26o THE PRESENT EVOLUTION OF MAN 



tuberculosis, malaria, measles, or any other disease or ill-condition 

 suffered by parents increases the vitality of their offspring. If, 

 then, this theory is correct, all races afflicted by any sort of disease 

 should steadily drift towards extinction. 



432. Lastly, if the theory that the vast majority of variations 

 are spontaneous, be true, and, therefore, if the doctrine of Natural 

 Selection be correct, then every prevalent and lethal disease should 

 be the cause of protective evolution against itself. Diseases which 

 are not prevalent or not lethal should cause no racial change. 

 Prevalent and lethal chronic diseases, since the survivors from 

 them are people who resist infection, or who, if they recover from 

 infection, do so only because their vitality is improved, not because 

 they undergo that peculiar reaction known as acquired immunity, 

 should cause an evolution of inborn immunity an evolution of an 

 inborn power of resisting infection. Prevalent and lethal acute 

 diseases, since the survivors from them do not, as a rule, resist 

 infection, but merely when infected tend to recover within a 

 definite time, should cause an evolution of the power of acquiring 

 immunity under the experience of disease. In other words, the 

 evolution should be such that, in the case of chronic maladies, 

 immunity should develop in the individual under the stimulus of 

 nutrition, whereas in the case of acute maladies it should develop 

 in him under the stimulus of use. Thus, if the theory of spon- 

 taneous variations and of Natural Selection be true, the individuals 

 of a race that has been long and severely afflicted by tuberculosis 

 should, as a rule, display a greater innate power of resisting infec- 

 tion, or, if infected, of withstanding the malady, than individuals 

 of races that have been less or not at all afflicted. A race afflicted 

 by measles should display no greater power of resisting infection 

 than any other race, but a much greater faculty of recovering by 

 the acquirement of immunity than races that have had less experi- 

 ence of the disease. On the other hand, races that have been 

 afflicted by such non-lethal diseases as chicken - pox l should 

 possess a power of resistance no greater than that displayed by 

 races that have had no previous experience. Moreover, in all 

 cases the resisting power, whether inborn immunity or the inborn 

 power of acquiring immunity, should be specific. That is, the 

 effect of each disease on human races should be an increased 



1 In 1905 " chicken-pox is said to have claimed ninety-three victims, but it 

 is at least probable that some of the latter were unrecognized cases of the graver 

 malady " (smallpox). Sixty -eighth Annual Report of the Registrar-General of Births, 

 Deaths, and Marriages in England and Wales, p. Ixxix. 



