THEORY OF EVOLUTION 61 



the same way as are the characters of the mu- 

 tant types — a fact that is. not generally appre- 

 ciated except by students of genetics, although 

 it is of the most far-reaching significance for 

 the theory of evolution. 



A mutant appeared in which the eye color 

 of the female was different from that of the 

 male. The eye color of the mutant female is 

 a dark eosin color, that of the male yellowish 

 eosin. From the beginning this difference was 

 as marked as it is to-day. Breeding experi- 

 ments show that eosin eye color differs from 

 the red color of the eye of the wild fly by a 

 single mutant factor. Here then at a single 

 step a type appeared that was sexually 

 dimorphic. 



Zoologists know that sexual dimorphism is 

 not uncommon in wild species of animals, and 

 Darwin proposed the theory of sexual selec- 

 tion to account for the difference between the 

 sexes. He assumed that the male preferred 

 certain kinds of females differing from himself 

 in a particular character, and thus in time 

 through sexual selection, the sexes came to 

 differ from each other. 



