THE CASPIAN SEA AND ITS NEIGHBORS 351
If we accept this conclusion, the diagram on page 349 may be interpreted as the climatic curve of the Aralo-Caspian basin. Except during the last two centuries, the details are uncertain. There have probably been notable fluctuations of which we have no record. One such is suggested by the dotted line between 900 and 1200 A. D. If Bruckner is right as to the date of the caravanserai at Baku, a short dry period must have ensued after the moister period indicated by the account of Istakhri. Making due allowance for the defects of our knowledge, there remains a strong presumption that the Aralo-Caspian basin has passed through a double series of great climatic changes during historic times. During the period commonly called antiquity, the climate was apparently damper and cooler than now. This first historic fluvial epoch gave place during the Dark Ages, Emerton's Transitional Epoch, to the first historic inter-fluvial epoch, during which the climate was warmer or drier than to-day. In the course of the next few centuries there was a change to the somewhat damper or cooler conditions of the mediœval fluvial epoch ; and this in turn has been succeeded by the modern dry epoch.
The most significant feature of the climatic curve of the Caspian Sea is that it is applicable to the whole of western and central Asia. Two examples will show how the climatic hypothesis illustrated in the diagram throws light on, and is confirmed by, hitherto inexplicable phenomena of distant regions. Around the little lake of Son Kul, which we visited with the Khirghiz in the western Tian Shan mountains a thousand miles east of the Caspian Sea, Professor Davis and I found in 1903 the remains of a number of old irrigation
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