GRAY AM' HOOKEB ON THE ROCKY MOUNTAIM FLORA, o' I 



seldom answer questions of this kind ; but ;> reference to the past maj 

 sometimes <lo bo. 



Although the vegetable palaeontologist goes farther hack, the botantsl 

 of our era, in the discussion of his problems, maj take the Tertiary 

 period for his point of departure. At least, the kej to the distribution 

 of the flora ot the temperate regions of the northern hemisphere — 

 with which we are concerned — is afforded i>\ the later Tertiary botany. 



Our knowledge — fragmentary, yet real — of (lie flora around us begins 

 with a period when it or its direct ancestors occupied the zone between 

 the arctic circle and the pole, and doubtless several lower degrees oi 

 latitude. There it must have flourished until the coming on of thai 

 change o( climate which culminated in the glacial period. It must at 



that time have encircled that portion of the earth much as the arctic 

 flora now does. During the period of maximum refrigeration, its north- 

 ern Limits, abutting upon an arctic flora then in low latitude, musl 

 have been so tar south in the Atlantic states that the vegetation <>r the 

 northern shore of the Gulf of Mexico probably resembled that of the 



southern shore of the (lull* ot" Saint Lawrence now. Of this northern 

 limit there cannot he much donbl : yet we could not hazard an opinion 

 asto where the warm-temperate vegetation of that daymerged into the 



Subtropical, as it now does in Southern Texas. 



The change between that period and the present, in the opposite di- 

 rection, has been an amelioration of climate which has carried the arc- 

 tic flora hack to the arctic circle, with which we now associate it. except- 

 ing the portions which, in the retreat, have ascended the mountains 

 and persisted there, forming the arctic-alpine vegetation. This, as we 

 have seen, is very scanty m the Atlantic district, where it has abided 



only en the most northern mountains: while the more elevated ranges 

 ot" the western part of the continent have afforded ampler refuge. 



A similar advance and ensuing retrogression, consequent upon the 

 Mining and going out of the Glacial epoch, must have taken place in 

 other parts of the northern hemisphere. Under these great and pro- 

 tracted movements of transference, we suppose that a common flora, 

 which was comparatively homogeneous round the new arctic /one. has 

 been differentiated into the several existing north-temperate floras, and 

 that their common features, and the occasional very unexpected identi- 

 ties or similarities (such as those between Japanese and North Ameri 

 can botany are thus explained. Their respective peculiarities are 

 thought to have resulted from the different vicissitudes and the different 

 climatic conditions to which the primeval stock has been exposed in 

 .. Europe, and America, and upon the opposite sides and great in 

 tenors of continents, the climates of which — greatly different dow— have 

 probably been so from verj earlj times. The plants which were most 

 adapted <-i adaptable to the on.- could not I ted t<> survive In 



another, or in any other than one of similar or analogous climate. But 

 this is not the place for considering the application of these principle* 



