From the head of the Persian Gulf extend two great plains, the one stretching northwestward along the Tigris and Euphrates, the other northeastward along the Karoun. These two plains constitute Turkish and Persian Arabistan respectively. They were the seat of one of the earliest and most highly developed civilizations of the world, or perhaps rather of two competing and rival civilizations. Once the region teemed with a vast population. Now it is largely desert. Both plains depend for their fertility not upon the rain, but upon the rivers which flow down from the mountains. When these were diked and dammed and carried every-whither by irrigating canals, the Babylonian plain and the steppe of Persian Arabistan were immensely fertile, capable of sustaining by their own products an enormous population. Now dikes and dams are broken and canals choked and the life-creating water runs to waste, part of the year causing inundations and turning vast regions into lakes and swamps, and the remainder of the time moving seaward through a single channel, shrunk far below its banks.