There are, in fact, two other causes besides climate which may prevent immigrants from multiplying in a new country.  The first of these is the presence of microbic diseases to which the old inhabitants are wholly or partially immune, but which find a virgin soil in the bodies of the newcomers.  The strongest example is the West Coast of Africa, of which Miss Mary Kingsley writes:  ’Yet remember, before you elect to cast your lot with the West Coasters, that 85 per cent, of them die of fever, or return home with their health permanently wrecked.  Also remember that there is no getting acclimatised to the Coast.  There are, it is true, a few men out there who, although they have been resident in West Africa for years, have never had fever, but you can count them on the fingers of one hand.’  There can be no acclimatisation where the weeding out is as drastic as this.  Either the anopheles mosquito or the European must quit.  There are parts of tropical America where the natives have actually been protected by the malaria, which keeps the white man at arm’s length.  But more often the microbe is on the side of the civilised race, killing off the natives who have not run the gauntlet of town-life.  The extreme reluctance of the barbarians who overran the Roman Empire to settle in the towns is easily accounted for if, as is probable, the towns killed them off whenever they attempted to live in them.  The difference is remarkable between the fate of a conquered race which has become accustomed to town-life, and that of one which has not.  There are no ‘native quarters’ in the towns of any country where the aborigines were nomads or tillers of the soil.  To the North American Indian, residence in a town is a sentence of death.  The American Indians were accustomed to none of our zymotic diseases except malaria.  In the north they were destroyed wholesale by tuberculosis; in Mexico and Peru, where large towns existed before the conquest, they fared better.  Fiji was devastated by measles; other barbarians by small-pox.  Negroes have acquired, through severe natural selection, a certain degree of immunisation in America; but even now it is said that ’every other negro dies of consumption.’  There are, however, two races, both long accustomed to town-life under horribly insanitary conditions, which have shown that they can live in almost any climate.  These are the Jews and the Chinese.  The medieval Ghetto exterminated all who were not naturally resistant to every form of microbic disease; the modern Jew, though often of poor physique, is hard to kill.  The same may be said of the Chinaman, who, when at home, lives under conditions which would kill most Europeans.