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.