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 
