434 



EXTINCTION OF SPECIES. 



[Ch. XLII. 



enjoy ;, by virtne of those powers of diifusion already men- 

 tioned (Chapters XXXVIII., XXXIX., XL.), of inyading 

 adjacent territories. 



assemblag-e 



maintain 



pend, as is well known, on the relations between the physio- 

 logical nature of each species, and the climate, exposure, 

 soil, and other physical conditions of the locality, and the 

 pov/er of each to compete with other organic beings in the 



Some plants live only on rocks, others in 



es. Of the latter, some 

 —others in salt marshes, 

 where their roots may copiously absorb saline particles. 

 Some prefer an alpine region in a warm latitude, where. 



struggle for life. 



meadows, a third class in marsl 



delio'ht in a fresh-water morass, 



summer 



the cool waters of melting snows. To others loose sand, 

 so fatal to the generality of species, affords the most proper 

 station. The Garex arenaria and the Elymus arenarius 

 acquire their full vigour on a sandy dune, obtaining an ascen- 

 dancy over the very plants which in a stiff clay would imme- 



them 



Wher 



it is extremely favourable to certain species, and agrees ill 

 with every other, the former get exclusive possession of the 

 ground, and as in the case of heaths, live in societies. In 



moss 



{Sphagnum) is fully developed in 

 peaty swamps, and becomes, like the heath, in the language 

 of botanists, a social plant. Such monopolies, however, are 

 not common, for they are checked by various causes. 'Not 

 only are many species endowed with equal powers to obtai 

 and keep possession of similar stations, but each plant, for 

 reasons not fully explained by the physiologist, has the 

 property of rendering the soil where it has grown less fitted 

 for the support of other individuals of its own species, or 



same family 



same spot. 



from being- im 



another family. Oaks, for example, render the soil more fertile 

 for the fir tribe, and firs prepare the soil for oaks. Every 

 aOTiculturist feels the force of this law of the organic world, 

 and regulates accordingly the rotation of liis crops. 



