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

42 DISTRIBUTION OF ANIMALS. [part iamong forms implying a climate very little different from thepresent ; and our own Crag formation furnishes evidence of agradual refrigeration of climate ; since its three divisions, theCoralline, Ked, and Norwich Crags, show a decreasing numberof southern, and an increasing number of northern species, as weapproach the Glacial epoch. Still later than these we have theshells of the drift, almost all of which are northern and manyof them arctic species. Among the mammalia indicative ofcold, are the mammoth and the reindeer.In gravels and cavedepositsof Post-Pliocene date we find the same two animals,which soon disappear asthe climate approached its present condition;and Professor Forbes has given a list of fifty shellswhich inhabited the British seas before the Glacial epoch andinhabit it still, but are all wanting in the glacial deposits. Thewhole of these are found in the Newer Pliocene strata of Sicilyand the south of Europe, where they escaped destruction duringthe glacial winter.There are also certain facts in the distribution of plants, whichare so well explained by the Glacial epoch that they may be saidto give an additional confirmation to it. All over the northernhemisphere within the glaciated districts, the summits of loftymountains produce plants identical with those of the polarregions. In the celebrated case of the "White Mountains an NewHampshire, United States (latitude 45°), all the plants on thesummit are arctic species, none of which exist in the lowlandsfor near a thousand miles further north. It has also been remarked.that the plants of each mountain are more especiallyrelated to those of the countries directly north of it. Thus,those of the Pyrenees and of Scotland are Scandinavian, andthose of the White Mountains are all species found in Labrador.Now, remembering that we have evidence of an exceedinglymild and uniform climate in the arctic regions during theMiocene period and aevident that with each degreegradual refrigeration from that time, it isof change more and more hardyplants would be successively driven southwards ;till at last theplains of the temperate zone would be inhabited by plants, whichwere once confined to alpine heights or to the arctic regions.