeth-like knobs. The palate is also covered
with hard rounded knobs. The edges of the lower mandible are
serrated with teeth much more prominent, coarser, and sharper than
in the duck. The common goose does not sift the water, but uses its
beak exclusively for tearing or cutting herbage, for which purpose
it is so well fitted, that it can crop grass closer than almost any
other animal. There are other species of geese, as I hear from Mr.
Bartlett, in which the lamellae are less developed than in the
common goose.
We thus see that a member of the duck family, with a beak
constructed like that of the common goose and adapted solely for
grazing, or even a member with a beak having less well-developed
lamellae, might be converted by small changes into a species like
the Egyptian goose,- this into one like the common duck,- and, lastly,
into one like the shoveller, provided with a beak almost exclusively
adapted for sifting the water; for this bird could hardly use any part
of its beak, except the hooked tip, for seizing or tearing solid food.
The beak of a goose, as I may add, might also be converted by small
changes into one provided with prominent, recurved teeth, like those
of the merganser (a member of the same family), serving for the widely
different purpose of securing live fish.
Returning to the whales: the Hyperoodon bidens is destitute of
true teeth in an efficient condition, but its palate is roughened,
according to Lacepide, with small, unequal, hard points of horn. There
is, therefore, nothing improbable in supposing that some early
cetacean form was provided with similar points of horn on the
palate, but rather more regularly placed, and which, like the knobs on
the beak of the goose, aided it in seizing or tearing its food. If so,
it will hardly be denied that the points might have been converted
through variation and natural selection into lamellae as well
developed as those of the Egyptian goose, in which case they would
have been used both for seizing objects and for sifting the water;
then into lamellae like those of the domestic duck; and so onwards,
until they became as well constructed as those of the shoveller, in
which case they would have served exclusively as a sifting
apparatus. From this stage, in which the lamellae would be
two-thirds of the length of the plates of baleen in the Balaenoptera
rostrata, gradations, which may be observed in still-existing
cetaceans, lead us onwards to the enormous plates of baleen in the
Greenland whale. Nor is there the least reason to doubt that each step
in this scale might have been as serviceable to certain ancient
cetaceans, with the functions of the parts slowly changing during
the progress of development, as are the gradations in the beaks of the
different existing members of the duck family. We should bear in
mind that each species of duck is subjected to a severe struggle for
existence, and that the structure of every part of its frame must be
well adapted to its conditions of life.
The Pleuronectidae, or flat-fish, are remarkable for their
asymmetrical bodies. They rest on one side,- in the greater number
of species on the left, but in some on the right side; and
occasionally reversed adult specimens occur. The lower, or
resting-surface, resembles at first sight the ventral surface of an
ordinary fish: it is of a white colour, less developed in many ways
than the upper side, with the lateral fins often of smaller size.
But the eyes offer the most remarkable peculiarity; for they are
both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then
symmetrical, with both sides equally coloured. Soon the eye proper
to the lower side begins to glide slowly round the head to the upper
side; but does not pass right through the skull, as was formerly
thought to be the case. It is obvious that unless the lower eye did
thus travel round, it could not be used by the fish whilst lying in
its habitual position on one side. The lower eye would, also, have
been liable to be abraded by the sandy bottom. That the Pleuronectidae
are admirably adapted by their flattened and asymmetrical structure
for their habits of life, is manifest from several species, such as
soles, flounders, &c., being extremely common. The chief advantages
thus gained seem to be protection from their enemies, and facility for
feeding on the ground. The different members, however, of the family
present, as Schiodte remarks, "a long series of forms exhibiting a
gradual transition from Hippoglossus pinguis, which does not in any
considerable degree alter the shape in which it leaves the ovum, to
the soles, which are entirely thrown to one side."
Mr. Mivart has taken up this case, and remarks that a sudden
spontaneous transformation in the position of the eyes is hardly
conceivable, in which I quite agree with him. He then adds: "If the
transit was gradual, then how such transit of one eye a minute
fraction of the journey towards the other side of the head could
benefit the individual is, indeed, far from clear. It seems, even,
that such an incipient transformation must rather have been
injurious." But he might have found an answer to this objection in the
excellent observations published in 1867 by Malm. The Pleuronectidae
whilst very young and still symmetrical, with their eyes standing on
opposite sides of the head, cannot long retain a vertical position,
owing to the excessive depth of their bodies, the small size of
their lateral fins, and to their being destitute of a swimbladder.
Hence soon growing tired, they fall to the bottom on one side.
Whilst thus at rest they often twist, as Malm observed, the lower
eye upwards, to see above them; and they do this so vigorously that
the eye is pressed hard against the upper part of the orbit. The
forehead between the eyes consequently becomes, as could be plainly
seen, temporarily contracted in breadth. On one occasion Malm saw a
young fish raise and depress the lower eye through an angular distance
of about seventy degrees.
We should remember that the skull at this early age is cartilaginous
and flexible, so that it readily yields to muscular action. It is also
known with the higher animals, even after early youth, that the
skull yields and is altered in shape, if the skin or muscles be
permanently contracted through disease or some accident. With
long-eared rabbits, if one ear lops forwards and downwards, its weight
drags forward all the bones of the skull on the same side, of which
I have given a figure. Malm states that the newly-hatched young of
perches, salmon, and several other symmetrical fishes, have the
habit of occasionally resting on one side at the bottom; and he has
observed that they often then strain their lower eyes so as to look
upwards; and their skulls are thus rendered rather crooked. These
fishes, however, are soon able to hold themselves in a vertical
position, and no permanent effect is thus produced. With the
Pleuronectidae, on the other hand, the older they grow the more
habitually they rest on one side, owing to the increasing flatness
of their bodies, and a permanent effect is thus produced on the form
of the head, and on the position of the eyes. Judging from analogy,
the tendency to distortion would no doubt be increased through the
principle of inheritance. Schiodte believes, in opposition to some
other naturalists, that the Pleuronectidae are not quite symmetrical
even in the embryo; and if this be so, we could understand how it is
that certain species, whilst young, habitually fall over and rest on
the left side, and other species on the right side. Malm adds, in
confirmation of the above view, that the adult Trachypterus
arcticus, which is not a member of the Pleuronectidae, rests on its
left side at the bottom, and swims diagonally through the water; and
in this fish, the two sides of the head are said to be somewhat
dissimilar. Our great authority on fishes, Dr. Gunther, concludes
his abstract of Malm's paper, by remarking that "the author gives a
very simple explanation of the abnormal condition of the
pleuronectoids."
We thus see that the first stages of the transit of the eye from one
side of the head to the other, which Mr. Mivart considers would be
injurious, may be attributed to the habit, no doubt beneficial to
the individual and to the species, of endeavouring to look upwards
with both eyes, whilst resting on one side at the bottom. We may
also attribute to the inherited effects of use the fact of the mouth
in several kinds of flat-fish being bent towards the lower surface,
with the jaw bones stronger and more effective on this, the eyeless
side of the head, than on the other, for the sake, as Dr. Traquair
supposes, of feeding with ease on the ground. Disuse, on the other
hand, will account for the less developed condition of the whole
inferior half of the body, including the lateral fins; though Yarrel
thinks that the reduced size of these fins is advantageous to the
fish, as "there is so much less room for their action, than with the
larger fins above." Perhaps the lesser number of teeth in the
proportion of four to seven in the upper halves of the two jaws of the
plaice, to twenty-five to thirty in the lower halves, may likewise
be accounted for by disuse. From the colourless state of the ventral
surface of most fishes and of many other animals, we may reasonably
suppose that the absence of colour in flat-fish on the side, whether
it be the right or left, which is undermost, is due to the exclusion
of light. But it cannot be supposed that the peculiar speckled
appearance of the upper side of the sole, so like the sandy bed of the
sea, or the power in some species, as recently shown by Pouchet, of
changing their colour in accordance with the surrounding surface, or
the presence of bony tubercles on the upper side of the turbot, are
due to the action of the light. Here natural selection has probably
come into play, as well as in adapting the general shape of the body
of these fishes, and many other peculiarities, to their habits of
life. We should keep in mind, as I have before insisted, that the
inherited effects of the increased use of parts, and perhaps of
their disuse, will be strengthened by natural selection. For all
spontaneous variations in the right direction will thus be
preserved; as will those individuals which inherit in the highest
degree the effects of the increased and beneficial use of any part.
How much to attribute in each particular case to the effects of use,
and how much to natural selection, it seems impossible to decide.
I may give another instance of a structure which apparently owes its
origin exclusively to use or habit. The extremity of the tail in
some American monkeys has been converted into a wonderfully perfect
prehensile organ, and serves as a fifth hand. A reviewer who agrees
with Mr. Mivart in every detail, remarks on this structure: "It is
impossible to believe that in any number of ages the first slight
incipient tendency to grasp could preserve the lives of the
individuals possessing it, or favour their chance of having and of
rearing offspring." But there is no necessity for any such belief.
Habit, and this almost implies that some benefit great or small is
thus derived, would in all probability suffice for the work. Brehm saw
the young of an African monkey (Cercopithecus) clinging to the under
surface of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor
Henslow kept in confinement some harvest mice (Mus messorius) which do
not possess a structurally prehensile tail; but he frequently observed
that they curled their tails round the branches of a bush placed in
the cage, and thus aided themselves in climbing. I have received an
analogous account from Dr. Gunther, who has seen a mouse thus
suspend itself. If the harvest mouse had been more strictly
arboreal, it would perhaps have had its tail rendered structurally
prehensile, as is the case with some members of the same order. Why
Cereopithecus, considering its habits whilst young, has not become
thus provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of more service to
it as a balancing organ in making its prodigious leaps, than as a
prehensile organ.
The mammary glands are common to the whole class of mammals, and are
indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing
positively about their manner of development. Mr. Mivart asks: "Is
it conceivable that the young of any animal was ever saved from
destruction by accidentally sucking a drop of scarcely nutritious
fluid from an accidentally hypertrophied cutaneous gland of its
mother? And even if one was so, what chance was there of the
perpetuation of such a variation?" But the case is not here put
fairly. It is admitted by most evolutionists that mammals are
descended from a marsupial form; and if so, the mammary glands will
have been at first developed within the marsupial sack. In the case of
the fish (Hippocampus) the eggs are hatched, and the young are
reared for a time, within a sack of this nature; and an American
naturalist, Mr. Lockwood, believes from what he has seen of the
development of the young, that they are nourished by a secretion
from the cutaneous glands of the sack. Now with the early
progenitors of mammals, almost before they deserved to be thus
designated, is it not at least possible that the young might have been
similarly nourished? And in this case, the individuals which
secreted a fluid, in some degree or manner the most nutritious, so
as to partake of the nature of milk, would in the long run have reared
a larger number of well-nourished offspring, than would the
individuals which secreted a poorer fluid; and thus the cutaneous
glands, which are the homologues of the mammary glands, would have
been improved or rendered more effective. It accords with the widely
extended principle of specialisation, that the glands over a certain
space of the sack should have become more highly developed than the
remainder; and they would then have formed a breast, but at first
without a nipple as we see in the Ornithorhynchus, at the base of
the mammalian series. Through what agency the glands over a certain
space became more highly specialised than the others, I will not
pretend to decide, whether in part through compensation of growth, the
effects of use, or of natural selection.
The development of the mammary glands would have been of no service,
and could not have been effected through natural selection, unless the
young at the same time were able to partake of the secretion. There is
no greater difficulty in understanding how young mammals have
instinctively learnt to suck the breast, than in understanding how
unhatched chickens have learnt to break the egg-shell by tapping
against it with their specially adapted beaks; or how a few hours
after leaving the shell they have learnt to pick up grains of food. In
such cases the most probable solution seems to be, that the habit
was at first acquired by practice at a more advanced age, and
afterwards transmitted to the offspring at an earlier age. But the
young kangaroo is said not to suck, only to cling to the nipple of its
mother, who has the power of injecting milk into the mouth of her
helpless, half-formed offspring. On this head, Mr. Mivart remarks:
"Did no special provision exist, the young one must infallibly be
choked by the intrusion of the milk into the windpipe. But there is
a special provision. The larynx is so elongated that it rises up
into the posterior end of the nasal passage, and is thus enabled to
give free entrance to the air for the lungs, while the milk passes
harmlessly on each side of this elongated larynx, and so safely
attains the gullet behind it." Mr. Mivart then asks how did natural
selection remove in the adult kangaroo (and in most other mammals,
on the assumption that they are descended from a marsupial form),
"this at least perfectly innocent and harmless structure?" It may be
suggested in answer that the voice, which is certainly of high
importance to many animals, could hardly have been used with full
force as long as the larynx entered the nasal passage; and Professor
Flower has suggested to me that this structure would have greatly
interfered with an animal swallowing solid food.
We will now turn for a short space to the lower divisions of the
animal kingdom. The Echinodermata (star-fishes, sea-urchins, &c.)
are furnished with remarkable organs, called pedicellariae, which
consist, when well developed, of a tridactyle forceps- that is, of one
formed of three serrated arms, neatly fitting together and placed on
the summit of a flexible stem, moved by muscles. These forceps can
firmly seize hold of any object; and Alexander Agassiz has seen an
Echinus or sea-urchin rapidly passing particles of excrement from
forceps to forceps down certain lines of its body, in order that its
shell should not be fouled. But there is no doubt that besides
removing dirt of all kinds, they subserve other functions; and one
of these apparently is defence.
With respect to these organs, Mr. Mivart, as on so many previous
occasions, asks: "What would be the utility of the first rudimentary
beginnings of such structures, and how could such incipient buddings
have ever preserved the life of a single Echinus?" He adds, "Not
even the sudden development of the snapping action could have been
beneficial without the freely moveable stalk, nor could the latter
have been efficient without the snapping jaws, yet no minute merely
indefinite variations could simultaneously evolve these complex
co-ordinations of structure; to deny this seems to do no less than
to affirm a startling paradox." Paradoxical as this may appear to
Mr. Mivart, tridactyle forcepses, immovably fixed at the base, but
capable of a snapping action, certainly exist on some starfishes;
and this is intelligible if they serve, at least in part, as a means
of defence. Mr. Agassiz, to whose great kindness I am indebted for
much information on the subject, informs me that there are other
star-fishes, in which one of the three arms of the forceps is
reduced to a support for the other two; and again, other genera in
which the third arm is completely lost. In Echinoneus, the shell is
described by M. Perrier as bearing two kinds of pedicellariae, one
resembling those of Echinus, and the other those of Spatangus; and
such cases are always interesting as affording the means of apparently
sudden transitions, through the abortion of one of the two states of
an organ.
With respect to the steps by which these curious organs have been
evolved, Mr. Agassiz infers from his own researches and those of
Muller, that both in star-fishes and sea-urchins the pedicellariae
must undoubtedly be looked at as modified spines. This may be inferred
from their manner of development in the individual, as well as from
a long and perfect series of gradations in different species and
genera, from simple granules to ordinary spines, to perfect tridactyle
pedicellariae. The gradations extend even to the manner in which
ordinary spines and pedicellariae with their supporting calcareous
rods are articulated to the shell. In certain genera of star-fishes,
"the very combinations needed to show that the pedicellariae are
only modified branching spines" may be found. Thus we have fixed
spines, with three equidistant, serrated, moveable branches,
articulated to near their bases; and higher up, on the same spine,
three other moveable branches. Now when the latter arise from the
summit of a spine they form in fact a rude tridactyle pedicellaria,
and such may be seen on the same spine together with the three lower
branches. In this case the identity in nature between the arms of
the pedicellariae and the moveable branches of a spine, is
unmistakable. It is generally admitted that the ordinary spines
serve as a protection; and if so, there can be no reason to doubt that
those furnished with serrated and moveable branches likewise serve for
the same purpose; and they would thus serve still more effectively
as soon as by meeting together they acted as a prehensile or
snapping apparatus. Thus every gradation, from an ordinary fixed spine
to a fixed pedicellaria, would be of service.
In certain genera of star-fishes these organs, instead of being
fixed or borne on an immoveable support, are placed on the summit of a
flexible and muscular, though short, stem; and in this case they
probably subserve some additional function besides defence. In the
sea-urchins the steps can be followed by which a fixed spine becomes
articulated to the shell, and is thus rendered moveable. I wish I
had space here to give a fuller abstract of Mr. Agassiz's
interesting observations on the development of the pedicellariae.
All possible gradations, as he adds, may likewise be found between the
pedicellariae of the star-fishes and the hooks of the ophiurians,
another group of Echinodermata; and again between the pedicellariae of
sea-urchins and the anchors of the Holothuriae, also belonging to
the same great class.
Certain compound animals, or zoophytes as they have been termed,
namely the Polyzoa, are provided with curious organs called
avicularia. These differ much in structure in the different species.
In their most perfect condition, they curiously resemble the head
and beak of a vulture in miniature, seated on a neck and capable of
movement, as is likewise the lower jaw or mandible. In one species
observed by me all the avicularia on the same branch often moved
simultaneously backwards and forwards, with the lower jaw widely open,
through an angle of about 90 degrees, in the course of five seconds;
and their movement caused the whole polyzoary to tremble. When the
jaws are touched with a needle they seize it so firmly that the branch
can thus be shaken.
Mr. Mivart adduces this case, chiefly on account of the supposed
difficulty of organs, namely the avicularia of the Polyzoa and the
pedicellariae of the Echinodermata, which he considers as "essentially
similar," having been developed through natural selection in widely
distinct divisions of the animal kingdom. But, as far as structure
is concerned, I can see no similarity between tridactyle pedicellariae
and avicularia. The latter resemble somewhat more closely the chelae
or pincers of crustaceans; and Mr. Mivart might have adduced with
equal appropriateness this resemblance as a special difficulty; or
even their resemblance to the head and beak of a bird. The
avicularia are believed by Mr. Busk, Dr. Smitt, and Dr. Nitsche-
naturalists who have carefully studied this group- to be homologous
with the zooids and their cells which compose the zoophyte; the
moveable lip or lid of the cell corresponding with the lower and
moveable mandible of the avicularium. Mr. Busk, however, does not know
of any gradations now existing between a zooid and an avicularium.
It is therefore impossible to conjecture by what serviceable
gradations the one could have been converted into the other: but it by
no means follows from this that such gradations have not existed.
As the chelae of crustaceans resemble in some degree the
avicularia of Polyzoa, both serving as pincers, it may be worth
while to show that with the former a long series of serviceable
gradations still exists. In the first and simplest stage, the terminal
segment of a limb shuts down either on the square summit of the
broad penultimate segment, or against one whole side; and is thus
enabled to catch hold of an object; but the limb still serves as an
organ of locomotion. We next find one corner of the broad
penultimate segment slightly prominent, sometimes furnished with
irregular teeth; and against these the terminal segment shuts down. By
an increase in the size of this projection, with its shape, as well as
that of the terminal segment, slightly modified and improved, the
pincers are rendered more and more perfect, until we have at last an
instrument as efficient as the chelae of a lobster; and all these
gradations can be actually traced.
Besides the avicularia, the Polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of
movement and easily excited. In one species examined by me the
vibracula were slightly curved and serrated along the outer margin;
and all of them on the same polyzoary often moved simultaneously; so
that, acting like long oars, they swept a branch rapidly across the
object-glass of my microscope. When a branch was placed on its face,
the vibracula became entangled, and they made violent efforts to
free themselves. They are supposed to serve as a defence, and may be
seen, as Mr. Busk remarks, "to sweep slowly and carefully over the
surface of the polyzoary, removing what might be noxious to the
delicate inhabitants of the cells when their tentacula are protruded."
The avicularia, like the vibracula, probably serve for defence, but
they also catch and kill small living animals, which it is believed
are afterwards swept by the currents within reach of the tentacula
of the zooids. Some species are provided with avicularia and
vibracula; some with avicularia alone, and a few with vibracula alone.
It is not easy to imagine two objects more widely different in
appearance than a bristle or vibraculum, and an avicularium like the
head of a bird; yet they are almost certainly homologous and have been
developed from the same common source, namely a zooid with its cell.
Hence we can understand how it is that these organs graduate in some
cases, as I am informed by Mr. Busk, into each other. Thus with the
avicularia of several species of Lepralia, the moveable mandible is so
much produced and is so like a bristle, that the presence of the upper
or fixed beak alone serves to determine even its avicularian nature.
The vibracula may have been directly developed from the lips of the
cells, without having passed through the avicularian stage; but it
seems more probable that they have passed through this stage, as
during the early stages of the transformation, the other parts of
the cell with the included zooid could hardly have disappeared at
once. In many cases the vibracula have a grooved support at the
base, which seems to represent the fixed beak; though this support
in some species is quite absent. This view of the development of the
vibracula, if trustworthy, is interesting; for supposing that all
the species provided with avicularia had become extinct, no one with
the most vivid imagination would ever have thought that the
vibracula had originally existed as part of an organ, resembling a
bird's head or an irregular box or hood. It is interesting to see
two such widely different organs developed from a common origin; and
as the moveable lip of the cell serves as a protection to the zooid,
there is no difficulty in believing that all the gradations, by
which the lip became converted first into the lower mandible of an
avicularium and then into an elongated bristle, likewise served as a
protection in different ways and under different circumstances.
In the vegetable kingdom Mr. Mivart only alludes to two cases,
namely the structure of the flowers of orchids, and the movements of
climbing plants. With respect to the former, he says, "The explanation
of their origin is deemed thoroughly unsatisfactory- utterly
insufficient to explain the incipient, infinitesimal beginnings of
structures which are of utility only when they are considerably
developed." As I have fully treated this subject in another work, I
will here give only a few details on one alone of the most striking
peculiarities of the flowers of orchids, namely their pollinia. A
pollinium when highly developed consists of a mass of pollen-grains,
affixed to an elastic footstalk or caudicle, and this to a little mass
of extremely viscid matter. The pollinia are by this means transported
by insects from one flower to the stigma of another. In some orchids
there is no caudicle to the pollen-masses, and the grains are merely
tied together by fine threads; but as these are not confined to
orchids, they need not here be considered; yet I may mention that at
the base of the orchidaceous series, in Cypripedium, we can see how
the threads were probably first developed. In other orchids the
threads cohere at one end of the pollen-masses; and this forms the
first or nascent trace of a caudicle. That this is the origin of the
caudicle, even when of considerable length and highly developed, we
have good evidence in the aborted pollen-grains which can sometimes be
detected embedded within the central and solid parts.
With respect to the second chief peculiarity, namely the little mass
of viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In
most flowers belonging to other orders the stigma secretes a little
viscid matter. Now in certain orchids similar viscid matter is
secreted, but in much larger quantities by one alone of the three
stigmas; and this stigma, perhaps in consequence of the copious
secretion, is rendered sterile. When an insect visits a flower of this
kind, it rubs off some of the viscid matter and thus at the same
time drags away some of the pollen-grains. From this simple condition,
which differs but little from that of a multitude of common flowers,
there are endless gradations,- to species in which the pollen-mass
terminates in a very short, free caudicle,- to others in which the
caudicle becomes firmly attached to the viscid matter, with the
sterile stigma itself much modified. In this latter case we have a
pollinium in its most highly developed and perfect condition. He who
will carefully examine the flowers of orchids for himself will not
deny the existence of the above series of gradations- from a mass of
pollen-grains merely tied together by threads, with the stigma
differing but little from that of an ordinary flower, to a highly
complex pollinium, admirably adapted for transportal by insects; nor
will he deny that all the gradations in the several species are
admirably adapted in relation to the general structure of each
flower for its fertilisation by different insects. In this, and in
almost every other case, the enquiry may be pushed further
backwards; and it may be asked how did the stigma of an ordinary
flower become viscid, but as we do not know the full history of any
one group of beings, it is as useless to ask, as it is hopeless to
attempt answering, such questions.
We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those
which I have called leaf-climbers, and to those provided with
tendrils. In these two latter classes the stems have generally, but
not always, lost the power of twining, though they retain the power of
revolving, which the tendrils likewise possess. The gradations from
leaf-climbers to tendril-bearers are wonderfully close, and certain
plants may be indifferently placed in either class. But in ascending
the series from simple twiners to leaf-climbers, an important
quality is added, namely sensitiveness to a touch, by which means
the foot-stalks of the leaves or flowers, or these modified and
converted into tendrils, are excited to bend round and clasp the
touching object. He who will read my memoir on these plants will, I
think, admit that all the many gradations in function and structure
between simple twiners and tendril-bearers are in each case beneficial
in a high degree to the species. For instance, it is clearly a great
advantage to a twining plant to become a leaf-climber; and it is
probable that every twiner which possessed leaves with long
foot-stalks would have been developed into a leaf-climber if the
footstalks had possessed in any slight degree the requisite
sensitiveness to a touch.
As twining is the simplest means of ascending a support, and forms
the basis of our series, it may naturally be asked how did plants
acquire this power in an incipient degree, afterwards to be improved
and increased through natural selection. The power of twining depends,
firstly, on the stems whilst young being extremely flexible (but
this is a character common to many plants which are not climbers);
and, secondly, on their continually bending to all points of the
compass, one after the other in succession, in the same order. By this
movement the stems are inclined to all sides, and are made to move
round and round. As soon as the lower part of a stem strikes against
any object and is stopped, the upper part still goes on bending and
revolving, and thus necessarily twines round and up the support. The
revolving movement ceases after the early growth of each shoot. As
in many widely separated families of plants, single species and single
genera possess the power of revolving, and have thus become twiners,
they must have independently acquired it, and cannot have inherited it
from a common progenitor. Hence I was led to predict that some
slight tendency to a movement of this kind would be found to be far
from uncommon with plants which did not climb; and that this had
afforded the basis for natural selection to work on and improve.
When I made this prediction, I knew of only one imperfect case,
namely, of the young flower-peduncles of a Maurandia which revolved
slightly and irregularly, like the stems of twining plants, but
without making any use of this habit. Soon afterwards Fritz Muller
discovered that the young stems of an Alisima and of a Linum,-
plants which do not climb and are widely separated in the natural
system,- revolved plainly, though irregularly; and he states that he
has reason to suspect that this occurs with some other plants. These
slight movements appear to be of no service to the plants in question;
anyhow, they are not of the least use in the way of climbing, which is
the point that concerns us. Nevertheless we can see that if the
stems of these plants had been flexible, and if under the conditions
to which they are exposed it had profited them to ascend to a
height, then the habit of slightly and irregularly revolving might
have been increased and utilised through natural selection, until they
had become converted into well-developed twining species.
With respect to the sensitiveness of the footstalks of the leaves
and flowers, and of tendrils, nearly the same remarks are applicable
as in the case of the revolving movements of twining plants. As a vast
number of species, belonging to widely distinct groups, are endowed
with this kind of sensitiveness, it ought to be found in a nascent
condition in many plants which have not become climbers. This is the
case: I observed that the young flower-peduncles of the above
Maurandia curved themselves a little toward the side which was
touched. Morren found in several species of Oxalis that the leaves and
their foot-stalks moved, especially after exposure to a hot sun,
when they were gently and repeatedly touched, or when the plant was
shaken. I repeated these observations on some other species of
Oxalis with the same result; in some of them the movement was
distinct, but was best seen in the young leaves; in others it was
extremely slight. It is a more important fact that according to the
high authority of Hofmeister, the young shoots and leaves of all
plants move after being shaken; and with climbing plants it is, as
we know, only during the early stages of growth that the foot-stalks
and tendrils are sensitive.
It is scarcely possible that the above slight movements, due to a
touch or shake, in the young and growing organs of plants, can be of
any functional importance to them. But plants possess, in obedience to
various stimuli, powers of movement, which are of manifest
importance to them; for instance, towards and more rarely from the
light,- in opposition to, and more rarely in the direction of, the
attraction of gravity. When the nerves and muscles of an animal are
excited by galvanism or by the absorption of strychnine, the
consequent movements may be called an incidental result, for the
nerves and muscles have not been rendered specially sensitive to these
stimuli. So with plants it appears that, from having the power of
movement in obedience to certain stimuli, they are excited in an
incidental manner by a touch, or by being shaken. Hence there is no
great difficulty in admitting that in the case of leaf-climbers and
tendril-bearers, it is this tendency which has been taken advantage of
and increased through natural selection. It is, however, probable,
from reasons which I have assigned in my memoir, that this will have
occurred only with plants which had already acquired the power of
revolving, and had thus become twiners.
I have already endeavoured to explain how plants became twiners,
namely, by the increase of a tendency to slight and irregular
revolving movements, which were at first of no use to them; this
movement, as well as that due to a touch or shake, being the
incidental result of the power of moving, gained for other and
beneficial purposes. Whether, during the gradual development of
climbing plants, natural selection has been aided by the inherited
effects of use, I will not pretend to decide; but we know that certain
periodical movements, for instance the so-called sleep of plants,
are governed by habit.
I have now considered enough, perhaps more than enough, of the
cases, selected with care by a skilful naturalist, to prove that
natural selection is incompetent to account for the incipient stages
of useful structures; and I have shown, as I hope, that there is no
great difficulty on this head. A good opportunity has thus been
afforded for enlarging a little on gradations of structure, often
associated with changed functions,- an important subject which was not
treated at sufficient length in the former editions of this work. I
will now briefly recapitulate the foregoing cases.
With the giraffe, the continued preservation of the individuals of
some extinct high-reaching ruminant, which had the longest necks,
legs, &c., and could browse a little above the average height, and the
continued destruction of those which could not browse so high, would
have sufficed for the production of this remarkable quadruped; but the
prolonged use of all the parts together with inheritance will have
aided in an important manner in their co-ordination. With the many
insects which imitate various objects, there is no improbability in
the belief that an accidental resemblance to some common object was in
each case the foundation for the work of natural selection, since
perfected through the occasional preservation of slight variations
which ma de the resemblance at all closer; and this will have been
carried on as long as the insect continued to vary, and as long as a
more and more perfect resemblance led to its escape from sharp-sighted
enemies. In certain species of whales there is a tendency to the
formation of irregular little points of horn on the palate; and it
seems to be quite within the scope of natural selection to preserve
all favourable variations, until the points were converted first
into lamellated knobs or teeth, like those on the beak of a goose,-
then into short lamellae, like those of the domestic ducks,- and
then into lamellae, as perfect as those of the shoveller-duck,- and
finally into the gigantic plates of baleen, as in the mouth of the
Greenland whale. In the family of the ducks, the lamellae are first
used as teeth, then partly as teeth, and partly as a sifting
apparatus, and at last almost exclusively for this latter purpose.
With such structures as the above lamellae of horn or whalebone,
habit or use can have done little or nothing, as far as we can
judge, towards their development. On the other hand, the transportal
of the lower eye of a flat-fish to the upper side of the head, and the
formation of a prehensile tail, may be attributed almost wholly to
continued use, together with inheritance. With respect to the mammae
of the higher animals, the most probable conjecture is that
primordially the cutaneous glands over the whole surface of a
marsupial sack secreted a nutritious fluid; and that these glands were
improved in function through natural selection, and concentrated
into a confined area, in which case they would have formed a mamma.
There is no more difficulty in understanding how the branched spines
of some ancient echinoderm, which served as a defence, became
developed through natural selection into tridactyle pedicellariae,
than in understanding the development of the pincers of crustaceans,
through slight, serviceable modifications in the ultimate and
penultimate segments of a limb, which was at first used solely for
locomotion. In the avicularia and vibracula of the Polyzoa we have
organs widely different in appearance developed from the same
source; and with the vibracula we can understand how the successive
gradations might have been of service. With the pollinia of orchids,
the threads which originally served to tie together the pollen-grains,
can be traced cohering into caudicles; and the steps can likewise be
followed by which viscid matter, such as that secreted by the
stigmas of ordinary flowers, and still subserving nearly but not quite
the same purpose, became attached to the free ends of the
caudicles;- all these gradations being of modest benefit to the plants
in question. With respect to climbing plants, I need not repeat what
has been so lately said.
It has often been asked, if natural selection be so potent, why
has not this or that structure been gained by certain species, to
which it would apparently have been advantageous? But it is
unreasonable to expect a precise answer to such questions, considering
our ignorance of the past history of each species, and of the
conditions which at the present day determine its numbers and range.
In most cases only general reasons, but in some few cases special
reasons, can be assigned. Thus to adapt a species to new habits of
life, many co-ordinated modifications are almost indispensable, and it
may often have happened that the requisite parts did not vary in the
right manner or to the right degree. Many species must have been
prevented from increasing in numbers through destructive agencies,
which stood in no relation to certain structures, which we imagine
would have been gained through natural selection from appearing to
us advantageous to the species. In this case, as the struggle for life
did not depend on such structures, they could not have been acquired
through natural selection. In many cases complex and long-enduring
conditions, often of a peculiar nature, are necessary for the
development of a structure; and the requisite conditions may seldom
have concurred. The belief that any given structure, which we think,
often erroneously, would have been beneficial to a species, would have
been gained under all circumstances through natural selection, is
opposed to what we can understand of its manner of action. Mr.
Mivart does not deny that natural selection has effected something;
but he considers it as "demonstrably insufficient" to account for
the phenomena which I explain by its agency. His chief arguments
have now been considered, and the others will hereafter be considered.
They seem to me to partake little of the character of demonstration,
and to have little weight in comparison with those in favour of the
power of natural selection, aided by the other agencies often
specified. I am bound to add, that some of the facts and arguments
here used by me, have been advanced for the same purpose in an able
article lately published in the Medico-Chirurgical Review.
At the present day almost all naturalists admit evolution under some
form. Mr. Mivart believes that species change through "an internal
force or tendency," about which it is not pretended that anything is
known. That species have a capacity for change will be admitted by all
evolutionists; but there is no need, as it seems to me, to invoke
any internal force beyond the tendency to ordinary variability,
which through the aid of selection by man has given rise to many
well-adapted domestic races, and which through the aid of natural
selection would equally well give rise by graduated steps to natural
races or species. The final result will generally have been, as
already explained, an advance, but in some few cases a
retrogression, in organisation.
Mr. Mivart is further inclined to believe, and some naturalists
agree with him, that new species manifest themselves "with
suddenness and by modifications appearing at once." For instance, he
supposes that the differences between the extinct three-toed Hipparion
and the horse arose suddenly. He thinks it difficult to believe that
the wing of a bird "was developed in any other way than by a
comparatively sudden modification of a marked and important kind"; and
apparently he would extend the same view to the wings of bats and
pterodactyles. This conclusion, which implies great breaks or
discontinuity in the series, appears to me improbable in the highest
degree.
Every one who believes in slow and gradual evolution, will of course
admit that specific changes may have been as abrupt and as great as
any single variation which we meet with under nature, or even under
domestication. But as species are more variable when domesticated or
cultivated than under their natural conditions, it is not probable
that such great and abrupt variations have often occurred under
nature, as are known occasionally to arise under domestication. Of
these latter variations several may be attributed to reversion; and
the characters which thus reappear were, it is probable, in many cases
at first gained in a gradual manner. A still greater number must be
called monstrosities, such as six-fingered men, porcupine men, Ancon
sheep, Niata cattle, &c.; and as they are widely different in
character from natural species, they throw very little light on our
subject. Excluding such cases of abrupt variations, the few which
remain would at best constitute, if found in a state of nature,
doubtful species, closely related to their parental types.
My reasons for doubting whether natural species have changed as
abruptly as have occasionally domestic races, and for entirely
disbelieving that they have changed in the wonderful manner
indicated by Mr. Mivart, are as follows. According to our
experience, abrupt and strongly marked variations occur in our
domesticated productions, singly and at rather long intervals of time.
If such occurred under nature, they would be liable, as formerly
explained, to be lost by accidental causes of destruction and by
subsequent inter-crossing; and so it is known to be under
domestication, unless abrupt variations of this kind are specially
preserved and separated by the care of man. Hence in order that a
new species should suddenly appear in the manner supposed by Mr.
Mivart, it is almost necessary to believe, in opposition to all
analogy, that several wonderfully changed individuals appeared
simultaneously within the same district. This difficulty, as in the
case of unconscious selection by man, is avoided on the theory of
gradual evolution, through the preservation of a large number of
individuals, which varied more or less in any favourable direction,
and of the destruction of a large number which varied in an opposite
manner.
That many species have been evolved in an extremely gradual
manner, there can hardly be a doubt. The species and even the genera
of many large natural families are so closely allied together, that it
is difficult to distinguish not a few of them. On every continent in
proceeding from north to south, from lowland to upland, &c., we meet
with a host of closely related or representative species; as we
likewise do on certain distinct continents, which we have reason to
believe were formerly connected. But in making these and the following
remarks, I am compelled to allude to subjects hereafter to be
discussed. Look at the many outlying islands round a continent, and
see how many of their inhabitants can be raised only to the rank of
doubtful species. So it is if we look to past times, and compare the
species which have just passed away with those still living within the
same areas; or if we compare the fossil species embedded in the
sub-stages of the same geological formation. It is indeed manifest
that multitudes of species are related in the closest manner to
other species that still exist, or have lately existed; and it will
hardly be maintained that such species have been developed in an
abrupt or sudden manner. Nor should it be forgotten, when we look to
the special parts of allied species, instead of to distinct species,
that numerous and wonderfully fine gradations can be traced,
connecting together widely different structures.
Many large groups of facts are intelligible only on the principle
that species have been evolved by very small steps: for instance,
the fact that the species included in the larger genera are more
closely related to each other, and present a greater number of
varieties than do the species in the smaller genera. The former are
also grouped in little clusters, like varieties round species, and
they present other analogies with varieties, as was shown in our
second chapter. On this same principle we can understand how it is
that specific characters are more variable than generic characters;
and how the parts which are developed in an extraordinary degree or
manner are more variable than other parts of the same species. Many
analogous facts, all pointing in the same direction, could be added.
Although very many species have almost certainly been produced by
steps not greater than those separating fine varieties; yet it may
be maintained that some have been developed in a different and
abrupt manner. Such an admission, however, ought not to be made
without strong evidence being assigned. The vague and in some respects
false analogies, as they have been shown to be by Mr. Chauncey Wright,
which have been advanced in favour of this view, such as the sudden
crystallisation of inorganic substances, or the falling of a
facetted spheroid from one facet to another, hardly deserve
consideration. One class of facts, however, namely, the sudden
appearance of new and distinct forms of life in our geological
formations, supports at first sight the belief in abrupt
development. But the value of this evidence depends entirely on the
perfection of the geological record, in relation to periods remote
in the history of the world. If the record is as fragmentary as many
geologists strenuously assert, there is nothing strange in new forms
appearing as if suddenly developed.
Unless we admit transformations as prodigious as those advocated
by Mr. Mivart, such as the sudden development of the wings of birds or
bats, or the sudden conversion of a Hipparion into a horse, hardly any
light is thrown by the belief in abrupt modifications on the
deficiency of connecting links in our geological formations. But
against the belief in such abrupt changes, embryology enters a
strong protest. It is notorious that the wings of birds and bats,
and the legs of horses or other quadrupeds, are undistinguishable at
an early embryonic period, and that they become differentiated by
insensibly fine steps. Embryological resemblances of all kinds can
be accounted for, as we shall hereafter see, by the progenitors of our
existing species having varied after early youth, and having
transmitted their newly acquired characters to their offspring, at a
corresponding age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species. Hence it is
that existing species during the early stages of their development
so often resemble ancient and extinct forms belonging to the same
class. On this view of the meaning of embryological resemblances,
and indeed on any view, it is incredible that an animal should have
undergone such momentous and abrupt transformations, as those above
indicated; and yet should not bear even a trace in its embryonic
condition of any sudden modification; every detail in its structure
being developed by insensibly fine steps.
He who believes that some ancient form was transformed suddenly
through an internal force or tendency into, for instance, one
furnished with wings, will be almost compelled to assume, in
opposition to all analogy, that many individuals varied
simultaneously. It cannot be denied that such abrupt and great changes
of structure are widely different from those which most species
apparently have undergone. He will further be compelled to believe
that many structures beautifully adapted to all the other parts of the
same creature and to the surrounding conditions, have been suddenly
produced; and of such complex and wonderful co-adaptations, he will
not be able to assign a shadow of an explanation. He will be forced to
admit that these great and sudden transformations have left no trace
of their action on the embryo. To admit all this is, as it seems to
me, to enter into the realms of miracle, and to leave those of
Science.
CHAPTER VIII
INSTINCT
MANY instincts are so wonderful that their development will probably
appear to the reader a difficulty sufficient to overthrow my whole
theory. I may here premise that I have nothing to do with the origin
of the mental powers, any more than I have with that of life itself.
We are concerned only with the diversities of instinct and of the
other mental faculties in animals of the same class.
I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by
this term; but every one understands what is meant, when it is said
that instinct impels the cuckoo to migrate and to lay her eggs in
other birds' nests. An action, which we ourselves require experience
to enable us to perform, when performed by an animal, more
especially by a very young one, without experience, and when performed
by many individuals in the same way, without their knowing for what
purpose it is performed, is usually said to be instinctive. But I
could show that none of these characters are universal. A little
dose of judgment or reason, as Pierre Huber expresses it, often
comes into play, even with animals low in the scale of nature.
Frederic Cuvier and several of the older metaphysicians have
compared instinct with habit. This comparison gives, I think, an
accurate notion of the frame of mind under which an instinctive action
is performed, but not necessarily of its origin. How unconsciously
many habitual actions are performed, indeed not rarely in direct
opposition to our conscious will! Yet they may be modified by the will
or reason. Habits easily become associated with other habits, with
certain periods of time, and states of the body. When once acquired,
they often remain constant throughout life. Several other points of
resemblance between instincts and habits could be pointed out. As in
repeating a well-known song, so in instincts, one action follows
another by a sort of rhythm; if a person be interrupted in a song,
or in repeating anything by rote, he is generally forced to go back to
recover the habitual train of thought; so P. Huber found it was with a
caterpillar, which makes a very complicated hammock; for if he took
a caterpillar which had completed its hammock up to, say, the sixth
stage of construction, and put it into a hammock completed up only
to the third stage, the caterpillar simply reperformed the fourth,
fifth, and sixth stages of construction. if, however, a caterpillar
were taken out of a hammock made up, for instance, to the third stage,
and were put into one finished up to the sixth stage, so that much
of its work was already done for it, far from deriving any benefit
from this, it was much embarrassed, and in order to complete its
hammock, seemed forced to start from the third stage, where it had
left off, and thus tried to complete the already finished work.
If we suppose any habitual action to become inherited- and it can be
shown that this does sometimes happen- then the resemblance between
what originally was a habit and an instinct becomes so close as not to
be distinguished. If Mozart, instead of playing the pianoforte at
three years old with wonderfully little practice, had played a tune
with no practice at all, he might truly be said to have done so
instinctively. But it would be a serious error to suppose that the
greater number of instincts have been acquired by habit in one
generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive-bee and of
many ants, could not possibly have been acquired by habit.
It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its
present conditions of life. Under changed conditions of life, it is at
least possible that slight modifications of instinct might be
profitable to a species; and if it can be shown that instincts do vary
ever so little, then I can see no difficulty in natural selection
preserving and continually accumulating variations of instinct to
any extent that was profitable. It is thus, as I believe, that all the
most complex and wonderful instincts have originated. As modifications
of corporeal structure arise from, and are increased by, use or habit,
and are diminished or lost by disuse, so I do not doubt it has been
with instincts. But I believe that the effects of habit are in many
cases of subordinate importance to the effects of the natural
selection of what may be called spontaneous variations of
instincts;- that is of variations produced by the same unknown
causes which produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural
selection, except by the slow and gradual accumulation of numerous
slight, yet profitable, variations. Hence, as in the case of corporeal
structures, we ought to find in nature, not the actual transitional
gradations by which each complex instinct has been acquired- for these
could be found only in the lineal ancestors of each species- but we
ought to find in the collateral lines of descent some evidence of such
gradations; or we ought at least to be able to show that gradations of
some kind are possible; and this we certainly can do. I have been
surprised to find, making allowance for the instincts of animals
having been but little observed except in Europe and North America,
and for no instinct being known amongst extinct species, how very
generally gradations, leading to the most complex instincts, can be
discovered. Changes of instinct may sometimes be facilitated by the
same species having different instincts at different periods of
life, or at different seasons of the year, or when placed under
different circumstances, &c; in which case either the one or the other
instinct might be preserved by natural selection. And such instances
of diversity of instinct in the same species can be shown to occur
in nature.
Again, as in the case of corporeal structure, and conformably to
my theory, the instinct of each species is good for itself, but has
never, as far as we can judge, been produced for the exclusive good of
others. One of the strongest instances of an animal apparently
performing an action for the sole good of another, with which I am
acquainted, is that of aphides voluntarily yielding, as was first
observed by Huber, their sweet excretion to ants: that they do so
voluntarily, the following facts show. I removed all the ants from a
group of about a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval, I felt sure that
the aphides would want to excrete. I watched them for some time
through a lens, but not one excreted; I then tickled and stroked
them with a hair in the same manner, as well as I could, as the ants
do with their antennae; but not one excreted. Afterwards I allowed
an ant to visit them, and it immediately seemed, by its eager way of
running about, to be well aware what a rich flock it had discovered;
it then began to play with its antennae on the abdomen first of one
aphis and then of another; and each, as soon as it felt the
antennae, immediately lifted up its abdomen and excreted a limpid drop
of sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action
was instinctive, and not the result of experience. It is certain, from
the observations of Huber, that the aphides show no dislike to the
ants: if the latter be not present they are at last compelled to eject
their excretion. But as the excretion is extremely viscid, it is no
doubt a convenience to the aphides to have it removed; therefore
probably they do not excrete solely for the good of the ants. Although
there is no evidence that any animal performs an action for the
exclusive good of another species, yet each tries to take advantage of
the instincts of others, as each takes advantage of the weaker
bodily structure of other species. So again instincts cannot be
considered as absolutely perfect; but as details on this and other
such points are not indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature,
and the inheritance of such variations, are indispensable for the
action of natural selection, as many instances as possible ought to be
given; but want of space prevents me. I can only assert that instincts
certainly do vary- for instance, the migratory instinct, both in
extent and direction, and in its total loss. So it is with the nests
of birds, which vary partly in dependence on the situations chosen,
and on the nature and temperature of the country inhabited, but
often from causes wholly unknown to us: Audubon has given several
remarkable cases of differences in the nests of the same species in
the northern and southern United States. Why, it has been asked, if
instinct be variable, has it not granted to the bee "the ability to
use some other material when wax was deficient"? But what other
natural material could bees use? They will work, as I have seen,
with wax hardened with vermilion or softened with lard. Andrew
Knight observed that his bees, instead of laboriously collecting
propolis, used a cement of wax and turpentine, with which he had
covered decorticated trees. It has lately been shown that bees,
instead of searching for pollen, will gladly use a very different
substance, namely oatmeal. Fear of any particular enemy is certainly
an instinctive quality, as may be seen in nestling birds, though it is
strengthened by experience, and by the sight of fear of the same enemy
in other animals. The fear of man is slowly acquired, as I have
elsewhere shown, by the various animals which inhabit desert
islands; and we see an instance of this even in England, in the
greater wildness of all our large birds in comparison with our small
birds; for the large birds have been most persecuted by man. We may
safely attribute the greater wildness of our large birds to this
cause; for in uninhabited islands large birds are not more fearful
than small; and the magpie, so wary in England, is tame in Norway,
as is the hooded crow in Egypt.
That the mental qualities of animals of the same kind, born in a
state of nature, vary much, could be shown by many facts. Several
cases could also be adduced of occasional and strange habits in wild
animals, which, if advantageous to the species, might have given rise,
through natural selection, to new instincts. But I am well aware
that these general statements, without the facts in detail, will
produce but a feeble effect on the reader's mind. I can only repeat my
assurance, that I do not speak without good evidence.
Inherited Changes of Habit or Instinct in Domesticated Animals
The possibility, or even probability, of inherited variations of
instinct in a state of nature will be strengthened by briefly
considering a few cases under domestication. We shall thus be
enabled to see the part which habit and the selection of so-called
spontaneous variations have played in modifying the mental qualities
of our domestic animals. It is notorious how much domestic animals
vary in their mental qualities. With cats, for instance, one naturally
takes to catching rats, and another mice, and these tendencies are
known to be inherited. One cat, according to Mr. St. John, always
brought home gamebirds, another hares or rabbits, and another hunted
on marshy ground and almost nightly caught woodcocks or snipes. A
number of curious and authentic instances could be given of various
shades of disposition and of taste, and likewise of the oddest tricks,
associated with certain frames of mind or periods of time, being
inherited. But let us look to the familiar case of the breeds of the
dogs: it cannot be doubted that young pointers (I have myself seen a
striking instance) will sometimes point and even back other dogs the
very first time that they are taken out; retrieving is certainly in
some degree inherited by retrievers; and a tendency to run round,
instead of at, a flock of sheep, by shepherd dogs. I cannot see that
these actions, performed without experience by the young, and in
nearly the same manner by each individual, performed with eager
delight by each breed, and without the end being known- for the
young pointer can no more know that he points to aid his master,
than the white butterfly knows why she lays her eggs on the leaf of
the cabbage- I cannot see that these actions differ essentially from
true instincts. If we were to behold one kind of wolf, when young
and without any training, as soon as it scented its prey, stand
motionless like a statue, and then slowly crawl forward with a
peculiar gait; and another kind of wolf rushing round, instead of
at, a herd of deer, and driving them to a distant point, we should
assuredly call these actions instinctive. Domestic instincts, as
they may be called, are certainly far less fixed than natural
instincts; but they have been acted on by far less rigorous selection,
and have been transmitted for an incomparably shorter period, under
less fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions
are inherited, and how curiously they become mingled, is well shown
when different breeds of dogs are crossed. Thus it is known that a
cross with a bull-dog has affected for many generations the courage
and obstinacy of greyhounds; and a cross with a greyhound has given to
a whole family of shepherd-dogs a tendency to hunt hares. These
domestic instincts, when thus tested by crossing, resemble natural
instincts, which in a like manner become curiously blended together,
and for a long period exhibit traces of the instincts of either
parent: for example, Le Roy describes a dog, whose great-grandfather
was a wolf, and this dog showed a trace of its wild parentage only
in one way, by not coming in a straight line to his master, when
called.
Domestic instincts are sometimes spoken of as actions which have
become inherited solely from long-continued and compulsory habit,
but this is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler-pigeon to tumble,- an action
which, as I have witnessed, is performed by young birds, that have
never seen a pigeon tumble. We may believe that some one pigeon showed
a slight tendency to this strange habit, and that the long-continued
selection of the best individuals in successive generations made
tumblers what they now are; and near Glasgow there are house-tumblers,
as I hear from Mr. Brent, which cannot fly eighteen inches high
without going head over heels. It may be doubted whether any one would
have thought of training a dog to point, had not some one dog
naturally shown a tendency in this line; and this is known
occasionally to happen, as I once saw, in a pure terrier: the act of
pointing is probably, as many have thought, only the exaggerated pause
of an animal preparing to spring on its prey. When the first
tendency to point was once displayed, methodical selection and the
inherited effects of compulsory training in each successive generation
would soon complete the work; and unconscious selection is still in
progress, as each man tries to procure, without intending to improve
the breed, dogs which stand and hunt best. On the other hand, habit
alone in some cases has sufficed; hardly any animal is more
difficult to tame than the young of the wild rabbit; scarcely any
animal is tamer than the young of the tame rabbit; but I can hardly
suppose that domestic rabbits have often been selected for tameness
alone; so that we must attribute at least the greater part of the
inherited change from extreme wildness to extreme tameness, to habit
and long-continued close confinement.
Natural instincts are lost under domestication: a remarkable
instance of this is seen in those breeds of fowls which very rarely or
never become "broody," that is, never wish to sit on their eggs.
Familiarity alone prevents our seeing how largely and how
permanently the minds of our domestic animals have been modified. It
is scarcely possible to doubt that the love of man has become
instinctive in the dog. All wolves, foxes, jackals, and species of the
cat genus, when kept tame, are most eager to attack poultry, sheep,
and pigs; and this tendency has been found incurable in dogs which
have been brought home as puppies from countries such as Tierra del
Fuego and Australia, where the savages do not keep these domestic
animals. How rarely, on the other hand, do our civilised dogs, even
when quite young, require to be taught not to attack poultry, sheep,
and pigs! No doubt they occasionally do make an attack, and are then
beaten; and if not cured, they are destroyed; so that habit and some
degree of selection have probably concurred in civilising by
inheritance our dogs. On the other hand, young chickens have lost,
wholly by habit, that fear of the dog and cat which no doubt was
originally instinctive with them; for I am informed by Captain
Hutton that the young chickens of the parent-stock, the Gallus
bankiva, when reared in India under a hen, are at first excessively
wild. So it is with young pheasants reared in England under a hen.
It is not that chickens have lost all fear, but fear only of dogs
and cats, for if the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal themselves in
the surrounding grass or thickets; and this is evidently done for
the instinctive purpose of allowing as we see in wild ground-birds,
their mother to fly away. But this instinct retained by our chickens
has become useless under domestication, for the mother-hen has
almost lost by disuse the power of flight.
Hence, we may conclude, that under domestication instincts have been
acquired, and natural instincts have been lost, partly by habit, and
partly by man selecting and accumulating, during successive
generations, peculiar mental habits and actions, which at first
appeared from what we must in our ignorance call an accident. In
some cases compulsory habit alone has sufficed to produce inherited
mental changes; in other cases, compulsory habit has done nothing, and
all has been the result of selection, pursued both methodically and
unconsciously: but in most cases habit and selection have probably
concurred.
Special Instincts
We shall, perhaps, best understand how instincts in a state of
nature have become modified by selection by considering a few cases. I
will select only three,- namely, the instinct which leads the cuckoo
to lay her eggs in other birds' nests; the slave-making instinct of
certain ants; and the cell-making power of the hive-bee. These two
latter instincts have generally and justly been ranked by
naturalists as the most wonderful of all known instincts.
Instincts of the Cuckoo.- It is supposed by some naturalists that
the more immediate cause of the instinct of the cuckoo is, that she
lays her eggs, not daily, but at intervals of two or three days; so
that, if she were to make her own nest and sit on her own eggs those
first laid would have to be left for some time unincubated, or there
would be eggs and young birds of different ages in the same nest. If
this were the case, the process of laying and hatching might be
inconveniently long, more especially as she migrates at a very early
period; and the first hatched young would probably have to be fed by
the male alone. But the American cuckoo is in this predicament; for
she makes her own nest, and has eggs and young successively hatched,
all at the same time. It has been both asserted and denied that the
American cuckoo occasionally lays her eggs in other birds' nests;
but I have lately heard from Dr. Merrell, of Iowa, that he once
found in Illinois a young cuckoo together with a young jay in the nest
of a blue jay (Garrulus cristatus); and as both were nearly full
feathered, there could be no mistake in their identification. I
could also give several instances of various birds which have been
known occasionally to lay their eggs in other birds' nests. Now let us
suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo, and that she occasionally laid an egg
in another bird's nest. If the old bird profited by this occasional
habit through being enabled to migrate earlier or through any other
cause; or if the young were made more vigorous by advantage being
taken of the mistaken instinct of another species than when reared
by their own mother, encumbered as she could hardly fail to be by
having eggs and young of different ages at the same time; then the old
birds or the fostered young would gain an advantage. And analogy would
lead us to believe, that the young thus reared would be apt to
follow by inheritance the occasional and aberrant habit of their
mother, and in their turn would be apt to lay their eggs in other
birds' nests, and thus be more successful in rearing their young. By a
continued process of this nature, I believe that the strange
instinct of our cuckoo has been generated. It has, also, recently been
ascertained on sufficient evidence, by Adolf Muller, that the cuckoo
occasionally lays her eggs on the bare ground, sits on them, and feeds
her young. This rare event is probably a case of reversion to the
long-lost, aboriginal instinct of nidification.
It has been objected that I have not noticed other related instincts
and adaptations of structure in the cuckoo, which are spoken of as
necessarily co-ordinated. But in all cases, speculation on an instinct
known to us only in a single species, is useless, for we have hitherto
had no facts to guide us. Until recently the instincts of the European
and of the nonparasitic American cuckoo alone were known. now, owing
to Mr. Ramsay's observations, we have learnt something about three
Australian species, which lay their eggs in other birds' nests. The
chief points to be referred to are three: first, that the common
cuckoo, with rare exceptions, lays only one egg in a nest, so that the
large and voracious young bird receives ample food. Secondly, that the
eggs are remarkably small, not exceeding those of the skylark,- a bird
about one-fourth as large as the cuckoo. That the small size of the
egg is a real cause of adaptation we may infer from the fact of the
non-parasitic American cuckoo laying full-sized eggs. Thirdly, that
the young cuckoo, soon after birth, has the instinct, the strength,
and a properly shaped back for ejecting its foster-brothers, which
then perish from cold and hunger. This has been boldly called a
beneficent arrangement, in order that the young cuckoo may get
sufficient food, and that its foster-brothers may perish before they
had acquired much feeling!
Turning now to the Australian species; though these birds
generally lay only one egg in a nest, it is not rare to find two or
even three eggs in the same nest. In the bronze cuckoo the eggs vary
greatly in size, from eight to ten times in length. Now if it had been
of an advantage to this species to have laid eggs even smaller than
those now laid, so as to have deceived certain foster-parents, or,
as is more probable, to have been hatched within a shorter period (for
it is asserted that there is a relation between the size of eggs and
the period of their incubation), then there is no difficulty in
believing that a race or species might have been formed which would
have laid smaller and smaller eggs; for these would have been more
safely hatched and reared. Mr. Ramsay remarks that two of the
Australian cuckoos, when they lay their eggs in an open nest, manifest
a decided preference for nests containing eggs similar in colour to
their own. The European species apparently manifests some tendency
towards a similar instinct, but not rarely departs from it, as is
shown by her laying her dull and pale-coloured eggs in the nest of the
Hedge-warbler with bright greenish-blue eggs. Had our cuckoo
invariably displayed the above instinct, it would assuredly have
been added to those which it is assumed must all have been acquired
together. The eggs of the Australian bronze cuckoo vary, according
to Mr. Ramsay, to an extraordinary degree in colour; so that in this
respect, as well as in size, natural selection might have secured
and fixed any advantageous variation.
In the case of the European cuckoo, the offspring of the
foster-parents are commonly ejected from the nest within three days
after the cuckoo is hatched; and as the latter at this age is in a
most helpless condition, Mr. Gould was formerly inclined to believe
that the act of ejection was performed by the foster-parents
themselves. But he has now received a trustworthy account of a young
cuckoo which was actually seen, whilst still blind and not able even
to hold up its own head, in the act of ejecting its foster-brothers.
One of these was replaced in the nest by the observer, and was again
thrown out. With respect to the means by which this strange and odious
instinct was acquired, if it were of great importance for the young
cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having
gradually acquired, during successive generations, the blind desire,
the strength, and structure necessary for the work of ejection; for
those young cuckoos which had such habits and structure best developed
would be the most securely reared. The first step towards the
acquisition of the proper instinct might have been more
unintentional restlessness on the part of the young bird, when
somewhat advanced in age and strength; the habit having been
afterwards improved, and transmitted to an earlier age. I can see no
more difficulty in this, than in the unhatched young of other birds
acquiring the instinct to break through their own shells;- or than
in young snakes acquiring in their upper jaws, as Owen has remarked, a
transitory sharp tooth for cutting through the tough egg-shell. For if
each part is liable to individual variations at all ages, and the
variations tend to be inherited at a corresponding or earlier age,-
propositions which cannot be disputed,- then the instincts and
structure of the young could be slowly modified as surely as those
of the adult; and both cases must stand or fall together with the
whole theory of natural selection.
