304 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



their areas of distribution, or so fatal as the three already 

 discussed, at least as regards malaria and tuberculosis, 

 evolution against few others is so marked as against 

 them. Moreover, most of the other very prevalent and 

 fatal diseases against which considerable evolution has 

 occurred are those affecting the alimentary tract, i. e. 

 are those in which, generally speaking, infection depends 

 on the character of the food and water supply, and 

 therefore, as regards them, since strangers who enter 

 the infected districts are usually Europeans, more careful 

 as to their food and water supply than the natives of 

 Africa and Asia, where these diseases chiefly prevail, 

 they usually suffer from them less in proportion to 

 their susceptibility than the natives. For instance, 

 Europeans actually suffer less from cholera in India 

 than do the natives, though their great susceptibility is 

 proved by the history of various pandemics, during 

 which the disease overpassed its normal boundaries, and 

 ravaged Europe. It is therefore difficult to measure 

 with any degree of accuracy the extent of the evolution 

 undergone by races that have had extended and dis- 

 astrous experience of these diseases, for we cannot with 

 accuracy contrast them with races which have had little 

 or no experience of them, and have therefore undergone 

 no evolution in relation to them. Nevertheless, in all 

 publications in which the subject is alluded to — official 

 reports, medical works, travels, &c. — we continually 

 meet statements, collectively so numerous that volumes 

 might be filled with them, that tend to show that the 

 evolution has been considerable. Thus we read that, 

 owing to malaria and dysentery, Muscat is uninhabitable 

 by Europeans during the summer months ; ^ .that in the 

 districts infested by yellow fever strangers are vastly 

 more liable to attack than natives ; ^ that in Ceylon the 

 mortality from dysentery among the troops reached the 

 1 Hirsch, vol. iii. p. 293. ^ md. vol. i. p. 340. 



