96 THE THEORIES OF EVOLUTION 



terflies or fishes, the manes, tufts of hair of certain 

 mammals, the song of birds, their dances and their 

 strutting, the ornaments of all description observed 

 in many animals serve no apparent purpose. The 

 fact that they generally appear in one of the sexes 

 alone, in the male, while the female is deprived of 

 them, and that, in many cases, they do not appear 

 except at breeding time, suggested to Darwin the 

 idea that their usefulness is related to the sexual func- 

 tion and has been developed through selection of a 

 special kind. 



Selection based upon sexual characters was desig- 

 nated by Darwin as sexual selection. Its purpose is 

 no longer to insure the survival of the fittest but to 

 give certain individuals of one sex a definite advan- 

 tage over other individuals of the same sex r enabling 

 them to reproduce themselves to the exclusion of the 

 others, or, for reasons to be explained later, to repro- 

 duce themselves under conditions favourable to the 

 procreation of a more numerous and more vigorous 

 progeny. As the males contend for the females, 

 sexual selection exerts its action upon them and they 

 are provided with fit weapons for this peaceful con- 

 test, bright colours and harmonious voices being 

 likely to appeal to females. 



Still these ornamental characters alone cannot de- 

 termine which males shall gain possession of the fe- 

 males; the male also wields real weapons of warfare 



