Sexual Selection 43 
are being justified: “My conviction as to the operation of natural 
selection remains unshaken,” and further, “If naturalists were to 
become more familiar with the idea of sexual selection, it would, 
I think, be accepted to a much greater extent, and already it is 
fully and favourably accepted by many competent judges.” Darwin 
was able to speak thus because he was already acquainted with an 
immense mass of facts, which, taken together, yield overwhelming 
evidence of the validity of the principle of sexual selection. 
Natural selection chooses out for reproduction the individuals 
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 tlie 
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 
seléction 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 securea-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- -oceurrence-of-secondary-sexual- characters ehiefly among 
them, and to find it: especially frequent in-polygamous- ‘species: And 
this is actually the case. 
Tf-we-were to try to guess—without knowing the facts—what 
means the male animals make use of to overcome their rivals in 
