278 ON THE GEOGRAPHICAL 



found there : and lastly, countries separated by a vast extent 

 of ocean, and at the same time insulated by climate, do not 

 possess in common a single phcenogamous plant* — as, the 

 southern extremity of Africa with that of New Holland, or 

 of South America. 



Among the principal agents in conveying the seeds of plants 

 to a distance from their original site, are the winds, and the 

 currents of the ocean. 



The seeds of West Indian plants are thrown by the gulf 

 stream upon the coast of Northern Europe, and sometimes 

 germinate there, but are destroyed by the frosts on the ap- 

 proach of winter. Accordingly, those species, which are 

 common to countries separated by a great extent of ocean, are 

 observed to be generally aquatic and marsh plants, especially 

 maritime, whose seeds are formed to bear a long exposure to 

 water. 



Some estimate of the influence of the winds in distribut- 

 ing the seeds of vegetables may be formed, from the fact of 

 the ashes of a volcano being frequently carried many hundred 

 miles from its crater. Botanists have observed that the crypto- 

 gamia, especially lichens, in their geographical distribution, do 

 not appear to follow the same laws with phcenogamous plants, 

 many of the former being found in every part of the globe. — 

 The excessive minuteness of the seeds of these plants autho- 

 rizes the conjecture, that the winds alone have accomplished 

 such an universal distribution. 



The fact of the European Alps affording on their summits 

 some arctic plants is not so readily disposed of, as the general 

 direction of this chain of mountains is parallel with the equa- 

 tor, and there is a wide interval, of several hundred miles, 

 between their most northern bend and the southern extremity 

 of the mountains of the Scandinavian peninsula. Here, with 

 the exception of the annual migration of birds, the winds 

 appear to be the only agents left us, and it seems improbable 



* Exceptions are extremely rare : among fifty thousand phrenogamous plants, 

 now known, Samolut valerandi is the only well authenticated instance of a plant, 

 which is common to almost every part of the globe. 



