220 NATURAL HISTORY. 



many of the Arctic animals and plants (the latter more 

 especially) would not retire northwards, but would be 

 driven from the low grounds to the more mountainous 

 parts of the country, when the temperature would still be 

 cold enough to suit them. Some of these, moreover, 

 would in this way succeed in maintaining a permanent 

 foothold in the country, since the elevation above the sea- 

 level to which they had retired, would secure them a suffi- 

 ciently low temperature for their existence. The above 

 processes must, of course, have been very slowly effected ; 

 but we need not doubt that Forbes's views on this point 

 were correct, and that the Scandinavian and Arctic plants 

 now living in the Scottish Highlands, in the Lake District, 

 and in Wales, are only the survivors of a much greater 

 number of northern types of life which invaded us during 

 the cold of the glacial period. We can also easily under- 

 stand, on this view, how it should be that the Highlands, 

 lying as they do nearer to the original home of the 

 northern invaders than either Cumberland or Wales, 

 should now possess a greater number of these Arctic 

 species than do the two districts last named. 



There remain for consideration, the three smaller floras 

 which Forbes distinguished as occurring in the British 

 area. The plants characteristic of these three floras 

 are, according to Forbes, ' derived assemblages of plants 

 south of the great Germanic group. As the south of 

 England and of Ireland were in all probability unsub- 

 merged during the glacial epoch, they may have come 

 over either before, or during, or after that epoch. There 

 are strong reasons for believing they migrated before.' 



The plants of the south-east of England, constituting 



