The geographical distribution of animals, with a study of the relations ...

chap, vii.] MAMMALIA OF THE NEW WORLD. 151portions of the earth's surface, and one which, as far as theTertiary period at least isconcerned, was of an exceptional character.Such a cause exists in the great and recent physical changeknown as " the Glacial epoch." We have proof in both Europeand North America, that just about the time these large animalswere disappearing, all the northern parts of these continentswere wrapped in a mantle of ice ;believe that the presence of thisand we have every reason tolarge quantity of ice (known tohave been thousands of feet if not some miles in thickness) musthave acted in various ways to have produced alterations oflevel of the ocean as well as vast local floods, which would havecombined with the excessive cold to destroy animal life.Thereis great difference of opinion among geologists and physicists asto the extent, nature, and duration of the Glacial epoch. Somebelieve it to have prevailed alternately in the northern andsouthern hemispheres ;others that it was simultaneous in both.Some think there was a succession of cold periods, each lastingmany thousands of years, but with intercalated warm periods ofequal duration ; others deny that there is any evidence of suchchanges, and maintain that the Glacial epoch was one continuousperiod of arctic conditions in the temperate zones, with somefluctuations perhaps but with no regular alternations of warmperiods. Some believe in a huge ice-cap covering the wholenorthern hemisphere from the pole to near 50° north latitudein the eastern, and 40° in the western hemisphere ;impute the observed effects either to glacierswhile othersfrom local centres,or to floating icebergs of vast size passing over the surface duringa period of submersion.Without venturing to decide which of these various theorieswill be ultimately proved to be correct, we may state, thatthere is an increasing belief among geologists in the long durationof this ice-period, and the vast extent and great thicknessattained by the ice-sheet. One of the most recent, andnot the least able, of the writers on this question (Mr. Belt) showsstrong reasons for adopting the view that the ice-period wassimultaneous in both hemispheres ; and he calculates that thevast amount of water abstracted from the ocean aid locked up