DISTRIBUTION OF BRITISH PLANTS. 329 



third is suggested as being the most probable one, that, 

 namely, of migration over land, which implies that a con- 

 tinuous surface must formerly (though perhaps at different 

 times) have connected the British Isles with different parts 

 of the adjacent countries. 



The arguments on which the theory rests are chiefly geo- 

 logical and zoological, and are only to be fully appreciated 

 by those who are skilled in those sciences ; but without at- 

 tempting to follow the reasoning through all its mysterious 

 windings, it will be enough for the present purpose to con- 

 vey the result to which it leads. 



In the botanical part of the argument Professor Forbes 

 has illustrated his theory by a modification of certain bota- 

 nical provinces into which the British flora has been mapped 

 out by Mr. Hewett Watson, who " first drew attention to 

 the various elements of which the flora is composed, and 

 grouped the species into botanical provinces." A classifi- 

 cation of the British mountain flora, with regard to different 

 zones of elevation, has been likewise worked out by the same 

 learned botanist. 



There are five distinct regions in which particular plants 

 are grouped (Plate XIX.), which, as was stated before, 

 " have their likenesses in distant and distinct lands :" 



