British North American Plants. 419 



of the present European flora. As far back as even the 

 Lignite Tertiary of Eocene age, there were in the New 

 England States and Manitoba, representatives of the modern 

 genera Populus, Cinnamomum, Fagus, Quercus, Plat anus, 

 Sassafras, Nyssa, Carpinus, Aristolochia, Onoclea, Illicium 

 and Sapindus, indicating a somewhat more southern climate 

 than now exists, and some of the species were apparently 

 specifically identical with plants in Canada at the present 

 day. If the view can then be entertained that the inter- 

 mingling of Asiatic and American species took place prior 

 to the elevation of the Rocky Mountains to their present 

 height, and prior, therefore, to the appearance of British 

 Columbia at that period, it will fully explain the more 

 recent establishment in that province of its present flora 

 and the absence of representatives of that flora in Japan. 



The Eocky Mountains, as well as the prairies, would be 

 effective barriers to the spread of many species across the 

 continent. The mountains would present high elevations, 

 special climatic conditions, and a rugged character, whilst 

 the prairies with their vast, open, level, generally treeless 

 stretches of country, would afford constant exposure to dry- 

 ing winds and the sun's rays, and in certain sections, to the 

 not infrequent recurrence of drought. Each of these condi- 

 tions would, in its turn, prove fatal to the progress of cer- 

 tain trees and plants, and thus, collectively, large numbers 

 would have barriers raised to their range. The prairies 

 have been formed since the elevation of the Eocky Moun- 

 tains, and are, in some places, still in process of formation. 

 Plants and trees which had not, prior to the formation of 

 these prairies, spread themselves across the continent, would 

 not now be able to extend their range, if the physical and 

 climatic conditions presented by these prairies were un- 

 favourable to progress. 



With regard to the second consideration that the migra- 

 tion has been westward from Europe, there is some argu- 

 ment to support it, difficult as it may be from the present 

 relative condition of land and water, to trace any connect- 

 ing lines of communication. In a paper in this Journal 

 (Canadian Naturalist, vol. 7, p. 221,) when discussing the 



