190 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



driest part of the peninsula. In fact, as has been pointed out in our discussion of the 

 jungle and the forest, a small rainfall is a distinct advantage, because it prevents the growth 

 of the great tropical forest wliich so effectively checks human progress. If the rainfall 

 of the past had been greater than that of the present, the effect would have been to diminish 

 rather than increase the density of population. 



There appears, then, to be no reason for thinking that Yucatan has suffered a diminution 

 of rainfall similar to that of Arizona or of the portions of Asia in temperate latitudes. 

 Nevertheless, the lakes and terraces of the Valley of Mexico and the great terraces of the 

 state of Oaxaca furnish strong evidence that a marked change of some sort has taken place 

 in those regions; and if such has been the case there, Yucatan, which Ues but 500 miles from 

 Oaxaca and from 1 to 4 degrees farther north, can scarcely have failed to be affected. 

 At these southern points, however, the effect need not necessarily have been of the same 

 type as that produced farther north — in New Mexico, for example— since the two places 

 he in different climatic zones. 



The general effect of changes of climate seems to be to shift the peculiarities of one lati- 

 tude into another or, perhaps more accurately, to cause the seasonal shifting of zones to 

 vary in amount and intensity. Inasmuch as the chief change during the past 2,000 to 

 3,000 years in the regions 30° to 40° north of the equator appears to have been in the 

 direction of aridity, the general shifting seems to have carried the conditions of more 

 southerly regions into those farther north. This would seem to mean that at the beginning 

 of the Christian era or earlier the zone of westerly storms, during the winter, but not 

 necessarily in summer, lay farther south than to-day, and thereby made the present 

 subtropical zone less arid than at present. The natural corollary of this would be that 

 the subtropical zone of aridity was also displaced southward. This would have led to a 

 diminution of winter rainfall and hence of vegetation along the northern edge of the equa- 

 torial zone in those parts where inblowing trade winds combine with equatorial low pressure 

 to produce abundant rain at all seasons. Thus jungle would have been caused to take the 

 place of genuine, dense forests in those particular regions, and jungle might in turn be 

 replaced by bush. In Yucatan and other parts of the extreme south of Mexico, or in 

 Central America, the transition from jungle to forest is often quite sudden. For instance, 

 in Yucatan it occurs within a distance of 30 to 40 miles. If the line of transition were 

 shoved southward 200 to 300 miles, it would cause jungle to prevail in practically all the 

 places where ruins are now found. 



Such a change as has just been described would not merely explain the location of 

 great ruins in regions now too densely forested to be habitable; it would also to a certain 

 extent relieve us of the necessity of assuming that the ancient Yucatecos possessed a degree 

 of energy and ability out of harmony with anything which now exists in regions so warm 

 and debiUtating as Yucatan. 



Before explaining this, however, it will be well to examine more closely the probable 

 mechanism of a shifting of the great chmatic zones. This can best be understood by 

 considering first what happens during our ordinarj' winters. Most of the rainfall of the 

 United States, as everyone knows, is derived from cyclonic storms— that is, from great 

 areas of low pressure and inblowing winds which may have a diameter of 1,000 or more miles, 

 and which sw^eep across the country with a general easterly trend in obedience to the 

 prevailing direction of the winds in temperate latitudes. The courses of these storms, so 

 far as they are understood, are determined by the differences in pressure between the 

 several more or less permanent areas of high or low barometer which center over oceans and 

 lands in various latitudes and with varying degrees of intensity at different seasons. In 

 general, storms move out from or around areas of high pressure and are drawn toward those 

 of low pressure. Anything which changes the location or intensity of the major pressure 

 areas changes the coui-se and intensity of storms. The North Atlantic Ocean, by reason 