Some species of Molothrus, a widely distinct genus of American
birds, allied to our starlings, have parasitic habits like those of
the cuckoo; and the species present an interesting gradation in the
perfection of their instincts. The sexes of Molothrus badius are
stated by an excellent observer, Mr. Hudson, sometimes to live
promiscuously together in flocks, and sometimes to pair. They either
build a nest of their own, or seize on one belonging to some other
bird, occasionally throwing out the nestlings of the stranger. They
either lay their eggs in the nest thus appropriated, or oddly enough
build one for themselves on the top of it. They usually sit on their
own eggs and rear their own young; but Mr. Hudson says it is
probable that they are occasionally parasitic, for he has seen the
young of this species following old birds of a distinct kind and
clamouring to be fed by them. The parasitic habits of another
species of Molothrus, the M. bonariensis, are much more highly
developed than those of the last, but are still far from perfect. This
bird, as far as it is known, invariably lays its eggs in the nests
of strangers; but it is remarkable that several together sometimes
commence to build an irregular untidy nest of their own, placed in
singularly ill-adapted situations, as on the leaves of a large
thistle. They never, however, as far as Mr. Hudson has ascertained,
complete a nest for themselves. They often lay so many eggs- from
fifteen to twenty- in the same foster-nest, that few or none can
possibly be hatched. They have, moreover, the extraordinary habit of
pecking holes in the eggs, whether of their own species or of their
foster-parents, which they find in the appropriated nests. They drop
also many eggs on the bare ground, which are thus wasted. A third
species, the M. pecoris of North America, has acquired instincts as
perfect as those of the cuckoo, for it never lays more than one egg in
a foster-nest, so that the young bird is securely reared. Mr. Hudson
is a strong disbeliever in evolution, but he appears to have been so
much struck by the imperfect instincts of the Molothrus bonariensis
that he quotes my words, and asks, "Must we consider these habits, not
as especially endowed or created instincts, but as small
consequences of one general law, namely, transition?"
Various birds, as has already been remarked, occasionally lay
their eggs in the nest of other birds. This habit is not very uncommon
with the Gallinaceae, and throws some light on the singular instinct
of the ostrich. In this family several hen-birds unite and lay first a
few eggs in one nest and then in another; and these are hatched by the
males. This instinct may probably be accounted for by the fact of
the hens laying a large number of eggs, but, as with the cuckoo, at
intervals of two or three days. The instinct, however, of the American
ostrich, as in the case of the Molothrus bonariensis, has not as yet
been perfected; for a surprising number of eggs lie strewed over the
plains, so that in one day's hunting I picked up no less than twenty
lost and wasted eggs.
Many bees are parasitic, and regularly lay their eggs in the nests
of other kinds of bees. This case is more remarkable than that of
the cuckoo; for these bees have not only had their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would have been
indispensable if they had stored up food for their own young. Some
species of Sphegidea (wasp-like insects) are likewise parasitic; and
M. Fabre has lately shown good reason for believing that, although the
Tachytes nigra generally makes its own burrow and stores it with
paralysed prey for its own larvae, yet that, when this insect finds
a burrow already made and stored by another species, it takes
advantage of the prize and becomes for the occasion parasitic. In this
case, as with that of the Molothrus or cuckoo, I can see no difficulty
in natural selection making an occasional habit permanent, if of
advantage to the species, and if the insect whose nest and stored food
are feloniously appropriated, be not thus exterminated.
Slave-making instinct.- This remarkable instinct was first
discovered in the Formica (Polyerges) rufescens by Pierre Huber, a
better observer even than his celebrated father. This ant is
absolutely dependent on its slaves; without their aid, the species
would certainly become extinct in a single year. The males and fertile
female do no work of any kind, and the workers or sterile females,
though most energetic and courageous in capturing slaves, do no
other work. They are incapable of making their own nests, or of
feeding their own larvae. When the old nest is found inconvenient, and
they have to migrate, it is the slaves which determine the
migration, and actually carry their masters in their jaws. So
utterly helpless are the masters, that when Huber shut up thirty of
them without a slave, but with plenty of the food which they like
best, and with their own larvae and pupae to stimulate them to work,
they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca),
and she instantly set to work, fed and saved the survivors; made
some cells and tended the larvae, and put all to rights. What can be
more extraordinary than these well-ascertained facts? If we had not
known of any other slave-making ant, it would have been hopeless to
speculate how so wonderful an instinct could have been perfected.
Another species, Formica sanguinea, was likewise first discovered by
P. Huber to be a slave-making ant. This species is found in the
southern parts of England, and its habits have been attended to by Mr.
F. Smith, of the British Museum, to whom I am much indebted for
information on this and other subjects. Although fully trusting to the
statements of Huber and Mr. Smith, I tried to approach the subject
in a sceptical frame of mind, as any one may well be excused for
doubting the existence of so extraordinary an instinct as that of
making slaves. Hence, I will give the observations which I made in
some little detail. I opened fourteen nests of F. sanguinea, and found
a few slaves in all. Males and fertile females of the slave species
(F. fusca) are found only in their own proper communities, and have
never been observed in the nests of F. sanguinea. The slaves are black
and not above half the size of their red masters, so that the contrast
in their appearance is great. When the nest is slightly disturbed, the
slaves occasionally come out, and like their masters are much agitated
and defend the nest: when the nest is much disturbed, and the larvae
and pupae are exposed, the slaves work energetically together with
their masters in carrying them away to a place of safety. Hence, it is
clear, that the slaves feel quite at home. During the months of June
and July, on three successive years, I watched for many hours
several nests in Surrey and Sussex, and never saw a slave either leave
or enter a nest. As, during these months, the slaves are very few in
number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June, and August, both in Surrey and
Hampshire, and has never seen the slaves, though present in large
numbers in August, either leave or enter the nest. Hence he
considers them as strictly household slaves. The masters, on the other
hand, may be constantly seen bringing in materials for the nest, and
food of all kinds. During the year 1860, however, in the month of
July, I came across a community with an unusually large stock of
slaves, and I observed a few slaves mingled with their masters leaving
the nest, and marching along the same road to a tall
Scotch-fir-tree, twenty-five yards distant, which they ascended
together, probably in search of aphides or cocci. According to
Huber, who had ample opportunities for observation, the slaves in
Switzerland habitually work with their masters in making the nest, and
they alone open and close the doors in the morning and evening; and,
as Huber expressly states, their principal office is to search for
aphides. This difference in the usual habits of the masters and slaves
in the two countries, probably depends merely on the slaves being
captured in greater numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one
nest to another, and it was a most interesting spectacle to behold the
masters carefully carrying their slaves in their jaws instead of being
carried by them, as in the case of F. rufescens. Another day my
attention was struck by about a score of the slave-makers haunting the
same spot, and evidently not in search of food; they approached and
were vigorously repulsed by an independent community of the
slave-species (F. fusca); sometimes as many as three of these ants
clinging to the legs of the slavemaking F. sanguinea. The latter
ruthlessly killed their small opponents, and carried their dead bodies
as food to their nest, twenty-nine yards distant; but they were
prevented from getting any pupae to rear as slaves. I then dug up a
small parcel of the pupae of F. fusca from another nest, and put
them down on a bare spot near the place of combat; they were eagerly
seized and carried off by the tyrants, who perhaps fancied that, after
all, they had been victorious in their late combat.
At the same time I laid on the same place a small parcel of the
pupae of another species, F. flava, with a few of these little
yellow ants still clinging to the fragments of their nest. This
species is sometimes, though rarely, made into slaves, as has been
described by Mr. Smith. Although so small a species, it is very
courageous, and I have seen it ferociously attack other ants. In one
instance I found to my surprise an independent community of F. flava
under a stone beneath a nest of the slavemaking F. sanguinea; and when
I had accidentally disturbed both nests, the little ants attacked
their big neighbours with surprising courage. Now I was curious to
ascertain whether F. sanguinea could distinguish the pupae of F.
fusca, which they habitually make into slaves, from those of the
little and furious F. flava, which they rarely capture, and it was
evident that they did at once distinguish them; for we have seen
that they eagerly and instantly seized the pupae of F. fusca,
whereas they were much terrified when they came across the pupae or
even the earth from the nest, of F. flava, and quickly ran away; but
in about a quarter of an hour, shortly after all the little yellow
ants had crawled away, they took heart and carried off the pupae.
One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying
the dead bodies of F. fusca (showing that it was not a migration)
and numerous pupae. I traced a long file of ants burthened with booty,
for about forty yards back, to a very thick clump of heath, whence I
saw the last individual of F. sanguinea emerge, carrying a pupa; but I
was not able to find the desolated nest in the thick heath. The
nest, however, must have been close at hand, for two or three
individuals of F. fusca were rushing about in the greatest
agitation, and one was perched motionless with its own pupa in its
mouth on the top of a spray of heath, an image of despair over its
ravaged home.
Such are the facts, though they did not need confirmation by me,
in regard to the wonderful instinct of making slaves. Let it be
observed what a contrast the instinctive habits of F. sanguinea
present with those of the continental F. rufescens. The latter does
not build its own nest, does not determine its own migrations, does
not collect food for itself or its young, and cannot even feed itself:
it is absolutely dependent on its numerous slaves. Formica
sanguinea, on the other hand, possesses much fewer slaves, and in
the early part of the summer extremely few: the masters determine when
and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves
seem to have the exclusive care of the larvae, and the masters alone
go on slave-making expeditions. In Switzerland the slaves and
masters work together, making and bringing materials for the nest
both, but chiefly the slaves, tend, and milk, as it may be called,
their aphides; and thus both collect food for the community. In
England the masters alone usually leave the nest to collect building
materials and food for themselves, their slaves and larvae. So that
the masters in this country receive much less service from their
slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants which are not slave-makers will, as
I have seen, carry off the pupae of other species, if scattered near
their nests, it is possible that such pupae originally stored as
food might become developed; and the foreign ants thus unintentionally
reared would then follow their proper instincts, and do what work they
could. If their presence proved useful to the species which had seized
them- if it were more advantageous to this species to capture
workers than to procreate them- the habit of collecting pupae,
originally for food, might by natural selection be strengthened and
rendered permanent for the very different purpose of raising slaves.
When the instinct was once acquired, if carried out to a much less
extent even than in our British F. sanguinea, which, as we have
seen, is less aided by its slaves than the same species in
Switzerland, natural selection might increase and modify the instinct-
always supposing each modification to be of use to the species-
until an ant was formed as abjectly dependent on its slaves as is
the Formica rufescens.
Cell-making instinct of the Hive-Bee.- I will not here enter on
minute details on this subject, but will merely give an outline of the
conclusions at which I have arrived. He must be a dull man who can
examine the exquisite structure of a comb, so beautifully adapted to
its end, without enthusiastic admiration. We hear from
mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape to hold the
greatest possible amount of honey, with the least possible consumption
of precious wax in their construction. It has been remarked that a
skilful workman with fitting tools and measures, would find it very
difficult to make cells of wax of the true form, though this is
effected by a crowd of bees working in a dark hive. Granting
whatever instincts you please, it seems at first quite inconceivable
how they can make all the necessary angles and planes, or even
perceive when they are correctly made. But the difficulty is not
nearly so great as it at first appears: all this beautiful work can be
shown, I think, to follow from a few simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has
shown that the form of the cell stands in close relation to the
presence of adjoining cells; and the following view may, perhaps, be
considered only as a modification of this theory. Let us look to the
great principle of gradation, and see whether Nature does not reveal
to us her method of work. At one end of a short series we have
humble-bees, which use their old cocoons to hold honey, sometimes
adding to them short tubes of wax, and likewise making separate and
very irregular rounded cells of wax. At the other end of the series we
have the cells of the hive-bee, placed in a double layer: each cell,
as is well known, is an hexagonal prism, with the basal edges of its
six sides bevelled so as to join an inverted pyramid, of three rhombs.
These rhombs have certain angles, and the three which form the
pyramidal base of a single cell on one side of the comb enter into the
composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the
hive-bee and the simplicity of those of the humble-bee we have the
cells of the Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble-bee, but more nearly related
to the latter; it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition, some large
cells of wax for holding honey. These latter cells are nearly
spherical and of nearly equal sizes, and are aggregated into an
irregular mass. But the important point to notice is, that these cells
are always made at that degree of nearness to each other that they
would have intersected or broken into each other if the spheres had
been completed; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which thus tend to
intersect. Hence, each cell consists of an outer spherical portion,
and of two, three, or more flat surfaces, according as the cell
adjoins two, three, or more other cells. When one cell rests on
three other cells, which, from the spheres being nearly of the same
size, is very frequently and necessarily the case, the three flat
surfaces are united into a pyramid; and this pyramid, as Huber has
remarked, is manifestly a gross imitation of the three-sided pyramidal
base of the cell of the hive-bee. As in the cells of the hive-bee,
so here, the three plane surfaces in any one cell necessarily enter
into the construction of three adjoining cells. It is obvious that the
Melipona saves wax, and what is more important, labour, by this manner
of building; for the flat walls between the adjoining cells are not
double, but are of the same thickness as the outer spherical portions,
and yet each flat portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona
had made its spheres at some given distance from each other, and had
made them of equal sizes and had arranged them symmetrically in a
double layer, the resulting structure would have been as perfect as
the comb of the hive-bee. Accordingly I wrote to Professor Miller of
Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
strictly correct:-
If a number of equal squares be described with their centres
placed in two parallel layers; with the centre of each sphere at the
distance of radius X the square root of 2, or radius X 1.41421 (or
at some lesser distance), from the centres of the six surrounding
spheres in the same layer; and at the same distance from the centres
of the adjoining spheres in the other and parallel layer; then, if
planes of intersection between the several spheres in both layers be
formed, there will result a double layer of hexagonal prisms united
together by pyramidal bases formed of three rhombs; and the rhombs and
the sides of the hexagonal prisms will have every angle identically
the same with the best measurements which have been made of the
cells of the hive-bee. But I hear from Prof. Wyman, who has made
numerous careful measurements, that the accuracy of the workmanship of
the bee has been greatly exaggerated; so much so, that whatever the
typical form of the cell may be, it is rarely, if ever, realised.
Hence we may safely conclude that, if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not
very wonderful, this bee would make a structure as wonderfully perfect
as that of the hive-bee. We must suppose that Melipona to have the
power of forming her cells truly spherical, and of equal sizes, and
this would not be very surprising, seeing that she already does so
to a certain extent, and seeing what perfectly cylindrical burrows
many insects make in wood, apparently by turning round on a fixed
point. We must suppose the Melipona to arrange her cells in level
layers, as she already does her cylindrical cells; and we must further
suppose, and this is the greatest difficulty, that she can somehow
judge accurately at what distance to stand from her fellow-labourers
when several are making their spheres; but she is already so far
enabled to judge of distance, that she always describes her spheres so
as to intersect to a certain extent; and then she unites the points of
intersection by perfectly flat surfaces. By such modifications of
instincts which in themselves are not very wonderful,- hardly more
wonderful than those which guide a bird to make its nest,- I believe
that the hive-bee has acquired, through natural selection, her
inimitable architectural powers.
But this theory can be tested by experiment. Following the example
of Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, rectangular strip of wax: the bees instantly began to
excavate minute circular pits in it; and as they deepened these little
pits, they made them wider and wider until they were converted into
shallow basins, appearing to the eye perfectly true or parts of a
sphere, and of about the diameter of a cell. It was most interesting
to observe that, wherever several bees had begun to excavate these
basins near together, they had begun their work at such a distance
from each other, that by the time the basins had acquired the above
stated width (i.e. about the width of an ordinary cell), and were in
depth about one-sixth of the diameter of the sphere of which they
formed a part, the rims of the basins intersected or broke into each
other. As soon as this occurred, the bees ceased to excavate, and
began to build up flat walls of wax on the lines of intersection
between the basins, so that each hexagonal prism was built upon the
scalloped edge of a smooth basin, instead of on the straight edges
of a three-sided pyramid as in the case of ordinary cells.
I then put into the hive, instead of a thick, rectangular piece of
wax, a thin and narrow, knife-edged ridge, coloured with vermilion.
The bees instantly began on both sides to excavate little basins
near to each other, in the same way as before; but the ridge of wax
was so thin, that the bottoms of the basins, if they had been
excavated to the same depth as in the former experiment, would have
broken into each other from the opposite sides. The bees, however, did
not suffer this to happen, and they stopped their excavations in due
time; so that the basins, as soon as they had been a little
deepened, came to have flat bases; and these flat bases, formed by
thin little plates of the vermilion wax left ungnawed, were
situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite sides of the
ridge of wax. In some parts, only small portions, in other parts,
large portions of a rhombic plate were thus left between the opposed
basins, but the work, from the unnatural state of things, had not been
neatly performed. The bees must have worked at very nearly the same
rate in circularly gnawing away and deepening the basins on both sides
of the ridge of vermilion wax, in order to have thus succeeded in
leaving flat plates between the basins, by stopping work at the planes
of intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip
of wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at
exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just commenced cell, which were
slightly concave on one side, where I suppose that the bees had
excavated too quickly, and convex on the opposed side where the bees
had worked less quickly. In one well-marked instance, I put the comb
back into the hive, and allowed the bees to go on working for a
short time, and again examined the cell, and I found that the
rhombic plate had been completed, and had become perfectly flat: it
was absolutely impossible, from the extreme thinness of the little
plate, that they could have effected this by gnawing away the convex
side; and I suspect that the bees in such cases stand on opposite
sides and push and bend the ductile and warm wax (which as I have
tried is easily done) into its proper intermediate plane, and thus
flatten it.
From the experiment of the ridge of vermilion wax we can see that,
if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the
proper distance from each other, by excavating at the same rate, and
by endeavouring to make equal spherical hollows, but never allowing
the spheres to break into each other. Now bees, as may be clearly seen
by examining the edge of a growing comb, do make a rough,
circumferential wall or rim all round the comb; and they gnaw this
away from the opposite sides, always working circularly as they deepen
each cell. They do not make the whole three-sided pyramidal base of
any one cell at the same time, but only that one rhombic plate which
stands on the extreme growing margin, or the two plates, as the case
may be; and they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these statements
differ from those made by the justly celebrated elder Huber, but I
am convinced of their accuracy; and if I had space, I would show
that they are conformable with my theory.
Huber's statement that the very first cell is excavated out of a
little parallel-sided wall of wax, is not, as far as I have seen,
strictly correct; the first commencement having always been a little
hood of wax; but I will not here enter on details. We see how
important a part excavation plays in the construction of the cells;
but it would be a great error to suppose that the bees cannot build up
a rough wall of wax in the proper position- that is, along the plane
of intersection between two adjoining spheres. I have several
specimens showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing comb, flexures
may sometimes be observed, corresponding in position to the planes
of the rhombic basal plates of future cells. But the rough wall of wax
has in every case to be finished off, by being largely gnawed away
on both sides. The manner in which the bees build is curious; they
always make the first rough wall from ten to twenty times thicker than
the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by supposing
masons first to pile up a broad ridge of cement, and then to begin
cutting it away equally on both sides near the ground, till a
smooth, very thin wall is left in the middle; the masons always piling
up the cut-away cement, and adding fresh cement on the summit of the
ridge. We shall thus have a thin wall steadily growing upward but
always crowned by a gigantic coping. From all the cells, both those
just commenced and those completed, being thus crowned by a strong
coping of wax, the bees can cluster and crawl over the comb without
injuring the delicate hexagonal walls. These walls, as Professor
Miller has kindly ascertained for me, vary greatly in thickness;
being, on an average of twelve measurements made near the border of
the comb, 1/352nd of an inch in thickness; whereas the basal
rhomboidal plates are thicker, nearly in the proportion of three to
two, having a mean thickness, from twenty-one measurements, of 1/229th
of an inch. By the above singular manner of building, strength is
continually given to the comb, with the utmost ultimate economy of
wax.
It seems at first to add to the difficulty of understanding how
the cells are made, that a multitude of bees all work together; one
bee after working a short time at one cell going to another, so
that, as Huber has stated, a score of individuals work even at the
commencement of the first cell. I was able practically to show this
fact, by covering the edges of the hexagonal walls of a single cell,
or the extreme margin of the circumferential rim of a growing comb,
with an extremely thin layer of melted vermilion wax; and I invariably
found that the colour was most delicately diffused by the bees- as
delicately as a painter could have done it with his brush- by atoms of
the coloured wax having been taken from the spot on which it had
been placed, and worked into the growing edges of the cells all round.
The work of construction seems to be a sort of balance struck
between many bees, all instinctively standing at the same relative
distance from each other, all trying to sweep equal spheres, and
then building up, or leaving ungnawed, the planes of intersection
between these spheres. It was really curious to note in cases of
difficulty, as when two pieces of comb met at an angle, how often
the bees would pull down and rebuild in different ways the same
cell, sometimes recurring to a shape which they had at first rejected.
When bees have a place on which they can stand in their proper
positions for working,- for instance, on a slip of wood, placed
directly under the middle of a comb growing downwards, so that the
comb has to be built over one face of the slip- in this case the
bees can lay the foundations of one wall of a new hexagon, in its
strictly proper place, projecting beyond the other completed cells. It
suffices that the bees should be enabled to stand at their proper
relative distances from each other and from the walls of the last
completed cells, and then, by striking imaginary spheres, they can
build up a wall intermediate between two adjoining spheres; but, as
far as I have seen, they never gnaw away and finish off the angles
of a cell till a large part both of that cell and of the adjoining
cells has been built. This capacity in bees of laying down under
certain circumstances a rough wall in its proper place between two
just-commenced cells, is important, as it bears on a fact, which seems
at first subversive of the foregoing theory; namely, that the cells on
the extreme margin of wasp-combs are sometimes strictly hexagonal; but
I have not space here to enter on this subject. Nor does there seem to
me any great difficulty in a single insect (as in the case of a
queen-wasp) making hexagonal cells, if she were to work alternately on
the inside and outside of two or three cells commenced at the same
time, always standing at the proper relative distance from the parts
of the cells just begun, sweeping spheres or cylinders, and building
up intermediate planes.
As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked,
how a long and graduated succession of modified architectural
instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I
think the answer is not difficult: cells constructed like those of the
bee or the wasp gain in strength, and save much in labour and space,
and in the materials of which they are constructed. With respect to
the formation of wax, it is known that bees are often hard pressed
to get sufficient nectar, and I am informed by Mr. Tegetmeier that
it has been experimentally proved that from twelve to fifteen pounds
of dry sugar are consumed by a hive of bees for the secretion of a
pound of wax; so that a prodigious quantity of fluid nectar must be
collected and consumed by the bees in a hive for the secretion of
the wax necessary for the construction of their combs. Moreover,
many bees have to remain idle for many days during the process of
secretion. A large store of honey is indispensable to support a
large stock of bees during the winter; and the security of the hive is
known mainly to depend on a large number of bees being supported.
Hence the saving of wax by largely saving honey and the time
consumed in collecting the honey must be an important element of
success to any family of bees. Of course the success of the species
may be dependent on the number of its enemies, or parasites, or on
quite distinct causes, and so be altogether independent of the
quantity of honey which the bees can collect. But let us suppose
that this latter circumstance determined, as it probably often has
determined, whether a bee allied to our humble-bees could exist in
large numbers in any country; and let us further suppose that the
community lived through the winter, and consequently required a
store of honey: there can in this case be no doubt that it would be an
advantage to our imaginary humble-bee if a slight modification in
her instincts led her to make her waxen cells near together, so as
to intersect a little; for a wall in common even to two adjoining
cells would save some little labour and wax. Hence it would
continually be more and more advantageous to our humble-bees, if
they were to make their cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the
Melipona; for in this case a large part of the bounding surface of
each cell would serve to bound the adjoining cells, and much labour
and wax would be saved. Again, from the same cause, it would be
advantageous to the Melipona, if she were to make her cells closer
together, and more regular in every way than at present; for then,
as we have seen, the spherical surfaces would wholly disappear and
be replaced by plane surfaces; and the Melipona would make a comb as
perfect as that of the hive-bee. Beyond this stage of perfection in
architecture, natural selection could not lead; for the comb of the
hive-bee, as far as we can see, is absolutely perfect in economising
labour and wax.
Thus, as I believe, the most wonderful of all known instincts,
that of the hive-bee, can be explained by natural selection having
taken advantage of numerous, successive, slight modifications of
simpler instincts; natural selection having, by slow degrees, more and
more perfectly led the bees to sweep equal spheres at a given distance
from each other in a double layer, and to build up and excavate the
wax along the planes of intersection; the bees, of course, no more
knowing that they swept their spheres at one particular distance
from each other, than they know what are the several angles of the
hexagonal prisms and of the basal rhombic plates; the motive power
of the process of natural selection having been the construction of
cells of due strength and of the proper size and shape for the larvae,
this being effected with the greatest possible economy of labour and
wax; that individual swarm which thus made the best cells with least
labour, and least waste of honey in the secretion of wax, having
succeeded best, and having transmitted their newly-acquired economical
instincts to new swarms, which in their turn will have had the best
chance of succeeding in the struggle for existence.
Objections to the Theory of Natural Selection as applied to
Instincts: Neuter and Sterile Insects
It has been objected to the foregoing view of the origin of
instincts that "the variations of structure and of instinct must
have been simultaneous and accurately adjusted to each other, as a
modification in the one without an immediate corresponding change in
the other would have been fatal." The force of this objection rests
entirely on the assumption that the changes in the instincts and
structure are abrupt. To take as an illustration the case of the
larger titmouse (Parus major) alluded to in a previous chapter; this
bird often holds the seeds of the yew between its feet on a branch,
and hammers with its beak till it gets at the kernel. Now what special
difficulty would there be in natural selection preserving all the
slight individual variations in the shape of the beak, which were
better and better adapted to break open the seeds, until a beak was
formed, as well constructed for this purpose as that of the
nuthatch, at the same time that habit, or compulsion, or spontaneous
variations of taste, led the bird to become more and more of a
seed-eater? In this case the beak is supposed to be slowly modified by
natural selection, subsequently to, but in accordance with, slowly
changing habits or taste; but let the feet of the titmouse vary and
grow larger from correlation with the beak, or from any other
unknown cause, and it is not improbable that such larger feet would
lead the bird to climb more and more until it acquired the
remarkable climbing instinct and power of the nuthatch. In this case a
gradual change of structure is supposed to lead to changed instinctive
habits. To take one more case: few instincts are more remarkable
than that which leads the swift of the Eastern Islands to make its
nest wholly of inspissated saliva. Some birds build their nests of
mud, believed to be moistened with saliva; and one of the swifts of
North America makes its nest (as I have seen) of sticks agglutinated
with saliva, and even with flakes of this substance. Is it then very
improbable that the natural selection of individual swifts, which
secreted more and more saliva, should at last produce a species with
instincts leading it to neglect other materials, and to make its
nest exclusively of inspissated saliva? And so in other cases. It
must, however, be admitted that in many instances we cannot conjecture
whether it was instinct or structure which first varied.
No doubt many instincts of very difficult explanation could be
opposed to the theory of natural selection- cases, in which we
cannot see how an instinct could have originated; cases, in which no
intermediate gradations are known to exist; cases of instincts of such
trifling importance, that they could hardly have been acted on by
natural selection; cases of instincts almost identically the same in
animals so remote in the scale of nature, that we cannot account for
their similarity by inheritance from a common progenitor, and
consequently must believe that they were independently acquired
through natural selection. I will not here enter on these several
cases, but will confine myself to one special difficulty, which at
first appeared to me insuperable, and actually fatal to the whole
theory. I allude to the neuters or sterile females in
insect-communities; for these neuters often differ widely in
instinct and in structure from both the males and fertile females, and
yet, from being sterile, they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I
will here take only a single case, that of working or sterile ants.
How the workers have been rendered sterile is a difficulty; but not
much greater than that of any other striking modification of
structure; for it can be shown that some insects and other
articulate animals in a state of nature occasionally become sterile;
and if such insects had been social, and it had been profitable to the
community that a number should have been annually born capable of
work, but incapable of procreation, I can see no especial difficulty
in this having been effected through natural selection. But I must
pass over this preliminary difficulty. The great difficulty lies in
the working ants differing widely from both the males and the
fertile females in structure, as in the shape of the thorax, and in
being destitute of wings and sometimes of eyes, and in instinct. As
far as instinct alone is concerned, the wonderful difference in this
respect between the workers and the perfect females, would have been
better exemplified by the hive-bee. If a working ant or other neuter
insect had been an ordinary animal, I should have unhesitatingly
assumed that all its characters had been slowly acquired through
natural selection; namely, by individuals having been born with slight
profitable modifications, which were inherited by the offspring; and
that these again varied and again were selected, and so onwards. But
with the working ant we have an insect differing greatly from its
parents, yet absolutely sterile; so that it could never have
transmitted successively acquired modifications of structure or
instinct to its progeny. It may well be asked how is it possible to
reconcile this case with the theory of natural selection?
First, let it be remembered that we have innumerable instances, both
in our domestic productions and in those in a state of nature, of
all sorts of differences of inherited structure which are correlated
with certain ages, and with either sex. We have differences correlated
not only with one sex, but with that short period when the
reproductive system is active, as in the nuptial plumage of many
birds, and in the hooked jaws of the male salmon. We have even
slight differences in the horns of different breeds of cattle in
relation to an artificially imperfect state of the male sex; for
oxen of certain breeds have longer horns than the oxen of other
breeds, relatively to the length of the horns in both the bulls and
cows of these same breeds. Hence I can see no great difficulty in
any character becoming correlated with the sterile condition of
certain members of insect communities: the difficulty lies in
understanding how such correlated modifications of structure could
have been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain
the desired end. Breeders of cattle wish the flesh and fat to be
well marbled together: an animal thus characterised has been
slaughtered, but the breeder has gone with confidence to the same
stock and has succeeded. Such faith may be placed in the power of
selection, that a breed of cattle, always yielding oxen with
extraordinarily long horns, could, it is probable, be formed by
carefully watching which individual bulls and cows, when matched,
produced oxen with the longest horns; and yet no ox would ever have
propagated its kind. Here is a better and real illustration: according
to M. Verlot, some varieties of the double annual Stock from having
been long and carefully selected to the right degree, always produce a
large proportion of seedlings bearing double and quite sterile
flowers; but they likewise yield some single and fertile plants. These
latter, by which alone the variety can be propagated, may be
compared with the fertile male and female ants, is ants, and the
double sterile plants with the neuters of the same community. As
with the varieties of the stock, so with social insects, selection has
been applied to the family, and not to the individual, for the sake of
gaining a serviceable end. Hence we may conclude that slight
modifications of structure or of instinct, correlated with the sterile
condition of certain members of the community, have proved
advantageous: consequently the fertile males and females have
flourished, and transmitted to their fertile offspring a tendency to
produce sterile members with the same modifications. This process must
have been repeated many times, until that prodigious amount of
difference between the fertile and sterile females of the same species
has been produced, which we see in many social insects.
But we have not as yet touched on the acme of the difficulty;
namely, the fact that the neuters of several ants differ, not only
from the fertile females and males, but from each other, sometimes
to an almost incredible degree, and are thus divided into two or
even three castes. The castes, moreover, do not commonly graduate into
each other, but are perfectly well defined; being as distinct from
each other as are any two species of the same genus, or rather as
any two genera of the same family. Thus in Eciton, there are working
and soldier neuters, with jaws and instincts extraordinarily
different: in Cryptocerus, the workers of one caste alone carry a
wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecoeystus, the workers of one caste
never leave the nest; they are fed by the workers of another caste,
and they have an enormously developed abdomen which secretes a sort of
honey, supplying the place of that excreted by the aphides, or the
domestic cattle as they may be called, which our European ants guard
and imprison.
It will indeed be thought that I have an overweening confidence in
the principle of natural selection, when I do not admit that such
wonderful and well-established facts at once annihilate the theory. In
the simpler case of neuter insects all of one caste, which, as I
believe, have been rendered different from the fertile males and
females through natural selection, we may conclude from the analogy of
ordinary variations, that the successive, slight, profitable
modifications did not first arise in all the neuters in the same nest,
but in some few alone; and that by the survival of the communities
with females which produced most INSTINCT is neuters having the
advantageous modifications, all the neuters ultimately came to be thus
characterised. According to this view we ought occasionally to find in
the same nest neuter insects, presenting gradations of structure;
and this we do find, even not rarely, considering how few neuter
insects out of Europe have been carefully examined. Mr. F. Smith has
shown that the neuters of several British ants differ surprisingly
from each other in size and sometimes in colour; and that the
extreme forms can be linked together by individuals taken out of the
same nest: I have myself compared perfect gradations of this kind.
It sometimes happens that the larger or the smaller sized workers
are the most numerous; or that both large and small are numerous,
whilst those of an intermediate size are scanty in numbers. Formica
lava has larger and smaller workers, with some few of intermediate
size; and, in this species, as Mr. F. Smith has observed, the larger
workers have simple eyes (ocelli), which though small can be plainly
distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these
workers, I can affirm that the eyes are far more rudimentary in the
smaller workers than can be accounted for merely by their
proportionally lesser size; and I fully believe, though I dare not
assert so positively, that the workers of intermediate size have their
ocelli in an exactly intermediate condition. So that here we have
two bodies of sterile workers in the same nest, differing not only
in size, but in their organs of vision, yet connected by some few
members in an intermediate condition. I may digress by adding, that if
the smaller workers had been the most useful to the community, and
those males and females had been continually selected, which
produced more and more of the smaller workers, until all the workers
were in this condition; we should then have had a species of ant
with neuters in nearly the same condition as those of Myrmica. For the
workers of Myrmica have not even rudiments of ocelli, though the
male and female ants of this genus have well-developed ocelli.
I may give one other case: so confidently did I expect
occasionally to find gradations of important structures between the
different castes of neuters in the same species, that I gladly availed
myself of Mr. F. Smith's offer of numerous specimens from the same
nest of the driver ant (Anomma) of West Africa. The reader will
perhaps best appreciate the amount of difference in these workers,
by my giving not the actual measurements, but a strictly accurate
illustration: the difference was the same as if we were to see a set
of workmen building a house, of whom many were five feet four inches
high, and many sixteen feet high; but we must in addition suppose that
the larger workmen had heads four instead of three times as big as
those of the smaller men, and jaws nearly five times as big. The jaws,
moreover, of the working ants of the several sizes differed
wonderfully in shape, and in the form and number of the teeth. But the
important fact for us is, that, though the workers can be grouped into
castes of different size, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as Sir J. Lubbock made drawings
for me, with the camera lucida, of the jaws which I dissected from the
workers of the several sizes. Mr. Bates, in his interesting Naturalist
on the Amazons, has described analogous cases.
With these facts before me, I believe that natural selection, by
acting on the fertile ants or parents, could form a species which
should regularly produce neuters, all of large size with one form of
jaw, or all of small size with widely different jaws; or lastly, and
this is the greatest difficulty, one set of workers of one size and
structure, and simultaneously another set of workers of a different
size and structure;- a graduated series having first been formed, as
in the case of the driver ant, and then the extreme forms having
been produced in greater and greater numbers, through the survival
of the parents which generated them, until none with an intermediate
structure were produced.
An analogous explanation has been given by Mr. Wallace, of the
equally complex case, of certain Malayan butterflies regularly
appearing under two or even three distinct female forms; and by
Fritz Muller, of certain Brazilian crustaceans likewise appearing
under two widely distinct male forms. But this subject need not here
be discussed.
I have now explained how, as I believe, the wonderful fact of two
distinctly defined castes of sterile workers existing in the same
nest, both widely different from each other and from their parents,
has originated. We can see how useful their production may have been
to a social community of ants, on the same principle that the division
of labour is useful to civilised man. Ants, however, work by inherited
instincts and by inherited organs or tools, whilst man works by
acquired knowledge and manufactured instruments. But I must confess,
that, with all my faith in natural selection, I should never have
anticipated that this principle could have been efficient in so high a
degree, had not the case of these neuter insects led me to this
conclusion. I have, therefore, discussed this case, at some little but
wholly insufficient length, in order to show the power of natural
selection, and likewise because this is by far the most serious
special difficulty which my theory has encountered. The case, also, is
very interesting, as it proves that with animals, as with plants,
any amount of modification may be effected by the accumulation of
numerous, slight, spontaneous variations, which are in any way
profitable, without exercise or habit having been brought into play.
For peculiar habits confined to the workers or sterile females,
however long they might be followed, could not possibly affect the
males and fertile females, which alone leave descendants. I am
surprised that no one has hitherto advanced this demonstrative case of
neuter insects, against the well-known doctrine of inherited habit, as
advanced by Lamarck.
Summary
I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that
instincts vary slightly in a state of nature. No one will dispute that
instincts are of the highest importance to each animal. Therefore
there is no real difficulty, under changing conditions of life, in
natural selection accumulating to any extent slight modifications of
instinct which are in any way useful. In many cases habit or use and
disuse have probably come into play. I do not pretend that the facts
given in this chapter strengthen in any great degree my theory; but
none of the cases of difficulty, to the best of my judgment,
annihilate it. On the other hand, the fact that instincts are not
always absolutely perfect and are liable to mistakes;- that no
instinct can be shown to have been produced for the good of other
animals, though animals take advantage of the instincts of others;-
that the canon in natural history, of "Natura non facit saltum," is
applicable to instincts as well as to corporeal structure, and is
plainly explicable on the foregoing views, but is otherwise
inexplicable, all tend to corroborate the theory of natural selection.
This theory is also strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but distinct
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining
nearly the same instincts. For instance, we can understand, on the
principle of inheritance, how it is that the thrush of tropical
South America lines its nest with mud, in the same peculiar manner
as does our British thrush; how it is that the hornbills of Africa and
India have the same extraordinary instinct of plastering up and
imprisoning the females in a hole in a tree, with only a small hole
left in the plaster through which the males feed them and their
young when hatched; how it is that the male wrens (Troglodytes) of
North America build "cocknests," to roost in, like the males of our
kittywrens,- a habit wholly unlike that of any other known bird.
Finally, it may not be a logical deduction, but to my imagination it
is far more satisfactory to look at such instincts as the young cuckoo
ejecting its foster-brothers,- ants making slaves,- the larvae of
ichneumonidea feeding within the live bodies of caterpillars,- not
as specially endowed or created instincts, but as small consequences
of one general law leading to the advancement of all organic
beings,- namely, multiply, vary, let the strongest live and the
weakest die.
CHAPTER IX
HYBRIDISM
THE view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order
to prevent their confusion. This view certainly seems at first
highly probable, for species living together could hardly have been
kept distinct had they been capable of freely crossing. The subject is
in many ways important for us, more especially as the sterility of
species when first crossed, and that of their hybrid offspring, cannot
have been acquired, as I shall show, by the preservation of successive
profitable degrees of sterility. It is an incidental result of
differences in the reproductive systems of the parent-species.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the
hybrids produced from them.
Pure species have of course their organs of reproduction in a
perfect condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive
organs functionally impotent, as may be clearly seen in the state of
the male element in both plants and animals; though the formative
organs themselves are perfect in structure, as far as the microscope
reveals. In the first case the two sexual elements which go to form
the embryo are perfect; in the second case they are either not at
all developed, or are imperfectly developed. This distinction is
important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction probably has been slurred
over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed
to be descended from common parents, when crossed, and likewise the
fertility of their mongrel offspring, is, with reference to my theory,
of equal importance with the sterility of species; for it seems to
make a broad and clear distinction between varieties and species.
Degrees of Sterility.- First, for the sterility of species when
crossed and of their hybrid offspring. It is impossible to study the
several memoirs and works of those two conscientious and admirable
observers, Kolreuter and Gartner, who almost devoted their lives to
this subject, without being deeply impressed with the high
generality of some degree of sterility. Kolreuter makes the rule
universal; but then he cuts the knot, for in ten cases in which he
found two forms, considered by most authors as distinct species, quite
fertile together, he unhesitatingly ranks them as varieties.
Gartner, also, makes the rule equally universal; and he disputes the
entire fertility of Kolreuter's ten cases. But in these and in many
other cases, Gartner is obliged carefully to count the seeds, in order
to show that there is any degree of sterility. He always compares
the maximum number of seeds produced by two species when first
crossed, and the maximum produced by their hybrid offspring, with
the average number produced by both pure parent-species in a state
of nature. But causes of serious error here intervene: a plant, to
be hybridised, must be castrated, and, what is often more important,
must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimented on by
Gartner were potted, and were kept in a chamber in his house. That
these processes are often injurious to the fertility of a plant cannot
be doubted; for Gartner gives in his table about a score of cases of
plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosae, in which
there is an acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired. Moreover,
as Gartner repeatedly crossed some forms, such as the common red and
blue pimpernels (Anagallis arvensis and caerulea), which the best
botanists rank as varieties, and found them absolutely sterile, we may
doubt whether many species are really so sterile, when intercrossed,
as he believed.
It is certain, on the one hand, that the sterility of various
species when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends
and sterility begins. I think no better evidence of this can be
required than that the two most experienced observers who have ever
lived, namely Kolreuter and Gartner, arrived at diametrically opposite
conclusions in regard to some of the very same forms. It is also
most instructive to compare- but I have not space here to enter on
details- the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same observer from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and
varieties. The evidence from this source graduates away, and is
doubtful in the same degree as is the evidence derived from other
constitutional and structural differences.
In regard to the sterility of hybrids in successive generations:
though Gartner was enabled to rear some hybrids, carefully guarding
them from a cross with either pure parent, for six or seven, and in
one case for ten generations, yet he asserts positively that their
fertility never increases, but generally decreases greatly and
suddenly. With respect to this decrease, it may first be noticed
that when any deviation in structure or constitution is common to both
parents, this is often transmitted in an augmented degree to the
offspring; and both sexual elements in hybrid plants are already
affected in some degree. But I believe that their fertility has been
diminished in nearly all these cases by an independent cause,
namely, by too close interbreeding. I have made so many experiments
and collected so many facts, showing on the one hand that an
occasional cross with a distinct individual or variety increases the
vigour and fertility of the offspring, and on the other hand that very
close interbreeding lessens their vigour and fertility, that I
cannot doubt the correctness of this conclusion. Hybrids are seldom
raised by experimentalists in great numbers; and as the
parent-species, or other allied hybrids, generally grow in the same
garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids, if left to themselves, will generally
be fertilised during each generation by pollen from the same flower;
and this would probably be injurious to their fertility, already
lessened by their hybrid origin. I am strengthened in this
conviction by a remarkable statement repeatedly made by Gartner,
namely, that if even the less fertile hybrids be artificially
fertilised with hybrid pollen of the same kind, their fertility,
notwithstanding the frequent ill effects from manipulation,
sometimes decidedly increases, and goes on increasing. Now, in the
process of artificial fertilisation, pollen is as often taken by
chance (as I know from my own experience) from the anthers of
another flower, as from the anthers of the flower itself which is to
be fertilised; so that a cross between two flowers, though probably
often on the same plant, would be thus effected. Moreover, whenever
complicated experiments are in progress, so careful an observer as
Gartner would have castrated his hybrids, and this would have
ensured in each generation a cross with pollen from a distinct flower,
either from the same plant or from another plant of the same hybrid
nature. And thus, the strange fact of an increase of fertility in
the successive generations of artificially fertilised hybrids, in
contrast with those spontaneously self-fertilised, may, as I
believe, be accounted for by too close interbreeding having been
avoided.
Now let us turn to the results arrived at by a third most
experienced hybridiser, namely, the Hon. and Rev. W. Herbert. He is as
emphatic in his conclusion that some hybrids are perfectly fertile- as
fertile as the pure parent-species- as are Kolreuter and Gartner
that some degree of sterility between distinct species is a
universal law of nature. He experimented on some of the very same
species as did Gartner. The difference in their results may, I
think, be in part accounted for by Herbert's great horticultural
skill, and by his having hot-houses at his command. Of his many
important statements I will here give only a single one as an example,
namely, that "every ovule in a pod of Crinum capense fertilised by
C. revolutum produced a plant, which I never saw to occur in a case of
its natural fecundation." So that here we have perfect or even more
than commonly perfect fertility, in a first cross between two distinct
species.
This case of the Crinum leads me to refer to a singular fact,
namely, that individual plants of certain species of Lobelia,
Verbascum and Passiflora, can easily be fertilised by pollen from a
distinct species, but not by pollen from the same plant, though this
pollen can be proved to be perfectly sound by fertilising other plants
or species. In the genus Hippeastrum, in Corydalis as shown by
Professor Hildebrand, in various orchids as shown by Mr. Scott and
Fritz Muller, all the individuals are in this peculiar condition. So
that with some species, certain abnormal individuals, and in other
species all the individuals, can actually be hybridised much more
readily than they can be fertilised by pollen from the same individual
plant! To give one instance, a bulb of Hippeastrum aulicum produced
four flowers; three were fertilised by Herbert with their own
pollen, and the fourth was subsequently fertilised by the pollen of
a compound hybrid descended from three distinct species: the result
was that "the ovaries of the three first flowers soon ceased to
grow, and after a few days perished entirely, whereas the pod
impregnated by the pollen of the hybrid made vigorous growth and rapid
progress to maturity, and bore good seed, which vegetated freely." Mr.
Herbert tried similar experiments during many years, and always with
the same result. These cases serve to show on what slight and
mysterious causes the lesser or greater fertility of a species
sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &c., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid
from Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, "reproduces itself as perfectly as if
it had been a natural species from the mountains of Chili." I have
taken some pains to ascertain the degree of fertility of some of the
complex crosses of rhododendrons, and I am assured that many of them
are perfectly fertile. Mr. C. Noble, for instance, informs me that
he raises stocks for grafting from a hybrid between Rhod. ponticum and
catawbiense, and that this hybrid "seeds as freely as it is possible
to imagine." Had hybrids when fairly treated, always gone on
decreasing in fertility in each successive generation, as Gartner
believed to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrid, and
such alone are fairly treated, for by insect agency the several
individuals are allowed to cross freely with each other, and the
injurious influence of close interbreeding is thus prevented. Any
one may readily convince himself of the efficiency of insect-agency by
examining the flowers of the more sterile kinds of hybrid
rhododendrons, which produce no pollen for he will find on their
stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully
tried than with plants. If our systematic arrangements can be trusted,
that is, if the genera of animals are as distinct from each other as
are the genera of plants, then we may infer that animals more widely
distinct in the scale of nature can be crossed more easily than in the
case of plants; but the hybrids themselves are, I think, more sterile.
It should, however, be borne in mind that, owing to few animals
breeding freely under confinement, few experiments have been fairly
tried: for instance, the canary-bird has been crossed with nine
distinct species of finches, but, as not one of these breeds freely in
confinement, we have no right to expect that the first crosses between
them and the canary, or that their hybrids, should be perfectly
fertile. Again, with respect to the fertility in successive
generations of the more fertile hybrid animals, I hardly know of an
instance in which two families of the same hybrid have been raised
at the same time from different parents, so as to avoid the ill
effects of close interbreeding. On the contrary, brothers and
sisters have usually been crossed in each successive generation, in
opposition to the constantly repeated admonition of every breeder. And
in this case, it is not at all surprising that the inherent
sterility in the hybrids should have gone on increasing.
Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the
hybrids from Cervulus vaginalis and reevesii, and from Phasianus
colchicus with P. torquatus, are perfectly fertile. M. Quatrefages
states that the hybrids from two moths (Bombyx cynthia and arrindia)
were proved in Paris to be fertile inter se for eight generations.
It has lately been asserted that two such distinct species as the hare
and rabbit, when they can be got to breed together, produce
offspring which are highly fertile when crossed with one of the
parent-species. The hybrids from the common and Chinese geese (A.
cygnoides), species which are so different that they are generally
ranked in distinct genera, have often bred in this country with either
pure parent, and in one single instance they have bred inter se.
This was effected by Mr. Eyton, who raised two hybrids from the same
parents, but from different hatches; and from these two birds he
raised no less than eight hybrids (grandchildren of the pure geese)
from one nest. In India, however, these cross-bred geese must be far
more fertile; for I am assured by two eminently capable judges, namely
Mr. Blyth and Capt. Hutton, that whole flocks of these crossed geese
are kept in various parts of the country; and as they are kept for
profit, where neither pure parent-species exists, they must
certainly be highly or perfectly fertile.
With our domesticated animals, the various races when crossed
together are quite fertile; yet in many cases they are descended
from two or more wild species. From this fact we must conclude
either that the aboriginal parent-species at first produced
perfectly fertile hybrids, or that the hybrids subsequently reared
under domestication became quite fertile. This latter alternative,
which was first propounded by Pallas, seems by far the most
probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild stocks;
yet, with perhaps the exception of certain indigenous domestic dogs of
South America, all are quite fertile together; but analogy makes me
greatly doubt whether the several aboriginal species would at first
have freely bred together and have produced quite fertile hybrids.
So again I have lately acquired decisive evidence that the crossed
offspring from the Indian humped and common cattle are inter se
perfectly fertile; and from the observations by Rutimeyer on their
important osteological differences, as well as from those by Mr. Blyth
on their differences in habits, voice, constitution, &c., these two
forms must be regarded as good and distinct species. The same
remarks may be extended to the two chief races of the pig. We must,
therefore, either give up the belief of the universal sterility of
species when crossed; or we must look at this sterility in animals,
not as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, considering all the ascertained facts on the
intercrossing of plants and animals, it may be concluded that some
degree of sterility, both in first crosses and in hybrids, is an
extremely general result; but that it cannot, under our present
state of knowledge, be considered as absolutely universal.
Laws governing the Sterility of first Crosses and of Hybrids
We will now consider a little more in detail the laws governing
the sterility of first crosses and of hybrids. Our chief object will
be to see whether or not these laws indicate that species have been
specially endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following
conclusions are drawn up chiefly from Gartner's admirable work on
the hybridisation of plants. I have taken much pains to ascertain
how far they apply to animals, and, considering how scanty our
knowledge is in regard to hybrid animals, I have been surprised to
find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both
of first crosses and of hybrids, graduates from zero to perfect
fertility. It is surprising in how many curious ways this gradation
can be shown; but only the barest outline of the facts can here be
given. When pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no more influence
than so much inorganic dust. From this absolute zero of fertility, the
pollen of different species applied to the stigma of some one
species of the same genus, yields a perfect gradation in the number of
seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen
produces. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even with the pollen of
the pure parents, a single fertile seed: but in some of these cases
a first trace of fertility may be detected, by the pollen of one of
the pure parent-species causing the flower of the hybrid to wither
earlier than it otherwise would have done; and the early withering
of the flower is well known to be a sign of incipient fertilisation.
From this extreme degree of sterility we have self-sterilised
hybrids producing a greater and greater number of seeds up to
perfect fertility.
The hybrids raised from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very
sterile; but the parallelism between the difficulty of making a
first cross, and the sterility of the hybrids thus produced- two
classes of facts which are generally confounded together- is by no
means strict. There are many cases, in which two pure species, as in
the genus Verbascum, can be united with unusual facility, and
produce numerous hybrid offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed
very rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same
genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more
easily affected by unfavourable conditions, than is that of pure
species. But the fertility of first crosses is likewise innately
variable; for it is not always the same in degree when the same two
species are crossed under the same circumstances; it depends in part
upon the constitution of the individuals which happen to have been
chosen for the experiment. So it is with hybrids, for their degree
of fertility is often found to differ greatly in the several
individuals raised from seed out of the same capsule and exposed to
the same conditions.
By the term systematic affinity is meant, the general resemblance
between species in structure and constitution. Now the fertility of
first crosses, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by
hybrids never having been raised between species ranked by
systematists in distinct families; and on the other hand, by very
closely allied species generally uniting with facility. But the
correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be given of
very closely allied species which will not unite, or only with extreme
difficulty; and on the other hand of very distinct species which unite
with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed;
and another genus, as Silene, in which the most persevering efforts
have failed to produce between extremely close species a single
hybrid. Even within the limits of the same genus, we meet with this
same difference; for instance, the many species of Nicotiana have been
more largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly
distinct species, obstinately failed to fertilise, or to be fertilised
by no less than eight other species of Nicotiana. Many analogous facts
could be given.
No one has been able to point out what kind or what amount of
difference, in any recognisable character, is sufficient to prevent
two species crossing. It can be shown that plants most widely
different in habit and general appearance, and having strongly
marked differences in every part of the flower, even in the pollen, in
the fruit, and in the cotyledons, can be crossed. Annual and perennial
plants, deciduous and evergreen trees, plants inhabiting different
stations and fitted for extremely different climates, can often be
crossed with ease.
By a reciprocal cross between two species, I mean the case, for
instance, of a female-ass being first crossed by a stallion, and
then a mare by a male-ass; these two species may then be said to
have been reciprocally crossed. There is often the widest possible
difference in the facility of making reciprocal crosses. Such cases
are highly important, for they prove that the capacity in any two
species to cross is often completely independent of their systematic
affinity, that is of any difference in their structure or
constitution, excepting in their reproductive systems. The diversity
of the result in reciprocal crosses between the same two species was
long ago observed by Kolreuter. To give an instance: Mirabilis
jalapa can easily be fertilised by the pollen of M. longiflora, and
the hybrids thus produced are sufficiently fertile; but Kolreuter
tried more than two hundred times, during eight following years, to
fertilise reciprocally M. longiflora with the pollen of M. jalapa, and
utterly failed. Several other equally striking cases could be given.
Thuret has observed the same fact with certain sea-weeds or Fuci.
Gartner, moreover, found that this difference of facility in making
reciprocal crosses is extremely common in a lesser degree. He has
observed it even between closely related forms (as Matthiola annua and
gilabra) which many botanists rank only as varieties. It is also a
remarkable fact, that hybrids raised from reciprocal crosses, though
of course compounded of the very same two species, the one species
having first been used as the father and then as the mother, though
they rarely differ in external characters, yet generally differ in
fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from Gartner: for
instance, some species have a remarkable power of crossing with
other species; other species of the same genus have a remarkable power
of impressing their likeness on their hybrid offspring; but these
two powers do not at all necessarily go together. There are certain
hybrids which, instead of having, as is usual, an intermediate
character between their two parents, always closely resemble one of
them; and such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are
born, which closely resemble one of their pure parents; and these
hybrids are almost always utterly sterile, even when the other hybrids
raised from seed from the same capsule have a considerable degree of
fertility. These facts show how completely the fertility of a hybrid
may be independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the
fertility of first causes and of hybrids, we see that when forms,
which must be considered as good and distinct species, are united,
their fertility graduates from zero to perfect fertility, or even to
fertility under certain conditions in excess; that their fertility,
besides being eminently susceptible to favourable and unfavourable
conditions, is innately variable; that it is by no means always the
same in degree in the first cross and in the hybrids produced from
this cross; that the fertility of hybrids is not related to the degree
in which they resemble in external appearance either parent; and
lastly, that the facility of making a first cross between any two
species is not always governed by their systematic affinity or
degree of resemblance to each other. This latter statement is
clearly proved by the difference in the result of reciprocal crosses
between the same two species, for, according as the one species or the
other is used as the father or the mother, there is generally some
difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from
reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species have
been endowed with sterility simply to prevent their becoming
confounded in nature? I think not. For why should the sterility be
so extremely different in degree, when various species are crossed,
all of which we must suppose it would be equally important to keep
from blending together? Why should the degree of sterility be innately
variable in the individuals of the same species? Why should some
species cross with facility, and yet produce very sterile hybrids; and
other species cross with extreme difficulty, and yet produce fairly
fertile hybrids? Why should there often be so great a difference in
the result of a reciprocal cross between the same two species? Why, it
may even be asked, has the production of hybrids been permitted? To
grant to species the special power of producing hybrids, and then to
stop their further propagation by different degrees of sterility,
not strictly related to the facility of the first union between
their parents, seems a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me
clearly to indicate that the sterility both of first crosses and of
hybrids is simply incidental or dependent on unknown differences in
their reproductive systems; the differences being of so peculiar and
limited a nature, that, in reciprocal crosses between the same two
species, the male sexual element of the one will often freely act on
the female sexual element of the other, but not in a reversed
direction. It will be advisable to explain a little more fully by an
example what I mean by sterility being incidental on other
differences, and not a specially endowed quality. As the capacity of
one plant to be grafted or budded on another is unimportant for
their welfare in a state of nature, I presume that no one will suppose
that this capacity is a specially endowed quality, but will admit that
it is incidental on differences in the laws of growth of the two
plants. We can sometimes see the reason why one tree will not take
on another, from differences in their rate of growth, in the
hardness of their wood, in the period of the flow or nature of their
sap, &c.; but in a multitude of cases we can assign no reason
whatever. Great diversity in the size of two plants, one being woody
and the other herbaceous, one being evergreen and the other deciduous,
and adaptation to widely different climates, do not always prevent the
two grafting together. As in hybridisation, so with grafting, the
capacity is limited by systematic affinity, for no one has been able
to graft together trees belonging to quite distinct families; and,
on the other hand, closely allied species, and varieties of the same
species, can usually, but not invariably, be grafted with ease. But
this capacity, as in hybridisation, is by no means absolutely governed
by systematic affinity. Although many distinct genera within the
same family have been grafted together, in other cases species of
the same genus will not take on each other. The pear can be grafted
far more readily on the quince, which is ranked as a distinct genus,
than on the apple, which is a member of the same genus. Even different
varieties of the pear take with different degrees of facility on the
quince; so do different varieties of the apricot and peach on
certain varieties of the plum.
As Gartner found that there was sometimes an innate difference in
different individuals of the same two species in crossing; so
Sageret believes this to be the case with different individuals of the
same two species in being grafted together. As in reciprocal
crosses, the facility of effecting an union is often very far from
equal, so it sometimes is in grafting; the common gooseberry, for
instance, cannot be grafted on the currant, whereas the current will
take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a different case
from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct classes of cases
run to a large extent parallel. Something analogous occurs in
grafting; for Thouin found that three species of Robinia, which seeded
freely on their own roots, and which could be grafted with no great
difficulty on a fourth species, when thus grafted were rendered
barren. On the other hand, certain species of Sorbus, when grafted
on other species yielded twice as much fruit as when on their own
roots. We are reminded by this latter fact of the extraordinary
cases of Hippeastrum, Passiflora, &c., which seed much more freely
when fertilised with the pollen of a distinct species, than when
fertilised with pollen from the same plant.
We thus see, that, although there is a clear and great difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and of
crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted
on each other as incidental on unknown differences in their vegetative
systems, so I believe that the still more complex laws governing the
facility of first crosses are incidental on unknown differences in
their reproductive systems. These differences in both cases, follow to
a certain extent, as might have been expected, systematic affinity, by
which term every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed. The facts by no means seem to
indicate that the greater or lesser difficulty of either grafting or
crossing various species has been a special endowment; although in the
case of crossing, the difficulty is as important for the endurance and
stability of specific forms, as in the case of grafting it is
unimportant for their welfare.
