The Descent of Man

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