THE PRESENT EVOLUTION OF MAN — PHYSICAL 281 



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 Exirope, — 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 civUized 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 stUl 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 baciUi 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. 



