14 INTRODUCTION
aridity. In drier regions the extent of land available for pasturage and cultivation has been seriously curtailed; and the habitability of the country has decreased. In certain moister districts among the mountains, on the other hand, the change has been beneficial : they have become less damp and snowy, and hence more habitable. Moreover, in both the drier and the moister regions the change of climate does not appear to have been all in one direction. After a period of rapidly decreasing rainfall and rising temperature during the early centuries of the Christian era, there is evidence of a slight reversal, and of a tendency toward more abundant rainfall and lower temperature during the Middle Ages.
In relatively dry regions increasing aridity is a dire calamity, giving rise to famine and distress. These, in turn, are fruitful causes of wars and migrations, which engender the fall of dynasties and empires, the rise of new nations, and the growth of new civilizations. If, on the contrary, a country becomes steadily less arid, and the conditions of life improve, prosperity and contentment are the rule. There is less temptation to war, and men's attention is left more free for the gentler arts and sciences which make for higher civilization.
The main outlines of the history of Central Asia agree with what would be expected from a knowledge of the changes of climate through which the country has passed. The favorable changes coincide with periods of prosperity and progress; the unfavorable with depression and depopulation. My own investigations show that the parallelism between climatic changes and history applies to an area extending at least three thousand miles, from Turkey on
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