4 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



to extend the work more than 1,300 niiles southward to southern Mexico and Yucatan, 

 and finally 400 niiles farther into Guatemala and Honduras. Here again the lines of 

 reasoning previously employed in Asia and later in the United States and northern Mexico 

 gave similar results. In Yucatan and Guatemala, however, a new type of phenomena 

 was also found, which confii-med the previous conclusions most interestingly. In that 

 tropical region the presence of magnificent ruins in the midst of dense forests seems to 

 indicate changes of chmate contrary to those of the regions farther north. On the whole 

 the country appears to be moister than it was several thousand years ago, instead of drier, 

 as in regions farther north. This at first sight appears contradictory, but in reality it 

 confirms a conclusion derived from other evidence, namely, that changes of climate are 

 probably characterized by the shifting of the world's great climatic zones from north to 

 south, and the reverse. 



Having reached this conclusion, we find ourselves able to test it by means of another 

 of wholly independent nature based on a comparison of historic events and climatic changes 

 in Europe and Asia. This second conclusion is that while the course of history depends 

 upon a vast number of factors and its details are due to what may be designated as pm-ely 

 human causes, yet in its broader outhnes it is profoundly affected by climatic changes. 

 These may alter economic conditions, they may disturb the adaptation of a race to its 

 environment by fostering special diseases, or they may force people to move out of a 

 habitat where they can no longer compete with nature. In Yucatan and the neighboring 

 parts of Central America the dense tropical forests and their deadly malarial fevers are 

 man's chief enemy. Our study of the terraces, ruins, trees, and other phenomena of the 

 United States and northern Mexico leads to the hypothesis that the forests of Yucatan 

 and the surrounding regions have alternately increased and diminished in size, the increase 

 coming at times when aridity prevailed in regions such as CaUfornia, and the decrease during 

 CJalifornia's moist times. Our hypothesis as to the relation of changes of climate and 

 history leads to the inference that civiUzation in Central America would thrive when the 

 forests diminished and would decline when they increased. A comparison of the cUmatic 

 periods indicated by the Sequoias with the events of the history of the ancient Mayas of 

 Yucatan, as recorded on monuments and in chronicles, shows that our expectations are 

 realized to a considerable degree. Thus both conclusions are strengthened. 



The degree of certainty given to oiu- conclusions by the agreement of the evidence of 

 old trees with that derived from other sources brings us to the point where we may reason- 

 ably attempt to ascertain the cause of changes of chmate. In such an attempt the first 

 matter to claim attention is a certain degree of coincidence between the phenomena of 

 chmate and those of the sun. There are many reasons for thinking that the well-known 

 sun-spot cycle of 11 years is related to a distinct climatic cycle. The longer, but less 

 thoroughly estabhshed 35-year cycle of Brlickner also appears to be correlated with the 

 activity of the sun. These things, as has often been pointed out, suggest that our minor 

 climatic fluctuations maybe due to sUght variations in the intensity of the sun's radiation. 

 The work of Langley, Abbott, and others proves that the sun's radiation actually does 

 vary, while that of Koppen, Newcomb, and many more proves that the temperature of 

 the earth's atmosphere shows a corresponding variation. The terrestrial variation is so 

 slight, however, and is so irregular outside of equatorial regions, that some of the best 

 authorities doubt whether it is sufficient to produce appreciable results. Opposed to this 

 is the fact that by almost universal consent students of glaciation beheve that a permanent 

 lowering of the earth's mean temperature to the extent of from 3° to 10° C. would produce 

 a glacial period. The most conservative estimates of the change in terrestrial temperature 

 between the minimum and maximum of sun-spots is about 0.5° C. This is so large a frac- 

 tion of the change needed to produce glaciation that it seems as if it must produce some 

 appreciable meteorological results. 



