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Descent of Man [ 1871 ]

Charles Darwin [ 1809 - 1882 ]

 

Chapter XVI - Birds-Concluded


  WE must now consider the transmission of characters, as limited by
age, in reference to sexual selection. The truth and importance of the
principle of inheritance at corresponding ages need not here be
discussed, as enough has already been said on the subject. Before
giving the several rather complex rules or classes of cases, under
which the differences in plumage between the young and the old, as far
as known to me, may be included, it will be well to make a few
preliminary remarks.
  With animals of all kinds when the adults differ in colour from
the young, and the colours of the latter are not, as far as we can
see, of any special service, they may generally be attributed, like
various embryological structures, to the retention of a former
character. But this view can be maintained with confidence, only
when the young of several species resemble each other closely, and
likewise resemble other adult species belonging to the same group; for
the latter are the living proofs that such a state of things was
formerly possible. Young lions and pumas are marked with feeble
stripes or rows of spots, and as many allied species both young and
old are similarly marked, no believer in evolution will doubt that the
progenitor of the lion and puma was a striped animal, and that the
young have retained vestiges of the stripes, like the kittens of black
cats, which are not in the least striped when grown up. Many species
of deer, which when mature are not spotted, are whilst young covered
with white spots, as are likewise some few species in the adult state.
So again the young in the whole family of pigs (Suidae), and in
certain rather distantly allied animals, such as the tapir, are marked
with dark longitudinal stripes; but here we have a character
apparently derived from an extinct progenitor, and now preserved by
the young alone. In all such cases the old have had their colours
changed in the course of time, whilst the young have remained but
little altered, and this has been effected through the principle of
inheritance at corresponding ages.
  This same principle applies to many birds belonging to various
groups, in which the young closely resemble each other, and differ
much from their respective adult parents. The young of almost all
the Gallinaceae, and of some distantly allied birds such as ostriches,
are covered with longitudinally striped down; but this character
points back to a state of things so remote that it hardly concerns us.
Young cross-bills (Loxia) have at first straight beaks like those of
other finches, and in their immature striated plumage they resemble
the mature red-pole and female siskin, as well as the young of the
goldfinch, greenfinch, and some other allied species. The young of
many kinds of buntings (Emberiza) resemble one another, and likewise
the adult state of the common bunting, E. miliaria. In almost the
whole large group of thrushes the young have their breasts spotted-
a character which is retained throughout life by many species, but
is quite lost by others, as by the Turdus migratorius. So again with
many thrushes, the feathers on the back are mottled before they are
moulted for the first time, and this character is retained for life by
certain eastern species. The young of many species of shrikes
(Lanius), of some woodpeckers, and of an Indian pigeon (Chalcophaps
indicus), are transversely striped on the under surface; and certain
allied species or whole genera are similarly marked when adult. In
some closely-allied and resplendent Indian cuckoos (Chrysococcyx), the
mature species differ considerably from one another in colour, but the
young cannot be distinguished. The young of an Indian goose
(Sarkidiornis melanonotus) closely resemble in plumage an allied
genus, Dendrocygna, when mature.* Similar facts will hereafter be
given in regard to certain herons. Young black-grouse (Tetrao
tetrix) resemble the young as well as the old of certain other
species, for instance the red-grouse or T. scoticus. Finally, as Mr.
Blyth, who has attended closely to this subject, has well remarked,
the natural affinities of many species are best exhibited in their
immature plumage; and as the true affinities of all organic beings
depend on their descent from a common progenitor, this remark strongly
confirms the belief that the immature plumage approximately shews us
the former or ancestral condition of the species.

  * In regard to thrushes, shrikes, and woodpeckers, see Mr. Blyth, in
Charlesworth's Mag. of Nat. Hist., vol. i., 1837, p. 304; also
footnote to his translation of Cuvier's Regne Animal, p. 159. I give
the case of Loxia on Mr. Blyth's information. On thrushes, see also
Audubon, Ornith. Biog., vol. ii., p. 195. On Chrysococcyx and
Chalcophaps, Blyth, as quoted in Jerdon's Birds of India, vol. iii.,
p. 485. On Sarkidiornis, Blyth, in Ibis, 1867, p. 175.

  Although many young birds, belonging to various families, thus
give us a glimpse of the plumage of their remote progenitors, yet
there are many other birds, both dull-coloured and bright-coloured, in
which the young closely resemble their parents. In such cases the
young of the different species cannot resemble each other more closely
than do the parents; nor can they strikingly resemble allied forms
when adult. They give us but little insight into the plumage of
their progenitors, excepting in so far that, when the young and the
old are coloured in the same general manner throughout a whole group
of species, it is probable that their progenitors were similarly
coloured.
  We may now consider the classes of cases, under which the
differences and resemblances between the plumage of the young and
the old, in both sexes or in one sex alone, may be grouped. Rules of
this kind were first enounced by Cuvier; but with the progress of
knowledge they require some modification and amplification. This I
have attempted to do, as far as the extreme complexity of the
subject permits, from information derived from various sources; but
a full essay on this subject by some competent ornithologist is much
needed. In order to ascertain to what extent each rule prevails, I
have tabulated the facts given in four great works, namely, by
Macgillivray on the birds of Britain, Audubon on those of North
America, Jerdon on those of India, and Gould on those of Australia.
I may here premise, first, that the several cases or rules graduate
into each other; and secondly, that when the young are said to
resemble their parents, it is not meant that they are identically
alike, for their colours are almost always less vivid, and the
feathers are softer and often of a different shape.

                 RULES OR CLASSES OF CASES.

  I  When the adult male is more beautiful or conspicuous than the
adult female, the young of both sexes in their first plumage closely
resemble the adult female, as with the common fowl and peacock; or, as
occasionally occurs. they resemble her much more closely than they
do the adult male.
  II  When the adult female is more conspicuous than the adult male,
as sometimes though rarely occurs, the young of both sexes in their
first plumage resemble the adult male.
  III  When the adult male resembles the adult female, the young of
both sexes have a peculiar first plumage of their own, as with the
robin.
  IV  When the adult male resembles the adult female, the young of
both sexes in their first plumage resemble the adults, as with the
kingfisher, many parrots, crows, hedge-warblers.
  V  When the adults of both sexes have a distinct winter and summer
plumage, whether or not the male differs from the female, the young
resemble the adults of both sexes in their winter dress, or much
more rarely in their summer dress, or they resemble the females alone.
Or the young may have an intermediate character; or again they may
differ greatly from the adults in both their seasonal plumages.
  VI  In some few cases the young in their first plumage differ from
each other according to sex; the young males resembling more or less
closely the adult males, and the young females more or less closely
the adult females.
  CLASS I  In this class, the young of both sexes more or less closely
resemble the adult female, whilst the adult male differs from the
adult female, often in the most conspicuous manner. Innumerable
instances in all Orders could be given; it will suffice to call to
mind the common pheasant, duck, and house-sparrow. The cases under
this class graduate into others. Thus the two sexes when adult may
differ so slightly, and the young so slightly from the adults, that it
is doubtful whether such cases ought to come under the present, or
under the third or fourth classes. So again the young of the two
sexes, instead of being quite alike, may differ in a slight degree
from each other, as in our sixth class. These transitional cases,
however, are few, or at least are not strongly pronounced, in
comparison with those which come strictly under the present class.
  The force of the present law is well shewn in those groups, in
which, as a general rule, the two sexes and the young are all alike;
for when in these groups the male does differ from the female, as with
certain parrots, kingfishers, pigeons, &c., the young of both sexes
resemble the adult female.* We see the same fact exhibited still
more clearly in certain anomalous cases; thus the male of Heliothrix
auriculata (one of the humming-birds) differs conspicuously from the
female in having a splendid gorget and fine ear-tufts, but the
female is remarkable from having a much longer tail than that of the
male; now the young of both sexes resemble (with the exception of
the breast being spotted with bronze) the adult female in all other
respects, including the length of her tail, so that the tail of the
male actually becomes shorter as he reaches maturity, which is a
most unusual circumstance.*(2) Again, the plumage of the male
goosander (Mergus merganser) is more conspicuously coloured than
that of the female, with the scapular and secondary wing-feather
much longer; but differently from what occurs, as far as I know, in
any other bird, the crest of the adult male, though broader than
that of the female, is considerably shorter, being only a little above
an inch in length; the crest of the female being two and a half inches
long. Now the young of both sexes entirely resemble the adult
female, so that their crests are actually of greater length, though
narrower, than in the adult male.*(3)

  * See, for instance, Mr. Gould's account (Handbook of the Birds of
Australia, vol. i., p. 133) of Cyanalcyon (one of the Kingfishers), in
which, however, the young male, though resembling the adult female, is
less brilliantly coloured. In some species of Dacelo the males have
blue tails, and the females brown ones; and Mr. R. B. Sharpe informs
me that the tail of the young male of D. gaudichaudi is at first
brown. Mr. Gould has described (ibid., vol. ii., pp. 14, 20, 37) the
sexes and the young of certain black cockatoos and of the king lory,
with which the same rule prevails. Also Jerdon (Birds of India, vol.
i., p. 260) on the Paloeornis rosa, in which the young are more like
the female than the male. See Audubon (Ornithological Biography,
vol. ii., p. 475) on the two sexes and the young of Columba passerina.

  *(2) I owe this information to Mr. Gould, who shewed me the
specimens; see also his Introduction to the Trochilidae, 1861, p. 120.
  *(3) Macgillivray, Hist. Brit. Birds, vol. v., pp. 207-214.

  When the young and the females closely resemble each other and
both differ from the males, the most obvious conclusion is that the
males alone have been modified. Even in the anomalous cases of the
Heliothrix and Mergus, it is probable that originally both adult sexes
were furnished- the one species with a much elongated tail, and the
other with a much elongated crest- these characters having since
been partially lost by the adult males from some unexplained cause,
and transmitted in their diminished state to their male offspring
alone, when arrived at the corresponding age of maturity. The belief
that in the present class the male alone has been modified, as far
as the differences between the male and the female together with her
young are concerned, is strongly supported by some remarkable facts
recorded by Mr. Blyth,* with respect to closely-allied species which
represent each other in distinct countries. For with several of
these representative species the adult males have undergone a
certain amount of change and can be distinguished; the females and the
young from the distinct countries being indistinguishable, and
therefore absolutely unchanged. This is the case with certain Indian
chats (Thamnobia), with certain honey-suckers (Nectarinia), shrikes
(Tephrodornis), certain kingfishers (Tanysiptera), Kalij pheasants
(Gallophasis), and tree-partridges (Arboricola).

