Page:George Archdall Reid 1896 The present evolution of man.djvu/293

Rh and in a country where the disease is not nearly so prevalent as in England.

We habitually speak of the fatal climate of the West Coast of Africa or of the Terai; but we are usually unaware that our own climate at the present day is nearly, if not quite, as fatal to the inhabitants of much the greater part of the world—of all the New World and of Africa, a considerable portion of Asia, and part of Europe,—and that therefore our race, which is able to persist under such adverse conditions, has undergone an evolution in relation to tuberculosis fully equal to the evolution against malaria undergone by the West Africans.

The micro-organisms of tuberculosis, since they are essentially earth-borne and entirely parasitic, are unable to travel any distance outside the living body, or to persist except under such conditions as enable them to pass almost immediately from one host to another. These conditions are best satisfied in the dwelling-houses of civilized peoples, particularly of those who dwell in cold or temperate climates. Here sputum, swarming with bacilli, falls from infected lungs to the ground, or on clothes and articles of furniture, and is dried there. The bacilli, much lighter for desiccation, but still retaining their vitality, are thereafter wafted into the air by every movement which causes the dust to rise. Where air-currents blow strongly through the houses they are mostly borne away to perish outside, for which cause tuberculosis is less prevalent in hot than in cold countries; but where such currents are practically absent—i.e. in the "well-built" houses of the cold and temperate zones, in which draughts are at a minimum—the bacilli either fall to rest again within the room, or are inhaled by its inhabitants; when such of the latter as are not already infected, and are not sufficiently resistant, contract the disease.