24 
THE SECRET OF THE BIG TREES. 
a living. Then, toward the end of the long period of drought, there 
occurred the tremendous outpouring of the Arabs, unified by Moham¬ 
medanism, as is universally agreed, and also spurred by hunger, as 
we infer from a study of climate. Thus the Dark Ages reached their 
climax. No period in all history, save that which centers 1200 B. C., 
was more chaotic; and that early period also appears to have been a 
time of greatly diminished rainfall. 
It is impossible here to trace further the correspondence of the two 
curves and their relation to history. The essential point is this: By 
means of a rigid mathematical test we have worked out the climatic 
changes of California. From ruins, lacustrine strands, traditions, 
famines, and many other lines of evidence we have worked out the 
changes in Asia. Thus by methods absolutely dissimilar we have 
constructed curves showing climatic fluctuations in parts of the world 
10.000 miles apart. In essentials the two agree in spite of differences 
in detail. It therefore seems probable not only that climatic pulsa¬ 
tions have taken place on a large scale during historic times, but that 
on the whole the more important changes have occurred at the same 
time all around the world, at least in the portion of the north tem¬ 
perate zone lying from 30° to 40? north of the Equator. This, in 
itself, does not prove that great historic changes have occurred in 
response to climatic pulsations, but it goes far in that direction. 
It introduces a new factor into that most profound and far-reaching 
of the problems of history—the cause of the rise and fall of nations* 
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