SECT. III. ALTITUDE. 435 



because, as we have seen, the physical climate de- 

 pends upon altitude. In consequence of which, 

 combined with the ridges and direction of the 

 mountains, America and Asia are much colder in 

 the same degrees of northern latitude than Europe. 

 American plants vegetating at 42 of northern lati- 

 tude will vegetate very well at 52 in Europe. The 

 same, or nearly so, may be said of Asia, which in 

 the former case is perhaps owing to the immense 

 tracts of woods and marshes covering the surface, 

 and in the latter to the more elevated and moun- 

 tainous situation of the country affecting the degree 

 of temperature. So also Africa is much hotter 

 under the tropics than America; because in the 

 latter the temperature is lowered by immense 

 chains of mountains traversing the equatorial re- 

 gions, while in the former it is increased by means 

 of the hot and burning sands that cover the greater 

 part of its surface. 



The effects of altitude are observable also even 

 in the case of aquatics, as modifying the habitats ; 

 thus some acquatics float always on the surface of 

 the water, as Lemna, while others are either par- 

 tially or wholly immersed. Such as grow in the 

 depths of the sea are not influenced by climate ; 

 but such as are near the surface are influenced by 

 climate, and have their habitats affected by it. 



2 F 



