The Descent of Man and Selection in Relation to Sex
by
Charles Darwin
Part 10 out of 17
degree; and this occurs with a host of species. If all the successive
variations were transmitted equally to both sexes, the females would be
indistinguishable from the males; and this likewise occurs with many birds.
If, however, dull colours were of high importance for the safety of the
female during incubation, as with many ground birds, the females which
varied in brightness, or which received through inheritance from the males
any marked accession of brightness, would sooner or later be destroyed.
But the tendency in the males to continue for an indefinite period
transmitting to their female offspring their own brightness, would have to
be eliminated by a change in the form of inheritance; and this, as shewn by
our previous illustration, would be extremely difficult. The more probable
result of the long-continued destruction of the more brightly-coloured
females, supposing the equal form of transmission to prevail, would be the
lessening or annihilation of the bright colours of the males, owing to
their continual crossing with the duller females. It would be tedious to
follow out all the other possible results; but I may remind the reader that
if sexually-limited variations in brightness occurred in the females, even
if they were not in the least injurious to them and consequently were not
eliminated, yet they would not be favoured or selected, for the male
usually accepts any female, and does not select the more attractive
individuals; consequently these variations would be liable to be lost, and
would have little influence on the character of the race; and this will aid
in accounting for the females being commonly duller-coloured than the
males.
In the eighth chapter instances were given, to which many might here be
added, of variations occurring at various ages, and inherited at the
corresponding age. It was also shewn that variations which occur late in
life are commonly transmitted to the same sex in which they first appear;
whilst variations occurring early in life are apt to be transmitted to both
sexes; not that all the cases of sexually-limited transmission can thus be
accounted for. It was further shewn that if a male bird varied by becoming
brighter whilst young, such variations would be of no service until the age
for reproduction had arrived, and there was competition between rival
males. But in the case of birds living on the ground and commonly in need
of the protection of dull colours, bright tints would be far more dangerous
to the young and inexperienced than to the adult males. Consequently the
males which varied in brightness whilst young would suffer much destruction
and be eliminated through natural selection; on the other hand, the males
which varied in this manner when nearly mature, notwithstanding that they
were exposed to some additional danger, might survive, and from being
favoured through sexual selection, would procreate their kind. As a
relation often exists between the period of variation and the form of
transmission, if the bright-coloured young males were destroyed and the
mature ones were successful in their courtship, the males alone would
acquire brilliant colours and would transmit them exclusively to their male
offspring. But I by no means wish to maintain that the influence of age on
the form of transmission, is the sole cause of the great difference in
brilliancy between the sexes of many birds.
When the sexes of birds differ in colour, it is interesting to determine
whether the males alone have been modified by sexual selection, the females
having been left unchanged, or only partially and indirectly thus changed;
or whether the females have been specially modified through natural
selection for the sake of protection. I will therefore discuss this
question at some length, even more fully than its intrinsic importance
deserves; for various curious collateral points may thus be conveniently
considered.
Before we enter on the subject of colour, more especially in reference to
Mr. Wallace's conclusions, it may be useful to discuss some other sexual
differences under a similar point of view. A breed of fowls formerly
existed in Germany (6. Bechstein, 'Naturgeschichte Deutschlands,' 1793, B.
iii. 339.) in which the hens were furnished with spurs; they were good
layers, but they so greatly disturbed their nests with their spurs that
they could not be allowed to sit on their own eggs. Hence at one time it
appeared to me probable that with the females of the wild Gallinaceae the
development of spurs had been checked through natural selection, from the
injury thus caused to their nests. This seemed all the more probable, as
wing-spurs, which would not be injurious during incubation, are often as
well-developed in the female as in the male; though in not a few cases they
are rather larger in the male. When the male is furnished with leg-spurs
the female almost always exhibits rudiments of them,--the rudiment
sometimes consisting of a mere scale, as in Gallus. Hence it might be
argued that the females had aboriginally been furnished with well-developed
spurs, but that these had subsequently been lost through disuse or natural
selection. But if this view be admitted, it would have to be extended to
innumerable other cases; and it implies that the female progenitors of the
existing spur-bearing species were once encumbered with an injurious
appendage.
In some few genera and species, as in Galloperdix, Acomus, and the Javan
peacock (Pavo muticus), the females, as well as the males, possess well-
developed leg-spurs. Are we to infer from this fact that they construct a
different sort of nest from that made by their nearest allies, and not
liable to be injured by their spurs; so that the spurs have not been
removed? Or are we to suppose that the females of these several species
especially require spurs for their defence? It is a more probable
conclusion that both the presence and absence of spurs in the females
result from different laws of inheritance having prevailed, independently
of natural selection. With the many females in which spurs appear as
rudiments, we may conclude that some few of the successive variations,
through which they were developed in the males, occurred very early in
life, and were consequently transferred to the females. In the other and
much rarer cases, in which the females possess fully developed spurs, we
may conclude that all the successive variations were transferred to them;
and that they gradually acquired and inherited the habit of not disturbing
their nests.
