392 Darwin , and after Darwin. 



can be no doubt that healthy and vigorous birds best 

 provide for their young, natural selection, by always 

 placing its premium on health and vigour in the males, 

 thus also incidentally promotes, through correlated 

 growth, their superior coloration. 



Again, with regard to the display which is practised 

 by male birds, and which constitutes the strongest 

 of all Mr. Darwin's arguments in favour of sexual 

 selection, Mr. Wallace points out that there is no 

 evidence of the females being in any way affected 

 thereby. On the other hand, he argues that this 

 display may be due merely to general excitement ; 

 and he lays stress upon the more special fact that 

 moveable feathers are habitually erected under the 

 influence of anger and rivalry, in order to make the 

 bird look more formidable in the eyes of antago- 

 nists. 



Furthermore, he adduces the consideration that, 

 even if the females are in any way affected by colour 

 and its display on the part of the males, and if, there- 

 fore, sexual selection be conceded a true principle in 

 theory, still we must remember that, as a matter of 

 fact, it can only operate in so far as it is allowed to 

 operate by natural selection. Now, according to Mr. 

 Wallace, natural selection must wholly neutralize any 

 such supposed influence of sexual selection. For, 

 unless the survivors in the general struggle for exis- 

 tence happen to be those which are also the most 

 highly ornamented, natural selection must neutralize 

 and destroy any influence that may be exerted by 

 female selection. But obviously the chances against 

 the otherwise best fitted males happening to be like- 

 wise the most highly ornamented must be many to 



