334 EVOLUTION AND GENETICS 



in harmony with the available data. The y chromosome is not 

 constant in occurrence ; inheritance of sex is the same whether the 

 y chromosome is present or absent, and finally sex-linked charac- 

 ters are transmitted by the x chromosomes. In view of these 

 facts it is impossible to accept the conclusion that genes of the 

 allosomes alone are directly responsible for the development of 

 the secondary sexual characters even in the insects. 



Although no adequate explanation of the entire phenomenon 

 of gynandromorphism is available, we cannot fail to see that the 

 chromosomes are not alone in their effect upon the organism as 

 determiners of sex. 



The secondary sexual characters of birds and mammals are 

 known to be dependent for their development upon hormones 

 secreted by various ductless glands. The gonads, although they 

 are more evidently cytogenic glands, also have the function of 

 secreting hormones which influence the development of the body, 

 and the other endocrine glands, such as the pituitary, may be 

 related so closely to sex as even to affect the development of the 

 genitalia. Obviously any effect produced by a hormone is likely 

 to be expressed in all possible parts of the individual, since the 

 hormones are distributed by the blood stream. The activity of 

 these substances would seem to be inimical to the development 

 of gynandromorphs. 



Insufficiency of hormones results in complete or partial failure 

 of the individual to develop the characters dependent upon them. 

 Since the characters of the sexes are in many cases antithetical 

 expressions of the same structure, the effect of insuflficiency is 

 often an approximation of the secondary sexual characters of the 

 opposite sex. 



An extreme degree of insufficiency is caused by removal of 

 the gonads. Since this is a common practice in the breeding of 

 animals for food its effects are well known. In all domestic 

 animals it results in the development of males which resemble 

 females instead of normal males. Removal of the ovaries is 

 almost without effect. Removal of the testes of most fowls likewise 

 results in the development of some female characteristics, but 

 early removal of the ovaries results in the almost complete develop- 

 ment of male characters in the female. 



Males of Sebright bantams have tail feathers similar to those 

 of most hens, but when the testes are removed they develop the 



