THE NORTHERN VEGETATION COMPARED, ETC. 141 



upon the prevailing winds. It is obvious, for instance, that a moun- 

 tain chain like the Alps, running from east to west, and thus forming 

 a barrier between the colder region northwards, and the warmer 

 southwards, will have a tendency to lower the temperature of the 

 northern plains, and to increase that of the southern, below or above 

 the mean which such localities would otherwise present ; while the 

 influence of a chain running north and south, like the Rocky Moun- 

 tains and the Andes, will be quite the reverse, and tend to increase 

 the natural differences between the eastern and western shores of 

 the continent, and, laying open the north to southern influences and 

 the south to thosi of the north, render its climate excessive, i. e., 

 its summer warmer and its winter colder. 



Again, the equalizing influence of a large sheet of water, the tem- 

 perature of which is less liable to sudden changes than the atmos- 

 pheric air, is very apparent in the uniformity of coast vegetation 

 over extensive tracts, provided the soil be of the same nature, and 

 also in the slower transition from one season into the other along the 

 shores ; the coasts having less extreme temperatures than the main 

 land. The absolute degree of temperature of the water acts with 

 equal power ; as the aquatic plants of the tropical regions, for in- 

 stance those of Guyana, differ as widely from those of Lake Supe- 

 rior, as the palms differ from the pine forests. * 



* One of the most prominent causes of the dispersion, not to say of the distribution 

 of plants, is certainly the direction and the swiftness of water-courses. On one hand 

 the rivers bring down from the summits or the elevated parts of the country a large 

 number of plants and seeds, whjch are stopped and take root farther below, on their 

 banks ; on the other, they spread in their neighborhood a greater or less amount 

 of moisture. This is, I think, the best cause to assign to the uniformity of vegeta- 

 tion over large plains, traversed by rivers, or to that of the sea-shores, or especially 

 to that of the low islands and peninsulas of little extent. We must also admit, how- 

 ever, that there are along the course of rivers a great variety of stations, which we 

 may find nowhere else, valleys, abrupt rocks, shaded places, constantly or alternately 

 lighted by the sun according to their bearing ; and that in this manner secondary agents 

 may have their influence in varying greatly the aspect of vegetation. 



It is also a curious but positive fact, that high mountain chains have a direct influ- 

 ence upon the dissemination of the species over the neighboring secondary chains, 

 even at a considerable distance. This fact is plainly shown in the Jura for instance, 

 ■where from the summits of the Dole to those of the Chasseral we observe a true Alpine 

 vegetation, less and less abundant the more we recede from the Alps in one or another 

 direction. At an equal elevation the summits of the northern Jura lose every trace of 

 Alpine plants which we find so abundantly upon its southern summits, especially upon 

 the ridges near the Alps, as the Dole, the Mount Tendre, for instance. The same takes 



