5) 
_ isolation is brought about by a flora being separated from anoth- 
er similar one by an entirely different botanical condition. Take 
for example the densely forested area of the A palachian Moun- 
_ tains, particularly at the south. That contains doubtless quite a 
number of trees which would grow in portions of our mountains, 
and it contains many other plants among herbs which would thrive 
here if it were not for our isolation. This is brought about by the 
Vast treeless region west of the Missouri river, whose climate is 
sufficiently different from that of the Appalachians to make it im- 
possible for those plants to grow there. This then becomes a vast 
_ barrier over five hundred miles in width; extending trom the Great 
Lakes to the sea, which will effectually prevent plants from go- 
ing from the Appalachians to the Rocky Mountains unless carried 
there ty animals, which in a state of nature would scarcely be pos- 
sible, or whose seeds might be carried there by the wind, which is 
almost an impossibility for anything but the most minute seeds, 
and an absolute impossibility for seeds of any size. We therefore 
do not find any plants in the Great Plateau which have come di- 
rectly from the Appalachians, at least not since the glacial period. 
Another kind of barriers is found in the mountain ranges. 
is almost a uniform fact that the mountain ranges of the Great 
SO ee ee ae ree es 
RY ae CE Ae Sa ee 
difficult than the transportation of the va 
to another, but it still. would be fraught with many drawbacks. 
In this manner we expect to find and do find a decided erase 
in the vegetation in the valleys as we progress from the rae 10 
the Pacific. For example the vegetation of the reso so = 
decidedly different from that of the rest of the Great Plateau. The 
