The Descent of Man

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 condi-