20 Geographical Botany of the United States. 



rections of Mr. Cornelius, a great similarity prevails ; and it is pro- 

 bable, that this continues westward to the mountains of Mexico. 



In tracing the peculiarities of vegetation of the portion of the 

 United States west of the Alleghanies, one circumstance should be 

 remembered, as having a marked and powerful influence. It is, 

 that this territory is watered by a magnificent stream, which holds 

 a southerly course, through 20 degrees of latitude, to the Gulf of 

 Mexico— that this stream is fed by numerous tributaries from vari- 

 ous parts of the Kocky Mountains on the west, and from the Alle- 

 ghanies on the east, which pour into it the waters of the western 

 part of New- York, as well as those of the mountains of Oregon. 

 When we reflect what a vast number of plants flourish on the banks 

 of these various streams, and that seeds are carried by the wa- 

 ters, and become fixed in the banks at other and distant places, wo 

 should not wonder if an uncommon uniformity of vegetation pre- 

 vailed throughout this extensive region ; — uncommon, when com- 

 pared with tracts of similar extent in other parts of the globe. An 

 attention to this prominent feature in the geography of the western 

 part of the United States, will also account for the fact mentioned 

 by many travellers, that certain tropical forms reach a more north- 

 ern latitude here than on the coast— and for the want of which, 

 erroneous views have been entertained concerning its climate, 



I have purposely omitted to notice, until the present time, the 



ed^that " vegetation, in ascending above the level of the sea, un- 

 dergoes modifications, analagous to those which attend its progress 

 from the line to either pole. With this distinction, that in the last 

 case, the phenomena succeed by almost imperceptible gradations, 

 while they crowd upon and follow each other in rapid succession, 

 on the ascent of mountains. The height of 4 or 5000 yards in the 

 hottest parts of the globe, produces changes as distinct as the 2000 

 leagues or more, which lie between the equator and the polar re- 

 gions » (Mirbel, in the 2d volume of Branded Journal) 



Observations of this kind, however, have been greatly extended 

 by Humboldt and Bonpland, and by Becandolle. It has been es- 

 timated by the latter, that in the climate of France, 180 or 200 

 yards of elevation, act upon the mean temperature nearly in the ra- 

 tio of a degree of latitude. 



In our own country, the observations upon the vegetation of the 

 mountains have not, heretofore, been conducted with all the accu- 



