The language divisions of the philologist do tally, it is manifest, in a broad sort of way with the main race classes of the ethnologist, and they carry out the same idea of age-long separations between great divisions of mankind. In the Glacial Age, ice, or at least a climate too severe for the free spreading of peoples, extended from the north pole into central Europe and across Russia and Siberia to the great tablelands of central Asia. After the last Glacial Age, this cold north mitigated its severities very slowly, and was for long without any other population than the wandering hunters who spread eastward and across Bering Strait. North and central Europe and Asia did not become sufficiently temperate for agriculture until quite recent times, times that is within the limit of 12,000 or possibly even 10,000 years, and a dense forest period intervened between the age of the hunter and the agricultural clearings. This forest period was also a very wet period. It has been called the Pluvial or Lacustrine Age, the rain or pond period. It has to be remembered that the outlines of the land of the world have changed greatly even in the last hundred centuries. Across European Russia, from the Baltic to the Caspian Sea, as the ice receded there certainly spread much water and many impassable swamps; the Caspian Sea and the Sea of Aral and parts of the Desert of Turkestan, are the vestiges of a great extent of sea that reached far up to the Volga valley and sent an arm westward to join the Black Sea. Mountain barriers much higher than they are now, and the arm of the sea that is now the region of the Indus, completed the separation of the early Caucasian races from the Mongolians and the Dravidians, and made the broad racial differentiation of those groups possible. 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.