The vocal organs and the feathers variously modified for producing sound,
as well as the proper instincts for using them, often differ in the two
sexes, but are sometimes the same in both. Can such differences be
accounted for by the males having acquired these organs and instincts,
whilst the females have been saved from inheriting them, on account of the
danger to which they would have been exposed by attracting the attention of
birds or beasts of prey? This does not seem to me probable, when we think
of the multitude of birds which with impunity gladden the country with
their voices during the spring. (7. Daines Barrington, however, thought
it probable ('Philosophical Transactions,' 1773, p. 164) that few female
birds sing, because the talent would have been dangerous to them during
incubation. He adds, that a similar view may possibly account for the
inferiority of the female to the male in plumage.) It is a safer
conclusion that, as vocal and instrumental organs are of special service
only to the males during their courtship, these organs were developed
through sexual selection and their constant use in that sex alone--the
successive variations and the effects of use having been from the first
more or less limited in transmission to the male offspring.
Many analogous cases could be adduced; those for instance of the plumes on
the head being generally longer in the male than in the female, sometimes
of equal length in both sexes, and occasionally absent in the female,--
these several cases occurring in the same group of birds. It would be
difficult to account for such a difference between the sexes by the female
having been benefited by possessing a slightly shorter crest than the male,
and its consequent diminution or complete suppression through natural
selection. But I will take a more favourable case, namely the length of
the tail. The long train of the peacock would have been not only
inconvenient but dangerous to the peahen during the period of incubation
and whilst accompanying her young. Hence there is not the least a priori
improbability in the development of her tail having been checked through
natural selection. But the females of various pheasants, which apparently
are exposed on their open nests to as much danger as the peahen, have tails
of considerable length. The females as well as the males of the Menura
superba have long tails, and they build a domed nest, which is a great
anomaly in so large a bird. Naturalists have wondered how the female
Menura could manage her tail during incubation; but it is now known (8.
Mr. Ramsay, in 'Proc. Zoolog. Soc.' 1868, p. 50.) that she "enters the nest
head first, and then turns round with her tail sometimes over her back, but
more often bent round by her side. Thus in time the tail becomes quite
askew, and is a tolerable guide to the length of time the bird has been
sitting." Both sexes of an Australian kingfisher (Tanysiptera sylvia) have
the middle tail-feathers greatly lengthened, and the female makes her nest
in a hole; and as I am informed by Mr. R.B. Sharpe these feathers become
much crumpled during incubation.
In these two latter cases the great length of the tail-feathers must be in
some degree inconvenient to the female; and as in both species the tail-
feathers of the female are somewhat shorter than those of the male, it
might be argued that their full development had been prevented through
natural selection. But if the development of the tail of the peahen had
been checked only when it became inconveniently or dangerously great, she
would have retained a much longer tail than she actually possesses; for her
tail is not nearly so long, relatively to the size of her body, as that of
many female pheasants, nor longer than that of the female turkey. It must
also be borne in mind that, in accordance with this view, as soon as the
tail of the peahen became dangerously long, and its development was
consequently checked, she would have continually reacted on her male
progeny, and thus have prevented the peacock from acquiring his present
magnificent train. We may therefore infer that the length of the tail in
the peacock and its shortness in the peahen are the result of the requisite
variations in the male having been from the first transmitted to the male
offspring alone.
We are led to a nearly similar conclusion with respect to the length of the
tail in the various species of pheasants. In the Eared pheasant
(Crossoptilon auritum) the tail is of equal length in both sexes, namely
sixteen or seventeen inches; in the common pheasant it is about twenty
inches long in the male and twelve in the female; in Soemmerring's
pheasant, thirty-seven inches in the male and only eight in the female; and
lastly in Reeve's pheasant it is sometimes actually seventy-two inches long
in the male and sixteen in the female. Thus in the several species, the
tail of the female differs much in length, irrespectively of that of the
male; and this can be accounted for, as it seems to me, with much more
probability, by the laws of inheritance,--that is by the successive
variations having been from the first more or less closely limited in their
transmission to the male sex than by the agency of natural selection,
resulting from the length of tail being more or less injurious to the
females of these several allied species.
We may now consider Mr. Wallace's arguments in regard to the sexual
coloration of birds. He believes that the bright tints originally acquired
through sexual selection by the males would in all, or almost all cases,
have been transmitted to the females, unless the transference had been
checked through natural selection. I may here remind the reader that
various facts opposed to this view have already been given under reptiles,
amphibians, fishes and lepidoptera. Mr. Wallace rests his belief chiefly,
but not exclusively, as we shall see in the next chapter, on the following
statement (9. 'Journal of Travel,' edited by A. Murray, vol. i. 1868, p.
78.), that when both sexes are coloured in a very conspicuous manner, the
nest is of such a nature as to conceal the sitting bird; but when there is
a marked contrast of colour between the sexes, the male being gay and the
female dull-coloured, the nest is open and exposes the sitting bird to
view. This coincidence, as far as it goes, certainly seems to favour the
belief that the females which sit on open nests have been specially
modified for the sake of protection; but we shall presently see that there
is another and more probable explanation, namely, that conspicuous females
have acquired the instinct of building domed nests oftener than dull-
coloured birds. Mr. Wallace admits that there are, as might have been
expected, some exceptions to his two rules, but it is a question whether
the exceptions are not so numerous as seriously to invalidate them.
