THE PRESENT EVOLUTION OF MAN — PHYSICAL 255 



by them, take up their abode in such long civilized com- 

 munities, they perish with almost as much certainty 

 as do Englishmen who migrate to the West Coast of 

 Africa or to the Terai. Thus, while most Englishmen 

 can dwell with impunity in London or any other 

 British town, a Pacific Islander who takes up his abode 

 there is in great danger of perishing from tuberculosis. 

 But while the members of crowded communities are, 

 on the whole, more liable to attack from zymotic 

 diseases of the non-malarial class than the members of 

 less crowded communities, or of nomadic communities, 

 and while, therefore, protective evolution against such 

 diseases has on the whole proceeded farther among the 

 former than among the latter, yet this difference in 

 liability to be attacked, and consequently in evolution, 

 between crowded and sparse communities is not 

 equally great as regards all these diseases, since the 

 microbes of some of them are much better able than 

 the microbes of others to travel from point to point, 

 and therefore to inflict the inhabitants of sparsely 

 peopled lands, and even nomadic tribes. Such diseases 

 are of the air-bome class. Their microbes have never 

 been microscopically observed, and possibly they never 

 will be so observed, for the fact that they are air-borne 

 seems to indicate that they are exceedingly minute. 

 Like the finest dust, they may be carried by the wind 

 to considerable distances, and therefore they are more 

 able to prevail among sparse populations than earth or 

 water-borne diseases, especially when, on the borders of 

 a sparsely inhabited country, there are more crowded 

 spots, among the inhabitants of which they are cDdemic. 

 Air-borne diseases may also be earth or water-borne, 

 but their chief mode of infection is through the air. 



The microbes of earth and water-borne diseases have 

 mostly been observed under the microscope. They are 

 too large and heavy to be carried far by air currents. 



