392 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



investigator in surmounting these difficulties seems to the writer to have 

 been clearly established in the course of the present chapter. 



Before closing, a few words must be said in regard to certain climatic 

 theories which have at different times been much in vogue among; geol- 

 ogists, and of which, thus far in the present work, only a passing notice 

 has been taken. Allusion is here made to the views held by Adhemar, 

 Croll, and others, according to which there has been during the geological 

 ages a recurrence or cj'clical succession either of cold and warni periods, 

 or, if not of these, of such as were favorable to the development of ice and 

 snow altei'nately on the two hemispheres. The essential feature of these 

 theories of recurrent periods, or cycles of conditions tending to bring about 

 glaciation, is that the northern and southern halves of the globe are, at dif- 

 ferent epochs, differently situated with regard to the sun. It was Adhemar's 

 idea that the southern hemisphere is now in a glaciated condition, because 

 the winter there is several days longer than in the other hemisphere. The 

 theory of Croll, on the other hand, is based on the fact that the eccentricity 

 of the earth's orbit is subjected to a cyclical change, in consequence of which 

 its distances from the sun in perihelion and aphelion vary to a considerable 

 amount. Mr. Croll believes that the result of the aphelion taking place in 

 winter, during a period of maximum eccentricity, would produce such a cli- 

 matic condition as would bring about a " Glacial epoch " in the hemisphere 

 thus affected. 



The first point, then, to which attention would naturally be turned, in 

 endeavoring to make out whether these cyclical theories had some sub- 

 stantial basis of fact on which to rest, would be, whether geological and 

 palajontological investigations proved that such recurrent changes as are 

 demanded by these theories have ever really taken place. 



If, as the reader will easily comprehend, there has been a cyclical recur- 

 rence of conditions resulting in periods of alternate glaciation and non- 

 glaciation of the two hemispheres, then all that has been stated in the 

 previous chapter in regard to persistent change in one direction, in conse- 

 quence of the diminution of the sun's radiating power, would have to be set 

 down as naught, unless it could be shown that evidence of refrigeration 

 similar in importance and amount to that presented as proof of a warmer 

 climate during variousgeological epochs had been overlooked or kept back 

 designedly. If there has been a recurrence of changes from cold to warm, 

 or from conditions favoring the formation of ice to those of a contrary nature. 



