From end to end of America the great mountains form a sharp dividing line. The aboriginal tribes on the Pacific slope are markedly different from those farther east across the mountains. Brinton sums the case up admirably: "As a rule the tribes of the western coast are not connected with any east of the mountains. What is more singular, although they differ surprisingly among themselves in language, they have marked anthropologic similarities, physical and psychical. Virchow has emphasized the fact that the skulls from the northern point of Vancouver's Island reveal an unmistakable analogy to those from the southern coast of California; and this is to a degree true of many intermediate points. Not that the crania have the same indices. On the contrary, they present great and constant differences within the same tribe; but these differences are analogous one to the other, and on fixed lines. "There are many other physical similarities which mark the Pacific Indians and contrast them with those east of the mountains. The eyes are less oblique, the nose flatter, the lips fuller, the chin more pointed, the face wider. There is more hair on the face and in the axilla, and the difference between the sexes is much more obvious. "The mental character is also in contrast. The Pacific tribes are more quiet, submissive, and docile; they have less courage, and less of that untamable independence which is so constant a feature in the history of the Algonquins and Iroquois." * * D. G. Brinton, "The American Race," pp. 103-4. Although mountains may guide migrations, the plains are the regions where people dwell in greatest numbers. The plains in the two great land masses of the Old World and the New have the same inverse or right- and left-handed symmetry as the mountains. In the north the vast stretches from the Mackenzie River to the Gulf of Mexico correspond to the plains of Siberia and Russia from the Lena to the Black Sea. Both regions have a vast sweep of monotonous tundras at the north and both become fertile granaries in the center. Before the white man introduced the horse, the ox, and iron ploughs, there prevailed an extraordinary similarity in the habits of the plains Indians from Texas to Alberta. All alike depended on the buffalo; all hunted him in much the same way; all used his skins for tents and robes, his bones for tools, and his horns for utensils. All alike made him the center of their elaborate rituals and dances. Because the plains of North America were easy to traverse, the relatively high culture of the ancient people of the South spread into the Mississippi Valley. Hence the Natchez tribe of Mississippi had a highly developed form of sun-worship and a well-defined caste system with three grades of nobility in addition to the common people. Even farther north, almost to the Ohio River, traces of the sun-worship of Mexico had penetrated along the easy pathway of the plains.