Quite different are the _folk migrations_ that occur at the very dawn of the history of the great cultural peoples. The difference between tribal and racial migrations is an important one. When a race migrates, it retains its peculiar characteristics, its traditions, its heroes, and its gods, and transplants these into the new territory. True, these various elements do not remain unchanged. They inevitably become fused with the culture of the original inhabitants, and it is from these fusions, when they are at all deep-going, that new peoples arise. None of the great cultural nations that mark the beginning of this age of heroes and gods, from the Babylonians down to the Greeks, the Romans, and the Germans, is homogeneous. Indeed, recent Babylonian investigations have shown that the Semitic immigration into Babylon was preceded by that of other peoples who were probably of different origin--namely, the Sumerians. We know of the latter only through linguistic traces in Babylonian inscriptions, of which, however, the religious parts, especially, show that the Sumerians exercised a great influence upon later civilization. Similarly, the settlement of the Greeks, Romans, and Germans in the territory which they eventually occupied, followed upon great earlier migrations to these regions. The people that finally formed the Greek race left the mountain country of Thrace and Thessaly in prehistoric times; wandering towards the sea, they fused with the original inhabitants of the regions into which they entered. In view of these migrations of early history, the theory of the desirability of racial purity, which has recently been so ardently championed in many quarters, is scarcely tenable. Political organization, on the one hand, and mythology and religion, on the other, represent important creations which for the most part sprang into existence only in the wake of migration and of the resultant fusion of peoples of different races. Though political organization has been mentioned as the first important feature distinguishing the heroic age from the preceding era, there is a second and not less significant differentia. This relates to the material conditions of life. Two things are of outstanding importance for the new culture. The first of these consists in what we ordinarily call agriculture--that is, the tilling of the soil by the aid of the _plough_, or, as it is therefore more properly called in contrast to the earlier hoe-culture, plough-culture. In addition, there is the _breeding of domestic animals_, particularly of food-supplying cattle, and, later, of sheep and goats.