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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