THE CAUSATION OF DISEASE. 235 



crowding of coloured men, has no effect upon them, while to the 

 white man it is terribly fatal. Did we not know the black to 

 be proof against yellow fever, we should doubtless assert with 

 confidence that the poison of this fever was one which must 

 inevitably provoke disease in all, but since such is the case, what 

 justification have we for asserting off-haud that the typhoidal 

 poison constitutes a necessarily fatal form of E, and that adapta- 

 tion to it is impossible, provided its stability were guaranteed ? 

 The aim of hygiene being to destroy, as far as may be, these 

 specific zymotic E's in the interests of humanity, we may 

 ask — Does it not, by checking adaptation, in some measure 

 defeat its own ends? The answer is decidedly — Yes. But 

 the good effects far outweigh the evil. Instance the case 

 of scarlatina. The poison is, as it were, hunted down by a 

 system of isolation and disinfection. If this system be carried 

 out with great care from year to year, the number of cases of 

 scarlatina will tend to diminish ; for, if fewer be exposed to 

 the poison, fewer can contract the fever, and in this way it 

 might be possible to exterminate the parasite altogether. The 

 effect of this would be meanwhile to increase the severity of the 

 cases which do occur — always supposing the virus to remain 

 the same — for the process of elimination will be interfered with, 

 and the standard of adaptation consequently lowered. Where- 

 fore the hygienist has two powerful arguments to justify his 

 course. First, there is the possibility of completely hunting 

 down the poison (and there is no doubt that with great care 

 this could be done) ; secondly, the poison being the product of 

 living organisms, and therefore essentially variable in cha- 

 racter, it necessarily constitutes a great check to adaptation by 

 natural variation.* 



* Mr. Parker {British Medical Journal, Nov. 5, 1878, Jan. 7, 1887) does not 

 regard the system of isolating cases of scarlatina as an unmixed blessing, for he 

 contends that the adult is quite as liable to contract the disease as the child, 

 and that the disease is more serious in adult life than in early youth. Facts, 

 however, speak strongly against both these views. Murchinson long ago asserted 

 that the adult is both less liable to contract the disease, and more likely to have 

 it in a mild degree, and the Registrar-General's report for the past year bears 

 out these statements. Mr. Parker further contends that " strict isolation will 

 gradually increase the number of persons unprotected by a previous attack, 

 and, therefore, the number of persons liable to be attacked during any given 

 epidemic." This, he tells us, is " his main contention." No one would con- 



