254 THE TRESENT EVOLUTION OF MAN — PHYSICAL 



In other words, only after the supply of new nutriment 

 had become secure can the descendants of saprophytic 

 micro-organisms have evolved entirely parasitic habits. 

 The evolution, therefore, of such zymotic diseases as are 

 not malarial in type must have proceeded concurrently 

 with the evolution of man's civilization. With the 

 advance of this civilization, as his race, abandoning 

 nomadic habits, gathered itself into larger and larger 

 communities, into villages, towns, and cities, the 

 nutritive supply of various species of micro-organisms, 

 which had become pathogenic, became more and more 

 abundant and secure, and therefore they more and 

 more abandoned their saprophytic traits and became 

 entirely parasitic. But with the evolution of the 

 non-malarial zymotic diseases there must have pro- 

 ceeded, in such races of men as were afflicted by them, 

 a crradual evolution through survival of the fittest of the 

 power of resisting them, concurrently with which there 

 probably occurred an evolution of attacking power in 

 the microbes. At the least, as we see, there certainly 

 occurred, in consequence of the continually increasing 

 acrcrreaation of men into fixed communities, a continual 

 increase of their opportunities for attacking; so that in 

 many communities as regards many of these diseases, 

 just as regards malaria in regions afflicted by it, no 

 man, not only recently, but for ages past, has escaped 

 attack except he were immune, or death except he 

 were resistant. In such communities, which, generally 

 speaking, are those which have long been civilized, 

 i. c. which have long led a settled and crowded existence, 

 the evolution of the power of making resistance to the 

 microbes, whether of the inborn or the acquired kind, 

 has proceeded so far that their members are able to 

 exist, under conditions so favourable to the microbes, 

 that when individuals from other communities, which 

 have had little or no experience of the diseases produced 



