"This is not to say that other causes do not contribute to lower the birth-rate of a country, for that is an almost world-wide phenomenon. But the desire to be separated from inferiors is as strong a motive to birth-control as the desire for luxury or to ape one's economic superiors. Races follow Gresham's law as to money: the poorer of two kinds in the same place tends to supplant the better. Mark you, _supplant_, not drive out. One of the most common fallacies is the idea that the natives whose places are taken by the lower immigrants are 'driven up' to more responsible positions. A few may be pushed up; more are driven to a new locality, as happened in the mining regions; _but most are prevented from coming into existence at all_. "What is the result, then, of the migration of 1,000,000 persons of lower level into a country where the average is of a higher level? Considering the world as a whole, there are, after a few years, 2,000,000 persons of the lower type in the world, and probably from 500,000 to 1,000,000 less of the higher type. The proportion of lower to higher in the country from which the migration goes may remain the same; but in the country receiving it, it has _risen_. Is the world as a whole the gainer? "Of course the euthenist[151] says at once that these immigrants are improved. We may grant that, although the improvement is probably much exaggerated. You cannot make bad stock into good by changing its meridian, any more than you can turn a cart-horse into a hunter by putting it into a fine stable, or make a mongrel into a fine dog by teaching it tricks. But such improvement as there is involves time, expense, and trouble; and, when it is done, has anything been gained? Will any one say that the races that have supplanted the old Nordic stock in New England are any better, or as good, as the descendants of that stock would have been if their birth-rate had not been lowered? "Further, in addition to the purely biological aspects of the matter, there are certain psychological ones. Although a cosmopolitan atmosphere furnishes a certain freedom in which strong congenital talents can develop, it is a question whether as many are not injured as helped by this. Indeed, there is considerable evidence to show that for the production of great men, a certain homogeneity of environment is necessary. The reason of this is very simple. In a homogeneous community, opinions on a large number of matters are fixed. The individual does not have to attend to such things, but is free to go ahead on some special line of his own, to concentrate to his limit on his work, even though that work be fighting the common opinions.