THE CAUSATION OF DISEASE. 323 



the Ss ; and since, as we have seen, the Ss is almost entirely 

 the outcome of heredity, it follows that heredity determines 

 the disease in the dog. Again, let each be exposed to the 

 virus of measles : the child will this time contract the disease, 

 while the dog will escape ; and here again the character of the 

 response is determined by heredity. 



Some may doubt the legitimacy of the above implication, 

 namely, that measles is hereditary. It is, however, allowed on 

 all hands that a special tendency to contract the different 

 specific fevers displays itself in particular families, and does 

 anyone doubt that this excessive proclivity is inherited ? The 

 above argument, I think, renders it clear that all cases of 

 specific fever are hereditary in the strictest sense of the word ; 

 and if it be thought that this view requires further justification, 

 let the reader compare any specific fever, say measles, with 

 tubercle. None will deny that tubercle is hereditary, but 

 how ? The answer is, by an inheritance of a peculiar 

 structure (or soil) favourable to the development of the tuber- 

 cular bacillus ; but let the specific pathogenic E be what it 

 may, there can be no doubt that a peculiar S is necessary, 

 and that this peculiarity is inherited. Exactly the same line 

 of argument applies, of course, to measles. 



So much for the influence of heredity in the production of 

 the specific fevers. There are probably few who will not 

 acknowledge this influence ; but will they so readily allow that 

 such a disorder as flea-bite has quite as much of the hereditary 

 element in it as phthisis ? There is, however, no escape 

 from this conclusion. Some persons, if exposed to the attack 

 of fleas, are furiously bitten, and suffer the direst incon- 

 venience, while others wholly escape, or, if bitten, suifer 

 little or no discomfort. There is, in fact, a very great 

 difference between different races and different individuals in 

 respect of the tendency to be attacked by the different 

 parasites,' microscopic or macroscopic ; and these differences are 

 due to Ss, which, as we have seen, is almost entirely the 

 work of heredity. But if the element of heredity in such 

 disorders is apt to be lost sight of, how much more likely is it 

 to be overlooked in traumatic injuries ? In such injuries the 

 cause resides chiefly in the E, of which the chief element in 



