THE CAUSATION OF DISEASE. 233 



a fresh, contagious disease into a country is wont to play great 

 havoc. This has been amply proved in the case of scarlatina, 

 small-pox, and measles. It must be fresh in the memory of 

 many how the North Canadian voyageurs were destroyed almost 

 to a man by small-pox soon after their landing at Gravesend, 

 on their return from the Nile expedition. Some of them, it is 

 true, had been vaccinated, but their susceptibility was probably 

 far greater than that of the average Englishman. Darwin 

 observes that on the island of St. Helena scarlatina is dreaded 

 as a plague.* Now it is possible that communities may from 

 the first differ from one another in their susceptibility to 

 various contagia, irrespective of any process of adaptation — 

 before, namely, any adaptation by a survival of the fittest 

 could have occurred ; but it is none the less certain that the 

 difference in respect of resisting power to a particular virus 

 between communities which have for a long time been exposed 

 to its influence, and such as never have been exposed to it 

 at all, is chiefly, if not entirely, due to such process. 



It may be thought that I am exaggerating the part 

 played by natural selection in respect of specific contagia. 

 A little thought will, I think, show that this is not the 

 case. Individuals differ prodigiously in the resisting power 

 which they offer to the various contagia, wherefore natural 

 selection — the survival of those best able to resist these 

 poisons — must inevitably occur. Some families there are who 

 show a great susceptibility to measles ; others are, in like 

 manner, particularly susceptible to scarlatina; others, again, 

 to typhoid, contracting the disorder readily and in the most 

 virulent form. Our present Royal family, for instance, 

 displays a peculiar susceptibility to the typhoidal and allied 

 poisons. The individuals thus unfortunately varying are being 

 continually weeded out with an unsparing hand, albeit they 

 may, in all other regards, be perfectly healthy ; and when it 

 is remembered that, of the many who are exposed to these 

 several poisons before the procreative period, such only are 

 suffered to leave offspring as can successfully battle against 

 them, it is clear that the incapacity to withstand the poison is 

 not, in such cases, transmitted. Surely this active elimination 

 * " Voyage of Beagle" p. 434. 



