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

5;chap, in.] CONDITIONS AFFECTING DISTEIBUTION. 41great valleys on the southern side of the Alps poured downstreams of ice which stretched farout into the plains of NorthernItaly, and have left their debris in the form of hugemountainous moraines, in some cases more than a thousand feethigh.In Canada and New Hampshire the marks of moving iceare found on the tops of mountains from 3,000 to 5,000 feethigh ; and the whole surface of the country around and to thenorth of the great lakes is scored by glaciers. Wherever theland was submerged during a part of this cold period, a depositcalled boulder-clay, or glacial-drift has been formed. This is amass of sand, clay, or gravel, full of angular or rounded stonesof all sizes, up to huge blocks as large as a cottage ; and especiallycharacterized by these stones being distributed confusedlythrough it, the largest being as often near the top as near thebottom, and never sorted into layers of different sizes as inmaterials carried by water. Such deposits are known to beformed by glaciers and icebergs ; when deposited on the land byglaciersthey form moraines, when carried into water and thusspread with more regularity over a wider area they form drift.This drift is rarely found except where there isother evidence ofice-action, and never south of the 40th parallel of latitude, towhich in the northern hemisphere signs of ice-action extend.In the southern hemisphere, inPatagonia and in New Zealand,exactly similar phenomena occur.A very interesting confirmation of the reality of this coldepoch is derived from the study of fossil remains. Both theplants and animals of the Miocene period indicate that theclimate of Central Europe was decidedly warmer or more equablethan it is now ; since the flora closely resembled that of theSouthern United States, with a likeness also to that of EasternAsia and Australia. Many of the shells were of tropical generaand there were numbers of large mammalia allied to theelephant, rhinoceros, and tapir. At the same time, or perhapssomewhat earlier, atemperate climate extended into the arcticregions, and allowed a magnificent vegetation of shrubs andforest trees, some of them evergreen, ta 'flourish within twelvedegrees of the Pole. In the Pliocene period we find ourselvesVol. I.—