by them, take up their abode in such long civilized communities, 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-borne 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 endemic. 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,
