1878. ] United States Survey of the Territories. 109 
Polar regions, and that of the southern part extending through 
New Mexico to the Cordillera of Mexico proper 
The first and second (Atlantic plus Mississippi and Pacific) 
regions are traversed by meridional. chains of mountains approxi- 
mately parallel to the Rocky mountains, namely, on the Atlantic 
side by the various systems often included under the general 
term Appalachian, which extend from Maine to Georgia, and on 
the Pacific side by the Sierra Nevada, which bound California on 
the east. The third and fourth of the regions present a continua- 
tion of the Rocky mountains of Colorado and Utah, flanked for a 
certain distance by an eastern prairie flora extending from the 
British Possessions to Texas, and a western desert or saline flora, 
extending from the Snake river to Arizona and Mexico. Thus 
the Colorado and Utah. floras might be expected to contain repre- 
sentatives of all the various vegetations of North America, except 
the small tropical region of Florida, which is confined to the ex- 
treme south-east of the continent. 
The most singular botanical feature of North America is un- 
questionably the marked contrast: between its two humid floras, 
namely, those of the Atlantic plus Mississippi, and the Pacific 
one; this has been ably illustrated and discussed by Dr. Gray, in 
various communications to the American Academy of Sciences, 
and elsewhere, and he has further largely traced the peculiarities 
of each to their source, thus laying the foundation for all future 
researches into the botanical peke i ii of North America; but 
the relations of the dry intermediate region either to these or to 
the floras of other countries had not been similarly treated, and 
this we hope that we have now materials for discussing. 
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3 
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7 
d 
Dr. Hooker sums up the results of the joint investigations of 
Dr. Gray and himself, aided by Dr. Gray’s previously intimate 
knowledge of the elements of the American flora, from the Mis- 
sissippi to the Pacific coast: 
Each of these, again, is subdivisible into three, as follows : 
. The Atlantic slope plus Mississippi region, subdivisible into 
(2) an Atlantic (£), a Mississippi valley, and (y) an interposed 
Mountain region with a temperate and subalpine flora. 
2. The Pacific slope, subdivisible into (4) a very humid, cool, 
forest-clad coast range ; (8) the great, hot, drier Californian valley 
ormed by the San Juan river flowing to o the north and the Sacra- 
nto river flowing to the south, both into the bay of San Fran- 