Origin and Causes of the Sterility of first Crosses and of Hybrids
At one time it appeared to me probable, as it has to others, that
the sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slightly lessened degrees of
fertility, which, like any other variation, spontaneously appeared
in certain individuals of one variety when crossed with those of
another variety. For it would clearly be advantageous to two varieties
or incipient species, if they could be kept from blending, on the same
principle that, when man is selecting at the same time two
varieties, it is necessary that he should keep them separate. In the
first place, it may be remarked that species inhabiting distinct
regions are often sterile when crossed; now it could clearly have been
of no advantage to such separated species to have been rendered
mutually sterile, and consequently this could not have been effected
through natural selection; but it may perhaps be argued, that, if a
species was rendered sterile with some one compatriot, sterility
with other species would follow as a necessary contingency. In the
second place, it is almost as much opposed to the theory of natural
selection as to that of special creation, that in reciprocal crosses
the male element of one form should have been rendered utterly
impotent on a second form, whilst at the same time the male element of
this second form is enabled freely to fertilise the first form; for
this peculiar state of the reproductive system could hardly have
been advantageous to either species.
In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps from
slightly lessened fertility to absolute sterility. It may be
admitted that it would profit an incipient species, if it were
rendered in some slight degree sterile when crossed with its parent
form or with some other variety; for thus fewer bastardised and
deteriorated offspring would be produced to commingle their blood with
the new species in process of formation. But he who will take the
trouble to reflect on the steps by which this first degree of
sterility could be increased through natural selection to that high
degree which is common with so many species, and which is universal
with species which have been differentiated to a generic or family
rank, will find the subject extraordinarily complex. After mature
reflection it seems to me that this could not have been effected
through natural selection. Take the case of any two species which,
when crossed, produced few and sterile offspring; now, what is there
which could favour the survival of those individuals which happened to
be endowed in a slightly higher degree with mutual infertility, and
which thus approached by one small step towards absolute sterility?
Yet an advance of this kind, if the theory of natural selection be
brought to bear, must have incessantly occurred with many species, for
a multitude are mutually quite barren. With sterile neuter insects
we have reason to believe that modifications in their structure and
fertility have been slowly accumulated by natural selection, from an
advantage having been thus indirectly given to the community to
which they belonged over other communities of the same species; but an
individual animal not belonging to a social community, if rendered
slightly sterile when crossed with some other variety, would not
thus itself gain any advantage or indirectly give any advantage to the
other individuals of the same variety, thus leading to their
preservation.
But it would be superfluous to discuss this question in detail;
for with plants we have conclusive evidence that the sterility of
crossed species must be due to some principle, quite independent of
natural selection. Both Gartner and Kolreuter have proved that in
genera including numerous species, a series can be formed from species
which when crossed yield fewer and fewer seeds, to species which never
produce a single seed, but yet are affected by the pollen of certain
other species, for the germen swells. It is here manifestly impossible
to select the more sterile individuals, which have already ceased to
yield seeds; so that this acme of sterility, when the germen alone
is affected, cannot have been gained through selection; and from the
laws governing the various grades of sterility being so uniform
throughout the animal and vegetable kingdoms, we may infer that the
cause, whatever it may be, is the same or nearly the same in an cases.
We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses
and in hybrids. In the case of first crosses, the greater or less
difficulty in effecting an union and in obtaining offspring apparently
depends on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be
the case with a plant having a pistil too long for the pollen-tubes to
reach the ovarium. It has also been observed that when the pollen of
one species is placed on the stigma of a distantly allied species,
though the pollen-tubes protrude, they do not penetrate the
stigmatic surface. Again, the male element may reach the female
element but be incapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret's experiments on Fuci.
No explanation can be given of these facts, any more than why
certain trees cannot be grafted on others. Lastly, an embryo may be
developed, and then perish at an early period. This latter alternative
has not been sufficiently attended to; but I believe, from
observations communicated to me by Mr. Rewitt, who has had great
experience in hybridising pheasants and fowls, that the early death of
the embryo is a very frequent cause of sterility in first crosses. Mr.
Salter has recently given the results of an examination of about 500
eggs produced from various crosses between three species of Gallus and
their hybrids; the majority of these eggs had been fertilised; and
in the majority of the fertilised eggs, the embryos had either been
partially developed and had then perished, or had become nearly
mature, but the young chickens had been unable to break through the
shell. Of the chickens which were born, more than four-fifths died
within the first few days, or at latest weeks, "without any obvious
cause, apparently from mere inability to live"; so that from the 500
eggs only twelve chickens were reared. With plants, hybridised embryos
probably often perish in a like manner; at least it is known that
hybrids raised from very distinct species are sometimes weak and
dwarfed, and perish at an early age; of which fact Max Wichura has
recently given some striking cases with hybrid willows. It may be here
worth noticing that in some cases of parthenogenesis, the embryos
within the eggs of silk moths which had not been fertilised, pass
through their early stages of development and then perish like the
embryos produced by a cross between distinct species. Until becoming
acquainted with these facts, I was unwilling to believe in the
frequent early death of hybrid embryos; for hybrids, when once born,
are generally healthy and long-lived, as we see in the case of the
common mule. Hybrids, however, are differently circumstanced before
and after birth: when born and living in a country where their two
parents live, they are generally placed under suitable conditions of
life. But a hybrid partakes of only half of the nature and
constitution of its mother; it may therefore before birth, as long
as it is nourished within its mother's womb, or within the egg or seed
produced by the mother, be exposed to conditions in some degree
unsuitable, and consequently be liable to perish at an early period;
more especially as all very young beings are eminently sensitive to
injurious or unnatural conditions of life. But after all, the cause
more probably lies in some imperfection in the original act of
impregnation, causing the embryo to be imperfectly developed, rather
than in the conditions to which it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual
elements are imperfectly developed, the case is somewhat different.
I have more than once alluded to a large body of facts showing that,
when animals and plants are removed from their natural conditions,
they are extremely liable to have their reproductive systems seriously
affected. This, in fact, is the great bar to the domestication of
animals. Between the sterility thus super-induced and that of hybrids,
there are many points of similarity. In both cases the sterility is
independent of general health, and is often accompanied by excess of
size or great luxuriance. In both cases the sterility occurs in
various degrees; in both, the male element is the most liable to be
affected; but sometimes the female more than the male. In both, the
tendency goes to a certain extent with systematic affinity, for
whole groups of animals and plants are rendered impotent by the same
unnatural conditions; and whole groups of species tend to produce
sterile hybrids. On the other hand, one species in a group will
sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce unusually
fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement, or any exotic plant
seed freely under culture; nor can he tell till he tries, whether
any two species of a genus will produce more or less sterile
hybrids. Lastly, when organic beings are placed during several
generations under conditions not natural to them, they are extremely
liable to vary, which seems to be partly due to their reproductive
systems having been specially affected, though in a lesser degree than
when sterility ensues. So it is with hybrids, for their offspring in
successive generations are eminently liable to vary, as every
experimentalist has observed.
Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids are produced by the unnatural
crossing of two species, the reproductive system, independently of the
general state of health, is affected in a very similar manner. In
the one case, the conditions of life have been disturbed, though often
in so slight a degree as to be inappreciable by us; in the other case,
or that of hybrids, the external conditions have remained the same,
but the organisation has been disturbed by two distinct structures and
constitutions, including of course the reproductive systems, having
been blended into one. For it is scarcely possible that two
organisations should be compounded into one, without some
disturbance occurring in the development, or periodical action, or
mutual relations of the different parts and organs one to another or
to the conditions of life. When hybrids are able to breed inter se,
they transmit to their offspring from generation to generation the
same compounded organisation, and hence we need not be surprised
that their sterility, though in some degree variable, does not
diminish; it is even apt to increase, this being generally the result,
as before explained, of too close interbreeding. The above view of the
sterility of hybrids being caused by two constitutions being
compounded into one has been strongly maintained by Max Wichura.
It must, however, be owned that we cannot understand, on the above
or any other view, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced
from reciprocal crosses; or the increased sterility in those hybrids
which occasionally and exceptionally resemble closely either pure
parent. Nor do I pretend that the foregoing remarks go to the root
of the matter; no explanation is offered why an organism, when
placed under unnatural conditions, is rendered sterile. All that I
have attempted to show is, that in two cases, in some respects allied,
sterility is the common result,- in the one case from the conditions
of life having been disturbed, in the other case from the organisation
having been disturbed by two organisations being compounded into one.
A similar parallelism holds good with an allied yet very different
class of facts. It is an old and almost universal belief founded on
a considerable body of evidence, which I have elsewhere given, that
slight changes in the conditions of life are beneficial to all
living things. We see this acted on by farmers and gardeners in
their frequent exchanges of seed, tubers, &c., from one soil or
climate to another, and back again. During the convalescence of
animals, great benefit is derived from almost any change in their
habits of life. Again, both with plants and animals, there is the
clearest evidence that a cross between individuals of the same
species, which differ to a certain extent, gives vigour and
fertility to the offspring; and that close interbreeding continued
during several generations between the nearest relations, if these
be kept under the same conditions of life, almost always leads to
decreased size, weakness, or sterility.
Hence it seems that, on the one hand, slight changes in the
conditions of life benefit all organic beings, and on the other
hand, that slight crosses, that is crosses between the males and
females of the same species, which have been subjected to slightly
different conditions, or which have slightly varied, give vigour and
fertility to the offspring. But, as we have seen, organic beings
long habituated to certain uniform conditions under a state of nature,
when subjected, as under confinement, to a considerable change in
their conditions, very frequently are rendered more or less sterile;
and we know that a cross between two forms, that have become widely or
specifically different, produce hybrids which are almost always in
some degree sterile. I am fully persuaded that this double parallelism
is by no means an accident or an illusion. He who is able to explain
why the elephant and a multitude of other animals are incapable of
breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being so
generally sterile. He will at the same time be able to explain how
it is that the races of some of our domesticated animals, which have
often been subjected to new and not uniform conditions, are quite
fertile together, although they are descended from distinct species,
which would probably have been sterile if aboriginally crossed. The
above two parallel series of facts seem to be connected together by
some common but unknown bond, which is essentially related to the
principle of life; this principle, according to Mr. Herbert Spencer,
being that life depends on, or consists in, the incessant action and
reaction of various forces, which, as throughout nature, are always
tending towards an equilibrium; and when this tendency is slightly
disturbed by any change, the vital forces gain in power.
Reciprocal Dimorphism and Trimorphism
This subject may be here briefly discussed, and will be found to
throw some light on hybridism. Several plants belonging to distinct
orders present two forms, which exist in about equal numbers and which
differ in no respect except in their reproductive organs; one form
having a long pistil with short stamens, the other a short pistil with
long stamens; the two having differently sized pollen-grains. With
trimorphic plants there are three forms likewise differing in the
lengths of the pistils and stamens, in the size and colour of the
pollen grains, and in some other respects; and as in each of the three
forms there are two sets of stamens, the three forms possess
altogether six sets of stamens and three kinds of pistils. These
organs are so proportioned in length to each other, that half the
stamens in two of the forms stand on a level with the stigma of the
third form. Now I have shown, and the result has been confirmed by
other observers, that, in order to obtain full fertility with these
plants, it is necessary that the stigma of the one form should be
fertilised by pollen taken from the stamens of corresponding height in
another form. So that with dimorphic species two unions, which may
be called legitimate, are fully fertile; and two, which may be
called illegitimate, are more or less infertile. With trimorphic
species six unions are legitimate, or fully fertile,- and twelve are
illegitimate, or more or less infertile.
The infertility which may be observed in various dimorphic and
trimorphic plants, when they are illegitimately fertilised, that is by
pollen taken from stamens not corresponding in height with the pistil,
differs much in degree, up to absolute and utter sterility; just in
the same manner as occurs in crossing distinct species. As the
degree of sterility in the latter case depends in an eminent degree on
the conditions of life being more or less favourable, so I have
found it with illegitimate unions. It is well known that if pollen
of a distinct species be placed on the stigma of a flower, and its own
pollen be afterwards, even after a considerable interval of time,
placed on the same stigma, its action is so strongly prepotent that it
generally annihilates the effect of the foreign pollen; so it is
with the pollen of the several forms of the same species, for
legitimate pollen is strongly prepotent over illegitimate pollen, when
both are placed on the same stigma. I ascertained this by
fertilising several flowers, first illegitimately, and twenty-four
hours afterwards legitimately with the pollen taken from a
peculiarly coloured variety, and all the seedlings were similarly
coloured; this shows that the legitimate pollen, though applied
twenty-four hours subsequently, had wholly destroyed or prevented
the action of the previously applied illegitimate pollen. Again, as in
making reciprocal crosses between the same two species, there is
occasionally a great difference in the result, so the same thing
occurs with trimorphic plants; for instance, the mid-styled form of
Lythrum galicaria was illegitimately fertilised with the greatest ease
by pollen from the longer stamens of the short-styled form, and
yielded many seeds; but the latter form did not yield a single seed
when fertilised by the longer stamens of the mid-styled form.
In all these respects, and in others which might be added, the forms
of the same undoubted species when illegitimately united behave in
exactly the same manner as do two distinct species when crossed.
This led me carefully to observe during four years many seedlings,
raised from several illegitimate unions. The chief result is that
these illegitimate plants, as they may be called, are not fully
fertile. It is possible to raise from dimorphic species, both
long-styled and short-styled illegitimate plants, and from
trimorphic plants all three illegitimate forms. These can then be
properly united in a legitimate manner. When this is done, there is no
apparent reason why they should not yield as many seeds as did their
parents when legitimately fertilised. But such is not the case. They
are all infertile, in various degrees; some being so utterly and
incurably sterile that they did not yield during four seasons a single
seed or even seed-capsule. The sterility of these illegitimate plants,
when united with each other in a legitimate manner, may be strictly
compared with that of hybrids when crossed inter se. If, on the
other hand, a hybrid is crossed with either pure parent-species, the
sterility is usually much lessened: and so it is when an
illegitimate plant is fertilised by a legitimate plant. In the same
manner as the sterility of hybrids does not always run parallel with
the difficulty of making the first cross between the two
parent-species, so the sterility of certain illegitimate plants was
unusually great, whilst the sterility of the union from which they
were derived was by no means great. With hybrids raised from the
same seed-capsule the degree of sterility is innately variable, so
it is in a marked manner with illegitimate plants. Lastly, many
hybrids are profuse and persistent flowerers, whilst other and more
sterile hybrids produce few flowers, and are weak, miserable dwarfs;
exactly similar cases occur with the illegitimate offspring of various
dimorphic and trimorphic plants.
Altogether there is the closest identity in character and
behaviour between illegitimate plants and hybrids. It is hardly an
exaggeration to maintain that illegitimate plants are hybrids,
produced within the limits of the same species by the improper union
of certain forms, whilst ordinary hybrids are produced from an
improper union between so-called distinct species. We have also
already seen that there is the closest similarity in all respects
between first illegitimate unions and first crosses between distinct
species. This will perhaps be made more fully apparent by an
illustration; we may suppose that a botanist found two well-marked
varieties (and such occur) of the long-styled form of the trimorphic
Lythrum salicaria, and that he determined to try by crossing whether
they were specifically distinct. He would find that they yielded
only about one-fifth of the proper number of seeds, and that they
behaved in all the other above-specified respects as if they had
been two distinct species. But to make the case sure, he would raise
plants from his supposed hybridised seed, and he would find that the
seedlings were miserably dwarfed and utterly sterile, and that they
behaved in all other respects like ordinary hybrids. He might then
maintain that he had actually proved, in accordance with the common
view, that his two varieties were as good and as distinct species as
any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are
important, because they show us, first, that the physiological test of
lessened fertility, both in first crosses and in hybrids, is no safe
criterion of specific distinction; secondly, because we may conclude
that there is some unknown bond which connects the infertility of
illegitimate unions with that of their illegitimate offspring, and
we are led to extend the same view to first crosses and hybrids;
thirdly, because we find, and this seems to me of especial importance,
that two or three forms of the same species may exist and may differ
in no respect whatever, either in structure or in constitution,
relatively to external conditions, and yet be sterile when united in
certain ways. For we must remember that it is the union of the
sexual elements of individuals of the same form, for instance, of
two long-styled forms, which results in sterility; whilst it is the
union of the sexual elements proper to two distinct forms which is
fertile. Hence the case appears at first sight exactly the reverse
of what occurs, in the ordinary unions of the individuals of the
same species and with crosses between distinct species. It is,
however, doubtful whether this is really so; but I will not enlarge on
this obscure subject.
We may, however, infer as probable from the consideration of
dimorphic and trimorphic plants, that the sterility of distinct
species when crossed and of their hybrid progeny, depends
exclusively on the nature of their sexual elements, and not on any
difference in their structure or general constitution. We are also led
to this same conclusion by considering reciprocal crosses, in which
the male of one species cannot be united, or can be united with
great difficulty, with the female of a second species, whilst the
converse cross can be effected with perfect facility. That excellent
observer, Gartner, likewise concluded that species when crossed are
sterile owing to differences confined to their reproductive systems.
Fertility of Varieties when Crossed, and of their Mongrel Offspring,
not universal
It may be urged, as an overwhelming argument, that there must be
some essential distinction between species and varieties, inasmuch
as the latter, however much they may differ from each other in
external appearance, cross with perfect facility, and yield
perfectly fertile offspring. With some exceptions, presently to be
given, I fully admit that this is the rule. But the subject is
surrounded by difficulties, for, looking to varieties produced under
nature, if two forms hitherto reputed to be varieties be found in
any degree sterile together, they are at once ranked by most
naturalists as species. For instance, the blue and red pimpernel,
which are considered by most botanists as varieties, are said by
Gartner to be quite sterile when crossed, and he subsequently ranks
them as undoubted species. If we thus argue in a circle, the fertility
of all varieties produced under nature will assuredly have to be
granted.
If we turn to varieties, produced, or supposed to have been
produced, under domestication, we are still involved in some doubt.
For when it is stated, for instance, that certain South American
indigenous domestic dogs do not readily unite with European dogs,
the explanation which will occur to every one, and probably the true
one, is that they are descended from aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic races,
differing widely from each other in appearance, for instance those
of the pigeon, or of the cabbage, is a remarkable fact; more
especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when
intercrossed. Several considerations however, render the fertility
of domestic varieties less remarkable. In the first place, it may be
observed that the amount of external difference between two species is
no sure guide to their degree of mutual sterility, so that similar
differences in the case of varieties would be no sure guide. It is
certain that with species the cause lies exclusively in differences in
their sexual constitution. Now the varying conditions to which
domesticated animals and cultivated plants have been subjected, have
had so little tendency towards modifying the reproductive system in
a manner leading to mutual sterility, that we have good grounds for
admitting the directly opposite doctrine of Pallas, namely, that
such conditions generally eliminate this tendency; so that the
domesticated descendants of species, which in their natural state
probably would have been in some degree sterile when crossed, become
perfectly fertile together. With plants, so far is cultivation from
giving a tendency towards sterility between distinct species, that
in several well-authenticated cases already alluded to, certain plants
have been affected in an opposite manner, for they have become
self-impotent whilst still retaining the capacity of fertilising,
and being fertilised by, other species. If the Pallasian doctrine of
the elimination of sterility through long-continued domestication be
admitted, and it can hardly be rejected, it becomes in the highest
degree improbable that similar conditions long-continued should
likewise induce this tendency; though in certain cases, with species
having a peculiar constitution, sterility might occasionally be thus
caused. Thus, as I believe, we can understand why with domesticated
animals varieties have not been produced which are mutually sterile;
and why with plants only a few such cases, immediately to be given,
have been observed.
The real difficulty in our present subject is not, as it appears
to me, why domestic varieties have not become mutually infertile
when crossed, but why this has so generally occurred with natural
varieties, as soon as they have been permanently modified in a
sufficient degree to take rank as species. We are far from precisely
knowing the cause; nor is this surprising, seeing how profoundly
ignorant we are in regard to the normal and abnormal action of the
reproductive system. But we can see that species, owing to their
struggle for existence with numerous competitors, will have been
exposed during long periods of time to more uniform conditions, than
have domestic varieties; and this may well make a wide difference in
the result. For we know how commonly wild animals and plants, when
taken from their natural conditions and subjected to captivity, are
rendered sterile; and the reproductive functions of organic beings
which have always lived under natural conditions would probably in
like manner be eminently sensitive to the influence of an unnatural
cross. Domesticated productions, on the other hand, which, as shown by
the mere fact of their domestication, were not originally highly
sensitive to changes in their conditions of life, and which can now
generally resist with undiminished fertility repeated changes of
conditions, might be expected to produce varieties, which would be
little liable to have their reproductive powers injuriously affected
by the act of crossing with other varieties which had originated in
a like manner.
I have not as yet spoken as if the varieties of the same species
were invariably fertile when intercrossed. But it is impossible to
resist the evidence of the existence of a certain amount of
sterility in the few following cases, which I will briefly abstract.
The evidence is at least as good as that from which we believe in
the sterility of a multitude of species. The evidence is, also,
derived from hostile witnesses, who in all other cases consider
fertility and sterility as safe criterions of specific distinction.
Gartner kept during several years a dwarf kind of maize with yellow
seeds, and a tall variety with red seeds growing near each other in
his garden; and although these plants have separated sexes, they never
naturally crossed. He then fertilised thirteen flowers of the one kind
with pollen of the other; but only a single head produced any seed,
and this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes.
No one, I believe, has suspected that these varieties of maize are
distinct species; and it is important to notice that the hybrid plants
thus raised were themselves perfectly fertile; so that even Gartner
did not venture to consider the two varieties as specifically
distinct.
Girou de Buzareingues crossed three varieties of gourd, which like
the maize has separated sexes, and he asserts that their mutual
fertilization is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimented on are ranked by Sageret, who mainly founds his
classification by the test of infertility, as varieties, and Naudin
has come to the same conclusion.
The following case is far more remarkable, and seems at first
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness as Gartner: namely, that the
yellow and white varieties when crossed produce less seed than the
similarly coloured varieties of the same species. Moreover, he asserts
that, when yellow and white varieties of one species are crossed
with yellow and white varieties of a distinct species, more seed is
produced by the crosses between the similarly coloured flowers, than
between those which are differently coloured. Mr. Scott also has
experimented on the species and varieties of Verbascum; and although
unable to confirm Gartner's results on the crossing of the distinct
species, he finds that the dissimilarly coloured varieties of the same
species yield fewer seeds in the proportion of 86 to 100, than the
similarly coloured varieties. Yet these varieties differ in no respect
except in the colour of their flowers; and one variety can sometimes
be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one particular
variety of the common tobacco was more fertile than the other
varieties, when crossed with a widely distinct species. He
experimented on five forms which are commonly reputed to be varieties,
and which he tested by the severest trial, namely, by reciprocal
crosses, and he found their mongrel offspring perfectly fertile. But
one of these five varieties, when used either as the father or mother,
and crossed with the Nicotiana glutinosa, always yielded hybrids not
so sterile as those which were produced from the four other
varieties when crossed with N. glutinosa. Hence the reproductive
system of this one variety must have been in some manner and in some
degree modified.
From these facts it can no longer be maintained that varieties
when crossed are invariably quite fertile. From the great difficulty
of ascertaining the infertility of varieties in a state of nature, for
a supposed variety, if proved to be infertile in any degree, would
almost universally be ranked as a species;- from man attending only to
external characters in his domestic varieties, and from such varieties
not having been exposed for very long periods to uniform conditions of
life;- from these several considerations we may conclude that
fertility does not constitute a fundamental distinction between
varieties and species when crossed. The general sterility of crossed
species may safely be looked at, not as a special acquirement or
endowment, but as incidental on changes of an unknown nature in
their sexual elements.
Hybrids and Mongrels compared, independently of their fertility
Independently of the question of fertility, the offspring of species
and of varieties when crossed may be compared in several other
respects. Gartner, whose strong wish it was to draw a distinct line
between species and varieties, could find very few, and, as it seems
to me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of
varieties. And, on the other hand, they agree most closely in many
important respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gartner admits that hybrids from
species which have long been cultivated are often variable in the
first generation; and I have myself seen striking instances of this
fact. Gartner further admits that hybrids between very closely
allied species are more variable than those from very distinct
species; and this shows that the difference in the degree of
variability graduates away. When mongrels and the more fertile hybrids
are propagated for several generations, an extreme amount of
variability in the offspring in both cases is notorious; but some
few instances of both hybrids and mongrels long retaining a uniform
character could be given. The variability, however, in the
successive generations of mongrels is, perhaps, greater than in
hybrids.
This greater variability in mongrels than in hybrids does not seem
at all surprising. For the parents of mongrels are varieties, and
mostly domestic varieties (very few experiments having been tried on
natural varieties), and this implies that there has been recent
variability, which would often continue and would augment that arising
from the act of crossing. The slight variability of hybrids in the
first generation, in contrast with that in the succeeding generations,
is a curious fact and deserves attention. For it bears on the view
which I have taken of one of the causes of ordinary variability;
namely, that the reproductive system from being eminently sensitive to
changed conditions of life, fails under these circumstances to perform
its proper function of producing offspring closely similar in all
respects to the parent-form. Now hybrids in the first generation are
descended from species (excluding those long-cultivated) which have
not had their reproductive systems in any way affected, and they are
not variable; but hybrids themselves have their reproductive systems
seriously affected, and their descendants are highly variable.
But to return to our comparison of mongrels and hybrids: Gartner
states that mongrels are more liable than hybrids to revert to
either parent-form; but this, if it be true, is certainly only a
difference in degree. Moreover, Gartner expressly states that
hybrids from long cultivated plants are more subject to reversion than
hybrids from species in their natural state; and this probably
explains the singular difference in the results arrived at by
different observers: thus Max Wichura doubts whether hybrids ever
revert to their parent-forms, and he experimented on uncultivated
species of willows; whilst Naudin, on the other hand, insists in the
strongest terms on the almost universal tendency to reversion in
hybrids, and he experimented chiefly on cultivated plants. Gartner
further states that when any two species, although most closely allied
to each other, are crossed with a third species, the hybrids are
widely different from each other; whereas if two very distinct
varieties of one species are crossed with another species, the hybrids
do not differ much. But this conclusion, as far as I can make out,
is founded on a single experiment; and seems directly opposed to the
results of several experiments made by Kolreuter.
Such alone are the unimportant differences which Gartner is able
to point out between hybrid and mongrel plants. On the other hand, the
degrees and kinds of resemblance in mongrels and in hybrids to their
respective parents, more especially in hybrids produced from nearly
related species, follow according to Gartner the same laws. When two
species are crossed, one has sometimes a prepotent power of impressing
its likeness on the hybrid. So I believe it to be with varieties of
plants; and with animals one variety certainly often has this
prepotent power over another variety. Hybrid plants produced from a
reciprocal cross, generally resemble each other closely; and so it
is with mongrel plants from a reciprocal cross. Both hybrids and
mongrels can be reduced to either pure parent-form, by repeated
crosses in successive generations with either parent.
These several remarks are apparently applicable to animals; but
the subject is here much complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency
in transmitting likeness running more strongly in one sex than in
the other, both when one species is crossed with another, and when one
variety is crossed with another variety. For instance, I think those
authors are right who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny resemble more closely
the ass than the horse; but that the prepotency runs more strongly
in the male than in the female ass, so that the mule, which is the
offspring of the male ass and mare, is more like an ass, than is the
hinny, which is the offspring of the female ass and stallion.
Much stress has been laid by some authors on the supposed fact, that
it is only with mongrels that the offspring are not intermediate in
character, but closely resemble one of their parents; but this does
sometimes occur with hybrids, yet I grant much less frequently than
with mongrels. Looking to the cases which I have collected of
cross-bred animals closely resembling one parent, the resemblances
seem chiefly confined to characters almost monstrous in their
nature, and which have suddenly appeared- such as albinism,
melanism, deficiency of tail or horns, or additional fingers and toes;
and do not relate to characters which have been slowly acquired
through selection. A tendency to sudden reversions to the perfect
character of either parent would, also, be much more likely to occur
with mongrels, which are descended from varieties often suddenly
produced and semi-monstrous in character, than with hybrids, which are
descended from species slowly and naturally produced On the whole, I
entirely agree with Dr. Prosper Lucas, who, after arranging an
enormous body of facts with respect to animals, comes to the
conclusion that the laws of resemblance of the child to its parents
are the same, whether the two parents differ little or much from
each other, namely, in the union of individuals of the same variety,
or of different varieties, or of distinct species.
Independently of the question of fertility and sterility, in all
other respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look
at species as having been specially created, and at varieties as
having been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.
Summary of Chapter
First crosses between forms, sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight
that the most careful experimentalists have arrived at diametrically
opposite conclusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and is eminently
susceptible to the action of favourable and unfavourable conditions.
The degree of sterility does not strictly follow systematic
affinity, but is governed by several curious and complex laws. It is
generally different, and sometimes widely different in reciprocal
crosses between the same two species. It is not always equal in degree
in a first cross and in the hybrids produced from this cross.
In the same manner as in grafting trees, the capacity in one species
or variety to take on another, is incidental on differences, generally
of an unknown nature, in their vegetative systems, so in crossing, the
greater or less facility of one species to unite with another is
incidental on unknown differences in their reproductive systems. There
is no more reason to think that species have been specially endowed
with various degrees of sterility to prevent their crossing and
blending in nature, than to think that trees have been specially
endowed with various and somewhat analogous degrees of difficulty in
being grafted together in order to prevent their inarching in our
forests.
The sterility of first crosses and of their hybrid progeny has not
been acquired through natural selection. In the case of first
crosses it seems to depend on several circumstances; in some instances
in chief part on the early death of the embryo. In the case of
hybrids, it apparently depends on their whole organisation having been
disturbed by being compounded from two distinct forms; the sterility
being closely allied to that which so frequently affects pure species,
when exposed to new and unnatural conditions of life. He who will
explain these latter cases will be able to explain the sterility of
hybrids. This view is strongly supported by a parallelism of another
kind: namely, that, firstly, slight changes in the conditions of
life add to the vigour and fertility of all organic beings; and
secondly, that the crossing of forms, which have been exposed to
slightly different conditions of life or which have varied, favours
the size, vigour, and fertility of their offspring. The facts given on
the sterility of the illegitimate unions of dimorphic and trimorphic
plants and of their illegitimate progeny, perhaps render it probable
that some unknown bond in all cases connects the degree of fertility
of first unions with that of their offspring. The consideration of
these facts on dimorphism, as well as of the results of reciprocal
crosses, clearly leads to the conclusion that the primary cause of the
sterility of crossed species is confined to differences in their
sexual elements. But why, in the case of distinct species, the
sexual elements should so generally have become more or less modified,
leading to their mutual infertility, we do not know; but it seems to
stand in some close relation to species having been exposed for long
periods of time to nearly uniform conditions of life.
It is not surprising that the difficulty in crossing any two
species, and the sterility of their hybrid offspring, should in most
cases correspond, even if due to distinct causes: for both depend on
the amount of difference between the species which are crossed. Nor is
it surprising that the facility of effecting a first cross, and the
fertility of the hybrids thus produced, and the capacity of being
grafted together- though this latter capacity evidently depends on
widely different circumstances- should all run, to a certain extent,
parallel with the systematic affinity of the forms subjected to
experiment; for systematic affinity includes resemblances of all
kinds.
First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring,
are very generally, but not, as is so often stated, invariably
fertile. Nor is this almost universal and perfect fertility
surprising, when it is remembered how liable we are to argue in a
circle with respect to varieties in a state of nature; and when we
remember that the greater number of varieties have been produced under
domestication by the selection of mere external differences, and
that they have not been long exposed to uniform conditions of life. It
should also be especially kept in mind, that long-continued
domestication tends to eliminate sterility, and is therefore little
likely to induce this same quality. Independently of the question of
fertility, in all other respects there is the closest general
resemblance between hybrids and mongrels,- in their variability, in
their power of absorbing each other by repeated crosses, and in
their inheritance of characters from both parent-forms. Finally, then,
although we are as ignorant of the precise cause of the sterility of
first crosses and of hybrids as we are why animals and plants
removed from their natural conditions become sterile, yet the facts
given in this chapter do not seem to me opposed to the belief that
species aboriginally existed as varieties.
CHAPTER X
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD
IN THE sixth chapter I enumerated the chief objections which might
be justly urged against the views maintained in this volume. Most of
them have now been discussed. One, namely the distinctness of specific
forms, and their not being blended together by innumerable
transitional links, is a very obvious difficulty. I assigned reasons
why such links do not commonly occur at the present day under the
circumstances apparently most favourable for their presence, namely,
on an extensive and continuous area with graduated physical
conditions. I endeavoured to show, that the life of each species
depends in a more important manner on the presence of other already
defined organic forms, than on climate, and, therefore, that the
really governing conditions of life do not graduate away quite
insensibly like heat or moisture. I endeavoured, also, to show that
intermediate varieties, from existing in lesser numbers than the forms
which they connect, will generally be beaten out and exterminated
during the course of further modification and improvement. The main
cause, however, of innumerable intermediate links not now occurring
everywhere throughout nature, depends on the very process of natural
selection, through which new varieties continually take the places
of and supplant their parent-forms. But just in proportion as this
process of extermination has acted on an enormous scale, so must the
number of intermediate varieties, which have formerly existed, be
truly enormous. Why then is not every geological formation and every
stratum full of such intermediate links? Geology assuredly does not
reveal any such finely-graduated organic chain; and this, perhaps,
is the most obvious and serious objection which can be urged against
the theory. The explanation lies, as I believe, in the extreme
imperfection of the geological record.
In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I
have found it difficult, when looking at any two species, to avoid
picturing to myself forms directly intermediate between them. But this
is a wholly false view; we should always look for forms intermediate
between each species and a common but unknown progenitor; and the
progenitor will generally have differed in some respects from all
its modified descendants. To give a simple illustration: the fantail
and pouter pigeons are both descended from the rock-pigeon; if we
possessed all the intermediate varieties which have ever existed, we
should have an extremely close series between both and the
rock-pigeon; but we should have no varieties directly intermediate
between the fantail and pouter; none, for instance, combining a tail
somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have
become so much modified, that, if we had no historical or indirect
evidence regarding their origin, it would not have been possible to
have determined, from a mere comparison of their structure with that
of the rock-pigeon, C. livia, whether they had descended from this
species or from some allied form, such as C. aenas.
So, with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links directly intermediate between them ever existed, but between
each and an unknown common parent. The common parent will have had
in its whole organisation much general resemblance to the tapir and to
the horse; but in some points of structure may have differed
considerably from both, even perhaps more than they differ from each
other. Hence, in all such cases, we should be unable to recognise
the parent-form of any two or more species, even if we closely
compared the structure of the parent with that of its modified
descendants, unless at the same time we had a nearly perfect chain
of the intermediate links.
It is just possible by theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case direct intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount
of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare
event; for in all cases the new and improved forms of life tend to
supplant the old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the natural and domestic varieties of
the same species at the present day; and these parent-species, now
generally extinct, have in their turn been similarly connected with
more ancient forms; and so on backwards, always converging to the
common ancestor of each great class. So that the number of
intermediate and transitional links, between all living and extinct
species, must have been inconceivably great. But assuredly, if this
theory be true, such have lived upon the earth.
On the Lapse of Time, as inferred from the rate of Deposition and
extent of Denudation
Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot have
sufficed for so great an amount of organic change, all changes
having been effected slowly. It is hardly possible for me to recall to
the reader who is not a practical geologist, the facts leading the
mind feebly to comprehend the lapse of time. He who can read Sir
Charles Lyell's grand work on the Principles of Geology, which the
future historian will recognise as having produced a revolution in
natural science, and yet does not admit how vast have been the past
periods of time, may at once close this volume. Not that it suffices
to study the Principles of Geology, or to read special treatises by
different observers on separate formations, and to mark how each
author attempts, to give an inadequate idea of the duration of each
formation, or even of each stratum. We can best gain some idea of past
time by knowing the agencies at work, and learning how deeply the
surface of the land has been denuded, and how much sediment has been
deposited. As Lyell has well remarked, the extent and thickness of our
sedimentary formations are the result and the measure of the
denudation which the earth's crust has elsewhere undergone.
Therefore a man should examine for himself the great piles of
superimposed strata, and watch the rivulets bringing down mud, and the
waves wearing away the sea-cliffs, in order to comprehend something
about the duration of past time, the monuments of which we see all
around us.
It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves
eat into them only when they are charged with sand or pebbles; for
there is good evidence that pure water effects nothing in wearing away
rock. At last the base of the cliff is undermined, huge fragments fall
down, and these, remaining fixed, have to be worn away atom by atom,
until after being reduced in size they can be rolled about by the
waves, and then they are more quickly ground into pebbles, sand, or
mud. But how often do we see along the bases of retreating cliffs
rounded boulders, all thickly clothed by marine productions, showing
how little they are abraded and how seldom they are rolled about!
Moreover, if we follow for a few miles any line of rocky cliff,
which is undergoing degradation, we find that it is only here and
there, along a short length or round a promontory, that the cliffs are
at the present time suffering. The appearance of the surface and the
vegetation show that elsewhere years have elapsed since the waters
washed their base.
We have, however, recently learnt from the observations of Ramsay,
in the van of many excellent observers- of Jukes, Geikie, Croll, and
others, that subaerial degradation is a much more important agency
than coast-action, or the power of the waves. The whole surface of the
land is exposed to the chemical action of the air and of the
rain-water with its dissolved carbolic acid, and in colder countries
to frost; the disintegrated matter is carried down even gentle
slopes during heavy rain, and to a greater extent than might be
supposed, especially in arid districts, by the wind; it is then
transported by the streams and rivers, which when rapid deepen their
channels, and triturate the fragments. On a rainy day, even in a
gently undulating country, we see the effects of subaerial degradation
in the muddy rills which flow down every slope. Messrs. Ramsay and
Whitaker have shown, and the observation is a most striking one,
that the great lines of escarpment in the Wealden district and those
ranging across England, which formerly were looked at as ancient
sea-coasts, cannot have been thus formed, for each line is composed of
one and the same formation, whilst our sea-cliffs are everywhere
formed by the intersection of various formations. This being the case,
we are compelled to admit that the escarpments owe their origin in
chief part to the rocks of which they are composed having resisted
subaerial denudation better than the surrounding surface; this surface
consequently has been gradually lowered, with the lines of harder rock
left projecting. Nothing impresses the mind with the vast duration
of time, according to our ideas of time, more forcibly than the
conviction thus gained that subaerial agencies which apparently have
so little power, and which seem to work so slowly, have produced great
results.
When thus impressed with the slow rate at which the land is worn
away through subaerial and littoral action, it is good, in order to
appreciate the past duration of time, to consider, on the one hand,
the masses of rock which have been removed over many extensive
areas, and on the other hand the thickness of our sedimentary
formations. I remember having been much struck when viewing volcanic
islands, which have been worn by the waves and pared all round into
perpendicular cliffs of one or two thousand feet in height; for the
gentle slope of the lava-streams, due to their formerly liquid
state, showed at a glance how far the hard, rocky beds had once
extended into the open ocean. The same story is told still more
plainly by faults,- those great cracks along which the strata have
been upheaved on one side, or thrown down on the other, to the
height or depth of thousands of feet; for since the crust cracked, and
it makes no great difference whether the upheaval was sudden, or, as
most geologists now believe, was slow and effected by many starts, the
surface of the land has been so completely planed down that no trace
of these vast dislocations is externally visible. The Craven fault,
for instance, extends for upwards of 30 miles, and along this line the
vertical displacement of the strata varies from 600 to 3000 feet.
Professor Ramsay has published an account of a downthrow in Anglesea
of 2300 feet; and he informs me that he fully believes that there is
one in Merionethshire of 12,000 feet; yet in these cases there is
nothing on the surface of the land to show such prodigious
movements; the pile of rocks on either side of the crack having been
smoothly swept away.
On the other hand, in all parts of the world the piles of
sedimentary strata are of wonderful thickness. In the Cordillera I
estimated one mass of conglomerate at ten thousand feet; and
although conglomerates have probably been accumulated at a quicker
rate than finer sediments, yet from being formed of worn and rounded
pebbles, each of which bears the stamp of time, they are good to
show how slowly the mass must have been heaped together. Professor
Ramsay has given me the maximum thickness, from actual measurement
in most cases, of the successive formations in different parts of
Great Britain; and this is the result:-
Palaeozoic strata (not including igneous beds) 57,154 feet
Secondary strata 13,190 feet
Tertiary strata 2,249 feet
-making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of the formations, which are
represented in England by thin beds, are thousands of feet in
thickness on the Continent. Moreover, between each successive
formation, we have, in the opinion of most geologists, blank periods
of enormous length. So that the lofty pile of sedimentary rocks in
Britain gives but an inadequate idea of the time which has elapsed
during their accumulation. The consideration of these various facts
impresses the mind almost in the same manner as does the vain
endeavour to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in an
interesting paper, remarks that we do not err "in forming too great
a conception of the length of geological periods," but in estimating
them by years. When geologists look at large and complicated
phenomena, and then at the figures representing several million years,
the two produce a totally different effect on the mind, and the
figures are at once pronounced too small. In regard to subaerial
denudation, Mr. Croll shows, by calculating the known amount of
sediment annually brought down by certain rivers, relatively to
their areas of drainage, that 1000 feet of solid rock, as it became
gradually disintegrated, would thus be removed from the mean level
of the whole area in the course of six million years. This seems an
astonishing result, and some considerations lead to the suspicion that
it may be too large, but even if halved or quartered it is still
very surprising. Few of us, however, know what a million really means:
Mr. Croll gives the following illustration: take a narrow strip of
paper, 83 feet 4 inches in length, and stretch it along the wall of
a large hall; then mark off at one end the tenth of an inch. This
tenth of an inch will represent one hundred years, and the entire
strip a million years. But let it be borne in mind, in relation to the
subject of this work, what a hundred years implies, represented as
it is by a measure utterly insignificant in a hall of the above
dimensions. Several eminent breeders, during a single lifetime, have
so largely modified some of the higher animals which propagate their
kind much more slowly than most of the lower animals, that they have
formed what well deserves to be called a new sub-breed. Few men have
attended with due care to any one strain for more than half a century,
so that a hundred years represents the work of two breeders in
succession. It is not to be supposed that species in a state of nature
ever change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer with
the effects which follow from unconscious selection, that is the
preservation of the most useful or beautiful animals, with no
intention of modifying the breed; but by this process of unconscious
selection, various breeds have been sensibly changed in the course
of two or three centuries.
Species, however, probably change much more slowly, and within the
same country only a few change at the same time. This slowness follows
from all the inhabitants of the same country being already so well
adapted to each other, that new places in the polity of nature do
not occur until after long intervals, due to the occurrence of
physical changes of some kind, or through the immigration of new
forms. Moreover variations or individual differences of the right
nature, by which some of the inhabitants might be better fitted to
their new places under the altered circumstances, would not always
occur at once. Unfortunately we have no means of determining,
according to the standards of years, how long a period it takes to
modify a species; but to the subject of time we must return.
On the Poorness of Palaeontological Collections
Now let us turn to our richest geological museums, and what a paltry
display we behold! That our collections are imperfect is admitted by
every one. The remark of that admirable palaeontologist, Edward
Forbes, should never be forgotten, namely, that very many fossil
species are known and named from single and often broken specimens, or
from a few specimens collected on some one spot. Only a small
portion of the surface of the earth has been geologically explored,
and no part with sufficient care, as the important discoveries made
every year in Europe prove. No organism wholly soft can be
preserved. Shells and bones decay and disappear when left on the
bottom of the sea, where sediment is not accumulating. We probably
take a quite erroneous view, when we assume that sediment is being
deposited over nearly the whole bed of the sea, at a rate sufficiently
quick to embed and preserve fossil remains. Throughout an enormously
large proportion of the ocean, the bright blue tint of the water
bespeaks its purity. The many cases on record of a formation
conformably covered, after an immense interval of time, by another and
later formation, without the underlying bed having suffered in the
interval any wear and tear, seem explicable only on the view of the
bottom of the sea not rarely lying for ages in an unaltered condition.
The remains which do become embedded, if in sand or gravel, will, when
the beds are upraised, generally be dissolved by the percolation of
rain-water charged with carbolic acid. Some of the many kinds of
animals which live on the beach between high and low water mark seem
to be rarely preserved. For instance, the several species of the
Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all
over the world in infinite numbers: they are all strictly littoral,
with the exception of a single Mediterranean species, which inhabits
deep water, and this has been found fossil in Sicily, whereas not
one other species has hitherto been found in any tertiary formation:
yet it is known that the genus Chthamalus existed during the Chalk
period. Lastly, many great deposits requiring a vast length of time
for their accumulation, are entirely destitute of organic remains,
without our being able to assign any reason: one of the most
striking instances is that of the Flysch formation, which consists
of shale and sandstone, several thousand, occasionally even six
thousand feet in thickness, and extending for at least 300 miles
from Vienna to Switzerland; and although this great mass has been most
carefully searched, no fossils, except a few vegetable remains, have
been found.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that
our evidence is fragmentary in an extreme degree. For instance,
until recently not a land-shell was known belonging to either of these
vast periods, with the exception of one species discovered by Sir C.
Lyell and Dr. Dawson in the carboniferous strata of North America; but
now land-shells have been found in the lias. In regard to
mammiferous remains, a glance at the historical table published in
Lyell's Manual will bring home the truth, how accidental and rare is
their preservation, far better than pages of detail. Nor is their
rarity surprising, when we remember how large a proportion of the
bones of tertiary mammals have been discovered either in caves or in
lacustrine deposits; and that not a cave or true lacustrine bed is
known belonging to the age of our secondary or palaeozoic formations.
But the imperfection in the geological record largely results
from another and more important cause than any of the foregoing;
namely, from the several formations being separated from each other by
wide intervals of time. This doctrine has been emphatically admitted
by many geologists and palaeontologists, who, like E. Forbes, entirely
disbelieve in the change of species. When we see the formations
tabulated in written works, or when we follow them in nature, it is
difficult to avoid believing that they are closely consecutive. But we
know, for instance, from Sir R. Murchison's great work on Russia, what
wide gaps there are in that country between the superimposed
formations; so it is in North America, and in many other parts of
the world. The most skilful geologist if his attention had been
confined exclusively to these large territories, would never have
suspected that, during the periods which were blank and barren in
his own country, great piles of sediment, charged with new and
peculiar forms of life, had elsewhere been accumulated. And if, in
each separate territory, hardly any idea can be formed of the length
of time which has elapsed between the consecutive formations, we may
infer that this could nowhere be ascertained. The frequent and great
changes in the mineralogical composition of consecutive formations,
generally implying great changes in the geography of the surrounding
lands, whence the sediment was derived, accord with the belief of vast
intervals of time having elapsed between each formation.
We can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each
other in close sequence. Scarcely any fact struck me more when
examining many hundred miles of the South American coasts, which
have been upraised several hundred feet within the recent period, than
the absence of any recent deposits sufficiently extensive to last
for even a short geological period. Along the whole west coast,
which is inhabited by a peculiar marine fauna, tertiary beds are so
poorly developed, that no record of several successive and peculiar
marine faunas will probably be preserved to a distant age. A little
reflection will explain why, along the rising coast of the western
side of South America, no extensive formations with recent or tertiary
remains can anywhere be found, though the supply of sediment must
for ages have been great, from the enormous degradation of the
coast-rocks and from muddy streams entering the sea. The
explanation, no doubt, is, that the littoral and sub-littoral deposits
are continually worn away, as soon as they are brought up by the
slow and gradual rising of the land within the grinding action of
the coast-waves.
We may, I think, conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during
successive oscillations of level as well as the subsequent subaerial
degradation. Such thick and extensive accumulations of sediment may be
formed in two ways; either in profound depths of the sea, in which
case the bottom will not be inhabited by so many and such varied forms
of life, as the more shallow seas; and the mass when upraised will
give an imperfect record of the organisms which existed in the
neighbourhood during the period of its accumulation. Or, sediment
may be deposited to any thickness and extent over a shallow bottom, if
it continue slowly to subside. In this latter case, as long as the
rate of subsidence and the supply of sediment nearly balance each
other, the sea will remain shallow and favourable for many and
varied forms, and thus a rich fossiliferous formation, thick enough,
when upraised, to resist a large amount of denudation, may be formed.
I am convinced that nearly all our ancient formations, which are
throughout the greater part of their thickness rich in fossils, have
thus been formed during subsidence. Since publishing my views on
this subject in 1845, I have watched the progress of geology, and have
been surprised to note how author after author, in treating of this or
that great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only ancient
tertiary formation on the west coast of South America, which has
been bulky enough to resist such degradation as it has yet suffered,
but which will hardly last to a distant geological age, was
deposited during a downward oscillation of level, and thus gained
considerable thickness.
All geological facts tell us plainly that each area has undergone
slow oscillations of level, and apparently these oscillations have
affected wide spaces. Consequently, formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation,
will have been formed over wide spaces during periods of subsidence,
but only where the supply of sediment was sufficient to keep the sea
shallow and to embed and preserve the remains before they had time
to decay. On the other hand, as long as the bed of the sea remains
stationary, thick deposits cannot have been accumulated in the shallow
parts, which are the most favourable to life. Still less can this have
happened during the alternate periods of elevation; or, to speak
more accurately, the beds which were then accumulated will generally
have been destroyed by being upraised and brought within the limits of
the coast-action.
These remarks apply chiefly to littoral and sub-littoral deposits.
In the case of an extensive and shallow sea, such as that within a
large part of the Malay Archipelago, where the depth varies from 30 or
40 to 60 fathoms, a widely extended formation might be formed during a
period of elevation, and yet not suffer excessively from denudation
during its slow upheaval; but the thickness of the formation could not
be great, for owing to the elevatory movement it would be less than
the depth in which it was formed; nor would the deposit be much
consolidated, nor be capped by overlying formations, so that it
would run a good chance of being worn away by atmospheric
degradation and by the action of the sea during subsequent
oscillations of level. It has, however, been suggested by Mr. Hopkins,
that if one part of the area, after rising and before being denuded,
subsided, the deposit formed during the rising movement, though not
thick, might afterwards become protected by fresh accumulations, and
thus be preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds of
considerable horizontal extent have rarely been completely
destroyed. But all geologists, excepting the few who believe that
our present metamorphic schists and plutonic rocks once formed the
primordial nucleus of the globe, will admit that these latter rocks
have been stript of their coverings to an enormous extent. For it is
scarcely possible that such rocks could have been solidified and
crystallized whilst uncovered; but if the metamorphic action
occurred at profound depths of the ocean, the former protecting mantle
of rock may not have been very thick. Admitting then that gneiss,
mica-schist, granite, diorite, &c, were once necessarily covered up,
how can we account for the naked and extensive areas of such rocks
in many parts of the world, except on the belief that they have
subsequently been completely denuded of all overlying strata? That
such extensive areas do exist cannot be doubted: the granitic region
of Parime is described by Humboldt as being as least nineteen times as
large as Switzerland. South of the Amazon, Boue colours an area
composed of rocks of this nature as equal to that of Spain, France,
Italy, part of Germany, and the British Islands, all conjoined. This
region has not been carefully explored, but from the concurrent
testimony of travellers, the granitic area is very large: thus, von
Eschwege gives a detailed section of these rocks, stretching from
Rio de Janeiro for 260 geographical miles inland in a straight line;
and I travelled for 150 miles in another direction, and saw nothing
but granitic rocks. Numerous specimens, collected along the whole
coast from near Rio de Janeiro to the mouth of the Plata, a distance
of 1100 geographical miles, were examined by me, and they all belonged
to this class. Inland, along the whole northern bank of the Plata I
saw, besides modern tertiary beds, only one small patch of slightly
metamorphosed rock, which alone could have formed a part of the
original capping of the granitic series. Turning to a well-known
region, namely, to the United States and Canada, as shown in Professor
H. D. Rogers's beautiful map, I have estimated the areas by cutting
out and weighing the paper, and I find that the metamorphic (excluding
"the semi-metamorphic") and granitic rocks exceed, in the proportion
of 19 to 12.5, the whole of the newer Palaeozoic formations. In many
regions the metamorphic and granitic rocks would be found much more
widely extended than they appear to be, if all the sedimentary beds
were removed which rest unconformably on them, and which could not
have formed part of the original mantle under which they were
crystallized. Hence it is probable that in some parts of the world
whole formations have been completely denuded, with not a wreck left
behind.
One remark is here worth a passing notice. During periods of
elevation the area of the land and of the adjoining shoal parts of the
sea will be increased, and new stations will often be formed:- all
circumstances favourable, as previously explained, for the formation
of new varieties and species; but during such periods there will
generally be a blank in the geological record. On the other hand,
during subsidence, the inhabited area and number of inhabitants will
decrease (excepting on the shores of a continent when first broken
up into an archipelago), and consequently during subsidence, though
there will be much extinction, few new varieties or species will be
formed; and it is during these very periods of subsidence, that the
deposits which are richest in fossils have been accumulated.
On the Absence of Numerous Intermediate Varieties in any Single
Formation
From these several considerations, it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if
we confine our attention to any one formation, it becomes much more
difficult to understand why we do not therein find closely graduated
varieties between the allied species which lived at its commencement
and at its close. Several cases are on record of the same species
presenting varieties in the upper and lower parts of the same
formation; thus, Trautschold gives a number of instances with
ammonites; and Hilgendorf has described a most curious case of ten
graduated forms of Planorbis multiformis in the successive beds of a
fresh-water formation in Switzerland. Although each formation has
indisputably required a vast number of years for its deposition,
several reasons can be given why each should not commonly include a
graduated series of links between the species which lived at its
commencement and close; but I cannot assign due proportional weight to
the following considerations.
Although each formation may mark a very long lapse of years, each
probably is short compared with the period requisite to change one
species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us from coming to
any just conclusion on this head. When we see a species first
appearing in the middle of any formation, it would be rash in the
extreme to infer that it had not elsewhere previously existed. So
again when we find a species disappearing before the last layers
have been deposited, it would be equally rash to suppose that it
then became extinct. We forget how small the area of Europe is
compared with the rest of the world; nor have the several stages of
the same formation throughout Europe been correlated with perfect
accuracy.
We may safely infer that with marine animals of all kinds there
has been a large amount of migration due to climatal and other
changes; and when we see a species first appearing in any formation,
the probability is that it only then first immigrated into that
area. It is well known, for instance, that several species appear
somewhat earlier in the palaeozoic beds of North America than in those
of Europe; time having apparently been required for their migration
from the American to the European seas. In examining the latest
deposits in various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit,
but have become extinct in the immediately surrounding sea; or,
conversely that some are now abundant in the neighbouring sea, but are
rare or absent in this particular deposit. It is an excellent lesson
to reflect on the ascertained amount of migration of the inhabitants
of Europe during the glacial epoch, which forms only a part of one
whole geological period; and likewise to reflect on the changes of
level, on the extreme change of climate, and on the great lapse of
time, all included within this same glacial period. Yet it may be
doubted whether, in any quarter of the world, sedimentary deposits,
including fossil remains, have gone on accumulating within the same
area during the whole of this period. It is not, for instance,
probable that sediment was deposited during the whole of the glacial
period near the mouth of the Mississippi, within that limit of depth
at which marine animals can best flourish: for we know that great
geographical changes occurred in other parts of America during this
space of time. When such beds as were deposited in shallow water
near the mouth of the Mississippi during some part of the glacial
period shall have been upraised, organic remains will probably first
appear and disappear at different levels, owing to the migrations of
species and to geographical changes. And in the distant future, a
geologist, examining these beds, would be tempted to conclude that the
average duration of life of the embedded fossils had been less than
that of the glacial period, instead of having been really far greater,
that is, extending from before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper
and lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the
slow process of modification; hence the deposit must be a very thick
one; and the species, undergoing change must have lived in the same
district throughout the whole time. But we have seen that a thick
formation, fossiliferous throughout its entire thickness, can
accumulate only during a period of subsidence; and to keep the depth
approximately the same, which is necessary that the same marine
species may live on the same space, the supply of sediment must nearly
counterbalance the amount of subsidence. But this same movement of
subsidence will tend to submerge the area whence the sediment is
derived, and thus diminish the supply, whilst the downward movement
continues. In fact, this nearly exact balancing between the supply
of sediment and the amount of subsidence is probably a rare
contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of widely different mineralogical composition, we may
reasonably suspect that the process of deposition has been more or
less interrupted. Nor will the closest inspection of a formation
give us any idea of the length of time which its deposition may have
consumed. Many instances could be given of beds only a few feet in
thickness, representing formations, which are elsewhere thousands of
feet in thickness, and which must have required an enormous period for
their accumulation; yet no one ignorant of this fact would have even
suspected the vast lapse of time represented by the thinner formation.
Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation,- facts, showing what wide, yet easily
overlooked, intervals have occurred in its accumulation. In other
cases we have the plainest evidence in great fossilised trees, still
standing upright as they grew, of many long intervals of time and
changes of level during the process of deposition, which would not
have been suspected, had not the trees been preserved: thus Sir C.
Lyell and Dr. Dawson found carboniferous beds 1400 feet thick in
Nova Scotia, with ancient root-bearing strata, one above the other
at no less than sixty-eight different levels. Hence, when the same
species occurs at the bottom, middle, and top of a formation, the
probability is that it has not lived on the same spot during the whole
period of deposition, but has disappeared and reappeared, perhaps many
times, during the same geological period. Consequently if it were to
undergo a considerable amount of modification during the deposition of
any one geological formation, a section would not include all the fine
intermediate gradations which must on our theory have existed, but
abrupt, though perhaps slight, changes of form.
It is all-important to remember that naturalists have no golden rule
by which to distinguish species and varieties; they grant some
little variability to each species, but when they meet with a somewhat
greater amount of difference between any two forms, they rank both
as species, unless they are enabled to connect them together by the
closest intermediate gradations; and this, from the reasons just
assigned, we can seldom hope to effect in any one geological
section. Supposing B and C to be two species, and a third, A, to be
found in an older and underlying bed; even if A were strictly
intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be closely
connected by intermediate varieties with either one or both forms. Nor
should it be forgotten, as before explained, that A might be the
actual progenitor of B and C, and yet would not necessarily be
strictly intermediate between them in all respects. So that we might
obtain the parent-species, and its several modified descendants from
the lower and upper beds of the same formation, and unless we obtained
numerous transitional gradations, we should not recognise their
blood-relationship, and should consequently rank them as distinct
species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different substages of the same
formation. Some experienced conchologists are now sinking many of
the very fine species of D'Orbigny and others into the rank of
varieties; and on this view we do find the kind of evidence of
change which on the theory we ought to find. Look again at the later
tertiary deposits, which include many shells believed by the
majority of naturalists to be identical with existing species; but
some excellent naturalists as Agassiz and Pictet, maintain that all
these tertiary species are specifically distinct, though the
distinction is admitted to be very slight; so that here, unless we
believe that these eminent naturalists have been misled by their
imaginations, and that these late tertiary species really present no
difference whatever from their living. representatives, or unless we
admit, in opposition to the judgment of most naturalists, that these
tertiary species are all truly distinct from the recent, we have
evidence of the frequent occurrence of slight modifications of the
kind required. It we look to rather wider intervals of time, namely,
to distinct but consecutive stages of the same great formation, we
find that the embedded fossils, though universally ranked as
specifically different, yet are far more closely related to each other
than are the species found in more widely separated formations; so
that here again we have undoubted evidence of change in the
direction required by the theory; but to this latter subject I shall
return in the following chapter.
