But western Asia, or more precisely the Iranian plateau, had another piedmont region beside the zone stretching along its northern border. This second piedmont zone of grass-land, or oasis, as Breasted has pointed out, bends in the form of a horseshoe along the western slope of the Iranian plateau, then northward and westward around the headwaters of the Tigris and the Euphrates, and southward through Syria.[84] Here it dries out in the great Syrian and Arabian deserts. But these also, as well as the Arabian plateau stretching along the Red Sea, may have been well watered and inhabitable in early post-glacial time. The Arabian plateau and its piedmont zone in those days may well have been an independent centre of agricultural development, which gave place to the nomadism so characteristic of the Semitic peoples only at a later date. Of the early history of Arabia we are still completely ignorant. But in the twilight of history we see those Semites coming into the Mesopotamian valley from the west while the Sumerians entered from the east. Those two streams of migration, mingling, founded the great Babylonian Empire, to which all oriental peoples looked up with an awe and reverence, as well as fear, which we can scarcely appreciate. Evidently, and this is the fact of chief importance to us, parts of the nearer east were highly civilized before anything better than savagery had developed in northern Europe. But far older than these cities of the Mesopotamian river valleys is the culture of the forests, glades, lakes, and riversides of the plateaus. Evidence seems steadily to accumulate that here we are to seek for the beginnings of agriculture and the domestication of animals which were slowly to change the face of the earth and the life and character of man. Hoe-tillage of the ground is evidently far older than cattle-raising or nomadic life. It had been brought to Anau before 8000 B. C., and had probably already been practised at Susa and elsewhere thousands of years earlier. But we cannot help asking whether other plants may not have been cultivated long before cereals. Roots and tubers are much more conspicuous than the smaller grains. These underground storehouses of nutriment adapted to give the plant a quick and sure start, during a short spring period of growth and flowering, are abundant everywhere. They still form the staple crop in many parts of the world. We remember the potatoes, sweet potatoes, yams, the cassava, and a host of others. In our northern regions we still cultivate beets, turnips, and carrots, though now becoming more and more food for cattle. These plants also are less closely limited to the steppes and plateaus. They occur all through the mountain or shore regions, and for this reason would have been likely to attract the attention of "collectors."