  * See his admirable paper in the Journal of the Asiatic Soc. of
Bengal, vol. xix., 1850, p. 223; see also Jerdon, Birds of India, vol.
i., introduction, p. xxix. In regard to Tanysiptera, Prof. Schlegel
told Mr. Blyth that he could distinguish several distinct races,
solely by comparing the adult males.

  In some analogous cases, namely with birds having a different summer
and winter plumage, but with the two sexes nearly alike, certain
closely-allied species can easily be distinguished in their summer
or nuptial plumage, yet are indistinguishable in their winter as
well as in their immature plumage. This is the case with some of the
closely-allied Indian wagtails or Motacillae. Mr. Swinhoe* informs
me that three species of Ardeola, a genus of herons, which represent
one another on separate continents, are "most strikingly different"
when ornamented with their summer plumes, but are hardly, if at all,
distinguishable during the winter. The young also of these three
species in their immature plumage closely resemble the adults in their
winter dress. This case is all the more interesting, because with
two other species of Ardeola both sexes retain, during the winter
and summer, nearly the same plumage as that possessed by the three
first species during the winter and in their immature state; and
this plumage, which is common to several distinct species at different
ages and seasons, probably shews us how the progenitors of the genus
were coloured. In all these cases, the nuptial plumage which we may
assume was originally acquired by the adult males during the
breeding-season, and transmitted to the adults of both sexes at the
corresponding season, has been modified, whilst the winter and
immature plumages have been left unchanged.

  * See also Mr. Swinhoe, in Ibis, July, 1863, p. 131; and a
previous paper, with an extract from a note by Mr. Blyth, in Ibis,
January, 1861, p. 25.

  The question naturally arises, how is it that in these latter
cases the winter plumage of both sexes, and in the former cases the
plumage of the adult females, as well as the immature plumage of the
young, have not been at all affected? The species which represent each
other in distinct countries will almost always have been exposed to
somewhat different conditions, but we can hardly attribute to this
action the modification of the plumage in the males alone, seeing that
the females and the young, though similarly exposed, have not been
affected. Hardly any fact shews us more clearly how subordinate in
importance is the direct action of the conditions of life, in
comparison with the accumulation through selection of indefinite
variations, than the surprising difference between the sexes of many
birds; for both will have consumed the same food, and have been
exposed to the same climate. Nevertheless we are not precluded from
believing that in the course of time new conditions may produce some
direct effect either on both sexes, or from their constitutional
differences chiefly on one sex. We see only that this is subordinate
in importance to the accumulated results of selection. Judging,
however, from a wide-spread analogy, when a species migrates into a
new country (and this must precede the formation of representative
species), the changed conditions to which they will almost always have
been exposed will cause them to undergo a certain amount of
fluctuating variability. In this case sexual selection, which
depends on an element liable to change- the taste or admiration of the
female- will have had new shades of colour or other differences to act
on and accumulate; and as sexual selection is always at work, it would
(from what we know of the results on domestic animals of man's
unintentional selection), be surprising if animals inhabiting separate
districts, which can never cross and thus blend their newly-acquired
characters, were not, after a sufficient lapse of time, differently
modified. These remarks likewise apply to the nuptial or summer
plumage, whether confined to the males, or common to both sexes.
  Although the females of the above closely-allied or representative
species, together with their young, differ hardly at all from one
another, so that the males alone can be distinguished, yet the females
of most species within the same genus obviously differ from each
other. The differences, however, are rarely as great as between the
males. We see this clearly in the whole family of the Gallinaceae: the
females, for instance, of the common and Japan pheasant, and
especially of the gold and Amherst pheasant- of the silver pheasant
and the wild fowl- resemble one another very closely in colour, whilst
the males differ to an extraordinary degree. So it is with the females
of most of the Cotingidae, Fringillidae, and many other families.
There can indeed be no doubt that, as a general rule, the females have
been less modified than the males. Some few birds, however, offer a
singular and inexplicable exception; thus the females of Paradisea
apoda and P. papuana differ from each other more than do their
respective males;* the female of the latter species having the under
surface pure white, whilst the female P. apoda is deep brown
beneath. So, again, as I hear from Professor Newton, the males of
two species of Oxynotus (shrikes), which represent each other in the
islands of Mauritius and Bourbon,*(2) differ but little in colour,
whilst the females differ much. In the Bourbon species the female
appears to have partially retained an immature condition of plumage,
for at first sight she "might be taken for the young of the
Mauritian species." These differences may be compared with those
inexplicable ones, which occur independently of man's selection in
certain sub-breeds of the game-fowl, in which the females are very
different, whilst the males can hardly be distinguished.*(3)

  * Wallace, The Malay Archipelago, vol. ii., 1869, p. 394.
  *(2) These species are described with coloured figures, by M. F.
Pollen, in Ibis, 1866, p. 275.
  *(3) Variation of Animals, &c., vol. i., p. 251.

  As I account so largely by sexual selection for the differences
between the males of allied species, how can the differences between
the females be accounted for in all ordinary cases? We need not here
consider the species which belong to distinct genera; for with
these, adaptation to different habits of life, and other agencies,
will have come into play. In regard to the differences between the
females within the same genus, it appears to me almost certain,
after looking through various large groups, that the chief agent has
been the greater or less transference to the female of the
characters acquired by the males through sexual selection. In the
several British finches, the two sexes differ either very slightly
or considerably; and if we compare the females of the greenfinch,
chaffinch, goldfinch, bullfinch, crossbill, sparrow, &c., we shall see
that they differ from one another chiefly in the points in which
they partially resemble their respective males; and the colours of the
males may safely be attributed to sexual selection. With many
gallinaceous species the sexes differ to an extreme degree, as with
the peacock, pheasant, and fowl, whilst with other species there has
been a partial or even complete transference of character from the
male to the female. The females of the several species of Polyplectron
exhibit in a dim condition, and chiefly on the tail, the splendid
ocelli of their males. The female partridge differs from the male only
in the red mark on her breast being smaller; and the female wild
turkey only in her colours being much duller. In the guinea-fowl the
two sexes are indistinguishable. There is no improbability in the
plain, though peculiarly spotted plumage of this latter bird having
been acquired through sexual selection by the males, and then
transmitted to both sexes; for it is not essentially different from
the much more beautifully spotted plumage, characteristic of the males
alone of the tragopan pheasants.
  It should be observed that, in some instances, the transference of
characters from the male to the female has been effected apparently at
a remote period, the male having subsequently undergone great changes,
without transferring to the female any of his later-gained characters.
For instance, the female and the young of the black-grouse (Tetrao
tetrix) resemble pretty closely both sexes and the young of the
red-grouse (T. scoticus); and we may consequently infer that the
black-grouse is descended from some ancient species, of which both
sexes were coloured in nearly the same manner as the red-grouse. As
both sexes of this latter species are more distinctly barred during
the breeding-season than at any other time, and as the male differs
slightly from the female in his more strongly-pronounced red and brown
tints,* we may conclude that his plumage has been influenced by sexual
selection, at least to a certain extent. If so, we may further infer
that nearly similar plumage of the female black-grouse was similarly
produced at some former period. But since this period the male
black-grouse has acquired his fine black plumage, with his forked
and outwardly-curled tail-feathers; but of these characters there
has hardly been any transference to the female, excepting that she
shews in her tail a trace of the curved fork.

  * Macgillivray, History of British Birds, vol. i., pp. 172-174.

  We may therefore conclude that the females of distinct though allied
species have often had their plumage rendered more or less different
by the transference in various degrees of characters acquired by the
males through sexual selection, both during former and recent times.
But it deserves especial attention that brilliant colours have been
transferred much more rarely than other tints. For instance, the
male of the red-throated blue-breast (Cyanecula suecica) has a rich
blue breast, including a sub-triangular red mark; now marks of
nearly the same shape have been transferred to the female, but the
central space is fulvous instead of red, and is surrounded by
mottled instead of blue feathers. The Gallinaceae offer many analogous
cases; for none of the species, such as partridges, quails,
guinea-fowls, &c., in which the colours of the plumage have been
largely transferred from the male to the female, are brilliantly
coloured. This is well exemplified with the pheasants, in which the
male is generally so much more brilliant than the female; but with the
Eared and Cheer pheasants (Crossoptilon auritum and Phasianus
wallichii) the sexes closely resemble each other and their colours are
dull. We may go so far as to believe that if any part of the plumage
in the males of these two pheasants had been brilliantly coloured,
it would not have been transferred to the females. These facts
strongly support Mr. Wallace's view that with birds which are
exposed to much danger during incubation, the transference of bright
colours from the male to the female has been checked through natural
selection. We must not, however, forget that another explanation,
before given, is possible; namely, that the males which varied and
became bright, whilst they were young and inexperienced, would have
been exposed to much danger, and would generally have been
destroyed; the older and more cautious males, on the other hand, if
they varied in a like manner, would not only have been able to
survive, but would have been favoured in their rivalry with other
males. Now variations occurring late in life tend to be transmitted
exclusively to the same sex, so that in this case extremely bright
tints would not have been transmitted to the females. On the other
hand, ornaments of a less conspicuous kind, such as those possessed by
the Eared and Cheer pheasants, would not have been dangerous, and if
they appeared during early youth, would generally have been
transmitted to both sexes.
  In addition to the effects of the partial transference of characters
from the males to the females, some of the differences between the
females of closely-allied species may be attributed to the direct or
definite action of the conditions of life.* With the males, any such
action would generally have been masked by the brilliant colours
gained through sexual selection; but not so with the females. Each
of the endless diversities in plumage which we see in our domesticated
birds is, of course, the result of some definite cause; and under
natural and more uniform conditions, some one tint, assuming that it
was in no way injurious, would almost certainly sooner or later
prevail. The free intercrossing of the many individuals belonging to
the same species would ultimately tend to make any change of colour,
thus induced, uniform in character.

  * See, on this subject, chap. xxiii. in the Variation of Animals and
Plants under Domestication.

