Page:George Archdall Reid 1896 The present evolution of man.djvu/264

man; but they are equally able to find their nutriment and to multiply in the environment outside him; 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 favourable; and since these conditions differ in different districts, the microbes in different districts differ in their pathogenic characteristics. Owing to their enormous 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 would 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