54 ORGANIC EVOLUTION 



kind are, as a rule, developed only at maturity, and most 

 frequently during only a part of the year, which is invariably 

 the breeding season ; (b) they are always more or less seri- 

 ously affected by emasculation ; (c) they are always, and only, 

 displayed in perfection during the act of courtship ; (d) then, 

 however, they are displayed with the most elaborate pains ; 

 yet always, and only, before the females ; (e) they appear, at 

 all events in many cases, to. have the effect of charming the 

 females into " accepting the male. These statements are 

 perhaps a little too emphatic, yet they indicate clearly the 

 reasons for believing in sexual selection. Remembering 

 the facts of individual preference in choice of mates ob- 

 served among domestic animals by their breeders, the real- 

 ity of sexual selection seems well established. 



Groos 1 has pointed out that the coyness of the females, in 

 those groups of animals in which sexual selection occurs, 

 may be developed through natural selection. He says : " As 

 the sexual impulse must have tremendous power, it is for 

 the interest of the preservation of the species that its dis- 

 charge should be rendered difficult. This result is partly 

 accomplished in the animal world by the necessity for great 

 and often long-continued excitement as a prelude to the 

 act of pairing. This thought at once throws light on the 

 peculiar hereditary arts of courtship, especially on the indul- 

 gence in flying, dancing, or singing by a whole flock at 

 once. But the hindrance to the sexual function that is 

 most efficacious, though hitherto unappreciated, is the 

 instinctive coyness of the female. This it is that necessi- 

 tates all the arts of courtship, and the probability is that 

 seldom or never does the female exert any choice. She is 



1 The Play of Animals, Preface. 



