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 and locked up 
