But neither of these two circumstances--a lesser developed sense of race-caste in the Iberian, nor the massing of the southern Indians in settled communities--explain the disappearance of a racial frontier. The true explanation lies in the fact that Nature has grafted in the human mind instinctive impulses which are far stronger than those designated as race-prejudice. Nature has spent her most painstaking efforts in establishing within the human organization a mechanism to ensure, above all other ends, that the individual shall continue. The instinct to propagate is the strongest of the instinctive impulses with which mankind has been fitted. It dominates and conquers the race instinct on all occasions save one. Sex impulse is the battery which breaks down race-barriers. Race instinct becomes the master of sexual impulse only when a pure stock has established itself as a complete and growing community in a new country. Sexual impulses are the endowments of individual men and women; they dominate and are manifested by individuals, whereas race antipathies are manifestations not of the individual, but of the mass. Race instinct comes into play only when men, women, and children of the same stock are organized into communities. Until such a community is organized sex instinct traffics freely across racial barriers; once organized, race instinct conquers or restrains hybridization. It is a right understanding of the conditions under which human instincts work that gives us the true key to the hybridization of Spaniard and American Indian. The Iberian pioneers exposed themselves to racial contact in Mexico and Peru under conditions which were bound to give their sex impulses a victory over their race instinct. No _Mayflower_ reached the Spanish coasts of America; only bands of adventurers, who established no independent home-like settlements to form the cradles of race-feeling. The sex instinct was left dominant, and by this force the racial barriers south of the Mexican rubicon were broken down. North of this Rubicon the American continent was colonized; south of it, there was not a colonization but a plantation. From an anthropologist's point of view, as we shall note later, colonization and plantation are totally different processes. RACE PROBLEMS IN AUSTRALIA AND NEW ZEALAND