142 ESSAYS OF A BIOLOGIST 



activities of man are so much more important than 

 those of other forms that even the cessation of activity 

 of the reproductive organs, for instance in the female at 

 the change of Hfe, or even their total removal, need not 

 prevent the continuation, albeit in a modified form, 

 of the sexual life in its varied indirect manifestations. 



Before attempting to probe the intricacies of the 

 mental side of the subject, we had better see what 

 we can learn of the physical. Let us first remind 

 ourselves of one or two facts gained from animal 

 experimentation. In the first place, in mammals 

 the activation of the sexual instincts of one or the 

 other sex appears to be completely or almost com- 

 pletely under the control of the internal secretions 

 of the reproductive organs. Steinach and others have 

 taken new-born male guinea-pigs and have removed 

 their testes and grafted ovaries in their place. The 

 result has been an animal almost completely feminized 

 both as regards body and mind. In some of the 

 animals milk was secreted, and when this occurred 

 they would act as foster-mothers to new-born guinea- 

 pigs of other parents. The reverse operation, the 

 masculinization of females, was equally successful, the 

 animals growing large and showing all the instincts of 

 a normal male and none of those of a normal female. 



A similar dependence of behaviour on gonad is 

 seen in fowls. Here nature makes a number of 

 experiments, which have recently been studied by 

 Dr. Crew of Edinburgh. When the ovaries of a hen 



