NORTH AMERICAN FLORA. 281 



Now if, as I have maintained, the eastern side of North 

 America and the eastern side of northern Asia are the favored 

 heirs of the old boreal flora, and if I have plausibly explained 

 how Europe lost so much of its portion of a common inher- 

 itance, it only remains to consider how the western side of 

 North America lost so much more. For that the missing types 

 once existed there, as well as in Europe, has already been in- 

 dicated in the few fossil explorations that have been made. 

 They have brought to light Magnolias, Elms, Beeches, Chest- 

 nuts, a Liquidambar, etc. And living witnesses remain in the 

 two Sequoias of California, whose ancestors, along with Taxo- 

 dium, which is similarly preserved on the Atlantic side, ap- 

 pear to have formed no small part of the Miocene flora of the 

 arctic regions. 



Several causes may have conspired in the destruction ; — 

 climatic differences between the two sides of the continent, 

 such as must early have been established (and we know that 

 a difference no greater than the present would be effective) ; 

 geographical configuration, probably confining the migration 

 to and fro to a long and narrow tract, little wider, perhaps, 

 than that to which it is now restricted ; the tremendous out- 

 pouring of lava and volcanic ashes just anterior to the Glacial 

 period, by which a large part of the region was thickly cov- 

 ered ; and, at length, competition from the Mexican -plateau 

 vegetation, — a vegetation beyond the reach of general glacial 

 movement from the north, and climatically well adapted to 

 the southwestern portion of the United States. 



It is now becoming obvious that the Mexican-plateau vege- 

 tation is the proximate source of most of the peculiar ele- 

 ments of the Californian flora, as also of the southern Rocky 

 Mountain -region and of the Great Basin between ; and that 

 these plants from the south have competed with those from 

 the north on the eastward plains and prairies. It is from this 

 source that are derived not only our Cactece but our Mimosece, 

 our Daleas and Petalostemons, our numerous and varied Ona- 

 gracece, our Loasacece, a large part of our Compositce, espe- 

 cially the JEupatoriacece, Helianthoidem, HelenioidecB^ and 

 Mutisiacece, which are so characteristic of the country, the 



