146 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



of the essential differences in the physical constitu- 

 tion of brains of these two types. We may specu- 

 late as to the nature of these differences, but it 

 cannot be pretended that we have facts on which 

 to support any speculations. The real explanation 

 may lie in the nature of the protoplasm of the 

 nervous material that makes up the brain, or it 

 may lie in the arrangement of the nervous elements 

 -we have at present no satisfactory methods of 

 investigation in either of these directions. 



But the mere fact that brains are of such different 

 types in respect to this susceptibility to mental 

 derangement is of basic importance, because it 

 makes clear the reason why two individuals, sub- 

 jected to exactly the same injurious influences, 

 react to them in wholly different ways. Moreover, 

 these inborn differences in organization give us the 

 clew to the proper forestalling of such disorders 

 namely, the elimination of all injurious stimuli. We 

 cannot change the make-up of the brain, but we can 

 change the environment which is constantly flood- 

 ing consciousness by means of stimuli of all kinds. 



One other fact of deep significance is plainly ob- 

 vious. A large proportion of the susceptible brains 

 are found in persons whose heredity is not above 

 criticism. If we examine into the parental characters 

 of these susceptibles, we find that nervous disorders, 

 diseases of nutrition, intoxication from alcohol, 

 syphilis, etc., are especially common. If we examine 

 into the parental characters of those who are rela- 



