THE SHIFTING OF CLIMATIC ZONES. 207 



that which now prevails there and would thus tend to cause jungle to take the place of 

 forests. 



The effect of such a change upon the summer rains, on the other hand, can not easily 

 be determined. Possibly the general strengthening of the winds would bring the equa- 

 torial rains farther north than is now the case, or would at least make them more abundant. 

 It is equally possible that the shoving southward of the zone of storms would be as prom- 

 inent a feature in summer as in winter, and hence that the equatorial rains would not 

 come so far north as at present. A possible test of this matter lies in a comparison of the 

 curve of the sequoia in California with the fluctuations of the lakes around the City of 

 Mexico, but the data are so imperfect that it is not conclusive. The California growth 

 represents the variations in a purely winter type of rainfall characteristic of the zone of 

 prevaiUng westerlies. The fluctuations of the lakes of Mexico, on the other hand, are due 

 to a rainfall which comes almost entirely, but not wholly, from May to October, and is 

 largely of the equatorial type. Hence, if a strengthening of the earth's circulation increases 

 the equatorial rains as well as those of the zone of westerlies, we should expect to find the 

 Mexican lakes high at the same time when the California trees grow rapidly. This would 

 not necessarily mean that in summer the rains of the equatorial tj^pe shifted farther north 

 than now, although this is a possibility. It would merely indicate that the more rapid 

 circulation of the air caused the equatorial rains to be heavier than at present. 



With this in mind let us sum up the variations of the lakes, as set forth in Chapter X, 

 and compare them with the curve of the sequoia as given in figure 72. Our first knowledge 

 of the Mexican lake suggests that in 1325 a. d., when the Aztecs founded the City of Mexico, 

 it stood quite high. At this time the California trees were growing very rapidly and grew 

 still more rapidly during the succeeding decade. Then their growth decreased until about 

 1420. Thereafter, for the space of forty years, the growth of the trees remained practi- 

 cally stationary — or, to put it in another way, a climatic change markedly checked a 

 previously rapid rate of decUne, but did not succeed in reversing it. Just what happened 

 in Mexico at this time we do not know. It merely appears that during the forty years of 

 the change in California, conditions were such that after some severe floods in the early 

 years of the reign of Montezuma, the Aztecs were at length led to build the first dike in 

 1446. The history of the inundations of Mexico City from the time of Montezuma to 

 1800 A. D. is summed up in table 10. The data here used are taken partly from Humboldt, 

 who speaks of fourteen chief floods between the time of Montezuma and his own day. 

 The chief of these, which occurred in 1553, 1604, and 1607, are mentioned (it will be 

 remembered) by Torquemada. Certain other floods are mentioned by Cavo,* whose 

 work terminates with the year 1765. From this work Mr. A. F. Bandelier has kindly 

 gleaned for me the references noted in the table. The inundations mentioned only by 

 Cavo are less important than those mentioned by the other authorities. 



The table scarcely requires explanation, but a few words as to the general course of 

 events may not be amiss. After the time of Montezuma the growth of the trees fell off 

 at a very rapid rate, and from about 1460 to 1490 we find them growing as slowly as at 

 any known period. No inundations are recorded during this time, but Humboldt tells 

 us that the Mexican lakes fell to so low a level that the city suffered much distress because 

 canoes laden with food could not come in as formerly from the surrounding country. There- 

 after the rate of growth of the trees increased rapidly until about 1560, and in this period 

 we find four inundations, the last of which, in 1553, was famous. It is noteworthy that it 

 occurred at about the time when the trees reached their maximum rate of growth. Next 

 the trees feU off slightly for thirty years, approximately from 1565 to 1595, and Mexico 

 City did not suffer greatly, although one inundation of no great note occurred in 1580. 

 Then the growth of the trees increased at a very rapid rate during the decade from 1600 



* Les Tres Siglos de Mejico (Jalapa, 1870, edited by Carlos Maria de Bustamentes). 



