256 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



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 aiij-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 there- 

 fore 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 other- 

 wise 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 loss 

 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 