There is in the first place much truth in the Duke of Argyll's remark (10.
'Journal of Travel,' edited by A. Murray, vol. i. 1868, p. 281.) that a
large domed nest is more conspicuous to an enemy, especially to all tree-
haunting carnivorous animals, than a smaller open nest. Nor must we forget
that with many birds which build open nests, the male sits on the eggs and
aids the female in feeding the young: this is the case, for instance, with
Pyranga aestiva (11. Audubon, 'Ornithological Biography,' vol. i. p.
233.), one of the most splendid birds in the United States, the male being
vermilion, and the female light brownish-green. Now if brilliant colours
had been extremely dangerous to birds whilst sitting on their open nests,
the males in these cases would have suffered greatly. It might, however,
be of such paramount importance to the male to be brilliantly coloured, in
order to beat his rivals, that this may have more than compensated some
additional danger.
Mr. Wallace admits that with the King-crows (Dicrurus), Orioles, and
Pittidae, the females are conspicuously coloured, yet build open nests; but
he urges that the birds of the first group are highly pugnacious and could
defend themselves; that those of the second group take extreme care in
concealing their open nests, but this does not invariably hold good (12.
Jerdon, 'Birds of India,' vol. ii. p. 108. Gould's 'Handbook of the Birds
of Australia,' vol. i. p. 463.); and that with the birds of the third group
the females are brightly coloured chiefly on the under surface. Besides
these cases, pigeons which are sometimes brightly, and almost always
conspicuously coloured, and which are notoriously liable to the attacks of
birds of prey, offer a serious exception to the rule, for they almost
always build open and exposed nests. In another large family, that of the
humming-birds, all the species build open nests, yet with some of the most
gorgeous species the sexes are alike; and in the majority, the females,
though less brilliant than the males, are brightly coloured. Nor can it be
maintained that all female humming-birds, which are brightly coloured,
escape detection by their tints being green, for some display on their
upper surfaces red, blue, and other colours. (13. For instance, the
female Eupetomena macroura has the head and tail dark blue with reddish
loins; the female Lampornis porphyrurus is blackish-green on the upper
surface, with the lores and sides of the throat crimson; the female
Eulampis jugularis has the top of the head and back green, but the loins
and the tail are crimson. Many other instances of highly conspicuous
females could be given. See Mr. Gould's magnificent work on this family.)
In regard to birds which build in holes or construct domed nests, other
advantages, as Mr. Wallace remarks, besides concealment are gained, such as
shelter from the rain, greater warmth, and in hot countries protection from
the sun (14. Mr. Salvin noticed in Guatemala ('Ibis,' 1864, p. 375) that
humming-birds were much more unwilling to leave their nests during very hot
weather, when the sun was shining brightly, as if their eggs would be thus
injured, than during cool, cloudy, or rainy weather.); so that it is no
valid objection to his view that many birds having both sexes obscurely
coloured build concealed nests. (15. I may specify, as instances of dull-
coloured birds building concealed nests, the species belonging to eight
Australian genera described in Gould's 'Handbook of the Birds of
Australia,' vol. i. pp. 340, 362, 365, 383, 387, 389, 391, 414.) The
female Horn-bill (Buceros), for instance, of India and Africa is protected
during incubation with extraordinary care, for she plasters up with her own
excrement the orifice of the hole in which she sits on her eggs, leaving
only a small orifice through which the male feeds her; she is thus kept a
close prisoner during the whole period of incubation (16. Mr. C. Horne,
'Proc. Zoolog. Soc.' 1869. p. 243.); yet female horn-bills are not more
conspicuously coloured than many other birds of equal size which build open
nests. It is a more serious objection to Mr. Wallace's view, as is
admitted by him, that in some few groups the males are brilliantly coloured
and the females obscure, and yet the latter hatch their eggs in domed
nests. This is the case with the Grallinae of Australia, the Superb
Warblers (Maluridae) of the same country, the Sun-birds (Nectariniae), and
with several of the Australian Honey-suckers or Meliphagidae. (17. On the
nidification and colours of these latter species, see Gould's 'Handbook to
the Birds of Australia,' vol. i. pp. 504, 527.)
