DISTRIBUTION OF PLANTS. 27 7 



tance from mountains; — whether exposed to the rays of tin 

 Mm. or protected by forests, ice. 



This relation of plants to water* confines them to particular 

 Situations, and any one plant can occupy but a small portion of 

 the surface of the soil, while at the same time a great Dum- 

 ber of species can exist together within a limited space. Un- 

 der certain circumstances, this may have considerable influ- 

 ence upon their range. 



These phenomena, with many others which are continu- 

 ally presenting themselves, carry usat once to the supposition, 

 that each species must have originated on a particular point 

 of the earth's surface, from which, in the course of successive 

 generations, it would have spread over the whole globe, hut 

 that it has been kept hack and confined within narrow limits, 

 by causes, of which the above mentioned are the most pro- 

 minent We find accordingly, that almost every practical 

 botanist, conversant with the subject, has followed. (.Hen un- 

 consciously, a mode of reasoning which ultimately leads to this 

 conclusion. 



That no species has ori^inaled on two differ* nt points of 

 the earth's surface is proved by a variety of circumstances ; 

 most of the instances where a plant occurs in two distant and 

 mingly insulated places being readily accounted for bj 

 existing causes. 



There has been much discussion relative to the quadrupeds 

 common to the eastern and western continents; hut it is now 

 admitted, that those species only are common, whose range 

 extends near to, or within, the arctic circle where the two 

 continents closely approach each other. 



The foregoing conclusion is also confirmed by tin vegeta- 

 tion of islands. In those which are situated at the distance of 

 from one to several miles from the main land, all the plants 

 are common to the neighbouring continent: while if at a 



greater distance, they frequently afford some species not to be 



Maritime planti arc confined to a soil impregnated with muriate of aoda, and 

 i few species appear to i"- peculiar to limestone rocks. 

 VOL. III. 4 A 



