142 HEREDITY AND SEX 



OPERATIONS ON BIRDS 



In striking contrast to these results with mammals 

 are those with birds, where in recent years we have 

 gained some definite information concerning the devel- 

 opment of secondary sexual characters. 



I am fortunate in being able to refer to several 

 cases — the most successful on record — carried out 

 by my friend, H. D. Goodale, at the Carnegie Lab- 

 oratory at Cold Spring Harbor. One ''case" is that 

 of a female Mallard duck from which the ovary was 

 completely removed when she was a very young bird. 

 Figure 16 illustrates the striking difference between 

 the normal male and the female Mallard. In the 

 spayed female the plumage is like that of the male. 



Darwin records a case in which a female duck in her 

 old age assumed the characteristics of a male, and 

 similar cases are recorded for pheasants and fowls. 



Goodale also removed the ovary from very young 

 chicks. He found that the female developed the 

 secondary sexual plumage of the cock. 



How shall we interpret these cases ? It is clear that 

 the female has the potentiality of producing the full 

 plumage of the male, but she does not do so as long as 

 the ovary is present. The ovary must therefore be 

 supposed to prevent, or inhibit, the development of 

 secondary sexual characters that appear therefore only 

 in the male. 



The converse operation — the removal of the male 

 glands from the male — is an operation that is very 

 common among poultry men. The birds grow larger 

 and fatter. They are known as capons. In this case 



