CLIMATE OF NORTH AMERICA HUNTINGTON. 405 



whelmed by invaders from both the Libyan and Arabian deserts. 

 The next great period of aridity culminated in the seventh century 

 or thereabouts. Its approach was marked by the barbarian invasions 

 of Europe, and its culmination by the Mohammedan outpouring 

 from Arabia. Finally, the third of the more important dry epochs 

 reached its climax in the thirteenth century A. D., when the hordes 

 of Ghenghis Khan ravaged Asia from China to the Mediterranean. 

 Besides these more intense periods of aridity there seem to have been 

 others of minor importance, but these need not here be considered. 

 The history of Asia during the last 3,000 years thus consists of three 

 main epochs of moist climate and advancing civilization, separated 

 by epochs of aridity and declining civilization accompanied by 

 migrations and wars. In comparing America with Asia it is interest- 

 ing to find that in the drier portions of the Western Hemisphere we 

 also have three main periods of prosperity and apparently of abundant 

 precipitation separated by epochs of decline and depopulation. 

 Perhaps the ancient farming population of America, the Hohokam, 

 may date from the period of moist climatic conditions at the time of 

 Christ and earlier. Their disappearance may have been due to the 

 aridity of the period which culminated in the seventh or eighth cen- 

 tury. Then the village people, the Pajaritans, may have flourished 

 in the Middle Ages, and may have been ousted by the twofold disaster 

 of prolonged drought and fierce invasion which would have come to 

 America about 1200 A. D. if conditions here were like those of Asia. 

 And, fuially, the occupation of places like Gran Quivira by the modern 

 Pueblo Indians may have been made possible by the propitious con- 

 ditions of relatively abundant rainfall which followed the aridity of 

 the thirteenth century. I do not advance this as more than a working 

 hypothesis, but as such it may suggest various lines of research 

 hitherto neglected. 



PART III. THE EVIDENCE OF THE TREES. 



The evidence of climatic changes thus far presented probably seems 

 much more conclusive to the "WTitcr than to the reader. I am well 

 aware that when the name of an author once becomes identified with 

 a theory his fellow workers are apt to discount his results. They 

 know that a man is prone to find what he looks for, and that when 

 certain facts are capable of two or more equally plausible explanations 

 he is likely to choose in accordance with his preconceived ideas rather 

 than accordmg to the weight of the evidence. In searchmg for some 

 method whereby this danger might be avoided I found no success 

 until an article by Prof. A. E. Douglass, of the University of Arizona, 

 appeared in the Monthly Weather Review for June, 1909, under the 

 title "Weather Cycles in the Growth of Big Trees." 

 85360°— SM 1912 27 



