68 INFLUENCE OF HORMONES 



in the two sexes. On the one hand, we have extra- 

 ordinary and very conspicuous pecuHarities in the 

 male, entirely absent in the female, such as the 

 antlers of stags, and the vivid plumage of the gold 

 pheasant ; on the other we have the sexes externally 

 ahke and only distinguished by their sexual organs, 

 as in mouse, rabbit, hare, and many other Rodents, 

 most Equidae, kingfisher, crows and rooks, many 

 parrots, many Reptiles, Amphibia, Fishes, and in- 

 vertebrate animals. In the majority of fishes, in 

 which fertilisation is external and no care is taken 

 of the eggs or young, there are no somatic sexual 

 differences. Moreover, somatic sexual characters 

 where they do occur have no common characteristics 

 either in structure or position in the body. It may 

 be said that any part of the soma may in different 

 cases present a sex-limited development. In the 

 stag the male peculiarity is an enormous development 

 of bone on the head, in the peacock it is the enlarge- 

 ment of the feathers of the tail. In some birds there 

 are spurs on the legs, in others spurs on the wings. 

 It is no explanation, therefore, to say that these 

 various organs and characters are the expression of 

 sex in the somatic cells. 



As I pointed out in my Sexual Dimorphism (1900), 

 the common characteristic of somatic sexual char- 

 acters is their adaptive relation to some function 

 in the sexual habits of the species in which they 

 occur. There is no universal characteristic of sex 

 except the difference between the gametes and the 

 reproductive organs (gonads) in which they are 

 produced. All other differences, therefore, including 

 genital ducts and copulatory or intromittent organs, 

 are somatic. When we examine these somatic 



