EXCURSIONS OF AN EVOLUTIONIST 



ent at once why we must probably select the 

 latest period of high eccentricity in the earth's 

 orbit as the period for which we have been seek- 

 ing. For that period which began 240,000 

 years ago, and terminated 80,000 years ago 

 presented such a set of astronomical circum- 

 stances as must have resulted in the repeated 

 glaciation of the northern hemisphere, after the 

 manner above described. And the antiquity 

 of that period seems to be sufficiently great to 

 allow for the geological changes which have 

 occurred since the Pleistocene age. If we were 

 to assign an earlier epoch of high eccentricity 

 for the Glacial period, it would then become 

 necessary to show why, with the present rela- 

 tions of land and sea on the globe, the latest 

 epoch of high eccentricity should not have pro- 

 duced a subsequent glacial period. But the Gla- 

 cial period which Agassiz first taught us to under- 

 stand, and which in recent years has been made 

 the subject of such minute study, is clearly the 

 latest glacial period that has occurred in the 

 northern hemisphere ; for it is the one of which 

 the traces are now everywhere around us ; it is 

 the one which has carved the mountains of 

 Scotland and New England in their present 

 beautiful outlines, and covered their sides with 

 boulders, and filled the valleys with romantic 

 tarns or magnificent lakes. If we adopt Mr. 

 Croll's theory of the causes of glaciation, we 



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