iduals that
are best equipped for the struggle for existence, and it does so at
every stage of development; it thus improves the species in all its
stages and forms. _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.

If we were to try to guess--without knowing the facts--what means the
male animals make use of to overcome their rivals in the struggle for
the possession of the female, we might name many kinds of means, but
it would be difficult to suggest any which is not actually employed in
some ani