Here are four different classes of geographic influences, all which may become active in modifying a people when it changes its habitat. Many of the characteristics acquired in the old home still live on, or at best yield slowly to the new environment. This is especially true of the direct physical and psychical effects. But a country may work a prompt and radical change in the social organization of an immigrant people by the totally new conditions of economic life which it presents. These may be either greater wealth or poverty of natural resources than the race has previously known, new stimulants or deterrents to commerce and intercourse, and new conditions of climate which affect the efficiency of the workman and the general character of production. From these a whole complex mass of secondary effects may follow. The Aryans and Mongols, leaving their homes in the cool barren highlands of Central Asia where nature dispensed her gifts with a miserly hand, and coming down to the hot, low, fertile plains of the Indian rivers, underwent several fundamental changes in the process of adaptation to their new environment. An enervating climate did its work in slaking their energies; but more radical still was the change wrought by the contrast of poverty and abundance, enforced asceticism and luxury, presented by the old and new home. The restless, tireless shepherds became a sedentary, agricultural people; the abstemious nomads,--spare, sinewy, strangers to indulgence--became a race of rulers, revelling in luxury, lording it over countless subjects; finally, their numbers increased rapidly, no longer kept down by the scant subsistence of arid grasslands and scattered oases. In a similar way, the Arab of the desert became transformed into the sedentary lord of Spain. In the luxuriance of field and orchard which his skilful methods of irrigation and tillage produced, in the growing predominance of the intellectual over the nomadic military life, of the complex affairs of city and mart over the simple tasks of herdsman or cultivator, he lost the benefit of the early harsh training and therewith his hold upon his Iberian empire. Biblical history gives us the picture of the Sheik Abraham, accompanied by his nephew Lot, moving up from the rainless plains of Mesopotamia with his flocks and herds into the better watered Palestine. There his descendants in the garden land of Canaan became an agricultural people; and the problem of Moses and the Judges was to prevent their assimilation in religion and custom to the settled Semitic tribes about them, and to make them preserve the ideals born in the starry solitudes of the desert. [Sidenote: Retrogression in new habitat.]