which require an Ice Age, single in its occurrence and 

 unique in the climatic history of the earth. The theo- 

 ries of this class proved unsatisfactory, and failed to 

 fully explain the admitted facts of Geology. This fail- 

 ure, in a measure, warranted a resort to the ingenious 

 theories of Class II; which theories require a recurrence 

 of glacial epochs. The lack of evidence of such recur- 

 rence has prevented the theories of this class from being 

 generally accepted. 



Class I. — (1) A decrease in the original heat of the 

 earth. 



(2) Changes in the elevation of the land areas, and 

 consequent variations in the distribution of land and 

 water. 



(3) A period of greater moisture in the atmosphere. 

 Class II. — (4) Changes in the obliquity of the axis 



of the earth. 



(5) A coincidence of an Aphelion winter with a 

 period of maximum eccentricity of the orbit of the 

 earth. 



(6) A combination of (2) and (5). 



These have been frequently reviewed by various au- 

 thorities. The general conclusions reached may be 

 summed up in the following opinions: 



Referring particularly to the first cause of the series, 

 and probably the oldest, Prof. Jas. D. Whitney says: 



"It is evident that the idea of connecting the phe- 

 nomena of the internal heat of the globe with terrestrial 

 climates, whether of the present or of past geological 

 ages, must be entirely abandoned, as it has been by most 

 writers on this subject. The hypothesis can not be 

 allowed to stand as even one of the possible theories of 

 climatic change. (The Climatic Changes of Later Geo- 

 logical Times, page 261. See also LeConte, Elements of 



