THE DEVELOPMENT Otf VEBTEBRATA. 293 



tions or hormones. It was contradicted from the beginning 

 by the long-known facts of the effects of castration on 

 secondary sexual characters. Darwin explained such cha- 

 racters by the selection of the males by the females, or by 

 the victory of the males which were strongest and provided 

 with the best weapons. But such selection among the 

 males would not explain the absence of the characters, 

 e.g. antlers of deer, in the females. The selection of 

 a male with antlers to mate with the female without any 

 would not cause the heredity of the antlers to be con- 

 fined to male progeny. The limitation of the heredity 

 must have preceded the selection. In fact the antlers 

 are inherited by both sexes but only developed in the 

 males, and this development does not take place if the 

 gonads are removed. 



The development of the antlers is therefore due not 

 entirely to the properties of the ovum from which the 

 animal is developed, but partly to the presence of the 

 testes, and it is now known that the influence of the testes 

 is a chemical one ; something absorbed from the testes 

 acts as a chemical stimulus on the skull which causes the 

 inherited tendency to produce antlers to become active. 

 The absence of antlers in the female, therefore, is to a 

 great extent, if not entirely, due to the fact that the 

 internal secretion of the testes is absent in the female, 

 and an internal secretion from the ovaries is present which 

 stimulates the development of female characters. 



Development, therefore, is not always a mere differen- 

 tiation of the mass of cells derived from the ovum, de- 

 pending on properties already contained in the ovum 

 before segmentation, but the degree of development in 

 one part of the body may depend on chemical compounds 

 derived from another part. This is a conception of de- 

 velopment quite different from that of Weissman and 

 his successors, who regard development as the predestined 

 course of heredity. 



18. Internal Secretions or Hormones. Many hormones 

 are known besides those derived from the sexual organs. 

 It has been proved that the respiration centre is stiniu- 



