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
