No i.\ GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 61 



seldom answer questions of this kind ; but a reference to the past may 

 sometimes do so. 



Although the vegetable pala3ontok)gist goes faithcr back, the botanist 

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

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

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

 with which we are concerned — is afforded by the later Tertiary botany. 



Our knowledge — fragmentary, yet real — of the 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 sev(?ral lower degrees of 

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

 cha-nge of 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, must 

 have been so far south in the Atlantic States that the vegetation of the 

 northern shore of the Gulf of Mexico i^robably resembled that of the 

 southern shore of the Gulf of Saint Lawrence now. Of this northern 

 limit there cannot be much doubt ; yet we could not hazard an opinion 

 as to where the warm-temperate vegetation of that day merged 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 back 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 in the Atlantic district, where it has abided 

 only on the most northern mountains ; while the more elevated ranges 

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



A similar advance and ensuing retrogression, consequent upon tbjfr 

 coming and going oiit of the Glacial epoch, must have taken place in^ 

 other parts of the northern liemisx)here. Under these great and pro- 

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

 which was comparatively homogeneous round the new arctic zone, 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 i)eculiarities are 

 thought to have resulted from the different vicissitudes and the different 

 climatic conditions to which the primeval stock has been exi)psed in 

 Asia, Europe, and America, and upon the opposite sides and great in- 

 teriors of continents, the climates of which — greatly different now— have 

 probably been so from very early times. The plants which were most 

 adapted or adaptable to the one could not be expected to 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 principles 



