VARIATION AND HEREDITY 237 



a reaction which we call the development of im- 

 munity. This power has been previously men- 

 tioned. There is another class of disease-process 

 for which we have not found a causative agent. 

 This consists in the overemphasis of a metabolic 

 process, itself normal, which only leads to trouble 

 when the balance of the organism is thereby upset. 

 Thus the laying down of fat, which is a normal and 

 necessary bodily function, may become overempha- 

 sized and lead to a condition of obesity or even 

 " fatty degeneration." The local overdevelopment 

 of connective and epithelial tissues produces tumors, 

 etc. 



A third class of disease is concerned with the 

 nervous system. Here the difference between nor- 

 mality and abnormality is very hard to define, but 

 since the days of witch-burning and exorcism have 

 passed, no one believes that insanity is anything more 

 than a condition, not a thing, whatever may be the 

 " derangements " of the nervous system in which 

 it has its origin. 



Owing perhaps to unconscious suggestion, due to 

 the methods of insurance companies, there is a 

 rather strong popular belief in the heritability of 

 disease, a belief which, in most cases, is unfounded. 

 If we recall the mechanism of inheritance and the 

 fact that the organism derives its individual heri- 

 tage through a single cell, the gamete, and reflect 

 that disease-conditions are practically always mani- 

 fested in the soma, it becomes evident that disease 

 cannot be inherited unless there is something passed 



