SEXUAL SELECTION operates only on individuals that are already capable of
reproduction, and does so only in relation to the attainment of
reproduction. It arises from the rivalry of one sex, usually the male, for
the possession of the other, usually the female. Its influence can
therefore only DIRECTLY affect one sex, in that it equips it better for
attaining possession of the other. But the effect may extend indirectly to
the female sex, and thus the whole species may be modified, without,
however, becoming any more capable of resistance in the struggle for
existence, for sexual selection only gives rise to adaptations which are
likely to give their possessor the victory over rivals in the struggle for
possession of the female, and which are therefore peculiar to the wooing
sex: the manifold "secondary sexual characters." The diversity of these
characters is so great that I cannot here attempt to give anything
approaching a complete treatment of them, but I should like to give a
sufficient number of examples to make the principle itself, in its various
modes of expression, quite clear.

One of the chief preliminary postulates of sexual selection is the unequal
number of individuals in the two sexes, for if every male immediately finds
his mate there can be no competition for the possession of the female.
Darwin has shown that, for the most part, the inequality between the sexes
is due simply to the fact that there are more males than females, and
therefore the males must take some pains to secure a mate. But the
inequality does not always depend on the numerical preponderance of the
males, it is often due to polygamy; for, if one male claims several
females, the number of females in proportion to the rest of the males will
be reduced. Since it is almost always the males that are the wooers, we
must expect to find the occurrence of secondary sexual characters chiefly
among them, and to find it especially frequent in polygamous species. And
this is actually the case.