430 THE ICE AGE IN NORTH AMERICA. 



that kept together at a low level, and made good their retreat, 

 form the main body of present arctic vegetation. Those that 

 took to the mountains had their line of retreat cut off, and 

 hold their positions on the mountain-tops under cover of the 

 frigid climate due to elevation. The conditions of these on 

 different continents or different mountains are similar but not 

 wholly alike. Some species proved better adapted to one, some 

 to another, part of the world : where less adapted, or less 

 adaptable, they have perished ; where better adapted, they con- 

 tinue — with or without some change — and hence the diversifi- 

 cation of Alpine plants as well as the general likeness through 

 all the northern hemisphere. 



All this exactly applies to the temperate-zone vegetation, 

 and to the trees that we are concerned with. The clew was 

 seized when the fossil botany of the high arctic regions came 

 to light ; when it was demonstrated that in the times next pre- 

 ceding the Glacial period— in the latest Tertiary — from Spitz- 

 bergen and Iceland to Greenland and Kamchatka, a climate 

 like that we now enjoy prevailed, and forests like those of New 

 England and Virginia, and of California, clothed the land. 

 We infer the climate from the trees ; and the trees give sure 

 indications of the climate. 



I had divined and published the explanation long before I 

 knew of the fossil plants. These, since made known, render 

 the inference sure, and give us a clear idea of just what the 

 climate was. At the time we speak of, Greenland, Spitzbergen, 

 and our arctic sea-shore, had the climate of Pennsylvania and 

 Virginia now. It would take too much time to enumerate the 

 sorts of trees that have been identified by their leaves and 

 fruits in the arctic later Tertiary deposits. 



I can only say, at large, that the same species have been 

 found all round the world ; that the richest and most exten- 

 sive finds are in Greenland ; that they comprise most of the 

 sorts which I have spoken of, as American trees which once 

 lived in Europe — magnolias, sassafras, hickories, gum-trees, 

 our identical Southern cypress (for all we can see of difference), 

 and especially Sequoias — not only the two which obviously 

 answer to the two big trees now peculiar to California, but sev- 

 eral others ; that they equally comprise trees now peculiar to 



