CHAPTER XX 



THE ORIGIN OF THE BRITISH FLORA 



THE British Isles form a group of islands resting on a 

 submarine platform, or shelf, attached to the continent of 

 Europe. The depth of the narrow seas which separate 

 them from the mainland and from each other is nowhere 

 more than 100 fathoms, and in many places it is much 

 less. Beyond the west coast of Ireland and the Western 

 Isles of Scotland this ledge shelves steeply down to the 

 deeps of the Atlantic Ocean. 



Islands are generally classified into two great groups, 

 according to their mode of origin : 



1. Continental Islands, which, like our own, are de- 

 tached portions of continents. They are never far 

 distant from the mainland, of which they once formed a 

 part, and they are united to it by a submerged platform 

 of varying depth. Their detachment was due to a general 

 subsidence of the land and the encroachment of the sea 

 over the low-lying parts. Continental islands repeat or 

 continue the geological structure of the adjacent conti- 

 nent, and their animal and vegetable life has been derived 

 from it, mainly before the intervening land-connections 

 were broken. The vegetation of such islands, therefore, 

 resembles in its main features the flora of the continent 

 to which it belongs. It is a reduced copy of the conti- 

 nental flora. 



2. Oceanic Islands. These are generally found in deep 

 water far away from the edges of any continental land. 

 They are of volcanic or coral origin, and as they have 

 never had any connection with the continent, their flora 

 and fauna must have reached them over the intervening 

 sea by wind or current or flying birds. 



Continental islands are further divided into two groups 



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