There is a great problem called the “desiccation of Inner Asia,” and it is intimately bound up with the riddle of mankind. It seems that great areas have become drier and drier even within historical times, and the Duab lies within the sphere of this influence. Thus the study of its deserts, mountains and glaciers and the questions relating to the great ice age are endowed with peculiar interest. Our exact knowledge being still very meagre we are obliged to do a little careful speculation now and again in trying to arrive at some idea. In the course of this book I shall not show undue hesitation in following the line of deductive reasoning, whenever interesting questions arise, which cannot as yet be surrounded and overpowered by a sufficient array of facts and figures. For this purpose I shall make a liberal use of the direct method consisting in the comparison of large features and striking phenomena with familiar European, and especially Alpine, conditions, for the Alps are the fundamental standard of comparison, whether openly avowed or unconsciously traditional, of all research concerned with mountains, valleys or glaciers.

In physical geography or physiography, which is the study of the life of the earth and of the expression of her features, a definite meaning has been attached to Central Asia by Richthofen. He defines it as the region hemmed in by the Altai, the Pamirs, Himalaya and the watershed towards the great Chinese rivers, or, in other words, Chinese Turkestan and Tibet. Western Turkestan and Iran he calls the peripheral districts.