For the people of the desert were as little static as the people of the hills. The economic life of the desert was based on the supply of camels, which were best bred on the rigorous upland pastures with their strong nutritive thorns. By this industry the Bedouins lived; and it in turn moulded their life, apportioned the tribal areas, and kept the clans revolving through their rote of spring, summer and winter pasturages, as the herds cropped the scanty growths of each in turn. The camel markets in Syria, Mesopotamia, and Egypt determined the population which the deserts could support, and regulated strictly their standard of living. So the desert likewise overpeopled itself upon occasion; and then there were heavings and thrustings of the crowded tribes as they elbowed themselves by natural courses towards the light. They might not go south towards the inhospitable sand or sea. They could not turn west; for there the steep hills of Hejaz were thickly lined by mountain peoples taking full advantage of their defensiveness. Sometimes they went towards the central oases of Aridh and Kasim, and, if the tribes looking for new homes were strong and vigorous, might succeed in occupying parts of them.