Although we have no direct evidence, the meteorological conditions suggest very strongly that the dry belt extended across Russia into Siberia as a marked period of desiccation, possibly worse than any droughts of the historic period. At present Siberia receives its rainfall mainly from depressions which cross Russia from the Baltic or Black Seas, and follow a well-marked track north of the central Asiatic mountains. But during the forest period these tracks were abandoned, and the majority of the depressions passed north-eastward off the coast of Norway into the Arctic Ocean. The result must have been a great diminution of rainfall over the continent. We shall see later (Chapter XIX) that this period of drought was of extraordinary importance in human history. For during the moist maritime phase central and eastern Europe, and probably also Asia, had become extensively peopled by neolithic nomads of Aryan and Semitic races, while the great river valleys of the south were in the possession of dense agricultural populations in a more advanced state of civilization. As the climate became progressively drier and the pasture diminished, the land was unable to support such a large nomadic population, and there was a great outburst of raiding and conquering expeditions directed southwards and westwards, resulting in a succession of empires in the rich Mesopotamian regions and neighbouring countries, which form the beginnings of our history. The beginnings of history in China also, about 2500 B.C., show that at this time the settled peoples of that country were in trouble with the nomads of the interior. BIBLIOGRAPHY Reid, C. “Submerged forests.” Cambridge University Press, 1913. Lewis, F. J. “The history of the Scottish peat-mosses and their relation to the Glacial period.” Edinburgh, _Scot. Geogr. Mag._, 22, 1906, p. 241. CHAPTER XVI THE “CLASSICAL” RAINFALL MAXIMUM, 1800 B.C. TO A.D. 500