The Descent of Man

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