1879. ] Distribution of the North American Flora. 157 
America, these, his fellow-emigrants, would remain as witnesses 
of his former presence, not only on the shores and in the forests 
of the older States, but in the interior prairie and. the newly set- 
tled valleys of the Rocky mountains themselves. 
Time does not permit me to dwell longer upon this subject of 
immigration during the historic period. I must now hasten to 
consider the flora of North America as it was for an indefinite 
period before the arrival of the Anglo-Saxon, embracing pre- 
_ historic and geological epochs; we have to regard this flora as a 
whole, and as subdivisible into local floras, characterized by the 
prevalence of certain assemblages of plants; to connect these 
local floras with the geographical features of the areas they 
occupy; to account for their position and composition by a refer- 
ence.to the countries from which their components may have been 
derived, and to the means of communication which exist, or ~~ 
in former times have existed with these countries. 
Before proceeding with this inquiry I will indicate, with the 
aid of the map, those prominent features of North American 
geography, which have regulated the distribution of its plants. 
Physical Conformation of America —In the Arctic regions the 
three northern continents approach, and the hydrography and 
geography of these regions favor the assumption that in former 
times they may have been connected. Next we observe that in 
the American continent (unlike the European and Asiatic), the 
great obstacles to the intermingling of floras, the mountain chains, 
are longitudinal ; as are the principal valleys, which are the great 
aids to their diffusion. If we now run a section across the conti- 
nent along its principal parallel (that near 40°), which approxi- 
-mately coincides with the isotherm of 55°, we find that it (see p. 
5), represents tolerably well any other parallel to it in those 
meridians in which there is the greatest development of a tem- 
perate vegetation. Commencing on the east, there is first the 
Atlantic seaboard, bounded to the westward by mountain ranges of 
moderate elevation (rarely attaining 6000 feet), which under various 
names extend from New Brunswick, in lat. 48°, to Alabama and 
Georgia, in lat. 34° (and which have been collectively called the 
Appalachian chain). Westward of this chain are the broad, low 
well-watered valleys of the Ohio, Mississippi and Missouri, the 
latter in its intersection with our principal parallel being nearly: 
midway across the continent and 1 300 Lexi st the ae ee 
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