With animals and plants that propagate rapidly and do not wander
much, there is reason to suspect, as we have formerly seen, that their
varieties are generally at first local; and that such local
varieties do not spread widely and supplant their parent-forms until
they have been modified and perfected in some considerable degree.
According to this view, the chance of discovering in a formation in
any one country all the early stages of transition between any two
forms, is small, for the successive changes are supposed to have
been local or confined to some one spot. Most marine animals have a
wide range; and we have seen that with plants it is those which have
the widest range, that oftenest present varieties; so that, with
shells and other marine animals, it is probable that those which had
the widest range, far exceeding the limits of the known geological
formations in Europe, have oftenest given rise, first to local
varieties and ultimately to new species; and this again would
greatly lessen the chance of our being able trace the stages of
transition in any one geological formation.
It is a more important consideration, leading to the same result, as
lately insisted on by Dr. Falconer, namely, that the period during
which each species underwent modification, though long as measured
by years, was probably short in comparison with that during which it
remained without undergoing any change.
It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties, and thus proved to be the same species,
until many specimens are collected from many places; and with fossil
species this can rarely be done. We shall, perhaps, best perceive
the improbability of our being enabled to connect species by numerous,
fine, intermediate, fossil links, by asking ourselves whether, for
instance, geologists at some future period will be able to prove
that our different breeds of cattle, sheep, horses, and dogs are
descended from a single stock or from several aboriginal stocks; or,
again, whether certain sea-shells inhabiting the shores of North
America, which are ranked by some conchologists as distinct species
from their European representatives, and by other conchologists as
only varieties, are really varieties, or are, as it is called,
specifically distinct. This could be effected by the future
geologist only by his discovering in a fossil state numerous
intermediate gradations; and such success is improbable in the highest
degree.
It has been asserted over and over again, by writers who believe
in the immutability of species, that geology yields no linking
forms. This assertion, as we shall see in the next chapter, is
certainly erroneous. As Sir J. Lubbock has remarked, "Every species is
a link between other allied forms." If we take a genus having a
score of species, recent and extinct, and destroy four-fifths of them,
no one doubts that the remainder will stand much more distinct from
each other. If the extreme forms in the genus happen to have been thus
destroyed, the genus itself will stand more distinct from other allied
genera. What geological research has not revealed, is the former
existence of infinitely numerous gradations, as fine as existing
varieties, connecting together nearly all existing and extinct
species. But this ought not to be expected; yet this has been
repeatedly advanced as a most serious objection against my views.
It may be worth while to sum up the foregoing remarks on the
causes of the imperfection of the geological record under an imaginary
illustration. The Malay Archipelago is about the size of Europe from
the North Cape to the Mediterranean, and from Britain to Russia; and
therefore equals all the geological formations which have been
examined with any accuracy, excepting those of the United States of
America. I fully agree with Mr. Godwin-Austen, that the present
condition of the Malay Archipelago, with its numerous large islands
separated by wide and shallow seas, probably represents the former
state of Europe, whilst most of our formations were accumulating.
The Malay Archipelago is one of the richest regions in organic beings;
yet if all the species were to be collected which have ever lived
there, how imperfectly would they represent the natural history of the
world!
But we have every reason to believe that the terrestrial productions
of the archipelago would be preserved in an extremely imperfect manner
in the formations which we suppose to be there accumulating. Not
many of the strictly littoral animals, or of those which lived on
naked submarine rocks, would be embedded; and those embedded in gravel
or sand would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at
a sufficient rate to protect organic bodies from decay, no remains
could be preserved.
Formations rich in fossils of many kinds, and of thickness
sufficient to last to an age as distant in futurity as the secondary
formations lie in the past, would generally be formed in the
archipelago only during periods of subsidence. These periods of
subsidence would be separated from each other by immense intervals
of time, during which the area would be either stationary or rising;
whilst rising, the fossiliferous formations on the steeper shores
would be destroyed, almost as soon as accumulated, by the incessant
coast-action, as we now see on the shores of South America. Even
throughout the extensive and shallow seas within the archipelago,
sedimentary beds could hardly be accumulated of great thickness during
the periods of elevation, or become capped and protected by subsequent
deposits, so as to have a good chance of enduring to a very distant
future. During the periods of subsidence, there would probably be much
extinction of life; during the periods of elevation, there would be
much variation, but the geological record would then be less perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with
a contemporaneous accumulation of sediment, would exceed the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional
gradations between any two or more species. If such gradations were
not all fully preserved, transitional varieties would merely appear as
so many new, though closely allied species. It is also probable that
each great period of subsidence would be interrupted by oscillations
of level, and that slight climatal changes would intervene during such
lengthy periods; and in these cases the inhabitants of the archipelago
would migrate, and no closely consecutive record of their
modifications could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy plainly leads to
the belief that it would be chiefly these far ranging species,
though only some of them, which would oftenest produce new
varieties; and the varieties would at first be local or confined to
one place, but if possessed of any decided advantage, or when
further modified and improved, they would slowly spread and supplant
their parent-forms. When such varieties returned to their ancient
homes, as they would differ from their former state in a nearly
uniform, though perhaps extremely slight degree, and as they would
be found embedded in slightly different sub-stages of the same
formation, they would, according to the principles followed by many
palaeontologists, be ranked as new and distinct species.
If then there be some degree of truth in these remarks, we have no
right to expect to find, in our geological formations, an infinite
number of those fine transitional forms which, on our theory, have
connected all the past and present species of the same group into
one long and branching chain of life. We ought only to look for a
few links, and such assuredly we do find- some more distantly, some
more closely, related to each other; and these links, let them be ever
so close, if found in different stages of the same formation, would,
by many palaeontologists, be ranked as distinct species. But I do
not pretend that I should ever have suspected how poor was the
record in the best preserved geological sections, had not the
absence of innumerable transitional links between the species which
lived at the commencement and close of each formation, pressed so
hardly on my theory.
On the sudden Appearance of whole Groups of allied Species
The abrupt manner in which whole groups of species suddenly appear
in certain formations, has been urged by several palaeontologists- for
instance, by Agassiz, Pictet, and Sedgwick- as a fatal objection to
the belief in the transmutation of species. If numerous species,
belonging to the same genera or families, have really started into
life at once, the fact would be fatal to the theory of evolution
through natural selection. For the development by this means of a
group of forms, all of which are descended from some one progenitor,
must have been an extremely slow process; and the progenitors must
have lived long before their modified descendants. But we
continually overrate the perfection of the geological record, and
falsely infer, because certain genera or families have not been
found beneath a certain stage, that they did not exist before that
stage. In all cases positive palaeontological evidence may be
implicitly trusted; negative evidence is worthless, as experience
has so often shown. We continually forget how large the world is,
compared with the area over which our geological formations have
been carefully examined; we forget that groups of species may
elsewhere have long existed, and have slowly multiplied, before they
invaded the ancient archipelagoes of Europe and the United States.
We do not make due allowance for the intervals of time which have
elapsed between our consecutive formations,- longer perhaps in many
cases than the time required for the accumulation of each formation.
These intervals will have given time for the multiplication of species
from some one parent-form: and in the succeeding formation, such
groups or species will appear as if suddenly created.
I may here recall a remark formerly made, namely, that it might
require a long succession of ages to adapt an organism to some new and
peculiar line of life, for instance, to fly through the air; and
consequently that the transitional forms would often long remain
confined to some one region; but that, when this adaptation had once
been effected, and a few species had thus acquired a great advantage
over other organisms, a comparatively short time would be necessary to
produce many divergent forms, which would spread rapidly and widely,
throughout the world. Professor Pictet, in his excellent review of
this work, in commenting on early transitional forms, and taking birds
as an illustration, cannot see how the successive modifications of the
anterior limbs of a supposed prototype could possibly have been of any
advantage. But look at the penguins of the Southern Ocean; have not
these birds their front limbs in this precise intermediate state of
"neither true arms nor true wings"? Yet these birds hold their place
victoriously in the battle for life; for they exist in infinite
numbers and of many kinds. I do not suppose that we here see the
real transitional grades through which the wings of birds have passed;
but what special difficulty is there in believing that it might profit
the modified descendants of the penguin, first to become enabled to
flap along the surface of the sea like the logger-headed duck, and
ultimately to rise from its surface and glide through the air?
I will now give a few examples to illustrate the foregoing
remarks, and to show how liable we are to error in supposing that
whole groups of species have suddenly been produced. Even in so
short an interval as that between the first and second editions of
Pictet's great work on Palaeontology, published in 1844-46 and in
1853-57, the conclusions on the first appearance and disappearance
of several groups of animals have been considerably modified; and a
third edition would require still further changes. I may recall the
well-known fact that in geological treatises, published not many years
ago, mammals were always spoken of as having abruptly come in at the
commencement of the tertiary series. And now one of the richest
known accumulations of fossil mammals belongs to the middle of the
secondary series; and true mammals have been discovered in the new red
sandstone at nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum; but now
extinct species have been discovered in India, South America and in
Europe, as far back as the miocene stage. Had it not been for the rare
accident of the preservation of the footsteps in the new red sandstone
of the United States, who would have ventured to suppose that no
less than at least thirty different bird-like animals, some of
gigantic size, existed during that period? Not a fragment of bone
has been discovered in these beds. Not long ago, palaeontologists
maintained that the whole class of birds came suddenly into
existence during the eocene period; but now we know, on the
authority of Professor Owen, that a bird certainly lived during the
deposition of the upper greensand; and still more recently, that
strange bird, the Archeopteryx, with a long lizard-like tail,
bearing a pair of feathers on each joint, and with its wings furnished
with two free claws, has been discovered in the oolitic slates of
Solenhofen. Hardly any recent discovery shows more forcibly than this,
how little we as yet know of the former inhabitants of the world.
I may give another instance, which, from having passed under my
own eyes, has much struck me. In a memoir On Fossil Sessile
Cirripedes, I stated that, from the large number of existing and
extinct tertiary species; from the extraordinary abundance of the
individuals of many species all over the world, from the Arctic
regions to the equator, inhabiting various zones of depths from the
upper tidal limits to 50 fathoms; from the perfect manner in which
specimens are preserved in the oldest tertiary beds; from the ease
with which even a fragment of a valve can be recognised; from all
these circumstances, I inferred that, had sessile cirripedes existed
during the secondary periods, they would certainly have been preserved
and discovered; and as not one species had then been discovered in
beds of this age, I concluded that this great group had been
suddenly developed at the commencement of the tertiary series. This
was a sore trouble to me, adding as I then thought one more instance
of the abrupt appearance of a great group of species. But my work
had hardly been published, when a skilful palaeontologist, M. Bosquet,
sent me a drawing of a perfect specimen of an unmistakable sessile
cirripede, which he had himself extracted from the chalk of Belgium.
And, as if to make the case as striking as possible, this cirripede
was a Chthamalus, a very common, large, and ubiquitous genus, of which
not one species has as yet been found even in any tertiary stratum.
Still more recently, a Pyrgoma, a member of a distinct subfamily of
sessile cirripedes, has been discovered by Mr. Woodward in the upper
chalk; so that we now have abundant evidence of the existence of
this group of animals during the secondary period.
The case most frequently insisted on by palaeontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down, according to Agassiz, in the Chalk
period. This group includes the large majority of existing species.
But certain Jurassic and Triassic forms are now commonly admitted to
be teleostean; and even some palaeozoic forms have thus been classed
by one high authority. If the teleosteans had really appeared suddenly
in the northern hemisphere at the commencement of the chalk
formation the fact would have been highly remarkable; but it would not
have formed an insuperable difficulty, unless it could likewise have
been shown that at the same period the species were suddenly and
simultaneously developed in other quarters of the world. It is
almost superfluous to remark that hardly any fossil-fish are known
from south of the equator; and by running through Pictet's
Palaeontology it will be seen that very few species are known from
several formations in Europe. Some few families of fish now have a
confined range; the teleostean fishes might formerly have had a
similarly confined range, and after having been largely developed in
some one sea, have spread widely. Nor have we any right to suppose
that the seas of the world have always been so freely open from
south to north as they are at present. Even at this day, if the
Malay Archipelago were converted into land, the tropical parts of
the Indian Ocean would form a large and perfectly enclosed basin, in
which any great group of marine animals might be multiplied: and
here they would remain confined, until some of the species became
adapted to a cooler climate, and were enabled to double the Southern
capes of Africa or Australia, and thus reach other and distant seas.
From these considerations, from our ignorance of the geology of
other countries beyond the confines of Europe and the United States,
and from the revolution in our palaeontological knowledge effected
by the discoveries of the last dozen years, it seems to me to be about
as rash to dogmatize on the succession of organic forms throughout the
world, as it would be for a naturalist to land for five minutes on a
barren point in Australia, and then to discuss the number and range of
its productions.
On the Sudden Appearance of Groups of allied Species in the lowest
known Fossiliferous Strata
There is another and allied difficulty, which is much more
serious. I allude to the manner in which species belonging to
several of the main divisions of the animal kingdom suddenly appear in
the lowest known fossiliferous rocks. Most of the arguments which have
convinced me that all the existing species of the same group are
descended from a single progenitor, apply with equal force to the
earliest known species. For instance, it cannot be doubted that all
the Cambrian and Silurian trilobites are descended from some one
crustacean, which must have lived long before the Cambrian age, and
which probably differed greatly from any known animal. Some of the
most ancient animals, as the Nautilus, Lingula, &c., do not differ
much from living species; and it cannot on our theory be supposed,
that these old species were the progenitors of all the species
belonging to the same groups which have subsequently appeared, for
they are not in any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that
before the lowest Cambrian stratum was deposited, long periods
elapsed, as long as, or probably far longer than, the whole interval
from the Cambrian age to the present day; and that during these vast
periods the world swarmed with living creatures. Here we encounter a
formidable objection; for it seems doubtful whether the earth, in a
fit state for the habitation of living creatures, has lasted long
enough. Sir W. Thompson concludes that the consolidation of the
crust can hardly have occurred less than 20 or more than 400 million
years ago, but probably not less than 98 or more than 200 million
years. These very wide limits show how doubtful the data are; and
other elements may have hereafter to be introduced into the problem.
Mr. Croll estimates that about 60 million years have elapsed since the
Cambrian period, but this, judging from the small amount of organic
change since the commencement of the Glacial epoch, appears a very
short time for the many and great mutations of life, which have
certainly occurred since the Cambrian formation; and the previous
140 million years can hardly be considered as sufficient for the
development of the varied forms of life which already existed during
the Cambrian period. It is, however, probable, as Sir William Thompson
insists, that the world at a very early period was subjected to more
rapid and violent changes in its physical conditions than those now
occurring; and such changes would have tended to induce changes at a
corresponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer. Several eminent geologists,
with Sir R. Murchison at their head, were until recently convinced
that we beheld in the organic remains of the lowest Silurian stratum
the first dawn of life. Other highly competent judges, as Lyell and E.
Forbes, have disputed this conclusion. We should not forget that
only a small portion of the world is known with accuracy. Not very
long ago M. Barrande added another and lower stage, abounding with new
and peculiar species, beneath the then known Silurian system; and now,
still lower down in the Lower Cambrian formation, Mr. Hicks has
found in South Wales beds rich in trilobites, and containing various
molluscs and annelids. The presence of phosphatic nodules and
bituminous matter, even in some of the lowest azoic rocks, probably
indicates life at these periods; and the existence of the Eozoon in
the Laurentian formation of Canada is generally admitted. There are
three great series of strata beneath the Silurian system in Canada, in
the lowest of which the Eozoon is found. Sir W. Logan states that
their "united thickness may possibly far surpass that of all the
succeeding rocks, from the base of the palaeozoic series to the
present time. We are thus carried back to a period so remote, that the
appearance of the so-called primordial fauna (of Barrande) may by some
be considered as a comparatively modern event." The Eozoon belongs
to the most lowly organised, of all classes of animals, but is
highly organised for its class; it existed in countless numbers,
and, as Dr. Dawson has remarked, certainly preyed on other minute
organic beings, which must have lived in great numbers. Thus the
words, which I wrote in 1859, about the existence of living beings
long before the Cambrian period, and which are almost the same with
those since used by Sir W. Logan, have proved true. Nevertheless,
the difficulty of assigning any good reason for the absence of vast
piles of strata rich in fossils beneath the Cambrian system is very
great. It does not seem probable that the most ancient beds have
been quite worn away by denudation, or that their fossils have been
wholly obliterated by metamorphic action, for if this had been the
case we should have found only small remnants of the formations next
succeeding them in age, and these would always have existed in
partially metamorphosed condition. But the descriptions which we
possess of the Silurian deposits over immense territories in Russia
and in North America, do not support the view, that the older a
formation is, the more invariably it has suffered extreme denudation
and metamorphism.
The case at present must remain inexplicable; and may be truly urged
as a valid argument against the views here entertained. To show that
it may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains which do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that
from first to last large islands or tracts of land, whence the
sediment was derived, occurred in the neighbourhood of the now
existing continents of Europe and North America. This same view has
since been maintained by Agassiz and others. But we do not know what
was the state of things in the intervals between the several
successive formations; whether Europe and the United States during
these intervals existed as dry land, or as a submarine surface near
land, on which sediment was not deposited, or as the bed of an open
and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but hardly one truly
oceanic island (with the exception of New Zealand, if this can be
called a truly oceanic island) is as yet known to afford even a
remnant of any palaeozoic or secondary formation. Hence we may perhaps
infer, that during the palaeozoic and secondary periods, neither
continents nor continental islands existed where our oceans now
extend; for had they existed, palaeozoic and secondary formations
would in all probability have been accumulated from sediment derived
from their wear and tear; and these would have been at least partially
upheaved by the oscillations of level, which must have intervened
during these enormously long periods. If then we may infer anything
from these facts, we may infer that, where our oceans now extend,
oceans have extended from the remotest period of which we have any
record; and on the other hand, that where continents now exist,
large tracts of land have existed, subjected no doubt to great
oscillations of level, since the Cambrian period. The coloured map
appended to my volume on coral reefs, led me to conclude that the
great oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But we have no reason to assume that things have
thus remained from the beginning of the world. Our continents seem
to have been formed by a preponderance, during many oscillations of
level, of the force of elevation; but may not the areas of
preponderant movement have changed in the lapse of ages? At a period
long antecedent to the Cambrian epoch, continents may have existed
where oceans are now spread out; and clear and open oceans may have
existed where our continents now stand. Nor should we be justified
in assuming that if, for instance, the bed of the Pacific Ocean were
now converted into a continent we should there find sedimentary
formations in a recognisable condition older than the Cambrian strata,
supposing such to have been formerly deposited; for it might well
happen that strata which had subsided some miles nearer to the
centre of the earth, and which had been pressed on by an enormous
weight of super-incumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to
the surface. The immense areas in some parts of the world, for
instance in South America, of naked metamorphic rocks, which must have
been heated under great pressure, have always seemed to me to
require some special explanation; and we may perhaps believe that we
see in these large areas, the many formations long anterior to the
Cambrian epoch in a completely metamorphosed and denuded condition.
The several difficulties here discussed, namely- that, though we
find in our geological formations many links between the species which
now exist and which formerly existed, we do not find infinitely
numerous fine transitional forms closely joining them all together;-
the sudden manner in which several groups of species first appear in
our European formations;- the almost entire absence, as at present
known, of formations rich in fossils beneath the Cambrian strata,- are
all undoubtedly of the most serious nature. We see this in the fact
that the most eminent palaeontologists, namely Cuvier, Agassiz,
Barrande, Pictet, Falconer, E. Forbes, &c., and all our greatest
geologists, as Lyell, Murchison, Sedgwick, &c., have unanimously,
often vehemently, maintained the immutability of species. But Sir
Charles Lyell now gives the support of his high authority to the
opposite side; and most geologists and palaeontologists are much
shaken in their former belief. Those who believe that the geological
record is in any degree perfect, will undoubtedly at once reject the
theory. For my part, following out Lyell's metaphor, I look at the
geological record as a history of the world imperfectly kept, and
written in a changing dialect; of this history we possess the last
volume alone, relating only to two or three countries. Of this volume,
only here and there a short chapter has been preserved; and of each
page, only here and there a few lines. Each word of the
slowly-changing language, more or less different in the successive
chapters, may represent the forms of life, which are entombed in our
consecutive formations, and which falsely appear to have been abruptly
introduced. On this view, the difficulties above discussed are greatly
diminished, or even disappear.
CHAPTER XI
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
LET us now see whether the several facts and laws relating to the
geological succession of organic beings accord best with the common
view of the immutability of species, or with that of their slow and
gradual modification, through variation and natural selection.
New species have appeared very slowly, one after another, both on
the land and in the waters. Lyell has shown that it is hardly possible
to resist the evidence on this head in the case of the several
tertiary stages; and every year tends to fill up the blanks between
the stages, and to make the proportion between the lost and existing
forms more gradual. In some of the most recent beds, though
undoubtedly of high antiquity if measured by years, only one or two
species are extinct, and only one or two are new, having appeared
there for the first time, either locally, or, as far as we know, on
the face of the earth. The secondary formations are more broken;
but, as Bronn has remarked, neither the appearance nor disappearance
of the many species embedded in each formation has been simultaneous.
Species belonging to different genera and classes have not changed
at the same rate, or in the same degree. In the older tertiary beds
a few living shells may still be found in the midst of a multitude
of extinct forms. Falconer has given a striking instance of a
similar fact, for an existing crocodile is associated with many lost
mammals and reptiles in the sub-Himalayan deposits. The Silurian
Lingula differs but little from the living species of this genus;
whereas most of the other Silurian molluscs and all the crustaceans
have changed greatly. The productions of the land seem to have changed
at a quicker rate than those of the sea, of which a striking
instance has been observed in Switzerland. There is some reason to
believe that organisms high in the scale, change more quickly than
those that are low: though there are exceptions to this rule. The
amount of organic change, as Pictet has remarked, is not the same in
each successive so-called formation. Yet if we compare any but the
most closely related formations, all the species will be found to have
undergone some change. When a species has once disappeared from the
lace of the earth, we have no reason to believe that the same
identical form ever reappears. The strongest apparent exception to
this latter rule is that of the so-called "colonies" of M. Barrande,
which intrude for a period in the midst of an older formation, and
then allow the pre-existing fauna to reappear; but Lyell's
explanation, namely, that it is a case of temporary migration from a
distinct geographical province, seems satisfactory.
These several facts accord well with our theory, which includes no
fixed law of development, causing all the inhabitants of an area to
change abruptly, or simultaneously, or to an equal degree. The process
of modification must be slow, and will generally affect only a few
species at the same time; for the variability of each species is
independent of that of all others. Whether such variations or
individual differences as may arise will be accumulated through
natural selection in a greater or less degree, thus causing a
greater or less amount of permanent modification, will depend on
many complex contingencies- on the variations being of a beneficial
nature, on the freedom of intercrossing, on the slowly changing
physical conditions of the country, on the immigration of new
colonists, and on the nature of the other inhabitants with which the
varying species come into competition. Hence it is by no means
surprising that one species should retain the same identical form much
longer than others; or, if changing, should change in a less degree.
We find similar relations between the existing inhabitants of distinct
countries; for instance, the land-shells and coleopterous insects of
Madeira have come to differ considerably from their nearest allies
on the continent of Europe, whereas the marine shells and birds have
remained unaltered. We can perhaps understand the apparently quicker
rate of change in terrestrial and in more highly organised productions
compared with marine and lower productions, by the more complex
relations of the higher beings to their organic and inorganic
conditions of life, as explained in a former chapter. When many of the
inhabitants of any area have become modified and improved, we can
understand, on the principle of competition, and from the
all-important relations of organism to organism in the struggle for
life, that any form which did not become in some degree modified and
improved, would be liable to extermination. Hence we see why all the
species in the same region do at last, if we look to long enough
intervals of time, become modified, for otherwise they would become
extinct.
In members of the same class the average amount of change, during
long and equal periods of time, may, perhaps, be nearly the same;
but as the accumulation of enduring formations, rich in fossils,
depends on great masses of sediment being deposited on subsiding
areas, our formations have been almost necessarily accumulated at wide
and irregularly intermittent intervals of time; consequently the
amount of organic change exhibited by the fossils embedded in
consecutive formations is not equal. Each formation, on this view,
does not mark a new and complete act of creation, but only an
occasional scene, taken almost at hazard, in an ever slowly changing
drama.
We can clearly understand why a species when once lost should
never reappear, even if the very same conditions of life, organic
and inorganic, should recur. For though the offspring of one species
might be adapted (and no doubt this has occurred in innumerable
instances) to fill the place of another species in the economy of
nature, and thus supplant it; yet the two forms- the old and the
new- would not be identically the same; for both would almost
certainly inherit different characters from their distinct
progenitors; and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail pigeons
were destroyed, that fanciers might make a new breed hardly
distinguishable from the present breed; but if the parent
rock-pigeon were likewise destroyed, and under nature we have every
reason to believe that parent-forms are generally supplanted and
exterminated by their improved off spring, it is incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from any other well-established
race of the domestic pigeon, for the successive variations would
almost certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group, when it has once disappeared, never reappears; that
is, its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions
are surprisingly few, so few that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain) admit its
truth; and the rule strictly accords with the theory. For all the
species of the same group, however long it may have lasted, are the
modified descendants one from the other, and all from a common
progenitor. In the genus Lingula, for instance, the species which have
successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to
the present day.
We have seen in the last chapter that whole groups of species
sometimes falsely appear to have been abruptly developed; and I have
attempted to give an explanation of this fact, which if true would
be fatal to my views. But such cases are certainly exceptional; the
general rule being a gradual increase in number, until the group
reaches its maximum, and then, sooner or later, a gradual decrease. If
the number of the species included within a genus, or the number of
the genera within a family, be represented by a vertical line of
varying thickness, ascending through the successive geological
formations, in which the species are found, the line will sometimes
falsely appear to begin at its lower end, not in a sharp point, but
abruptly; it then gradually thickens upwards, often keeping of equal
thickness for a space, and ultimately thins out in the upper beds,
marking the decrease and final extinction of the species. This gradual
increase in number of the species of a group is strictly conformable
with the theory, for the species of the same genus, and the genera
of the same family, can increase only slowly and progressively; the
process of modification and the production of a number of allied forms
necessarily being a slow and gradual process,- one species first
giving rise to two or three varieties, these being slowly converted
into species, which in their turn produce by equally slow steps
other varieties and species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
On Extinction
We have as yet only spoken incidentally of the disappearance of
species and of groups of species. On the theory of natural
selection, the extinction of old forms and the production of new and
improved forms are intimately connected together. The old notion of
all the inhabitants of the earth having been swept away by
catastrophes at successive periods is very generally given up, even by
those geologists, as Elie de Beaumont, Murchison, Barrande, &c., whose
general views would naturally lead them to this conclusion. On the
contrary, we have every reason to believe, from the study of the
tertiary formations, that species and groups of species gradually
disappear, one after another, first from one spot, then from
another, and finally from the world. In some few cases however, as
by the breaking of an isthmus and the consequent irruption of a
multitude of new inhabitants into an adjoining sea, or by the final
subsidence of an island, the process of extinction may have been
rapid. Both single species and whole groups of species last for very
unequal periods; some groups, as we have seen, have endured from the
earliest known dawn of life to the present day; some have
disappeared before the close of the palaeozoic period. No fixed law
seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that
the extinction of a whole group of species is generally a slower
process than their production: if their appearance and disappearance
be represented, as before, by a vertical line of varying thickness the
line is found to taper more gradually at its upper end, which marks
the progress of extermination, than at its lower end, which marks
the first appearance and the early increase in number of the
species. In some cases, however, the extermination of whole groups, as
of ammonites, towards the close of the secondary period, has been
wonderfully sudden.
The extinction of species has been involved in the most gratuitous
mystery. Some authors have even supposed that, as the individual has a
definite length of life, so have species a definite duration. No one
can have marvelled more than I have done at the extinction of species.
When I found in La Plata the tooth of a horse embedded with the
remains of Mastodon, Megatherium, Toxodon, and other extinct monsters,
which all co-existed with still living shells at a very late
geological period, I was filled with astonishment; for, seeing that
the horse, since its introduction by the Spaniards into South America,
has run wild over the whole country and has increased in numbers at an
unparalleled rate, I asked myself what could so recently have
exterminated the former horse under conditions of life apparently so
favourable. But my astonishment was groundless. Professor Owen soon
perceived that the tooth, though so like that of the existing horse,
belonged to an extinct species. Had this horse been still living,
but in some degree rare, no naturalist would have felt the least
surprise at its rarity; for rarity is the attribute of a vast number
of species of all classes, in all countries. If we ask ourselves why
this or that species is rare, we answer that something is unfavourable
in its conditions of life; but what that something is we can hardly
ever tell. On the supposition of the fossil horse still existing as
a rare species, we might have felt certain, from the analogy of all
other mammals, even of the slow-breeding elephant, and from the
history of the naturalisation of the domestic horse in South
America, that under more favourable conditions it would in a very
few years have stocked the whole continent. But we could not have told
what the unfavourable conditions were which checked its increase,
whether some one or several contingencies, and at what period of the
horse's life, and in what degree they severally acted. If the
conditions had gone on, however slowly, becoming less and less
favourable, we assuredly should not have perceived the fact, yet the
fossil horse would certainly have become rarer and rarer, and
finally extinct;- its place being seized on by some more successful
competitor.
It is most difficult always to remember that the increase of every
creature is constantly being checked by unperceived hostile
agencies; and that these same unperceived agencies are amply
sufficient to cause rarity, and finally extinction. So little is
this subject understood, that I have heard surprise repeatedly
expressed at such great monsters as the Mastodon and the more
ancient dinosaurians having become extinct; as if mere bodily strength
gave victory in the battle of life. Mere size, on the contrary,
would in some cases determine, as has been remarked by Owen, quicker
extermination from the greater amount of requisite food. Before man
inhabited India or Africa, some cause must have checked the
continued increase of the existing elephant. A highly capable judge,
Dr. Falconer, believes that it is chiefly insects which, from
incessantly harassing and weakening the elephant in India, check its
increase; and this was Bruce's conclusion with respect to the
African elephant in Abyssinia. It is certain that insects and
bloodsucking bats determine the existence of the larger naturalized
quadrupeds in several parts of S. America.
We see in many cases in the more recent tertiary formations, that
rarity precedes extinction; and we know that this has been the
progress of events with those animals which have been exterminated,
either locally or wholly, through man's agency. I may repeat what I
published in 1845, namely, that to admit that species generally become
rare before they become extinct- to feel no surprise at the rarity
of a species, and yet to marvel greatly when the species ceases to
exist, is much the same as to admit that sickness in the individual is
the forerunner of death- to feel no surprise at sickness, but, when
the sick man dies, to wonder and to suspect that he died by some
deed of violence.
The theory of natural selection is grounded on the belief that
each new variety and ultimately each new species, is produced and
maintained by having some advantage over those with which it comes
into competition; and the consequent extinction of the less-favoured
forms almost inevitably follows. It is the same with our domestic
productions; when a new and slightly improved variety has been raised,
it at first supplants the less improved varieties in the same
neighbourhood; when much improved it is transported far and near, like
our short-horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance of
old forms, both those naturally and those artificially produced, are
bound together. In flourishing groups, the number of new specific
forms which have been produced within a given time has at some periods
probably been greater than the number of the old specific forms
which have been exterminated; but we know that species have not gone
on indefinitely increasing, at least during the later geological
epochs, so that, looking to later times, we may believe that the
production of new forms has caused the extinction of about the same
number of old forms.
The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like
each other in all respects. Hence the improved and modified
descendants of a species will generally cause the extermination of the
parent-species; and if many new forms have been developed from any one
species, the nearest allies of that species, i.e. the species of the
same genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species, that is a
new genus, comes to supplant an old genus, belonging to the same
family. But it must often have happened that a new species belonging
to some one group has seized on the place occupied by a species
belonging to a distinct group, and thus have caused its extermination.
If many allied forms be developed from the successful intruder, many
will have to yield their places; and it will generally be the allied
forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct
class, which have yielded their places to other modified and
improved species, a few of the sufferers may often be preserved for
a long time, from being fitted to some peculiar line of life, or
from inhabiting some distant and isolated station, where they will
have escaped severe competition. For instance, some species of
Trigonia, a great genus of shells in the secondary formations, survive
in the Australian seas; and a few members of the great and almost
extinct group of ganoid fishes still inhabit our fresh waters.
Therefore the utter extinction of a group is generally, as we have
seen, a slower process than its production.
With respect to the apparently sudden extermination of whole
families or orders, as of trilobites at the close of the palaeozoic
period and of ammonites at the close of the secondary period, we
must remember what has been already said on the probable wide
intervals of time between our consecutive formations; and in these
intervals there may have been much slow extermination. Moreover, when,
by sudden immigration or by unusually rapid development, many
species of a new group have taken possession of an area, many of the
older species will have been exterminated in a correspondingly rapid
manner; and the forms which thus yield their places will commonly be
allied, for they will partake of the same inferiority in common.
Thus, as it seems to me, the manner in which single species and
whole groups of species become extinct accords well with the theory of
natural selection. We need not marvel at extinction; if we must
marvel, let it be at our own presumption in imagining for a moment
that we understand the many complex contingencies on which the
existence of each species depends. If we forget for an instant that
each species tends to increase inordinately, and that some check is
always in action, yet seldom perceived by us, the whole economy of
nature will be utterly obscured. Whenever we can precisely say why
this species is more abundant in individuals than that; why this
species and not another can be naturalised in a given country; then,
and not until then, we may justly feel surprise why we cannot
account for the extinction of any particular species or group of
species.
On the Forms of Life changing almost simultaneously throughout the
World
Scarcely any palaeontological discovery is more striking than the
fact that the forms of life change almost simultaneously throughout
the world. Thus our European Chalk formation can be recognised in many
distant regions, under the most different climates, where not a
fragment of the mineral chalk itself can be found; namely in North
America, in equatorial South America, in Tierra del Fuego, at the Cape
of Good Hope, and in the peninsula of India. For at these distant
points, the organic remains in certain beds present an unmistakable
resemblance to those of the Chalk. It is not that the same species are
met with; for in some cases not one species is identically the same,
but they belong to the same families, genera, and sections of
genera, and sometimes are similarly characterised in such trifling
points as mere superficial sculpture. Moreover, other forms, which are
not found in the Chalk of Europe, but which occur in the formations
either above or below, occur in the same order at these distant points
of the world. In the several successive palaeozoic formations of
Russia, Western Europe, and North America, a similar parallelism in
the forms of life has been observed by several authors; so it is,
according to Lyell, with the European and North American tertiary
deposits. Even if the few fossil species which are common to the Old
and New Worlds were kept wholly out of view, the general parallelism
in the successive forms of life, in the palaeozoic and tertiary
stages, would still be manifest, and the several formations could be
easily correlated.
These observations, however, relate to the marine inhabitants of the
world: we have not sufficient data to judge whether the productions of
the land and of fresh water at distant points change in the same
parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to
Europe from La Plata, without any information in regard to their
geological position, no one would have suspected that they had
co-existed with seashells all still living; but as these anomalous
monsters co-existed with the mastodon and horse, it might at least
have been inferred that they had lived during one of the later
tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same year, or to the same century, or even
that it has a very strict geological sense; for if all the marine
animals now living in Europe, and all those that lived in Europe
during the pleistocene period (a very remote period as measured by
years, including the whole glacial epoch) were compared with those now
existing in South America or in Australia, the most skilful naturalist
would hardly be able to say whether the present or the pleistocene
inhabitants of Europe resembled most closely those of the southern
hemisphere. So, again, several highly competent observers maintain
that the existing productions of the United States are more closely
related to those which lived in Europe during certain late tertiary
stages, than to the present inhabitants of Europe; and if this be
so, it is evident that fossiliferous beds now deposited on the
shores of North America would hereafter be liable to be classed with
somewhat older European beds. Nevertheless, looking to a remotely
future epoch, there can be little doubt that all the more modern
marine formations, namely, the upper pliocene, the pleistocene and
strictly modern beds of Europe, North and South America, and
Australia, from containing fossil remains in some degree allied, and
from not including those forms which are found only in the older
underlying deposits, would be correctly ranked as simultaneous in a
geological sense.
The fact of the forms of life changing simultaneously, in the
above large sense, at distant parts of the world, has greatly struck
these admirable observers, MM. de Verneuil and d'Archiae. After
referring to the parallelism of the palaeozoic forms of life in
various parts of Europe, they add, "If, struck by this strange
sequence, we turn our attention to North America, and there discover a
series of analogous phenomena, it will appear certain that all these
modifications of species, their extinction, and the introduction of
new ones, cannot be owing to mere changes in marine currents or
other causes more or less local and temporary, but depend on general
laws which govern the whole animal kingdom." M. Barrande has made
forcible remarks to precisely the same effect. It is, indeed, quite
futile to look to changes of currents, climate, or other physical
conditions, as the cause of these great mutations in the forms of life
throughout the world, under the most different climates. We must, as
Barrande has remarked, look to some special law. We shall see this
more clearly when we treat of the present distribution of organic
beings, and find how slight is the relation between the physical
conditions of various countries and the nature of their inhabitants.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural
selection. New species are formed by having some advantage over
older forms; and the forms, which are already dominant, or have some
advantage over the other forms in their own country, give birth to the
greatest number of new varieties or incipient species. We have
distinct evidence on this head, in the plants which are dominant, that
is, which are commonest and most widely diffused, producing the
greatest number of new varieties. It is also natural that the
dominant, varying, and far-spreading species, which have already
invaded to a certain extent the territories of other species, should
be those which would have the best chance of spreading still
further, and of giving rise in new countries to other new varieties
and species. The process of diffusion would often be very slow,
depending on climatal and geographical changes, on strange
accidents, and on the gradual acclimatisation of new species to the
various climates through which they might have to pass, but in the
course of time the dominant forms would generally succeed in spreading
and would ultimately prevail. The diffusion would, it is probable,
be slower with the terrestrial inhabitants of distinct continents than
with the marine inhabitants of the continuous sea. We might
therefore expect to find, as we do find, a less strict degree of
parallelism in the succession of the productions of the land than with
those of the sea.
Thus, as it seems to me, the parallel, and, taken in a large
sense, simultaneous, succession of the same forms of life throughout
the world, accords well with the principle of new species having
been formed by dominant species spreading widely and varying; the
new species thus produced being themselves dominant, owing to their
having had some advantage over their already dominant parents, as well
as over other species, and again spreading, varying, and producing new
forms. The old forms which are beaten and which yield their places
to the new and victorious forms, will generally be allied in groups,
from inheriting some inferiority in common; and therefore, as new
and improved groups spread throughout the world, old groups
disappear from the world; and the succession of forms everywhere tends
to correspond both in their first appearance and final disappearance.
There is one other remark connected with this subject worth
making. I have given my reasons for believing that most of our great
formations, rich in fossils, were deposited during periods of
subsidence; and that blank intervals of vast duration, as far as
fossils are concerned, occurred during the periods when the bed of the
sea was either stationary or rising, and likewise when sediment was
not thrown down quickly enough to embed and preserve organic
remains. During these long and blank intervals I suppose that the
inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration from
other parts of the world. As we have reason to believe that large
areas are affected by the same movement, it is probable that
strictly contemporaneous formations have often been accumulated over
very wide spaces in the same quarter of the world; but we are very far
from having any right to conclude that this has invariably been the
case, and that large areas have invariably been affected by the same
movements. When two formations have been deposited in two regions
during nearly, but not exactly, the same period, we should find in
both, from the causes explained in the foregoing paragraphs, the
same general succession in the forms of life; but the species would
not exactly correspond; for there will have been a little more time in
the one region than in the other for modification, extinction, and
immigration.
I suspect that cases of this nature occur in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of
England and France, is able to draw a close general parallelism
between the successive stages in the two countries; but when he
compares certain stages in England with those in France, although he
finds in both a curious accordance in the numbers of the species
belonging to the same genera, yet the species themselves differ in a
very, difficult to account for, considering the proximity of the two
areas,- unless, indeed, it be assumed that an isthmus separated two
seas inhabited by distinct, but contemporaneous, faunas. Lyell has
made similar observations on some of the later tertiary formations.
Barrande, also, shows that there is a striking general parallelism
in the successive Silurian deposits of Bohemia and Scandinavia;
nevertheless he finds a surprising amount of difference in the
species. If the several formations in these regions have not been
deposited during the same exact periods,- a formation in one region
often corresponding with a blank interval in the other,- and if in
both regions the species have gone on slowly changing during the
accumulation of the several formations and during the long intervals
of time between them; in this case the several formations in the two
regions could be arranged in the same order, in accordance with the
general succession of the forms of life, and the order would falsely
appear to be strictly parallel; nevertheless the species would not
be all the same in the apparently corresponding stages in the two
regions.
On the Affinities of Extinct Species to each other, and to Living
Forms
Let us now look to the mutual affinities of extinct and living
species. All fall into a few grand classes; and this fact is once
explained on the principle of descent. The more ancient any form is,
the more, as a general rule, it differs from living forms. But, as
Buckland long ago remarked, extinct species can all be classed
either in still existing groups, or between them. That the extinct
forms of life help to fill up the intervals between existing genera,
families, and orders, is certainly true; but as this statement has
often been ignored or even denied, it may be well to make some remarks
on this subject, and to give some instances. If we confine our
attention either to the living or to the extinct species of the same
class, the series is far less perfect than if we combine both into one
general system. In the writings of Professor Owen we continually
meet with the expression of generalised forms, as applied to extinct
animals; and in the writings of Agassiz, of prophetic or synthetic
types; and these terms imply that such forms are in fact
intermediate or connecting links. Another distinguished
palaeontologist, M. Gaudry, has shown in the most striking manner that
many of the fossil mammals discovered by him in Attica serve to
break down the intervals between existing genera. Cuvier ranked the
ruminants and pachyderms as two of the most distinct orders of
mammals: but so many fossil links have been disentombed that Owen
has had to alter the whole classification, and has placed certain
pachyderms in the same sub-order with ruminants; for example, he
dissolves by gradations the apparently wide interval between the pig
and the camel. The Ungulata or hoofed quadrupeds are now divided
into the even-toed or odd-toed divisions; but the Maerauchenia of S.
America connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing
horse and certain older ungulate forms. What a wonderful connecting
link in the chain of mammals is the Typotherium from S. America, as
the name given to it by Professor Gervais expresses, and which
cannot be placed in any existing order. The Sirenia form a very
distinct group of mammals, and one of the most remarkable
peculiarities in the existing dugong and lamentin is the entire
absence of hind limbs without even a rudiment being left; but the
extinct Halitherium had, according to Professor Flower, an ossified
thighbone "articulated to a well-defined acetabulum in the pelvis,"
and it thus makes some approach to ordinary hoofed quadrupeds, to
which the Sirenia are in other respects allied. The cetaceans or
whales are widely different from all other mammals but the tertiary
Zeuglodon and Squalodon, which have been placed by some naturalists in
an order by themselves, are considered by Professor Huxley to be
undoubtedly cetaceans, "and to constitute connecting links with the
aquatic carnivora."
Even the wide interval between birds and reptiles has been shown
by the naturalist just quoted to be partially bridged over in the most
unexpected manner, on the one hand, by the ostrich and extinct
Archeopteryx, and on the other hand, by the Compsognathus, one of
the dinosaurians- that group which includes the most gigantic of all
terrestrial reptiles. Turning to the Invertebrata, Barrande asserts, a
higher authority could not be named, that he is every day taught that,
although palaeozoic can certainly be classed under existing groups,
yet that at this ancient period the groups were not so distinctly
separated from each other as they now are.
Some writers have objected to any extinct species, or group of
species, being considered as intermediate between any two living
species, or groups of species. If by this term it is meant that an
extinct form is directly intermediate in all its characters between
two living forms or groups, the objection is probably valid. But in
a natural classification many fossil species certainly stand between
living species, and some extinct genera between living genera, even
between genera belonging to distinct families. The most common case,
especially with respect to very distinct groups, such as fish and
reptiles, seems to be, that, supposing them to be distinguished at the
present day by a score of characters, the ancient members are
separated by a somewhat lesser number of characters; so that the two
groups formerly made a somewhat nearer approach to each other than
they now do.
It is a common belief that the more ancient a form is, by so much
the more it tends to connect by some of its characters groups now
widely separated from each other. This remark no doubt must be
restricted to those groups which have undergone much change in the
course of geological ages; and it would be difficult to prove the
truth of the proposition, for every now and then even a living animal,
as the Lepidosiren, is discovered having affinities directed towards
very distinct groups. Yet if we compare the older reptiles and
batrachians, the older fish, the older cephalopods, and the eocene
mammals, with the more recent members of the same classes, we must
admit that there is truth in the remark.
Let us see how far these several facts and inferences accord with
the theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the
fourth chapter. We may suppose that the numbered letters in italics
represent genera, and the lines diverging from them the species in
each genus. The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and all the
forms beneath the uppermost line may be considered as extinct. The
three existing genera a14, q14, p14, will form a small family; b14,
and f14 a closely allied family or sub-family; and o14, e14, m14, a
third family. These three families, together with the many extinct
genera on the several lines of descent diverging from the
parent-form (A) will form an order, for all will have inherited
something in common from their ancient progenitor. On the principle of
the continued tendency to divergence of character, which was
formerly illustrated by this diagram, the more recent any form is, the
more it will generally differ from its ancient progenitor. Hence we
can understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the
descendants from a species being thus enabled to seize on many and
different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.
All the many forms, extinct and recent, descended from (A), make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to
have perished at different periods, and some to have endured to the
present day.
By looking at the diagram we can see that if many of the extinct
forms supposed to be imbedded in the successive formations, were
discovered at several points low down in the series, the three
existing families on the uppermost line would be rendered less
distinct from each other. If, for instance, the genera a1, a5, a10,
f8, m3, m6, m9, were disinterred, these three families would be so
closely linked together that they probably would have to be united
into one great family, in nearly the same manner as has occurred
with ruminants and certain pachyderms. Yet he who objected to consider
as intermediate the extinct genera, which thus link together the
living genera of three families, would be partly justified, for they
are intermediate, not directly, but only by a long and circuitous
course through many widely different forms. If many extinct forms were
to be discovered above one of the middle horizontal lines or
geological formations- for instance, above No. VI.- but none from
beneath this line, then only two of the families (those on the left
hand, a14, &c., and b14, &c.) would have to be united into one; and
there would remain two families, which would be less distinct from
each other than they were before the discovery of the fossils. So
again if the three families formed of eight genera (a14 to m14), on
the uppermost line, be supposed to differ from each other by
half-a-dozen important characters, then the families which existed
at the period marked VI. would certainly have differed from each other
by a less number of characters; for they would at this early stage
of descent have diverged in a less degree from their common
progeneitor. Thus it comes that ancient and extinct genera are often
in a greater or less degree intermediate in character between their
modified descendants, or between their collateral relations.
Under nature the process will be far more complicated than
represented in the diagram; for the groups will have been more
numerous; they will have endured for extremely unequal lengths of
time, and will have been modified in various degrees. As we possess
only the last volume of the geological record, and that in a very
broken condition, we have no right to expect, except in rare cases, to
fill up the wide intervals in the natural system, and thus to unite
distinct families or orders. All that we have a right to expect is,
that those groups which have, within known geological periods,
undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ
less from each other in some of their characters than do the existing
members of the same groups; and this by the concurrent evidence of our
best palaeontologists is frequently the case.
Thus, on the theory of descent with modification, the main facts
with respect to the mutual affinities of the extinct forms of life
to each other and to living forms, are explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna during any one
great period in the earth's history will be intermediate in general
character between that which preceded and that which succeeded it.
Thus the species which lived at the sixth great stage of descent in
the diagram are the modified offspring of those which lived at the
sixth stage of descent, and are the parents of those which became
still more modified at the seventh stage; hence they could hardly fail
to be nearly intermediate in character between the forms of life above
and below. We must, however, allow for the entire extinction of some
preceding forms, and in any one region for the immigration of new
forms from other regions, and for a large amount of modification
during the long and blank intervals between the successive formations.
Subject to these allowances, the fauna of each geological period
undoubtedly is intermediate in character, between the preceding and
succeeding faunas. I need give only one instance, namely, the manner
in which the fossils of the Devonian system, when this system was
first discovered, were at once recognized by palaeontologists as
intermediate in character between those of the overlying
carboniferous, and underlying Silurian systems. But each fauna is
not necessarily exactly intermediate, as unequal intervals of time
have elapsed between consecutive formations.
It is no real objection to the truth of the statement that the fauna
of each period as a whole is nearly intermediate in character
between preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, the species of mastodons and
elephants, when arranged by Dr. Falconer in two series,- the first
place according to their mutual affinities, and in the second place
according to their periods of existence,- do not accord in
arrangement. The species extreme in character are not the oldest or
the most recent; nor are those which are intermediate in character,
intermediate in age. But supposing for an instant, in this and other
such cases, that the record of the first appearance and
disappearance of the species was complete, which is far from the case,
we have no reason to believe that forms successively produced
necessarily endure for corresponding lengths of time. A very ancient
form may occasionally have lasted much longer than a form elsewhere
subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things
with great; if the principal living and extinct races of the
domestic pigeon were arranged in serial affinity, this arrangement
would not closely accord with the order in time of their production,
and even less with the order of their disappearance; for the parent
rock-pigeon still lives; and many varieties between the rock-pigeon
and the carrier have become extinct; and carriers which are extreme in
the important character of length of beak originated earlier than
short-beaked tumblers, which are at the opposite end of the series
in this respect.
Closely connected with the statement, that the organic remains
from an intermediate formation are in some degree intermediate in
character, is the fact, insisted on by all palaeontologists, that
fossils from two consecutive formations are far more closely related
to each other, than are the fossils from two remote formations. Pictet
gives as a well-known instance, the general resemblance of the organic
remains from the several stages of the Chalk formation, though the
species are distinct in each stage. This fact alone, from its
generality, seems to have shaken Professor Pictet in his belief in the
immutability of species. He who is acquainted with the distribution of
existing species over the globe, will not attempt to account for the
close resemblance of distinct species in closely consecutive
formations, by the physical conditions of the ancient areas having
remained nearly the same. Let it be remembered that the forms of life,
at least those inhabiting the sea, have changed almost
simultaneously throughout the world, and therefore under the most
different climates and conditions. Consider the prodigious
vicissitudes of climate during the pleistocene period, which
includes the whole glacial epoch, and note how little the specific
forms of the inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fossil remains
from closely consecutive formations being closely related, though
ranked as distinct species, is obvious. As the accumulation of each
formation has often been interrupted, and as long blank intervals have
intervened between successive formations, we ought not to expect to
find, as I attempted to show in the last chapter, in any one or in any
two formations, all the intermediate varieties between the species
which appeared at the commencement and close of these periods: but
we ought to find after intervals, very long as measured by years,
but only moderately long as measured geologically, closely allied
forms, or, as they have been called by some authors, representative
species; and these assuredly we do find. We find, in short, such
evidence of the slow and scarcely sensible mutations of specific
forms, as we have the right to expect.
On the State of Development of Ancient compared with Living Forms
We have seen in the fourth chapter that the degree of
differentiation and specialisation of the parts in organic beings,
when arrived at maturity, is the best standard, as yet suggested, of
their degree of perfection or highness. We have also seen that, as the
specialisation of parts is an advantage to each being, so natural
selection will tend to render the organisation of each being more
specialised and perfect, and in this sense higher; not but that it may
leave many creatures with simple and unimproved structures fitted
for simple conditions of life, and in some cases will even degrade
or simplify the organisation, yet leaving such degraded beings
better fitted for their new walks of life. In another and more general
manner, new species become superior to their predecessors; for they
have to beat in the struggle for life all the older forms, with
which they come into close competition. We may therefore conclude that
if under a nearly similar climate the eocene inhabitants of the
world could be put into competition with the existing inhabitants, the
former would be beaten and exterminated by the latter, as would the
secondary by the eocene, and the palaeozoic by the secondary forms. So
that by this fundamental test of victory in the battle for life, as
well as by the standard of the specialisation of organs, modern
forms ought, on the theory of natural selection, to stand higher
than ancient forms. Is this the case? A large majority of
palaeontologists would answer in the affirmative; and it seems that
this answer must be admitted as true, though difficult of proof.
It is no valid objection to this conclusion, that certain
brachiopods have been but slightly modified from an extremely remote
geological epoch; and that certain land and fresh-water shells have
remained nearly the same, from the time when, as far as is known, they
first appeared. It is not an insuperable difficulty that
Foraminifera have not, as insisted on by Dr. Carpenter, progressed
in organisation since even the Laurentian epoch; for some organisms
would have to remain fitted for simple conditions of life, and what
could be better fitted for this end than these lowly organised
Protozoa? Such objections as the above would be fatal to my view, if
it included advance in organisation as a necessary contingent. They
would likewise be fatal, if the above Foraminifera, for instance,
could be proved to have first come into existence during the
Laurentian epoch, or the above brachiopods during the Cambrian
formation; for in this case, there would not have been time sufficient
for the development of these organisms up to the standard which they
had then reached. When advanced up to any given point, there is no
necessity, on the theory of natural selection, for their further
continued progress; though they will, during each successive age, have
to be slightly modified, so as to hold their places in relation to
slight changes in their conditions. The foregoing objections hinge
on the question whether we really know how old the world is, and at
what period the various forms of life first appeared; and this may
well be disputed.
The problem whether organisation on the whole has advanced is in
many ways excessively intricate. The geological record, at all times
imperfect, does not extend far enough back to show with unmistakable
clearness that within the known history of the world organisation
has largely advanced. Even at the present day, looking to members of
the same class, naturalists are not unanimous which forms ought to
be ranked as highest: thus, some look at the selaceans or sharks, from
their approach in some important points of structure to reptiles, as
the highest fish; others look at the teleosteans as the highest. The
ganoids stand intermediate between the selaceans and teleosteans;
the latter at the present day are largely preponderant in number;
but formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said
that fishes have advanced or retrograded in organisation. To attempt
to compare members of distinct types in the scale of highness seem
hopeless; who will decide whether a cuttle-fish be higher than a
bee- that insect which the great von Baer believed to be "in fact more
highly organised than a fish, although upon another type"? In the
complex struggle for life it is quite credible that crustaceans, not
very high in their own class, might beat cephalopods, the highest
molluscs; and such crustaceans, though not highly developed, would
stand very high in the scale of invertebrate animals, if judged by the
most decisive of all trials- the law of battle. Besides these inherent
difficulties in deciding which forms are the most advanced in
organisation, we ought not solely to compare the highest members of
a class at any two periods- though undoubtedly this is one and perhaps
the most important element in striking a balance- but we ought to
compare all the members, high and low, at the two periods. At an
ancient epoch the highest and lowest molluscoidal animals, namely,
cephalopods and brachiopods, swarmed in numbers; at the present time
both groups are greatly reduced, whilst others, intermediate in
organisation, have largely increased; consequently some naturalists
maintain that molluscs were formerly more highly developed than at
present; but a stronger case can be made out on the opposite side,
by considering the vast reduction of brachiopods, and the fact that
our existing cephalopods, though few in number, are more highly
organised than their ancient representatives. We ought also to compare
the relative proportional numbers at any two periods of the high and
low classes throughout the world: if, for instance, at the present day
fifty thousand kinds of vertebrate animals exist, and if we knew
that at some former period only ten thousand kinds existed, we ought
to look at this increase in number in the highest class, which implies
a great displacement of lower forms, as a decided advance in the
organisation of the world. We thus see how hopelessly difficult it
is to compare with perfect fairness such extremely complex
relations, the standards of organisation of the imperfectly-known
faunas of successive periods.
We shall appreciate this difficulty more clearly, by looking to
certain existing faunas and floras. From the extraordinary manner in
which European productions have recently spread over New Zealand,
and have seized on places which must have been previously occupied
by the indigenes, we must believe, that if all the animals and
plants of Great Britain were set free in New Zealand, a multitude of
British forms would in the course of time become thoroughly
naturalised there, and would exterminate many of the natives. On the
other hand, from the fact that hardly a single inhabitant of the
southern hemisphere has become wild in any part of Europe, we may well
doubt whether, if all the productions of New Zealand were set free
in Great Britain, any considerable number would be enabled to seize on
places now occupied by our native plants and animals. Under this point
of view, the productions of Great Britain stand much higher in the
scale than those of New Zealand. Yet the most skilful naturalist, from
an examination Of the species of the species of the two countries,
could not have foreseen this result.
Agassiz and several other highly competent judges insist that
ancient animals resemble to a certain extent the embryos of recent
animals belonging to the same classes; and that the geological
succession of extinct forms is nearly parallel with the
embryological development of existing forms. This view accords
admirably well with our theory. In a future chapter I shall attempt to
show that the adult differs from its embryo, owing to variations
having supervened at a not early age, and having been inherited at a
corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive
generations, more and more difference to the adult. Thus the embryo
comes to be left as a sort of picture, preserved by nature, of the
former and less modified condition of the species. This view may be
true, and yet may never be capable of proof. Seeing, for instance,
that the oldest known mammals, reptiles, and fishes strictly belong to
their proper classes, though some of these old forms are in a slight
degree less distinct from each other than are the typical members of
the same groups at the present day, it would be vain to look for
animals having the common embryological character of the Vertebrata,
until beds rich in fossils are discovered far beneath the lowest
Cambrian strata- a discovery of which the chance is small.
On the Succession of the same Types within the same Areas, during
the later Tertiary periods
Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of
that continent. In South America a similar relationship is manifest,
even to an uneducated eye, in the gigantic pieces of armour, like
those of the armadillo, found in several parts of La Plata; and
Professor Owen has shown in the most striking manner that most of
the fossil mammals, buried there in such numbers, are related to
South American types. This relationship is even more clearly seen in
the wonderful collection of fossil bones made by MM. Lund and
Clausen in the caves of Brazil. I was so much impressed with these
facts that I strongly insisted, in 1839 and 1845, on this "law of
the succession of types,"- on "this wonderful relationship in the same
continent between the dead and the living." Professor Owen has
subsequently extended the same generalisation to the mammals of the
Old World. We see the same law in this author's restorations of the
extinct and gigantic birds of New Zealand. We see it also in the birds
of the caves of Brazil. Mr. Woodward has shown that the same law holds
good with sea-shells, but, from the wide distribution of most
molluscs, it is not well displayed by them. Other cases could be
added, as the relation between the extinct and living land-shells of
Madeira; and between the extinct and living brackish watershells of
the Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same
types within the same areas mean? He would be a bold man who, after
comparing the present climate of Australia and of parts of South
America, under the same latitude, would attempt to account, on the one
hand through dissimilar physical conditions, for the dissimilarity
of the inhabitants of these two continents; and, on the other hand
through similarity of conditions, for the uniformity of the same types
in each continent during the later tertiary periods. Nor can it be
pretended that it is an immutable law that marsupials should have been
chiefly or solely produced in Australia; or that Edentata and other
American types should have been solely produced in South America.
