That large tracts of Asia have been subject to a gradual process of desiccation has been made clear to us by the reports of the successive explorations of Sir Aurel Stein, who has shown us that regions, which are now uninhabited desert, once held a flourishing population. It has been suggested by Ellsworth Huntington,[208] who accompanied the Pumpelly expedition to Turkestan, that the process of desiccation has been neither continuous nor progressive, but has been subject to intermittent action and the alternation of dry and wet periods. The evidence which he has adduced of the rise and fall of the level of the Caspian sea seems to bear out his thesis, which has been further strengthened by his later observations in Palestine and on the shores of the Dead Sea.[209] It is part of Ellsworth Huntington’s hypothesis that during these periods of drought, or light precipitation, the population of the steppe lands, which had grown in numbers during the previous years of heavier rainfall, have found it difficult to obtain adequate pasturage for their flocks and herds, and have in consequence dispersed to more favoured regions. To this he attributes the great raids from the steppe and desert into the more fertile zones adjoining them, which have been so marked a feature in the history of the Near East. He points out that a relatively small diminution of rainfall may make all the difference between a sufficient and inadequate crop of grass, and should the crop be insufficient, the flocks and herds, the sole means of support for the steppe-folks, would inevitably perish unless driven to moister regions. How serious even one dry year may be has recently been brought home to us by the Russian famine in 1921. This thesis has been severely attacked, especially by Peisker.[210] Still, though Huntington’s conclusions may require modification in detail, his main contention seems to have withstood the attacks made upon it. Mr. Brooks[211] has recently shown us that the climate of Europe has passed through considerable changes since the ice age, and that such changes come down to relatively recent times and may yet be in progress. He attributes these largely to changes in coast line, and to the relative masses of land and water. The Pumpelly reports[212] show that considerable changes of level have taken place in Turkestan, and but small changes are needed to connect the Aralo-Caspian basin, by means of the Obi valley, with the Arctic Ocean. All this tends to show that we may expect considerable variation in the climate of this region, while Huntington’s evidence of changes in the level of the Caspian Sea seems to prove that such variations have not been always in the same direction. Mr. Cook is, however, inclined to see in this the destruction of forests and their conversion into grass-lands by the primitive process of cultivation which he terms Milpa agriculture.[213]