The Descent of Man

etween the sexes of the same species are, also,
of the same general nature as the differences in
colour between the distinct species of the same
group. For when in a group in which the sexes
are usually alike, the male differs considerably
from the female, he is not coloured in a quite
new style. Hence we may infer that within the
same group the special colours of both sexes
when they are alike, and the colours of the male,
when he differs slightly or even considerably
from the female, have been in most cases
determined by the same general cause; this
being sexual selection.
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