For we know that Europe in ancient times was peopled by numerous
marsupials; and I have shown in the publications above alluded to,
that in America the law of distribution of terrestrial mammals was
formerly different from what it now is. North America formerly
partook strongly of the present character of the southern half of
the continent; and the southern half was formerly more closely allied,
than it is at present, to the northern half. In a similar manner we
know, from Falconer and Cautley's discoveries, that Northern India was
formerly more closely related in its mammals to Africa than it is at
the present time. Analogous facts could be given in relation to the
distribution of marine animals.
On the theory of descent with modification, the great law of the
long enduring, but not immutable, succession of the same types
within the same areas, is at once explained; for the inhabitants of
each quarter of the world will obviously tend to leave in that
quarter, during the next succeeding period of time, closely allied
though in some degree modified descendants. If the inhabitants of
one continent formerly differed greatly from those of another
continent, so will their modified descendants still differ in nearly
the same manner and degree. But after very long intervals of time, and
after great geographical changes, permitting much intermigration,
the feebler will yield to the more dominant forms, and there will be
nothing immutable in the distribution of organic beings.
It may be asked in ridicule, whether I suppose that the
Megatherium and other allied huge monsters, which formerly lived in
South America, have left behind them the sloth, armadillo, and
anteater, as their degenerate descendants. This cannot for an
instant be admitted. These huge animals have become wholly extinct,
and have left no progeny. But in the caves of Brazil, there are many
extinct species which are closely allied in size and in all other
characters to the species still living in South America; and some of
these fossils may have been the actual progenitors of the living
species. It must not be forgotten that, on our theory, all the species
of the same genus are the descendants of some one species; so that, if
six genera, each having eight species, be found in one geological
formation, and in a succeeding formation there be six other allied
or representative genera each with the same number of species, then we
may conclude that generally only one species of each of the older
genera has left modified descendants, which constitute the new
genera containing the several species; the other seven species of each
old genus having died out and left no progeny. Or, and this will be
a far commoner case, two or three species in two or three alone of the
six older genera will be the parents of the new genera: the other
species and the other old genera having become utterly extinct. In
failing orders, with the genera and species decreasing in numbers as
is the case with the Edentata of South America, still fewer genera and
species will leave modified blood-descendants.
Summary of the preceding and present Chapters
I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been
geologically explored with care; that only certain classes of
organic beings have been largely preserved in a fossil state; that the
number both of specimens and of species, preserved in our museums,
is absolutely as nothing compared with the number of generations which
must have passed away even during a single formation; that, owing to
subsidence being almost necessary for the accumulation of deposits
rich in fossil species of many kinds, and thick enough to outlast
future degradation, great intervals of time must have elapsed
between most of our successive formations; that there has probably
been more extinction during the periods of subsidence, and more
variation during the periods of elevation, and during the latter the
record will have been less perfectly kept; that each single
formation has not been continuously deposited; that the duration of
each formation is probably short compared with the average duration of
specific forms; that migration has played an important part in the
first appearance of new forms in any one area and formation; that
widely ranging species are those which have varied most frequently,
and have oftenest given rise to new species; that varieties have at
first been local; and lastly, although each species must have passed
through numerous transitional stages, it is probable that the periods,
during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods
during which each remained in an unchanged condition. These causes,
taken conjointly, will to a large extent explain why- though we do
find many links- we do not find interminable varieties, connecting
together all extinct and existing forms by the finest graduated steps.
It should also be constantly borne in mind that any linking variety
between two forms, which might be found, would be ranked, unless the
whole chain could be perfectly restored, as a new and distinct
species; for it is not pretended that we have any sure criterion by
which species and varieties can be discriminated.
He who rejects this view of the imperfection of the geological
record, will rightly reject the whole theory. For he may ask in vain
where are the numberless transitional links which must formerly have
connected the closely allied or representative species, found in the
successive stages of the same great formation? He may disbelieve in
the immense intervals of time which must have elapsed between our
consecutive formations; he may overlook how important a part migration
has played, when the formations of any one great region, as those of
Europe, are considered; he may urge the apparent, but often falsely
apparent, sudden coming in of whole groups of species. He may ask
where are the remains of those infinitely numerous organisms which
must have existed long before the Cambrian system was deposited? We
now know that at least one animal did then exist; but I can answer
this last, question only by supposing that where our oceans now extend
they have extended for an enormous period, and where our oscillating
continents now stand they have stood since the commencement of the
Cambrian system; but that, long before that epoch, the world presented
a widely different aspect; and that the older continents formed of
formations older than any known to us, exist now only as remnants in a
metamorphosed condition, or lie still buried under the ocean.
Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with
modification through variation and natural selection. We can thus
understand how it is that new species come in slowly and successively;
how species of different classes do not necessarily change together,
or at the same rate, or in the same degree; yet in the long run that
all undergo modification to some extent. The extinction of old forms
is the almost inevitable consequence of the productions of new
forms. We can understand why, when a species has once disappeared,
it never reappears. Groups of species increase in numbers slowly,
and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species belonging to large and dominant
groups tend to leave many modified descendants, which form new
sub-groups and groups. As these are formed, the species of the less
vigorous groups, from their inferiority inherited from a common
progenitor, tend to become extinct together, and to leave no
modified offspring on the face of the earth. But the utter
extinction of a whole group of species has sometimes been a slow
process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has
been broken.
We can understand how it is that dominant forms which spread
widely and yield the greatest number of varieties tend to people the
world with allied, but modified, descendants; and these will generally
succeed in displacing the groups which are their inferiors in the
struggle for existence. Hence, after long intervals of time, the
productions of the world appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient
and recent, make together a few grand classes. We can understand, from
the continued tendency to divergence of character, why the more
ancient a form is, the more it generally differs from those now
living; why ancient and extinct forms often tend to fill up gaps
between existing forms, sometimes blending two groups, previously
classed as distinct, into one; but more commonly bringing them only
a little closer together. The more ancient a form is, the more often
it stands in some degree intermediate between groups now distinct; for
the more ancient a form is, the more nearly it will be related to, and
consequently resemble, the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate
between existing forms; but are intermediate only by a long and
circuitous course through other extinct and different forms. We can
clearly see why the organic remains of closely consecutive
formations are closely allied; for they are closely linked together by
generation. We can clearly see why the remains of an intermediate
formation are intermediate in character.
The inhabitants of the world at each successive period in its
history have beaten their predecessors in the race for life, and
are, in so far, higher in the scale, and their structure has generally
become more specialised; and this may account for the common belief
held by so many palaeontologists, that organisation on the whole has
progressed. Extinct and ancient animals resemble to a certain extent
the embryos of the more recent animals belonging to the same
classes, and this wonderful fact receives a simple explanation
according to our views. The succession of the same types of
structure within the same areas during the later geological periods
ceases to be mysterious, and is intelligible on the principle of
inheritance.
If then the geological record be as imperfect as many believe, and
it may at least be asserted that the record cannot be proved to be
much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand, an
the chief laws of palaeontology plainly proclaim, as it seems to me,
that species have been produced by ordinary generation: old forms
having been supplanted by new and improved forms of life, the products
of Variation and the Survival of the Fittest.
CHAPTER XII
GEOGRAPHICAL DISTRIBUTION
IN considering the distribution of organic beings over the face of
the globe, the first great fact which strikes us is, that neither
the similarity nor the dissimilarity of the inhabitants of various
regions can be wholly accounted for by climatal and other physical
conditions. Of late, almost every author who has studied the subject
has come to this conclusion. The case of America alone would almost
suffice to prove its truth; for if we exclude the arctic and
northern temperate parts, all authors agree that one of the most
fundamental divisions in geographical distribution is that between the
New and Old Worlds; yet if we travel over the vast American continent,
from the central parts of the United States to its extreme southern
point, we meet with the most diversified conditions; humid
districts, arid deserts, lofty mountains, grassy plains, forests,
marshes, lakes, and great rivers, under almost every temperature.
There is hardly a climate or condition in the Old World which cannot
be paralleled in the New- at least as closely as the same species
generally require. No doubt small areas can be pointed out in the
Old World hotter than any in the New World; but these are not
inhabited by a fauna different from that of the surrounding
districts; for it is rare to find a group of organisms confined to a
small area, of which the conditions are peculiar in only a slight
degree. Notwithstanding this general parallelism in the conditions
of the Old and New Worlds, how widely different are their living
productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between
latitudes 25 and 35, we shall find parts extremely similar in all
their conditions, yet it would not be possible to point out three
faunas and floras more utterly dissimilar. Or, again, we may compare
the productions of South America south of lat. 35 with those north
of 25, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet
they are incomparably more closely related to each other than they are
to the productions of Australia or Africa under nearly the same
climate. Analogous facts could be given with respect to the
inhabitants of the sea.
A second great fact which strikes us in our general review is,
that barriers of any kind, or obstacles to free migration, are related
in a close and important manner to the differences between the
productions of various regions. We see this in the great difference in
nearly all the terrestrial productions of the New and Old Worlds,
excepting in the northern parts, where the land almost joins, and
where, under a slightly different climate, there might have been
free migration for the northern temperate forms, as there now is for
the strictly arctic productions. We see the same fact in the great
difference between the inhabitants of Australia, Africa, and South
America under the same latitude; for these countries are almost as
much isolated from each other as is possible. On each continent, also,
we see the same fact; for on the opposite sides of lofty and
continuous mountain-ranges, of great deserts and even of large rivers,
we find different productions; though as mountain-chains, deserts,
&c., are not as impassable, or likely to have endured so long, as
the oceans separating continents, the differences are very inferior in
degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabitants
of the eastern and western shores of South America are very
distinct, with extremely few shells, Crustacea, or Echinodermata in
common; but Dr. Gunther has recently shown that about thirty per cent.
of the fishes are the same on the opposite sides of the isthmus of
Panama; and this fact has led naturalists to believe that the
isthmus was formerly open. Westward of the shores of America, a wide
space of open ocean extends, with not an island as a halting-place for
emigrants; here we have a barrier of another kind, and as soon as this
is passed we meet in the eastern islands of the Pacific with another
and totally distinct fauna. So that three marine faunas range far
northward and southward in parallel lines not far from each other,
under corresponding climates; but from being separated from each other
by impassable barriers, either of land or open sea, they are almost
wholly distinct. On the other hand, proceeding still farther
westward from the eastern islands of the tropical parts of the
Pacific, we encounter no impassable barriers, and we have
innumerable islands as halting-places, or continuous coasts, until,
after travelling over a hemisphere, we come to the shores of Africa;
and over this vast space we meet with no well-defined and distinct
marine faunas. Although so few marine animals are common to the
above-named three approximate faunas of eastern and western America
and the eastern Pacific islands, yet many fishes range from the
Pacific into the Indian Ocean, and many shells are common to the
eastern islands of the Pacific and the eastern shores of Africa on
almost exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statement, is
the affinity of the productions of the same continent or of the same
sea, though the species themselves are distinct at different points
and stations. It is a law of the widest generality, and every
continent offers innumerable instances. Nevertheless the naturalist,
in travelling, for instance, from north to south, never fails to be
struck by the manner in which successive groups of beings,
specifically distinct, though nearly related, replace each other. He
hears from closely allied, yet distinct kinds of birds, notes nearly
similar, and sees their nests similarly constructed, but not quite
alike, with eggs coloured in nearly the same manner. The plains near
the Straits of Magellan are inhabited by one species of Rhea (American
ostrich) and northward the plains of La Plata by another species of
the same genus; and not by a true ostrich or emu, like those
inhabiting Africa and Australia under the same latitude. On these
same plains of La Plata we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits, and belonging to
the same order of rodents, but they plainly display an American type
of structure. We ascend the lofty peaks of the Cordillera, and we find
an alpine species of bizcacha; we look to the waters, and we do not
find the beaver or musk-rat, but the coypu and capybara, rodents of
the S. American type. Innumerable other instances could be given. If
we look to the islands off the American shore, however much they may
differ in geological structure, the inhabitants are essentially
American, though they may be all peculiar species. We may look back to
past ages, as shown in the last chapter, and we find American types
then prevailing on the American continent and in the American seas. We
see in these facts some deep organic bond, throughout space and
time, over the same areas of land and water, independently of physical
conditions. The naturalist must be dull who is not led to enquire
what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we
see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a subordinate
degree to the definite influence of different physical conditions. The
degrees of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been more
or less effectually prevented, at periods more or less remote;- on the
nature and number of the former immigrants;- and on the action of
the inhabitants on each other in leading to the preservation of
different modifications; the relation of organism to organism in the
struggle for life being, as I have already often remarked, the most
important of all relations. Thus the high importance of barriers
comes into play by checking migration; as does time for the slow
process of modification through natural selection. Widely-ranging
species, abounding in individuals, which have already triumphed over
many competitors in their own widely-extended homes, will have the
best chance of seizing on new places, when they spread into new
countries. In their new homes they will be exposed to new
conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and
will produce groups of modified descendants. On this principle of
inheritance with modification we can understand how it is that
sections of genera, whole genera, and even families, are confined to
the same areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability of
each species is an independent property, and will be taken advantage
of by natural selection, only so far as it profits each individual
in its complex struggle for life, so the amount of modification in
different species will be no uniform quantity. If a number of species,
after having long competed with each other in their old home, were
to migrate in a body into a new and afterwards isolated country,
they would be little liable to modification; for neither migration nor
isolation in themselves effect anything. These principles come into
play only by bringing organisms into new relations with each other and
in a lesser degree with the surrounding physical conditions. As we
have seen in the last chapter that some forms have retained nearly the
same character from an enormously remote geological period, so certain
species have migrated over vast spaces, and have not become greatly or
at all modified.
According to these views, it is obvious that the several species
of the same genus, though inhabiting the most distant quarters of
the world, must originally have proceeded from the same source, as
they are descended from the same progenitor. In the case of those
species which have undergone during the whole geological periods
little modification, there is not much difficulty in believing that
they have migrated from, the same region; for during the vast
geographical and climatal changes which have supervened since
ancient times, almost any amount of migration is possible. But in many
other cases, in which we have reason to believe that the species of
a genus have been produced within comparatively recent times, there is
great difficulty on this head. It is also obvious that the
individuals of the same species, though now inhabiting distant and
isolated regions, must have proceeded from one spot, where their
parents were first produced: for, as has been explained, it is
incredible that individuals identically the same should have been
produced from parents specifically distinct.
Single Centres of supposed Creation.- We are thus brought to the
question which has been largely discussed by naturalists, namely,
whether species have been created at one or more points of the earth's
surface. Undoubtedly there are many cases of extreme difficulty in
understanding how the same species could possibly have migrated from
some one point to the several distant and isolated points, where now
found. Nevertheless the simplicity of the view that each species was
first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with
subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a
species is continuous; and that when a plant or animal inhabits two
points so distant from each other, or with an interval of such a
nature, that the space could not have been easily passed over by
migration, the fact is given as something remarkable and
exceptional. The incapacity of migrating across a wide sea is more
clear in the case of terrestrial mammals than perhaps with any other
organic beings; and, accordingly, we find no inexplicable instances of
the same mammals inhabiting distant points of the world. No
geologist feels any difficulty in Great Britain possessing the same
quadrupeds with the rest of Europe, for they were no doubt once
united. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the
same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal
plants are identically the same at these distant points of the
northern and southern hemispheres. The answer, as I believe, is,
that mammals have not been able to migrate, whereas some plants,
from their varied means of dispersal, have migrated across the wide
and broken interspaces. The great and striking influence of barriers
of all kinds, is intelligible only on the view that the great majority
of species have been produced on one side, and have not been able to
migrate to the opposite side. Some few families, many sub-families,
very many genera, and a still greater number of sections of genera,
are confined to a single region; and it has been observed by several
naturalists that the most natural genera, or those genera in which the
species are most closely related to each other, are generally confined
to the same, country, or if they have a wide range that their range is
continuous. What a strange anomaly it would be, if a directly
opposite rule were to prevail, when we go down one step lower in the
series, namely, to the individuals of the same species, and these
had not been, at least at first, confined to some one region!
Hence it seems to me, as it has to many other naturalists, that
the view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers of
migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we cannot
explain how the same species could have passed from one point to the
other. But the geographical and climatal changes which have
certainly occurred within recent geological times, must have
rendered discontinuous the formerly continuous range of many
species. So that we are reduced to consider whether the exceptions
to continuity of range are so numerous and of so grave a nature,
that we ought to give up the belief, rendered probable by general
considerations, that each species has been produced within one area,
and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species,
now living at distant and separated points, nor do I for a moment
pretend that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few of the
most striking classes of facts; namely, the existence of the same
species on the summits of distant mountain ranges, and at distant
points in the arctic and antarctic regions; and secondly (in the
following chapter), the wide distribution of fresh-water
productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the nearest mainland, though separated by
hundreds of miles of open sea. If the existence of the same species at
distant and isolated points of the earth's surface, can in many
instances be explained on the view of each species having migrated
from a single birthplace; then, considering our ignorance with respect
to former climatal and geographical changes and to the various
occasional means of transport, the belief that a single birthplace
is the law, seems to me incomparably the safest.
In discussing this subject, we shall be enabled at the same time
to consider a point equally important for us, namely, whether the
several species of a genus which must on our theory all be descended
from a common progenitor, can have migrated, undergoing modification
during their migration, from some one area. If, when most of the
species inhabiting one region are different from those of another
region, though closely allied to them, it can be shown that
migration from the one region to the other has probably occurred at
some former period, our general view will be much strengthened; for
the explanation is obvious on the principle of descent with
modification. A volcanic island, for instance, upheaved and formed
at the distance of a few hundreds of miles from a continent, would
probably receive from it in the course of time a few colonists, and
their descendants, though modified, would still be related by
inheritance to the inhabitants of that continent. Cases of this nature
are common, and are, as we shall hereafter see, inexplicable on the
theory of independent creation. This view of the relation of the
species of one region to those of another, does not differ much from
that advanced by Mr. Wallace, who concludes that "every species has
come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs
from another though allied question,- namely, whether all the
individuals of the same species are descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With organic beings which never
intercross, if such exist, each species must be descended from a
succession of modified varieties, that have supplanted each other, but
have never blended with other individuals or varieties of the same
species; so that, at each successive stage of modification, all the
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be
kept nearly uniform by intercrossing; so that many individuals will go
on simultaneously changing, and the whole amount of modification at
each stage will not be due to descent from a single parent. To
illustrate what I mean: our English race-horses differ from the horses
of every other breed; but they do not owe their difference and
superiority to descent from any single pair, but to continued care
in the selecting and training of many individuals during each
generation.
Before discussing the three classes of facts, which I have
selected as presenting the greatest amount of difficulty on the theory
of "single centres of creation," I must say a few words on the means
of dispersal.
Means of Dispersal
Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration. A
region now impassable to certain organisms from the nature of its
climate, might have been a high road for migration, when the climate
was different. I shall, however, presently have to discuss this branch
of the subject in some detail. Changes of level in the land must
also have been highly influential: a narrow isthmus now separates
two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend together, or may formerly
have blended. Where the sea now extends, land may at a former period
have connected islands or possibly even continents together, and
thus have allowed terrestrial productions to pass from one to the
other No geologist disputes that great mutations of level have
occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must have been
recently connected with Europe or Africa, and Europe likewise with
America. Other authors have thus hypothetically bridged over every
ocean, and united almost every island with some mainland. If indeed
the arguments used by Forbes are to be trusted, it must be admitted
that scarcely a single island exists which has not recently been
united to some continent. This view cuts the Gordian knot of the
dispersal of the same species to the more distant points, and
removes many a difficulty; but to the best of my judgment we are not
authorised in admitting such enormous geographical changes within
the period of existing species. It seems to me that we have abundant
evidence of great oscillations in the level of the land or sea; but
not of such vast change in the position and extension of our
continents, as to have united them within the recent period to each
other and to the several intervening oceanic islands. I freely admit
the former existence of many islands, now buried beneath the sea,
which may have served as halting-places for plants and for many
animals during their migration. In the coral-producing oceans such
sunken islands are now marked by rings of coral or atolls standing
over them. Whenever it is fully admitted, as it will some day be, that
each species has proceeded from a single birthplace, and when in the
course of time we know something definite about the means of
distribution, we shall be enabled to speculate with security on the
former extension of the land. But I do not believe that it will ever
be proved that within the recent period most of our continents which
now stand quite separate have been continuously, or almost
continuously united with each other, and with the many existing
oceanic islands. Several facts in distribution,- such as the great
difference in the marine faunas on the opposite sides of almost
every continent,- the close relation of the tertiary inhabitants of
several lands and even seas to their present inhabitants,- the
degree of affinity between the mammals inhabiting islands with those
of the nearest continent, being in part determined (as we shall
hereafter see) by the depth of the intervening ocean,- these and other
such facts are opposed to the admission of such prodigious
geographical revolutions within the recent period, as are necessary on
the view advanced by Forbes and admitted by his followers. The
nature and relative proportions of the inhabitants of oceanic
islands are likewise opposed to the belief of their former
continuity with continents. Nor does the almost universally volcanic
composition of such islands favour the admission that they are the
wrecks of sunken continents;- if they had originally existed as
continental mountain ranges, some at least of the islands would have
been formed, like other mountain summits, of granite, metamorphic
schists, old fossiliferous and other rocks, instead of consisting of
mere piles of volcanic matter.
I must now say a few words on what are called accidental means,
but which more properly should be called occasional means of
distribution. I shall here confine myself to plants. In botanical
works, this or that plant is often stated to be ill adapted for wide
dissemination; but the greater or less facilities for transport across
the sea may be said to be almost wholly unknown. Until I tried, with
Mr. Berkeley's aid, a few experiments, it was not even known how far
seeds could resist the injurious action of sea-water. To my surprise I
found that out of 87 kinds, 64 germinated after an immersion of 28
days, and a few survived an immersion of 137 days. It deserves
notice that certain orders were far more injured than others: nine
leguminosae were tried, and, with one exception, they resisted the
salt-water badly; seven species of the allied orders,
Hydrophyllaceae and Polemoniacae, were all killed by a month's
immersion. For convenience' sake I chiefly tried small seeds without
the capsule or fruit; and as all of these sank in a few days they
could not have been floated across wide spaces of the sea, whether
or not they were injured by the salt-water. Afterwards I tried some
larger fruits, capsules, &c., and some of these floated for a long
time. It is well known what a difference there is in the buoyancy of
green and seasoned timber; and it occurred to me that floods would
often wash into the sea dried plants or branches with seed-capsules or
fruit attached to them. Hence I was led to dry the stems and
branches of 94 plants with ripe fruit, and to place them on sea-water.
The majority sank rapidly, but some which, whilst green, floated for a
short time, when dried floated much longer; for instance, ripe
hazel-nuts sank immediately, but when dried they floated for 90
days, and afterwards when planted germinated; an asparagus-plant
with ripe berries floated for 23 days, when dried it floated for 85
days, and the seeds afterwards germinated; the ripe seeds of
Helosciadium sank in two days, when dried they floated for above 90
days, and afterwards germinated. Altogether, out of the 94 dried
plants, 18 floated for above 28 days; and some of the 18 floated for a
very much longer period. So that as 64/87 kinds of seeds germinated
after an immersion of 28 days; and as 18/94 distinct species with ripe
fruit (but not all the same species as in the foregoing experiment)
floated, after being dried, for above 28 days, we may conclude, as far
as anything can be inferred from these scanty facts, that the seeds of
14/100 kinds of plants of any country might be floated by sea currents
during 28 days, and would retain their power of germination. In
Johnston's Physical Atlas, the average rate of the several Atlantic
currents is 33 miles per diem (some currents running at the rate of
miles per diem); on this average, the seeds of 14/100 plants belonging
to one country might be floated across 924 miles of sea to another
country, and when stranded, if blown by an inland gale to a favourable
spot, would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but
in a much better manner, for he placed the seeds in a box in the
actual sea, so that they were alternately wet and exposed to the air
like really floating plants. He tried 98 seeds, mostly different
from mine; but he chose many large fruits and likewise seeds from
plants which live near the sea; and this would have favoured both
the average length of their flotation and their resistance to the
injurious action of the salt-water. On the other hand, he did not
previously dry the plants or branches with the fruit; and this, as
we have seen, would have caused some of them to have floated much
longer. The result was that 18/98ths of his seeds of different kinds
floated for 42 days, and were then capable of germination. But I do
not doubt that plants exposed to the waves would float for a less time
than those protected from violent movement as in our experiments.
Therefore it would perhaps be safer to assume that the seeds of
about 10/100 plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then
germinate. The fact of the larger fruits often floating longer than
the small, is interesting; as plants with large seeds or fruit
which, as Alph. de Candolle has shown, generally have restricted
ranges, could hardly be transported by any other means.
Seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral islands in the Pacific
procure stones for their tools, solely from the roots of drifted
trees, these stones being a valuable royal tax. I find that when
irregularly shaped are embedded in the roots of trees, small parcels
of earth are frequently enclosed in their interstices and behind
them,- so perfectly that not a particle could be washed away during
the longest transport: out of one small portion of earth thus
completely enclosed by the roots of an oak about 50 years old, three
dicotyledonous plants germinated: I am certain of the accuracy of this
observation. Again, I can show that the carcases of birds, when
floating on the sea, sometimes escape being immediately devoured:
and many kinds of seeds in the crops of floating birds long retain
their vitality: peas and vetches, for instance, are killed by even a
few days' immersion in sea-water; but some taken out of the crop of
a pigeon, which had floated on artificial sea-water for 30 days, to my
surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how
frequently birds of many kinds are blown by gales to vast distances
across the ocean. We may safely assume that under such circumstances
their rate of flight would often be 35 miles an hour; and some authors
have given a far higher estimate. I have never seen an instance of
nutritious seeds passing through the intestines of a bird, but hard
seeds of fruit pass uninjured through even the digestive organs of a
turkey. In the course of two months, I picked up in my garden 12 kinds
of seeds, out of the excrement of small birds, and these seemed
perfect, and some of them, which were tried, germinated. But the
following fact is more important: the crops of birds do not secrete
gastric juice, and do not, as I know by trial, injure in the least the
germination of seeds; now, after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains do not
pass into the gizzard for twelve or even eighteen hours. A bird in
this interval might easily be blown to the distance of 500 miles,
and hawks are known to look out for tired birds, and the contents of
their torn crops might thus readily get scattered. Some hawks and owls
bolt their prey whole, and, after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds
of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having
been thus retained for two days and fourteen hours. Fresh-water
fish, I find, eat seeds of many land and water plants; fish are
frequently devoured by birds, and thus the seeds might be
transported from place to place. I forced many kinds of seeds into the
stomachs of dead fish, and then gave their bodies to fishing-eagles,
storks, and pelicans; these birds, after an interval of many hours,
either rejected the seeds in pellets or passed them in their
excrement; and several of these seeds retained the power of
germination. Certain seeds, however, were always killed by this
process.
Locusts are sometimes blown to great distances from the land; I
myself caught one 370 miles from the coast of Africa, and have heard
of others caught at greater distances. The Rev. R. T. Lowe informed
Sir C. Lyell that in November, 1844, swarms of locusts visited the
island of Madeira. They were in countless numbers, as thick as the
flakes of snow in the heaviest snowstorm, and extended upwards as
far as could be seen with a telescope. During two or three days they
slowly careered round and round in an immense ellipse, at least five
or six miles in diameter, and at night alighted on the taller trees,
which were completely coated with them. They then disappeared over the
sea, as suddenly as they had appeared, and have not since visited
the island. Now, in parts of Natal it is believed by some farmers,
though on insufficient evidence, that injurious seeds are introduced
into their grass-land in the dung left by the great flights of locusts
which often visit that country. In consequence of this belief Mr.
Weale sent me in a letter a small packet of the dried pellets, out
of which I extracted under the microscope several seeds, and raised
from them seven grass plants, belonging to two species, of two genera.
Hence a swarm of locusts, such as that which visited Madeira, might
readily be the means of introducing several kinds of plants into an
island lying far from the mainland.
Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one grains, and
in another case twenty-two grains of dry argillaceous earth from the
foot of a partridge, and in the earth there was a pebble as large as
the seed of a vetch. Here is a better case: the leg of a woodcock
was sent to me by a friend, with a little cake of dry earth attached
to the shank, weighing only nine grains; and this contained a seed
of the toad-rush (Juncus bufonius) which germinated and flowered.
Mr. Swaysland, of Brighton, who during the last forty years has paid
close attention to our migratory birds, informs me that he has often
shot wagtails (Motacillae), wheat-ears, and whinchats (Saxicolae),
on their first arrival on our shores, before they had alighted; and he
has several times noticed little cakes of earth attached to their
feet. Many facts could be given showing how generally soil is
charged with seeds. For instance, Prof. Newton sent me the leg of a
red-legged partridge (Caccabis rufa) which had been wounded and
could not fly, with a ball of hard earth adhering to it, and
weighing six and a half ounces. The earth had been kept for three
years, but when broken, watered and placed under a bell glass, no less
than 82 plants sprung from it: these consisted of 12 monocotyledons,
including the common oat, and at least one kind of grass, and of 70
dicotyledons, which consisted, judging from the young leaves, of at
least three distinct species. With such facts before us, can we
doubt that the many birds which are annually blown by gales across
great spaces of ocean, and which annually migrate- for instance, the
millions of quails across the Mediterranean- must occasionally
transport a few seeds embedded in dirt adhering to their feet or
beaks? But I shall have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and
stones, and have even carried brushwood, bones, and the nest of a
land-bird, it can hardly be doubted that they must occasionally, as
suggested by Lyell, have transported seeds from one part to another of
the arctic and antarctic regions; and during the Glacial period from
one part of the now temperate regions to another. In the Azores,
from the large number of plants common to Europe, in comparison with
the species on the other islands of the Atlantic, which stand nearer
to the mainland, and (as remarked by Mr. H. C. Watson) from their
somewhat northern character in comparison with the latitude, I
suspected that these islands had been partly stocked by ice-borne
seeds, during the Glacial epoch. At my request Sir C. Lyell wrote to
M. Hartung to inquire whether he had observed erratic boulders on
these islands, and he answered that he had found large fragments of
granite and other rocks, which do not occur in the archipelago.
Hence we may safely infer that icebergs formerly landed their rocky
burthens on the shores of these mid-ocean islands and it is at least
possible that they may have brought thither some few seeds of
northern plants.
Considering that these several means of transport, and that other
means, which without doubt remain to be discovered, have been in
action year after year for tens of thousands of years, it would, I
think, be a marvellous fact if many plants had not thus become
widely transported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents of the
sea are not accidental, nor is the direction of prevalent gales of
wind. It should be observed that scarcely any means of transport would
carry seeds for very great distances: for seeds do not retain their
vitality when exposed for a great length of time to the action of
sea-water; nor could they be long carried in the crops or intestines
of birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island
to island, or from a continent to a neighbouring island, but not
from one distant continent to another. The floras of distant
continents would not by such means become mingled; but would remain as
distinct as they now are. The currents, from their course, would never
bring seeds from North America to Britain, though they might and do
bring seeds from the West Indies to our western shores, where, if
not killed by their very long immersion in salt water, they could
not endure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to the
western shores of Ireland and England; but seeds could be
transported by these rare wanderers only by one means, namely, by
dirt adhering to their feet or beaks, which is in itself a rare
accident. Even in this case, how small would be the chance of a seed
falling on favourable soil, and coming to maturity! But it would be
a great error to argue that because a well-stocked island, like
Great Britain, has not, as far as is known (and it would be very
difficult to prove this), received within the last few centuries,
through occasional means of transport, immigrants from Europe or any
other continent, that a poorly-stocked island, though standing more
remote from the mainland, would not receive colonists by similar
means. Out of a hundred kinds of seeds or animals transported to an
island, even if far less well-stocked than Britain, perhaps not more
than one would be so well fitted to its new home, as to become
naturalised. But this is no valid argument against what would be
effected by occasional means of transport, during the long lapse of
geological time, whilst the island was being upheaved, and before it
had become fully stocked with inhabitants. On almost bare land, with
few or no destructive insects or birds living there, nearly every seed
which chanced to arrive, if fitted for the climate, would germinate
and survive.
Dispersal during the Glacial Period
The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where
Alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points without the
apparent possibility of their having migrated from one point to the
other. It is indeed a remarkable fact to see so many plants of the
same species living on the snowy regions of the Alps or Pyrenees,
and in the extreme northern parts of Europe; but it is far more
remarkable, that the plants on the White Mountains, in the United
States of America, are all the same with those of Labrador, and
nearly all the same, as we hear from Asa Gray, with those on the
loftiest mountains of Europe. Even as long ago as 1747, such facts led
Gmelin to conclude that the same species must have been
independently created at many distinct points; and we might have
remained in this same belief, had not Agassiz and others called
vivid attention to the Glacial period, which, as we shall
immediately see, affords a simple explanation of these facts. We
have evidence of almost every conceivable kind, organic and inorganic,
that, within a very recent geological period, central Europe and North
America suffered under an arctic climate. The ruins of a house burnt
by fire do not tell their tale more plainly than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were
lately filled. So greatly has the climate of Europe changed, that in
northern Italy, gigantic moraines, left by old glaciers, are now
clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.
The former influence of the glacial climate on the distribution of
the inhabitants of Europe, as explained by Edward Forbes, is
substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period slowly to come on, and then
pass away, as formerly occurred. As the cold came on, and as each more
southern zone became fitted for the inhabitants of the north, these
would take the places of the former inhabitants of the temperate
regions. The latter, at the same time, would travel further and
further southward, unless they were stopped by barriers, in which case
they would perish. The mountains would become covered with snow and
ice, and their former Alpine inhabitants would descend to the
plains. By the time that the cold had reached its maximum, we should
have an arctic fauna and flora, covering the central parts of
Europe, as far south as the Alps and Pyrenees, and even stretching
into Spain. The now temperate regions of the United States would
likewise be covered by arctic plants and animals and these would be
nearly the same with those of Europe; for the present circumpolar
inhabitants, which we suppose to have everywhere travelled
southward, are remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending, as the warmth increased and the snow still
further disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same species, which had lately lived together on the
European and North American lowlands, would again be found in the
arctic regions of the Old and New Worlds, and on many isolated
mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain range are more especially related to the arctic forms
living due north or nearly due north of them: for the first
migration when the cold came on, and the re-migration on the returning
warmth, would generally have been due south and north. The Alpine
plants, for example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Ramond, are more
especially allied to the plants of northern Scandinavia; those of
the United States to Labrador; those of the mountains of Siberia to
the arctic regions of that country. These views, grounded as they
are on the perfectly well-ascertained occurrence of a former Glacial
period, seem to me to explain in so satisfactory a manner the
present distribution of the Alpine and arctic productions of Europe
and America, that when in other regions we find the same species on
distant mountain-summits, we may almost conclude, without other
evidence, that a colder climate formerly permitted their migration
across the intervening lowlands, now become too warm for their
existence.
As the arctic forms moved first southward and afterwards backwards
to the north, in unison with the changing climate, they will not
have been exposed during their long migration to any great diversity
of temperature; and as they all migrated in a body together, their
mutual relations will not have been much disturbed. Hence, in
accordance with the principles inculcated in this volume, these
forms will not have been liable to much modification. But with the
Alpine productions, left isolated from the moment of the returning
warmth, first at the bases and ultimately on the summits of the
mountains, the case will have been somewhat different; for it is not
likely that all the same arctic species will have been left on
mountain ranges far distant from each other, and have survived there
ever since; they will also in all probability, have become mingled
with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which
during the coldest period will have been temporarily driven down to
the plains; they will, also, have been subsequently exposed to
somewhat different climatal influences. Their mutual relations will
thus have been in some degree disturbed; consequently they will have
been liable to modification; and they have been modified; for if we
compare the present Alpine plants and animals of the several great
European mountain ranges one with another, though many of the
species remain identically the same, some exist as varieties, some
as doubtful forms or sub-species, and some as distinct yet closely
allied species representing each other on the several ranges.
In the foregoing illustration I have assumed that at the
commencement of our imaginary Glacial period, the arctic productions
were as uniform round the polar regions as they are at the present
day. But it is also necessary to assume that many sub-arctic and
some few temperate forms were the same round the world, for some of
the species which now exist on the lower mountain-slopes and on the
plains of North America and Europe are the same; and it may be asked
how I account for this degree of uniformity in the sub-arctic and
temperate forms round the world, at the commencement of the real
Glacial period. At the present day, the sub-arctic and northern
temperate productions of the Old and New Worlds are separated from
each other by the whole Atlantic Ocean and by the northern part of the
Pacific. During the Glacial period, when the inhabitants of the Old
and New Worlds lived farther southward than they do at present, they
must have been still more completely separated from each other by
wider spaces of ocean; so that it may well be asked how the same
species could then or previously have entered the two continents.
The explanation, I believe, lies in the nature of the climate before
the commencement of the Glacial period. At this, the newer Pliocene
period, the majority of the inhabitants of the world were specifically
the same as now, and we have good reason to believe that the climate
was warmer than at the present day. Hence we may suppose that the
organisms which now live under latitude 60, lived during the
Pliocene period farther north under the Polar Circle, in latitude
66-67; and that the present arctic productions then lived on the
broken land still nearer to the pole. Now, if we looked at a
terrestrial globe, we see under the Polar Circle that there is
almost continuous land from western Europe, through Siberia, to
eastern America. And this continuity of the circumpolar land, with the
consequent freedom under a more favourable climate for intermigration,
will account for the supposed uniformity of the sub-arctic and
temperate productions of the Old and New Worlds, at a period
anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents
have long remained in nearly the same relative position, though
subjected to great oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some still earlier and
still warmer period, such as the older Pliocene period, a large number
of the same plants and animals inhabited the almost continuous
circumpolar land; and that these plants and animals, both in the Old
and New Worlds, began slowly to migrate southwards as the climate
became less warm, long before the commencement of the Glacial
period. We now see, as I believe, their descendants, mostly in a
modified condition, in the central parts of Europe and the United
States. On this view we can understand the relationship with very
little identity, between the productions of North America and Europe,-
a relationship which is highly remarkable, considering the distance of
the two areas, and their separation by the whole Atlantic Ocean. We
can further understand the singular fact remarked on by several
observers that the productions of Europe and America during the
later tertiary stages were more closely related to each other than
they are at the present time; for during these warmer periods the
northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as
soon as the species in common, which inhabited the New and Old Worlds,
migrated south of the Polar Circle, they will have been completely cut
off from each other. This separation, as far as the more temperate
productions are concerned, must have taken place long ages ago. As the
plants and animals migrated southwards, they will have become
mingled in the one great region with the native American
productions, and would have had to compete with them; and in the
other great region, with those of the Old World. Consequently we
have here everything favourable for much modification,- for far more
modification than with the Alpine productions, left isolated, within a
much more recent period, on the several mountain-ranges and on the
arctic lands of Europe and N. America. Hence it has come, that when we
compare the now living productions of the temperate regions of the New
and Old Worlds, we find very few identical species (though Asa Gray
has lately shown that more plants are identical than was formerly
supposed), but we find in every great class many forms, which some
naturalists rank as geographical races, and others as distinct
species; and a host of closely allied or representative forms which
are ranked by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern
migration of a marine fauna, which, during the Pliocene or even a
somewhat earlier period, was nearly uniform along the continuous
shores of the Polar Circle, will account, on the theory of
modification, for many closely allied forms now living in marine areas
completely sundered. Thus, I think, we can understand the presence
of some closely allied, still existing and extinct tertiary forms,
on the eastern and western shores of temperate North America; and
the still more striking fact of many closely allied crustaceans (as
described in Dana's admirable work), some fish and other marine
animals, inhabiting the Mediterranean and the seas of Japan,- these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.
These cases of close relationship in species either now or
formerly inhabiting the seas on the eastern and western shores of
North America, the Mediterranean and Japan, and the temperate lands of
North America and Europe, are inexplicable on the theory of
creation. We cannot maintain that such species have been created
alike, in correspondence with the nearly similar physical conditions
of the areas; for if we compare, for instance, certain parts of
South America with parts of South Africa or Australia, we see
countries closely similar in all their physical conditions, with their
inhabitants utterly dissimilar.
Alternate Glacial Periods in the North and South
But we must return to our more immediate subject. I am convinced
that Forbes's view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of
Britain to the Oural range, and southward to the Pyrenees. We may
infer from the frozen mammals and nature of the mountain vegetation,
that Siberia was similarly affected. In the Lebanon, according to
Dr. Hooker, perpetual snow formerly covered the central axis, and
fed glaciers which rolled 400 feet down the valleys. The same observer
has recently found great moraines at a low level on the Atlas range in
N. Africa. Along the Himalaya, at points 900 miles apart, glaciers
have left the marks of their former low descent; and in Sikkim, Dr.
Hooker saw maize growing on ancient and gigantic moraines. Southward
of the Asiatic continent, on the opposite side of the equator, we
know, from the excellent researches of Dr. J. Haast and Dr. Hector,
that in New Zealand immense glaciers formerly descended to a low
level; and the same plants found by Dr. Hooker on widely separated
mountains in this island tell the same story of a former cold
period. From facts communicated to me by the Rev. W. B. Clarke, it
appears also that there are traces of former glacial action on the
mountains of the south-eastern corner of Australia.
Looking to America; in the northern half, ice-borne fragments of
rock have been observed on the eastern side of the continent, as far
south as lat. 36-37, and on the shores of the Pacific, where the
climate is now so different, as far south as lat. 46. Erratic boulders
have, also, been noticed on the Rocky Mountains. In the Cordillera
of South America, nearly under the equator, glaciers once extended far
below their present level. In Central Chile I examined a vast mound of
detritus with great boulders, crossing the Portillo valley, which
there can hardly be a doubt once formed a huge moraine; and Mr. D.
Forbes informs me that he found in various parts of the Cordillera,
from lat. 13 deg. to 30 deg. S., at about the height of 19,000 feet,
deeply furrowed rocks, resembling those with which he was familiar
in Norway, and likewise great masses of detritus, including grooved
pebbles. Along this whole space of the Cordillera true glaciers do not
exist even at much more considerable heights. Farther south on both
sides of the continent, from lat. 41 deg. to the southernmost
extremity, we have the clearest evidence of former glacial action,
in numerous immense boulders transported far from their parent source.
From these several facts, namely from the glacial action having
extended all round the northern and southern hemispheres- from the
period having been in a geological sense recent in both hemispheres-
from its having lasted in both during a great length of time, as may
be inferred from the amount of work affected- and lastly from glaciers
having recently descended to a low level along the whole line of the
Cordillera, it at one time appeared to me that we could not avoid
the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now Mr. Croll,
in a series of admirable memoirs, has attempted to show that a glacial
condition of climate is the result of various physical causes, brought
into operation by an increase in the eccentricity of the earth's
orbit. All these causes tend towards the same end; but the most
powerful appears to be the indirect influence of the eccentricity of
the orbit upon oceanic currents. According to Mr. Croll, cold
periods regularly occur every ten or fifteen thousand years; and these
at long intervals are extremely severe, owing to certain
contingencies, of which the most important, as Sir C. Lyell has shown,
is the relative position of the land and water. Mr. Croll believes
that the last great Glacial period occurred about 240,000 years ago,
and endured with slight alterations of climate for about 160,000
years. With respect to more ancient Glacial periods, several
geologists are convinced from direct evidence that such occurred
during the Miocene and Eocene formations, not to mention still more
ancient formations. But the most important result for us, arrived at
by Mr. Croll, is that whenever the northern hemisphere passes
through a cold period the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder, chiefly
through changes in the direction of the ocean currents. So
conversely it will be with the northern hemisphere, whilst the
southern passes through a Glacial period. This conclusion throws so
much light on geographical distribution that I am strongly inclined to
trust in it; but I will first give the facts, which demand an
explanation.
In South America, Dr. Hooker has shown that besides many closely
allied species, between forty and fifty of the flowering plants of
Tierra del Fuego, forming no inconsiderable part of its scanty
flora, are common to North America and Europe, enormously remote as
these areas in opposite hemispheres are from each other. On the
lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the Organ mountains of
Brazil, some few temperate European, some Antarctic, and some Andean
genera were found by Gardner, which do not exist in the low
intervening hot countries. On the Silla of Caraccas, the illustrious
Humboldt long ago found species belonging to genera characteristic
of the Cordillera.
In Africa, several forms characteristic of Europe and some few
representatives of the flora of the Cape of Good Hope occur in the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the
mountains several representative European forms are found, which
have not been discovered in the intertropical parts of Africa. Dr.
Hooker has also lately shown that several of the plants living on
the upper parts of the lofty island of Fernando Po and on the
neighbouring Cameroon mountains, in the Gulf of Guinea, are closely
related to those on the mountains of Abyssinia, and likewise to
those of temperate Europe. It now also appears, as I hear from Dr.
Hooker, that some of these same temperate plants have been
discovered by the Rev. R. T. Lowe on the mountains of the Cape Verde
Islands. This extension of the same temperate forms, almost under
the equator, across the whole continent of Africa and to the mountains
of the Cape Verde Archipelago, is one of the most astonishing facts
ever recorded in the distribution of plants.
On the Himalaya, and on the isolated mountain-ranges of the
peninsula of India, on the heights of Ceylon, and on the volcanic
cones of Java, many plants occur, either identically the same or
representing each other, and at the same time representing plants of
Europe, not found in the intervening hot lowlands. A list of the
genera of plants collected on the loftier peaks of Java, raises a
picture of a collection made on a hillock in Europe! Still more
striking is the fact that peculiar Australian forms are represented by
certain plants growing on the summits of the mountains of Borneo. Some
of these Australian forms, as I hear from Dr. Hooker, extend along the
heights of the peninsula of Malacca, and are thinly scattered on the
one hand over India, and on the other hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man,
occur on the lowlands; and a long list can be given, as I am
informed by Dr. Hooker, of European genera, found in Australia, but
not in the intermediate torrid regions. In the admirable
Introduction to the Flora of New Zealand, by Dr. Hooker, analogous and
striking facts are given in regard to the plants of that large island.
Hence we see that certain plants growing on the more lofty mountains
of the tropics in all parts of the world, and on the temperate
plains of the north and south, are either the same species or
varieties of the same species. It should, however, be observed that
these plants are not strictly arctic forms; for, as Mr. H. C. Watson
has remarked, "in receding from polar towards equatorial latitudes,
the Alpine or mountain floras really become less and less Arctic."
Besides these identical and closely allied forms, many species
inhabiting the same widely sundered areas, belong to genera not now
found in the intermediate tropical lowlands.
These brief remarks apply to plants alone; but some few analogous
facts could be given in regard to terrestrial animals. In marine
productions, similar cases likewise occur; as an example, I may
quote a statement by the highest authority, Prof. Dana, that "It is
certainly a wonderful fact that New Zealand should have a closer
resemblance in its Crustacea to Great Britain, its antipode, than to
any other part of the world." Sir J. Richardson, also, speaks of the
reappearance on the shores of New Zealand, Tasmania, &c., of
northern forms of fish. Dr. Hooker informs me that twenty-five species
of Algae are common to New Zealand and to Europe, but have not been
found in the intermediate tropical seas.
From the foregoing facts, namely, the presence of temperate forms on
the highlands across the whole of equatorial Africa, and along the
Peninsula of India, to Ceylon and the Malay Archipelago, and in a less
well-marked manner across the wide expanse of tropical South
America, it appears almost certain that at some former period, no
doubt during the most severe part of a Glacial period, the lowlands of
these great continents were everywhere tenanted under the equator by
considerable number of temperate forms. At this period the
equatorial climate at the level of the sea was probably about the same
with that now experienced at the height of from five to six thousand
feet under the same latitude, or perhaps even rather cooler. During
this, the coldest period, the lowlands under the equator must have
been clothed with a mingled tropical and temperate vegetation, like
that described by Hooker as growing luxuriantly at the height of
from four to five thousand feet on the lower slopes of the Himalaya,
but with perhaps a still greater preponderance of temperate forms.
So again in the mountainous island of Fernando Po, in the Gulf of
Guinea, Mr. Mann found temperate European forms beginning to appear at
the height of about five thousand feet. On the mountains of Panama, at
the height of only two thousand feet, Dr. Seemann found the vegetation
like that of Mexico, "with forms of the torrid zone harmoniously
blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period,
the southern hemisphere was actually warmer, throws any clear light on
the present apparently inexplicable distribution of various
organisms in the temperate parts of both hemispheres, and on the
mountains of the tropics. The Glacial period, as measured by years,
must have been very long; and when we remember over what vast spaces
some naturalised plants and animals have spread within a few
centuries, this period will have been ample for any amount of
migration. As the cold became more and more intense, we know that
arctic forms invaded the temperate regions; and, from the facts just
given, there can hardly be a doubt that some of the more vigorous,
dominant, and widest-spreading temperate forms invaded the
equatorial lowlands. The inhabitants of these hot lowlands would at
the same time have migrated to the tropical and subtropical regions of
the south, for the southern hemisphere was at this period warmer. On
the decline of the Glacial period, as both hemispheres gradually
recovered their former temperatures, the northern temperate forms
living on the lowlands under the equator, would have been driven to
their former homes or have been destroyed, being replaced by the
equatorial forms returning from the south. Some, however, of the
northern temperate forms would almost certainly have ascended any
adjoining high land, where, if sufficiently lofty, they would have
long survived like the arctic forms on the mountains of Europe. They
might have survived, even if the climate was not perfectly fitted
for them, for the change of temperature must have been very slow,
and plants undoubtedly possess a certain capacity for acclimatisation,
as shown by their transmitting to their offspring different
constitutional powers of resisting heat and cold.
In the regular course of events the southern hemisphere would in
its turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms
would invade the equatorial lowlands. The northern forms which had
before been left on the mountains would now descend and mingle with
the southern forms. These latter, when the warmth returned, would
return to their former homes, leaving some few species on the
mountains, and carrying southward with them some of the northern
temperate forms which had descended from their mountain fastnesses.
Thus, we should have some few species identically the same in the
northern and southern temperate zones and on the mountains of the
intermediate tropical regions. But the species left during a long time
on these mountains, or in opposite hemispheres, would have to
compete with many new forms and would be exposed to somewhat different
physical conditions; hence they would be eminently liable to
modification, and would generally now exist as varieties or as
representative species; and this is the case. We must, also, bear in
mind the occurrence in both hemispheres of former Glacial periods; for
these will account, in accordance with the same principles, for the
many quite distinct species inhabiting the same widely separated
areas, and belonging to genera not now found in the intermediate
torrid zones.
It is a remarkable fact strongly insisted on by Hooker in regard
to America, and by Alph. de Candolle in regard to Australia, that many
more identical or slightly modified species have migrated from the
north to the south, than in a reversed direction. We see, however, a
few southern forms on the mountains of Borneo and Abyssinia. I suspect
that this preponderant migration from the north to the south is due to
the greater extent of land in the north, and to the northern forms
having existed in their own homes in greater numbers, and having
consequently been advanced through natural selection and competition
to a higher stage of perfection, or dominating power, than the
southern forms. And thus, when the two sets became commingled in the
equatorial regions, during the alternations to the Glacial periods,
the northern forms were the more powerful and were able to hold
their places on the mountains, and afterwards to migrate southward
with the southern forms; but not so the southern in regard to the
northern forms. In the same manner at the present day, we see that
very many European productions cover the ground in La Plata, New
Zealand, and to a lesser degree in Australia, and have beaten the
natives; whereas extremely few southern forms have become
naturalised in any part of the northern hemisphere, though hides,
wool, and other objects likely to carry seeds have been largely
imported into Europe during the last two or three centuries from La
Plata and during the last forty or fifty years from Australia. The
Neilgherrie mountains in India, however, offer a partial exception;
for here, as I hear from Dr. Hooker, Australian forms are rapidly
sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere yielded to
the more dominant forms generated in the larger areas and more
efficient workshops of the north. In many islands the native
productions are nearly equalled, or even outnumbered, by those which
have become naturalised; and this is the first stage towards their
extinction. Mountains are islands on the land, and their inhabitants
have yielded to those produced within the larger areas of the north,
just in the same way as the inhabitants of real islands have
everywhere yielded and are still yielding to continental forms
naturalised through man's agency.
The same principles apply to the distribution of terrestrial animals
and of marine productions, in the northern and southern temperate
zones, and on the intertropical mountains. When, during the height
of the Glacial period, the ocean-currents were widely different to
what they now are, some of the inhabitants of the temperate seas might
have reached the equator; of these a few would perhaps at once be able
to migrate southward, by keeping to the cooler currents, whilst
others might remain and survive in the colder depths until the
southern hemisphere was in its turn subjected to a glacial climate and
permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by arctic productions
exist to the present day in the deeper parts of the northern temperate
seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes
on the intermediate mountain-ranges, are removed on the views above
given. The exact lines of migration cannot be indicated. We cannot say
why certain species and not others have migrated; why certain
species have been modified and have given rise to new forms, whilst
others have remained unaltered. We cannot hope to explain such
facts, until we can say why one species and not another becomes
naturalised by man's agency in a foreign land; why one species
ranges twice or thrice as far, and is twice or thrice as common, as
another species within their own homes.
Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points
so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remarkable case.
Some of these species are so distinct, that we cannot suppose that
there has been time since the commencement of the last Glacial
period for their migration and subsequent modification to the
necessary degree. The facts seem to indicate that distinct species
belonging to the same genera have migrated in radiating lines from a
common centre; and I am inclined to look in the southern, as in the
northern hemisphere, to a former and warmer period, before the
commencement of the last Glacial period, when the Antarctic lands, now
covered with ice, supported a highly peculiar and isolated flora. It
may be suspected that before this flora was exterminated during the
last Glacial epoch, a few forms had been already widely dispersed to
various points of the southern hemisphere by occasional means of
transport, and by the aid as halting-places, of now sunken islands.
Thus the southern shores of America, Australia, and New Zealand may
have become slightly tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language
almost identical with mine, on the effects of great alterations of
climate throughout the world on geographical distribution. And we have
now seen that Mr. Croll's conclusion that successive Glacial periods
in the one hemisphere coincide with warmer periods in the opposite
hemisphere, together with the admission of the slow modification of
species, explains a multitude of facts in the distribution of the same
and of the allied forms of life in all parts of the globe. The
living waters have flowed during one period from the north and
during another from the south, and in both cases have reached the
equator; but the stream of life has flowed with greater force from the
north than in the opposite direction, and has consequently more freely
inundated the south. As the tide leaves its drift in horizontal lines,
rising higher on the shores where the tide rises highest, so have
the living waters left their living drift on our mountain summits,
in a line gently rising from the arctic lowlands to a great altitude
under the equator. The various beings thus left stranded may be
compared with savage races of man, driven up and surviving in the
mountain fastnesses of almost every land, which serves as a record,
full of interest to us, of the former inhabitants of the surrounding
lowlands.
CHAPTER XIII
GEOGRAPHICAL DISTRIBUTION Continued
Fresh-water Productions
AS LAKES and river-systems are separated from each other by barriers
of land, it might have been thought that fresh-water productions would
not have ranged widely within the same country, and as the sea is
apparently a still more formidable barrier, that they would never have
extended to distant countries. But the case is exactly the reverse.
Not only have many fresh-water species, belonging to different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. When first collecting in the fresh
waters of Brazil, I well remember feeling much surprise at the
similarity of the fresh-water insects, shells &c., and at the
dissimilarity of the surrounding terrestrial beings, compared with
those of Britain.
But the wide ranging power of fresh-water productions can, I
think, in most cases be explained by their having become fitted, in
a manner highly useful to them, for short and frequent migrations from
pond to pond, or from stream to stream, within their own countries;
and liability to wide dispersal would follow from this capacity as
an almost necessary consequence. We can here consider only a few
cases; of these, some of the most difficult to explain are presented
by fish. It was formerly believed that the same fresh-water species
never existed on two continents distant from each other. But Dr.
Gunther has lately shown that the Galaxias attenuatus inhabits
Tasmania, New Zealand, the Falkland Islands, and the mainland of South
America. This is a wonderful case, and probably indicates dispersal
from an Antarctic centre during a former warm period. This case,
however, is rendered in some degree less surprising by the species
of this genus having the power of crossing by some unknown means
considerable spaces of open ocean: thus there is one species common to
New Zealand and to the Auckland Islands, though separated by a
distance of about 230 miles. On the same continent fresh-water fish
often range widely, and as if capriciously; for in two adjoining
river-systems some of the species may be the same, and some wholly
different.
It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the
ova retain their vitality for a considerable time after removal from
the water. Their dispersal may, however, be mainly attributed to
changes in the level of the land within the recent period, causing
rivers to flow into each other. Instances, also, could be given of
this having occurred during floods, without any change of level. The
wide difference of the fish on the opposite sides of most
mountain-ranges, which are continuous, and which consequently must
from an early period have completely prevented the inosculation of the
river-systems on the two sides, leads to the same conclusion. Some
fresh-water fish belong to very ancient forms, and in such cases there
will have been ample time for great geographical changes, and
consequently time and means for much migration. Moreover, Dr.
Gunther has recently been led by several considerations to infer
that with fishes the same forms have a long endurance. Salt-water fish
can with care be slowly accustomed to live in fresh water; and,
according to Valenciennes' there is hardly a single group of which
an the members are confined to fresh water, so that a marine species
belonging to a fresh-water group might travel far along the shores
of the sea, and could, it is probable, become adapted without much
difficulty to the fresh waters of a distant land.
Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent,
and must have proceeded from a single source, prevail throughout the
world. Their distribution at first perplexed me much, as their ova are
not likely to be transported by birds; and the ova, as well as the
adults, are immediately killed by sea-water. I could not even
understand how some naturalised species have spread rapidly throughout
the same country. But two facts, which I have observed- and many
others no doubt will be discovered- throw some light on this
subject. When ducks suddenly emerge from a pond covered with
duck-weed, I have twice seen these little plants adhering to their
backs; and it has happened to me, in removing a little duck-weed
from one aquarium to another, that I have unintentionally stocked
the one with fresh-water shells from the other. But another agency
is perhaps more effectual: I suspended the feet of a duck in an
aquarium, where many ova of fresh-water shells were hatching; and I
found that numbers of the extremely minute and just-hatched shells
crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just-hatched
molluscs, though aquatic in their nature, survived on the duck's feet,
in damp air, from twelve to twenty-hours; and in this length of time a
duck or heron might fly at least six or seven hundred miles, and if
blown across the sea to an oceanic island, or to any other distant
point, would be sure to alight on a pool or rivulet. Sir Charles Lyell
informs me that a Dytiscus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a
water-beetle of the same family, a Colymbetes, once flew on board
the "Beagle," when forty-five miles distant from the nearest land: how
much farther it might have been blown by a favouring gale no one can
tell.
