ORIGIN OP THE BRITISH FLORA 209 



to spread into new territories. Most endemic forms are 

 merely the survivals of nearly extinct races ; new forma, 

 if successful, are virile and aggressive, and spread very 

 quickly. 



Now, the British Isles form a group of recently detached 

 continental islands. The separation from the mainland 

 of Europe began when the Ice Age was passing away and 

 the great ice-sheets began their retreat along the low- 

 lands, now occupied by the North Sea, northwards 

 towards the mountains of Scandinavia. It is probable 

 that the passage of the Straits of Dover was forced when 

 the Rhine was dammed back by the great glaciers block- 

 ing the outlet towards the north at the close of the 

 Glacial Epoch. Sufficient land-connections, however, 

 must have remained, interrupted by no greater barrier 

 than rivers, long enough after the Ice Age to allow of the 

 establishment of the present flora in these islands in all 

 its essential details before their final separation from the 

 Continent by several miles of sea. It is probable, how- 

 ever, that before the colonization was complete these 

 islands had already become isolated, and that the last 

 arrivals came in the form of light seeds driven by the 

 wind over the narrow straits, or carried on the feet of birds 

 over the intervening sea. 



We are thus prepared for the fact that the flora of 

 Great Britain is practically identical with that of Europe, 

 though somewhat reduced. The connection with the 

 Continent is, geologically, so recent that we should 

 hardly expect to find a single flower in Great Britain 

 which is not also found in similar situations on the Con- 

 tinent, and, as a matter of fact, not a single truly in- 

 digenous species peculiar to our flora and unknown on 

 the Continent is to be found throughout the land. Among 

 our rarer plants, however, a few varieties have been 

 described which are said not to occur elsewhere, but it is 

 extremely doubtful whether these are really distinct races, 

 or, if they are, whether they are really absent from the 

 European flora. Among these plants are : Helianthemum 

 Breweri, a variety of the spotted rock-rose, found only in 

 Anglesea and two or three localities in Ireland ; (Enanthe 

 fluviatilis, a floating aquatic closely allied to CE. PHellan- 

 drium, the great water-dropwort ; and Hieracium iricum, 

 a mountain hawk weed. 



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