populations least, and, generally speaking, only by airborne diseases; the more dense and settled populations most, and, generally speaking, by the earth and waterborne diseases as well as by the air-borne diseases: and therefore the whole of its inhabitants from time immemorial have been undergoing evolution in relation to one or more of these diseases, the sparse populations least, and in relation, generally speaking, to the air-borne diseases only, the more dense populations most, and in relation, generally speaking, to the earth and water-borne diseases, as well as in relation to the air-borne diseases.
In the New World, in North and South America and the adjacent islands, in Australia and the' islands of the Pacific, as well as in certain oceanic islands of the Old World, such as the Andamans, the case, until recently, was very different. We have every reason to believe, that before the discovery and invasion by Europeans of these vast regions, zymotic diseases of non-malarial type were unknown, or almost unknown, to their inhabitants, and therefore we have every reason to conclude, à priori, that their inhabitants before that date had undergone no evolution in relation to such diseases, a conclusion which is amply confirmed by à posteriori considerations. Diseases of the malarial type, the micro-organisms of which are perfectly able to maintain a wholly saprophytic existence, and are therefore able to persist in places in which a human population is scanty or entirely absent, were as prevalent in the New World of old as at the present day in territories where the environment was favourable to them, and of old, as at the present day, the native inhabitants of such territories were more resistant to them than strangers from outside the infected areas. Here similar diseases caused a similar evolution in both the Old World and the New. But zymotic diseases of the non-malarial type, the microbes of which are unable to
