THE PRESENT EVOLUTION OF MAN — PHYSICAL 257 



afflicts, to a great extent, only dense populations, i. e. 

 populations so dense that the infected and the healthy 

 are brought into frequent contact with one another ; 

 and it afflicts to a very great extent only those dense 

 populations which inhabit a particular kind of dwelling, 

 a dwelling, namely, which is more or less air-tight, and 

 the inhabitants of which therefore breathe vitiated air. 

 Tuberculosis is never air-borne for more than a few 

 yards, but since it is a disease of long duration, it may 

 be carried under present conditions of travel round the 

 globe in the bodies of those suffering from it. If its 

 victims, when arrived in lands to which it is strange, 

 take up their abode in more or less air-tight dwellings 

 similar to those in which they contracted it, then the 

 disease will spread to their co-dwellers, and will, under 

 the circumstances, be more fatal to them than to the 

 inhabitants of the land whence it came ; but if its 

 victims, when arrived in lands to which it ,is strange, 

 take up their abode in dwellings which are freely swept 

 by all currents of air, and especially if they take up 

 their abode with nomadic peoples, then their co-dwellers 

 will comparatively seldom be infected, and the disease 

 will not spread to a great extent. 



Now we have every reason to suppose that the 

 inhabitants of the Old World, of Europe, Asia, and 

 Africa, and their adjacent islands, have been afflicted 

 for thousands of years by zymotic diseases of the non- 

 malarial type, and therefore that for thousands of years 

 evolution in relation to them has been taking place ; 

 and so dense from remote antiquity has been the 

 population of the Old World, so few and' of such 

 limited extent, so bordered by densely-peopled 

 countries, and so intersected by trade routes, its 

 sparsely-inhabited tracts, that from time immemorial 

 the whole of its inhabitants have been afflicted by one 

 or more of these diseases : the sparse and nomadic 



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