412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



at the time of Christ, for example, or those which center 1000 or 

 1600 A. D. The agreement is so close that it can scarcely be a 

 matter of chance. Further discussion of the subject must be deferred 

 for the present. We can here merely sum up the main conclusions. 

 The study of the trees of New Mexico and California in the first place 

 seems to confirm the conclusions derived from the ruins and physi- 

 ographic evidences found in the drier parts of North America. 

 It thus shows that the methods upon which those conclusions are 

 based are sound, and that the results derived from such methods 

 whether in America or Asia are vahd. In the second place, the trees 

 confirm the theory of pulsatory climatic changes. They apparently 

 show that the climate of the earth is subject to pulsations having a 

 period of centuries. In the third place, the rate of growth of the trees 

 indicates that m the distant historic past the moist epochs were on 

 the whole moister than the similar epochs in more recent times. 

 Fourth and last, we are led to conclude that the main chmatic 

 changes of America are synchronous with those of Asia and are of 

 the same kind. This does not mean that changes in tropical countries 

 are like those in the Temperate Zone. It does indicate, however, 

 that in the temperate continental regions of the world, in both the 

 eastern and western hemispheres, periods of exceptional aridity 

 or of exceptional moisture have occurred at approximately the 

 same time, and have sometimes lasted for centuries. Hitherto tliis 

 point has been open to question, and therefore historians and other 

 students of man have been skeptical as to the possibihty that chmatic 

 changes could have been of sufficient importance profoundly to 

 influence history. With the fuUer apphcation of the methods here 

 discussed, we shall soon be able to determine the exact nature and 

 degree of chmatic changes throughout historic time, and then we 

 shall have the basis for a true appreciation of their effect upon history. 



