222 THE EVOLUTION THEORY 



on the part of the females either to the song or to the beauty of their 

 mates. And since, according to the theory, only those characters of 

 the males could be increased which decided the choice, it therefore 

 seems to me that this mutual exclusiveness of the two kinds of dis- 

 tinguishing characters is another indication of the reality of sexual 

 selection. It proves — so at least I am inclined to believe — that the 

 excitement of the female has been essentially affected by only one of 

 the characters of the male, that in the bird of Paradise it was mainly 

 the brilliance of the plumage which roused excitement, while in the 

 nightingale it was mainly the song. 



It might be objected to this that there are brilliant butterflies 

 which also possess scent-scales. This is really the case ; thus a mag- 

 nificent blue iridescent Apatura from Brazil has on the posterior 

 Avings a large yellow brush of scent-hairs, and even the beautiful 

 blue males of our Lycrenids have scent-scales in addition to their 

 beautiful colour. But this can hardly be considered as a contradiction, 

 but is rather an exception, which is the easier to explain since the 

 odoriferous apparatus is a relatively simple arrangement, which did 

 not require such a long series of generations for its evolution 

 as the complicated song-box and brain-mechanism of the singing- 

 birds. 



Moreover, it may also be that the scent-scales have arisen later 

 than the decorative colouring, and they would do so the more easily 

 since the brilliant blue, when once it was perfectly developed, and was 

 common to all the males of the species in an equal degree, was no longer 

 distinctive, and would have no specially exciting effect, while a novel 

 preferential character in the male would have a much stronger effect. 

 In the same way, the different parts of the body would be furnished 

 in succession with decorative and, therefore, exciting distinctive 

 characters. To understand this effect on the opposite sex we need 

 only think of analogous phenomena in human kind, and of the 

 strongly exciting effect that the sight of the secondary sexual 

 characters of the woman has upon the man. 



By the successive additions of new decorative characters after 

 the older ones became general and reached a climax, the origin of 

 the extraordinary diversity of the decorative plumage in one and the 

 same species of bird, can be readily understood, and the same is true 

 of the complicated decorative coloration of the butterflies in so far as it 

 depends on sexual selection, and not on other factors. The details did 

 not arise all at once, but one after the other, and every character 

 went on increasing till it had reached its limit of increase, but when- 

 ever it was common in its highest development to all the males it 



