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. If, however, the desert had not this strength, its peoples were pushed gradually north, up between Medina of the Hejaz and Kasim of Nejd, till they found themselves at the fork of two roads. They could strike eastward, by Wadi Rumh or Jebel Sham-mar, to follow eventually the Batn to Shamiya, where they would become riverine Arabs of the Lower Euphrates; or they could climb, by slow degrees, the ladder of western oases—Henakiya, Kheibar, Teima, Jauf, and the Sirhan—till fate saw them nearing Jebel Druse, in Syria, or watering their herds about Tadmor of the northern desert, on their way to Aleppo or Assyria. Nor then did the pressure cease: the inexorable trend northward continued. The tribes found themselves driven to the very edge of cultivation in Syria or Mesopotamia. Opportunity and their bellies persuaded them of the advantages of possessing goats, and then of possessing sheep; and lastly they began to sow, if only a little barley for their animals. They were now no longer Bedouin, and began to suffer like the villagers from the ravages of the nomads behind. Insensibly, they made common cause with the peasants already on the soil, and found out that they, too, were peasantry. So we see clans, born in the highlands of Yemen, thrust by stronger clans into the desert, where, unwillingly, they became nomad to keep themselves alive. We see them wandering, every year moving a little further north or a little further east as chance has sent them down one or other of the well-roads of the wilderness, till finally this pressure drives them from the desert again into the sown, with the like unwillingness of their first shrinking experiment in nomad life. This was the circulation which kept vigour in the Semitic body. There were few, if indeed there was a single northern Semite, whose ancestors had not at some dark age passed through the desert. The mark of nomadism, that most deep and biting social discipline, was on each of them in his degree.