18 



and mammal. 



SEX 



All the different modes of 

 sexual reproduction, all the different illus- 

 trations of sex-dimorphism, all the forms of 

 mating, have to be studied (Figs. 1 and 2). 



Fia. 2. Minute male and large female spider, Nephila njga> 

 f natural size. (After Vinson.) 



The more precise physiological study passes 

 insensibly into medical inquiries as to the 

 normal and abnormal exercise of sexual 

 functions, as to the phenomena of adolescence, 



