_Civilization in Mesopotamia_.--The Tigris and Euphrates, two great rivers having their sources in mountain regions, pouring their floods for centuries into the Persian Gulf, made a broad, fertile valley along their lower courses. The soil was of inexhaustible fertility and easy of cultivation. The climate was almost rainless, and agriculture was dependent upon artificial irrigation. The upper portion of this great river valley was formed of undulating plains stretching away to the north, where, almost treeless, they furnished great pasture ranges for flocks and herds, which also added to the permanency of the food supply and helped to develop the wealth and prosperity of the country. It was in this climate, so favorable for the development of early man, and with this fertile soil yielding such bountiful productions, that the ancient Chaldean civilization started, which was followed by the Babylonian and Assyrian civilizations, each of which developed a great empire. These empires, ruling in turn, not only represented centres of civilization and wealth, but they acquired the overlordship of territories far and wide, their monarchs ruling eastward toward India and westward toward Phoenicia. In early times ancient Chaldea, located on the lower Euphrates, was divided into two parts, the lower portion known as Sumer, and the other, the upper, known as Akkad. While in the full development of these civilizations the Semitic race was dominant, there is every appearance that much of the culture of these primitive peoples came from farther east.