266 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



guides, men who are infected with disease, especially 

 with tuberculosis. Only by preserving intact, as far as 

 possible, the conditions under which their ancestry 

 lived, and for which their evolution has therefore fitted 

 them, can we hope to save for a time these — in many 

 cases — noble and interesting races. 



The decay of the races of the New World affords the 

 most convincing, as it does the most terrible, proof that 

 man's present evolution is mainly against zymotic 

 disease. The extent to which that evolution has pro- 

 ceeded in the races of the Old World, as contrasted with 

 those of the New, affords proof, moreover, of the 

 immense antiquity of man's civilization in the former, 

 for zymotic diseases, as we see, can have arisen only long 

 after men had become very numerous and had begun 

 to gather into settled communities; since before that 

 time saprophytic micro-organisms would obviously have 

 been unable to assume entirely parasitic ways of life in 

 relation to him. The oldest histories extant tell of 

 plagues and pestilences, of water and air-borne diseases, 

 and when they were written earth-borne diseases, such 

 as tuberculosis, were doubtless present also ; yet since 

 these, though death-dealing beyond the others — at least 

 as regards tuberculosis— never prevail in a strikingly 

 epidemic form, they are not mentioned. 



We have now to consider man's evolution in relation 

 to particular zymotic diseases, but here the proofs are 

 often much less convincing than when we consider the 

 proofs of his evolution in relation to zymotic disease in 

 general. This again is just what might be expected, 

 for since many zymotic diseases are not very prevalent 

 or not very fatal, evolution against them is frequently 

 not very marked; and it is only by comparing the 

 peoples of countries in which they have been most 

 prevalent and fatal with those of countries in which 

 they have been least prevalent and fatal, or have been 



