Page:George Archdall Reid 1896 The present evolution of man.djvu/266

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 proceeded, in such races of men as were afflicted by them, a gradual 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 aggregation 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.e. 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