CHAPTER X. 



SOUTHERN MEXICO AS A TEST CASE. 



The testing of a theory can be accomiDhshed in at least three ways. First, it can be 

 applied to new regions; second, it can be confronted by new lines of evidence; and, finally, 

 it can be investigated by new observers employing different methods or at any rate coming 

 to the problem from a different point of view. In the present investigation we have 

 already used the Southwest as a new region to be compared with the old regions of Asia 

 and the lands of the Mediterranean. Let us now take still a third great region and once 

 more make a test. Southern Mexico lies at a distance of from 1,200 to 1,500 miles from 

 Arizona and New Mexico. This is a small matter compared with the 8,000 or 10,000 miles 

 which separate those regions from the parts of Asia where our chief conclusions as to 

 that continent were reached. Nevertheless, the difference between Ai-izona and southern 

 Mexico is greater than between Arizona and Turkestan. This is because, although Arizona 

 has summer rain like that of Mexico, its most important precipitation is the winter type 

 characteristic of the zone where westerly winds and subtropical aridity are the dominant 

 features of winter and summer respectively. In going from Arizona to southern Mexico, 

 on the contrary, we cross the trade-wind zone and enter the edge of the zone of equatorial 

 rains and calms. Hence we are able to subject our theories to a more severe test than 

 would be possible even if we completely encircled the globe, but remained in the same 

 zone of climate. The change is so great that we are able not only to test our theory in a 

 distinctly new region, but also to confront it with certain new lines of evidence. 



The investigations in Mexico to be described below were made during the spring of 

 1912. Those here discussed were confined chiefly to the City of Mexico, in latitude 19.5°, 

 and to Oaxaca and Mitla, in latitude 16°. In both of these places evidences of changes 

 of climate appeared to an unexpected degree. In discussing this matter, let us take up, 

 first, the recent fluctuations of the lakes near the City of Mexico; second, the evidences 

 of a change in the conditions of the Basin of Mexico during the time of ancient civiliza- 

 tions; and third, the alluvial terraces found near Mexico and in Oaxaca. A foiirth type 

 of evidence, namely, the peculiar location of the ruins of Yucatan, together with those of 

 Guatemala and Honduras, is so new and important that it will be left for later chapters 

 after we have considered the trees of California. 



In the Monthly Weather Revieiv, for November 1908, I have discussed the City of 

 Mexico and Lake Tezcuco in their relation to changes of climate. In considering this 

 matter here, I shall largely follow that article, but shall add new facts which have come 

 to hght since it was written. The City of Mexico lies 7,400 feet above the sea near the 

 salt lake of Tezcuco and the tributary fresh lakes of Xochimilco and others. The basin 

 containing these lakes is similar in its general features to that of the Great Salt Lake in 

 Utah, Lop Nor in Central Asia, and Seistan in Eastern Persia. Accurate historic records 

 of the country extend back to the time of the Spanish invasion in 1519, and before that we 

 have fairly reUable traditions for at least 200 years more. Taking merely the 600 years 

 for which we now have data, we find that during that time there appears to have been a 

 slight but appreciable change of climate in Mexico similar to that which has apparently 

 occurred in Asia. The evidence is somewhat masked because the natural course of events 

 has been interrupted by various works of man, such as the dikes, canals, and tunnels 

 which have been built since 1446 to regulate the waters of Tezcuco and its three tributary 



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