252 THE PRESENT EVOLUTION OF MAN — -PHYSICAL 



man ; but they are equally able to find their nutriment 

 and to multiply in the environment outside liim; they 

 are therefore to be found in totally unpeopled districts, 

 but are limited to certain well-defined areas, where the 

 conditions of heat, moisture, nutriment, &c. are favour- 

 able; and since these conditions differ in different 

 districts, the microbes in different districts differ in 

 their pathogenic characteristics. Owing to their enor- 

 mous numbers and to the generally sparsely populated 

 condition of the countries infested by them, only an 

 infinitesimal proportion of the microbes can take up 

 their abode in human beings, and therefore the various 

 species can have undergone no evolution in relation to 

 man; but because no human being dwelling within 

 their areas of distribution escapes infection unless he is 

 immune, or death unless he is resistant, the races 

 within those areas have undergone great evolution in 

 relation to the microbes — an evolution which differs in 

 degree in different districts, being greatest where the 

 environment is most favourable to the microbes, where 

 they are most abundant and virulent, and least where 

 the environment is least favourable to them, where they 

 are least abundant and virulent. In consequence of 

 this evolution various races of mankind are able to 

 inhabit areas such as the West Coast _of Africa, and 

 that strip of forest land in India known as the Terai, 

 within which a continued existence is impossible to 

 races that have not undergone it. It is probable that 

 the races inhabiting the worst of these areas did not 

 develop the whole of their resisting powers within 

 them, for the virulence of the microbes infesting them 

 is such that, in their presence, any race which had not 

 developed some powers of resistance woxild probably 

 have been destroyed before a sufficient evolution of 

 such powers could have occurred. Probably, then, part 

 of the resisting power of these races was evolved in 



