chap, vii.] MAMMALIA OF THE NEW WORLD. 151 



portions of the earth's surface, and one which, as far as the 

 Tertiary period at least is concerned, was of an exceptional cha- 

 racter. Such a cause exists in the great and recent physical change 

 known as " the Glacial epoch." We have proof in both Europe 

 and North America, that just about the time these large animals 

 were disappearing, all the northern parts of these continents 

 were wrapped in a mantle of ice ; and we have every reason to 

 believe that the presence of this large quantity of ice (known to 

 have been thousands of feet if not some miles in thickness) must 

 have acted in various ways to have produced alterations of 

 level of the ocean as well as vast local floods, which would have 

 combined with the excessive cold to destroy animal life. There 

 is great difference of opinion among geologists and physicists as 

 to the extent, nature, and duration of the Glacial epoch. Some 

 believe it to have prevailed alternately in the northern and 

 southern hemispheres ; others that it was simultaneous in both. 

 Some think there was a succession of cold periods, each lasting 

 many thousands of years, but with intercalated warm periods of 

 equal duration ; others deny that there is any evidence of such 

 changes, and maintain that the Glacial epoch was one continuous 

 period of arctic conditions in the temperate zones, with some 

 fluctuations perhaps but with no regular alternations of warm 

 periods. Some believe in a huge ice-cap covering the whole 

 northern hemisphere from the pole to near 50° north latitude 

 in the eastern, and 40° in the western hemisphere ; while others 

 impute the observed effects either to glaciers from local centres, 

 or to floating icebergs of vast size passing over the surface during 

 a period of submersion. 



Without venturing to decide which of these various theories 

 will be ultimately proved to be correct, we may state, that 

 there is an increasing belief among geologists in the long dura- 

 tion of this ice-period, and the vast extent and great thick- 

 ness attained by the ice-sheet. One of the most recent, and 

 not the least able, of the writers on this question (Mr. Belt) shows 

 strong reasons for adopting the view that the ice-period was 

 simultaneous in both hemispheres ; and he calculates that the 

 vast amount of water abstracted from the ocean aid locked up 



