The ceaseless human movements making for new combinations have stimulated development. They have lifted the level of culture, and worked towards homogeneity of race and civilization on a higher plane. Since the period of the great discoveries inaugurated by Columbus enabled the historical movement to compass the world, whole continents, like North America and Australia, have been reclaimed to civilization by colonization. The process of assimilation is often ruthless in its method. Hence it has been attended by a marked reduction in the number of different ethnic stocks, tribes, languages, dialects, social and cultural types through wide-spread elimination of the weak, backward or unfit.[227] These have been wiped out, either by extermination or the slower process of absorption. The Indian linguistic stocks in the United States have been reduced from fifty-three to thirty-two; and of those thirty-two, many survive as a single tribe or the shrinking remnant of one.[228] In Africa the slave trade has caused the annihilation of many small tribes.[229] The history of the Hottentots, who have been passive before the active advance of the English, Dutch and Kaffirs about them, shows a race undergoing a widespread process of hybridization[230] and extermination.[231] Strong peoples, like the English, French, Russians and Chinese, occupy ever larger areas. Where an adverse climate precludes genuine colonization, as it did for the Spanish in Central and South America, and for the English and Dutch in the Indies, they make their civilization, if not their race, permeate the acquired territory, and gradually impose on it their language and economic methods. The Poles, who once boasted a large and distinguished nationality, are being Germanized and Russified to their final national extinction. The Finns, whose Scandinavian offshoot has been almost absorbed in Sweden,[232] are being forcibly dissolved in the Muscovite dominion by powerful reaegents, by Russian schoolmasters, a Russian priesthood, Russian military service. [Sidenote: No new ethnic types.]