Note 234 — Chapter IV · —The Movements Of Peoples In Their Geographical Significance

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.