  No one doubts that both sexes of many birds have had their colours
adapted for the sake of protection; and it is possible that the
females alone of some species may have been modified for this end.
Although it would be a difficult, perhaps an impossible process, as
shewn in the last chapter, to convert one form of transmission into
another through selection, there would not be the least difficulty
in adapting the colours of the female, independently of those of the
male, to surrounding objects, through the accumulation of variations
which were from the first limited in their transmission to the
female sex. If the variations were not thus limited, the bright
tints of the male would be deteriorated or destroyed. Whether the
females alone of many species have been thus specially modified, is at
present very doubtful. I wish I could follow Mr. Wallace to the full
extent; for the admission would remove some difficulties. Any
variations which were of no service to the female as a protection
would be at once obliterated, instead of being lost simply by not
being selected, or from free intercrossing, or from being eliminated
when transferred to the male and in any way injurious to him. Thus the
plumage of the female would be kept constant in character. It would
also be a relief if we could admit that the obscure tints of both
sexes of many birds had been acquired and preserved for the sake of
protection,- for example, of the hedge-warbler or kitty-wren (Accentor
modularis and Troglodytes vulgaris), with respect to which we have
no sufficient evidence of the action of sexual selection. We ought,
however, to be cautious in concluding that colours which appear to
us dull, are not attractive to the females of certain species; we
should bear in mind such cases as that of the common house-sparrow, in
which the male differs much from the female, but does not exhibit
any bright tints. No one probably will dispute that many
gallinaceous birds which live on the open ground, have acquired
their present colours, at least in part, for the sake of protection.
We know how well they are thus concealed; we know that ptarmigans,
whilst changing from their winter to their summer plumage, both of
which are protective, suffer greatly from birds of prey. But can we
believe that the very slight differences in tints and markings
between, for instance, the female black-grouse and red-grouse serve as
a protection? Are partridges, as they are now coloured, better
protected than if they had resembled quails? Do the slight differences
between the females of the common pheasant, the Japan and gold
pheasants, serve as a protection, or might not their plumages have
been interchanged with impunity? From what Mr. Wallace has observed of
the habits of certain gallinaceous birds in the East, he thinks that
such slight differences are beneficial. For myself, I will only say
that I am not convinced.
  Formerly when I was inclined to lay much stress on protection as
accounting for the duller colours of female birds, it occurred to me
that possibly both sexes and the young might aboriginally have been
equally bright coloured; but that subsequently, the females from the
danger incurred during incubation, and the young from being
inexperienced, had been rendered dull as a protection. But this view
is not supported by any evidence, and is not probable; for we thus
in imagination expose during past times the females and the young to
danger, from which it has subsequently been necessary to shield
their modified descendants. We have, also, to reduce, through a
gradual process of selection, the females and the young to almost
exactly the same tints and markings, and to transmit them to the
corresponding sex and period of life. On the supposition that the
females and the young have partaken during each stage of the process
of modification of a tendency to be as brightly coloured as the males,
it is also a somewhat strange fact that the females have never been
rendered dull-coloured without the young participating in the same
change; for there are no instances, as far as I can discover, of
species with the females dull and the young bright coloured. A partial
exception, however, is offered by the young of certain woodpeckers,
for they have "the whole upper part of the head tinged with red,"
which afterwards either decreases into a mere circular red line in the
adults of both sexes, or quite disappears in the adult females.*

  * Audubon, Ornith. Biography, vol. i., p. 193. Macgillivray, History
of British Birds, vol. iii., p. 85. See also the case before given
of Indopicus carlotta.

  Finally, with respect to our present class of cases, the most
probable view appears to be that successive variations in brightness
or in other ornamental characters, occurring in the males at a
rather late period of life have alone been preserved; and that most or
all of these variations, owing to the late period of life at which
they appeared, have been from the first transmitted only to the
adult male offspring. Any variations in brightness occurring in the
females or in the young, would have been of no service to them, and
would not have been selected; and moreover, if dangerous, would have
been eliminated. Thus the females and the young will either have
been left unmodified, or (as is much more common) will have been
partially modified by receiving through transference from the males
some of his successive variations. Both sexes have perhaps been
directly acted on by the conditions of life to which they have long
been exposed: but the females from not being otherwise much
modified, will best exhibit any such effects. These changes and all
others will have been kept uniform by the free intercrossing of many
individuals. In some cases, especially with ground birds, the
females and the young may possibly have been modified, independently
of the males, for the sake of protection, so as to have acquired the
same dull-coloured plumage.
  CLASS II  When the adult female is more conspicuous than the adult
male, the young of both sexes in their first plumage resemble the
adult male.- This class is exactly the reverse of the last, for the
females are here brighter coloured or more conspicuous than the males;
and the young, as far as they are known, resemble the adult males
instead of the adult females. But the difference between the sexes
is never nearly so great as with many birds in the first class, and
the cases are comparatively rare. Mr. Wallace, who first called
attention to the singular relation which exists between the less
bright colours of the males and their performing the duties of
incubation, lays great stress on this point,* as a crucial test that
obscure colours have been acquired for the sake of protection during
the period of nesting. A different view seems to me more probable.
As the cases are curious and not numerous, I will briefly give all
that I have been able to find.

  * Westminster Review, July, 1867, and A. Murray, Journal of
Travel, 1868, p. 83.

  In one section of the genus Turnix, quail-like birds, the female
is invariably larger than the male (being nearly twice as large in one
of the Australian species), and this is an unusual circumstance with
the Gallinaceae. In most of the species the female is more
distinctly coloured and brighter than the male,* but in some few
species the sexes are alike. In Turnix taigoor of India the male
"wants the black on the throat and neck, and the whole tone of the
plumage is lighter and less pronounced than that of the female." The
female appears to be noisier, and is certainly much more pugnacious
than the male; so that the females and not the males are often kept by
the natives for fighting, like game-cocks. As male birds are exposed
by the English bird-catchers for a decoy near a trap, in order to
catch other males by exciting their rivalry, so the females of this
Turnix are employed in India. When thus exposed the females soon begin
their "loud purring call, which can be heard a long way off, and any
females within ear-shot run rapidly to the spot, and commence fighting
with the bird." In this way from twelve to twenty birds, all
breeding females, may be caught in the course of a single day. The
natives assert that the females after laying their eggs associate in
flocks, and leave the males to sit on them. There is no reason to
doubt the truth of this assertion, which is supported by some
observations made in China by Mr. Swinhoe.*(2) Mr. Blyth believes,
that the young of both sexes resemble the adult male.

  * For the Australian species, see Gould's Handbook, &c., vol. ii.,
pp. 178, 180, 186, and 188. In the British Museum specimens of the
Australian plain-wanderer (Pedionomus torquatus) may be seen,
shewing similar sexual differences.
  *(2) Jerdon, Birds of India, vol. iii., p. 596. Mr. Swinhoe, in
Ibis, 1865, p. 542; 1866, pp. 131, 405.

  The females of the three species of painted snipes (see Rhynchoea,
fig. 62) "are not only larger but much more richly coloured than the
males."* With all other birds in which the trachea differs in
structure in the two sexes it is more developed and complex in the
male than in the female; but in the Rhynchoea australis it is simple
in the male, whilst in the female it makes four distinct
convolutions before entering the lungs.*(2) The female therefore of
this species has acquired an eminently masculine character. Mr.
Blyth ascertained, by examining many specimens, that the trachea is
not convoluted in either sex of R. bengalensis, which species
resembles R. australis so closely, that it can hardly be distinguished
except by its shorter toes. This fact is another striking instance
of the law that secondary sexual characters are often widely different
in closely-allied forms, though it is a very rare circumstance when
such differences relate to the female sex. The young of both sexes
of R. bengalensis in their first plumage are said to resemble the
mature male.*(3) There is also reason to believe that the male
undertakes the duty of incubation, for Mr. Swinhoe*(4) found the
females before the close of the summer associated in flocks, as occurs
with the females of the Turnix.

  * Jerdon, Birds of India, vol. iii., p. 677.
  *(2) Gould's Handbook of the Birds of Australia, vol. ii., p. 275.
  *(3) The Indian Field, Sept., 1858, p. 3.
  *(4) Ibis, 1866, p. 298.

  The females of Phalaropus fulicarius and P. hyperboreus are
larger, and in their summer plumage "more gaily attired than the
males." But the difference in colour between the sexes is far from
conspicuous. According to Professor Steenstrup, the male alone of P.
fulicarius undertakes the duty of incubation; this is likewise shewn
by the state of his breast-feathers during the breeding-season. The
female of the dotterel plover (Eudromias morinellus) is larger than
the male, and has the red and black tints on the lower surface, the
white crescent on the breast, and the stripes over the eyes, more
strongly pronounced. The male also takes at least a share in
hatching the eggs; but the female likewise attends to the young.* I
have not been able to discover whether with these species the young
resemble the adult males more closely than the adult females; for
the comparison is somewhat difficult to make on account of the
double moult.

  * For these several statements, see Mr. Gould's Birds of Great
Britain. Prof. Newton informs me that he has long been convinced, from
his own observations and from those of others, that the males of the
above-named species take either the whole or a large share of the
duties of incubation, and that they "shew much greater devotion
towards their young, when in danger, than do the females." So it is,
as he informs me, with Limosa lapponica and some few other waders,
in which the females are larger and have more strongly contrasted
colours than the males.

