268 HEREDITY AND DISEASE 



It may seem unsatisfactory to refer the origin of constitutional 

 diseases, such as insanity and obesity, to a germinal predis- 

 position i.e. to the terra ignota of the fertilised egg-cell. But 

 no other course is at present open. We are only doing in regard 

 to diseases what we must do in regard to all variations. The 

 little that can be safely said of their causes has already been 

 said in Chapter III. Variability is one of the fundamental 

 properties of the living organism, and the germ- cells are potential 

 organisms. In their relation to the body which is their mortal 

 vehicle, and in their own history, there is ample opportunity for 

 variations to arise, and among these variations we must rank 

 predispositions to disease. In short, such predispositions form 

 part of the puzzle of individuality. 



li we take a peculiarity like colour-blindness we know 

 practically nothing in regard to its origin. It is not known 

 to be associated with any structural defect of the eye ; it is 

 certainly not acquired ; it arises in a certain percentage of 

 the population, usually in males ; it is a good example of a 

 germinal variation which is exceedingly heritable. In the same 

 way, passing to disease, we cannot tell what a predisposition 

 to diabetes insipidus precisely means ; we know it in its 

 expressions in the body, but its origin is as obscure as that of 

 colour-blindness ; it is a germinal variation which is exceedingly 

 heritable. 



To illustrate the matter further, we may point out that the 

 inheritance of physiological " idiosyncrasies " which do not 

 express themselves as diseases is well established. Thus, the 

 inability to digest the proteids of eggs and milk may be a heritable 

 " family idiosyncrasy." It seems quite analogous to those 

 idiosyncrasies which under appropriate conditions manifest 

 themselves as diseases. A tendency to excessive " freckling " 

 seems to be hereditary; it implies an inborn imperfection in 

 the skin; under appropriate stimulation it may express itself 

 as Kaposi's disease. Scores of similar cases are well known, 



