This one safe conclusion, however, that the Semitic settlers of Babylonia were not the sole occupants, but by their side dwelt another race, or possibly a variety of races, possessing entirely different traits, is one of considerable importance. At various times the non-Semitic hordes of Elam and the mountain districts to the east of Babylonia swept over the valley, and succeeded, for a longer or shorter period, in securing a firm foothold. The ease with which these conquerors accommodated themselves to their surroundings, continuing the form of government which they found there, making but slight changes in the religious practices, can best be accounted for on the supposition that the mixture of different races in the valley had brought about an interchange and interlacing of traits which resulted in the approach of one type to the other. Again, it has recently been made probable that as early at least as 2000, or even 2500 B.C., Semitic invaders entering Babylonia from the side of Arabia drove the native Babylonian rulers from the throne;[18] and at a still earlier period intercourse between Babylonia and distant nations to the northeast and northwest was established, which left its traces on the political and social conditions. At every point we come across evidence of this composite character of Babylonian culture, and the question as to the origin of the latter may, after all, resolve itself into the proposition that the contact of different races gave the intellectual impetus which is the first condition of a forward movement in civilization; and while it is possible that, at one stage, the greater share in the movement falls to the non-Semitic contingent, the Semites soon obtained the intellectual ascendency, and so absorbed the non-Semitic elements as to give to the culture resulting from the combination, the homogeneous character it presents on the surface. IV. Our present knowledge of Babylonian history reaches back to the period of about 4000 B.C. At that time we find the Euphrates Valley divided into a series of states or principalities, parcelling North and South Babylonia between them. These states group themselves around certain cities. In fact, the Babylonian principalities arise from the extension of the city's jurisdiction, just as the later Babylonian empire is naught but the enlargement, on a greater scale, of the city of Babylon.