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, ete. 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 Cactew but our Mimosee, 
our Daleas and Petalostemons, our numerous and varied Ona- 
gracee, our Loasacee, a large part of our Composite, espe- 
cially the Hupatoriacee, Helianthoidew, Helenioidew, and 
Mutisiacee, which are so characteristic of the country, the 
