HELATIOX OF VEGETATION TO CONTINENTAL MOVEMENTS. 109 



discussions of these subjects, and those in the recently pubhshed essay 

 of Seward, as well as the posthumous report of Lesquereux on the flora 

 of the Dakota group. I have already referred to the special conditions 

 of the later Paleozoic in these respects, and am inclined to attribute the 

 great geographic uniformity of its vegetation principally to the then unfin- 

 ished condition of our continents, affording less local difference of eleva- 

 tion and greater uniformity in the distribution of ocean currents, though 

 the larger proportion of carbonic dioxide in the atmosphere may have 

 been also a determining cause. Yet, while there was little climatal dif- 

 ference of flora, there w^as continued change in time ; so that wherever 

 fossil plants occur, we can distinguish the vegetation of the Low^er, Middle • 

 and Upper Devonian, of the Lower Carboniferous, of the Coal Forma- 

 tion, of the Upper Coal Formation, and the Permian. The great earth - 

 movements of the Permian seem to have extinguished this flora by cre- 

 ating adverse climatic conditions, and in the Mesozoic age it was 

 replaced by a new assemlilage of plants, seemingly of southern origin, 

 and adapted to an insular condition of our hemisphere. The later Cre- 

 taceous flora, Avith its wealth of modern exogenous genera, seems to have 

 originated in the north and propagated itself southward, and the condi- 

 tion of things which led to a temperate flora in Greenland was connected 

 with the occurrence of a great mediterranean sea between the Rocky 

 mountains and the Appalachians, which determined the equatorial cur- 

 rent upward through the interior of the American continent and threw 

 its full force on Greenland, then probably less elevated than now. The 

 geographic conditions of these ages of the later Cretaceous and early 

 Cenozoic, we are now able to some extent to trace, and find them to corre- 

 spond with the climatal conditions indicated by the plants. On the 

 other hand, the changing physical conditions were correlated with those 

 changes in the vegetation which have enabled us to recognize so dis- 

 tinctly the lower, middle and later Cretaceous floras, and those of the 

 early, middle and later Cenozoic* 



While we have no evidence of a tropical climate in the northern part 

 of America in the Cretaceous or the Cenozoic periods, we have proof from 

 fossil plants of the continuance for long periods of a temperate chmate as 

 far north as Greenland, and that this passed gradually into the cooler 

 temperature of the ]\Iiocene and Pliocene. We can also correlate these 

 climatal conditions on the one hand with known geographic changes, 

 and on the other witli the distribution of animals and plants. 



The validity of such deductions does not altogether depend on the 

 accuracy of the reference of fossil species to existing genera or families. 

 In many cases there can be little doubt as to this, as in the species of 



* Trans. Royal Society of Canada, 1893. Paper on New Plants from Vancouver Island. 

 XV— Bum,. Geoi.. Soc. Aw., Voi-. u, 1893. 



