232 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



beginning in Asia and extending to America, we deduce certain consequences, and on com- 

 parison with the actual facts we find that on tlie whole the facts and the consequences 

 essentially agree. The first step in our reasoning was the simple theory that the climate 

 of early historic times was different from that of the present. The second was the further 

 hypothesis that the change from the past to the present has not been regular but pulsatory. 

 This in turn led to the supposition that climatic changes may have been one of the factors 

 which have borne a part in producing certain important historical results. These three 

 steps were taken in Asia and the eastern countries of the INIediterranean basin. The 

 next step was to employ the same methods of research in similar regions of America, with 

 the result that they led to the same general conclusions. Having reached this point, it 

 became necessary to drop the former methods and make an entirely new investigation 

 absolutely unconnected with the preceding steps and wholly independent of the personal 

 opinions of the investigator. This was done by means of the growth of trees. Its results 

 agreed with those of the other methods of investigation in indicating the pulsatory natui-e 

 of climatic changes. The results also led to two new conclusions. The fu-st was that in 

 continental regions lying in similar latitudes and in similar relation to the sea, climatic 

 pulsations of similar phase occur at the same time both in the eastern and in the western 

 hemispheres. The corollary of this is that all parts of the earth must be subject to climatic 

 changes at the same time, although the nature and degree of the change may vary greatly 

 from place to place. A second conclusion derived from the trees was that chmatic changes 

 are due primarily to a strengthening or weakening of the atmospheric cii'culation, and that 

 their general effect at one extreme is so to weaken the movements of the air that storms 

 are mild and seasonal variations slight. At the other extreme, on the contrary, storms 

 appear to be strong and seasonal variations to be great, because the various climatic zones 

 of the earth are moved far from their ordinary location, especially in winter. Finally, we 

 have taken all our conclusions as to the nature of climatic changes and their relation to 

 historic events and have applied them to a new I'egion in Central America. The result is 

 a considerable degree of agreement between our expectations and the facts, no matter 

 which of our sets of dates is used. If the interpretation of Maya history contained in our 

 table is correct, the agreement becomes truly remarkable. In view of the present uncer- 

 tainty as to Maya chronology, however, we must once more emphasize the fact that this 

 agreement can not be regarded as proving the accuracy of the various steps leading to 

 our present results, for there are still many points to be investigated. It seems, however, 

 to show clearly that even in this last expansion of our hypothesis we find nothing con- 

 trary to it. As we go ])ack toward the earlier parts of the hypothesis each step becomes 

 more and more firmly established. Our main conclusion does not rest u]oon Maya history, 

 but upon the trees of California and upon hundreds of pieces of evidence in the arid 

 Southwest and in Asia. All these seem strongly to indicate that the climate of the past 

 was different from that of the present and that the change from that time to this has been 

 pulsatory. 



