In Africa slavery has been intimately associated with agriculture as a source of wealth, and therefore has lent motive to intertribal wars. Captives were enslaved and then gradually absorbed into the tribe of their masters. Thus war and slavery contributed greatly to that widespread blending of races which characterizes negro Africa. Slaves became a medium of exchange and an article of commerce with other continents. The negro slave trade had its chief importance in the eyes of ethnologists and historians because, in distributing the black races in white continents, it has given a "negro question" to the United States, superseded the native Indian stock of the Antilles by negroes, and left a broad negro strain in the blood of Colombia, Venezuela, and Brazil. This particular historical movement, which during the two centuries of its greatest activity involved larger numbers than the Tartar invasion of Russia or the Turkish invasion of Europe, for a long period gave to black Africa the only historical importance which it possessed for the rest of the world.[159] [Sidenote: Fusion by deported and military colonies.] In higher stages of political development, war aiming at the subjugation of large territories finds another means to fuse the subject peoples and assimilate them to a common standard of civilization. The purpose is unification and the obliteration of local differences. These are also the unconscious ends of evolution by historical movement. With this object, conquerors the world over have used a system of tribal and racial exchanges. It was the policy of the Incas of ancient Peru to remove conquered tribes to distant parts of the realm, and supply their places with colonists from other districts who had long been subjected and were more or less assimilated.[160] In 722 B.C. the Assyrian king, Sargon, overran Samaria, carried away the Ten Tribes of Israel beyond the Tigris and scattered them among the cities of Media, where they probably merged with the local population. To the country left vacant by their wholesale deportation he transplanted people from Babylon and other Mesopotamian cities.[161] The descendants of these, mingled with the poorer class of Jews still left there, formed the despised Samaritans of the time of Christ. The Kingdom of Judah later was despoiled by Nebuchadnezzar of much of its population, which was carried off to Babylon.