  Turning now to the ostrich Order: the male of the common cassowary
(Casuarius galeatus) would be thought by any one to be the female,
from his smaller size and from the appendages and naked skin about his
head being much less brightly coloured; and I am informed by Mr.
Bartlett that in the Zoological Gardens, it is certainly the male
alone who sits on the eggs and takes care of the young.* The female is
said by Mr. T. W. Wood*(2) to exhibit during the breeding-season a
most pugnacious disposition; and her wattles then become enlarged
and more brilliantly coloured. So again the female of one of the
emus (Dromoeus irroratus) is considerably larger than the male, and
she possesses a slight top-knot, but is otherwise indistinguishable in
plumage. She appears, however, "to have greater power, when angry or
otherwise excited, of erecting, like a turkey-cock, the feathers of
her neck and breast. She is usually the more courageous and
pugilistic. She makes a deep hollow guttural boom especially at night,
sounding like a small gong. The male has a slenderer frame and is more
docile, with no voice beyond a suppressed hiss when angry, or a
croak." He not only performs the whole duty of incubation, but has
to defend the young from their mother; "for as soon as she catches
sight of her progeny she becomes violently agitated, and
notwithstanding the resistance of the father appears to use her utmost
endeavours to destroy them. For months afterwards it is unsafe to
put the parents together, violent quarrels being the inevitable
result, in which the female generally comes off conqueror."*(3) So
that with this emu we have a complete reversal not only of the
parental and incubating instincts, but of the usual moral qualities of
the two sexes; the females being savage, quarrelsome, and noisy, the
males gentle and good. The case is very different with the African
ostrich, for the male is somewhat larger than the female and has finer
plumes with more strongly contrasted colours; nevertheless he
undertakes the whole duty of incubation.*(4)

  * The natives of Ceram (Wallace, Malay Archipelago, vol. ii., p.
150) assert that the male and female sit alternately on the eggs;
but this assertion, as Mr. Bartlett thinks, may be accounted for by
the female visiting the nest to lay her eggs.
  *(2) The Student, April, 1870, p. 124.
  *(3) See the excellent account of the habits of this bird under
confinement, by Mr. A. W. Bennett, in Land and Water, May, 1868, p.
233.
  *(4) Mr. Sclater, on the incubation of the Struthiones, Proc.
Zool. Soc., June 9, 1863. So it is with the Rhea darwinii: Captain
Musters says (At Home with the Patagonians, 1871, p. 128), that the
male is larger, stronger and swifter than the female, and of
slightly darker colours; yet he takes sole charge of the eggs and of
the young, just as does the male of the common species of Rhea.

  I will specify the few other cases known to me, in which the
female is more conspicuously coloured than the male, although
nothing is known about the manner of incubation. With the carrion-hawk
of the Falkland Islands (Milvago leucurus) I was much surprised to
find by dissection that the individuals, which had all their tints
strongly pronounced, with the cere and legs orange-coloured, were
the adult females; whilst those with duller plumage and grey legs were
the males or the young. In an Australian tree-creeper (Climacteris
erythrops) the female differs from the male in "being adorned with
beautiful, radiated, rufous markings on the throat, the male having
this part quite plain." Lastly, in an Australian night-jar "the female
always exceeds the male in size and in the brilliance of her tints;
the males, on the other hand, have two white spots on the primaries
more conspicuous than in the female."*

  * For the Milvago, see Zoology of the Voyage of the Beagle: Birds,
1841, p. 16. For the Climacteris and night-jar (Eurostopodus), see
Gould's Handbook of the Birds of Australia, vol. i., pp. 602 and 97.
The new Zealand shieldrake (Tadorna variegata) offers a quite
anomalous case; the head of the female is pure white, and her back
is redder than that of the male; the head of the male is of a rich
dark bronzed colour, and his back is clothed with finely pencilled
slate-coloured feathers, so that altogether he may be considered as
the more beautiful of the two. He is larger and more pugnacious than
the female, and does not sit on the eggs. So that in all these
respects this species comes under our first class of cases; but Mr.
Sclater (Proceedings of the Zoological Society, 1866, p. 150) was much
surprised to observe that the young of both sexes, when about three
months old, resembled in their dark heads and necks the adult males,
instead of the adult females; so that it would appear in this case
that the females have been modified, whilst the males and the young
have retained a former state of plumage

  We thus see that the cases in which female birds are more
conspicuously coloured than the males, with the young in their
immature plumage resembling the adult males instead of the adult
females, as in the previous class, are not numerous, though they are
distributed in various Orders. The amount of difference, also, between
the sexes is incomparably less than that which frequently occurs in
the last class; so that the cause of the difference, whatever it may
have been, has here acted on the females either less energetically
or less persistently than on the males in the last class. Mr.
Wallace believes that the males have had their colours rendered less
conspicuous for the sake of protection during the period of
incubation; but the difference between the sexes in hardly any of
the foregoing cases appears sufficiently great for this view to be
safely accepted. In some of the cases, the brighter tints of the
female are almost confined to the lower surface, and the males, if
thus coloured, would not have been exposed to danger whilst sitting on
the eggs. It should also be borne in mind that the males are not
only in a slight degree less conspicuously coloured than the
females, but are smaller and weaker. They have, moreover, not only
acquired the maternal instinct of incubation, but are less
pugnacious and vociferous than the females, and in one instance have
simpler vocal organs. Thus an almost complete transposition of the
instincts, habits, disposition, colour, size, and of some points of
structure, has been effected between the two sexes.
  Now if we might assume that the males in the present class have lost
some of that ardour which is usual to their sex, so that they no
longer search eagerly for the females; or, if we might assume that the
females have become much more numerous than the males- and in the case
of one Indian Turnix the females are said to be "much more commonly
met with than the males"*- then it is not improbable that the
females would have been led to court the males, instead of being
courted by them. This indeed is the case to a certain extent with some
birds, as we have seen with the peahen, wild turkey, and certain kinds
of grouse. Taking as our guide the habits of most male birds, the
greater size and strength as well as the extraordinary pugnacity of
the females of the Turnix and emu, must mean that they endeavour to
drive away rival females, in order to gain possession of the male; and
on this view all the facts become clear; for the males would
probably be most charmed or excited by the females which were the most
attractive to them by their bright colours, other ornaments, or
vocal powers. Sexual selection would then do its work, steadily adding
to the attractions of the females; the males and the young being
left not at all, or but little modified.

  * Jerdon, Birds of India, vol. iii., p. 598.

  CLASS Ill  When the adult male resembles the adult female, the young
of both sexes have a peculiar first plumage of their own.- In this
class the sexes when adult resemble each other, and differ from the
young. This occurs with many birds of many kinds. The male robin can
hardly be distinguished from the female, but the young are widely
different, with their mottled dusky-olive and brown plumage. The
male and female of the splendid scarlet ibis are alike, whilst the
young are brown; and the scarlet colour, though common to both
sexes, is apparently a sexual character, for it is not well
developed in either sex under confinement; and a loss of colour
often occurs with brilliant males when they are confined. With many
species of herons the young differ greatly from the adults; and the
summer plumage of the latter, though common to both sexes, clearly has
a nuptial character. Young swans are slate-coloured, whilst the mature
birds are pure white; but it would be superfluous to give additional
instances. These differences between the young and the old
apparently depend, as in the last two classes, on the young having
retained a former or ancient state of plumage, whilst the old of
both sexes have acquired a new one. When the adults are bright
coloured, we may conclude from the remarks just made in relation to
the scarlet ibis and to many herons, and from the analogy of the
species in the first class, that such colours have been acquired
through sexual selection by the nearly mature males; but that,
differently from what occurs in the first two classes, the
transmission, though limited to the same age, has not been limited
to the same sex. Consequently, the sexes when mature resemble each
other and differ from the young.
  CLASS IV  When the adult male resembles the adult female, the
young of both sexes in their first plumage resemble the adults.- In
this class the young and the adults of both sexes, whether brilliantly
or obscurely coloured, resemble each other. Such cases are, I think,
more common than those in the last class. We have in England instances
in the kingfisher, some woodpeckers, the jay, magpie, crow, and many
small dull-coloured birds, such as the hedge-warbler or kitty-wren.
But the similarity in plumage between the young and the old is never
complete, and graduates away into dissimilarity. Thus the young of
some members of the kingfisher family are not only less vividly
coloured than the adults, but many of the feathers on the lower
surface are edged with brown,*- a vestige probably of a former state
of the plumage. Frequently in the same group of birds, even within the
same genus, for instance in an Australian genus of parrakeets
(Platycercus), the young of some species closely resemble, whilst
the young of other species differ considerably from, their parents
of both sexes, which are alike.*(2) Both sexes and the young of the
common jay are closely similar; but in the Canada jay (Perisoreus
canadensis) the young differ so much from their parents that they were
formerly described as distinct species.*(3)

  * Jerdon, Birds of India, vol. i., pp. 222, 228. Gould's Handbook to
the Birds of Australia, vol. i., pp. 124, 130.
  *(2) Gould, ibid., vol. ii., pp. 37, 46, 56.
  *(3) Audubon, Ornith. Biography, vol. ii., p. 55.

  I may remark before proceeding that, under the present and next
two classes of cases, the facts are so complex and the conclusions
so doubtful, that any one who feels no especial interest in the
subject had better pass them over.

  The brilliant or conspicuous colours which characterise many birds
in the present class, can rarely or never be of service to them as a
protection; so that they have probably been gained by the males
through sexual selection, and then transferred to the females and
the young. It is, however, possible that the males may have selected
the more attractive females; and if these transmitted their characters
to their offspring of both sexes, the same results would follow as
from the selection of the more attractive males by the females. But
there is evidence that this contingency has rarely, if ever,
occurred in any of those groups of birds in which the sexes are
generally alike; for, if even a few of the successive variations had
failed to be transmitted to both sexes, the females would have
slightly exceeded the males in beauty. Exactly the reverse occurs
under nature; for, in almost every group in which the sexes
generally resemble each other, the males of some few species are in
a slight degree more brightly coloured than the females. It is again
possible that the females may have selected the more beautiful
males, these males having reciprocally selected the more beautiful
females; but it is doubtful whether this double process of selection
would be likely to occur, owing to the greater eagerness of one sex
than the other, and whether it would be more efficient than
selection on one side alone. It is, therefore, the most probable
view that sexual selection has acted, in the present class, as far
as ornamental characters are concerned, in accordance with the general
rule throughout the animal kingdom, that is, on the males; and that
these have transmitted their gradually-acquired colours, either
equally or almost equally, to their offspring of both sexes.
  Another point is more doubtful, namely, whether the successive
variations first appeared in the males after had become nearly mature,
or whilst quite young. In either case sexual selection must have acted
on the male when he had to compete with rivals for the possession of
the female; and in both cases the characters thus acquired have been
transmitted to both sexes and all ages. But these characters if
acquired by the males when adult, may have been transmitted at first
to the adults alone, and at some subsequent period transferred to
the young. For it is known that, when the law of inheritance at
corresponding ages fails, the offspring often inherit characters at an
earlier age than that at which they first appeared in their
parents.* Cases apparently of this kind have been observed with
birds in a state of nature. For instance Mr. Blyth has seen
specimens of Lanius rufus and of Colymbus glacialis which had
assumed whilst young, in a quite anomalous manner, the adult plumage
of their parents.*(2) Again, the young of the common swan (Cygnus
olor) do not cast off their dark feathers and become white until
eighteen months or two years old; but Dr. F. Forel has described the
case of three vigorous young birds, out of a brood of four, which were
born pure white. These young birds were not albinos, as shewn by the
color of their beaks and legs, which nearly resembled the same parts
in the adults.*(3)

  * Variation of Animals and Plants under Domestication, vol. ii.,
p. 79.
  *(2) Charlesworth's Magazine of Natural History, vol. i., 1837,
pp. 305, 306.
  *(3) Bulletin de la Soc. Vaudoise des Sc. Nat., vol. x., 1869, p.
132. The young of the Polish swan, Cygnus immutabilis of Yarrell,
are always white; but this species, as Mr. Sclater informs me, is
believed to be nothing more than a variety of the domestic swan
(Cygnus olor).

