100 THE THEORIES OF EVOLUTION 



instanced of male birds (bullfinches, goldfinches, 

 finches, linnets) which try to attract the female by- 

 assuming attitudes in which their ornaments, whether 

 scanty or plentiful, are seen to good advantage. 



Such characters, either attractive or useful in 

 fights between males, transmitted to the offspring 

 and accumulated as other characters would be, under 

 the influence of natural selection, constitute finally 

 the marked differences which we observe between 

 males and females. 



Darwin's theory of sexual selection, formulated in 

 order to supplement the theory of natural selection, 

 found immediate acceptance with naturalists and for 

 many years was admitted without discussion. It is 

 still adhered to by many exclusive selectionists like 

 Weismann. Weismann, however, has introduced 

 into it several modifications and made a few reserva- 

 tions. For instance, he draws the line more strictly 

 between what can and what cannot be explained by 

 ordinary natural selection (for example, the weapons 

 used in the fight between males) and thus restricts 

 the sphere of influence of sexual selection. Further- 

 more, he does not always believe in an intelligent 

 choice on the part of the female, nor in a choice based 

 upon aesthetic considerations, but he thinks that cer- 

 tain symptoms of sexual excitation in the male may 

 simply influence the female and produce in her a sim- 

 ilar excitation. This is the way certain odours prob- 



