60 SEX 



has no vital activity after it is once formed ; 

 it is the structural side that is important. 

 On the other hand, the differences in the 

 blood of a male and a female, which have 

 been shown to be of profound importance, 

 may not have any detectable structural 

 expression. Similarly, there are many subtle 

 differences in instincts and impulses, in 

 physiological habit, and in length of life, 

 which are very real though we cannot say 

 much about their structural expression. 



Another consideration that must be kept 

 in mind in the classification of sex-differences 

 is the degree of permanence in their expres- 

 sion. An adult peacock can never be confused 

 with a peahen, but there are many birds, 

 e. g. some ducks, w r hich show great dissimi- 

 larity between the sexes at the breeding 

 season and great similarity at other times of 

 year. In many fishes, such as sticklebacks, 

 the males are conspicuously different from 

 the females at the breeding time, but in- 

 conspicuously different at other times. In 

 short, there are many nuptial characters 

 which wax and wane according to the sexual 

 state of the organism. 



ILLUSTRATIONS OF SEX-CHARACTERS. Con- 

 venient surveys of sex-differences are to be 

 found in Darwin's Descent of Man and in 

 Cunningham's Sexual Dimorphism. For our 

 purpose, we do not require more than a few 

 typical cases. Among mammals illustrations 

 abound. The male gorilla is larger and 

 stronger than the female, with more powerful 

 canine teeth, and with a well-marked crest 

 along the top of the head, which is absent in 



