For countless centuries Asia has experienced a slow upward movement, not only affecting her own topography, but likewise that of her European dependency. There was a time when the great sandy desert of Gobi was the bed of a sea which communicated through the Caspian with the Baltic, as may be proved not only by existing geographical facts, but also from geological considerations. It is only necessary, for this purpose, to inspect the imperfect maps that have been published of the Silurian and even tertiary periods. The vertical displacement of Europe, during and since the latter period, has indisputably been more than 2000 feet in many places. The effects of such movements on the flora and fauna of a region must, in the course of time, be very important, for an elevation of 350 feet is equal to one degree of cold in the mean annual temperature, or to sixty miles on the surface northward. Nor has this slow disturbance ended. Again and again, in historic times, have its results operated fearfully on Europe, by forcibly precipitating the Asiatic nomades along the great path-zone; again and again, through such changes of level, have they been rendered waterless, and thus driven into a forced emigration. Some of their rivers, as the Oxus and Jaxartes, have, within the records of history, been dry for several years. To these topographical changes, rather than to political influences, we must impute many of the most celebrated tribal invasions. It has been the custom to refer these events to an excessive overpopulation periodically occurring in Central Asia, or to the ambition of warlike chieftains. Doubtless those regions are well adapted to human life, and hence liable to overpopulation, considering the pursuits man there follows, and doubtless there have been occasions on which those nations have been put in motion by their princes; but the modern historian cannot too carefully bear in mind the laws which regulate the production of men, and also the body of evidence which proves that the crust of the earth is not motionless, but rising in one place and sinking in another. The grand invasions of Europe by Asiatic hordes have been much more violent and abrupt than would answer to a steady pressure resulting from overpopulation, and too extensive for mere warlike incitement; they answer more completely to the experience of some irresistible necessity arising from an insuperable physical cause, which could drive in hopeless despair from their homes the young and the old, the vigorous and feeble, with their cattle, and waggons, and flocks. Such a cause is the shifting of the soil and disturbance of the courses of water. The tribes compelled to migrate were forced along the path-zone, their track being, therefore, on a parallel of latitude, and not on a meridian; and hence, for the reasons set forth in the preceding chapter, their movements and journey of easier accomplishment. [Sidenote: Rate and extent of these movements.]