other districts, where the microbes were less virulent, and from which they subsequently migrated.
But as regards diseases for the persistence of the microbes of which a human prey is essential, it is clear that not only have the races of men which they have afflicted undergone evolution in relation to them, but their microbes must also have undergone evolution in relation to man, since these microbes, some species of which are now wholly parasitic, must have descended from ancestors that were originally wholly saprophytic, and which, at a time when man had not yet appeared on the scene, were able to maintain existence perfectly in his absence. This evolution, this passage of the microbes from the saprophytic to the parasitic type, must have occurred at a date long subsequent to man's appearance. He cannot have inherited any of them from his brute ancestors, for, so far as we know, they never afflict animals in a wild state, though some species of animals in a domesticated state are afflicted by some of them. Again, nomadic tribes in a sparsely populated country are never afflicted by them, except sometimes when they are infected from more populous countries, and even then they are never afflicted by tuberculosis, the most death-dealing of all diseases, except they contract it under circumstances to which nomadic tribes are not usually subjected—e.g. close dwellings. An abundant and settled population is essential for their evolution and persistence—abundant as regards all of them, because otherwise the scant supply of nutriment would prevent the evolution and persistence of a parasitic species; settled as regards most of them, because otherwise, since they are earth, air, or waterborne, the removal of the nutritive supply would also prevent their evolution and persistence; and this is especially true as regards tuberculosis, the microbes of which require a specially favourable environment.
