234 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



somewhat earlier times which are known as glacial stages. Lastly, the fourth type of cli- 

 matic change is that which appears so distinctly in the cm-ve of the sequoia. That curve 

 gives strong evidence of distinct chmatic cycles measured in units of hundreds of years. 

 So far as we can ascertain, all four of our types of changes, from those of the 11-year period 

 up to glacial epochs, seem to be of essentially the same nature : at one extreme they appear 

 to be characterized by an expansion of the polar zone of climate and a corresponding com- 

 pression of the remaining zones toward the equator, and at the other by a contraction of 

 the polar zone and a poleward expansion of the others. 



The types of changes which have just been mentioned do not embrace all the chmatic 

 vicissitudes to which the earth is subject. Back of the minor cycles measured by years 

 or decades, back of the historic cycles measured by centuries, back of the chmatic stages 

 measured by millenniums, and back of glacial epochs measured by tens of thousands of 

 years, there he glacial periods of still vaster dimensions composed of a series of epochs and 

 measured in hundreds of thousands of years. They appear to differ from one another 

 in a way quite unlike that in which one of the smaller epochs or cycles differs from another. 

 During one glacial period the conditions favorable to glaciation may be localized in polar, 

 or at least far northern latitudes, as was the case during the Pleistocene period; while during 

 another the exact reverse may be true, and the glaciation may be locahzed within from 

 20° to 40° of the equator, as happened in Permian times. This suggests that while the 

 smaller changes, from glacial epochs down to cycles of a few years, may all be of the same 

 nature, and may be due to the same cause, such pronounced phenomena as the Permian 

 redistribution of the climatic zones as a whole are probably due to another cause. This 

 idea, then, of a twofold cause of the chmatic instability of the earth may serve as a guide 

 in our future studies. 



In attempting to ascertain the causes of any group of facts two methods may be pur- 

 sued. In the fii-st place, we may search for phenomena whose effects are beyond the 

 range of observation, but which the processes of reasoning lead us to beheve may be asso- 

 ciated with the phenomena which we wish to explain. This process has led to two chief 

 climatic hypotheses: one is Croll's theory of the precession of the equinoxes, and the 

 other is the carbonic-acid theory, whose inception we owe to Arrhenius, and which has 

 been so well elaborated by Chamberlain. In developing both theories a long and extremely 

 complicated process of reasoning has been necessary in order to reach a final conclu- 

 sion . It has been impossible, even on a small scale, to test most of the steps involved in the 

 reasoning. 



The other method is the discovery of phenomena which can actually be seen to vary in 

 harmony with the facts which we desire to explain. This method has also given rise to 

 two climatic theories. In the first place, it is evident to the most unskilled observer that 

 the extent, elevation, and relief of the land are of the highest importance in causmg differ- 

 ences in climate. From this has arisen the theory that the chief changes in the chmate of 

 the earth are due to variations in the location, form, and extent of the continents, accom- 

 ])anied by corresponding changes in the circulation of the air and of oceanic waters. In 

 the second place, no one doubts that the amount of energy received from the sun is the fun- 

 damental factor in determining the climate of the different parts of the world. If the 

 amount of radiation received from the sun should change appreciably, the chmate of the earth 

 would certainly be modified. From this has arisen the solar theory. The two methods 

 of investigation which have just been indicated must in practise be carried on together; 

 nevertheless, there is a distinct and unportant difference. A highly theoretical conception, 

 such as the precession of the equinoxes or the abstraction of carbon dioxide from the 

 atmosphere, is more liable to error than is an observational conception, such as the climatic 

 effect of the altitude and form of the lands, or the effect of changes in solar radiation upon 

 terrestrial temperature. 



