NATURAL SELECTIOy; SEXUAL SELECTION 



71 



In order to explain certain important plicnomena outside 

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

 of selective activity is recognized by many l^iologists. Tliis is 

 the theory of Sexual Selection first propounded by Darwin. 



Fig. 40. — Male and female humming bird; showing sex dimorphism, (.\fter GouKl.) 



DifTerences between male and female individuals of tlie same 

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

 of these differences are wliat miglit be called tlie necessary ones 

 due to the particular functions assumed hy eacli individual in this 

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

 mental ones of the primary reproductive ones, such others as 



