THE tribes which wander from oasis to oasis in the Arabian desert have been called Semites. Like the northern nomads, they too, when starving from widespread droughts, have advanced in hordes upon the richer lands in the river valleys, or have filtered into them as wandering traders. Since 3000 B.C., at least, the Aryan and the Semite have more than once engaged in a tug-of-war, of which the Mediterranean lands and those of Western Asia have been the prize. The first encounter took place between the Sumerians in the south and the Semitic Akkadians in the northern part of the plain. The Sumerians had drained the swamps, built irrigation canals, and established nourishing little city-states. These city-states were always fighting against each other in boundary disputes. But about 2750 B.C. they had to combine, though too late, against a Semitic invasion from Akkad. These Semites had drifted into Akkad from the desert, and they now proceeded to invade and conquer the wealthy Sumerian cities. Under the first great Semitic conqueror, Sargon, they won an empire which extended from the Tigris to the coast of the Levant. The empire did not last long, but in Sumer the two races mingled together, and the Sumerian culture and religion were shared by both. The next empire was that of Babylon. This city lying in the northern part of Akkad, just below the point where the Euphrates and the Tigris come nearest to each other, had been unimportant in early times. About 2200 B.C., however, a new dynasty arose and established a short-lived but brilliant empire. The city, which was the centre of roads from the desert, inherited the traditions of Sumer, and became the religious as well as the commercial centre of Western Asia. The Sumerians had developed a bustling urban life long before the rise of Babylon. Writing was already familiar and clay tablets were used on which impressions were made with a blunt square-tipped reed. The wedge-shaped signs made by the reed in the clay are called cuneiform. Contracts were sealed by rolling over the clay tablet a cylinder seal on which signs or pictures were cut in relief. Some of these pictures are marvels of delicate craftsmanship. Printing on movable types is merely an extension of the principle of the cylinder seal. Schools existed in which boys learnt the four hundred signs of their alphabet and the calendar of twelve lunar months with one intercalary month. From this people we derive our week of seven days, with twenty-four hours to the day, and the division of the hour into sixty minutes.