If we look to the birds of England we shall see that there is no close and
general relation between the colours of the female and the nature of the
nest which is constructed. About forty of our British birds (excluding
those of large size which could defend themselves) build in holes in banks,
rocks, or trees, or construct domed nests. If we take the colours of the
female goldfinch, bullfinch, or blackbird, as a standard of the degree of
conspicuousness, which is not highly dangerous to the sitting female, then
out of the above forty birds the females of only twelve can be considered
as conspicuous to a dangerous degree, the remaining twenty-eight being
inconspicuous. (18. I have consulted, on this subject, Macgillivray's
'British Birds,' and though doubts may be entertained in some cases in
regard to the degree of concealment of the nest, and to the degree of
conspicuousness of the female, yet the following birds, which all lay their
eggs in holes or in domed nests, can hardly be considered, by the above
standard, as conspicuous: Passer, 2 species; Sturnus, of which the female
is considerably less brilliant than the male; Cinclus; Motallica boarula
(?); Erithacus (?); Fruticola, 2 sp.; Saxicola; Ruticilla, 2 sp.; Sylvia, 3
sp.; Parus, 3 sp.; Mecistura; Anorthura; Certhia; Sitta; Yunx; Muscicapa, 2
sp.; Hirundo, 3 sp.; and Cypselus. The females of the following 12 birds
may be considered as conspicuous according to the same standard, viz.,
Pastor, Motacilla alba, Parus major and P. caeruleus, Upupa, Picus, 4 sp.,
Coracias, Alcedo, and Merops.) Nor is there any close relation within the
same genus between a well-pronounced difference in colour between the
sexes, and the nature of the nest constructed. Thus the male house sparrow
(Passer domesticus) differs much from the female, the male tree-sparrow (P.
montanus) hardly at all, and yet both build well-concealed nests. The two
sexes of the common fly-catcher (Muscicapa grisola) can hardly be
distinguished, whilst the sexes of the pied fly-catcher (M. luctuosa)
differ considerably, and both species build in holes or conceal their
nests. The female blackbird (Turdus merula) differs much, the female ring-
ouzel (T. torquatus) differs less, and the female common thrush (T.
musicus) hardly at all from their respective males; yet all build open
nests. On the other hand, the not very distantly-allied water-ouzel
(Cinclus aquaticus) builds a domed nest, and the sexes differ about as much
as in the ring-ouzel. The black and red grouse (Tetrao tetrix and T.
scoticus) build open nests in equally well-concealed spots, but in the one
species the sexes differ greatly, and in the other very little.
Notwithstanding the foregoing objections, I cannot doubt, after reading Mr.
Wallace's excellent essay, that looking to the birds of the world, a large
majority of the species in which the females are conspicuously coloured
(and in this case the males with rare exceptions are equally conspicuous),
build concealed nests for the sake of protection. Mr. Wallace enumerates
(19. 'Journal of Travel,' edited by A. Murray, vol. i. p. 78.) a long
series of groups in which this rule holds good; but it will suffice here to
give, as instances, the more familiar groups of kingfishers, toucans,
trogons, puff-birds (Capitonidae), plantain-eaters (Musophagae,
woodpeckers, and parrots. Mr. Wallace believes that in these groups, as
the males gradually acquired through sexual selection their brilliant
colours, these were transferred to the females and were not eliminated by
natural selection, owing to the protection which they already enjoyed from
their manner of nidification. According to this view, their present manner
of nesting was acquired before their present colours. But it seems to me
much more probable that in most cases, as the females were gradually
rendered more and more brilliant from partaking of the colours of the male,
they were gradually led to change their instincts (supposing that they
originally built open nests), and to seek protection by building domed or
concealed nests. No one who studies, for instance, Audubon's account of
the differences in the nests of the same species in the Northern and
Southern United States (20. See many statements in the 'Ornithological
Biography.' See also some curious observations on the nests of Italian
birds by Eugenio Bettoni, in the 'Atti della Societa Italiana,' vol. xi.
1869, p. 487.), will feel any great difficulty in admitting that birds,
either by a change (in the strict sense of the word) of their habits, or
through the natural selection of so-called spontaneous variations of
instinct, might readily be led to modify their manner of nesting.
This way of viewing the relation, as far as it holds good, between the
bright colours of female birds and their manner of nesting, receives some
support from certain cases occurring in the Sahara Desert. Here, as in
most other deserts, various birds, and many other animals, have had their
colours adapted in a wonderful manner to the tints of the surrounding
surface. Nevertheless there are, as I am informed by the Rev. Mr.
Tristram, some curious exceptions to the rule; thus the male of the
Monticola cyanea is conspicuous from his bright blue colour, and the female
almost equally conspicuous from her mottled brown and white plumage; both
sexes of two species of Dromolaea are of a lustrous black; so that these
three species are far from receiving protection from their colours, yet
they are able to survive, for they have acquired the habit of taking refuge
from danger in holes or crevices in the rocks.
With respect to the above groups in which the females are conspicuously
coloured and build concealed nests, it is not necessary to suppose that
each separate species had its nidifying instinct specially modified; but
only that the early progenitors of each group were gradually led to build
domed or concealed nests, and afterwards transmitted this instinct,
together with their bright colours, to their modified descendants. As far
as it can be trusted, the conclusion is interesting, that sexual selection
together with equal or nearly equal inheritance by both sexes, have
indirectly determined the manner of nidification of whole groups of birds.
