74 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.vi. 



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

 of the bop of the White Mountains and of Labrador arc mainly the same ; but we need 

 not Buppose that u is bo be< ause birds have carried seeds from the one to the other. 



I bake it that bhe brne explanation of the whole problem conies from a just general 

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

 to be found by Looking bo the north, bo bhe state of things at the art tic zone — tirst. as 



it now is. and then as it has been. 



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

 bation. In bhis, 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 ey< — as our maps 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 8peoies existed, We may well believe that any ocean-gaps now in the way of 

 equable distribution may have been bridged over. There is tiow 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 ? Why. just what has happened in the Glacial period, when the refrigera- 

 tion somehow pushed all these plants before it down to Southern 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 W T hite 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 off, and hold their positions on 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 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 tin trees that we 

 ate < onccrned with. The clew was seized when the fossil botany of the high arctic 

 regions (ante 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. 



Iliad divined and published the explanation long before 1 knew of the fossil plants. 

 These, Since made known, lender the inference sure, and give us a clear idea of just 

 what the ciiniat" 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 l»een identified by their haves 



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 finds 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, our identical Southern Cypress (for all we can see 

 of difference), and especially Sequoias, not mil \ the two Which obviously answer to the 

 two Big-trees, now peculiar to California, i>ut several others ; that theyeqnallj coto- 

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



one of th«;uinot evident ly dist iugui^hahle from the Japan species, which alone stir- 



