10 INTRODUCTION. 



terior of large trunks of trees, old towers, &c. which are thickly 

 clothed with Mosses and Lichens. Many of these avoid the 

 light ; and their presence indicates the north side of the body to 

 which they are attached. To others, again, the light in all its 

 strength is genial ; and they frequent the southern aspect ; whilst 

 other forms, intermediate in habits, frequent the eastern and 

 western sides ; so that, on going round such a tower or large 

 trunk, we observe a succession of different species which may be 

 compared to that which is presented in the various latitudes, 

 passing from the equator towards the pole. A similar succession, 

 on a larger scale, is seen on ascending a high mountain between 

 the tropics, such as the Peak of Teneriffe. The lower portion 

 exhibits the vegetation of the surrounding country, in all the 

 luxuriance and richness of an island in the torrid zone. Higher 

 up, the traveller meets with productions similar to those found 

 on the borders of temperate regions ; and to these succeed those 

 of the medium temperate zone. Above these are perceived the 

 alpine plants, which in northern Europe are found at a com- 

 paratively trifling elevation ; and to these succeeds the dreariness 

 of perpetual snow. These five distinct zones are well marked 

 on the Peak of Teneriffe j each having a certain set of plants 

 peculiar to it, as the plants of Northern and Southern Europe, 

 and of Northern and Central Africa, are to those regions re- 

 spectively. 



Thus we see that on no part of the earth's surface, under no 

 peculiarities of soil or climate, is vegetation of some kind or 

 other impossible. Every distinct tribe of Plants flourishes natu- 

 rally under peculiar conditions, some preferring a warm atmo- 

 sphere, others a cool one ; some only luxuriating in moisture, 

 and others in the opposite condition of dryness ; some requiring 

 the most intense light, and others only growing in darkness. 

 There are some plants which are very deficient in the power of 

 adapting themselves to slight changes in these conditions ; and 

 these are accordingly restricted to certain localities favourable to 