According to Mr. Wallace, even in the groups in which the females, from
being protected in domed nests during incubation, have not had their bright
colours eliminated through natural selection, the males often differ in a
slight, and occasionally in a considerable degree from the females. This
is a significant fact, for such differences in colour must be accounted for
by some of the variations in the males having been from the first limited
in transmission to the same sex; as it can hardly be maintained that these
differences, especially when very slight, serve as a protection to the
female. Thus all the species in the splendid group of the Trogons build in
holes; and Mr. Gould gives figures (21. See his Monograph of the
Trogonidae, 1st edition.) of both sexes of twenty-five species, in all of
which, with one partial exception, the sexes differ sometimes slightly,
sometimes conspicuously, in colour,--the males being always finer than the
females, though the latter are likewise beautiful. All the species of
kingfishers build in holes, and with most of the species the sexes are
equally brilliant, and thus far Mr. Wallace's rule holds good; but in some
of the Australian species the colours of the females are rather less vivid
than those of the male; and in one splendidly-coloured species, the sexes
differ so much that they were at first thought to be specifically distinct.
(22. Namely, Cyanalcyon, Gould's 'Handbook to the Birds of Australia,'
vol. i. p. 133; see, also, pp. 130, 136.) Mr. R.B. Sharpe, who has
especially studied this group, has shewn me some American species (Ceryle)
in which the breast of the male is belted with black. Again, in
Carcineutes, the difference between the sexes is conspicuous: in the male
the upper surface is dull-blue banded with black, the lower surface being
partly fawn-coloured, and there is much red about the head; in the female
the upper surface is reddish-brown banded with black, and the lower surface
white with black markings. It is an interesting fact, as shewing how the
same peculiar style of sexual colouring often characterises allied forms,
that in three species of Dacelo the male differs from the female only in
the tail being dull-blue banded with black, whilst that of the female is
brown with blackish bars; so that here the tail differs in colour in the
two sexes in exactly the same manner as the whole upper surface in the two
sexes of Carcineutes.
With parrots, which likewise build in holes, we find analogous cases: in
most of the species, both sexes are brilliantly coloured and
indistinguishable, but in not a few species the males are coloured rather
more vividly than the females, or even very differently from them. Thus,
besides other strongly-marked differences, the whole under surface of the
male King Lory (Aprosmictus scapulatus) is scarlet, whilst the throat and
chest of the female is green tinged with red: in the Euphema splendida
there is a similar difference, the face and wing coverts moreover of the
female being of a paler blue than in the male. (23. Every gradation of
difference between the sexes may be followed in the parrots of Australia.
See Gould's 'Handbook,' etc., vol. ii. pp. 14-102.) In the family of the
tits (Parinae), which build concealed nests, the female of our common blue
tomtit (Parus caeruleus), is "much less brightly coloured" than the male:
and in the magnificent Sultan yellow tit of India the difference is
greater. (24. Macgillivray's 'British Birds,' vol. ii. p. 433. Jerdon,
'Birds of India,' vol. ii. p. 282.)
Again, in the great group of the woodpeckers (25. All the following facts
are taken from M. Malherbe's magnificent 'Monographie des Picidees,'
1861.), the sexes are generally nearly alike, but in the Megapicus validus
all those parts of the head, neck, and breast, which are crimson in the
male are pale brown in the female. As in several woodpeckers the head of
the male is bright crimson, whilst that of the female is plain, it occurred
to me that this colour might possibly make the female dangerously
conspicuous, whenever she put her head out of the hole containing her nest,
and consequently that this colour, in accordance with Mr. Wallace's belief,
had been eliminated. This view is strengthened by what Malherbe states
with respect to Indopicus carlotta; namely, that the young females, like
the young males, have some crimson about their heads, but that this colour
disappears in the adult female, whilst it is intensified in the adult male.
Nevertheless the following considerations render this view extremely
doubtful: the male takes a fair share in incubation (26. Audubon's
'Ornithological Biography,' vol. ii. p. 75; see also the 'Ibis,' vol. i. p.
268.), and would be thus almost equally exposed to danger; both sexes of
many species have their heads of an equally bright crimson; in other
species the difference between the sexes in the amount of scarlet is so
slight that it can hardly make any appreciable difference in the danger
incurred; and lastly, the colouring of the head in the two sexes often
differs slightly in other ways.
The cases, as yet given, of slight and graduated differences in colour
between the males and females in the groups, in which as a general rule the
sexes resemble each other, all relate to species which build domed or
concealed nests. But similar gradations may likewise be observed in groups
in which the sexes as a general rule resemble each other, but which build
open nests.
As I have before instanced the Australian parrots, so I may here instance,
without giving any details, the Australian pigeons. (27. Gould's
'Handbook to the Birds of Australia,' vol. ii. pp. 109-149.) It deserves
especial notice that in all these cases the slight differences in plumage
between the sexes are of the same general nature as the occasionally
greater differences. A good illustration of this fact has already been
afforded by those kingfishers in which either the tail alone or the whole
upper surface of the plumage differs in the same manner in the two sexes.
Similar cases may be observed with parrots and pigeons. The differences in
colour between the sexes of the same species are, also, of the same general
nature as the differences in colour between the distinct species of the
same group. For when in a group in which the sexes are usually alike, the
male differs considerably from the female, he is not coloured in a quite
new style. Hence we may infer that within the same group the special
colours of both sexes when they are alike, and the colours of the male,
when he differs slightly or even considerably from the female, have been in
most cases determined by the same general cause; this being sexual
selection.
