62 THEORY OF EVOLUTION 



In the case of eosin eye color no such process 

 as that postulated by Darwin to account for 

 the differences between the sexes was involved ; 



FIG. 26. Clover butterfly (Colias philodice) with two types 

 of females, above; and one type of male, below. 



for the single mutation that brought about the 

 change also brought in the dimorphism with it. 

 In recent years zoologists have carefully 

 studied several cases in which two types of fe- 

 male are found in the same species. In the 

 common clover butterfly, there is a yellow and 

 a white type of female, while the male is yellow 

 (fig. 26). It has been shown that a single 

 factor difference determines whether the female 



