THE CAUSATION OF DISEASE. I 57 



One often sees, e.g., two individuals — we will call them A. and 

 B. — exposed to an exactly similar E. A. contracts disease, 

 while B. escapes unhurt. In such a case, we say A. has a 

 greater proclivity or predisposition to the disease. The two 

 cases stand thus — 



(A.) S + E = Disease. (B.) S + E = Health. 



Since the result differs, it follows from the law of causation 

 that the material conditions must be different, and, the E being 

 the same in both cases, the S of A. must differ from that of B. 

 Hence the power of the same E to call forth a given result 

 differs in the two cases. This E is, perhaps, as powerless to 

 produce disease in the case of B. as is a spark to explode damp 

 gunpowder. In short, the nature of S determines whether it 

 shall or shall not take a morbid action. 



There is no more conclusive way of proving how differently 

 different individuals respond to the same E, than by studying 

 the actions of drugs upon different individuals, since, in this 

 case, we can regulate the specific E most accurately, and be 

 quite certain, moreover, that we are exposing different indi- 

 viduals to exactly the same. 



So differently, indeed, do different individuals respond to the 

 same dose of the same drug, that we have thus a ready means 

 of testing and proving individual differences of structure, 

 which are quite undiscoverable by microscopic or other modes 

 of examination; just as, in fact (I am using Darwin's simile), 

 we can discover chemical differences in different fluids by the 

 different ways in which they respond to different re-agents.* 



The nature of the S determines, therefore, whether a particular 



E shall or shall not throw it into morbid action. 



* By means of drugs we are able to subject a number of different individuals 

 to exactly the same specific E, and with what diversity of results everybody 

 knows. These differences in S are termed idiosyncrasies. In this connection, 

 Darwin makes the following remarks : " There appears to me a strong analogy 

 between the same infection or contagion producing the same result, or one 

 closely similar in two distinct animals, and the testing of two distinct fluids 

 by the same chemical re-agent "(" Descent of Man," p. 7). And, just as we 

 infer similarity or difference between two fluids by the similarity or difference 

 in their behaviour towards certain re-agents, so likewise can we infer similarity 

 or difference among different living organisms, for it is, as above remarked, the 

 nature of S which determines whether it shall or shall not take on a particular 

 action, when exposed to a particular E. 



