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 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 



