142 ESSAYS OF A BIOLOGIST 



sexual centres in the brain that there should occur a 

 continuous liberation of secretion from the repro- 

 ductive organs into the blood. Again, the mental 

 activities of man are so much more important than 

 those of other forms that even the cessation of activ- 

 ity of the reproductive organs, for instance in the fe- 

 male at the change of life, or even their total removal, 

 need not prevent the continuation, albeit in a modi- 

 fied 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 our- 

 selves of one or two facts gained from animal experi- 

 mentation. In the first place, in mammals the ac- 

 tivation of the sexual instincts of one or the other sex 

 appears to be completely or almost completely under 

 the control of the internal secretions of the repro- 

 ductive 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 grow- 

 ing 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 



