CHAP. VII. BOTANICAL GEOGRAPHY. SOS 



earth's surface to another ; and also the means by which 

 their migration is most effectually provided for. 

 (310.) Obstacles to Migration. 



1. Seas. The salt of sea- water produces an in- 

 jurious effect upon the seeds of plants, and completely 

 destroys the vitality of those which are long subjected 

 to its influence. In proportion therefore to the extent 

 of sea which surrounds a tract of land, the chances are 

 diminished by which the seeds of plants may be wafted 

 to or from it in a state fitted for germination. This 

 is remarkably exemplified in the flora of St. Helena, 

 which is so peculiar, that not more than two or three 

 of its indigenous species have been found on the con- 

 tinent of America, and not one of them on the con- 

 tinent of Africa. Generally speaking, the floras of all 

 islands resemble those of the continents to which they 

 are nearest, in proportion to their greater proximity to 

 those continents. England does not possess fifty species 

 which have not also been detected in France; and pro- 

 bably, the number peculiar to our flora is even still 

 less than this. The floras of the opposite shores of the 

 Mediterranean are very nearly the same. 



2. Deserts. These are a very effectual barrier to 

 the migration of species; and hence there are scarcely 

 any species described in the " Flora Atlantica" which 

 are to be met with in Senegal ; the great desert of 

 Sahara completely intercepting the botanical intercom- 

 munication of the two districts. 



3. Mountain Chains. Where mountain chains 

 possess lofty summits, the cold of those regions presents 

 a barrier to the migration of plants across them. In 

 general however they are not so effectual as seas and 

 deserts, on account of their being intersected by trans- 

 verse valleys. 



4. Partial Obstacles are offered by extensive forests 

 and marshes ; for although there are numerous species 

 which prefer such tracts as " stations, " to which they 

 are best adapted, there are others which cannot live 



