Entry into the fourth racial region was not from Indo-Malaysia but from the Arctic peninsulas of the eastern hemisphere. During the period of dispersion, continuous connections probably existed along these high latitudes, joining America with northwest Europe, on the one hand, and with northeast Asia, on the other. It was possible, therefore, during these early ages, for dolichocephalic Mediterranean people to continue the course of their northwesterly migrations until they reached the Atlantic shores of the new world; and for brachycephalic Asiatics to pursue their way northeastward until they came to the Pacific coast of the continent. That the western hemisphere was originally occupied in this manner by emigrants from Europe and Asia appears probable from the prevalence among the American aborigines of the long-head type in the east and the broad-head type in the west. The incursion along the Atlantic could not have lasted as long as that proceeding by way of the Pacific, for the ancient land-bridge joining northwest Europe with northeast America was broken long before the prehistoric period, and the islands left between were too far apart to afford further access from this direction. On the Pacific side, however, the old miocene bridge, with its temporary glacial extensions, probably endured until quaternary times, and after this, approaches still remained across the narrow Behring strait and along the Aleutian island chain. We should think of Asia, therefore, as the source of the main stream of migration that spread southeastwards over the new world.
In these early days, before the knowledge of ocean navigation, America was not as now a Durchgangsland, but rather a cul-de-sac. There was entry on either side from the north, but no exit in any other direction. The aboriginal people pouring in from above must, therefore, have been pushed down by the later comers through the constricted central space, like the sands in an hourglass, to spread out along the equator and become contracted again in the apex of the continent. Moreover, as the main stream of migration proceeded from Asia, the emigrants from Europe were probably confined from the first to the eastern edge of the hemisphere. Cut off completely from further contact with other people, the American aborigines, long-heads and round-heads alike, were henceforth subjected to the influences of their new surroundings and modified accordingly.
Geographically speaking, the American continent differs from the other regions of the earth, and at the same time possesses certain positive characteristics of its own. This isolation of the western hemisphere, taken together with such environmental uniformity as exists within its borders, had the effect of differentiating the aboriginal inhabitants from their European and Asiatic ancestors and blending them gradually into one racial variety, possessing pronounced Oriental affinities.