With respect to plants, it has long been known what enormous
ranges many fresh-water, and even marsh species, have, both over
continents and to the most remote oceanic islands. This is
strikingly illustrated, according to Alph. de Candolle, in those large
groups of terrestrial plants, which have very few aquatic members; for
the latter seem immediately to acquire, as if in consequence, a wide
range. I think favourable means of dispersal explain this fact. I
have before mentioned that earth occasionally adheres in some quantity
to the feet and beaks of birds. Wading birds, which frequent the muddy
edges of ponds, if suddenly flushed, would be the most likely to
have muddy feet. Birds of this order wander more than those of any
other; and they are occasionally found on the most remote and barren
islands of the open ocean; they would not be likely to alight on the
surface of the sea, so that any dirt on their feet would not be washed
off; and when gaining the land, they would be sure to fly to their
natural fresh-water haunts. I do not believe that botanists are
aware how charged the mud of ponds is with seeds; I have tried
several little experiments, but will here give only the most
striking case: I took in February three tablespoonfuls of mud from
three different points, beneath water, on the edge of a little pond:
this mud when dried weighed only 63/4 ounces; I kept it covered up
in my study for six months, pulling up and counting each plant as it
grew; the plants were of many kinds, and were altogether 537 in
number; and yet the viscid mud was all contained in a breakfast cup!
Considering these facts, I think it would be an inexplicable
circumstance if water-birds did not transport the seeds of fresh-water
plants to unstocked ponds and streams, situated at very distant
points. The same agency may have come into play with the eggs of some
of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I
have stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have
gone on daily devouring fish; they then take flight and go to other
waters, or are blown across the sea; and we have seen that seeds
retain their power of germination, when rejected many hours afterwards
in pellets or in the excrement. When I saw the great size of the seeds
of that fine water-lily, the Nelumbium, and remembered Alph. de
Candolle's remarks on the distribution of this plant, I thought that
the means of its dispersal must remain inexplicable; but Audubon
states that he found the seeds of the great southern water-lily
(probably, according to Dr. Hooker, the Nelumbium luteum) in a heron's
stomach. Now this bird must often have flown with its stomach thus
well stocked to distant ponds, and then getting a hearty meal of fish,
analogy makes me believe that it would have rejected the seeds in a
pellet in a fit state for germination.
In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance,
on a rising islet, it will be unoccupied; and a single seed or egg
will have a good chance of succeeding. Although there will always be a
struggle for life between the inhabitants of the same pond, however
few in kind, yet as the number even in a well-stocked pond is small in
comparison with the number of species inhabiting an equal area of
land, the competition between them will probably be less severe than
between terrestrial species; consequently an intruder from the
waters of a foreign country would have a better chance of seizing on
new place, than in the case of terrestrial colonists. We should also
remember that many fresh-water productions are low in the scale of
nature, and we have reason to believe that such beings become modified
more slowly than the high; and this will give time for the migration
of aquatic species. We should not forget the probability of many
fresh-water forms laving formerly ranged continuously over immense
areas, and then having become extinct at intermediate points. But
the wide distribution of fresh-water plants and of the lower
animals, whether retaining the same identical form or in some degree
modified, apparently depends in main part on the wide dispersal of
their seeds and eggs by animals, more especially by fresh-water birds,
which have great powers of flight, and naturally travel from one piece
of water to another.
On the Inhabitants of Oceanic Islands
We now come to the last of the three classes of facts, which I
have selected as presenting the greatest amount of difficulty with
respect to distribution, on the view that not only all the individuals
of the same species have migrated from some one area, but that
allied species, although now inhabiting the most distant points,
have proceeded from a single area,- the birthplace of their early
progenitors. I have already given my reasons for disbelieving in
continental extensions within the period of existing species, on so
enormous a scale that all the many islands of the several oceans
were thus stocked with their present terrestrial inhabitants. This
view removes many difficulties, but it does not accord with all the
facts in regard to the productions of islands. In the following
remarks I shall not confine myself to the mere question of
dispersal, but shall consider some other cases bearing on the truth of
the two theories of independent creation and of descent with
modification.
The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. New
Zealand, for instance, with its lofty mountains and diversified
stations, extending over 780 miles of latitude, together with the
outlying islands of Auckland, Campbell and Chatham, contain altogether
only 960 kinds of flowering plants; if we compare this moderate number
with the species which swarm over equal areas in South-Western
Australia or at the Cape of Good Hope, we must admit that some
cause, independently of different physical conditions, has given
rise to so great a difference in number. Even the uniform county of
Cambridge has 847 plants, and the little island of Anglesea 764, but a
few ferns and a few introduced plants are included in these numbers,
and the comparison in some other respects is not quite fair. We have
evidence that the barren island of Ascension aboriginally possessed
less than half-a-dozen flowering plants; yet many species have now
become naturalised on it, as they have in New Zealand and on every
other oceanic island which can be named. In St. Helena there is reason
to believe that the naturalised plants and animals have nearly or
quite exterminated many native productions. He who admits the
doctrine of the creation of each separate species, will have to
admit that a sufficient number of the best adapted plants and
animals were not created for oceanic islands; for man has
unintentionally stocked them far more fully and perfectly than did
nature.
Although in oceanic islands the species are few in number, the
proportion of endemic kinds (i.e., those found nowhere else in the
world) is often extremely large. If we compare, for instance, the
number of endemic landshells in Madeira, or of endemic birds in the
Galapagos Archipelago, with the number found on any continent, and
then compare the area of the island with that of the continent, we
shall see that this is true. This fact might have been theoretically
expected, for, as already explained, species occasionally arriving
after long intervals of time in the new and isolated district, and
having to compete with new associates, would be eminently liable to
modification, and would often produce groups of modified
descendants. But it by no means follows that, because in an island
nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend partly on the species which are not
modified having immigrated in a body, so that their mutual relations
have not been much disturbed; and partly on the frequent arrival of
unmodified immigrants from the mother-country, with which the
insular forms have intercrossed. It should be borne in mind that the
offspring of such crosses would certainly gain in vigour, so that even
an occasional cross would produce more effect than might have been
anticipated. I will give a few illustrations of the foregoing remarks:
in the Galapagos Islands there are 9.6 land-birds; of these 21 (or
perhaps 93) are peculiar, whereas of the 11 marine birds only 2 are
peculiar; and it is obvious that marine birds could arrive at these
islands much more easily and frequently than land-birds. Bermuda, on
the other hand, which lies at about the same distance from North
America as the Galapagos Islands do from South America, and which
has a very peculiar soil, does not possess a single endemic
landbird, and we know from Mr. J. M. Jones's admirable account of
Bermuda, that very many North American birds occasionally or even
frequently visit this, island. Almost every year, as I am informed
by Mr. E. V. Harcourt, many European and African birds are blown to
Madeira; this island is inhabited by 99 kinds of which one alone is
peculiar, though very closely related to a European form; and three or
four other species are confined to this island and to the Canaries. So
that the islands of Bermuda and Madeira have been stocked from the
neighbouring continents with birds, which for long ages have there
struggled together, and have become mutually co-adapted. Hence when
settled in their new homes, each kind will have been kept by the
others to its proper place and habits, and will consequently have
been but little liable to modification. Any tendency to modification
will also have been checked by intercrossing with the unmodified
immigrants, often arriving from the mother-country. Madeira again is
inhabited by a wonderful number of peculiar land-shells, whereas not
one species of sea-shell is peculiar to its shores: now, though we
do not know how seashells are dispersed, yet we can see that their
eggs or larvae, perhaps attached to seaweed or floating timber, or
to the feet of wading-birds, might be transported across three or four
hundred miles of open sea far more easily than land-shells. The
different orders of insects inhabiting Madeira present nearly
parallel cases.
Oceanic islands are sometimes deficient in animals of certain
whole classes, and their places are occupied by other classes; thus in
the Galapagos Islands reptiles, and in New Zealand gigantic wingless
birds, take, or recently took, the place of mammals. Although New
Zealand is here spoken of as an oceanic island, it is in some degree
doubtful whether it should be so ranked; it is of large size, and is
not separated from Australia by a profoundly deep sea; from its
geological character and the direction of its mountain-ranges, the
Rev. W. B. Clarke has lately maintained that this island, as well as
New Caledonia, should be considered as appurtenances of Australia.
Turning to plants, Dr. Hooker has shown that in the Galapagos
Islands the proportional numbers of the different orders are very
different from what they are elsewhere. All such differences in
number, and the absence of certain whole groups of animals and
plants, are generally accounted for by supposed differences in the
physical conditions of the islands; but this explanation is not a
little doubtful. Facility of immigration seems to have been fully as
important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands not
tenanted by a single mammal, some of the endemic plants have
beautifully hooked seeds; yet few relations are more manifest than
that hooks serve for the transportal of seeds in the wool or fur of
quadrupeds. But a hooked seed might be carried to an island by other
means; and the plant then becoming modified would form an endemic
species, still retaining its hooks, which would form a useless
appendage like the shrivelled wings under the soldered wing-covers
of many insular beetles. Again, islands often possess trees or
bushes belonging to orders which elsewhere include only herbaceous
species; now trees, as Alph. de Candolle has shown, generally have,
whatever the cause may be, confined ranges. Hence trees would be
little likely to reach distant oceanic islands; and an herbaceous
plant, which had no chance of successfully competing with the many
fully developed trees growing on a continent, might, when
established on an island, gain an advantage over other herbaceous
plants by growing taller and taller and overtopping them. In this
case, natural selection would tend to add to the stature of the plant,
to whatever order it belonged, and thus first convert it into a bush
and then into a tree.
Absence of Batrachians and Terrestrial Mammals on Oceanic Islands
With respect to the absence of whole orders of animals on oceanic
islands, Bory St. Vincent long ago remarked that batrachians (frogs,
toads, newts) are never found on any of the many islands with which
the great oceans are studded. I have taken pains to verify this
assertion, and have found it true, with the exception of New
Zealand, New Caledonia, the Andaman Islands, and perhaps the Solomon
Islands and the Seychelles. But I have already remarked that it is
doubtful whether New Zealand and New Caledonia ought to be classed
as oceanic islands; and this is still more doubtful with respect to
the Andaman and Solomon groups and the Seychelles. This general
absence of frogs, toads, and newts on so many true oceanic islands
cannot be accounted for by their physical conditions: indeed it
seems that islands are peculiarly fitted for these animals; for
frogs have been introduced into Madeira, the Azores, and Mauritius,
and have multiplied so as to become a nuisance. But as these animals
and their spawn are immediately killed (with the exception, as far
as known, of one Indian species) by sea-water, there would be great
difficulty in their transportal across the sea, and therefore we can
see why they do not exist on strictly oceanic islands. But why, on the
theory of creation, they should not have been created there, it
would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched
the oldest voyages, and have not found a single instance, free from
doubt, of a terrestrial mammal (excluding domesticated animals kept by
the natives) inhabiting an island situated above 300 miles from a
continent or great continental island; and many islands situated at
a much less distance are equally barren. The Falkland Islands, which
are inhabited by a wolf-like fox, come nearest to an exception; but
this group cannot be considered as oceanic, as it lies on a bank in
connection with the mainland at the distance of about 280 miles;
moreover, icebergs formerly brought boulders to its western shores,
and they may have formerly transported foxes, as now frequently
happens in the arctic regions. Yet it cannot be said that small
islands will not support at least small mammals, for they occur in
many parts of the world on very small islands, when lying close to a
continent; and hardly an island can be named on which our smaller
quadrupeds have not become naturalised and greatly multiplied. It
cannot be said, on the ordinary view of creation, that there has not
been time for the creation of mammals; many volcanic islands are
sufficiently ancient, as shown by the stupendous degradation which
they have suffered, and by their tertiary strata: there has also
been time for the production of endemic species belonging to other
classes; and on continents it is known that new species of mammals
appear and disappear at a quicker rate than other and lower animals.
Although terrestrial mammals do not occur on oceanic islands, aerial
mammals do occur on almost every island. New Zealand possesses two
bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne
Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it
may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily
be answered; for no terrestrial mammal can be transported across a
wide space of sea, but bats can fly across. Bats have been seen
wandering by day far over the Atlantic Ocean; and two North American
species either regularly or occasionally visit Bermuda, at the
distance of 600 miles from the mainland. I hear from Mr. Tomes, who
has specially studied this family, that many species have enormous
ranges, and are found on continents and on far distant islands.
Hence we have only to suppose that such wandering species have been
modified in their new homes in relation to their new position, and
we can understand the presence of endemic bats on oceanic islands,
with the absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the depth of
the sea separating islands from each other or from the nearest
continent, and the degree of affinity of their mammalian
inhabitants. Mr. Windsor Earl has made some striking observations on
this head, since greatly extended by Mr. Wallace's admirable
researches, in regard to the great Malay Archipelago, which is
traversed near Celebes by a space of deep ocean, and this separates
two widely distinct mammalian faunas. On either side the islands stand
on a moderately shallow submarine bank, and these islands are
inhabited by the same or by closely allied quadrupeds. I have not as
yet had time to follow up this subject in all quarters of the world;
but as far as I have gone, the relation holds good. For instance,
Britain is separated by a shallow channel from Europe, and the mammals
are the same on both sides; and so it is with all the islands near the
shores of Australia. The West Indian Islands, on the other hand, stand
on a deeply submerged bank, nearly 1000 fathoms in depth, and here
we find American forms, but the species and even the genera are
quite distinct. As the amount of modification which animals of all
kinds undergo partly depends on the lapse of time, and as the
islands which are separated from each other or from the mainland by
shallow channels, are more likely to have been continuously united
within a recent period than the islands separated by deeper
channels, we can understand how it is that a relation exists between
the depth of the sea separating two mammalian faunas, and the degree
of their affinity,- a relation which is quite inexplicable on the
theory of independent acts of creation.
The foregoing statements in regard to the inhabitants of oceanic
islands,- namely, the fewness of the species, with a large
proportion consisting of endemic forms,- the members of certain
groups, but not those of other groups in the same class, having been
modified,- the absence of certain whole orders, as of batrachians
and of terrestrial mammals, notwithstanding the presence of aerial
bats,- the singular proportions of certain orders of plants,-
herbaceous forms having been developed into trees, &c.,- seem to me to
accord better with the belief in the efficiency of occasional means of
transport, carried on during a long course of time, than with the
belief in the former connection of all oceanic islands with the
nearest continent; for on this latter view it is probable that the
various classes would have immigrated more uniformly, and from the
species having entered in a body their mutual relations would not have
been much disturbed, and consequently they would either have not
been modified, or all the species in a more equable manner.
I do not deny that there are many and serious difficulties in
understanding how many Of the inhabitants of the inhabitants of the
more remote islands, whether still retaining the same specific form or
subsequently modified, have reached their present homes. But the
probability of other islands having once existed as halting-places, of
which not a wreck now remains, must not be overlooked. I will
specify one difficult case. Almost all oceanic islands, even the
most isolated and smallest, are inhabited by landshells, generally
by endemic species, but sometimes by species found elsewhere,-
striking instances of which have been given by Dr. A. A. Gould in
relation to the Pacific. Now it is notorious that land-shells are
easily killed by sea-water; their eggs, at least such as I have tried,
sink in it and are killed. Yet there must be some unknown, but
occasionally efficient means for their transportal. Would the
just-hatched young sometimes adhere to the feet of birds roosting on
the ground, and thus get transported? It occurred to me that
landshells, when hibernating and having a membranous diaphragm over
the mouth of the shell, might be floated in chinks of drifted timber
across moderately wide arms of the sea. And I find that several
species in this state withstand uninjured an immersion in sea-water
during seven days: one shell, the Helix pomatia, after having been
thus treated and again hibernating was put into sea-water for twenty
days, and perfectly recovered. During this length of time the shell
might have been carried by a marine current of average swiftness, to a
distance of 660 geographical miles. As this Helix has a thick
calcareous operculum, I removed it, and when it had formed a new
membranous one, I again immersed it for fourteen days in sea-water,
and again it recovered and crawled away. Baron Aucapitaine has since
tried similar experiments: he placed 100 landshells, belonging to
ten species, in a box pierced with holes, and immersed it for a
fortnight in the sea. Out of the hundred shells, twenty-seven
recovered. The presence of an operculum seems to have been of
importance, as out of twelve specimens of Cyclostoma elegans, which is
thus furnished, eleven revived. It is remarkable, seeing how well
the Helix pomatia resisted with me the salt-water, that not one of
fifty-four specimens belonging to four other species of Helix tried by
Aucapitaine, recovered. It is, however, not at all probable that
land-shells have often been thus transported; the feet of birds
offer a more probable method.
On the Relations of the Inhabitants of Islands to those of the
nearest Mainland
The most striking and important fact for us is the affinity of the
species which inhabit islands to those of the nearest mainland,
without being actually the same. Numerous instances could be given.
The Galapagos Archipelago, situated under the equator, lies at the
distance of between 500 and 600 miles from the shores of South
America. Here almost every product of the land and of the water
bears the unmistakable stamp of the American continent. There are
twenty-six land-birds; of these, twenty-one, or perhaps twenty-three
are ranked as distinct species, and would commonly be assumed to
have been here created; yet the close affinity of most of these
birds to American species is manifest in every character, in their
habits, gestures, and tones of voice. So it is with the other animals,
and with a large proportion of the plants, as shown by Dr. Hooker in
his admirable Flora of this archipelago. The naturalist, looking at
the inhabitants of these volcanic islands in the Pacific, distant
several hundred miles from the continent, feels that he is standing on
American land. Why should this be so? Why should the species which are
supposed to have been created in the Galapagos Archipelago, and
nowhere else, bear so plainly the stamp of affinity to those created
in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in
the proportions in which the several classes are associated
together, which closely resemble; the conditions of the South American
coast: in fact, there is a considerable dissimilarity in all these
respects. On the other hand, there is a considerable degree of
resemblance in the volcanic nature of the soil, in the climate,
height, and size of the islands, between the Galapagos and Cape
Verde Archipelagoes: but what an entire and absolute difference in
their inhabitants! The inhabitants of the Cape Verde Islands are
related to those of Africa, like those of the Galapagos to America.
Facts such as these admit of no sort of explanation on the ordinary
view of independent creation; whereas on the view here maintained,
it is obvious that the Galapagos Islands would be likely to receive
colonists from America, whether by occasional means of transport or
(though I do not believe in this doctrine) by formerly continuous
land, and the Cape Verde Islands from Africa; such colonists would
be liable to modification,- the principle of inheritance still
betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost
universal rule that the endemic productions of islands are related
to those of the nearest continent, or of the nearest large island. The
exceptions are few, and most of them can be explained. Thus although
Kerguelen Land stands nearer to Africa than to America, the plants are
related, and that very closely, as we know from Dr. Hooker's
account, to those of America: but on the view that this island has
been mainly stocked by seeds brought with earth and stones on
icebergs, drifted by the prevailing currents, this anomaly disappears.
New Zealand in its endemic planes is much more closely related to
Australia, the nearest mainland, than to any other region: and this is
what might have been expected; but it is also plainly related to
South America, which, although the next nearest continent, is so
enormously remote, that the fact becomes an anomaly. But this
difficulty partially disappears on the view that New Zealand, South
America, and the other southern lands have been stocked in part from a
nearly intermediate though distant point, namely from the antarctic
islands, when they were clothed with vegetation, during a warmer
tertiary period, before the commencement of the last Glacial period.
The affinity, which though feeble, I am assured by Dr. Hooker is real,
between the flora of the south-western corner of Australia and of
the Cape of Good Hope, is a far more remarkable case; but this
affinity is confined to the plants, and will, no doubt, some day be
explained.
The same law which has determined the relationship between the
inhabitants of islands and the nearest mainland, is sometimes
displayed on a small scale, but in a most interesting manner, within
the limits of the same archipelago. Thus each separate island of the
Galapagos Archipelago is tenanted, and the fact is a marvellous one,
by many different species; but these species are related to each other
in a very much closer manner than to the inhabitants of the American
continent, or of any other quarter of the world. This is what might
have been expected, for islands situated so near to each other would
almost necessarily receive immigrants from the same original source,
and from each other. But how is it that many of the immigrants have
been differently modified, though only in a small degree, in islands
situated within sight of each other, having the same geological
nature, the same height, climate, &c.? This long appeared to me a
great difficulty: but it arises in chief part from the deeply-seated
error of considering the physical conditions of a country as the
most important; whereas it cannot be disputed that the nature of the
other species with which each has to compete, is at least as
important, and generally a far more important element of success.
Now if we look to the species which inhabit the Galapagos Archipelago,
and are likewise found in other parts of the world, we find that
they differ considerably in the several islands. This difference might
indeed have been expected if the islands had been stocked by
occasional means of transport- a seed, for instance, of one plant
having been brought to one island, and that of another plant to
another island, though all proceeding from the same general source.
Hence, when in former times an immigrant first settled on one of the
islands, or when it subsequently spread from one to another, it
would undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set
of organisms; a plant, for instance, would find the ground best fitted
for it occupied by somewhat different species in the different
islands, and would be exposed to the attacks of somewhat different
enemies. If then it varied, natural selection would probably favour
different varieties in the different islands. Some species, however,
might spread and yet retain the same character throughout the group,
just as we see some species spreading widely throughout a continent
and remaining the same.
The really surprising fact in this case of the Galapagos
Archipelago, and in a lesser degree in some analogous cases, is that
each new species after being formed in any one island, did not
spread quickly to the other islands. But the islands, though in
sight of each other, are separated by deep arms of the sea, in most
cases wider than the British Channel, and there is no reason to
suppose that they have at any former period been continuously
united. The currents of the sea are rapid and sweep between the
islands, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they
appear on a map. Nevertheless some of the species, both of those found
in other parts of the world and of those confined to the
archipelago, are common to the several islands; and we may infer
from their present manner of distribution, that they have spread
from one island to the others. But we often take, I think, an
erroneous view of the probability of closely-allied species invading
each other's territory, when put into free intercommunication.
Undoubtedly, if one species has any advantage over another, it will in
a very brief time wholly or in part supplant it; but if both are
equally well fitted for their own places, both will probably hold
their separate places for almost any length of time. Being familiar
with the fact that many species, naturalised through man's agency,
have spread with astonishing rapidity over wide areas, we are apt to
infer that most species would thus spread; but we should remember that
the species which become naturalised in new countries are not
generally closely allied to the aboriginal inhabitants, but are very
distinct forms, belonging in a large proportion of cases, as shown
by Alph. de Candolle, to distinct genera. In the Galapagos
Archipelago, many even of the birds, though so well adapted for flying
from island to island, differ on the different islands; thus there are
three closely-allied species of mocking-thrush, each confined to its
own island. Now let us suppose the mocking-thrush of Chatham Island to
be blown to Charles Island, which has its own mocking-thrush, why
should it succeed in establishing itself there? We may safely infer
that Charles Island is well stocked with its own species, for
annually more eggs are laid and young birds hatched, than can possibly
be reared; and we may infer that the mocking-thrush peculiar to
Charles's Island is at least as well fitted for its home as is the
species peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston
have communicated to me a remarkable fact bearing on this subject;
namely, that Madeira and the adjoining islet of Porto Santo possess
many distinct but representative species of land-shells, some of which
live in crevices of stone; and although large quantities of stone
are annually transported from Porto Santo to Madeira, yet this
latter island has not become colonised by the Porto Santo species;
nevertheless both islands have been colonised by European land-shells,
which no doubt had some advantage over the indigenous species. From
these considerations I think we need not greatly marvel at the endemic
species which inhabit the several islands of the Galapagos
Archipelago, not having all spread from island to island. On the
same continent, also, preoccupation has probably played an important
part in checking the commingling of the species which inhabit
different districts with nearly the same physical conditions. Thus,
the south-east and south-west corners of Australia have nearly the
same physical conditions, and are united by continuous land, yet
they are inhabited by a vast number of distinct mammals, birds, and
plants; so it is, according to Mr. Bates, with the butterflies and
other animals inhabiting the great, open, and continuous valley of the
Amazons.
The same principle which governs the general character of the
inhabitants of oceanic islands, namely, the relation to the source
whence colonists could have been most easily derived, together with
their subsequent modification, is of the widest application throughout
nature. We see this on every mountain summit, in every lake and marsh.
For Alpine species, excepting in as far as the same species have
become widely spread during the Glacial epoch, are related to those of
the surrounding lowlands; thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, &c., all strictly
belonging to American forms; and it is obvious that a mountain, as
it became slowly unheaved, would be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has allowed the
same forms to prevail throughout large portions of the world. We see
this same principle in the character of most of the blind animals
inhabiting the caves of America and of Europe. Other analogous facts
could be given. It will, I believe, be found universally true, that
wherever in two regions, let them be ever so distant, many closely
allied or representative species occur, there will likewise be found
some identical species; and wherever many closely-allied species
occur, there will be found many forms which some naturalists rank as
distinct species, and others as mere varieties; these doubtful forms
showing us the steps in the progress of modification.
The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the
existence at remote points of the world of closely-allied species,
is shown in another and more general way. Mr. Gould remarked to me
long ago, that in those genera of birds which range over the world,
many of the species have very wide ranges. I can hardly doubt that
this rule is generally true, though difficult of proof. Amongst
mammals, we see it strikingly displayed in bats, and in a lesser
degree in the Felidae and Canidae. We see the same rule in the
distribution of butterflies and beetles. So it is with most of the
inhabitants of fresh water, for many of the genera in the most
distinct classes range over the world, and many of the species have
enormous ranges. It is not meant that all, but that some of the
species have very wide ranges in the genera which range very widely.
Nor is it meant that the species in such genera have on an average a
very wide range; for this will largely depend on how far the process
of modification has gone; for instance, two varieties of the same
species inhabit America and Europe, and thus the species has an
immense range; but, if variation were to be carried a little
further, the two varieties would be ranked as distinct species, and
their range would be greatly reduced. Still less is it meant, that
species which have the capacity of crossing barriers and ranging
widely, as in the case of certain powerfully-winged birds, will
necessarily range widely; for we should never forget that to range
widely implies not only the power of crossing barriers, but the more
important power of being victorious in distant lands in the struggle
for life with foreign associates. But according to the view that all
the species of a genus, though distributed to the most remote points
of the world, are descended from a single progenitor, we ought to
find, and I believe as a general rule we do find, that some at least
of the species range very widely.
We should bear in mind that many genera in all classes are of
ancient origin, and the species in this case will have had ample
time for dispersal and subsequent modification. There is also reason
to believe from geological evidence, that within each great class
the lower organisms change at a slower rate than the higher;
consequently they will have had a better chance of ranging widely
and of still retaining the same specific character. This fact,
together with that of the seeds and eggs of most lowly organised forms
being very minute and better fitted for distant transportal,
probably accounts for a law which has long been observed, and which
has lately been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the more widely
it ranges.
The relations just discussed,- namely, lower organisms ranging
more widely than the higher,- some of the species of widely-ranging
genera themselves ranging widely,- such facts, as Alpine,
lacustrine, and marsh productions being generally related to those
which live on the surrounding low lands and dry lands,- the striking
relationship between the inhabitants of islands and those of the
nearest mainland, the still closer relationship of the distinct
inhabitants of the islands in the same archipelago,- are
inexplicable on the ordinary view of the independent creation of
each species, but are explicable if we admit colonisation from the
nearest or readiest source, together with the subsequent adaptation of
the colonists to their new homes.
Summary of the last and present Chapters
In these chapters I have endeavoured to show, that if we make due
allowance for our ignorance of the full effects of changes of
climate and of the level of the land, which have certainly occurred
within the recent period, and of other changes which have probably
occurred,- if we remember how ignorant we are with respect to the many
curious means of occasional transport,- if we bear in mind, and this
is a very important consideration, how often a species may have ranged
continuously over a wide area, and then have become extinct in the
intermediate tracts,- the difficulty is not insuperable in believing
that all the individuals of the same species, wherever found, are
descended from common parents. And we are led to this conclusion,
which has been arrived at by many naturalists under the designation of
single centres of creation, by various general considerations, more
especially from the importance of barriers of all kinds, and from
the analogical distribution of subgenera, genera, and families.
With respect to distinct species belonging to the same genus,
which on our theory have spread from one parent-source; if we make the
same allowances as before for our ignorance, and remember that some
forms of life have changed very slowly, enormous periods of time
having been thus granted for their migration, the difficulties are far
from insuperable; though in this case, as in that of the individuals
of the same species, they are often great.
As exemplifying the effects of climatal changes on distribution, I
have attempted to show how important a part the last Glacial period
has played, which affected even the equatorial regions, and which,
during the alternations of the cold in the north and south, allowed
the productions of opposite hemispheres to mingle, and left some of
them stranded on the mountain-summits in all parts of the world. As
showing how diversified are the means of occasional transport, I
have discussed at some little length the means of dispersal of
fresh-water productions.
If the difficulties be not insuperable in admitting that in the long
course of time all the individuals of the same species, and likewise
of the several species belonging to the same genus, have proceeded
from some one source; then all the grand leading facts of
geographical distribution are explicable on the theory of migration,
together with subsequent modification and the multiplication of new
forms. We can thus understand the high importance of barriers, whether
of land or water, in not only separating, but in apparently forming
the several zoological and botanical provinces. We can thus understand
the concentration of related species within the same areas; and how it
is that under different latitudes, for instance in South America,
the inhabitants of the plains and mountains, of the forests,
marshes, and deserts, are linked together in so mysterious a manner,
and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual relation
of organism to organism is of the highest importance, we can see why
two areas having nearly the same physical conditions should often be
inhabited by very different forms of life; for according to the length
of time which has elapsed since the colonists entered one of the
regions, or both; according to the nature of the communication which
allowed certain forms and not others to enter, either in greater or
lesser numbers; according or not, as those which entered happened to
come into more or less direct competition with each other and with the
aborigines; and according as the immigrants were capable of varying
more or less rapidly, there would ensue in the two or more regions,
independently of their physical conditions, infinitely diversified
conditions of life,- there would be an almost endless amount of
organic action and reaction,- and we should find some groups of beings
greatly, and some only slightly modified,- some developed in great
force, some existing in scanty numbers- and this we do find in the
several great geographical provinces of the world.
On these same principles we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but that of
these, a large proportion should be endemic or peculiar; and why, in
relation to the means of migration, one group of beings should have
all its species peculiar, and another group, even within the same
class, should have all its species the same with those in an adjoining
quarter of the world. We can see why whole groups of organisms, as
batrachians and terrestrial mammals, should be absent from oceanic
islands, whilst the most isolated islands should possess their own
peculiar species of aerial mammals or bats. We can see why, in
islands, there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the
sea between such islands and the mainland. We can clearly see why
all the inhabitants of an archipelago, though specifically distinct on
the several islets, should be closely related to each other; and
should likewise be related, but less closely, to those of the
nearest continent, or other source whence immigrants might have been
derived. We can see why, if there exists very closely allied or
representative species in two areas, however distant from each
other, some identical species will almost always there be found.
As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space; the laws
governing the succession of forms in past times being nearly the
same with those governing at the present time the differences in
different areas. We see this in many facts. The endurance of each
species and group of species is continuous in time; for the apparent
exceptions to the rule are so few, that they may fairly be
attributed to our not having as yet discovered in an intermediate
deposit certain forms which are absent in it, but which occur both
above and below: so in space, it certainly is the general rule that
the area inhabited by a single species, or by a group of species, is
continuous, and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by former migrations under
different circumstances, or through occasional means of transport,
or by the species having become extinct in the intermediate tracts.
Both in time and space species and groups of species have their points
of maximum development. Groups of species, living during the same
period of time, or living within the same area, are often
characterised by trifling features in common, as of sculpture or
colour. In looking to the long succession of past ages, as in
looking to distant provinces throughout the world, we find that
species in certain classes differ little from each other, whilst
those in another class, or only in a different section of the same
order, differ greatly from each other. In both time and space the
lowly organised members of each class generally change less than the
highly organised; but there are in both cases marked exceptions to the
rule. According to our theory, these several relations throughout
time and space are intelligible; for whether we look to the allied
forms of life which have changed during successive ages, or to those
which have changed after having migrated into distant quarters, in
both cases they are connected by the same bond of ordinary generation;
in both cases the laws of variation have been the same, and
modifications have been accumulated by the same means of natural
selection.
CHAPTER XIV
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS
Classification
FROM the most remote period in the history of the world organic
beings have been found to resemble each other in descending degrees,
so that they can be classed in groups under groups. This
classification is not arbitrary like the grouping of the stars in
constellations. The existence of groups would have been of simpler
significance, if one group had been exclusively fitted to inhabit
the land and another the water; one to feed on flesh, another on
vegetable matter, and so on; but the case is widely different, for
it is notorious how commonly members of even the same subgroup have
different habits. In the second and fourth chapters, on Variation
and on Natural Selection, I have attempted to show that within each
country it is the widely ranging, the much diffused and common, that
is the dominant species, belonging to the larger genera in each class,
which vary most. The varieties, or incipient species, thus produced,
ultimately become converted into new and distinct species; and
these, on the principle of inheritance, tend to produce other new
and dominant species. Consequently the groups which are now large,
and which generally include many dominant species, tend to go on
increasing in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly
tend to diverge in character. This latter conclusion is supported by
observing the great diversity of forms which, in any small area,
come into the closest competition, and by certain facts in
naturalisation.
I attempted also to show that there is a steady tendency in the
forms which are increasing in number and diverging in character, to
supplant and exterminate the preceding, less divergent and less
improved forms. I request the reader to turn to the diagram
illustrating the action, as formerly explained, of these several
principles; and he will see that the inevitable result is, that the
modified descendants proceeding from one progenitor become broken up
into groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species, and
the whole of the genera along this upper line form together one class,
for all are descended from one ancient parent, and, consequently, have
inherited something in common. But the three genera on the left hand
have, on this same principle, much in common, and form a sub-family,
distinct from that containing the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than when
grouped in sub-families; and they form a family distinct from that
containing the three genera still farther to the right hand, which
diverged at an earlier period. And all these genera, descended from
(A), form an order distinct from the genera descended from (I). So
that we here have many species descended from a single progenitor
grouped into genera; and the genera into sub-families, families, and
orders, all under one great class. The grand fact of the natural
subordination of organic beings in groups under groups, which, from
its familiarity, does not always sufficiently strike us, is in my
judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus
arranged. In this case there is of course no relation to genealogical
succession, and no cause can at present be assigned for their falling
into groups. But with organic beings the case is different, and the
view above given accords with their natural arrangement in group under
group; and no other explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera,
and families in each class, on what is called the Natural System.
But what is meant by this system? Some authors look at it merely as
a scheme for arranging together those living objects which are most
alike, and for separating those which are most unlike; or as an
artificial method of enunciating, as briefly as possible, general
propositions,- that is, by one sentence to give the characters common,
for instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and then, by
adding a single sentence, a full description is given of each kind
of dog. The ingenuity and utility of this system are indisputable. But
many naturalists think that something more is meant by the Natural
System; they believe that it reveals the plan of the Creator; but
unless it be specified whether order in time or space, or both, or
what else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Expressions such as that
famous one by Linnaeus, which we often meet with in a more or less
concealed form, namely, that the characters do not make the genus, but
that the genus gives the characters, seem to imply that some deeper
bond is included in our classifications than mere resemblance. I
believe that this is the case, and that community of descent- the
one known cause of close similarity in organic beings- is the bond,
which though observed by various degrees of modification, is partially
revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme
for enunciating general propositions and of placing together the forms
most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined
the habits of life, and the general place of each being in the economy
of nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse
to a shrew, of a dugong to a whale, of a whale to a fish, as of any
importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely " adaptive or
analogical characters "; but to the consideration of these
resemblances we shall recur. It may even be given as a general rule,
that the less any part of the organisation is concerned with special
habits, the more important it becomes for classification. As an
instance: Owen, in speaking of the dugong, says, "The generative
organs, being those which are most remotely related to the habits
and food of an animal, I have always regarded as affording very
clear indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an
essential character." With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are of little
significance; whereas the organs of reproduction, with their product
the seed and embryo, are of paramount importance! So again in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service
in classification. This depends on their constancy throughout many
allied groups; and their constancy chiefly depends on any slight
deviations not having been preserved and accumulated by natural
selection, which acts only on serviceable characters.
That the mere physiological importance of an organ does not
determine its classificatory value, is almost proved by the fact
that in allied groups, in which the same organ, as we have every
reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have
worked long at any group without being struck with this fact; and it
has been fully acknowledged in the writings of almost every author. It
will suffice to quote the highest authority, Robert Brown, who, in
speaking of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this, but, as
apprehend, in every natural family, is very unequal, and in some cases
seems to be entirely lost." Again, in another work he says, the genera
of the Connaraceae "differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or valvular
aestivation. Any one of these characters singly is frequently of
more than generic importance, though here even when all taken together
they appear insufficient to separate Cnestis from Connarus." To give
an example amongst insects: in one great division of the
Hymenoptera, the antennae, as Westwood has remarked, are most constant
in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no
one will say that the antennae in these two divisions of the same
order are of unequal physiological importance. Any number of instances
could be given of the varying importance for classification of the
same important organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of
high physiological or vital importance; yet, undoubtedly, organs in
this condition are often of much value in classification. No one
will dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between ruminants and
pachyderms. Robert Brown has strongly insisted on the fact that the
position of the rudimentary florets is of the highest importance in
the classification of the grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance,
but which are universally admitted as highly serviceable in the
definition of whole groups. For instance, whether or not there is an
open passage from the nostrils to the mouth, the only character,
according to Owen, which absolutely distinguishes fishes and reptiles-
the inflection of the angle of the lower jaw in marsupials- the manner
in which the wings of insects are folded- mere colour in certain
Algae- mere pubescence on parts of the flower in grasses- the nature
of the dermal covering, as hair or feathers, in the Vertebrata. If the
Ornithorhynchus had been covered with feathers instead of hair, this
external and trifling character would have been considered by
naturalists as an important aid in determining the degree of
affinity of this strange creature to birds.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of more
or less importance. The value indeed of an aggregate of characters
is very evident in natural history. Hence, as has often been remarked,
a species may depart from its allies in several characters, both of
high physiological importance, and of almost universal prevalence, and
yet leave us in no doubt where it should be ranked. Hence, also, it
has been found that a classification founded on any single
character, however, important that may be, has always failed; for no
part of the organisation is invariably constant. The importance of
an aggregate of characters, even when none are important, alone
explains the aphorism enunciated by Linnaeus, namely, that the
characters do not give the genus, but the genus gives the
characters; for this seems founded on the appreciation of many
trifling points of resemblance, too slight to be defined. Certain
plants, belonging to the Malpighiaceae, bear perfect and degraded
flowers; in the latter, as A. de Jussieu has remarked, " The greater
number of the characters proper to the species, to the genus, to the
family, to the class, disappear, and thus laugh at our
classification." When Aspicarpa produced in France, during several
years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper
type of the order, yet M. Richard sagaciously saw, as Jussieu
observes, that this genus should still be retained amongst the
Malpighiaceae. This case well illustrates the spirit of our
classifications.
Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which
they use in defining a group or in allocating any particular
species. If they find a character nearly uniform, and common to a
great number of forms, and not common to others, they use it as one of
high value; if common to some lesser number, they use it as of
subordinate value. This principle has been broadly confessed by some
naturalists to be the true one; and by none more clearly than by
that excellent botanist, Auguste de Saint-Hilaire. If several trifling
characters are always found in combination, though no apparent bond of
connection can be discovered between them, especial value is set on
them. As in most groups of animals, important organs, such as those
for propelling the blood, or for Aerating it, or those for propagating
the race, are found nearly uniform, they are considered as highly
serviceable in classification; but in some organs all these, the
most important vital organs, are found to offer characters of quite
subordinate value. Thus, as Fritz Muller has lately remarked, in the
same group of crustaceans, Cypridina is furnished with a heart, whilst
in two closely allied genera, namely Cypris and Cytherea, there is
no such organ; one species of Cypridina has well-developed
branchiae, whilst another species is destitute of them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural
classification of course includes all ages. But it is by no means
obvious, on the ordinary view, why the structure of the embryo
should be more important for this purpose than that of the adult,
which alone plays its full part in the economy of nature. Yet it has
been strongly urged by those great naturalists, Milne Edwards and
Agassiz, that embryological characters are the most important of
all; and this doctrine has very generally been admitted as true.
Nevertheless, their importance has sometimes been exaggerated, owing
to the adaptive characters of larvae not having been excluded; in
order to show this, Fritz Muller arranged by the aid of such
characters alone the great class of crustaceans, and the arrangement
did not prove a natural one. But there can be no doubt that
embryonic, excluding larval characters, are of the highest value for
classification, not only with animals but with plants. Thus the main
divisions of flowering plants are founded on differences in the
embryo,- on the number and position of the cotyledons, and on the mode
of development of the plumule and radicle. We shall immediately see
why these characters possess so high a value in classification,
namely, from the natural system being genealogical in its arrangement.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of
characters common to all birds; but with crustaceans, any such
definition has hitherto been found impossible. There are crustaceans
at the opposite ends of the series, which have hardly a character in
common; yet the species at both ends, from being plainly allied to
others, and these to others, and so onwards, can be recognised as
unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups
of species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary.
Several of the best botanists, such as Mr. Bentham and others, have
strongly insisted on their arbitrary value. Instances could be given
amongst plants and insects, of a group first ranked by practised
naturalists as only a genus, and then raised to the rank of a
sub-family or family; and this has been done, not because further
research has detected important structural differences, at first
overlooked, but because numerous allied species with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in
classification may be explained, if I do not greatly deceive myself,
on the view that the Natural System is founded on descent with
modification;- that the characters which naturalists consider as
showing true affinity between any two or more species, are those which
have been inherited from a common parent, all true classification
being genealogical;- that community of descent is the hidden bond
which naturalists have been unconsciously seeking, and not some
unknown plan of creation, or the enunciation of general
propositions, and the mere putting together and separating objects
more or less alike.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination
and relation to each other, must be strictly genealogical in order
to be natural; but that the amount of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different
degrees of modification which they have undergone; and this is
expressed by the forms being ranked under different genera,
families, sections, or orders. The reader will best understand what is
meant, if he will take the trouble to refer to the diagram in the
fourth chapter.
We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still
earlier form. In three of these genera (A, F, and I), a species has
transmitted modified descendants to the present day, represented by
the fifteen genera (a14 to z14) on the uppermost horizontal line. Now
all these modified descendants from a single species, are related in
blood or descent in the same degree; they may metaphorically be called
cousins to the same millionth degree; yet they differ widely and in
different degrees from each other. The forms descended from A, now
broken up into two or three families, constitute a distinct order from
those descended from I, also broken up into two families. Nor can
the existing species, descended from A, be ranked in the same genus
with the parent A; or those from I, with the parent I. But the
existing genus f14 may be supposed to have been but slightly modified;
and it will then rank with the parent-genus F; just as some few
still living organisms belong to Silurian genera. So that the
comparative value of the differences between these organic beings,
which are all related to each other in the same degree in blood, has
come to be widely different. Nevertheless their genealogical
arrangement remains strictly true, not only at the present time, but
at each successive period of descent. All modified descendants from
A will have inherited something in common from their common parent, as
will all the descendants from I; so will it be with each subordinate
branch of descendants, at each successive stage. If, however, we
suppose any descendant of A, or of I, to have become so much
modified as to have lost all traces of its parentage, in this case,
its place in the natural system will be lost, as seems to have
occurred with some few existing organisms. All the descendants of
the genus F, along its whole line of descent, are supposed to have
been but little modified, and they form a single genus. But this
genus, though much isolated, will still occupy its proper intermediate
position. The representation of the groups, as here given in the
diagram on a flat surface, is much too simple. The branches ought to
have diverged in all directions. If the names of the groups had been
simply written down in a linear series, the representation would have
been still less natural; and it is notoriously not possible to
represent in a series, on a flat surface, the affinities which we
discover in nature amongst the beings of the same group. Thus, the
Natural System is genealogical in its arrangement, like a pedigree:
but the amount of modification which the different groups have
undergone has to be expressed by ranking them under different
so-called genera, sub-families, families, sections, orders, and
classes.
It may be worth while to illustrate this view of classification,
by taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken
throughout the world; and if all extinct languages, and all
intermediate and slowly changing dialects, were to be included, such
an arrangement would be the only possible one. Yet it might be that
some ancient languages had altered very little and had given rise to
few new languages, whilst others had altered much owing to the
spreading, isolation, and state of civilisation of the several
co-descended races, and had thus given rise to many new dialects and
languages. The various degrees of difference between the languages
of the same stock, would have to be expressed by groups subordinate to
groups; but the proper or even the only possible arrangement would
still be genealogical; and this would be strictly natural, as it would
connect together all languages, extinct and recent, by the closest
affinities, and would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the sub-varieties
under the varieties; and in some cases, as with the domestic pigeon,
with several other grades of difference. Nearly the same rules are
followed as in classifying species. Authors have insisted on the
necessity of arranging varieties on a natural instead of an
artificial system; we are cautioned, for instance, not to class two
varieties of the pineapple together, merely because their fruit,
though the most important part, happens to be nearly identical; no one
puts the Swedish and common turnip together, though the esculent and
thickened stems are so similar. Whatever part is found to be most
constant, is used in classing varieties: thus the great
agriculturist Marshall says the horns are very useful for this purpose
with cattle, because they are less variable than the shape or colour
of the body, &c.; whereas with sheep the horns are much less
serviceable, because less constant. In classing varieties, I
apprehend that if we had a real pedigree, a genealogical
classification would be universally preferred; and it has been
attempted in some cases. For we might feel sure, whether there had
been more or less modification, that the principle of inheritance
would keep the forms together which were allied in the greatest number
of points. In tumbler pigeons, though some of the sub-varieties differ
in the important character of the length of the beak, yet all are kept
together from having the common habit of tumbling; but the short-faced
breed has nearly or quite lost this habit; nevertheless, without any
thought on the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects.
With species in a state of nature, every naturalist has in fact
brought descent into his classification; for he includes in his lowest
grade, that of species, the two sexes; and how enormously these
sometimes differ in the most important characters, is known to every
naturalist: scarcely a single fact can be predicated in common of
the adult males and hermaphrodites of certain cirripedes, and yet no
one dreams of separating them. As soon as the three orchidean forms,
Monachanthus, Myanthus, and Catasetum, which had previously been
ranked as three distinct genera, were known to be sometimes produced
on the same plant, they were immediately considered as varieties;
and now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes as
one species the various larval stages of the same individual,
however much they may differ from each other and from the adult, as
well as the so-called alternate generations of Steenstrup, which can
only in a technical sense be considered as the same individual. He
includes monsters and varieties, not from their partial resemblance to
the parent-form, but because they are descended from it.
As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larvae are sometimes extremely different; and as it has been used in
classing varieties which have undergone a certain, and sometimes a
considerable amount of modification, may not this same element of
descent have been unconsciously used in grouping species under
genera, and genera under higher groups, all under the so-called
natural system? I believe it has been unconsciously used; and thus
only can I understand the several rules and guides which have been
followed by our best systematists. As we have no written pedigrees, we
are forced to trace community of descent by resemblances of any
kind. Therefore we chose those characters which are the least likely
to have been modified, in relation to the conditions of life to
which each species has been recently exposed. Rudimentary structures
on this view are as good as, or even better than, other parts of the
organisation. We care not how trifling a character may be- let it be
the mere inflection of the angle of the jaw, the manner in which an
insect's wing is folded, whether the skin be covered by hair or
feathers- if it prevail throughout many and different species,
especially those having very different habits of life, it assumes high
value; for we can account for its presence in so many forms with
such different habits, only by inheritance from a common parent. We
may err in this respect in regard to single points of structure, but
when several characters, let them be ever so trifling, concur
throughout a large group of beings having different habits, we may
feel almost sure, on the theory of descent, that these characters
have been inherited from a common ancestor; and we know that such
aggregated characters have especial value in classification.
We can understand why a species or a group of species may depart
from its allies, in several of its most important characteristics, and
yet be safely classed with them. This may be safely done, and is
often done, as long as a sufficient number of characters, let them
be ever so unimportant, betrays the hidden bond of community of
descent. Let two forms have not a single character in common, yet,
if these extreme forms are connected together by a chain of
intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of
high physiological importance- those which serve to preserve life
under the most diverse conditions of existence- are generally the most
constant, we attach especial value to them; but if these same
organs, in another group or section of a group, are found to differ
much, we at once value them less in our classification. We shall
presently see why embryological characters are of such high
classificatory importance. Geographical distribution may sometimes
be brought usefully into play in classing large genera, because all
the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
Analogical Resemblances.- We can understand, on the above views, the
very important distinction between real affinities and analogical or
adaptive resemblances. Lamarck first called attention to this subject,
and he has been ably followed by Macleay and others. The resemblance
in the shape of the body and in the fin-like anterior limbs between
dugongs and whales, and between these two orders of mammals and
fishes, are analogical. So is the resemblance between a mouse and a
shrewmouse (Sorex), which belong to different orders; and the still
closer resemblance, insisted on by Mr. Mivart, between the mouse and a
small marsupial animal (Antechinus) of Australia. These latter
resemblances may be accounted for, as it seems to me, by adaptation
for similarly active movements through thickets and herbage,
together with concealment from enemies.
Amongst insects there are innumerable similar instances; thus
Linnaeus, misled by external appearances, actually classed an
homopterous insect as a moth. We see something of the same kind even
with our domestic varieties, as in the strikingly similar shape of the
body in the improved breeds of the Chinese and common pig, which are
descended from distinct species; and in the similarly thickened
stems of the common and specifically distinct Swedish turnip. The
resemblance between the greyhound and the race-horse is hardly more
fanciful than the analogies which have been drawn by some authors
between widely different animals.
On the view of characters being of real importance for
classification, only in so far as they reveal descent, we can
clearly understand why analogical or adaptive characters, although
of the utmost importance to the welfare of the being, are almost
valueless to the systematist. For animals, belonging to two most
distinct lines of descent, may have become adapted to similar
conditions, and thus have assumed a close external resemblance; but
such resemblances will not reveal- will rather tend to conceal their
blood-relationship. We can thus understand the apparent paradox,
that the very same characters are analogical when one group is
compared with another, but give true affinities when the members of
the same group are compared together: thus, the shape of the body
and fin-like limbs are only analogical when whales are compared with
fishes, being adaptations in both classes for swimming through the
water; but between the several members of the whale family, the
shape of the body and the fin-like limbs offer characters exhibiting
true affinity; for as these parts are so nearly similar throughout the
whole family, we cannot doubt that they have been inherited from a
common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite
distinct beings between single parts or organs, which have been
adapted for the same functions. A good instance is afforded by the
close resemblance of the jaws of the dog and Tasmanian wolf or
Thylacinus,- animals which are widely sundered in the natural
system. But this resemblance is confined to general appearance, as
in the prominence of the canines, and in the cutting shape of the
molar teeth. For the teeth really differ much: thus the dog has on
each side of the upper jaw four pre-molars and only two molars; whilst
the Thylacinus has three pre-molars and four molars. The molars also
differ much in the two animals in relative size and structure. The
adult dentition is preceded by a widely different milk dentition.
Any one may of course deny that the teeth in either case have been
adapted for tearing flesh, through the natural selection of successive
variations; but if this be admitted in the one case, it is
unintelligible to me that it should be denied in the other. I am
glad to find that so high an authority as Professor Flower has come to
this same conclusion.
The extraordinary cases given in a former chapter, of widely
different fishes possessing electric organs,- of widely different
insects possessing luminous organs,- and of orchids and asclepiads
having pollen-masses with viscid discs, come under this same head of
analogical resemblances. But these cases are so wonderful that they
were introduced as difficulties or objections to our theory. In all
such cases some fundamental difference in the growth or development of
the parts, and generally in their matured structure, can be
detected. The end gained is the same, but the means, though
appearing superficially to be the same, are essentially different. The
principle formerly alluded to under the term of analogical variation
has probably in these cases often come into play; that is, the members
of the same class, although only distantly allied, have inherited so
much in common in their constitution, that they are apt to vary
under similar exciting causes in a similar manner; and this would
obviously aid in the acquirement through natural selection of parts or
organs, strikingly like each other, independently of their direct
inheritance from a common progenitor.
As species belonging to distinct classes have often been adapted
by successive slight modifications to live under nearly similar
circumstances,- to inhabit, for instance, the three elements of
land, air, and water,- we can perhaps understand how it is that a
numerical parallelism has sometimes been observed between the
sub-groups of distinct classes. A naturalist, struck with a
parallelism of this nature, by arbitrarily raising or sinking the
value of the groups in several classes (and all our experience shows
that their valuation is as yet arbitrary), could easily extend the
parallelism over a wide range; and thus the septenary, quinary,
quarternary and ternary classifications have probably arisen.
There is another and curious class of cases in which close
external resemblance does not depend on adaptation to similar habits
of life, but has been gained for the sake of protection. I allude to
the wonderful manner in which certain butterflies imitate, as first
described by Mr. Bates, other and quite distinct species. This
excellent observer has shown that in some districts of S. America,
where, for instance, an Ithomia abounds in gaudy swarms, another
butterfly, namely, a leptalis, is often found mingled in the same
flock; and the latter so closely resembles the Ithomia in every
shade and stripe of colour and even in the shape of its wings, that
Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived. When the
mockers and the mocked are caught and compared, they are found to be
very different in essential structure, and to belong not only to
distinct genera, but often to distinct families. Had this mimicry
occurred in only one or two instances, it might have been passed
over as a strange coincidence. But, if we proceed from a district
where one Leptalis imitates an Ithomia, another mocking and mocked
species, belonging to the same two genera, equally close in their
resemblance, may be found. Altogether no less than ten genera are
enumerated, which include species that imitate other butterflies.
The mockers and mocked always inhabit the same region; we never find
an imitator living remote from the form which it imitates. The mockers
are almost invariably rare insects; the mocked in almost every case
abound in swarms. In the same district in which a species of
laptalis closely imitates an Ithomia, there are sometimes other
Lepidoptera mimicking the same Ithomia: so that in the same place,
species of three genera of butterflies and even a moth are found all
closely resembling a butterfly belonging to a fourth genus. It
deserves especial notice that many of the mimicking forms of the
leptalis, as well as of the mimicked forms, can be shown by a
graduated series to be merely varieties of the same species; whilst
others are undoubtedly distinct species. But why, it may be asked, are
certain forms treated as the mimicked and others as the mimickers? Mr.
Bates satisfactorily answers this question, by showing that the form
which is imitated keeps the usual dress of the group to which it
belongs, whilst the counterfeiters have changed their dress and do not
resemble their nearest allies.
We are next led to inquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and
quite distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt,
hit on the true explanation. The mocked forms, which always abound
in numbers, must habitually escape destruction to a large extent,
otherwise they could not exist in such swarms; and a large amount of
evidence has now been collected, showing that they are distasteful
to birds and other insect-devouring animals. The mocking forms, on the
other hand, that inhabit the same district, are comparatively rare,
and belong to rare groups; hence they must suffer habitually from some
danger, for otherwise, from the number of eggs laid by all
butterflies, they would in three or four generations swarm over the
whole country. Now if a member of one of these persecuted and rare
groups were to assume a dress so like that of a well-protected species
that it continually deceived the practised eye of an entomologist,
it would often deceive predaceous birds and insects, and thus often
escape destruction. Mr. Bates may almost be said to have actually
witnessed the process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms of Leptalis
which mimic so many other butterflies, varied in an extreme degree. In
one district several varieties occurred, and of these one alone
resembled to a certain extent, the common Ithomia of the same
district. In another district there were two or three varieties, one
of which was much commoner than the others, and this closely mocked
another form of Ithomia. From facts of this nature, Mr. Bates
concludes that the leptalis first varies; and when a variety happens
to resemble in some degree any common butterfly inhabiting the same
district, this variety, from its resemblance to a flourishing and
little-persecuted kind, has a better chance of escaping destruction
from predaceous birds and insects, and is consequently oftener
preserved;- "the less perfect degrees of resemblance being
generation after generation eliminated, and only the others left to
propagate their kind." So that here we have an excellent
illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay
Archipelago and Africa, and with some other insects. Mr. Wallace has
also detected one such case with birds, but we have none with the
larger quadrupeds. The much greater frequency of imitation with
insects than with other animals, is probably the consequence of
their small size; insects cannot defend themselves, excepting indeed
the kinds furnished with a sting, and I have never heard of an
instance of such kinds mocking other insects, though they are
mocked; insects cannot easily escape by flight from the larger animals
which prey on them; therefore, speaking metaphorically, they are
reduced, like most weak creatures, to trickery and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But starting with
species already somewhat like each other, the closest resemblance,
if beneficial, could readily be gained by the above means; and if
the imitated form was subsequently and gradually modified through
any agency, the imitating form would be led along the same track,
and thus be altered to almost any extent, so that it might
ultimately assume an appearance or colouring wholly unlike that of the
other members of the family to which it belonged. There is, however,
some difficulty on this head, for it is necessary to suppose in some
cases that ancient members belonging to several distinct groups,
before they had diverged to their present extent, accidentally
resembled a member of another and protected group in a sufficient
degree to afford some slight protection; this having given the basis
for the subsequent acquisition of the most perfect resemblance.
On the Nature of the Affinities connecting Organic Beings.- As the
modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to
which they belong large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups within each
class thus tend to go on increasing in size; and they consequently
supplant many smaller and feebler groups. Thus we can account for
the fact that all organisms, recent and extinct, are included under
a few great orders, and under still fewer classes. As showing how
few the higher groups are in number, and how widely they are spread
throughout the world, the fact is striking that the discovery of
Australia has not added an insect belonging to a new class; and that
in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three families of small size.
In the chapter on Geological Succession I attempted to show, on
the principle of each group having generally diverged much in
character during the long-continued process of modification, how it is
that the more ancient forms of life often present characters in some
degree intermediate between existing groups. As some few of the old
and intermediate forms have transmitted to the present day descendants
but little modified, these constitute our so-called osculant or
aberrant species. The more aberrant any form is, the greater must be
the number of connecting forms which have been exterminated and
utterly lost. And we have some evidence of aberrant groups having
suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and lepidosiren, for example,
would not have been less aberrant had each been represented by a
dozen species, instead of as at present by a single one, or by two
or three. We can, I think, account for this fact only by looking at
aberrant groups as forms which have been conquered by more
successful competitors, with a few members still preserved under
unusually favourable conditions.
Mr. Waterhouse has remarked that, when a member belonging to one
group of animals exhibits an affinity to a quite distinct group,
this affinity in most cases is general and not special; thus,
according to Mr. Waterhouse, of all rodents, the bizcacha is most
nearly related to marsupials; but in the points in which it
approaches this order, its relations are general, that is, not to
any one marsupial species more than to another. As these points of
affinity are believed to be real and not merely adaptive, they must be
due in accordance with our view to inheritance from a common
progenitor. Therefore we must suppose either that all rodents,
including the bizcacha, branched off from some ancient marsupial,
which will naturally have been more or less intermediate in
character with respect to all existing marsupials; or that both
rodents and marsupials branched off from a common progenitor, and that
both groups have since undergone much modification in divergent
directions. On either view we must suppose that the bizcacha has
retained, by inheritance, more of the, characters of its ancient
progenitor than have other rodents; and therefore it will not be
specially related to any one existing marsupial, but indirectly to all
or nearly all marsupials, from having partially retained the character
of their common progenitor, or of some early member of the group. On
the other hand, of all marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the
general order of rodents. In this case, however, it may be strongly
suspected as the resemblance is only analogical, owing to the
Phascolomys having become adapted to habits like those of a rodent.
