Environment determines the nomadic habits of the dweller of desert and steppe.  The distribution of pasture and water fixes the scope and the rate of his wandering; these in turn depend upon geographic conditions and vary with the season.  The Papago Indians of southern Arizona range with their cattle over a territory 100 by 150 miles in extent, and wander across the border into Mexico.  When their main water supply, derived from wells or artificial reservoirs near their summer villages, is exhausted, they migrate to the water-holes, springs or streams in the canons.  There the cattle graze out on the plains and return to the canons to drink.[1062] Every Mongol tribe and clan has its seasonal migration.  In winter the heavier precipitation and fuller streams enable them to collect in considerable groups in protected valleys; but the dry summer disperses them over the widest area possible, in order to utilize every water-hole and grass spot.  The hotter regions of the plains are abandoned in summer for highlands, where the short period of warmth yields temporary pastures and where alone water can be found.  The Kirghis of Russian Turkestan resort in summer to the slopes and high valleys of the Altai Mountains, where their auls or tent villages may be seen surrounded by big flocks of sheep, goats, camels, horses and cattle.[1063] The Pamir in the warm months is the gathering place for the nomads of Central Asia.  The naked desert of Arabia yields a rare herbage during the rainy season, when the Bedouin tribes resort to it for pasturage;[1064] but during the succeeding drought they scatter to the hills of Yemen, Syria and Palestine,[1065] or migrate to the valley of the Nile and Euphrates.[1066] The Arabs of the northern Sahara, followed by small flocks of sheep and goats, vibrate between the summer pastures on the slopes of the Atlas Mountains and the scant, wiry grass tufts found in winter on the borders of the desert.[1067] When the equatorial rains begin in June, the Arabs of the Atbara River follow them north-westward into the Nubian desert, and let their camel herds graze on the delicate grass which the moisture has conjured up from the sandy soil.  The country about Cassala, which is flooded during the monsoon rains by the rivers from the Abyssinian Mountains, is reserved for the dry season.[1068] In the same way the Tartar tribes of the Dnieper, Don, Volga and Ural Rivers in the thirteenth century moved down these rivers in winter to the sea coast, and in summer up-stream to the hills and mountains.[1069] So for the past hundred years the Boers of the South African grasslands have migrated in their tent wagons from the higher to the lower pastures, according to the season of the year, invading even the Karroo Desert after the short summer rains.[1070]

[Sidenote:  Marauding expeditions.]