  It may be worth while to illustrate the above three modes by
which, in the present class, the two sexes and the young may have come
to resemble each other, by the curious case of the genus Passer.* In
the house-sparrow (P. domesticus) the male differs much from the
female and from the young. The young and the females are alike, and
resemble to a large extent both sexes and the young of the sparrow
of Palestine (P. brachydactylus), as well as of some allied species.
We may therefore assume that the female and young of the house-sparrow
approximately shew us the plumage of the progenitor of the genus.
Now with the tree-sparrow (P. montanus) both sexes and the young
closely resemble the male of the house-sparrow; so that they have
all been modified in the same manner, and all depart from the
typical colouring of their early progenitor. This may have been
effected by a male ancestor of the tree-sparrow having varied,
firstly, when nearly mature; or, secondly, whilst quite young, and
by having in either case transmitted his modified plumage to the
females and the young; or, thirdly, he may have varied when adult
and transmitted his plumage to both adult sexes, and, owing to the
failure of the law of inheritance at corresponding ages, at some
subsequent period to his young.

  * I am indebted to Mr. Blyth for information in regard to this
genus. The sparrow of Palestine belongs to the sub-genus Petronia.

  It is impossible to decide which of these three modes has
generally prevailed throughout the present class of cases. That the
males varied whilst young, and transmitted their variations to their
offspring of both sexes, is the most probable. I may here add that I
have, with little success, endeavoured, by consulting various works,
to decide how far the period of variation in birds has generally
determined the transmission of characters to one sex or to both. The
two rules, often referred to (namely, that variations occurring late
in life are transmitted to one and the same sex, whilst those which
occur early in life are transmitted to both sexes), apparently hold
good in the first,* second, and fourth classes of cases; but they fail
in the third, often in the fifth,*(2) and in the sixth small class.
They apply, however, as far as I can judge, to a considerable majority
of the species; and we must not forget the striking generalisation
by Dr. W. Marshall with respect to the protuberances on the heads of
birds. Whether or not the two rules generally hold good, we may
conclude from the facts given in the eighth chapter, that the period
of variation is one important element in determining the form of
transmission.

  * For instance, the males of Tanagra aestiva and Fringilla cyanea
require three years, the male of Fringilla ciris four years, to
complete their beautiful plumage. (See Audubon, Ornith. Biography,
vol. i., pp. 233, 280, 378). The harlequin duck takes three years
(ibid., vol. iii., p. 614). The male of the gold pheasant, as I hear
from Mr. Jenner Weir, can be distinguished from the female when
about three months old, but he does not acquire his full splendour
until the end of the September in the following year.
  *(2) Thus the Ibis tantalus and Grus americanus take four years, the
flamingo several years, and the Ardea ludovicana two years, before
they acquire their perfect plumage. See Audubon, ibid., vol. i., p.
221; vol. iii., pp. 133, 139, 211.

  With birds it is difficult to decide by what standard we ought to
judge of the earliness or lateness of the period of variation, whether
by the age in reference to the duration of life, or to the power of
reproduction, or to the number of moults through which the species
passes. The moulting of birds, even within the same family,
sometimes differs much without any assignable cause. Some birds
moult so early, that nearly all the body feathers are cast off
before the first wing-feathers are fully grown; and we cannot
believe that this was the primordial state of things. When the
period of moulting has been accelerated, the age at which the
colours of the adult plumage are first developed will falsely appear
to us to be earlier than it really is. This may be illustrated by
the practice followed by some bird-fanciers, who pull out a few
feathers from the breast of nestling bullfinches, and from the head or
neck of young gold-pheasants, in order to ascertain their sex; for
in the males, these feathers are immediately replaced by coloured
ones.* The actual duration of life is known in but few birds, so
that we can hardly judge by this standard. And, with reference to
the period at which the power of reproduction is gained, it is a
remarkable fact that various birds occasionally breed whilst retaining
their immature plumage.*(2) The fact of birds breeding in their
immature plumage seems opposed to the belief that sexual selection has
played as important a part, as I believe it has, in giving
ornamental colours, plumes, &c., to the males, and, by means of
equal transmission, to the females of many species. The objection
would be a valid one, if the younger and less ornamental males were as
successful in winning females and propagating their kind, as the older
and more beautiful males. But we have no reason to suppose that this
is the case. Audubon speaks of the breeding of the immature males of
Ibis tantalus as a rare event, as does Mr. Swinhoe, in regard to the
immature males of Oriolus.*(3) If the young of any species in their
immature plumage were more successful in winning partners than the
adults, the adult plumage would probably soon be lost, as the males
would prevail, which retained their immature dress for the longest
period, and thus the character of the species would ultimately be
modified.*(4) If, on the other hand, the young never succeeded in
obtaining a female, the habit of early reproduction would perhaps be
sooner or later eliminated, from being superfluous and entailing waste
of power.

  * Mr. Blyth, in Charlesworth's Magazine of Natural History, vol. i.,
1837, p. 300. Mr. Bartlett has informed me in regard to gold
pheasants.
  *(2) I have noticed the following cases in Audubon's Ornith.
Biography. The red-start of America (Muscapica ruticilla, vol. i.,
p. 203). The Ibis tantalus takes four years to come to full
maturity, but sometimes breeds in the second year (vol. iii., p. 133).
The Grus americanus takes the same time, but breeds before acquiring
its full plumage (vol. iii., p. 211). The adults of Ardea caerulea are
blue, and the young white; and white, mottled, and mature blue birds
may all be seen breeding together (vol. iv., p. 58): but Mr. Blyth
informs me that certain herons apparently are dimorphic, for white and
coloured individuals of the same age may be observed. The harlequin
duck (Anas histrionica, Linn.) takes three years to acquire its full
plumage, though many birds breed in the second year (vol. iii., p.
614). The white-beaded eagle (Falco leucocephalus, vol. iii., p.
210) is likewise known to breed in its immature state. Some species of
Oriolus (according to Mr. Blyth and Mr. Swinhoe, in Ibis, July,
1863, p. 68) likewise breed before they attain their full plumage.
  *(3) See the last footnote.
  *(4) Other animals, belonging to quite distinct classes, are
either habitually or occasionally capable of breeding before they have
fully acquired their adult characters. This is the case with the young
males of the salmon. Several amphibians have been known to breed
whilst retaining their larval structure. Fritz Muller has shewn (Facts
and Arguments for Darwin, Eng. trans., 1869, p. 79) that the males
of several amphipod crustaceans become sexually mature whilst young;
and I infer that this is a case of premature breeding, because they
have not as yet acquired their fully-developed claspers. All such
facts are highly interesting, as bearing on one means by which species
may undergo great modifications of character.
  The plumage of certain birds goes on increasing in beauty during
many years after they are fully mature; this is the case with the
train of the peacock, with some of the birds of paradise, and with the
crest and plumes of certain herons, for instance, the Ardea
ludovicana.* But it is doubtful whether the continued development of
such feathers is the result of the selection of successive
beneficial variations (though this is the most probable view with
birds of paradise) or merely of continuous growth. Most fishes
continue increasing in size, as long as they are in good health and
have plenty of food; and a somewhat similar law may prevail with the
plumes of birds.

  * Jerdon, Birds of India, vol. iii., p. 507, on the peacock. Dr.
Marshall thinks that the older and more brilliant males of birds of
paradise, have an advantage over the younger males; see Archives
Neerlandaises, tom. vi., 1871.- On Ardea, Audubon, ibid., vol. iii.,
p. 139.

  CLASS V  When the adults of both sexes have a distinct winter and
summer plumage, whether or not the male differs from the female, the
young resemble the adults of both sexes in their winter dress, or much
more rarely in their summer dress, or they resemble the females alone.
Or the young may have an intermediate character; or, again, they may
differ greatly from the adults in both their seasonal plumages.- The
cases in this class are singularly complex; nor is this surprising, as
they depend on inheritance, limited in a greater or less degree in
three different ways, namely, by sex, age, and the season of the year.
In some cases the individuals of the same species pass through at
least five distinct states of plumage. With the species, in which
the male differs from the female during the summer season alone, or,
which is rarer, during both seasons,* the young generally resemble the
females,- as with the so-called gold-finch of North America, and
apparently with the splendid Maluri of Australia.*(2) With these
species, the sexes of which are alike during both the summer and
winter, the young may resemble the adults, firstly, in their winter
dress; secondly, and this is of much rarer occurrence, in their summer
dress; thirdly, they may be intermediate between these two states;
and, fourthly, they may differ greatly from the adults at all seasons.
We have an instance of the first of these four cases in one of the
egrets of India (Buphus coromandus), in which the young and the adults
of both sexes are white during the winter, the adults becoming
golden-buff during the summer.
  * For illustrative cases, see vol. iv. of Macgillivray's History
of British Birds; on Tringa, &c., pp. 229, 271; on the Machetes, p.
172; on the Charadrius hiaticula, p. 118; on the Charadrius pluvialis,
p. 94.
  *(2) For the goldfinch of N. America, Fringilla tristis, Linn.,
see Audubon, Ornithological Biography vol. i., p. 172. For the Maluri,
Gould's Handbook of the Birds of Australia, vol. i., p. 318.

  With the gaper (Anastomus oscitans) of India we have a similar case,
but the colours are reversed: for the young and the adults of both
sexes are grey and black during the winter, the adults becoming
white during the summer.* As an instance of the second case, the young
of the razor-bill (Alca torda, Linn.), in an early state of plumage,
are coloured like the adults during the summer; and the young of the
white-crowned sparrow of North America (Fringilla leucophrys), as soon
as fledged, have elegant white stripes on their heads, which are
lost by the young and the old during the winter.*(2) With respect to
the third case, namely, that of the young having an intermediate
character between the summer and winter adult plumages, Yarrell*(3)
insists that this occurs with many waders. Lastly, in regard to the
young differing greatly from both sexes in their adult summer and
winter plumages, this occurs with some herons and egrets of North
America and India,- the young alone being white.

