THE CAUSATION OF DISEASE. 333 



come of structural modifications wrought through E, then it 

 becomes obvious that S + E, or disease, is produced by E, 

 and by E alone. And that such is the origin of S, there can, 

 I think, be little doubt. Organic evolution results from the 

 summation during countless generations of individual niodifica- 

 tions of structure. But whence these modifications ? I have 

 again and again insisted that all are ultimately traceable to E. 

 To assume the existence of " spontaneous " variations is to 

 fling aside the law of causation, which tells us that nothing can 

 happen without a cause ; and what, besides E, is there that can 

 cause a modification of S ? One other cause, and one other 

 only, can be named — viz., sexual reproduction, for this leads 

 to the production of an organism which is exactly like neither 

 parent, but some sort of a mean of the two. Yet I think 

 that, even here, it may truly be said that E is the cause of the 

 structural modification. 



In order to render this evident, let us suppose that all 

 human beings were born exactly alike, save for sexual differ- 

 ences, and that the E were exactly the same for all ; it would 

 logically follow that all individuals would — except for these dif- 

 ferences — be exactly alike at corresponding ages. And if the E 

 continued the same for each individual during successive genera- 

 tions, this likeness would remain permanent in the race ; for the 

 material conditions attending the coming-into-being, and the 

 evolution of every individual of the same sex being exactly the 

 same, each successive generation of individuals of the same sex 

 would necessarily be exactly the same also. To deny this is to 

 deny the law of causation. But, given modifications of E, 

 modifications of S necessarily ensue, and it will then be impos- 

 sible to get any two individuals exactly alike. It follows from 

 this that the E is the primary cause of natural variations ; 

 once given such natural variations, however, the union of unlike 

 individuals will still further increase the tendency to vary. 



Now, the question we have to ask ourselves is this : Are we 

 to consider the sexual union of unlike individuals as a separate 

 and entirely independent cause of the variations thus resulting, 

 or are we to conclude that E is here also the real cause of 

 the variations ? I contend that the latter is the correct view. 

 To make this clear, we will suppose two individuals — male 



