have been the expansion of European peoples over the retarded regions of the world.  These peoples, coming into contact with inferior races, and armed generally with a race pride which was antagonistic to hybrid marriages, preserved their blood from extensive intermixture.  Hybridism, where it existed, was an ephemeral feature restricted to pioneer days, when white women were scarce, or to regions of extreme heat or cold, where white women and children could with difficulty survive.  Even in Spanish America, where ethnic blendings were most extensive, something of the old Spanish pride of race has reasserted itself.

[Sidenote:  Checks to differentiation.]

Improved communication maintains or increases the ranks of the intruders from the home supply.  The negroes in North America, imported as they were en masse, then steadily recruited by two centuries of the slave trade, while their race integrity was somewhat protected by social ostracism, have not been seriously modified physically by several generations of residence in a temperate land.  Their changes have been chiefly cultural.  The Englishman has altered only superficially in the various British colonial lands.  Constant intercourse and the progress of inventions have enabled him to maintain in diverse regions approximate uniformity of physical well-being, similar social and political ideals.  The changed environment modifies him in details of thought, manner, and speech, but not in fundamentals.

Moreover, civilized man spreading everywhere and turning all parts of the earth’s surface to his uses, has succeeded to some extent in reducing its physical differences.  The earth as modified by human action is a conspicuous fact of historical development.[234] Irrigation, drainage, fertilization of soils, terrace agriculture, denudation of forests and forestration of prairies have all combined to diminish the contrasts between diverse environments, while the acclimatization of plants, animals and men works even more plainly to the same end of uniformity.  The unity of the human race, varied only by superficial differences, reflects the unity of the spherical earth, whose diversities of geographical feature nowhere depart greatly from the mean except in point of climate.  Differentiation due to geography, therefore, early reached its limits.  For assimilation no limit can be forseen.

[Sidenote:  Geographical origins.]

In view of this constant differentiation on the one hand, and assimilation on the other, the historical movement has made it difficult to trace race types to their origin; and yet this is a task in which geography must have a hand.  Borrowed civilizations and purloined languages are often so many disguises which conceal the truth of ethnic relationships.  A long migration to a radically different habitat, into an outskirt or detached location protected from the swamping effects of cross-breeding, results eventually in a divergence