It is not probable, as has already been remarked, that differences in
colour between the sexes, when very slight, can be of service to the female
as a protection. Assuming, however, that they are of service, they might
be thought to be cases of transition; but we have no reason to believe that
many species at any one time are undergoing change. Therefore we can
hardly admit that the numerous females which differ very slightly in colour
from their males are now all commencing to become obscure for the sake of
protection. Even if we consider somewhat more marked sexual differences,
is it probable, for instance, that the head of the female chaffinch,--the
crimson on the breast of the female bullfinch,--the green of the female
greenfinch,--the crest of the female golden-crested wren, have all been
rendered less bright by the slow process of selection for the sake of
protection? I cannot think so; and still less with the slight differences
between the sexes of those birds which build concealed nests. On the other
hand, the differences in colour between the sexes, whether great or small,
may to a large extent be explained on the principle of the successive
variations, acquired by the males through sexual selection, having been
from the first more or less limited in their transmission to the females.
That the degree of limitation should differ in different species of the
same group will not surprise any one who has studied the laws of
inheritance, for they are so complex that they appear to us in our
ignorance to be capricious in their action. (28. See remarks to this
effect in 'Variation of Animals and Plants under Domestication,' vol. ii.
chap. xii.)
As far as I can discover there are few large groups of birds in which all
the species have both sexes alike and brilliantly coloured, but I hear from
Mr. Sclater, that this appears to be the case with the Musophagae or
plantain-eaters. Nor do I believe that any large group exists in which the
sexes of all the species are widely dissimilar in colour: Mr. Wallace
informs me that the chatterers of S. America (Cotingidae) offer one of the
best instances; but with some of the species, in which the male has a
splendid red breast, the female exhibits some red on her breast; and the
females of other species shew traces of the green and other colours of the
males. Nevertheless we have a near approach to close sexual similarity or
dissimilarity throughout several groups: and this, from what has just been
said of the fluctuating nature of inheritance, is a somewhat surprising
circumstance. But that the same laws should largely prevail with allied
animals is not surprising. The domestic fowl has produced a great number
of breeds and sub-breeds, and in these the sexes generally differ in
plumage; so that it has been noticed as an unusual circumstance when in
certain sub-breeds they resemble each other. On the other hand, the
domestic pigeon has likewise produced a vast number of distinct breeds and
sub-breeds, and in these, with rare exceptions, the two sexes are
identically alike.
Therefore if other species of Gallus and Columba were domesticated and
varied, it would not be rash to predict that similar rules of sexual
similarity and dissimilarity, depending on the form of transmission, would
hold good in both cases. In like manner the same form of transmission has
generally prevailed under nature throughout the same groups, although
marked exceptions to this rule occur. Thus within the same family or even
genus, the sexes may be identically alike, or very different in colour.
Instances have already been given in the same genus, as with sparrows, fly-
catchers, thrushes and grouse. In the family of pheasants the sexes of
almost all the species are wonderfully dissimilar, but are quite alike in
the eared pheasant or Crossoptilon auritum. In two species of Chloephaga,
a genus of geese, the male cannot be distinguished from the females, except
by size; whilst in two others, the sexes are so unlike that they might
easily be mistaken for distinct species. (29. The 'Ibis,' vol. vi. 1864,
p. 122.)
The laws of inheritance can alone account for the following cases, in which
the female acquires, late in life, certain characters proper to the male,
and ultimately comes to resemble him more or less completely. Here
protection can hardly have come into play. Mr. Blyth informs me that the
females of Oriolus melanocephalus and of some allied species, when
sufficiently mature to breed, differ considerably in plumage from the adult
males; but after the second or third moults they differ only in their beaks
having a slight greenish tinge. In the dwarf bitterns (Ardetta), according
to the same authority, "the male acquires his final livery at the first
moult, the female not before the third or fourth moult; in the meanwhile
she presents an intermediate garb, which is ultimately exchanged for the
same livery as that of the male." So again the female Falco peregrinus
acquires her blue plumage more slowly than the male. Mr. Swinhoe states
that with one of the Drongo shrikes (Dicrurus macrocercus) the male, whilst
almost a nestling, moults his soft brown plumage and becomes of a uniform
glossy greenish-black; but the female retains for a long time the white
striae and spots on the axillary feathers; and does not completely assume
the uniform black colour of the male for three years. The same excellent
observer remarks that in the spring of the second year the female spoon-
bill (Platalea) of China resembles the male of the first year, and that
apparently it is not until the third spring that she acquires the same
adult plumage as that possessed by the male at a much earlier age. The
female Bombycilla carolinensis differs very little from the male, but the
appendages, which like beads of red sealing-wax ornament the wing-feathers
(30. When the male courts the female, these ornaments are vibrated, and
"are shewn off to great advantage," on the outstretched wings: A. Leith
Adams, 'Field and Forest Rambles,' 1873, p. 153.), are not developed in her
so early in life as in the male. In the male of an Indian parrakeet
(Palaeornis javanicus) the upper mandible is coral-red from his earliest
youth, but in the female, as Mr. Blyth has observed with caged and wild
birds, it is at first black and does not become red until the bird is at
least a year old, at which age the sexes resemble each other in all
respects. Both sexes of the wild turkey are ultimately furnished with a
tuft of bristles on the breast, but in two-year-old birds the tuft is about
four inches long in the male and hardly apparent in the female; when,
however, the latter has reached her fourth year, it is from four to five
inches in length. (31. On Ardetta, Translation of Cuvier's 'Regne
Animal,' by Mr. Blyth, footnote, p. 159. On the Peregrine Falcon, Mr.