The elder De Candolle has made nearly similar observations on the
general nature of the affinities of distinct families of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from a common progenitor,
together with their retention by inheritance of some characters in
common, we can understand the excessively complex and radiating
affinities by which all the members of the same family or higher group
are connected together. For the common progenitor of a whole family,
now broken up by extinction into distinct groups and sub-groups,
will have transmitted some of its characters, modified in various ways
and degrees, to all the species; and they will consequently be related
to each other by circuitous lines of affinity of various lengths (as
may be seen in the diagram so often referred to), mounting up through
many predecessors. As it is difficult to show the blood relationship
between the numerous kindred of any ancient and noble family even by
the aid of a genealogical tree, and almost impossible to do so without
this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a
diagram, the various affinities which they perceive between the many
living and extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account for the distinctness
of whole classes from each other- for instance, of birds from all
other vertebrate animals- by the belief that many ancient forms of
life have been utterly lost, through which the early progenitors of
birds were formerly connected with the early progenitors of the
other and at that time less differentiated vertebrate classes. There
has been much less extinction of the forms of life which once
connected fishes with batrachians. There has been still less within
some whole classes, for instance the Crustacea, for here the most
wonderfully diverse forms are still linked together by a long and only
partially broken chain of affinities. Extinction has only defined
the groups: it has by no means made them; for if every form which
has ever lived on this earth were suddenly to reappear, though it
would be quite impossible to give definitions by which each group
could be distinguished, still a natural classification, or at least
a natural arrangement, would be possible. We shall see this by turning
to the diagram; the letters, A to L, may represent eleven Silurian
genera, some of which have produced large groups of modified
descendants, with every link in each branch and sub-branch still
alive; and the links not greater than those between existing
varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could
be distinguished from their more immediate parents and descendants.
Yet the arrangement in the diagram would still hold good and would be
natural; for, on the principle of inheritance, all the forms
descended, for instance, from A, would have something in common. In a
tree we can distinguish this or that branch, though at the actual fork
the two unite and blend together. We could not, as I have said, define
the several groups; but we could pick out types, or forms,
representing most of the characters of each group, whether large or
small, and thus give a general idea of the value of the differences
between them. This is what we should be driven to, if we were ever to
succeed in collecting all the forms in any one class which have lived
throughout all time and space. Assuredly we shall never succeed in
making so perfect a collection: nevertheless, in certain classes, we
are tending towards: this end; and Milne Edwards has lately insisted,
in an able paper, on the high importance of looking to types, whether
or not we can separate and define the groups to which such types
belong.
Finally we have seen that natural selection, which follows from
the struggle for existence, and which almost inevitably leads to
extinction and divergence of character in the descendants from any one
parent species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages under one species, although they may
have but few characters in common; we use descent in classing
acknowledged varieties, however different they may be from their
parents; and I believe that this element of descent is the hidden bond
of connection which naturalists have sought under the term of the,
Natural System. On this idea of the natural system, being, in so far
as it has been perfected, genealogical in its arrangement, with the
grades of difference expressed by the terms genera, families,
orders, &c., we can understand the rules which we are compelled to
follow in our classification. We can understand why we value certain
resemblances far more than others; why we use rudimentary and
useless organs, or others of trifling physiological importance; why,
in finding the relations between one group and another, we summarily
reject analogical or adaptive characters, and yet use these same
characters within the limits of the same group. We can clearly see how
it is that all living and extinct forms can be grouped together within
a few great classes; and how the several members of each class are
connected together by the most complex and radiating lines of
affinities. We shall never, probably, disentangle the inextricable web
of the affinities between the members of any one class; but when we
have a distinct object in view, and do not look to some unknown plan
of creation, we may hope to make sure but slow progress.
Professor Haeckel in his Generelle Morphologie and in other works,
has recently brought his great knowledge and abilities to bear on what
he calls phylogeny, or the lines of descent of all organic beings.
In drawing up the several series he trusts chiefly to embryological
characters, but receives aid from homologous and rudimentary organs,
as well as from the successive periods at which the various forms of
life are believed to have first appeared in our geological formations.
He has thus boldly made a great beginning, and shows us how
classification will in the future be treated.
Morphology
We have seen that the members of the same class, independently of
their habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity
of type"; or by saying that the several parts and organs in the
different species of the class are homologous. The whole subject is
included under the general term of Morphology. This is one of the
most interesting departments of natural history, and may almost be
said to be its very soul. What can be more curious than that the
hand of a man, formed for grasping, that of a mole for digging, the
leg of the horse, the paddle of the porpoise, and the wing of the bat,
should all be constructed on the same pattern, and should include
similar bones, in the same relative positions? How curious it is, to
give a subordinate though striking instance, that the hind-feet of the
kangaroo, which are so well fitted for bounding over the open
plains, those of the climbing, leaf-eating koala, equally well
fitted for grasping the branches of trees,- those of the
ground-dwelling, insect or root-eating, bandicoots,- and those of some
other Australian marsupials,- should all be constructed on the same
extraordinary type, namely with the bones of the second and third
digits extremely slender and enveloped within the same skin, so that
they appear like a single toe furnished with two claws.
Notwithstanding this similarity of pattern, it is obvious that the
hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the
more striking by the American opossums, which follow nearly the same
habits of life as some of their Australian relatives, having feet
constructed on the ordinary plan. Professor Flower, from whom these
statements are taken, remarks in conclusion: "We may call this
conformity to type, without getting much nearer to an explanation of
the phenomenon"; and he then adds "but is it not powerfully suggestive
of true relationship, of inheritance from a common ancestor?"
Geoffroy St-Hilaire has strongly insisted on the high importance
of relative position or connection in homologous parts; they may
differ to almost any extent in form and size, and yet remain connected
together in the same invariable order. We never find, for instance,
the bones of the arm and fore-arm, or of the thigh and leg,
transposed. Hence the same names can be given to the homologous
bones in widely different animals. We see the same great law in the
construction of the mouths of insects: what can be more different than
the immensely long spiral proboscis of a sphinxmoth, the curious
folded one of a bee or bug, and the great jaws of a beetle?- yet all
these organs, serving for such widely different purposes, are formed
by infinitely numerous modifications of an upper lip, mandibles, and
two pairs of maxillae. The same law governs the construction of the
mouths and limbs of crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this
similarity of pattern in members of the same class, by utility or by
the doctrine of final causes. The hopelessness of the attempt has been
expressly admitted by Owen in his most interesting work on the
Nature of Limbs. On the ordinary view of the independent creation of
each being, we can only say that so it is;- that it has pleased the
Creator to construct all the animals and plants in each great class on
a uniform plan; but this is not a scientific explanation.
The explanation is to a large extent simple on the theory of the
selection of successive slight modifications,- each modification being
profitable in some way to the modified form, but often affecting by
correlation other parts of the organisation. In changes of this
nature, there will be little or no tendency to alter the original
pattern, or to transpose the parts. The bones of a limb might be
shortened and flattened to any extent, becoming at the same time
enveloped in thick membrane, so as to serve as a fin; or a webbed
hand might have all its bones, or certain bones, lengthened to any
extent, with the membrane connecting them increased, so as to serve as
a wing; yet all these would not tend to alter the framework of the
bones or the relative connection of the parts. If we suppose that an
early progenitor- the archetype as it may be called- of all mammals,
birds, and reptiles, had its limbs constructed on the existing general
pattern, for whatever purpose they served, we can at once perceive the
plain signification of the homologous construction of the limbs
throughout the class. So with the mouths of insects, we have only to
suppose that their common progenitor had an upper lip, mandibles,
and two pairs of maxillae, these parts being perhaps very simple in
form; and then natural selection will account for the infinite
diversity in the structure and functions of the mouths of insects.
Nevertheless, it is conceivable that the general pattern of an organ
might become so much obscured as to be finally lost, by the
reduction and ultimately by the complete abortion of certain parts, by
the fusion of other parts, and by the doubling or multiplication of
others,- variations which we know to be within the limits of
possibility. In the paddles of the gigantic extinct sea-lizards, and
in the mouths of certain suctorial crustaceans, the general pattern
seems thus to have become partially obscured.
There is another and equally curious branch of our subject;
namely, serial homologies, or the comparison of the different parts or
organs in the same individual, and not of the same parts or organs
in different members of the same class. Most physiologists believe
that the bones of the skull are homologous- that is, correspond in
number and in relative connexion- with the elemental parts of a
certain number of vertebrae. The anterior and posterior limbs in all
the higher vertebrate classes are plainly homologous. So it is with
the wonderfully complex jaws and legs of crustaceans. It is familiar
to almost every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their intimate
structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we
often get direct evidence of the possibility of one organ being
transformed into another; and we can actually see, during the early or
embryonic stages of development in flowers, as well as in
crustaceans and many other animals, that organs, which when mature
become extremely different are at first exactly alike.
How inexplicable are the cases of serial homologies on the
ordinary view of creation! Why should the brain be enclosed in a box
composed of such numerous and such extraordinarily shaped pieces of
bone, apparently representing vertebrae? As Owen has remarked, the
benefit derived from the yielding of the separate pieces in the act of
parturition by mammals, will by no means explain the same
construction in the skulls of birds and reptiles. Why should similar
bones have been created to form the wing and the leg of a bat, used as
they are for such totally different purposes, namely flying and
walking? Why should one crustacean, which has an extremely complex
mouth formed of many parts, consequently always have fewer legs; or
conversely, those with many legs have simpler mouths? Why should the
sepals, petals, stamens, and pistils, in each flower, though fitted
for such distinct purposes, be all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent,
answer these questions. We need not here consider how the bodies of
some animals first became divided into a series of segments, or how
they became divided into right and left sides, with corresponding
organs, for such questions are almost beyond investigation. It is,
however, probable that some serial structures are the result of
cells multiplying by division, entailing the multiplication of the
parts developed from such cells. It must suffice for our purpose to
bear in mind that an indefinite repetition of the same part or organ
is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the
Vertebrata probably possessed many vertebrae; the unknown progenitor
of the Articulata, many segments; and the unknown progenitor of
flowering plants, many leaves arranged in one or more spires. We
have also formerly seen that parts many times repeated are eminently
liable to vary, not only in number, but in form. Consequently such
parts, being already present in considerable numbers, and being
highly variable, would naturally afford the materials for adaptation
to the most different purposes; yet they would generally retain,
through the force of inheritance, plain traces of their original or
fundamental resemblance. They would retain this resemblance all the
more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would tend from the
first to be similar; the parts being at an early stage of growth
alike, and being subjected to nearly the same conditions. Such
parts, whether more or less modified, unless their common origin
became wholly obscured, would be serially homologous.
In the great class of molluscs, though the parts in distinct species
can be shown to be homologous, only a few serial homologies, such as
the valves of chitons, can be indicated; that is, we are seldom
enabled to say that one part is homologous with another part in the
same individual. And we can understand this fact; for in molluscs,
even in the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part as we find in the other great
classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at first
appears, as has lately been well shown in a remarkable paper by Mr.
E. Ray Lankester, who has drawn an important distinction between
certain classes of cases which have all been equally ranked by
naturalists as homologous. He proposes to call the structures which
resemble each other in distinct animals, owing to their descent from a
common progenitor with subsequent modification, homogenous; and the
resemblances which cannot thus be accounted for, he proposes to call
homoplastic. For instance, he believes that the hearts of birds and
mammals are as a whole homogenous,- that is, have been derived from
a common progenitor; but that the four cavities of the heart in the
two classes are homoplastic,- that is, have been independently
developed. Mr. Lankester also adduces the close resemblance of the
parts on the right and left sides of the body, and in the successive
segments of the same individual animal; and here we have parts
commonly called homologous, which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures
are the same with those which I have classed, though in a very
imperfect manner, as analogous modifications or resemblances. Their
formation may be attributed in part to distinct organisms, or to
distinct parts of the same organism, having varied in an analogous
manner; and in part to similar modifications, having been preserved
for the same general purpose or function,- of which many instances
have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and
pistils in flowers as metamorphosed leaves; but it would in most
cases be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebrae, jaws and legs, &c., as having been
metamorphosed, not one from the other, as they now exist, but from
some common and simpler element. Most naturalists, however, use such
language only in a metaphorical sense; they are far from meaning
that during a long course of descent, primordial organs of any kind-
vertebrae in the one case and legs in the other- have actually been
converted into skulls or jaws. Yet so strong is the appearance of this
having occurred, that naturalists can hardly avoid employing
language having this plain signification. According to the views
here maintained, such language may be used literally; and the
wonderful fact of the jaws, for instance, of a crab retaining numerous
characters which they probably would have retained through
inheritance, if they had really been metamorphosed from true though
extremely simple legs, is in part explained.
Development and Embryology
This is one of the most important subjects in the whole round of
history. The metamorphoses of insects, with which every one is
familiar, are generally effected abruptly by a few stages; but the
transformations are in reality numerous and gradual, though
concealed. A certain ephemerous insect (Chloeon) during its
development, moults, as shown by Sir J. Lubbock, above twenty times,
and each time undergoes a certain amount of change; and in this case
we see the act of metamorphosis performed in a primary and gradual
manner. Many insects, and especially certain crustaceans, show us what
wonderful changes of structure can be effected during development.
Such changes, however, reach their acme in the so-called alternate
generations of some of the lower animals. It is, for instance, an
astonishing fact that a delicate branching coralline, studded with
polypi and attached to a submarine rock, should produce, first by
budding and then by transverse division, a host of huge floating
jelly-fishes; and that these should produce eggs, from which are
hatched swimming animalcules, which attach themselves to rocks and
become developed into branching corallines; and so on in an endless
cycle. The belief in the essential identity of the process of
alternate generation and of ordinary metamorphosis has been greatly
strengthened by Wagner's discovery of the larva or maggot of a fly,
namely the Cecidomyia, producing asexually other larvae, and these
others, which finally are developed into mature males and females,
propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner's remarkable discovery was
first announced, I was asked how was it possible to account for the
larvae of this fly having acquired the power of asexual
reproduction. As long as the case remained unique no answer could be
given. But already Grimm has shown that another fly, a Chironomus,
reproduces itself in nearly the same manner, and he believes that this
occurs frequently in the Order. It is the pupa, and not the larva,
of the Chironomus which has this power; and Grimm further shows that
this case, to a certain extent, "unites that of the Cecidomyia with
the parthenogenesis of the Coccidae";- the term parthenogenesis
implying that the mature females of the Coccidae are capable of
producing fertile eggs without the concourse of the males. Certain
animals belonging to several classes are now known to have the power
of ordinary reproduction at an unusually early age; and we have only
to accelerate parthenogenetic production by gradual steps to an
earlier and earlier age,- Chironomus showing us an almost exactly
intermediate stage, viz., that of the pupa- and we can perhaps account
for the marvellous case of the Cecidomyia.
It has already been stated that various parts in the same individual
which are exactly alike during an early embryonic period, become
widely different and serve for widely different purposes in the
adult state. So again it has been shown that generally the embryos
of the most distinct species belonging to the same class are closely
similar, but become, when fully developed, widely dissimilar. A better
proof of this latter fact cannot be given than the statement by von
Baer that "The embryos of mammalia, of birds, lizards, and snakes,
probably also of chelonia are in their earliest states exceedingly
like one another, both as a whole and in the mode of development of
their parts; so much so, in fact, that we can often distinguish the
embryos only by their size. In my possession are two little embryos in
spirit, whose names I have omitted to attach, and at present I am
quite unable to say to what class they belong. They may be lizards
or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these
animals. The extremities, however, are still absent in these
embryos. But even if they had existed in the earliest stage of their
development we should learn nothing, for the feet of lizards and
mammals, the wings and feet of birds, no less than the hands and
feet of man, all arise from the same fundamental form." The larvae
of most crustaceans, at corresponding stages of development, closely
resemble each other, however different the adult may become; and so it
is with very many other animals. A trace of the law of embryonic
resemblance occasionally lasts till a rather late age; thus birds of
the same genus, and of allied genera, often resemble each other in
their immature plumage; as we see in the spotted feathers in the
young of the thrush group. In the cat tribe, most of the species
when adult are striped or spotted in lines; and stripes or spots can
be plainly distinguished in the whelp of the lion and the puma. We
occasionally though rarely see something of the same kind in plants;
thus the first leaves of the ulex or furze, and the first leaves of
the phyllodineous aeacias, are pinnate or divided like the ordinary
leaves of the leguminosae.
The points of structure, in which the embryos of widely different
animals within the same class resemble each other, often have no
direct relation to their conditions of existence. We cannot, for
instance, suppose that in the embryos of the vertebrata the peculiar
looplike courses of the arteries near the branchial slits are
related to similar conditions,- in the young mammal which is nourished
in the womb of its mother, in the egg of the bird which is hatched
in a nest, and in the spawn of a frog under water. We have no more
reason to believe in such a relation, than we have to believe that the
similar bones in the hand of a man, wing of a bat, and fin of a
porpoise, are related to similar conditions of life. No one supposes
that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals.
The case, however, is different when an animal during any part of
its embryonic career is active, and has to provide for itself. The
period of activity may come on earlier or later in life; but
whenever it comes on, the adaptation of the larva to its conditions of
life is just as perfect and as beautiful as in the adult animal. In
how important a manner this has acted, has recently been well shown by
Sir J. Lubbock in his remarks on the close similarity of the larvae of
some insects belonging to very different orders, and on the
dissimilarity of the larvae of other insects within the same order,
according to their habits of life. Owing to such adaptations, the
similarity of the larvae of allied animals is sometimes greatly
obscured; especially when there is a division of labour during the
different stages of development, as when the same larva has during one
stage to search for food, and during another stage has to search for a
place of attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other than
do the adults. In most cases, however, the larvae, though active,
still obey, more or less closely, the law of common embryonic
resemblance. Cirripedes afford a good instance of this; even the
illustrious Cuvier did not perceive that a barnacle was a
crustacean: but a glance at the larva shows this in an unmistakable
manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, though differing widely in external
appearance, have larvae in all their stages barely distinguishable.
The embryo in the course of development generally rises in
organisation; I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by the organisation
being higher or lower. But no one probably will dispute that the
butterfly is higher than the caterpillar. In some cases, however,
the mature animal must be considered as lower in the scale than the
larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larvae in the first stage have three pairs of
locomotive organs, a simple single eye, and a probosciformed mouth,
with which they feed largely, for they increase much in size. In the
second stage, answering to the chrysalis stage of butterflies, they
have six pairs of beautifully constructed natatory legs, a pair of
magnificent compound eyes, and extremely complex antennae; but they
have a closed and imperfect mouth, and cannot feed: their function
at this stage is, to search out by their well-developed organs of
sense, and to reach by their active powers of swimming, a proper
place on which to become attached and to undergo their final
metamorphosis. When this is completed they are fixed for life: their
legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two
eyes are now reconverted into a minute, single, simple eye-spot. In
this last and complete state, cirripedes may be considered as either
more highly or more lowly organised than they were in the larval
condition. But in some genera the larvae become developed into
hermaphrodites having the ordinary structure, and into what I have
called complemental males; and in the latter the development has
assuredly been retrograde, for the male is a mere sack, which lives
for a short time and is destitute of mouth, stomach, and every other
organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure between
the embryo and the adult, that we are tempted to look at this
difference as in some necessary manner contingent on growth. But there
is no reason why, for instance, the wing of a bat, or the fin of a
porpoise, should not have been sketched out with all their parts in
proper proportion, as soon as any part became visible. In some whole
groups of animals and in certain members of other groups this is the
case, and the embryo does not at any period differ widely from the
adult: thus Owen has remarked in regard to cuttlefish, "There is no
metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed." Landshells and fresh-water
crustaceans are born having their proper forms, whilst the marine
members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified
habits, or are inactive from being placed in the midst of proper
nutriment or from being fed by their parents; but in some few cases,
as in that of Aphis, if we look to the admirable drawings of the
development of this insect, by Professor Huxley, we see hardly any
trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail.
Thus Fritz Muller has made the remarkable discovery that certain
shrimp-like crustaceans (allied to Penaeus) first appear under the
simple nauplius-form, and after passing through two or more
zoea-stages, and then through the mysis-stage, finally acquire their
mature structure: now in the whole great malacostracan order, to
which these crustaceans belong, no other member is as yet known to
be first developed under the nauplius-form, though many appear as
zoeas; nevertheless Muller assigns reasons for his belief, that if
there had been no suppression of development, all these crustaceans
would have appeared as nauplii.
How, then, can we explain these several facts in embryology,-
namely, the very general, though not universal, difference in
structure between the embryo and the adult;- the various parts in
the same individual embryo, which ultimately become very unlike and
serve for diverse purposes, being at an early period of growth alike;-
the common, but not invariable, resemblance between the embryos or
larvae of the most distinct species in the same class;- the embryo
often retaining, whilst within the egg or womb, structures which are
of no service to it, either at that or at a later period of life; on
the other hand, larvae, which have to provide for their own wants,
being perfectly adapted to the surrounding conditions;- and lastly the
fact of certain larvae standing higher in the scale of organisation
than the mature animal into which they are developed? I believe that
all these facts can be explained, as follows.
It is commonly assumed, perhaps from monstrosities affecting the
embryo at a very early period, that slight variations or individual
differences necessarily appear at an equally early period. We have
little evidence on this head, but what we have certainly points the
other way; for it is notorious that breeders of cattle, horses, and
various fancy animals, cannot positively tell, until some time after
birth, what will be the merits or demerits of their young animals.
We see this plainly in our own children; we cannot tell whether a
child will be tall or short, or what its precise features will be. The
question is not, at what period of life each variation may have been
caused, but at what period the effects are displayed. The cause may
have acted, and I believe often has acted, on one or both parents
before the act of generation. It deserves notice that it is of no
importance to a very young animal, as long as it remains in its
mother's womb or in the egg, or as long as it is nourished and
protected by its parent, whether most of its characters are acquired
little earlier or later in life. It would not signify, for instance,
to a bird which obtained its food by having a much-curved beak whether
or not whilst young it possessed a beak of this shape, as long as it
was fed by its parents.
I have stated in the first chapter, that at whatever age a
variation first appears in the parent, it tends to re-appear at a
corresponding age in the offspring. Certain variations can only appear
at corresponding ages; for instance, peculiarities in the caterpillar,
cocoon, or imago states of the silk-moth; or, again, in the full-grown
horns of cattle. But variations, which, for all that we can see
might have first appeared either earlier or later in life, likewise
tend to reappear at a corresponding age in the offspring and parent. I
am far from meaning that this is invariably the case, and I could give
several exceptional cases of variations (taking the word in the
largest sense) which have supervened at an earlier age in the child
than in the parent.
These two principles, namely, that slight variations generally
appear at a not very early period of life, and are inherited at a
corresponding not early period, explain, as I believe, all the
above-specified leading facts in embryology. But first let us look
to a few analogous cases in our domestic varieties. Some authors who
have written on dogs, maintain that the greyhound and bulldog,
though so different, are really closely allied varieties, descended
from the same wild stock; hence I was curious to see how far their
puppies differed from each other: I was told by breeders that they
differed just as much as their parents, and this, judging by the
eye, seemed almost to be the case; but on actually measuring the old
dogs and their six-days-old puppies, I found that the puppies had
not acquired nearly their full amount of proportional difference.
So, again, I was told that the foals of cart- and race-horses-
breeds which have been almost wholly formed by selection under
domestication- differed as much as the full-grown animals; but
having had careful measurements made of the dams and of three-days-old
colts of race and heavy cart-horses, I find that this is by no means
the case.
As we have conclusive evidence that the breeds of the pigeon are
descended from a single wild species, I compared the young within
twelve hours after being hatched; I carefully measured the proportions
(but will not here give the details) of the beak, width of mouth,
length of nostril and of eyelid, size of feet and length of leg, in
the wild parent-species, in pouters, fantails, runts, barbs,
dragons, carriers, and tumblers. Now some of these birds, when mature,
differ in so extraordinary a manner in the length and form of beak,
and in other characters, that they would certainly have been ranked as
distinct genera if found in a state of nature. But when the nestling
birds of these several breeds were placed in a row, though most of
them could just be distinguished, the proportional differences in
the above specified points were incomparably less than in the
full-grown birds. Some characteristic points of difference- for
instance, that of the width of mouth- could hardly be detected in
the young. But there was one remarkable exception to this rule, for
the young of the short-faced tumbler differed from the young of the
wild rock-pigeon and of the other breeds, in almost exactly the same
proportions as in the adult state.
These facts are explained by the above two principles. Fanciers
select their dogs, horses, pigeons, &c., for breeding, when nearly
grown up: they are indifferent whether the desired qualities are
acquired earlier or later in life, if the full-grown animal
possesses them. And the cases just given, more especially that of
the pigeons, show that the characteristic differences which have
been accumulated by man's selection, and which give value to his
breeds, do not generally appear at a very early period of life, and
are inherited at a corresponding not early period. But the case of the
shortfaced tumbler, which when twelve hours old possessed its proper
characters, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been
inherited, not at a corresponding, but at an earlier age.
Now let us apply these two principles to species in a state of
nature. Let us take a group of birds, descended from some ancient
form and modified through natural selection for different habits.
Then, from the many slight successive variations having supervened
in the several species at a not early age, and having been inherited
at a corresponding age, the young will have been but little
modified, and they will still resemble each other much more closely
than do the adults,- just as we have seen with the breeds of the
pigeon. We may extend this view to widely distinct structures and to
whole classes. The fore-limbs, for instance, which once served as legs
to a remote progenitor, may have become, through a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; but on the above two principles the
fore-limbs will not have been much modified in the embryos of these
several forms; although in each form the fore-limb will differ greatly
in the adult state. Whatever influence long-continued use or disuse
may have had in modifying the limbs or other parts of any species,
this will chiefly or solely have affected it when nearly mature,
when it was compelled to use its full powers to gain its own living;
and the effects thus produced will have been transmitted to the
offspring at a corresponding nearly mature age. Thus the young will
not be modified, or will be modified only in a slight degree,
through the effects of the increased use or disuse of parts.
With some animals the successive variations may have supervened at a
very early period of life, or the steps may have been inherited at
an earlier age than that at which they first occurred. In either of
these cases, the young or embryo will closely resemble the mature
parent-form, as we have seen with the short-faced tumbler. And this is
the rule of development in certain whole groups, or in certain
sub-groups alone, as with cuttle-fish, land-shells, fresh-water
crustaceans, spiders, and some members of the great class of
insects. With respect to the final cause of the young in such groups
not passing through any metamorphosis, we can see that this would
follow from the following contingencies; namely, from the young having
to provide at a very early age for their own wants, and from their
following the same habits of life with their parents; for in this
case, it would be indispensable for their existence that they should
be modified in the same manner as their parents. Again, with respect
to the singular fact that many terrestrial and fresh-water animals
do not undergo any metamorphosis, whilst marine members of the same
groups pass through various transformations, Fritz Muller has
suggested that the process of slowly modifying and adapting an
animal to live on the land or in fresh water, instead of in the sea,
would be greatly simplified by its not passing through any larval
stage; for it is not probable that places well adapted for both the
larval and mature stages, under such new and greatly changed habits of
life, would commonly be found unoccupied or ill-occupied by other
organisms. In this case the gradual acquirement at an earlier and
earlier age of the adult structure would be favoured by natural
selection; and all traces of former metamorphoses would finally be
lost.
If, on the other hand, it profited the young of an animal to
follow habits of life slightly different from those of the
parent-form, and consequently to be constructed on a slightly
different plan, or if it profited a larva already different from its
parent to change still further, then, on the principle of
inheritance at corresponding ages, the young or the larvae might be
rendered by natural selection more and more different from their
parents to any conceivable extent. Differences in the larva might,
also, become correlated with successive stages of its development;
so that the larva, in the first stage might come to differ greatly
from the larva in the second stage, as is the case with many
animals. The adult might also become fitted for sites or habits, in
which organs of locomotion or of the senses, &c., would be useless;
and in this case the metamorphosis would be retrograde.
From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass
through stages of development, perfectly distinct from the
primordial condition of their adult progenitors. Most of our best
authorities are now convinced that the various larval and pupal stages
of insects have thus been acquired through adaptation, and not
through inheritance from some ancient form. The curious case of
Sitaris- a beetle which passes through certain unusual stages of
development- will illustrate how this might occur. The first larval
form is described by M. Fabre, as an active, minute insect,
furnished with six legs, two long antennae, and four eyes. These
larvae are hatched in the nests of bees; and when the male-bees emerge
from their burrows, in the spring, which they do before the females,
the larvae spring on them, and afterwards crawl on to the females
whilst paired with the males. As soon as the female bee deposits her
eggs on the surface of the honey stored in the cells, the larvas of
the Sitaris leap on the eggs and devour them. Afterwards they
undergo a complete change; their eyes disappear; their legs and
antennae become rudimentary, and they feed on honey; so that they
now more closely resemble the ordinary larvae of insects; ultimately
they undergo a further transformation, and finally emerge as the
perfect beetle. Now, if an insect, undergoing transformations like
those of the Sitaris, were to become the progenitor of a whole new
class of insects, the course of development of the new class would
be widely different from that of our existing insects; and the first
larval stage certainly would not represent the former condition of
any adult and ancient form.
On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the
condition of the progenitor of the whole group in its adult state.
In the great class of the Crustacea, forms wonderfully distinct from
each other, namely, suctorial parasites, cirripedes, entomostraca, and
even the malacostraca, appear at first as larvae under the
nauplius-form; and as these larvae live and feed in the open sea,
and are not adapted for any peculiar habits of life, and from other
reasons assigned by Fritz Muller it is probable that at some very
remote period an independent adult animal, resembling the nauplius,
existed, and subsequently produced, along several divergent lines of
descent, the above-named great crustacean groups. So again it is
probable, from what we know of the embryos of mammals, birds,
fishes, and reptiles, that these animals are the modified
descendants of some ancient progenitor, which was furnished in its
adult state with branchiae, a swimbladder, four finlike limbs, and a
long tail, all fitted for an aquatic life.
As all the organic beings, extinct and recent, which have ever
lived, can be arranged within a few great classes; and as all within
each class have, according to our theory, been connected together by
fine gradations, the best, and, if our collections were nearly
perfect, the only possible arrangement, would be genealogical; descent
being the hidden bond of connexion which naturalists have been seeking
under the term of the Natural System. On this view we can understand
how it is that, in the eyes of most naturalists, the structure of
the embryo is even more important for classification than that of
the adult. In two or more groups of animals, however much they may
differ from each other in structure and habits in their adult
condition, if they pass through closely similar embryonic stages, we
may feel assured that they all are descended from one parent-form, and
are therefore closely related. Thus, community in embryonic
structure reveals community of descent; but dissimilarity in embryonic
development does not prove discommunity of descent, for in one of
two groups the developmental stages may have been suppressed, or may
have been so greatly modified through adaptation to new habits of
life, as to be no longer recognisable. Even in groups, in which the
adults have been modified to an extreme degree, community of origin is
often revealed by the structure of the larvae; we have seen, for
instance, that cirripedes, though externally so like shell-fish, are
at once known by their larvae to belong to the great class of
crustaceans. As the embryo often shows us more or less plainly the
structure of the less modified and ancient progenitor of the group, we
can see why ancient and extinct forms so resemble in their adult state
the embryos of existing species of the same class. Agassiz believes
this to be a universal law of nature; and we may hope hereafter to see
the law proved true. It can, however, be proved true only in those
cases in which the ancient state of the progenitor of the group has
not been wholly obliterated, either by successive variations having
supervened at a very early period of growth, or by such variations
having been inherited at an earlier age than that at which they
first appeared. It should also be borne in mind, that the law may be
true, but yet, owing to the geological record not extending far enough
back in time, may remain for a long period, or for ever, incapable
of demonstration. The law will not strictly hold good in those cases
in which an ancient form became adapted in its larvae state to some
special line of life, and transmitted the same larval state to a whole
group of descendants; for such larvae will not resemble any still more
ancient form in its adult state.
Thus, as it seems to me, the leading facts in embryology, which
are second to none in importance, are explained on the principle of
variations in the many descendants from some one ancient progenitor,
having appeared at a not very early period of life, and having been
inherited at a corresponding period. Embryology rises greatly in
interest, when we look at the embryo as a picture, more or less
obscured, of the progenitor, either in its adult or larval state, of
all the members of the same great class.
Rudimentary, Atrophied, and Aborted Organs
Organs or parts in this strange condition, bearing the plain stamp
of inutility, are extremely common, or even general, throughout
nature. It would be impossible to name one of the higher animals in
which some part or other is not in a rudimentary condition. In the
mammalia, for instance, the males possess rudimentary mammae; in
snakes one lobe of the lungs is rudimentary; in birds the
"bastardwing" may safely be considered as a rudimentary digit, and
in some species the whole wing is so far rudimentary that it cannot be
used for flight. What can be more curious than the presence of teeth
in foetal whales, which when grown up have not a tooth in their heads;
or the teeth, which never cut through the gums, in the upper jaws of
unborn calves?
Rudimentary organs plainly declare their origin and meaning in
various ways. There are beetles belonging to closely allied species,
or even to the same identical species, which have either full-sized
and perfect wings, or mere rudiments of membrane, which not rarely lie
under wing-covers firmly soldered together; and in these cases it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality: this occasionally occurs
with the mammae of male mammals, which have been known to become
well developed and to secrete milk. So again in the udders in the
genus Bos, there are normally four developed and two rudimentary
teats; but the latter in our domestic cows sometimes become well
developed and yield milk. In regard to plants the petals are sometimes
rudimentary, and sometimes well-developed in the individuals of the
same species. In certain plants having separated sexes Kolreuter found
that by crossing a species, in which the male flowers included a
rudiment of a pistil, with an hermaphrodite species, having of
course a well-developed pistil, the rudiment in the hybrid offspring
was much increased in size; and this clearly shows that the
rudimentary and perfect pistils are essentially alike in nature. An
animal may possess various parts in a perfect state, and yet they
may in one sense be rudimentary, for they are useless: thus the
tadpole of the common salamander or water-newt, as Mr. G. H. Lewes
remarks, "has gills, and passes its existence in the water; but the
Salamandra atra, which lives high up among the mountains, brings forth
its young full-formed. This animal never lives in the water. Yet if we
open a gravid female, we find tadpoles inside her with exquisitely
feathered gills; and when placed in water they swim about like the
tadpoles of the water-newt. Obviously this aquatic organisation has no
reference to the future life of the animal, nor has it any
adaptation to its embryonic condition; it has solely reference to
ancestral adaptations, it repeats a phase in the development of its
progenitors."
An organ, serving for two purposes, may become rudimentary or
utterly aborted for one, even the more important purpose, and remain
perfectly efficient for the other. Thus in plants, the office of the
pistil is to allow the pollen-tubes to reach the ovules within the
ovarium. The pistil consists of a stigma supported on a style; but
in some Compositae, the male florets, which of course cannot be
fecundated, have a rudimentary pistil, for it is not crowned with a
stigma; but the style remains well developed and is clothed in the
usual manner with hairs, which serve to brush the pollen out of the
surrounding and conjoined anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a distinct one: in
certain fishes the swimbladder seems to be rudimentary for its
proper function of giving buoyancy, but has become converted into a
nascent breathing organ or lung. Many similar instances could be
given.
Useful organs, however little they may be developed, unless we
have reason to suppose that they were formerly more highly
developed, ought not to be considered as rudimentary. They may be in a
nascent condition, and in progress towards further development.
Rudimentary organs, on the other hand, are either quite useless,
such as teeth which never cut through the gums, or almost useless,
such as the wings of an ostrich, which serve merely as sails. As
organs in this condition would formerly, when still less developed,
have been of even less use than at present, they cannot formerly
have been produced through variation and natural selection, which acts
solely by the preservation of useful modifications. They have been
partially retained by the power of inheritance, and relate to a former
state of things. It is, however, often difficult to distinguish
between rudimentary and nascent organs; for we can judge only by
analogy whether a part is capable of further development, in which
case alone it deserves to be called nascent. Organs in this
condition will always be somewhat rare; for beings thus provided
will commonly have been supplanted by their successors with the same
organ in a more perfect state, and consequently will have become
long ago extinct. The wing of the penguin is of high service, acting
as a fin; it may, therefore, represent the nascent state of the
wing: not that I believe this to be the case; it is more probably a
reduced organ, modified for a new function: the wing of the Apteryx,
on the other hand, is quite useless, and is truly rudimentary. Owen
considers the simple filamentary limbs of the lepidosiren as the
"beginnings of organs which attain full functional development in
higher vertebrates"; but, according to the view lately advocated by
Dr. Gunther, they are probably remnants, consisting of the
persistent axis of a fin, with the lateral rays of branches aborted.
The mammary glands of the Ornithorhynchus may be considered, in
comparison with the udders of a cow, as in a nascent condition. The
ovigerous frena of certain cirripedes, which have ceased to give
attachment to the ova and are feebly developed, are nascent branchiae.
Rudimentary organs in the individuals of the same species are very
liable to vary in the degree of their development and in other
respects. In closely allied species, also, the extent to which the
same organ has been reduced occasionally differs much. This latter
fact is well exemplified in the state of the wings of female moths
belonging to the same family. Rudimentary organs may be utterly
aborted; and this implies, that in certain animals or plants, parts
are entirely absent which analogy would lead us to expect to find in
them, and which are occasionally found in monstrous individuals.
Thus in most of the Scrophulariaceae the fifth stamen is utterly
aborted; yet we may conclude that a fifth stamen once existed, for a
rudiment of it is found in many species of the family, and this
rudiment occasionally becomes perfectly developed, as may sometimes be
seen in the common snap-dragon. In tracing the homologies of any
part in different members of the same class, nothing is more common,
or, in order fully to understand the relations of the parts, more
useful than the discovery of rudiments. This is well shown in the
drawings given by Owen of the leg-bones of the horse, ox, and
rhinoceros.
It is an important fact that rudimentary organs, such as teeth in
the upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a
universal rule, that a rudimentary part is of greater size in the
embryo relatively to the adjoining parts, than in the adult; so that
the organ at this early age is less rudimentary, or even cannot be
said to be in any degree rudimentary. Hence rudimentary organs in
the adult are often said to have retained their embryonic condition.
I have now given the leading facts with respect to rudimentary
organs. In reflecting on them, every one must be struck with
astonishment; for the same reasoning power which tells us that most
parts and organs are exquisitely adapted for certain purposes, tells
us with equal plainness that these rudimentary or atrophied organs are
imperfect and useless. In works on natural history, rudimentary organs
are generally said to have been created "for the sake of symmetry," or
in order "to complete the scheme of nature." But this is not an
explanation, merely a re-statement of the fact. Nor is it consistent
with itself; thus the boa constrictor has rudiments of hind-limbs
and of a pelvis, and if it be said that these bones have been retained
"to complete the scheme of nature," why, as Professor Weismann asks,
have they not been retained by other snakes, which do not possess even
a vestige of these same bones? What would be thought of an
astronomer who maintained that the satellites revolve in elliptic
courses round their planets "for the sake of symmetry," because the
planets thus revolve round the sun? An eminent physiologist accounts
for the presence of rudimentary organs, by supposing that they serve
to excrete matter in excess, or matter injurious to the system; but
can we suppose that the minute papilla, which often represents the
pistil in male flowers, and which is formed of mere cellular tissue,
can thus act? Can we suppose that rudimentary teeth, which are
subsequently absorbed, are beneficial to the rapidly growing embryonic
calf by removing matter so precious as phosphate of lime? When a
man's fingers have been amputated, imperfect nails have been known
to appear on the stumps, and I could as soon believe that these
vestiges of nails are developed in order to excrete horny matter, as
that the rudimentary nails on the fin of the manatee have been
developed for this same purpose.
On the view of descent with modification, the origin of
rudimentary organs is comparatively simple; and we can understand to a
large extent the laws governing their imperfect development. We have
plenty of cases of rudimentary organs in our domestic productions,- as
the stump of a tail in tailless breeds,- the vestige of an ear in
earless breeds of sheep,- the reappearance of minute dangling horns in
hornless breeds of cattle, more especially, according to Youatt, in
young animals,- and the state of the whole flower in the
cauliflower. We often see rudiments of various parts in monsters;
but I doubt whether any of these cases throw light on the origin of
rudimentary organs in a state of nature, further than by showing
that rudiments can be produced; for the balance of evidence clearly
indicates that species under nature do not undergo great and abrupt
changes. But we learn from the study of our domestic productions
that the disuse of parts leads to their reduced size; and that the
result is inherited.
It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more
and more complete reduction of a part, until at last it became
rudimentary,- as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced by beasts of prey to take flight, and have
ultimately lost the power of flying. Again, an organ, useful under
certain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this
case natural selection will have aided in reducing the organ, until it
was rendered harmless and rudimentary.
Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for
one purpose, might be modified and used for another purpose. An
organ might, also, be retained for one alone of its former
functions. Organs, originally formed by the aid of natural
selection, when rendered useless may well be variable, for their
variations can no longer be cheeked by natural selection. All this
agrees well with what we see under nature. Moreover, at whatever
period of life either disuse or selection reduces an organ, and this
will generally be when the being has come to maturity and has to exert
its full powers of action, the principle of inheritance at
corresponding ages will tend to reproduce the organ in its reduced
state at the same mature age, but will seldom effect it in the embryo.
Thus we can understand the greater size of rudimentary organs in the
embryo relatively to the adjoining parts. and their lesser relative
size in the adult. If, for instance, the digit of an adult animal
was used less and less during many generations, owing to some change
of habits, or if an organ or gland was less and less functionally
exercised, we may infer that it would become reduced in size in the
adult descendants of this animal, but would retain nearly its original
standard of development in the embryo.
There remains, however, this difficulty. After an organ has ceased
being used, and has become in consequence much reduced, how can it
be still further reduced in size until the merest vestige is left; and
how can it be finally quite obliterated? It is scarcely possible
that disuse can go on producing any further effect after the organ has
once been rendered functionless. Some additional explanation is here
requisite which I cannot give. If, for instance, it could be proved
that every part of the organisation tends to vary in a greater
degree towards diminution than towards augmentation of size, then we
should be able to understand how an organ which has become useless
would be rendered, independently of the effects of disuse,
rudimentary and would at last be wholly suppressed; for the variations
towards diminished size would no longer be checked by natural
selection. The principle of the economy of growth, explained in a
former chapter, by which the materials forming any part, if not useful
to the possessor, are saved as far as possible, will perhaps come into
play in rendering a useless part rudimentary. But this principle
will almost necessarily be confined to the earlier stages of the
process of reduction; for we cannot suppose that a minute papilla, for
instance, representing in a male flower the pistil of the female
flower, and formed merely of cellular tissue, could be further
reduced or absorbed for the sake of economising nutriment.
Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a
former state of things, and have been retained solely through the
power of inheritance,- we can understand, on the genealogical view
of classification, how it is that systematists, in placing organisms
in their proper places in the natural system, have often found
rudimentary parts as useful as, or even sometimes more useful than,
parts of high physiological importance. Rudimentary organs may be
compared with the letters in a word, still retained in the spelling,
but become useless in the pronunciation, but which serve as a clue for
its derivation. On the view of descent with modification, we may
conclude that the existence of organs in a rudimentary, imperfect, and
useless condition, or quite aborted, far from presenting a strange
difficulty, as they assuredly do on the old doctrine of creation,
might even have been anticipated in accordance with the views here
explained.
Summary
In this chapter I have attempted to show, that the arrangement of
all organic beings throughout all time in groups under groups- that
the nature of the relationships by which all living and extinct
organisms are united by complex, radiating, and circuitous lines of
affinities into a few grand classes,- the rules followed and the
difficulties encountered by naturalists in their classifications,- the
value set upon characters, if constant and prevalent, whether of
high or of the most trifling importance, or, as with rudimentary
organs, of no importance,- the wide opposition in value between
analogical or adaptive characters, and characters of true affinity;
and other such rules;- all naturally follow if we admit the common
parentage of allied forms, together with their modification through
variation and natural selection, with the contingencies of
extinction and divergence of character. In considering this view of
classification, it should be borne in mind that the element of descent
has been universally used in ranking together the sexes, ages,
dimorphic forms, and acknowledged varieties of the same species,
however much they may differ from each other in structure. If we
extend the use of this element of descent,- the one certainly known
cause of similarity in organic beings,- we shall understand what is
meant by the Natural System: it is genealogical in its attempted
arrangement, with the grades of acquired difference marked by the
terms, varieties, species, genera, families, orders, and classes.
On this same view of descent with modification, most of the great
facts in Morphology become intelligible,- whether we look to the
same pattern displayed by the different species of the same class in
their homologous organs, to whatever purpose applied; or to the serial
and lateral homologies in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the leading
facts in Embryology; namely, the close resemblance in the individual
embryo of the parts which are homologous, and which when matured
become widely different in structure and function; and the resemblance
of the homologous parts or organs in allied though distinct species,
though fitted in the adult state for habits as different as is
possible. Larvae are active embryos, which have been specially
modified in a greater or less degree in relation to their habits of
life, with their modifications inherited at a corresponding early age.
On these same principles,- and bearing in mind that when organs are
reduced in size, either from disuse or through natural selection, it
will generally be at that period of life when the being has to provide
for its own wants, and bearing in mind how strong is the force of
inheritance- the occurrence of rudimentary organs might even have been
anticipated. The importance of embryological characters and of
rudimentary organs in classification is intelligible, on the view
that a natural arrangement must be genealogical.
Finally, the several classes of facts which have been considered
in this chapter, seem to me to proclaim so plainly, that the
innumerable species, genera and families, with which this world is
peopled, are all descended, each within its own class or group, from
common parents, and have all been modified in the course of descent,
that I should without hesitation adopt this view, even if it were
unsupported by other facts or arguments.
CHAPTER XV
RECAPITULATION AND CONCLUSION
AS THIS whole volume is one long argument, it may be convenient to
the reader to have the leading facts and inferences briefly
recapitulated.
That many and serious objections may be advanced against the
theory of descent with modification through variation and natural
selection, I do not deny. I have endeavoured to give to them their
full force. Nothing at first can appear more difficult to believe than
that the more complex organs and instincts have been perfected, not by
means superior to, though analogous with, human reason, but by the
accumulation of innumerable slight variations, each good for the
individual possessor. Nevertheless, this difficulty, though
appearing to our imagination insuperably great, cannot be considered
real if we admit the following propositions, namely, that all parts of
the organisation and instincts offer, at least, individual
differences- that there is a struggle for existence leading to the
preservation of profitable deviations of structure or instinct- and,
lastly, that gradations in the state of perfection of each organ may
have existed, each good of its kind. The truth of these propositions
cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially
amongst broken and failing groups of organic beings, which have
suffered much extinction, but we see so many strange gradations in
nature, that we ought to be extremely cautious in saying that any
organ or instinct, or any whole structure, could not have arrived at
its present state by many graduated steps. There are, it must be
admitted, cases of special difficulty opposed to the theory of natural
selection; and one of the most curious of these is the existence in
the same community of two or three defined castes of workers or
sterile female ants; but I have attempted to show how these
difficulties can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost
universal fertility of varieties when crossed, I must refer the
reader to the recapitulation of the facts given at the end of the
ninth chapter, which seem to me conclusively to show that this
sterility is no more a special endowment than is the incapacity of two
distinct kinds of trees to be grafted together; but that it is
incidental on differences confined to the reproductive systems of
the intercrossed species. We see the truth of this conclusion in the
vast difference in the results of crossing the same two species
reciprocally,- that is, when one species is first used as the father
and then as the mother. Analogy from the consideration of dimorphic
and trimorphic plants clearly leads to the same conclusion, for when
the forms are illegitimately united, they yield few or no seed, and
their offspring are more or less sterile; and these forms belong to
the same undoubted species, and differ from each other in no respect
except in their reproductive organs and functions.
Although the fertility of varieties when intercrossed and of their
mongrel offspring has been asserted by so many authors to be
universal, this cannot be considered as quite correct after the
facts given on the high authority of Gartner and Kolreuter. Most of
the varieties which have been experimented on have been produced under
domestication; and as domestication (I do not mean mere confinement)
almost certainly tends to eliminate that sterility which, judging from
analogy, would have affected the parent-species if intercrossed, we
ought not to expect that domestication would likewise induce sterility
in their modified descendants when crossed. This elimination of
sterility apparently follows from the same cause which allows our
domestic animals to breed freely under diversified circumstances;
and this again apparently follows from their having been gradually
accustomed to frequent changes in their conditions of life.
A double and parallel series of facts seems to throw much light on
the sterility of species, when first crossed, and of their hybrid
offspring. On the one side, there is good reason to believe that
slight changes in the conditions of life give vigour and fertility
to all organic beings. We know also that a cross between the
distinct individuals of the same variety, and between distinct
varieties, increases the number of their offspring, and certainly
gives to them increased size and vigour. This is chiefly owing to
the forms which are crossed having been exposed to somewhat
different conditions of life; for I have ascertained by a laborious
series of experiments that if all the individuals of the same
variety be subjected during several generations to the same
conditions, the good derived from crossing is often much diminished or
wholly disappears. This is one side of the case. On the other side, we
know that species which have long been exposed to nearly uniform
conditions, when they are subjected under confinement to new and
greatly changed conditions, either perish, or if they survive, are
rendered sterile, though retaining perfect health. This does not
occur, or only in a very slight degree, with our domesticated
productions, which have long been exposed to fluctuating conditions.
Hence when we find that hybrids produced by a cross between two
distinct species are few in number, owing to their perishing soon
after conception or at a very early age, or if surviving that they are
rendered more or less sterile, it seems highly probable that this
result is due to their having been in fact subjected to a great change
in their conditions of life, from being compounded of two distinct
organisations. He who will explain in a definite manner why, for
instance, air elephant or a fox will not breed under confinement in
its native country, whilst the domestic pig or dog will breed freely
under the most diversified conditions, will at the same time be able
to give a definite answer to the question why two distinct species,
when crossed, as well as their hybrid offspring, are generally
rendered more or less sterile, whilst two domesticated varieties
when crossed and their mongrel offspring are perfectly fertile.
Turning to geographical distribution, the difficulties encountered
on the theory of descent with modification are serious enough. All the
individuals of the same species, and all the species of the same
genus, or even higher group, are descended from common parents; and
therefore, in however distant and isolated parts of the world they may
now be found, they must in the course of successive generations have
travelled from some one point to all the others. We are often wholly
unable even to conjecture how this could have been effected. Yet, as
we have reason to believe that some species have retained the same
specific form for very long periods of time, immensely long as
measured by years, too much stress ought not to be laid on the
occasional wide diffusion of the same species; for during very long
periods there will always have been a good chance for wide migration
by many means. A broken or interrupted range may often be accounted
for by the extinction of the species in the intermediate regions. It
cannot be denied that we are as yet very ignorant as to the full
extent of the various climatal and geographical changes which have
affected the earth during modern periods; and such changes will
often have facilitated migration. As an example, I have attempted to
show how potent has been the influence of the Glacial period on the
distribution of the same and of allied species throughout the world.
We are as yet profoundly ignorant of the many occasional means of
transport. With respect to distinct species of the same genus
inhabiting distant and isolated regions, as the process of
modification has necessarily been slow, all the means of migration
will have been possible during a very long period; and consequently
the difficulty of the wide diffusion of the species of the same
genus is in some degree lessened.
As according to the theory of natural selection an interminable
number of intermediate forms must have existed, linking together all
the species in each group by gradations as fine as are our existing
varieties, it may be asked: Why do we not see these linking forms
all around us? Why are not all organic beings blended together in an
inextricable chaos? With respect to existing forms, we should remember
that we have no right to expect (excepting in rare cases) to
discover directly connecting links between them, but only between each
and some extinct and supplanted form. Even on a wide area, which has
during a long period remained continuous, and of which the climatic
and other conditions of life change insensibly in proceeding from a
district occupied by one species into another district occupied by a
closely allied species, we have no just right to expect often to
find intermediate varieties in the intermediate zones. For we have
reason to believe that only a few species of a genus ever undergo
change; the other species becoming utterly extinct and leaving no
modified progeny. Of the species which do change, only a few within
the same country change at the same time; and all modifications are
slowly effected. I have also shown that the intermediate varieties
which probably at first existed in the intermediate zones, would be
liable to be supplanted by the allied forms on either hand; for the
latter, from existing in greater numbers, would generally be
modified and improved at a quicker rate than the intermediate
varieties, which existed in lesser numbers; so that the intermediate
varieties would, in the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species,
why is not every geological formation charged with such links? Why
does not every collection of fossil remains afford plain evidence of
the gradation and mutation of the forms of life? Although geological
research has undoubtedly revealed the former existence of many
links, bringing numerous forms of life much closer together, it does
not yield the infinitely many fine gradations between past and present
species required on the theory; and this is the most obvious of the
many objections which may be urged against it. Why, again, do whole
groups of allied species appear, though this appearance is often
false, to have come in suddenly on the successive geological stages?
Although we now know that organic beings appeared on this globe, at
a period incalculably remote, long before the lowest bed of the
Cambrian system was deposited, why do we not find beneath this
system great piles of strata stored with the remains of the
progenitors of the Cambrian fossils? For on the theory, such strata
must somewhere have been deposited at these ancient and utterly
unknown epochs of the world's history.
I can answer these questions and objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. The number of specimens in all our museums is
absolutely as nothing compared with the countless generations of
countless species which have certainly existed. The parent-form of any
two or more species would not be in all its characters directly
intermediate between its modified offspring, any more than the
rock-pigeon is directly intermediate in crop and tail between its
descendants, the pouter and fantail pigeons. We should not be able
to recognise a species as the parent of another and modified
species, if we were to examine the two ever so closely, unless we
possessed most of the intermediate links; and owing to the
imperfection of the geological record, we have no just right to expect
to find so many links. If two or three, or even more linking forms
were discovered, they would simply be ranked by many naturalists as so
many new species, more especially if found in different geological
sub-stages, let their differences be ever so slight. Numerous existing
doubtful forms could be named which are probably varieties; but who
will pretend that in future ages so many fossil links will be
discovered, that naturalists will be able to decide whether or not
these doubtful forms ought to be called varieties? Only a small
portion of the world has been geologically explored. Only organic
beings of certain classes can be preserved in a fossil condition, at
least in any great number. Many species when once formed never undergo
any further change but become extinct without leaving modified
descendants; and the periods, during which species have undergone
modification, though long as measured by years, have probably been
short in comparison with the periods during which they retain the same
form. It is the dominant and widely ranging species which vary most
frequently and vary most, and varieties are often at first local- both
causes rendering the discovery of intermediate links in any one
formation less likely. Local varieties will not spread into other
and distant regions until they are considerably modified and improved;
and when they have spread, and are discovered in a geological
formation, they appear as if suddenly created there, and will be
simply classed as new species. Most formations have been
intermittent in their accumulation; and their duration has probably
been shorter than the average duration of specific forms. Successive
formations are in most cases separated from each other by blank
intervals of time of great length; for fossiliferous formations
thick enough to resist future degradations can as a general rule be
accumulated only where much sediment is deposited on the subsiding bed
of the sea. During the alternate periods of elevation and of
stationary level the record will generally be blank. During these
latter periods there will probably be more variability in the forms of
life; during periods of subsidence, more extinction.
With respect to the absence of strata rich in fossils beneath the
Cambrian formation, I can recur only to the hypothesis given in the
tenth chapter; namely, that though our continents and oceans have
endured for an enormous period in nearly their present relative
positions, we have no reason to assume that this has always been the
case; consequently formations much older than any now known may lie
buried beneath the great oceans. With respect to the lapse of time not
having been sufficient since our planet was consolidated for the
assumed amount of organic change, and this objection, as urged by
Sir William Thompson, is probably one of the gravest as yet
advanced, I can only say, firstly, that we do not know at what rate
species change as measured by years, and secondly, that many
philosophers are not as yet willing to admit that we know enough of
the constitution of the universe and of the interior of our globe to
speculate with safety on its past duration.
That the geological record is imperfect all will admit; but that
it is imperfect to the degree required by our theory, few will be
inclined to admit. If we look to long enough intervals of time,
geology plainly declares that species have all changed; and they
have changed in the manner required by the theory, for they have
changed slowly and in a graduated manner. We clearly see this in the
fossil remains from consecutive formations invariably being much
more closely related to each other, than are the fossils from widely
separated formations.
Such is the sum of the several chief objections and difficulties
which may be justly urged against the theory; and I have now briefly
recapitulated the answers and explanations which, as far as I can see,
may be given. I have felt these difficulties far too heavily during
many years to doubt their weight. But it deserves especial notice
that the more important objections relate to questions on which we are
confessedly ignorant; nor do we know how ignorant we are. We do not
know all the possible transitional gradations between the simplest and
the most perfect organs; it cannot be pretended that we know all the
varied means of Distribution during the long lapse of years, or that
we know how imperfect is the Geological Record. Serious as these
several objections are, in my judgment they are by no means sufficient
to overthrow the theory of descent with subsequent modification.
Now let us turn to the other side of the argument. Under
domestication we see much variability, caused, or at least excited, by
changed conditions of life; but often in so obscure a manner, that
we are tempted to consider the variations as spontaneous.
Variability is governed by many complex laws,- by correlated growth,
compensation, the increased use and disuse of parts, and the
definite action of the surrounding conditions. There is much
difficulty in ascertaining how largely our domestic productions have
been modified; but we may safely infer that the amount has been large,
and that modifications can be inherited for long periods. As long as
the conditions of life remain the same, we have reason to believe that
a modification, which has already been inherited for many
generations, may continue to be inherited for an almost infinite
number of generations. On the other hand, we have evidence that
variability when it has once come into play, does not cease under
domestication for a very long period; nor do we know that it ever
ceases, for new varieties are still occasionally produced by our
oldest domesticated productions.
Variability is not actually caused by man; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation and causes it to vary. But man can and does select
the variations given to him by nature, and thus accumulates them in
any desired manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful or pleasing to
him without any intention of altering the breed. It is certain that he
can largely influence the character of a breed by selecting, in each
successive generation, individual differences so slight as to be
inappreciable except by an educated eye. This unconscious process of
selection has been the great agency in the formation of the most
distinct and useful domestic breeds. That many breeds produced by
man have to a large extent the character of natural species, is
shown by the inextricable doubts whether many of them are varieties or
aboriginally distinct species.
There is no reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the survival of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see a powerful and
ever-acting form of Selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is common,
to all organic beings. This high rate of increase is proved by
calculation,- by the rapid increase of many animals and plants
during succession of peculiar seasons, and when naturalised in new
countries. More individuals are born than can possibly survive. A
grain in the balance may determine which individuals shall live and
which shall die,- which variety or species shall increase in number,
and which shall decrease, or finally become extinct. As the
individuals of the same species come in all respects into the
closest competition with each other, the struggle will generally be
most severe between them; it will be almost equally severe between the
varieties of the same species, and next in severity between the
species of the same genus. On the other hand the struggle will often
be severe between beings remote in the scale of nature. The
slightest advantage in certain individuals, at any age or during any
season, over those with which they come into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will, in the long run, turn the balance.
