=Conditions in the Tigris-Euphrates Valley.= Physical conditions in the Tigris-Euphrates valley were in many ways similar to those along the Nile. A warm but invigorating climate, fertile, alluvial soil, deposited by the great rivers and renewed each year by the floods, and the protection of the desert on the west favored the development of a virile civilization, as early if not earlier than that of Egypt. Starting from the same northern mountains, the two great rivers find their way to the Persian Gulf by widely different courses. The Tigris flows southeast in a comparatively direct course of eleven hundred miles. Its name, "The Arrow," suggests the rapidity of its descent. The Euphrates, on the contrary, makes a long detour westward toward the Mediterranean and then turns to the southeast, where for the greater part of the last half of its one thousand eight hundred miles it flows through the desert. The lands lying between the lower waters of these great rivers were by nature fitted to become the home of the earliest civilization. At a very early period these level plains attracted the nomadic tribes from the neighboring desert. Here they found soil that was exceedingly fertile, but covered to a great extent by the overflow of the great rivers. To be made productive it had to be drained in the flood and irrigated in the dry season by an extensive system of canals and reservoirs. Hence this region furnished powerful incentives to develop an energetic, enterprising civilization. The absence of natural barriers in the level plains of Babylonia and the uniformity of its physical contour meant that in time all the Tigris-Euphrates valley would inevitably be brought under one rule.