  * I am indebted to Mr. Blyth for information as to the Buphus; see
also Jerdon, Birds of India, vol. iii., p. 749. On the Anastomus,
see Blyth, in Ibis, 1867, p. 173.
  *(2) On the Alca, see Macgillivray, Hist. Brit. Birds, vol. v., p.
347. On the Fringilla leucophrys, Audubon, ibid., vol. ii., p. 89. I
shall have hereafter to refer to the young of certain herons and
egrets being white.
  *(3) History of British Birds, vol. i., 1839, p. 159.

  I will make only a few remarks on these complicated cases. When
the young resemble the females in their summer dress, or the adults of
both sexes in their winter dress, the cases differ from those given
under Classes I and Ill only in the characters originally acquired
by the males during the breeding-season, having been limited in
their transmission to the corresponding season. When the adults have a
distinct summer and winter plumage, and the young differ from both,
the case is more difficult to understand. We may admit as probable
that the young have retained an ancient state of plumage; we can
account by sexual selection for the summer or nuptial plumage of the
adults, but how are we to account for their distinct winter plumage?
If we could admit that this plumage serves in all cases as a
protection, its acquirement would be a simple affair; but there
seems no good reason for this admission. It may be suggested that
the widely different conditions of life during the winter and summer
have acted in a direct manner on the plumage; this may have had some
effect, but I have not much confidence in so great a difference as
we sometimes see between the two plumages, having been thus caused.
A more probable explanation is, that an ancient style of plumage,
partially modified through the transference of some characters from
the summer plumage, has been retained by the adults during the winter.
Finally, all the cases in our present class apparently depend on
characters acquired by the adult males, having been variously
limited in their transmission according to age, season, and sex; but
it would not be worth while to attempt to follow out these complex
relations.
  CLASS VI  The young in their first plumage differ from each other
according to sex; the young males resembling more or less closely
the adult males, and the young females more or less closely the
adult females.- The cases in the present class, though occurring in
various groups, are not numerous; yet it seems the most natural
thing that the young should at first somewhat resemble the adults of
the same sex, and gradually become more and more like them. The
adult male blackcap (Sylvia atricapilla) has a black head, that of the
female being reddish-brown; and I am informed by Mr. Blyth, that the
young of both sexes can be distinguished by this character even as
nestlings. In the family of thrushes an unusual number of similar
cases have been noticed; thus, the male blackbird (Turdus merula)
can be distinguished in the nest from the female. The two sexes of the
mocking bird (Turdus polyglottus, Linn.) differ very little from
each other, yet the males can easily be distinguished at a very
early age from the females by showing more pure white.* The males of a
forest-thrush and of a rock-thrush (Orocetes erythrogastra and
Petrocincla cyanea) have much of their plumage of a fine blue,
whilst the females are brown; and the nestling males of both species
have their main wing and tail-feathers edged with blue whilst those of
the female are edged with brown.*(2) In the young blackbird the
wing-feathers assume their mature character and become black after the
others; on the other hand, in the two species just named the
wing-feather become blue before the others. The most probable view
with reference to the cases in the present class is that the males,
differently from what occurs in Class I, have transmitted their
colours to their male offspring at an earlier age than that at which
they were first acquired; for, if the males had varied whilst quite
young, their characters would probably have been transmitted to both
sexes.*(3)

  * Audubon, Ornith. Biography, vol. i., p. 113.
  *(2) Mr. C. A. Wright, in Ibis, vol. vi., 1864, p. 65. Jerdon, Birds
of India, vol. i., p. 515. See also on the blackbird, Blyth in
Charlesworth's Magazine of Natural History, vol. i., 1837, p. 113.
  *(3) The following additional cases may be mentioned; the young
males of Tanagra rubra can be distinguished from the young females
(Audubon, Ornith. Biography, vol. iv., p. 392), and so it is within
the nestlings of a blue nuthatch, Dendrophila frontalis of India
(Jerdon, Birds of India, vol. i., p. 389). Mr, Blyth also informs me
that the sexes of the stonechat, Saxicola rubicola, are
distinguishable at a very early age. Mr. Salvin gives (Proc. Zoolog.
Soc., 1870, p. 206) the case of a humming-bird, like the following one
of Eustephanus.

   In Aithurus polytmus, a humming-bird, the male is splendidly
coloured black and green, and two of the tail-feathers are immensely
lengthened; the female has an ordinary tail and inconspicuous colours;
now the young males, instead of resembling the adult female, in
accordance with the common rule, begin from the first to assume the
colours proper to their sex, and their tail-feathers soon become
elongated. I owe this information to Mr. Gould, who has given me the
following more striking and as yet unpublished case. Two humming-birds
belonging to the genus Eustephanus, both beautifully coloured, inhabit
the small island of Juan Fernandez, and have always been ranked as
specifically distinct. But it has lately been ascertained that the one
which is of a rich chestnut-brown colour with a golden-red head, is
the male, whilst the other which is elegantly variegated with green
and white with a metallic green head is the female. Now the young from
the first somewhat resemble the adults of the corresponding sex, the
resemblance gradually becoming more and more complete.
  In considering this last case, if as before we take the plumage of
the young as our guide, it would appear that both sexes have been
rendered beautiful independently; and not that one sex has partially
transferred its beauty to the other. The male apparently has
acquired his bright colours through sexual selection in the same
manner as, for instance, the peacock or pheasant in our first class of
cases; and the female in the same manner as the female Rhynchaea or
Turnix in our second class of cases. But there is much difficulty in
understanding how this could have been effected at the same time
with the two sexes of the same species. Mr. Salvin states, as we
have seen in the eighth chapter, that with certain humming-birds the
males greatly exceed the females in number, whilst with other
species inhabiting the same country the females greatly exceed the
males. If, then, we might assume that during some former lengthened
period the males of the Juan Fernandez species had greatly exceeded
the females in number, but that during another lengthened period the
females had far exceeded the males, we could understand how the
males at one time, and the females at another, might have been
rendered beautiful by the selection of the brighter coloured
individuals of either sex; both sexes transmitting their characters to
their young at a rather earlier age than usual. Whether this is the
true explanation I will not pretend to say; but the case is too
remarkable to be passed over without notice.

  We have now seen in all six classes, that an intimate relation
exists between the plumage of the young and the adults, either of
one sex or both. These relations are fairly well explained on the
principle that one sex- this being in the great majority of cases
the male- first acquired through variation and sexual selection bright
colours or other ornaments, and transmitted them in various ways, in
accordance with the recognised laws of inheritance. Why variations
have occurred at different periods of life, even sometimes with
species of the same group, we do not know, but with respect to the
form of transmission, one important determining cause seems to be
the age at which the variations first appear.
  From the principle of inheritance at corresponding ages, and from
any variations in colour which occurred in the males at an early age
not being then selected- on the contrary being often eliminated as
dangerous- whilst similar variations occurring at or near the period
of reproduction have been preserved, it follows that the plumage of
the young will often have been left unmodified, or but little
modified. We thus get some insight into the colouring of the
progenitors of our existing species. In a vast number of species in
five out of our six classes of cases, the adults of one sex or of both
are bright coloured, at least during the breeding-season, whilst the
young are invariably less brightly coloured than the adults, or are
quite dull coloured; for no instance is known, as far as I can
discover, of the young of dull-coloured species displaying bright
colours, or of the young of bright-coloured species being more
brilliant than their parents. In the fourth class, however, in which
the young and the old resemble each other, there are many species
(though by no means all), of which the young are bright-coloured,
and as these form old groups, we may infer that their early
progenitors were likewise bright. With this exception, if we look to
the birds of the world, it appears that their beauty has been much
increased since that period, of which their immature plumage gives
us a partial record.

  On the Colour of the Plumage in relation to Protection.- It will
have been seen that I cannot follow Mr. Wallace in the belief that
dull colours, when confined to the females, have been in most cases
specially gained for the sake of protection. There can, however, be no
doubt, as formerly remarked, that both sexes of many birds have had
their colours modified, so as to escape the notice of their enemies;
or in some instances, so as to approach their prey unobserved, just as
owls have had their plumage rendered soft, that their flight may not
be overheard. Mr. Wallace remarks* that "it is only in the tropics,
among forests which never lose their foliage, that we find whole
groups of birds, whose chief colour is green." It will be admitted
by every one, who has ever tried, how difficult it is to distinguish
parrots in a leaf-covered tree. Nevertheless, we must remember that
many parrots are ornamented with crimson, blue, and orange tints,
which can hardly be protective. Woodpeckers are eminently arboreal,
but besides green species, there are many black, and black-and-white
kinds- all the species being apparently exposed to nearly the same
dangers. It is therefore probable that with tree-haunting birds,
strongly-pronounced colours have been acquired through sexual
selection, but that a green tint has been acquired oftener than any
other, from the additional advantage of protection.

  * Westminster Review, July, 1867, p. 5.

  In regard to birds which live on the ground, every one admits that
they are coloured so as to imitate the surrounding surface. How
difficult it is to see a partridge, snipe, woodcock, certain
plovers, larks, and night-jars when crouched on ground. Animals
inhabiting deserts offer the most striking cases, for the bare surface
affords no concealment, and nearly all the smaller quadrupeds,
reptiles, and birds depend for safety on their colours. Mr. Tristram
has remarked in regard to the inhabitants of the Sahara, that all
are protected by their "isabelline or sand-colour."* Calling to my
recollection the desert-birds of South America, as well as most of the
ground-birds of Great Britain, it appeared to me that both sexes in
such cases are generally coloured nearly alike. Accordingly, I applied
to Mr. Tristram with respect to the birds of the Sahara, and he has
kindly given me the following information. There are twenty-six
species belonging to fifteen genera, which manifestly have their
plumage coloured in a protective manner; and this colouring is all the
more striking, as with most of these birds it differs from that of
their congeners. Both sexes of thirteen out of the twenty-six
species are coloured in the same manner; but these belong to genera in
which this rule commonly prevails, so that they tell us nothing
about the protective colours being the same in both sexes of
desert-birds. Of the other thirteen species, three belong to genera in
which the sexes usually differ from each other, yet here they have the
sexes alike. In the remaining ten species, the male differs from the
female; but the difference is confined chiefly to the under surface of
the plumage, which is concealed when the bird crouches on the
ground; the head and back being of the same sand-coloured hue in the
two sexes. So that in these ten species the upper surfaces of both
sexes have been acted on and rendered alike, through natural
selection, for the sake of protection; whilst the lower surfaces of
the males alone have been diversified, through sexual selection, for
the sake of ornament. Here, as both sexes are equally well
protected, we clearly see that the females have not been prevented
by natural selection from inheriting the colours of their male
parents; so that we must look to the law of sexually-limited
transmission.

