258 DISCUSSION OF THE DESICCATION QUESTION 



proper, in the present connection, to state briefly the principal theories of cli- 

 matic change which have been advanced by various investigators in this line 

 of inquiry, and to make a few remarks on their probability, and their compar- 

 ative value as fitting the facts which have been developed in the preceding 

 pages. If we find, on examination of that which has been published on the 

 subject, that strong evidence has been presented by the most eminent physicists 

 and astronomers in favor of the existence of a cause which must necessarily 

 bring about the precise results wliich we have found, as the result of cai'eful 

 geological inquiry, to have been, and to be still, taking place, we shall be 

 authorized to feel that we are supported in our interpretation of the focts. 

 For that the facts are very differently interpreted by different writers on this 

 subject cannot fail to have become evident to the reader. 



Even a cursory examination of the theories which have been put forward 

 to account for climatic changes on the earth will show that the important 

 ones may, with propriety, be classed in three rather distinct groups. To the 

 first belong those theories which are of so vague a character that they may 

 be said with truth to adapt themselves to any possible kind of change, or 

 series of changes, of climate. If the conditions which they assume as having 

 taken place could really be proved to have done so, there would be no diffi- 

 culty about the facts, because they would fit any conceivable scheme of 

 occurrences. The second group of theories or, rather, the second theory, for, 

 in reality, there is only one of this character, looks to a persistent change 

 of climate in one direction, a secular variation of the same kind, always in 

 action and not liable to any interruptions, whether of a periodical or of an ir- 

 regular character. The third group includes those theories which are based on 

 considerations connected with astronomical phenomena of regular recurrence, 

 and which are therefore supposed to bring about recurrent or cyclical changes 

 in the climates of the two hemispheres, according as they are differently 

 situated with respect to the sun. The various forms of this class of theories, 

 as developed by Adhemar, Schmick, and Croll, have been almost exclusively 

 applied to explain the occurrence of the so-called " Glacial epoch," and tiiey 

 have hardly any significance in their application to the phenomena of cli- 

 matic chancre reo-arded as extending through all the geoloijical aares. These 

 theories will, therefore, not be discussed in this place ; but their considera- 

 tion will be reserved for the next chapter, where attention will be called to 

 them in connection with the facts wliich they are supposed to explain. It 

 will there be sliovvn, as the writer believes, that the theories in question have 



