its borders. Topographically, the Asiatic section is dominated on the south by the Tibetan table-land, culminating in the Pamir region, called the 'roof of the world,' from which the land falls off rather abruptly on the west, and more gradually toward the north and east to the level of the sea. The bleak plateaux are thus succeeded by grassy plains and deforested steppes, which in turn are bordered by a comparatively narrow Tiaga, or wooded belt, extending to the Arctic Tundra and in places to the Pacific shore. The heart of the continent contains deserts and enclosed seas, while the surrounding lands are furrowed by forest-bordered streams flowing to the north, and by more open rivers of uncertain course draining toward the east. In consequence of these climatic and topographic differences, the Mongolic race has in the course of time become subdivided into a number of geographic groups. There are plateaux people, desert folk and steppe tribes, forest dwellers and typical Hyperboreans, and the settled inhabitants of the eastern valleys. Migration has moreover been succeeded by miscegenation, so that the lines of heredity and environment have become confused. Withal, however, enough Mongolian traits have persisted everywhere over the region, and during all the centuries that have elapsed since the original type was constituted, to allow us to set the Yellow people in a separate racial category, and distinguish the typical Mongol from his human fellows by his round head with high cheek-bones; the texture and pigment of his skin; his coarse straight hair, which is cylindrical in cross section; his thin colorless lips; and his small oblique black eyes.
South of the Himalayan line the peninsular portion of the old world spreads out like a fan from the Indo-Malaysian cradle-land to the Atlantic coast of the continent. On the east, the Indian section of this territory is connected with the equatorial region through the southern peninsulas, which once formed part of the Indo-African continent; but cut off from the Asiatic area by the lofty Himalayan range. On the west the conditions are reversed, the Mediterranean and European sections of this territory being cut off from the equatorial region by the Sahara, and connected with the Asiatic area through what is called the open gateway of the east, lying between the Ural mountains and the Caspian sea. There was access to the Indo-Mediterranean-European section, therefore, from two sides; from the equatorial region on the southeast, and from the Asiatic area on the northwest. The ancestry of the so-called Caucasians can accordingly be traced back to two sources. From the southeast, dolichocephalic Negroids pushed westward from Indo-Africa into the Mediterranean region and overran Europe in very early times. Somewhat later successive streams of brachycephalic Asiatics poured in through the open gateway of the east and mingled with the primitive inhabitants of these parts. From