No. 1.1 GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 75- 



Tives ; that we have evidence, not merely of Pinea and Maples, Poplars, BirchcH, Liu- 

 dens, and whatever else characterize the temi»erate-zone forests of our era, hut also of 

 particular species of these, so like those of our own time and country that wc may fairly 

 reckon them as the ancestors of several of ours. Long genealogies always deal more 

 or less in conjecture, but we appear to be within the limits of scientific inference when 

 we announce that onr existing temperate trees came from the north, and within the 

 bounds of nigh probability when we claim not a few of them as the originals of pres- 

 ent species. Remains of the same plants have been found fossil in our temperate re- 

 gion, as well as in Europe. 



Here, then, we have reached a fair answer to the question, how the same or similar 

 species of our trees came to be so dispersed over such widely separated continents. The 

 lands all diverge from a polar centre, and their jiroximate iJortions, however different 

 from their present configuration and extent, and however changed at different times, 

 were once the home of those trees, where they fionrished in a temperate climate. The 

 cold period which followed, and which doubtless came on by very slow degrees during 

 ages of time, must, long before its culmination, have brought down to our latitudes, 

 with the similar climate, the forest they jjossess now, or rather the ancestors of it. 

 During this long (and we may believe first) occupancy of Eurojje and the United 

 States were deposited in pools and shallow waters the cast leaves, fruits, and, occa- 

 sionally, branches, which are embedded in what are called Miocene Tertiary, or later 

 deposits, most abundant in Europe, from which the American character of the vege- 

 tation of the period is inferred. Geologists give the same name to these beds in 

 Greenland and Southern Europe, because they contain the remains of identical and 

 very similar species of plants, and they used to regard them as of the same age, on 

 account of this identity. But in fact this identity is good evidence that they cannot 

 be synchronous. The beds in the lower latitudes must be later, and were forming 

 when Greenland probably had very nearly the climate which it has now. 



Wherefore the high, and not the low, latitudes must be assumed as the birth-place 

 of our present flora ;* and the present arctic vegetation is best regarded as a derivative 

 of the temperate. This flora, which when circumpolar was as nearly homogeneous 

 round the high latitudes as the arctic vegetation is now, when slowly translated into 

 lower latitudes, would preserve its homogeneousness enough to account for the actual 

 distribution of the same and similar species round the world, and for the original en- 

 dowment of Europe with what we now call American types. It would also vary or 

 be selected from by the increasing differentiation of climate in the divergent conti- 

 nents, and on their different sides, in a way which might well account for the present 

 diversification. From an early ijeriod the system of the winds, the great ocean cur- 

 rents (however they maj^ have oscillated north and south), and the general proportions 

 and features of the continents in our latitude (at least of the American continent) 

 were much the same as now, so that species of plants, ever so little adapted or pre- 

 disposed to cold winters and hot summers, would abide and be developed on the east- 

 em side of continents, therefore in the Atlantic United States and in Jajian and Man- 

 churia ; those with preference for milder winters would incline to the western sides ; 

 those disposed to tolerate dryness would tend to interiors, or to regions lacking sum- 

 mer rain. So that if the same thousand species were thrust promiscuously into these 

 several districts, and carried slowly onward in the way supposed, they ^^•ould inevit- 

 ably be sifted in such a manner that the survival of the fittest for each district might 

 explain the present diversity. 



Besides, there are resiftings to take into the account. The glacial period or refrig- 

 eration from the north, which at its inception forced the temperate flora into our lati- 



* This takes for granted, after Nordenskiold, that there was no preceding Glacial 

 period, as neither paleontology nor the study of arctic sedimentary strata afford any 

 evidence of it. Or if there were any, it was too remote in time to concern the present 

 question. 



