24 



hood iu connection witli the subject of the distribution of the 

 [)lants of Great Britain generally, prefacing our lecture with just 

 so much on this latter head, as might serve to a certain extent to 

 explain the circumstances under which a given number of species 

 are found growing in any particular locality. 



We may begin by observing that the far greater portion of 

 our British plants — not to say all— are identical with species 

 found on the Continent; and assuming that each species 

 originated — without saying anything as to the mod.c of its origin 

 — in one particular spot, or centre, as it is called, and not in 

 several places at once, this being the hypothesis best supported, 

 and the one most generally adopted, they have clearly come to 

 lis from thence in the first instance. 



But when we set ourselves to consider the way in which they 

 have arrived on our shores, we find a difficulty. There is a wide 

 sea between us and the continent, and no natural transport, 

 such as often serves to disseminate plants and carry them from 

 one part of the country to another, is sufficient in all cases to 

 explain how they may have passed this barri(?r. The difficulty, 

 too, is greatly increased when we take the case of endemic, or 

 very local plants, which instead of being more or less generally 

 diff"used over the British Islands, are confined to particular 

 stations, or occupy very limited areas, being cut off by considerable 

 intervening spaces from any assemblages of the same species in 

 countries beyond Britain. 



• It was with an especial view to these local plants, and to 

 shew how such assemblages of individuals, though now so far 

 asunder, may still have sprung originally from one and the same 

 centre, that the late Professor E. Forbes put forth, as is well 

 known, an ingenious theory. He supposed that the areas in 

 which they are found growing, though now disconnected, with 

 the sea rolling between them, were once united by lands that 

 have since gone down and been submerged through the agency 

 of those causes, by which the relative distribution of land and 



