326 THE CAUSATION OF DISEASE. 



overlooked ? It is because when we speak of a disease being 

 hereditary, we have in mind the inheritance of something 

 peculiar to a few individuals. We do not concern ourselves 

 about the inheritance of measles, for since practically all are 

 susceptible (though in different degrees) to the poison, we 

 look upon this as quite a natural thing. But the inheritance 

 of the typical tubercular diathesis belongs to a few only ; it is 

 regarded as abnormal, and therefore much attention is paid to 

 it. And yet in a theoretical sense we are perhaps not alto- 

 gether justified in regarding the diathesis as abnormal, for the 

 negro in whom the tubercular diathesis is pronounced, might 

 live quite healthily in his native negro E. Nor is it necessary 

 to assume that the peculiar condition of tissue favourable to 

 the production of tubarcle must be the same in each case, for 

 it is conceivable that the bacillus might thrive in two or more 

 different kinds of soil. 



Although, as we have seen, all diseases are, strictly speaking, 

 hereditary, we shall find that even in regard to those diseases 

 which are universally acknowledged to be so, such as 

 insanity and rheumatism, many important considerations are 

 overlooked. One point entirely lost sight of by pathologists 

 when inquiring into hereditary history is that a disease may 

 gradually evolve, the predisposition to it increasing from 

 generation to generation ; the S may, for instance, gradually 

 change from S, to S s , when the disease will break out under 

 a specific E 3 ; that is, S 8 + E 3 =n. There is no doubt that 

 such a disease-tendency may evolve in the course of successive 

 generations by the continual operation on these several genera- 

 tions of an E capable of generating such a predisposition. Now 

 in such a case we might be inclined to regard the disease 

 tendency as quite outside the sphere of hereditary influence, 

 and, as far as I am aware, this point is always overlooked by 

 statisticians. That they err in so doing let me demonstrate 

 by the numerical method. Alcohol, it is well known, is a 

 constant cause of insanity, and this peculiar effect of it is 

 probably due to the fact that it has a special action upon nerve 

 centres, notably the higher nerve centres of the cortex. Now 

 let us suppose five generations to have led intemperate lives, 

 and in each generation the predisposition to have increased by 



