1879. | Distribution of the North American Flora. 165 
After a detailed comparison of the botany of Japan and North 
America, and proving their affinity, Prof. Gray refers to the fact 
that many of the existing genera and even species of both floras 
coéxisted in the high latitudes of America during Miocene times, 
as shown by Heer and other palzontologists; during which 
period he further assumes that the three northern continents were 
conjoined, or so nearly contiguous as to allow of a commingling 
of their floras. 
The glacial period followed, carrying an arctic climate south to 
the latitude of the Ohio, but so gradually, that these plants were 
not exterminated, but wholly or in part driven southward, fol- 
lowed in the rear by the arctic vegetation. As the temperature 
rose with the retreating ice, this flora returned northward, leaving 
the arctic and sub-arctic plants on the mountains of both East 
and West America. 
‘He next shows that the retreat northward was to a somewhat 
higher latitude than the same plants now attain; and this he 
accounts for by a reference to the fluvial epoch of Dana,’ when 
the region of the great lakes was submerged five hundred feet 
below their present level. This diminished area and lowered 
elevation of the land, by inducing a milder climate than now 
obtains inthe lake region, favored the extension of the flora to a 
higher latitude than it now attains, and hence effected a second 
commingling of American and Asiatic plants. Lastly, Dana's 
Terrace epoch supervened, when the previously depressed northern 
region was again raised, cooling the climate, finally dissociating 
the Asiatic and American floras, and giving to the arctic and sub- 
arctic plants of the continent their present limits. 
It remains now to account for the great rarity of East Asiatic 
types in America west of the prairies, and the presence in those 
meridians of Mexican and still more southern ones. Hitherto 
there have been no other attempts at a solution of this problem 
than such unsupported speculations as that the western half of 
the continent, though so much the loftier, was submerged during 
the southern migration of the northern miocene plants; or that 
the climate of the West was unsuited to the habits of these, which 
1 Whilst these pages were still in the press, Prof. Gray has informed me that he 
now lays little stress on the conditions suppesed to be due to the terrace and fluvial 
~ epochs; and that he is rather disposed to consider the separation of the northern 
floras by the Glacial epoch to have been final. 