Blyth, in Charlesworth's 'Mag. of Nat. Hist.' vol. i. 1837, p. 304. On
Dicrurus, 'Ibis,' 1863, p. 44. On the Platalea, 'Ibis,' vol. vi. 1864, p.
366. On the Bombycilla, Audubon's 'Ornitholog. Biography,' vol. i. p.
229. On the Palaeornis, see, also, Jerdon, 'Birds of India,' vol. i. p.
263. On the wild turkey, Audubon, ibid. vol. i. p. 15; but I hear from
Judge Caton that in Illinois the female very rarely acquires a tuft.
Analogous cases with the females of Petrocossyphus are given by Mr. R.
Sharpe, 'Proceedings of the Zoological Society,' 1872, p. 496.)
These cases must not be confounded with those where diseased or old females
abnormally assume masculine characters, nor with those where fertile
females, whilst young, acquire the characters of the male, through
variation or some unknown cause. (32. Of these latter cases Mr. Blyth has
recorded (Translation of Cuvier's 'Regne Animal,' p. 158) various instances
with Lanius, Ruticilla, Linaria, and Anas. Audubon has also recorded a
similar case ('Ornitholog. Biography,' vol. v. p. 519) with Pyranga
aestiva.) But all these cases have so much in common that they depend,
according to the hypothesis of pangenesis, on gemmules derived from each
part of the male being present, though latent, in the female; their
development following on some slight change in the elective affinities of
her constituent tissues.
A few words must be added on changes of plumage in relation to the season
of the year. From reasons formerly assigned there can be little doubt that
the elegant plumes, long pendant feathers, crests, etc., of egrets, herons,
and many other birds, which are developed and retained only during the
summer, serve for ornamental and nuptial purposes, though common to both
sexes. The female is thus rendered more conspicuous during the period of
incubation than during the winter; but such birds as herons and egrets
would be able to defend themselves. As, however, plumes would probably be
inconvenient and certainly of no use during the winter, it is possible that
the habit of moulting twice in the year may have been gradually acquired
through natural selection for the sake of casting off inconvenient
ornaments during the winter. But this view cannot be extended to the many
waders, whose summer and winter plumages differ very little in colour.
With defenceless species, in which both sexes, or the males alone, become
extremely conspicuous during the breeding-season,--or when the males
acquire at this season such long wing or tail-feathers as to impede their
flight, as with Cosmetornis and Vidua,--it certainly at first appears
highly probable that the second moult has been gained for the special
purpose of throwing off these ornaments. We must, however, remember that
many birds, such as some of the Birds of Paradise, the Argus pheasant and
peacock, do not cast their plumes during the winter; and it can hardly be
maintained that the constitution of these birds, at least of the
Gallinaceae, renders a double moult impossible, for the ptarmigan moults
thrice in the year. (33. See Gould's 'Birds of Great Britain.') Hence it
must be considered as doubtful whether the many species which moult their
ornamental plumes or lose their bright colours during the winter, have
acquired this habit on account of the inconvenience or danger which they
would otherwise have suffered.
I conclude, therefore, that the habit of moulting twice in the year was in
most or all cases first acquired for some distinct purpose, perhaps for
gaining a warmer winter covering; and that variations in the plumage
occurring during the summer were accumulated through sexual selection, and
transmitted to the offspring at the same season of the year; that such
variations were inherited either by both sexes or by the males alone,
according to the form of inheritance which prevailed. This appears more
probable than that the species in all cases originally tended to retain
their ornamental plumage during the winter, but were saved from this
through natural selection, resulting from the inconvenience or danger thus
caused.
I have endeavoured in this chapter to shew that the arguments are not
trustworthy in favour of the view that weapons, bright colours, and various
ornaments, are now confined to the males owing to the conversion, by
natural selection, of the equal transmission of characters to both sexes,
into transmission to the male sex alone. It is also doubtful whether the
colours of many female birds are due to the preservation, for the sake of
protection, of variations which were from the first limited in their
transmission to the female sex. But it will be convenient to defer any
further discussion on this subject until I treat, in the following chapter,
of the differences in plumage between the young and old.
CHAPTER XVI.
BIRDS--concluded.
The immature plumage in relation to the character of the plumage in both
sexes when adult--Six classes of cases--Sexual differences between the
males of closely-allied or representative species--The female assuming the
characters of the male--Plumage of the young in relation to the summer and
winter plumage of the adults--On the increase of beauty in the birds of the
world--Protective colouring--Conspicuously coloured birds--Novelty
appreciated--Summary of the four chapters on Birds.
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. (1.
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.) 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.