With animals having separated sexes, there will be in most cases a
struggle between the males for the possession of the females. The most
vigorous males, or those which have most successfully struggled with
their conditions of life, will generally leave most progeny. But
success will often depend on the males having special weapons, or
means of defence, or charms; add a slight advantage will lead to
victory.
As geology plainly proclaims that each land has undergone great
physical changes, we might have expected to find that organic beings
have varied under nature, in the same way as they have varied under
domestication. And if there has been any variability under nature,
it would be an unaccountable fact if natural selection had not come
into play. It has often been asserted, but the assertion is
incapable of proof, that the amount of variation under nature is a
strictly limited quantity. Man, though acting on external characters
alone and often capriciously, can produce within a short period a
great result by adding up mere individual differences in his
domestic productions; and every one admits that species present
individual differences. But, besides such differences, all naturalists
admit that natural varieties exist, which are considered
sufficiently distinct to be worthy of record in systematic works. No
one has drawn any clear distinction between individual differences and
slight varieties; or between more plainly marked varieties and
sub-species, and species. On separate continents, and on different
parts of the same continent when divided by barriers of any kind,
and on outlying islands, what a multitude of forms exist, which some
experienced naturalists rank as varieties, others as geographical
races or sub-species, and others as distinct, though closely allied
species!
If, then, animals and plants do vary, let it be ever so slightly
or slowly, why should not variations or individual differences,
which are in any way beneficial, be preserved and accumulated
through natural selection, or the survival of the fittest? If man
can by patience select variations useful to him, why, under changing
and complex conditions of life, should not variations useful to
nature's living products often arise, and be preserved or selected?
What limit can be put to this power, acting during long ages and
rigidly scrutinising the whole constitution, structure, and habits
of each creature,- favouring the good and rejecting the bad? I can see
no limit to this power, in slowly and beautifully adapting each form
to the most complex relations of life. The theory of natural
selection, even if we look no farther than this, seems to be in the
highest degree probable. I have already recapitulated, as fairly as
I could, the opposed difficulties and objections; now let us turn to
the special facts and arguments in favour of the theory.
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can
see why it is that no line of demarcation can be drawn between
species, commonly supposed to have been produced by special acts of
creation, and varieties which are acknowledged to have been produced
by secondary laws. On this same view we can understand how it is
that in a region where many species of a genus have been produced, and
where they now flourish, these same species should present many
varieties; for where the manufactory of species has been active, we
might expect, as a general rule, to find it still in action; and
this is the case if varieties be incipient species. Moreover, the
species of the larger genera, which afford the greater number of
varieties or incipient species, retain to a certain degree the
character of varieties; for they differ from each other by a less
amount of difference than do the species of smaller genera. The
closely allied species also of the larger genera apparently have
restricted ranges, and in their affinities they are clustered in
little groups round other species- in both respects resembling
varieties. These are strange relations on the view that each species
was independently created, but are intelligible if each existed
first as a variety.
As each species tends by its geometrical rate of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by as much as they become
more diversified in habits and structure, so as to be able to seize on
many and widely different places in the economy of nature, there
will be a constant tendency in natural selection to preserve the
most divergent offspring of any one species. Hence, during a
long-continued course of modification, the slight differences,
characteristic of varieties of the same species, tend to be
augmented into the greater differences characteristic of the species
of the same genus. New and improved varieties will inevitably supplant
and exterminate the older, less improved, and intermediate
varieties; and thus species are rendered to a large extent defined and
distinct objects. Dominant species belonging to the larger groups
within each class tend to give birth to new and dominant forms; so
that each large group tends to become still larger, and at the same
time more divergent in character. But as all groups cannot thus go
on increasing in size, for the world would not hold them, the more
dominant groups beat the less dominant. This tendency in the large
groups to go on increasing in size and diverging in character,
together with the inevitable contingency of much extinction,
explains the arrangement of all the forms of life in groups
subordinate to groups, all within a few great classes, which has
prevailed throughout all time. This grand fact of the grouping of
all organic beings under what is called the Natural System, is utterly
inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden
modifications; it can act only by short and slow steps. Hence, the
canon of "Natura non facit saltum," which every fresh addition to
our knowledge tends to confirm, is on this theory intelligible. We can
see why throughout nature the same general end is gained by an
almost infinite diversity of means, for every peculiarity when once
acquired is long inherited, and structures already modified in many
different ways have to be adapted for the same general purpose. We
can, in short, see why nature is prodigal in variety, though niggard
in innovation. But why this should be a law of nature if each
species has been independently created no man can explain.
Many other facts are, as it seems to me, explicable on this
theory. How strange it is that a bird, under the form of a woodpecker,
should prey on insects on the ground; that upland geese which rarely
or never swim, should possess webbed feet; that a thrush-like bird
should dive and feed on sub-aquatic insects; and that a petrel
should have the habits and structure fitting it for the life of an
awk! and so in endless other cases. But on the view of each species
constantly trying to increase in number, with natural selection always
ready to adapt the slowly varying descendants of each to any
unoccupied or ill-occupied place in nature, these facts cease to be
strange, or might even have been anticipated.
We can to a certain extent understand how it is that there is so
much beauty throughout nature; for this may be largely attributed to
the agency of selection. That beauty, according to our sense of it, is
not universal, must be admitted by every one who will look at some
venomous snakes, at some fishes, and at certain hideous bats with a
distorted resemblance to the human face. Sexual selection has given
the most brilliant colours, elegant patterns, and other ornaments to
the males, and sometimes to both sexes of many birds, butterflies, and
other animals. With birds it has often rendered the voice of the
male musical to the female, as well as to our ears. Flowers and
fruit have been rendered conspicuous by brilliant colours in
contrast with the green foliage, in order that the flowers may be
readily seen, visited and fertilised by insects, and the seeds
disseminated by birds. How it comes that certain colours, sounds,
and forms should give pleasure to man and the lower animals,- that is,
that is, how the sense of beauty in its simplest form was first
acquired,- we do not know any more than how certain odours and
flavours were first rendered agreeable.
As natural selection acts by competition, it adapts and improves the
inhabitants of each country only in relation to their
co-inhabitants; so that we need feel no surprise at the species of any
one country, although on the ordinary view supposed to have been
created and specially adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as far. as we
can judge, absolutely perfect, as in the case even of the human eye;
or if some of them be abhorrent to our ideas of fitness. We need not
marvel at the sting of the bee, when used against an enemy, causing
the bee's own death; at drones being produced in such great numbers
for one single act, and being then slaughtered by their sterile
sisters; at the astonishing waste of pollen by our fir-trees; at the
instinctive hatred of the queen-bee for her own fertile daughters;
at the Ichneumonidae feeding within the living bodies of caterpillars;
or at other such cases. The wonder indeed is, on the theory of natural
selection, that more cases of the want of absolute perfection have
not been detected.
The complex and little known laws governing the production of
varieties are the same, as far as we can judge, with the laws which
have governed the production of distinct species. In both cases
physical conditions seem to have produced some direct and definite
effect, but how much we cannot say. Thus, when varieties enter any new
station, they occasionally assume some of the characters proper to the
species of that station. With both varieties and species, use and
disuse seem to have produced a considerable effect; for it is
impossible to resist this conclusion when we look, for instance, at
the logger-headed duck, which has wings incapable of flight, in nearly
the same condition as in the domestic duck; or when we look at the
burrowing tucu-tucu, which is occasionally blind, and then at
certain moles, which are habitually blind and have their eyes
covered with skin; or when we look at the blind animals inhabiting the
dark caves of America and Europe. With varieties and species,
correlated variation seems to have played an important part, so that
when one part has been modified other parts have been necessarily
modified. With both parties and.species, reversions to long-lost
characters occasionally occur. How inexplicable on the theory of
creation is the occasional appearance of stripes on the shoulders
and legs of the several species of the horse-genus and of their
hybrids! How simply is this fact explained if we believe that these
species are all descended from a striped progenitor, in the same
manner as the several domestic breeds of the pigeon are descended
from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently
created, why should specific characters, or those by which the species
of the same genus differ from each other, be more variable than
generic characters in which they all agree? Why, for instance,
should the colour of a flower be more likely to vary in any one
species of a genus, if the other species possess differently
coloured flowers, than if all possessed the same coloured flowers?
If species are only well-marked varieties, of which the characters
have become in a high degree permanent, we can understand this fact;
for they have already varied since they branched off from a common
progenitor in certain characters, by which they have come to be
specifically distinct from each other; therefore these same characters
would be more likely again to vary than the generic characters which
have been inherited without change for an immense period. It is
inexplicable on the theory of creation why a part developed in a
very unusual manner in species alone of a genus, and therefore, as
we may naturally infer, of great importance to that species, should be
eminently liable to variation; but, on our view, this part has
undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and
therefore we might expect the part generally to be still variable. But
a part may be developed in the most unusual manner, like the wing of a
bat, and yet not be more variable than any other structure, if the
part be common to many subordinate forms, that is, if it has been
inherited for a very long period; for in this case, it will have
been rendered constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural
selection of successive slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation
throws on the admirable architectural powers of the hive-bee. Habit no
doubt often comes into play in modifying instincts; but it certainly
is not indispensable, as we see in the case of neuter insects, which
leave no progeny to inherit the effects of long-continued habit. On
the view of all the species of the same genus having descended from
a common parent, and having inherited much in common, we can
understand how it is that allied species, when placed under widely
different conditions of life, yet follow nearly the same instincts;
why the thrushes of tropical and temperate South America, for
instance, line their nests with mud like our British species. On the
view of instincts having been slowly acquired through natural
selection, we need not marvel at some instincts being not perfect
and liable to mistakes, and at many instincts causing other animals to
suffer.
If species be only well-marked and permanent varieties, we can at
once see why their crossed offspring should follow the same complex
laws in their degrees and kinds of resemblance to their parents,- in
being absorbed into each other by successive crosses, and in other
such points,- as do the crossed offspring of acknowledged varieties.
This similarity would be a strange fact, if species had been
independently created and varieties had been produced through
secondary laws.
If we admit that the geological record is imperfect to an extreme
degree, then the facts, which the record does give, strongly support
the theory of descent with modification. New species have come on
the stage slowly and at successive intervals; and the amount of
change, after equal intervals of time, is widely different in
different groups. The extinction of species and of whole groups of
species which has played so conspicuous a part in the history of the
organic world almost inevitably follows from the principle of
natural selection; for old forms are supplanted by new and improved
forms. Neither single species nor groups of species reappear when
the chain of ordinary generation is once broken. The gradual diffusion
of dominant forms, with the slow modification of their descendants,
causes the forms of life, after long intervals of time, to appear as
if they had changed simultaneously throughout the world. The fact of
the fossil remains of each formation being in some degree
intermediate in character between the fossils in the formations
above and below, is simply explained by their intermediate position in
the chain of descent. The grand fact that all extinct beings can be
classed with all recent beings, naturally follows from the living
and the extinct being the offspring of common parents. As species have
generally diverged in character during their long course of descent
and modification, we can understand why it is that the more ancient
forms, or early progenitors of each group, so often occupy a
position in some degree intermediate between existing groups. Recent
forms are generally looked upon as being, on the whole, higher in
the scale of organisation than ancient forms; and they must be higher,
insofar as the later and more improved forms have conquered the
older and less improved forms in the struggle for life; they have also
generally had their organs more specialised for different functions.
This fact is perfectly compatible with numerous beings still retaining
simple and but little improved structures, fitted for simple
conditions of life; it is likewise compatible with some forms having
retrograded in organisation, by having become at each stage of descent
better fitted for new and degraded habits of life. Lastly, the
wonderful law of the long endurance of allied forms on the same
continent,- of marsupials in Australia, of Edentata in America, and
other such cases,- is intelligible, for within the same country the
existing and the extinct will be closely allied by descent.
Looking to geographical distribution, if we admit that there has
been during the long course of ages much migration from one part of
the world to another, owing to former climatal and geographical
changes and to the many occasional and unknown means of dispersal,
then we can understand, on the theory of descent with modification,
most of the great leading facts in Distribution. We can see why
there should be so striking a parallelism in the distribution of
organic beings throughout space, and in their geological succession
throughout time; for in both cases the beings have been connected by
the bond of ordinary generation, and the means of modification have
been the same. We see the full meaning of the wonderful fact, which
has struck every traveller, namely, that on the same continent,
under the most diverse conditions, under heat and cold, on mountain
and lowland, on deserts and marshes, most of the inhabitants within
each great class are plainly related; for they are the descendants
of the same progenitors and early colonists. On this same principle of
former migration, combined in most cases with modification, we can
understand, by the aid of the Glacial period, the identity of some few
plants, and the close alliance of many others, on the most distant
mountains, and in the northern and southern temperate zones; and
likewise the close alliance of some of the inhabitants of the sea in
the northern and southern temperate latitudes, though separated by the
whole intertropical ocean. Although two countries may present physical
conditions as closely similar as the same species ever require, we
need feel no surprise at their inhabitants being widely different,
if they have been for a long period completely sundered from each
other; for as the relation of organism to organism is the most
important of all relations, and as the two countries will have
received colonists at various periods and in different proportions,
from some other country or from each other, the course of modification
in the two areas will inevitably have been different.
On this view of migration, with subsequent modification, we see
why oceanic islands are inhabited by only few species, but of these,
why many are peculiar or endemic forms. We clearly see why species
belonging to those groups of animals which cannot cross wide spaces of
the ocean, as frogs and terrestrial mammals, do not inhabit oceanic
islands; and why, on the other hand, new and peculiar species of
bats, animals which can traverse the ocean, are found on islands far
distant from any continent. Such cases as the presence of peculiar
species of bats on oceanic islands and the absence of all other
terrestrial mammals, are facts utterly inexplicable on the theory of
independent acts of creation.
The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the
same parent-forms formerly inhabited both areas; and we almost
invariably find that wherever many closely allied species inhabit
two areas, some identical species are still common to both. Wherever
many closely allied yet distinct species occur, doubtful forms and
varieties belonging to the same groups likewise occur. It is a rule of
high generality that the inhabitants of each area are related to the
inhabitants of the nearest source whence immigrants might have been
derived. We see this in the striking relation of nearly all plants and
animals of the Galapagos Archipelago, of Juan Fernandez, and of the
other American islands, to the plants and animals of the
neighbouring American mainland; and of those of the Cape de Verde
Archipelago, and of the other African islands to the African mainland.
It must be admitted that these facts receive no explanation on the
theory of creation.
The fact, as we have seen, that all past and present organic
beings can be arranged within a few great classes, in groups
subordinate to groups, and with the extinct groups often falling in
between the recent groups, is intelligible on the theory of natural
selection with its contingencies of extinction and divergence of
character. On these same principles we see how it is, that the
mutual affinities of the forms within each class are so complex and
circuitous. We see why certain characters are far more serviceable
than others for classification;- why adaptive characters, though of
paramount importance to the beings, are of hardly any importance in
classification; why characters derived from rudimentary parts,
though of no service to the beings, are often of high classificatory
value; and why embryological characters are often the most valuable of
all. The real affinities of all organic beings, in contradistinction
to their adaptive resemblances, are due to inheritance or community of
descent. The Natural System is a genealogical arrangement, with the
acquired grades of difference, marked by the terms, varieties,
species, genera, families, &c.; and we have to discover the lines of
descent by the most permanent characters whatever they may be and of
however slight vital importance.
The similar framework of bones in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse,- the same number of
vertebrae forming the neck of the giraffe and of the elephant,- and
innumerable other such facts, at once explain themselves on the theory
of descent with slow and slight successive modifications. The
similarity of pattern in the wing and in the leg of a bat, though used
for such different purpose,- in the jaws and legs of a crab,- in the
petals, stamens, and pistils of a flower, is likewise, to a large
extent, intelligible on the view of the gradual modification of
parts or organs, which were aboriginally alike in an early
progenitor in each of these classes. On the principle of successive
variations not always supervening at an early age, and being inherited
at a corresponding not early period of life, we clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
similar, and so unlike the adult forms. We may cease marvelling at the
embryo of an air-breathing mammal or bird having branchial slits and
arteries running in loops, like those of a fish which has to breathe
the air dissolved in water by the aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often have
reduced organs when rendered useless under changed habits or
conditions of life; and we can understand on this view the meaning
of rudimentary organs. But disuse and selection will generally act
on each creature, when it has come to maturity and has to play its
full part in the struggle for existence, and will thus have little
power on an organ during early life; hence the organ will not be
reduced or rendered rudimentary at this early age. The calf, for
instance, has inherited teeth, which never cut through the gums of the
upper jaw, from an early progenitor having well-developed teeth; and
we may believe, that the teeth in the mature animal were formerly
reduced by disuse, owing to the tongue and palate, or lips, having
become excellently fitted through natural selection to browse
without their aid; whereas in the calf, the teeth have been left
unaffected, and on the principle of inheritance at corresponding
ages have been inherited from a remote period to the present day. On
the view of each organism with all its separate parts having been
specially created, how utterly inexplicable is it that organs
bearing the plain stamp of inutility, such as the teeth in the
embryonic calf or the shrivelled wings under the soldered wingcovers
of many beetles, should so frequently occur. Nature may be said to
have taken pains to reveal her scheme of modification, by means of
rudimentary organs, of embryological and homologous structures, but we
are too blind to understand her meaning.
I have now recapitulated the facts and considerations which have
thoroughly convinced me that species have been modified, during a long
course of descent. This has been effected chiefly through the
natural selection of numerous successive, slight, favourable
variations; aided in an important manner by the inherited effects of
the use and disuse of parts; and in an unimportant manner, that is
in relation to adaptive structures, whether past or present, by the
direct action of external conditions, and by variations which seem
to us in our ignorance to arise spontaneously. It appears that I
formerly underrated the frequency and value of these latter forms of
variation, as leading to permanent modifications of structure
independently of natural selection. But as my conclusions have
lately been much misrepresented, and it has been stated that I
attribute the modification of species exclusively to natural
selection, I may be permitted to remark that in the first edition of
this work, and subsequently, I placed in a most conspicuous
position- namely, at the close of the Introduction- the following
words: "I am convinced that natural selection has been the main but
not the exclusive means of modification." This has been of no avail.
Great is the power of steady misrepresentation; but the history of
science shows that fortunately this power does not long endure.
It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the
several large classes of facts above specified. It has recently been
objected that this is an unsafe method of arguing; but it is a
method used in judging of the common events of life, and has often
been used by the greatest natural philosophers. The undulatory
theory of light has thus been arrived at; and the belief in the
revolution of the earth on its own axis was until lately supported
by hardly any direct evidence. It is no valid objection that science
as yet throws no light on the far higher problem of the essence or
origin of life. Who can explain what is the essence of the
attraction of gravity? No one now objects to following out the
results consequent on this unknown element of attraction;
notwithstanding that Leibnitz formerly accused Newton of introducing
"occult qualities and miracles into philosophy."
I see no good reason why the views given in this volume should shock
the religious feelings of any one. It is satisfactory, as showing
how transient such impressions are, to remember that the greatest
discovery ever made by man, namely, the law of the attraction of
gravity, was also attacked by Leibnitz, "as subversive of natural, and
inferentially of revealed, religion." A celebrated author and divine
has written to me that "he has gradually learnt to see that it is
just as noble a conception of the Deity to believe that He created a
few original forms capable of self-development into other and
needful forms, as to believe that He required a fresh act of
creation to supply the voids caused by the action of His laws."
Why, it may be asked, until recently did nearly all the most eminent
living naturalists and geologists disbelieve in the mutability of
species? It cannot be asserted that organic beings in a state of
nature are subject to no variation; it cannot be proved that the
amount of variation in the course of long ages is a limited quality;
no clear distinction has been, or can be, drawn between species and
well-marked varieties. It cannot be maintained that species when
intercrossed are invariably sterile, and varieties invariably fertile;
or that sterility is a special endowment and sign of creation. The
belief that species were immutable productions was almost
unavoidable as long as the history of the world was thought to be of
short duration; and now that we have acquired some idea of the lapse
of time, we are too apt to assume, without proof, that the
geological record is so perfect that it would have afforded us plain
evidence of the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one
species has given birth to clear and distinct species, is that we
are always slow in admitting great changes of which we do not see
the steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the agencies which we
see still at work. The mind cannot possibly grasp the full meaning
of the term of even a million years; it cannot add up and perceive the
full effects of many slight variations, accumulated during an almost
infinite number of generations.
Although I am fully convinced of the truth of the views given in
this volume under the form of an abstract, I by no means expect to
convince experienced naturalists whose minds are stocked with a
multitude of facts all viewed, during a long course of years, from a
point of view directly opposite to mine. It is so easy to hide our
ignorance under such expressions as the "plan of creation" or "unity
of design," &c., and to think that we give an explanation when we only
restate a fact. Any one whose disposition leads him to attach more
weight to unexplained difficulties than to the explanation of a
certain number of facts will certainly reject the theory. A few
naturalists, endowed with much flexibility of mind, and who have
already begun to doubt the immutability of species, may be
influenced by this volume; but I look with confidence to the
future,- to young and rising naturalists, who will be able to view
both sides of the question with impartiality. Whoever is led to
believe that species are mutable will do good service by
conscientiously expressing his conviction; for thus only can the
load of prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that
a multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently
created. This seems to me a strange conclusion to arrive at. They
admit that a multitude of forms, which till lately they themselves
thought were special creations, and which are still thus looked at
by the majority of naturalists, and which consequently have all the
external characteristic features of true species,- they admit that
these have been produced by variation, but they refuse to extend the
same view to other and slightly different forms. Nevertheless they
do not pretend that they can define, or even conjecture, which are the
created forms of life, and which are those produced by secondary laws.
They admit variation as a vera causa in one case, they arbitrarily
reject it in another, without assigning any distinction in the two
cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors
seem no more startled at a miraculous act of creation than at an
ordinary birth. But do they really believe that at innumerable
periods in the earth's history certain elemental atoms have been
commanded suddenly to flash into living tissues? Do they believe
that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants
created as eggs or seed, or as full grown? and in the case of mammals,
were they created bearing the false marks of nourishment from the
mother's womb? Undoubtedly some of these same questions cannot be
answered by those who believe in the appearance or creation of only
a few forms of life, or of some one form alone. It has been maintained
by several authors that it is as easy to believe in the creation of
a million beings as of one; but Maupertuis' philosophical axiom "of
least action" leads the mind more willingly to admit the smaller
number; and certainly we ought not to believe that innumerable
beings within each great class have been created with plain, but
deceptive, marks of descent from a single parent.
As a record of a former state of things, I have retained in the
foregoing paragraphs, and elsewhere, several sentences which imply
that naturalists believe in the separate creation of each species; and
I have been much censured for having thus expressed myself. But
undoubtedly this was the general belief when the first edition of
the present work appeared. I formerly spoke to very many naturalists
on the subject of evolution, and never once met with any sympathetic
agreement. It is probable that some did then believe in evolution, but
they were either silent, or expressed themselves so ambiguously that
it was not easy to understand their meaning. Now things are wholly
changed, and almost every naturalist admits the great principle of
evolution. There are, however, some who still think that species
have suddenly given birth, through quite unexplained means, to new and
totally different forms: but, as I have attempted to show, weighty
evidence can be opposed to the admission of great and abrupt
modifications. Under a scientific point of view, and as leading to
further investigation, but little advantage is gained by believing
that new forms are suddenly developed in an inexplicable manner from
old and widely different forms, over the old belief in the creation of
species from the dust of the earth.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more
distinct the forms are which we consider, by so much the arguments
in favour of community of descent become fewer in number and less in
force. But some arguments of the greatest weight extend very far.
All the members of whole classes are connected together by a chain
of affinities, and all can be classed on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very
wide intervals between existing orders.
Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed condition; and this in
some cases implies an enormous amount of modification in the
descendants. Throughout whole classes various structures are formed on
the same pattern, and at a very early age the embryos closely
resemble each other. Therefore I cannot doubt that the theory of
descent with modification embraces all the members of the same great
class or kingdom. I believe that animals are descended from at most
only four or five progenitors, and plants from an equal or lesser
number.
Analogy would lead me one step farther, namely, to the belief that
all animals and plants are descended from some one prototype. But
analogy may be a deceitful guide. Nevertheless all living things
have much in common, in their chemical composition, their cellular
structure, their laws of growth, and their liability to injurious
influences. We see this even in so trifling a fact as that the same
poison often similarly affects plants and animals; or that the
poison secreted by the gallfly produces monstrous growths on the
wild rose or oak-tree. With all organic beings excepting perhaps
some of the very lowest, sexual production seems to be essentially
similar. With all, as far as is at present known the germinal
vesicle is the same; so that all organisms start from a common origin.
If we look even to the two main divisions- namely, to the animal and
vegetable kingdoms- certain low forms are so far intermediate in
character that naturalists have disputed to which kingdom they
should be referred. As Professor Asa Gray has remarked, "The spores
and other reproductive bodies of many of the lower algae may claim
to have first a characteristically animal, and then an unequivocally
vegetable existence." Therefore, on the principle of natural selection
with divergence of character, it does not seem incredible that, from
such low and intermediate form, both animals and plants may have
been developed; and, if we admit this, we must likewise admit that all
the organic beings which have ever lived on this earth may be
descended from some one primordial form. But this inference is chiefly
grounded on analogy and it is immaterial whether or not it be
accepted. No doubt it is possible, as Mr. G. H. Lewes has urged,
that at the first commencement of life many different forms were
evolved; but if so we may conclude that only a very few have left
modified descendants. For, as I have recently remarked in regard to
the members of each great kingdom, such as the Vertebrata, Articulata,
&c., we have distinct evidence in their embryological homologous and
rudimentary structures that within each kingdom all the members are
descended from a single progenitor.
When the views advanced by me in this volume, and by Mr. Wallace, or
when analogous views on the origin of species are generally
admitted, we can dimly foresee that there will be a considerable
revolution in natural history. Systematists will be able to pursue
their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be a true
species. This, I feel sure and I speak after experience, will be no
slight relief. The endless disputes whether or not some fifty
species of British brambles are good species will cease.
Systematists will have only to decide (not that this will be easy)
whether any form be sufficiently constant and distinct from other
forms, to be capable of definition; and if definable, whether the
differences be sufficiently important to deserve a specific name.
This latter point will become a far more essential consideration
than it is at present; for differences, however slight, between any
two forms if not blended by intermediate gradations, are looked at
by most naturalists as sufficient to raise both forms to the rank of
species.
Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed, to be connected at the present day by
intermediate gradations, whereas species were formerly thus connected.
Hence, without rejecting the consideration of the present existence of
intermediate gradations between any two forms we shall be led to
weigh more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names; and in this case scientific and common language will
come into accordance. In short, we shall have to treat species in
the same manner as those naturalists treat genera, who admit that
genera are merely artificial combinations made for convenience. This
may not be a cheering prospect; but we shall at least be free from the
vain search for the undiscovered and undiscoverable essence of the
term species.
The other and more general departments of natural history will
rise greatly in interest. The terms used by naturalists, of
affinity, relationship, community of type, paternity, morphology,
adaptive characters, rudimentary and aborted organs, &c., will cease
to be metaphorical, and will have a plain signification. When we no
longer look at an organic being as a savage looks at a ship, as
something wholly beyond his comprehension; when we regard every
production of nature as one which has had a long history; when we
contemplate every complex structure and instinct as the summing up
of many contrivances, each useful to the possessor, in the same way as
any great mechanical invention is the summing up of the labour, the
experience, the reason, and even the blunders of numerous workmen;
when we thus view each organic being, how far more interesting- I
speak from experience- does the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation, on the effects of use
and disuse, on the direct action of external conditions, and so forth.
The study of domestic productions will rise immensely in value. A
new variety raised by man will be a more important and interesting
subject for study than one more species added to the infinitude of
already recorded species. Our classifications will come to be, as
far as they can be so made, genealogies; and will then truly give what
may be called the plan of creation. The rules for classifying will
no doubt become simpler when we have a definite object in view. We
possess no pedigrees or armorial bearings; and we have to discover
and trace the many diverging lines of descent in our natural
genealogies, by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to the nature of
long-lost structures. Species and groups of species which are called
aberrant, and which may fancifully be called living fossils, will
aid us in forming a picture of the ancient forms of life. Embryology
will often reveal to us the structure, in some degree obscured, of the
prototype of each great class.
When we feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not
very remote period descended from one parent, and have migrated from
some one birth-place; and when we better know the many means of
migration, then, by the light which geology now throws, and will
continue to throw, on former changes of climate and of the level of
the land, we shall surely be enabled to trace in an admirable manner
the former migrations of the inhabitants of the whole world. Even at
present, by comparing the differences between the inhabitants of the
sea on the opposite sides of a continent, and the nature of the
various inhabitants on that continent, in relation to their apparent
means of immigration, some light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme
imperfection of the record. The crust of the earth with its imbedded
remains must not be looked at as a well-filled museum, but as a poor
collection made at hazard and at rare intervals. The accumulation of
each great fossiliferous formation will be recognised as having
depended on an unusual concurrence of favourable circumstances, and
the blank intervals between the successive stages as having been of
vast duration. But we shall be able to gauge with some security the
duration of these intervals by a comparison of the preceding and
succeeding organic forms. We must be cautious in attempting to
correlate as strictly contemporaneous two formations, which do not
include many identical species, by the general succession of the forms
of life. As species are produced and exterminated by slowly acting and
still existing causes, and not by miraculous acts of creation; and
as the most important of all causes of organic change is one which
is almost independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism,-
the improvement of one organism entailing the improvement or the
extermination of others; it follows, that the amount of organic change
in the fossils of consecutive formations probably serves as a fair
measure of the relative though not actual lapse of time. A number of
species, however, keeping in a body might remain for a long period
unchanged, whilst within the same period several of these species by
migrating into new countries and coming into competition with
foreign associates, might become modified; so that we must not
overrate the accuracy of organic change as a measure of time.
In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well
laid by Mr. Herbert Spencer, that of the necessary acquirement of each
mental power and capacity by gradation. Much light will be thrown on
the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with
the view that each species has been independently created. To my
mind it accords better with what we know of the laws impressed on
matter by the Creator, that the production and extinction of the
past and present inhabitants of the world should have been due to
secondary causes, like those determining the birth and death of the
individual. When I view all beings not as special creations, but as
the lineal descendants of some few beings which lived long before
the first bed of the Cambrian system was deposited, they seem to me to
become ennobled. Judging from the past, we may safely infer that not
one living species will transmit its unaltered likeness to a distant
futurity. And of the species now living very few will transmit progeny
of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species
in each genus, and all the species in many genera, have left no
descendants, but have become utterly extinct. We can so far take a
prophetic glance into futurity as to foretell that it will be the
common and widely-spread species, belonging to the larger and dominant
groups within each class, which will ultimately prevail and
procreate new and dominant species. As all the living forms of life
are the lineal descendants of those which lived long before the
Cambrian epoch, we may feel certain that the ordinary succession by
generation has never once been broken, and that no cataclysm has
desolated the whole world. Hence we may look with some confidence to
secure future of great length. And as natural selection works solely
by and for the good of each being, all corporeal and mental endowments
will tend to progress towards perfection.
It is interesting to contemplate a tangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp
earth, and to reflect that these elaborately constructed forms, so
different from each other, and dependent upon each other in so complex
a manner, have all been produced by laws acting around us. These laws,
taken in the largest sense, being Growth with Reproduction;
Inheritance which is almost implied by reproduction; Variability
from the indirect and direct action of the conditions of life and from
use and disuse: a Ratio of Increase so high as to lead to a Struggle
for Life, and as a consequence to Natural Selection, entailing
Divergence of Character and the Extinction of less-improved forms.
Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is
grandeur in this view of life, with its several powers, having been
originally breathed by the Creator into a few forms or into one; and
that, whilst this planet has gone cycling on according to the fixed
law of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being evolved.
THE END
GLOSSARY
GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT
VOLUME
I am indebted to the kindness of Mr. W. S. Dallas for this Glossary,
which has been given because several readers have complained to me
that some of the terms used were unintelligible to them. Mr. Dallas
has endeavoured to give the explanations of the terms in as popular
a form as possible.
ABERRANT, Forms or groups of animals or plants which deviate in
important characters from their nearest allies, so as not to be easily
included in the same group with them, are said to be aberrant.
ABERRATION (in Optics), In the refraction of light by a convex
lens the rays passing through different parts of the lens are
brought to a focus at slightly different distances,- this is called
spherical aberration; at the same time the coloured rays are separated
by the prismatic action of the lens and likewise brought to a focus at
different distances, this is chromatic aberration.
ABNORMAL, Contrary to the general rule.
ABORTED, An organ is said to be aborted, when its development has
been arrested at a very early stage.
ALBINISM, Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and
its appendages. Albinism is the state of being an albino.
ALGAE, A class of plants including the ordinary seaweeds and the
filamentous fresh-water weeds.
ALTERNATION OF GENERATIONS, This term is applied to a peculiar
mode of reproduction which prevails among many of the lower animals,
in which the egg produces a living form quite different from its
parent, but from which the parent-form is reproduced by a process of
budding, or by the division of the substance of the first product of
the egg.
AMMONITES, A group of fossil, spiral, chambered shells, allied to
the existing pearly nautilus, but having the partitions between the
chambers waved in complicated patterns at their junction with the
outer wall of the shell.
ANALOGY, That resemblance of structures which depends upon
similarity of function, as in the wings of insects and birds. Such
structures are said to be analogous, and to be analogues of each
other.
ANIMALCULE, A minute animal: generally applied to those visible only
by the microscope.
ANNELIDS, A class of worms in which the surface of the body exhibits
a more or less distinct division into rings or segments, generally
provided with appendages for locomotion and with gills. It includes
the ordinary marine worms, the earthworms, and the leeches.
ANTENNAE, Jointed organs appended to the head in insects,
Crustacea and centipedes, and not belonging to the mouth.
ANTHERS, The summits of the stamens of flowers, in which the
pollen or fertilising dust is produced.
APLACENTALIA, APLACENTATA or Aplacental Mammals. See MAMMALIA.
ARCHETYPAL, Of or belonging to the Archetype, or ideal primitive
form upon which all the beings of a group seem to be organised.
ARTICULATA, A great division of the animal kingdom characterised
generally by having the surface of the body divided into rings
called segments, a greater or less number of which are furnished
with jointed legs (such as insects, crustaceans and centipedes).
ASYMMETRICAL, Having the two sides unlike.
ATROPHIED, Arrested in development at a very early age.
BALANUS, The genus including the common acorn shells which live in
abundance on the rocks of the sea-coast.
BATRACRIANS, A class of animals allied to the reptiles, but
undergoing a peculiar metamorphosis, in which the young animal is
generally aquatic and breathes by gills. (Examples, frogs, toads,
and newts.)
BOULDERS, Large transported blocks of stone generally imbedded in
clays or gravel.
BRACHIOPODA, A class of marine Mollusca, or softbodied animals,
furnished with a bivalve shell, attached to submarine objects by a
stalk which passes through an aperture in one of the valves, and
furnished with fringed arms, by the action of which food is carried to
the mouth.
BRANCHIAE, Gills or organs for respiration in water.
BRANCHIAL, Pertaining to gills or branchiae.
CAMBRIAN SYSTEM, A series of very ancient Palaeozoic rocks, between
the Laurentian and the Silurian. Until recently these were regarded as
the oldest fossiliferous rocks.
CANIDAE, The dog-family, including the dog, wolf, fox, jackal, &c.
CARAPACE, The shell enveloping the anterior part of the body in
crustaceans generally; applied also to the hard shelly pieces of the
cirripedes.
CARBONIFEROUS, This term is applied to the great formation which
includes among other rocks, the coal-measures. It belongs to the
oldest, or Palaeozoic , system of formations.
CAUDAL, Of or belonging to the tail.
CEPHALOPODS, The highest class of the Molluscs or soft-bodied
animals, characterised by having the mouth surrounded by a greater
or less number of fleshy arms or tentacles, which, in most living
species, are furnished with sucking-cups. (Examples, cuttle-fish,
nautilus.)
CETACEA, An order of Mammalia, including the whales, dolphins,
&c., having the form of the body fish-like, the skin naked, and only
the forelimbs developed.
CHELONIA, An order of reptiles including the turtles, tortoises,
&c.
CIRRIPEDES, An order of crustaceans including the barnacles and
acorn-shells. Their young resemble those of many other crustaceans
in form; but when mature they are always attached to other objects,
either directly or by means of a stalk, and their bodies are
enclosed by a calcareous shell composed of several pieces, two of
which can open to give issue to a bunch of curled, jointed
tentacles, which represent the limbs.
COCCUS, The genus of insects including the cochineal. In these the
male is a minute, winged fly, and the female generally a motionless,
berry-like mass.
COCOON, A case usually of silky material, in which insects are
frequently enveloped during the second or resting-stage (pupa) of
their existence. The term "cocoon-stage" is here used as equivalent to
"pupa-stage."
COELOSPERMOUS, A term applied to those fruits of the Umbelliferae
which have the seed hollowed on the inner face.
COLEOPTERA beetles, an order of insects, having a biting mouth and
the first pair of wings more or less horny, forming sheaths for the
second pair, and usually meeting in a straight line down the middle of
the back.
COLUMN, A peculiar organ in the flowers of orchids, in which the
stamens, style and stigma (or the reproductive parts) are united.
COMPOSITAE, or COMPOSITOUS PLANTS, Plants in which the inflorescence
consists of numerous small flowers (florets) brought together into a
dense head, the base of which is enclosed by a common envelope.
(Examples, the daisy, dandelion, &c.)
CONFERVAE, The filamentous weeds of fresh water.
CONGLOMERATE, A rock made up of fragments of rock or pebbles,
cemented together by some other material.
COROLLA The second envelope of a flower usually composed of
coloured, leaf-like organs (petals), which may be united by their
edges either in the basal part or throughout.
CORRELATION, The normal coincidence of one phenomenon, character,
&c., with another.
CORYMB, A bunch of flowers in which those springing from the lower
part of the flower stalk are supported on long stalks so as to be
nearly on a level with the upper ones.
COTYLEDONS, The first or seed-leaves of plants.
CRUSTACEANS, A class of articulated animals, having the skin of
the body generally more or less hardened by the deposition of
calcareous matter, breathing by means of gills. (examples, crab,
lobster, shrimp, &c.)
CURCULIO, The old generic term for the beetles known as weevils,
characterised by their four-jointed feet, and by the head being
produced into a sort of beak, upon the sides of which the antennae are
inserted.
CUTANEOUS, Of or belonging to the skin.
DEGRADATION, The wearing down of land by the action of the sea or of
meteoric agencies.
DENUDATION, The wearing away of the surface of the land by water.
DEVONIAN SYSTEM or formation, A series of Palaeozoic rocks,
including the Old Red Sandstone.
DICOTYLEDONS, or DICOTYLEDONOUS PLANTS, A class of plants
characterised by having two seed-leaves, by the formation of new
wood between the bark and the old wood (exogenous growth) and by the
reticulation of the veins of the leaves. The parts of the flowers
are generally in multiples of five.
DIFFERENTATION, The separation or discrimination of parts or
organs which in simpler forms of life are more or less united.
DIMORPHIC, Having two distinct forms.- Dimorphism is the condition
of the appearance of the same species under two dissimilar forms.
DIOECIOUS, Having the organs of the sexes upon distinct individuals.
DIORITE, A peculiar form of greenstone.
DORSAL, Of or belonging to the back.
EDENTATA, A peculiar order of quadrupeds, characterised by the
absence of at least the middle incisor (front) teeth in both jaws.
(Examples, the sloths and armadillos.)
ELYTRA, The hardened fore-wings of beetles, serving as sheaths for
the membranous hind-wings, which constitute the true organs of flight.
EMBRYO, The young animal undergoing development within the egg or
womb.
EMBRYOLOGY, The study of the development of the embryo.
ENDEMIC, Peculiar to a given locality.
ENTOMOSTRACA, A division of the class Crustacea, having all the
segments of the body usually distinct, gills attached to the feet or
organs of the mouth, and the feet fringed with fine hairs. They are
generally of small size.
EOCENE, The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells
identical with species now living.
EPHEMEROUS INSECTS, Insects allied to the May-fly.
FAUNA, The totality of the animals naturally inhabiting a certain
country or region, or which have lived during a given geological
period.
FELIDAE, The cat-family.
FERAL, Having become wild from a state of cultivation or
domestication.
FLORA, The totality of the plants growing naturally in a country, or
during a given geological period.
FLORETS, Flowers imperfectly developed in some respects, and
collected into a dense spike or head, as in the grasses, the
dandelion, &c.
FOETAL, Of or belonging to the foetus, or embryo in course of
development.
FORAMINIFERA, A class of animals of very low organisation, and
generally of small size, having a jellylike body, from the surface
of which delicate filaments can be given off and retracted for the
prehension of external objects, and having a calcareous or sandy
shell, usually divided into chambers, and perforated with small
apertures.
FOSSILIFEROUS, Containing fossils.
FOSSORIAL, Having a faculty of digging. The Fossorial Hymenoptera
are a group of wasp-like insects, which burrow in sandy soil to make
nests for their young.
FRENUM (pl. FRENA), A small band or fold of skin.
FUNGI (sing. FUNGUS), A class of cellular plants, of which
mushrooms, toadstools, and moulds, are familiar examples.
FURCULA, The forked bone formed by the union of the collar-bones
in many birds, such as the common fowl.
GALLINACEOUS BIRDS, An order of birds of which the common fowl,
turkey, and pheasant, are well-known examples.
GALLUS, The genus of birds which includes the common fowl.
GANGLION, A swelling or knot from which nerves are given off as from
a centre.
GANOID FISHES, Fishes covered with peculiar enamelled bony scales.
Most of them are extinct.
GERMINAL VESICLE, A minute vesicle in the eggs of animals, from
which development of the embryo proceeds.
GLACIAL PERIOD, A period of great cold and of enormous extension
of ice upon the surface of the earth. It is believed that glacial
periods have occurred repeatedly during the geological history of
the earth, but the term is generally applied to the close of the
Tertiary epoch, when nearly the whole of Europe was subjected to an
arctic climate.
GLAND, An organ which secretes or separates some peculiar product
from the blood or sap of animals or plants.
GLOTTIS, The opening of the windpipe into the oesophagus or gullet.
GNEISS, A rock approaching granite in composition, but more or
less laminated, and really produced by the alteration of a sedimentary
deposit after its consolidation.
GRALLATORES, The so-called wading-birds (storks, cranes, snipes,
&c.), which are generally furnished with long legs, bare of feathers
above the heel, and have no membranes between the toes.
GRANITE, A rock consisting essentially of crystals of felspar and
mica in a mass of quartz.
HABITAT, The locality in which a plant or animal naturally lives.
HEMIPTERA, An order or sub-order of insects, characterised by the
possession of a jointed beak or rostrum, and by having the
fore-wings horny in the basal portion and membranous at the extremity,
where they cross each other. This group includes the various species
of bugs.
HERMAPHRODITE, Possessing the organs of both sexes.
HOMOLOGY, That relation between parts which results from their
development from corresponding embryonic parts, either in different
animals, as in the case of the arm of man, the fore-leg of a
quadruped, and the wing of a bird; or in the same individual, as in
the case of the fore and hind legs in quadrupeds, and the segments
or rings and their appendages of which the body of a worm, a
centipede, &c., is composed. The latter is called serial homology. The
parts which stand in such a relation to each other are said to be
homologous, and one such part or organ is called the homologue of
the other. In different plants the parts of the flower are homologous,
and in general these parts are regarded as homologous with leaves.
HOMOPTERA, An order or sub-order of insects having (like the
Hemiptera) a jointed beak, but in which the fore-wings are either
wholly membranous or wholly leathery, The Cicadae, frog-hoppers, and
Aphides, are well-known examples.
HYBRID, The offspring of the union of two distinct species.
HYMENOPTERA, An order of insects possessing biting jaws and
usually four membranous wings in which there are a few veins. Bees and
wasps are familiar examples of this group.
HYPERTROPHIED, Excessively developed.
ICHNEUMONIDAE, A family of hymenopterous insects, the members of
which lay their eggs in the bodies or eggs of other insects.
IMAGO, The perfect (generally winged) reproductive state of an
insect.
INDIGENES, The aboriginal animal or vegetable inhabitants of a
country or region.
INFLORESCENCE, The mode of arrangement of the flowers of plants.
INFUSORIA, A class of microscopic animalcules, so called from
their having originally been observed in infusions of vegetable
matters. They consist of a gelatinous material enclosed in a
delicate membrane, the whole or part of which is furnished with
short vibrating hairs (called cilia), by means of which the
animalcules swim through the water or convey the minute particles of
their food to the orifice of the mouth.
INSECTIVOROUS, Feeding on insects.
INVERTEBRATA, or INVERTEBRATE ANIMALS, Those animals which do not
possess a backbone or spinal column.
LACUNAE, Spaces left among the tissues in some of the lower
animals and serving in place of vessels for the circulation of the
fluids of the body.
LAMELLATED, Furnished with lamellae or little plates.
LARVA (pl. LARVAE), The first condition of an insect at its
issuing from the egg, when it is usually in the form of a grub,
caterpillar, or maggot.
LARYNX, The upper part of the windpipe opening into the gullet.
LAURENTIAN, A group of greatly altered and very ancient rocks, which
is greatly developed along the course of the St. Laurence, whence
the name. It is in these that the earliest known traces of organic
bodies have been found.
LEGUMINOSAE, An order of plants represented by the common peas and
beans, having an irregular flower in which one petal stands up like
a wing, and the stamens and pistil are enclosed in a sheath formed
by two other petals. The fruit is a pod (or legume).
LEMURIDAE, A group of four-handed animals, distinct from the monkeys
and approaching the insectivorous quadrupeds in some of their
characters and habits. Its members have the nostrils curved or
twisted, and a claw instead of a nail upon the first finger of the
hind hands.
LEPIDOPTERA, An order of insects, characterised by the possession of
a spiral proboscis, and of four large more or less scaly wings. It
includes the well-known butterflies and moths.
LITTORAL, Inhabiting the seashore.
LOESS, A marly deposit of recent (Post-Tertiary) date, which
occupies a great part of the valley of the Rhine.
MALACOSTRACA, The higher division of the Crustacea, including the
ordinary crabs, lobsters, shrimps, &c., together with the woodlice and
sand-hoppers.
MAMMALIA, The highest class of animals, including the ordinary hairy
quadrupeds, the whales and man, and characterised by the production of
living young which are nourished after birth by milk from the teats
(Mammae, Mammary glands) of the mother. A striking difference in
embryonic development has led to the division of this class into two
great groups; in one of these, when the embryo has attained a
certain stage, a vascular connection, called the placenta, is formed
between the embryo and the mother; in the other this is wanting, and
the young are produced in a very incomplete state. The former,
including the greater part of the class, are called Placental mammals;
the latter, or Aplacental mammals, include the marsupials and
monotremes (Ornithorhynchus).
MAMMIFEROUS, Having mammae or teats (see MAMMALIA)
MANDIBLES, in insects, the first or uppermost pair of jaws, which
are generally solid, horny, biting organs. In birds the term is
applied to both jaws with their horny coverings. In quadrupeds the
mandible is properly the lower jaw.
MARSUPIALS, An order of Mammalia in which the young are born in a
very incomplete state of development, and carried by the mother, while
sucking, in a ventral pouch (marsupium), such as the kangaroos,
opossums, &c. (see MAMMALIA).
MAXILLAE, in insects, the second or lower pair of jaws, which are
composed of several joints and furnished with peculiar jointed
appendages called palpi, or feelers.
MELANISM, The opposite of albinism; an undue development of
colouring material in the skin and its appendages.
METAMORPHIC ROCKS, Sedimentary rocks which have undergone
alteration, generally by the action of heat, subsequently to their
deposition and consolidation.
MOLLUSCA, One of the great divisions of the animal kingdom,
including those animals which have a soft body, usually furnished with
a shell, and in which the nervous ganglia, or centres, present no
definite general arrangement. They are generally known under the
denomination of "shellfish"; the cuttle-fish, and the common snails,
whelks, oysters, mussels, and cockles, may serve as examples of them.
MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS, Plants in which the seed
sends up only a single seed-leaf (or cotyledon); characterised by
the absence of consecutive layers of wood in the stem (endogenous
growth), by the veins of the leaves being generally straight, and by
the parts of the flowers being generally in multiples of three.
(Examples, grasses, lilies, orchids, palms, &c.)
MORAINES, The accumulations of fragments of rock brought down by
glaciers.
MORPHOLOGY, The law of form or structure independent of function.
MYSIS-STAGE, A stage in the development of certain crustaceans
(prawns), in which they closely resemble the adults of a genus (Mysis)
belonging to a slightly lower group.
NASCENT, Commencing development.
NATATORY, Adapted for the purpose of swimming.
NAUPLIUS-FORM, The earliest stage in the development of many
Crustacea, especially belonging to the lower groups. In this stage the
animal has a short body, with indistinct indications of a division
into segments, and three pairs of fringed limbs. This form of the
common fresh-water cyclops was described as a distinct genus under the
name of Nauplius.
NEURATION, The arrangement of the veins or nervures in the wings
of insects.
NEUTERS, Imperfectly developed females of certain social insects
(such as ants and bees), which perform all the labours of the
community. Hence, they are also called workers.
NICTITATING MEMBRANE, A semi-transparent membrane, which can be
drawn across the eye in birds and reptiles, either to moderate the
effects of a strong light or to sweep particles of dust, &c., from the
surface of the eye.
OCELLI, The simple eyes or stemmata of insects, usually situated
on the crown of the head between the great compound eyes.
OESOPHAGUS, The gullet.
OOLITIC, A great series of secondary rocks, so called from the
texture of some of its members, which appear to be made up of a mass
of small egg-like calcareous bodies.
OPERCULUM, A calcareous plate employed by many Molluscae to close
the aperture of their shell. The opercular valves of cirripedes are
those which close the aperture of the shell.
ORBIT, The bony cavity for the reception of the eye.
ORGANISM, An organised being, whether plant or animal.
ORTHOSPERMOUS, A term applied to those fruits of the Umbelliferae
which have the seed straight.
OSCULANT, Forms or groups apparently intermediate between and
connecting other groups are said to be osculant.
OVA, Eggs.
OVARIUM or OVARY (in plants), The lower part of the pistil or female
organ of the flower, containing the ovules or incipient seeds; by
growth after the other organs of the flower have fallen, it usually
becomes converted into the fruit.
OVIGEROUS, Egg-bearing.
OVULES (of plants), The seeds in the earliest condition.
PACHYDERMS, A group of Mammalia, so called from their thick skins,
and including the elephant, rhinoceros, hippopotamus, &c.
PALAEOZOIC, The oldest system of fossiliferous rocks.
PALPI, Jointed appendages to some of the organs of the mouth in
insects and Crustacea.
PAPILIONACEAE, An order of plants (see LEGUMINOSAE), The flowers
of these plants are called papilionaceous, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings of
a butterfly.
PARASITE, An animal or plant living upon or in, and at the expense
of, another organism.
PARTHENOGENESIS, The production of living organisms from
unimpregnated eggs or seeds.
PEDUNCULATED, Supported upon a stem or stalk. The pedunculated oak
has its acorns borne upon a footstool.
PELORIA or PELORISM, The appearance of regularity of structure in
the flowers of plants which normally bear irregular flowers.
PELVIS, The bony arch to which the hind limbs of vertebrate
animals are articulated.
PETALS, The leaves of the corolla, or second circle of organs in a
flower. They are usually of delicate texture and brightly coloured.
PHYLLODINEOUS, Having flattened, leaf-like twigs or leafstalks
instead of true leaves.
PIGMENT, The colouring material produced generally in the
superficial parts of animals. The cells secreting it are called
pigment-cells.
PINNATE, Bearing leaflets on each side of a central stalk.
PISTILS, The female organs of a flower, which occupy a position in
the centre of the other floral organs. The pistil is generally
divisible into the ovary or germen, the style and the stigma.
PLACENTALIA, PLACENTATA, or Placental Mammals, See MAMMALIA.
PLANTIGRADES, Quadrupeds which walk upon the whole sole of the foot,
like the bears.
PLASTIC, Readily capable of change.
PLEISTOCENE PERIOD, The latest portion of the Tertiary epoch.
PLUMULE (in plants), The minute bud between the seed-leaves of
newly-germinated plants.
PLUTONIC ROCKS, Rocks supposed to have been produced by igneous
action in the depths of the earth.
POLLEN, The male element in flowering plants; usually a fine dust
produced by the anthers, which, by contact with the stigma effects the
fecundation of the seeds. This impregnation is brought about by
means of tubes (pollen-tubes) which issue from the pollen-grains
adhering to the stigma, and penetrate through the tissues until they
reach the ovary.
POLYANDROUS (flowers), Flowers having many stamens.
POLYGAMOUS PLANTS, Plants in which some flowers are unisexual and
others hermaphrodite. The unisexual (male and female) flowers, may
be on the same or on different plants.
POLYMORPHIC, Presenting many forms.
POLYZOARY, The common structure formed by the cells of the
Polyzoa, such as the well-known seamats.
PREHENSILE, Capable of grasping.
PREPOTENT, Having a superiority of power.
PRIMARIES, The feathers forming the tip of the wing of a bird, and
inserted upon that part which represents the hand of man.
PROCESSES, Projecting portions of bones, usually for the
attachment of muscles, ligaments, &c.
PROPOLIS, A resinous material collected by the hivebees from the
opening buds of various trees.
PROTEAN, Exceedingly variable.
PROTOZOA, The lowest great division of the animal kingdom. These
animals are composed of a gelatinous material, and show scarcely any
trace of distinct organs. The Infusoria, Foraminifera, and sponges,
with some other forms, belong to this division.
PUPA (pl. Pupae), The second stage in the development of an
insect, from which it emerges in the perfect (winged) reproductive
form. In most insects the pupal stage is passed in perfect repose. The
chrysalis is the pupal state of butterflies.
RADICLE, The minute root of an embryo plant.
RAMUS, One half of the lower jaw in the Mammalia. The portion
which rises to articulate with the skull is called the ascending
ramus.
RANGE, The extent of country over which a plant or animal is
naturally spread. Range in time expresses the distribution of a
species or group through the fossiliferous beds of the earth's crust.
RETINA, The delicate inner coat of the eye, formed by nervous
filaments spreading from the optic nerve, and serving for the
perception of the impressions produced by light.
RETROGRESSION, Backward development. When an animal, as it
approaches maturity, becomes less perfectly organised than might be
expected from its early stages and known relationships, it is said
to undergo a retrograde development or metamorphosis.
RHIZOPODS, A class of lowly organised animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the form
of root-like processes or filaments, which serve for locomotion and
the prehension of food. The most important order is that of the
Foraminifera.
RODENTS, The gnawing Mammalia, such as the rats, rabbits, and
squirrels. They are especially characterised by the possession of a
single pair of chisel-like cutting teeth in each jaw, between which
and the grinding teeth there is a great gap.
RUBUS, The bramble genus.
RUDIMENTARY, Very imperfectly developed.
RUMINANTS, The group of quadrupeds which ruminate or chew the cud,
such as oxen, sheep, and deer. They have divided hoofs, and are
destitute of front teeth in the upper jaw.
SACRAL, Belonging to the sacrum, or the bone composed usually of two
or more united vertebrae to which the sides of the pelvis in
vertebrate animals are attached.
SARCODE, The gelatinous material of which the bodies of the lowest
animals (Protozoa) are composed.
SCUTELLAE The horny plates with which the feet of birds are
generally more or less covered, especially in front.
SEDIMENTARY FORMATIONS, Rocks deposited as sediments from water.
SEGMENTS, The transverse rings of which the body of an articulate
animal or annelid is composed.
SEPALS, The leaves or segments of the calyx, or outermost envelope
of an ordinary flower. They are usually green, but sometimes
brightly coloured.
SERRATURES, Teeth like those of a saw.
SESSILE, Not supported on a stem or footstalk.
SILURIAN SYSTEM, A very ancient system of fossiliferous rocks
belonging to the earlier part of the Palaeozoic series.
SPECIALISATION, The setting apart of a particular organ for the
performance of a particular function.
SPINAL CORD, The central portion of the nervous system in the
Vertebrata, which descends from the brain through the arches of the
vertebrae, and gives off nearly all the nerves to the various organs
of the body.
STAMENS, The male organs of flowering plants, standing in a circle
within the petals. They usually consist of a filament and an anther,
the anther being the essential part in which the pollen, or
fecundating dust, is formed.
STERNUM, The breast-bone.
STIGMA, The apical portion of the pistil in flowering plants.
STIPULES, Small leafy organs placed at the base of the footstalks of
the leaves in many plants.
STYLE, The middle portion of the perfect pistil, which rises like
a column from the ovary and supports the stigma at its summit.
SUBCUTANEOUS, Situated beneath the skin.
SUCTORIAL, Adapted for sucking.
SUTURES (in the skull), The lines of junction of the bones of
which the skull is composed.
TARSUS (pl. TARSI), The jointed feet of articulate animals, such
as insects.
TELEOSTEAN FISHES, Fishes of the kind familiar to us in the
present day, having the skeleton usually completely ossified and the
scales horny.
TENTACULA or TENTACLES, Delicate fleshy organs of prehension or
touch possessed by many of the lower animals.
TERTIARY, The latest geological epoch, immediately preceding the
establishment of the present order of things.
TRACHEA, The windpipe or passage for the admission of air to the
lungs.
TRIDACTYLE, Three-fingered, or composed of three movable parts
attached to a common base.
TRILOBITES, A peculiar group of extinct crustaceans, somewhat
resembling the woodlice in external form, and, like some of them,
capable of rolling themselves up into a ball. Their remains are
found only in the Palaeozoic rocks, and most abundantly in those of
Silurian age.
TRIMORPHIC, Presenting three distinct forms.
UMBELLIFERAE, An order of plants in which the flowers, which
contain five stamens and a pistil with two styles, are supported
upon footstalks which spring from the top of the flower stem and
spread out like the wires of an umbrella, so as to bring all the
flowers in the same head (umbel) nearly to the same level.
(Examples, parsley and carrot.)
UNGULATA, Hoofed quadrupeds.
UNICELLULAR, Consisting of a single cell.
VASCULAR, Containing blood-vessels.
VERMIFORM, Like a worm.
VERTEBRATA: or VERTEBRATE ANIMALS, The highest division of the
animal kingdom, so called from the presence in most cases of a
backbone composed of numerous joints or vertebrae, which constitutes
the centre of the skeleton and at the same time supports and
protects the central parts of the nervous system.
WHORLS, The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.
WORKERS, See neuters.
ZOEA-STAGE, The earliest stage in the development of many of the
higher Crustacea, so called from the name of Zoea applied to these
young animals when they were supposed to constitute a peculiar genus.
ZOOIDS, In many of the lower animals (such as the corals, Medusae,
&c.) reproduction takes place in two ways, namely, by means of eggs
and by a process of budding with or without separation from the parent
of the product of the latter, which is often very different from
that of the egg. The individuality of the species is represented by
the whole of the form produced between two sexual reproductions; and
these forms, which are apparently individual animals, have been called
zooide.
END GLOSSARY
