THE CAUSATION OF DISEASE. 327 



two ; then in the fifth generation the predisposition of an 

 individual at birth will be represented by S s . Later on in 

 life this will increase to S 10 , when, on a slight exciting 

 cause ( = Ej), he will develop the disease and the formula will 

 stand thus: S 10 + E t = 1 1. Many may object that we have 

 no right, in such a case, to term the disease inherited, seeing 

 that it has not actually occurred in the ancestors of the sufferer. 

 Yet if we suppose an individual inherits S 2 from an ancestor 

 who has actually suffered from insanity, and that he develops the 

 disease from an exposure to E y , the case would universally be 

 acknowledged to be hereditary ; but surely the former case, 

 with an inherited tendency of S 8 , has a greater heritage of 

 disease. 



The subject of the gradual evolution of disease through many 

 generations has already been treated of, and let it be remem- 

 bered that in all diseases thus arising the tendency is to over- 

 look the influence of heredity. 



2. In searching into the influence of heredity in the 

 causation of any particular disease, the fact is neglected that 

 a separate disease-tendency in each of the parents may give 

 rise to a third and different tendency in the offspring. This 

 hybrid result would not by many be regarded as hereditary, 

 seeing that in such a case the parents do not suffer from 

 exactly the same conditions, but inasmuch as each element 

 in the resulting disease is inherited, so in a sense is the entire 

 •disease ; and the same holds true of the combination in the 

 ■offspring of two healthy tendencies in the parents, whereby an 

 abnormality results (supposing such a thing to be possible, 

 ■vide p. 79). 



3. Another very common source of error is the difficulty 

 of obtaining a proper family history. Some important incident 

 in this history may be purposely concealed, or, what is much 

 more likely, overlooked. In order to discover the full weight 

 of hereditary influence in the causation of any particular dis- 

 ease, it is necessary to go into the family history of many 

 ■generations, a course quite impossible. When we reflect that 

 the zebra-like stripes of the horse are descended from ancestors 

 belonging to many thousands of generations back, it is obvious 

 that the hereditary character of these stripes could not be 



z 2 