  * Ibis, 1859, vol. i., p. 429, et seq. Dr. Rohlfs, however,
remarks to me in a letter that according to his experience of the
Sahara, this statement is too strong.

  In all parts of the world both sexes of many soft-billed birds,
especially those which frequent reeds or sedges, are obscurely
coloured. No doubt if their colours had been brilliant, they would
have been much more conspicuous to their enemies; but whether their
dull tints have been specially gained for the sake of protection
seems, as far as I can judge, rather doubtful. It is still more
doubtful whether such dull tints can have been gained for the sake
of ornament. We must, however, bear in mind that male birds, though
dull-coloured, often differ much from their females (as with the
common sparrow), and this leads to the belief that such colours have
been gained through sexual selection, from being attractive. Many of
the soft-billed birds are songsters; and a discussion in a former
chapter should not be forgotten, in which it was shewn that the best
songsters are rarely ornamented with bright tints. It would appear
that female birds, as a general rule, have selected their mates either
for their sweet voices or gay colours, but not for both charms
combined. Some species, which are manifestly coloured for the sake
of protection, such as the jack-snipe, woodcock, and night-jar, are
likewise marked and shaded, according to our standard of taste, with
extreme elegance. In such cases we may conclude that both natural
and sexual selection have acted conjointly for protection and
ornament. Whether any bird exists which does not possess some
special attraction, by which to charm the opposite sex, may be
doubted. When both sexes are so obscurely coloured that it would be
rash to assume the agency of sexual selection, and when no direct
evidence can be advanced shewing that such colours serve as a
protection, it is best to own complete ignorance of the cause, or,
which comes to nearly the same thing, to attribute the result to the
direct action of the conditions of life.
  Both sexes of many birds are conspicuously, though not brilliantly
coloured, such as the numerous black, white, or piebald species; and
these colours are probably the result of sexual selection. With the
common blackbird, capercailzie, blackcock, black scoter-duck
(Oidemia), and even with one of the birds of paradise (Lophorina
atra), the males alone are black, whilst the females are brown or
mottled; and there can hardly be a doubt that blackness in these cases
has been a sexually selected character. Therefore it is in some degree
probable that the complete or partial blackness of both sexes in
such birds as crows, certain cockatoos, storks, and swans, and many
marine birds, is likewise the result of sexual selection,
accompanied by equal transmission to both sexes; for blackness can
hardly serve in any case as a protection. With several birds, in which
the male alone is black, and in others in which both sexes are
black, the beak or skin about the head is brightly coloured, and the
contrast thus afforded adds much to their beauty; we see this in the
bright yellow beak of the male blackbird, in the crimson skin over the
eyes of the blackcock and capercailzie, in the brightly and
variously coloured beak of the scoter-drake (Oidemia), in the red beak
of the chough (Corvus graculus, Linn.), of the black swan, and the
black stork. This leads me to remark that it is not incredible that
toucans may owe the enormous size of their beaks to sexual
selection, for the sake of displaying the diversified and vivid
stripes of colour, with which these organs are ornamented.* The
naked skin, also, at the base of the beak and round the eyes is
likewise often brilliantly coloured; and Mr. Gould, in speaking of one
species,*(2) Says that the colours of the beak "are doubtless in the
finest and most brilliant state during the time of pairing." There
is no greater improbability that toucans should be encumbered with
immense beaks, though rendered as light as possible by their
cancellated structure, for the display of fine colours (an object
falsely appearing to us unimportant), than that the male Argus
pheasant and some other birds should be encumbered with plumes so long
as to impede their flight.

  * No satisfactory explanation has ever been offered of the immense
size, and still less of the bright colours, of the toucan's beak.
Mr. Bates (The Naturalist on the Amazons, vol. ii., 1863, p. 341)
states that they use their beaks for reaching fruit at the extreme
tips of the branches; and likewise, as stated by other authors, for
extracting eggs and young birds from the nests of other birds. But, as
Mr. Bates admits, the beak "can scarcely be considered a very
perfectly-formed instrument for the end to which it is applied." The
great bulk of the beak, as shown by its breadth, depth, as well as
length, is not intelligible on the view that it serves merely as an
organ of prehension. Mr. Belt believes (The Naturalist in Nicaragua,
p. 197) that the principal use of the beak is as a defence against
enemies, especially to the female whilst nesting in a hole in a tree.
  *(2) Rhamphastos carinatus, Gould's Monograph of Ramphastidae.

   In the same manner, as the males alone of various species are
black, the females being dull-coloured; so in a few cases the males
alone are either wholly or partially white, as with the several
bell-birds of South America (Chasmorhynchus), the Antarctic goose
(Bernicla antarctica), the silver pheasant, &c., whilst the females
are brown or obscurely mottled. Therefore, on the same principle as
before, it is probable that both sexes of many birds, such as white
cockatoos, several egrets with their beautiful plumes, certain ibises,
gulls, terns, &c., have acquired their more or less completely white
plumage through sexual selection. In some of these cases the plumage
becomes white only at maturity. This is the case with certain gannets,
tropic-birds, &c., and with the snow-goose (Anser hyperboreus). As the
latter breeds on the "barren grounds," when not covered with snow, and
as it migrates southward during the winter, there is no reason to
suppose that its snow-white adult plumage serves as a protection. In
the Anastomus oscitans, we have still better evidence that the white
plumage is nuptial character, for it is developed only during the
summer; the young in their immature state, and the adults in their
winter dress, being grey and black. With many kinds of gulls
(Larus), the head and neck become pure white during the summer,
being grey or mottled during the winter and in the young state. On the
other hand, with the smaller gulls, or sea-mews (Gavia), and with some
terns (Sterna), exactly the reverse occurs; for the heads of the young
birds during the first year, and of the adults during the winter,
are either pure white, or much paler coloured than during the
breeding-season. These latter cases offer another instance of the
capricious manner in which sexual selection appears often to have
acted.*

  * On Larus, Gavia, and Sterna, see Macgillivray, History of
British Birds, vol. v., pp. 515, 584, 626. On the Anser hyperboreus,
Audubon, Ornithological Biography, vol. iv., p. 562. On the Anastomus,
Mr. Blyth, in Ibis, 1867, p. 173.

  That aquatic birds have acquired a white plumage so much oftener
than terrestrial birds, probably depends on their large size and
strong powers of flight, so that they can easily defend themselves
or escape from birds of prey, to which moreover they are not much
exposed. Consequently, sexual selection has not here been interfered
with or guided for the sake of protection. No doubt with birds which
roam over the open ocean, the males and females could find each
other much more easily, when made conspicuous either by being
perfectly white or intensely black; so that these colours may possible
serve the same end as the call-notes of many land-birds.* A white or
black bird when it discovers and flies down to a carcase floating on
the sea or cast up on the beach, will be seen from a great distance,
and will guide other birds of the same and other species, to the prey;
but as this would be a disadvantage to the first finders, the
individuals which were the whitest or blackest would not thus
procure more food than the less strongly coloured individuals. Hence
conspicuous colours cannot have been gradually acquired for this
purpose through natural selection.

  * It may be noticed that with vultures, which roam far and wide high
in the air, like marine birds over the ocean, three or four species
are almost wholly or largely white, and that many others are black. So
that here again conspicuous colours may possibly aid the sexes in
finding each other during the breeding-season.

  As sexual selection depends on so fluctuating an element as taste,
we can understand how it is that, within the same group of birds
having nearly the same habits, there should exist white or nearly
white, as well as black, or nearly black species,- for instance,
both white and black cockatoos, storks, ibises, swans, terns, and
petrels. Piebald birds likewise sometimes occur in the same groups
together with black and white species; for instance, the
black-necked swan, certain terns, and the common magpie. That a strong
contrast in colour is agreeable to birds, we may conclude by looking
through any large collection, for the sexes often differ from each
other in the male having the pale parts of a purer white, and the
variously coloured dark parts of still darker tints than the female.
  It would even appear that mere novelty, or slight changes for the
sake of change, have sometimes acted on female birds as a charm,
like changes of fashion with us. Thus the males of some parrots can
hardly be said to be more beautiful than the females, at least
according to our taste, but they differ in such points, as in having a
rose-coloured collar instead of "a bright emeraldine narrow green
collar"; or in the male having a black collar instead of "a yellow
demi-collar in front," with a pale roseate instead of a plum-blue
head.* As so many male birds have elongated tail-feathers or elongated
crests for their chief ornament, the shortened tail, formerly
described in the male of a humming-bird, and the shortened crest of
the male goosander, seem like one of the many changes of fashion which
we admire in our own dresses.

  * See Jerdon on the genus Palaeornis, Birds of India, vol. i., pp.
258-260.

  Some members of the heron family offer a still more curious case
of novelty in colouring having, as it appears, been appreciated for
the sake of novelty. The young of the Ardea asha are white, the adults
being dark slate-coloured; and not only the young, but the adults in
their winter plumage, of the allied Buphus coromandus are white,
this colour changing into a rich golden-buff during the
breeding-season. It is incredible that the young of these two species,
as well as of some other members of the same family,* should for any
special purpose have been rendered pure white and thus made
conspicuous to their enemies; or that the adults of one of these two
species should have been specially rendered white during the winter in
a country which is never covered with snow. On the other hand we
have good reason to believe that whiteness has been gained by many
birds as a sexual ornament. We may therefore conclude that some
early progenitor of the Ardea asha and the Buphus acquired a white
plumage for nuptial purposes, and transmitted this colour to their
young; so that the young and the old became white like certain
existing egrets; and that the whiteness was afterwards retained by the
young, whilst it was exchanged by the adults for more
strongly-pronounced tints. But if we could look still further back
to the still earlier progenitors of these two species, we should
probably see the adults dark-coloured. I infer that this would be
the case, from the analogy of many other birds, which are dark
whilst young, and when adult are white; and more especially from the
case of the Ardea gularis, the colours of which are the reverse of
those of A. asha, for the young are dark-coloured and the adults
white, the young having retained a former state of plumage. It appears
therefore that, during a long line of descent, the adult progenitors
of the Ardea asha, the Buphus, and of some allies, have undergone
the following changes of colour: first, a dark shade; secondly, pure
white; and thirdly, owing to another change of fashion (if I may so
express myself), their present slaty reddish, or golden-buff tints.
These successive changes are intelligible only on the principle of
novelty having been admired by birds for its own sake.

