THE SIGNIFICANCE OF CENTRAL ASIA 13
when the main features of one have been comprehended, but little need be added as to the others. The last basin, the so-called Aralo-Caspian depression, possesses many of the characteristics of its more arid neighbors, but its great size and the absence of mountains to the north give it a diversity of climate unknown to the others. I shall not consider it except in relation to the problem stated in the next paragraph.
In the study of the five basins along the lines of the definition of geography given by Professor Davis, I discovered a number of facts which lead to a new application of the geographic principle of cause and effect. In order to understand the present condition, that is the geography, of Central Asia, we must look upon it not as the result of the long-continued action of fixed physical conditions, but as the result of changing conditions. During the recorded occupation of the country by man there appear to have been widespread changes of climate. It has long been surmised by historians that certain parts of Asia have been growing more arid, but the surmise has lacked scientific confirmation. Indeed, meteorological data seem to stand directly opposed to it, for they show that there is no evidence of any appreciable change since records have been kept instrumentally. The oldest records, however, date back little more than a hundred years, and hence cannot be considered as proving anything in regard to antiquity. The data which I obtained in Central Asia, on the other hand, confirm the surmise of the historians. There is strong reason to believe that during the last two thousand years there has been a widespread and pronounced tendency toward
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