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
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