CLIMATE AND HISTORY. 315 
Again there was a change. The bed of the lake of Sistan was once more filled 
with water to a height greater than that wliich is now reached, but less than in the 
previous epoch of high water, for Zahidan was not covered as it had been before. 
Between the time of Istakhri and the present the Helmund was diverted from a 
southwestward to a northward course, and this was probabh- the cause of the increase 
in the size of the lake. This is the more probable because from historical and 
archeological evidence it is known that Zahidan was built soon after the time of 
Istakhri. To suppl)- so large a city with water a large amount must have been 
withdrawn from the Helmund before it reached the God-i-Zirrah and turned in 
tke direction of Sistan. For some centuries, until its destruction by Tiniur at the 
end of the fourteenth centur>', Zahidan continued to flourish. It is probable that 
the lake stood at a high level for a considerable portion of this time, for it was able to 
fonn, or at least to rejuvenate, a well-defined shoreline, with broad beaches and 
high bluffs. During the last five centuries, since the fall of Zahidan, there has been 
a gradual decrease in the size of the lake and in the density of the population that 
surrounds it. How this could take place without a dimunition in the water supply 
it is hard to understand. The historj- of Sistan, so far as it can be made out, seems 
to indicate a gradual desiccation of the countr}- from early historical times down 
even to the present. The evidence of archeology, historj', and tradition in the 
surrounding countries points in the same direction. At Sistan history and physiog- 
raphy appear to join hands, for the change from the conditions of greater water- 
supply during antiquity to the desiccation of to-day is apparently the change from 
the last fluvial epoch to the present interfluvial epoch. 
