258 THE PllESENT EVOLUTION OF MAX — PHYSICAL 



populations least, and, generally speaking, only by air- 

 borne diseases; the more dense and settled populations 

 most, and, generally speaking, by the earth and water- 

 borne diseases as well as by the air-borne diseases : and 

 therefore the whole of its inhabitants from time imme- 

 morial have been undergoing evolution in relation to 

 one or more of these diseases, the sparse pojDulations 

 least, and in relation, generally speaking, to the air-borne 

 diseases only, the more dense jjojjulations most, and in re- 

 lation, 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, a 2>riori, that their inhabitants before that 

 date had undergone no evolution in relation to such 

 diseases, a conclusion which is amply confirmed by a 

 l^jsteriori considerations. Diseases of the malarial type, 

 the micro-organisms of which are perfectly able to 

 maintain a wholly saprophytic existence, and are there- 

 fore able to j)ersist in places in which a human popula- 

 tion is scanty or entirely absent, were as jorevalent in 

 the New World of old as at the present day in terri- 

 tories 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 