  * The young of Ardea rufescens and A. caerulea of the United
States are likewise white, the adults being coloured in accordance
with their specific names. Audubon (Ornithological Biography, vol.
iii., p. 416; vol. iv., p. 58) seems rather pleased at the thought
that this remarkable change of plumage will greatly "disconcert the
systematists."

  Several writers have objected to the whole theory of sexual
selection, by assuming that with animals and savages the taste of
the female for certain colours or other ornaments would not remain
constant for many generations; that first one colour and then
another would be admired, and consequently that no permanent effect
could be produced. We may admit that taste is fluctuating, but it is
not quite arbitrary. It depends much on habit, as we see in mankind;
and we may infer that this would hold good with birds and other
animals. Even in our own dress, the general character lasts long,
and the changes are to a certain extent graduated. Abundant evidence
will be given in two places in a future chapter, that savages of
many races have admired for many generations the same cicatrices on
the skin, the same hideously perforated lips, nostrils, or ears,
distorted heads, &c.; and these deformities present some analogy to
the natural ornaments of various animals. Nevertheless, with savages
such fashions do not endure for ever, as we may infer from the
differences in this respect between allied tribes on the same
continent. So again the raisers of fancy animals certainly have
admired for many generations and still admire the same breeds; they
earnestly desire slight changes, which are considered as improvements,
but any great or sudden change is looked at as the greatest blemish.
With birds in a state of nature we have no reason to suppose that they
would admire an entirely new style of colouration, even if great and
sudden variations often occurred, which is far from being the case. We
know that dovecot pigeons do not willingly associate with the
variously coloured fancy breeds; that albino birds do not commonly get
partners in marriage; and that the black ravens of the Feroe Islands
chase away their piebald brethren. But this dislike of a sudden change
would not preclude their appreciating slight changes, any more than it
does in the case of man. Hence with respect to taste, which depends on
many elements, but partly on habit and partly on a love of novelty,
there seems no improbability in animals admiring for a very long
period the same general style of ornamentation or other attractions,
and yet appreciating slight changes in colours, form, or sound.

  Summary of the Four Chapters on Birds.- Most male birds are highly
pugnacious during the breeding-season, and some possess weapons
adapted for fighting with their rivals. But the most pugnacious and
the best armed males rarely or never depend for success solely on
their power to drive away or kill their rivals, but have special means
for charming the female. With some it is the power of song, or of
giving forth strange cries, or instrumental music, and the males in
consequence differ from the females in their vocal organs, or in the
structure of certain feathers. From the curiously diversified means
for producing various sounds, we gain a high idea of the importance of
this means of courtship. Many birds endeavour to charm the females
by love-dances or antics, performed on the ground or in the air, and
sometimes at prepared places. But ornaments of many kinds, the most
brilliant tints, combs and wattles, beautiful plumes, elongated
feathers, top-knots, and so forth, are by far the commonest means.
In some cases mere novelty appears to have acted as a charm. The
ornaments of the males must be highly important to them, for they have
been acquired in not a few cases at the cost of increased danger
from enemies, and even at some loss of power in fighting with their
rivals. The males of very many species do not assume their
ornamental dress until they arrive at maturity, or they assume it only
during the breeding-season, or the tints then become more vivid.
Certain ornamental appendages become enlarged, turgid, and brightly
coloured during the act of courtship. The males display their charms
with elaborate care and to the best effect; and this is done in the
presence of the females. The courtship is sometimes a prolonged
affair, and many males and females congregate at an appointed place.
To suppose that the females do not appreciate the beauty of the males,
is to admit that their splendid decorations, all their pomp and
display, are useless; and this is incredible. Birds have fine powers
of discrimination, and in some few instances it can be shewn that they
have a taste for the beautiful. The females, moreover, are known
occasionally to exhibit a marked preference or antipathy for certain
individual males.
  If it be admitted that the females prefer, or are unconsciously
excited by the more beautiful males, then the males would slowly but
surely be rendered more and more attractive through sexual
selection. That it is this sex which has been chiefly modified, we may
infer from the fact that, in almost every genus where the sexes
differ, the males differ much more from one another than do the
females; this is well shewn in certain closely-allied representative
species, in which the females can hardly be distinguished, whilst
the males are quite distinct. Birds in a state of nature offer
individual differences which would amply suffice for the work of
sexual selection; but we have seen that they occasionally present more
strongly marked variations which recur so frequently that they would
immediately be fixed, if they served to allure the female. The laws of
variation must determine the nature of the initial changes, and will
have largely influenced the final result. The gradations, which may be
observed between the males of allied species, indicate the nature of
the steps through which they have passed. They explain also in the
most interesting manner how certain characters have originated, such
as the indented ocelli on the tail-feathers of the peacock, and the
ball-and-socket ocelli on the wing-feathers of the Argus pheasant.
It is evident that the brilliant colours, top-knots, fine plumes, &c.,
of many male birds cannot have been acquired as a protection;
indeed, they sometimes lead to danger. That they are not due to the
direct and definite action of the conditions of life, we may feel
assured, because the females have been exposed to the same conditions,
and yet often differ from the males to an extreme degree. Although
it is probable that changed conditions acting during a lengthened
period have in some cases produced a definite effect on both sexes, or
sometimes on one sex alone, the more important result will have been
an increased tendency to vary or to present more strongly-marked
individual differences; and such differences will have afforded an
excellent ground-work for the action of sexual selection.
  The laws of inheritance, irrespectively of selection, appear to have
determined whether the characters acquired by the males for the sake
of ornament, for producing various sounds, and for fighting
together, have been transmitted to the males alone or to both sexes,
either permanently, or periodically during certain seasons of the
year. Why various characters should have been transmitted sometimes in
one way and sometimes in another, is not in most cases known; but
the period of variability seems often to have been the determining
cause. When the two sexes have inherited all characters in common they
necessarily resemble each other; but as the successive variations
may be differently transmitted, every possible gradation may be found,
even within the same genus, from the closest similarity to the
widest dissimilarity between the sexes. With many closely-allied
species, following nearly the same habits of life, the males have come
to differ from each other chiefly through the action of sexual
selection; whilst the females have come to differ chiefly from
partaking more or less of the characters thus acquired by the males.
The effects, moreover, of the definite action of the conditions of
life, will not have been masked in the females, as in the males, by
the accumulation through sexual selection of strongly-pronounced
colours and other ornaments. The individuals of both sexes, however
affected, will have been kept at each successive period nearly uniform
by the free intercrossing of many individuals.
  With species, in which the sexes differ in colour, it is possible or
probable that some of the successive variations often tended to be
transmitted equally to both sexes; but that when this occurred the
females were prevented from acquiring the bright colours of the males,
by the destruction which they suffered during incubation. There is
no evidence that it is possible by natural selection to convert one
form of transmission into another. But there would not be the least
difficulty in rendering a female dull-coloured, the male being still
kept bright-coloured, by the selection of successive variations, which
were from the first limited in their transmission to the same sex.
Whether the females of many species have actually been thus
modified, must at present remain doubtful. When, through the law of
the equal transmission of characters to both sexes, the females were
rendered as conspicuously coloured as the males, their instincts
appear often to have been modified so that they were led to build
domed or concealed nests.
  In one small and curious class of cases the characters and habits of
the two sexes have been completely transposed, for the females are
larger, stronger, more vociferous and brighter coloured than the
males. They have, also, become so quarrelsome that they often fight
together for the possession of the males, like the males of other
pugnacious species for the possession of the females. If, as seems
probable, such females habitually drive away their rivals, and by
the display of their bright colours or other charms endeavour to
attract the males, we can understand how it is that they have
gradually been rendered, by sexual selection and sexually-limited
transmission, more beautiful than the males- the latter being left
unmodified or only slightly modified.
  Whenever the law of inheritance at corresponding ages prevails but
not that of sexually-limited transmission, then if the parents vary
late in life- and we know that this constantly occurs with our
poultry, and occasionally with other birds- the young will be left
unaffected, whilst the adults of both sexes will be modified. If
both these laws of inheritance prevail and either sex varies late in
life, that sex alone will be modified, the other sex and the young
being unaffected. When variations in brightness or in other
conspicuous characters occur early in life, as no doubt often happens,
they will not be acted on through sexual selection until the period of
reproduction arrives; consequently if dangerous to the young, they
will be eliminated through natural selection. Thus we can understand
how it is that variations arising late in life have so often been
preserved for the ornamentation of the males; the females and the
young being left almost unaffected, and therefore like each other.
With species having a distinct summer and winter plumage, the males of
which either resemble or differ from the females during both seasons
or during the summer alone, the degrees and kinds of resemblance
between the young and the old are exceedingly complex; and this
complexity apparently depends on characters, first acquired by the
males, being transmitted in various ways and degrees, as limited by
age, sex, and season.
  As the young of so many species have been but little modified in
colour and in other ornaments we are enabled to form some judgment
with respect to the plumage of their early progenitors; and we may
infer that the beauty of our existing species, if we look to the whole
class, has been largely increased since that period, of which the
immature plumage gives us an indirect record. Many birds, especially
those which live much on the ground, have undoubtedly been obscurely
coloured for the sake of protection. In some instances the upper
exposed surface of the plumage has been thus coloured in both sexes,
whilst the lower surface in the males alone has been variously
ornamented through sexual selection. Finally, from the facts given
in these four chapters, we may conclude that weapons for battle,
organs for producing sound, ornaments of many kinds, bright and
conspicuous colours, have generally been acquired by the males through
variation and sexual selection, and have been transmitted in various
ways according to the several laws of inheritance- the females and the
young being left comparatively but little modified.*

  * I am greatly indebted to the kindness of Mr. Sclater for having
looked over these four chapters on birds, and the two following ones
on mammals. In this way I have been saved from making mistakes about
the names of the species, and from stating anything as a fact which is
known to this distinguished naturalist to be erroneous. But, of
course, he is not at all answerable for the accuracy of the statements
quoted by me from various authorities.

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