Immigration restriction is a species of segregation on a large scale, by which inferior stocks can be prevented from both diluting and supplanting good stocks. Just as we isolate bacterial invasions, and starve out the bacteria by limiting the area and amount of their food-supply, so we can compel an inferior race to remain in its native habitat, where its own multiplication in a limited area, as with all organisms, eventually limit its numbers and therefore its influence. - Lothrop Stoddard