NATURAL SELECTION; SEXUAL SELECTION 



71 



In order to explain certain important phenomena outside 

 the apparent range of natural selection, a theory of another sort 

 of selective activity is recognized by many biologists. This is 

 the theory of Sexual Selection first propounded by Darwin. 



FIG. 40. Male and female humming bird; showing sex dimorphism. (After Gould.) 



Differences between male and female individuals of the same 

 species are the rule rather than the exception (Fig. 40). Many 

 of these differences are what might be called the necessary ones 

 due to the particular functions assumed by each individual in this 

 differentiation of sex. Of this nature are, besides those funda- 

 mental ones of the primary reproductive ones, such others as 



