74 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.\I. 



conveyance of seeds across oceans, or even from one mountain to another. The plants 

 of the top of the White Mountains and of Labrador are mainly the same ; hut we need 

 not supijose that it is so because birds have carried seeds from the one to the other. 



I take it that the true explanation of the whole jiroblem comes from a just general 

 view, and not through piecemeal suppositious of chances. And I am clear that it is 

 to be found by looking to the north, to the state of things at the arctic zone — first, as 

 it now is, and then as it has been. 



North of our forest regions comes the zone unwooded from cold — the zone of arctic 

 vegetation. In this, as a rule, the species are the same round the world ; as exceptions, 

 some are restricted to a part of the circle. 



The polar projection of the earth down to the northern tropic, as here exhibited, 

 shows to the eye — as our luaps do not — how all the lands come together into one region, 

 and how natural it may be for the same species, under homogeneous conditions, to 

 spread over it. When we know, moreover, that sea and land have varied greatly since 

 these species existed, we may well believe that any ocean-gaps now in the way of 

 equable distribution may have been bridged over. There is now only one considera- 

 ble gap. 



What would happen if a cold period were to come on from the north, and were very 

 slowly to carry the present arctic climate, or something like it, down far into the tem- 

 perate zone ? ^Vhy, just what has happened in the Glacial period, when the refrigera- 

 tion somehow pushed all these plants before it down to Suuthern Europe, to Middle 

 Asia, to the middle and southern part of the United States, and, at length receding, 

 left some parts of them stranded on the Pyrenees, the Alps, the Appenines, the Cau- 

 casus, on our White and Rocky Mountains, or wherever they could escape the increas- 

 ing warmth as well by ascending mountains as by receding northward at lower levels. 

 Those 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 oif, and hold their positions on mountain tops under cover of the frigid 

 climate due to elevation. The conditions of these ou 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 continue, with or without some change, 

 and hence the diversification 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 wo 

 are concerned with. The clew was seized when the fossil botauy of the high arctic 

 regions came to light ; when it was demonstrated that in the times next preceding the 

 Glacial period — in the latest Tertiary — from Spitzbergen and Iceland to Greenland and 

 Kamtschatka 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 jilauts. 

 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 arc- 

 tic 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 extensive tinds are in Greenland ; that they comprise most 

 of the sorts which I have spoken of as American trees which once lived in Europe — Mag- 

 nolias, Sassafras, Hickories, Gum-trees, ouridentical 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 several others ; that they equally com- 

 prise trees now peculiar to Japan and China, three kinds of Gingko-trees, for instauce, 

 one of them not evidently distinguishable from the Japan species, which alone sur- 



