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 great enough to obliterate almost every cue to the ancient kinship. The long-headed Teutonic race of northern Europe is regarded now by ethnologists as an offshoot of the long-headed brunette Mediterranean race of African origin, which became bleached out under the pale suns of Scandinavian skies. The present distribution of the various Teutonic stocks is a geographical fact; their supposed cradle in the Mediterranean basin is a geographical hypothesis. The connecting links must also be geographical. They must prove the former presence of the migrating folk in the intervening territory. A dolichocephalic substratum of population, with a negroid type of skull, has in fact been traced by archaeologists all over Europe through the early and late Stone Ages. The remains of these aboriginal inhabitants are marked in France, even in sparsely tenanted districts like the Auvergne Plateau, which is now occupied by the broad-headed Alpine race; and they are found to underlie, in point of time, other brachycephalic areas, like the Po Valley, Bavaria and Russia.[235]