94 THE STORY OF PLANT LIFE 



the pre-Glacial period are found in the plant-beds of 

 these milder intervals. 



Some northern plants occur in early Glacial periods 

 such as the Arctic Willow and Dwarf Birch. Except 

 the Birch and Alder there were no forest trees 

 apparently. In the Interglacial beds the plants are 

 mainly those living to-day with some southern types 

 such as Acer monspessulanum, Naias graminea, N. 

 minor. 



The northern types are absent. In late Glacial 

 times there were numerous arctic types including 

 the Arctic Willow. 



On the return of milder conditions these last 

 northern types were driven to the summits of lofty 

 hills in this country, where they are represented 

 to-day by the Arctic Alpine flora of Saxifrages, etc. 



The southern types were now able, on retreat of 

 the ice, to take possession of these islands again. 

 Previous to the Ice Age the country was united to 

 the continent, but after an elevation it became discon- 

 nected, and after the cold conditions were succeeded 

 by milder, introduction from elsewhere would appear 

 to have taken place, as Mr. Clement Reid considers, 

 rather by accidental means — wind, the agency of 

 birds, currents, etc. — than by migration. And the 

 existence to-day in Cornwall and Ireland of a southern 

 or Lusitanian flora is to be attributed rather to such 

 agencies than to a possible former land connection 

 between Spain and Portugal and France with the 

 British Isles, and the persistence of these temperate 



