and presumably also, owing to their greater size, they are less numerous than the microbes of the air-borne diseases. They are, therefore, less able to afflict sparse populations than the latter, and therefore the difference in protective evolution between the inhabitants of thickly and thinly populated lands is greater as regards them than as regards air-borne diseases.
Water-borne diseases are naturally better able to travel from point to point than earth-borne diseases, yet, journeying as they do by the great trade routes, along which the population is usually settled and dense, or down considerable rivers, on the banks of which the population is in a like condition, even they seldom afflict sparse populations. As regards trade routes, in places along them where the population is scanty, they seldom spread. As regards rivers, when carried down in the waters, they usually pass from a dense and settled population to a yet denser and more settled. They therefore seldom afflict sparse populations; and therefore the members of sparse and nomadic communities, on that ground alone, should exhibit much less protective evolution against them than the members of denser and settled communities; but because these water-borne diseases are never or very rarely so prevalent, even in the densest communities, as to cause the elimination from age to age of all individuals who are weak against them, this difference is not so marked as might otherwise be expected. Moreover, since these diseases only prevail when other conditions (of heat, &c.) besides that of density of population are favourable, the contrast here does not lie merely between dense and scanty populations, but also between one dense population and another. This is also true, but to a much less extent, as regards air-borne diseases; and it is also true, but to a much greater extent, as regards earth-borne diseases. The chief of these latter is tuberculosis; it
