482 
GEOGRAPHY. 
BOOK V. 
The effect of elevation is not, in Europe, the same with all 
plants ; there are many that grow indifferently upon the plains 
and upon mountains as high as perpetual snow. De Can- 
dolle speaks of 700 instances, with which he is acquainted, of 
the prevalence of this law. But, on the other hand, there are 
many plants, the limits of which are strictly circumscribed by 
elevation or equivalent temperature; as, for example, the 
chestnut does not rise higher in the Swiss Alps, in the parallel 
of 45°, than 2400 feet : on Etna, in latitude 38"^, it reaches 
no higher than 4000 feet. Many of the plants found on 
plains in the north of Europe occupy the mountains of the 
south. The olive, in 44° of latitude, its most northern 
range, will not grow at a greater elevation than 1200 feet. 
In general it is found that, as we approach the equator, 
vegetation becomes more and more affected by elevation ; and 
that as we recede from it the effects of elevation gradually 
cease. 
The cause of the influence of elevation upon plants is 
ascribed, in the first place, to reduced temperature ; secondly, 
to a greater intensity of solar light ; and, thirdly, to a decrease 
in humidity. The rate at which temperature decreases as we 
ascend from the surface of the earth varies according to 
latitude : Humboldt has shown that, in the temperate and 
torrid zones, the decrement of heat is essentially different. In 
the equatorial zone, the temperature of the region lying at 
the height of between 3000 and 6000 feet, — on which the 
clouds repose that are visible to the natives of the plains, — 
decreases much more slowly than either above or below that 
elevation ; but, in the temperate zone, the decrease is more 
gradual. In proof of this the following table has been formed 
by Humboldt : — 
