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The Descent of Man and Selection in Relation to Sex

C >> Charles Darwin >> The Descent of Man and Selection in Relation to Sex




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.

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