SELECTION 171 



nuptial flight. There is also sexual selec- 

 tion and germinal selection, which may be 

 conveniently considered in separate sections. 



SEXUAL SELECTION. In extension of his 

 theory of natural selection Darwin proposed 

 a theory of sexual selection to account for 

 the frequent occurrence of markedly con- 

 trasted secondary sex-characters, familiar 

 in cases like peacock and peahen, stag and 

 hind. There are two modes of this sexual 

 selection: the combats between rival males 

 and the preferential mating where the fe- 

 male chooses or seems to choose. 



The issue is clearer in the case of the com- 

 bats of males. For when the younger or 

 weaker candidates are killed, or expelled 

 from the herd, or left unmated, there seems 

 little reason to doubt the discriminateness 

 of the elimination. 



As to preferential mating, there is no doubt 

 that the males, especially among birds, some- 

 times show off their varied attractions, but, 

 as Wallace has consistently maintained, 

 there is very little convincing evidence that 

 the female chooses a partner out of a number 

 of suitors. Still less is there evidence that 

 she chooses because of any particular excel- 

 lence in colour or in song or in dance. In 

 some cases, however, there is evidence that 

 certain males are left unmated, and that 