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, etc., the young of both sexes resemble the adult
female. (2. See, for instance, Mr. Gould's account ('Handbook to 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 Palaeornis 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.) 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. (3. I owe
this information to Mr. Gould, who shewed me the specimens; see also his
'Introduction to the Trochilidae,' 1861, p. 120.) Again, the plumage of
the male goosander (Mergus merganser) is more conspicuously coloured than
that of the female, with the scapular and secondary wing-feathers 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. (4.
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 (5. 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.), 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).
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 (6. 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.) 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.
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 (7. Wallace, 'The Malay Archipelago,' vol. ii. 1869, p. 394.); 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 (8. These
species are described with coloured figures, by M. F. Pollen, in 'Ibis,'
1866, p. 275.), 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. (9. 'Variation
of Animals,' etc., 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, etc., 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 (10. Macgillivray, 'History of British
Birds,' vol. i. pp. 172-174.), 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.
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, etc., 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. (11. See, on this subject, chap. xxiii. in the
'Variation of Animals and Plants under Domestication.') 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.
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. (12. 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 (13. 'Westminster
Review,' July 1867, and A. Murray, 'Journal of Travel,' 1868, p. 83.), 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.
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 (14. For the Australian species, see Gould's
'Handbook,' etc., 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.), 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 caged 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. (15. Jerdon, 'Birds of India,' vol. iii. p. 596.
Mr. Swinhoe, in 'Ibis,' 1865, p. 542; 1866, pp. 131, 405.) Mr. Blyth
believes, that the young of both sexes resemble the adult male.
[Fig. 62. Rhynchaea capensis (from Brehm).]
The females of the three species of Painted Snipes (Rhynchaea, Fig. 62)
"are not only larger but much more richly coloured than the males." (16.
Jerdon, 'Birds of India,' vol. iii. p. 677.) 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 Rhynchaea australis it
is simple in the male, whilst in the female it makes four distinct
convolutions before entering the lungs. (17. Gould's 'Handbook to the
Birds of Australia,' vol. ii. p. 275.) 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. (18. 'The Indian
Field,' Sept. 1858, p. 3.) There is also reason to believe that the male
undertakes the duty of incubation, for Mr. Swinhoe (19. 'Ibis,' 1866, p.
298.) found the females before the close of the summer associated in
flocks, as occurs with the females of the Turnix.
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. (20. 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.) 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.
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. (21. 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.) The female
is said by Mr. T.W. Wood (22. The 'Student,' April 1870, p. 124.) 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."
(23. 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.)
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. (24. 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." (25. 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 to 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" (26. Jerdon, 'Birds of India,' vol. iii. p. 598.)--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.
CLASS III.
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 (27.
Jerdon, 'Birds of India,' vol. i. pp. 222, 228. Gould's 'Handbook to the
Birds of Australia,' vol. i. pp. 124, 130.),--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. (28. Gould, ibid. vol. ii. pp. 37, 46, 56.) 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. (29. 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 large 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. (30. 'Variation of Animals and Plants under
Domestication,' vol. ii. p. 79.) 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. (31. 'Charlesworth's Magazine of Natural History,' vol. i. 1837,
pp. 305, 306.) 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 colour of their beaks and legs,
which nearly resembled the same parts in the adults. (32. '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. (33. I am indebted to Mr.
Blyth for information in regard to this genus. The sparrow of Palestine
belongs to the sub-genus Petronia.) 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.
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 (34. 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.), second,
and fourth classes of cases; but they fail in the third, often in the fifth
(35. 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.), 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.
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. (36. Mr. Blyth, in Charlesworth's
'Magazine of Natural History,' vol. i. 1837, p. 300. Mr. Bartlett has
informed me in regard to gold pheasants.) 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. (37. I have noticed the following cases
in Audubon's 'Ornith. Biography.' The redstart 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-headed 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.)
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, etc., 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 ornamented 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. (38. See footnote 37 above.) 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.
(39. 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.) 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.
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. (40. 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.) 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.
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 (41. For illustrative cases, see vol. iv. of Macgillivray's
'History of British Birds;' on Tringa, etc., pp. 229, 271; on the Machetes,
p. 172; on the Charadrius hiaticula, p. 118; on the Charadrius pluvialis,
p. 94.), the young generally resemble the females,--as with the so-called
goldfinch of North America, and apparently with the splendid Maluri of
Australia. (42. 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 those 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.
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. (43. 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.) 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. (44. 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.) With respect to the third case,
namely, that of the young having an intermediate character between the
summer and winter adult plumages, Yarrell (45. 'History of British Birds,'
vol. i. 1839, p. 159.) 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 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
III. 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. (46. Audubon, 'Ornith.
Biography,' vol. i. p. 113.) 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. (47. 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.) 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-feathers 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. (48. 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 (49. 'Westminster Review,' July 1867, p. 5.) 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.
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." (50.
'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.) 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.
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. (51. 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 shewn 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.) 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 (52. Rhamphastos carinatus, Gould's 'Monograph of Ramphastidae.'),
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.
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, etc., 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, etc., 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, etc., 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. (53. 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. (54.
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.) 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.
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. (55. See Jerdon on the genus Palaeornis,
'Birds of India,' vol. i. pp. 258-260.) 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.
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 (56. 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."),
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
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