Again the blown-sand Desert of Sahara—it is not a dried-up sea, but a wind desert, and was once fertile and rich in life—becoming more and more dry and sandy, cut the Caucasians off from the sparse primitive Negro population in the central forest region of Africa.
The Persian Gulf extended very far to the north of its present head, and combined with the Syrian desert to cut off the Semitic peoples from the eastern areas, while on the other hand the south of Arabia, much more fertile than it is to-day, may have reached across what is now the Gulf of Aden towards Abyssinia and Somaliland. The Mediterranean and Red Sea were probably still joined at Suez. The Himalayas and the higher and vaster massif of central Asia and the northward extension of the Bay of Bengal up to the present Ganges valley divided off the Dravidians from the Mongolians, the canoe was the chief link between Dravidian and Southern Mongol, and the Gobi system of seas and lakes which presently became the Gobi desert, and the great system of mountain chains which follow one another across Asia from the center to the north-east, split the Mongolian races into the Chinese and the Ural-Altaic language groups.
Bering Strait, when this came into existence, before or after the Pluvial Period, isolated the Amer-Indians.
These ancient separations must have remained effectual well into Neolithic times. The barriers between Africa, Asia, and Europe were lowered or bridged by that time, but mixing had not gone far. The practical separation of the west from Dravidian India and China continued indeed down almost into historical times; but the Semite, the Hamite, and the Aryan were already in close contact and vigorous reaction again in the very dawn of history.
We are not suggesting here, be it noted, that these ancient separations were absolute separations, but that they were effectual enough at least to prevent any great intermixture of blood or any great intermixture of speech in those days of man's social beginnings. There was, nevertheless, some amount of meeting and exchange even then, some drift of knowledge that spread the crude patterns and use of various implements, and the seeds of a primitive agriculture about the world.