southern Sumerian lands. With each invasion first this and then that section of the Semitic peoples comes into history. But each of such swarmings still leaves a tribal nucleus behind to supply fresh invasions in the future.

The history of the more highly organized empires of the horse and iron period, the empires of roads and writing, shows Arabia thrust like a wedge between Egypt, Palestine, and the Euphrates-Tigris country, and still a reservoir of nomadic tribes who raid and trade and exact tribute for the immunity and protection of caravans. There are temporary and flimsy subjugations. Egypt, Persia, Macedonia, Rome, Syria, Constantinople, and again Persia claim some unreal suzerainty in turn over Arabia, profess some unsubstantial protection. Under Trajan there was a Roman province of "Arabia," which included the then fertile region of the Hauran and extended as far as Petra. Now and then some Arab chief and