138 LAKE SUPERIOR. 



ish, thus exhibiting all over the globe a close connection between 

 the modifications of temperature from the equator to the poles, and 

 the geographical distribution of vegetable and animal life. The 

 more powerful influence of temperature upon vegetation does not, 

 however, preclude the influence of other agents ; even the manner 

 in which the same amount of heat is distributed over the earth in a 

 given time, will produce differences. It is well known, that coun- 

 tries in which the summers are short but very warm, and the winters 

 very long and cold, have a vegetation totally different from those 

 where the seasons are more equable and succeed each other by 

 gradual changes, although the mean annual temperature of both be 

 the same. Next in importance we may perhaps consider the degree 

 of moisture of the atmosphere, which differs widely in different re- 

 gions ; the damp valleys of the Mississippi, for instance, present the 

 most striking contrast with the rolling country farther west. Again, 

 the swamps and the sandy plains, the rocky hills and the loamy soils, 

 the snow-clad barrens and the frozen gravel of the North, even under 

 circumstances otherwise most similar, afford the greatest diversity of 

 vegetation. There is still another way in which moisture may act 

 in a particular manner ; as vegetation is not influenced simply by 

 the annual amount of moisture, but also by the quantity of water 

 that falls at one time, and the periods at which it falls. A low tem- 

 perature in a moist climate will indeed produce some remarkable 

 peculiarities ; for instance where early winters cause an extensive 

 sheet of snow to be accumulated over the ground, and to protect 

 vegetation from the destroying influence of frost ; as is the case in 

 the Alps, where the most delicate flowers prosper admirably under 

 their white blankets, and show themselves in full development as soon 

 as the snow melts away, late in the spring, when the warm season is 

 already fairly set in. Light, again, independently of heat, will also 

 show its influence ; shaded places are favorable to plants which would 

 be killed under the more direct influence of the rays of light. 



Atmospheric pressure would at first seem to have only a very sub- 

 ordinate influence upon vegetation. But comparing Alpine vegetation 

 with that of higher latitudes, which from their situation must have 

 climates otherwise very similar, we shall be led to the conclusion that 

 atmospheric pressure has its share in bringing about the diversity o f 



