The Descent of Man

les, 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