212 BEITISH PLANTS 



have kept their station there to this day, although the 

 sea has long since submerged the road by which they 

 came. 



The epoch preceding the Glacial Period was remarkable 

 for the warm conditions that then prevailed over northern 

 Europe. In England an almost tropical vegetation 

 flourished, and a temperate climate extended intd the 

 regions which are now within the Arctic Circle. The cause 

 of this great oscillation between the warmth of the pre- 

 Glacial Period and the cold of the Ice Age is not known. 

 During this period Great Britain formed a continuous 

 part of the Continent. Man had not yet appeared. 

 Animals similar to those now found in tropical and sub- 

 tropical regions lived in the country — e.g., the lion, 

 crocodile, hippopotamus, and rhinoceros ; while forming 

 a part of the flora were such plants as sarsaparilla (Smilax), 

 magnolia, bamboo, swamp-cypress [Taxodium), sumach, 

 tulip-tree (Liriodendron), fig, the evergreen oak, walnut, 

 and laurel — -plants whose nearest relatives are now found 

 native only in the warmer regions of the globe. It seems, 

 then, natural to suppose that the Lusitanian vegetation 

 is a remnant of this southern flora which crept up north- 

 wards with the increasing waves of warmth, and passed 

 to Great Britain over the territories now submerged 

 beneath the English Channel. During the Ice Age 

 Ireland, Scotland, and all England north of the Thames 

 were refrigerated ; but it is supposed that in some shel- 

 tered spots in the southern peninsulas of England and 

 western Ireland, and especially in the pre-glacial exten- 

 sions westwards, conditions remained sufficiently mild to 

 preserve remnants of the southern flora out of which has 

 ultimately developed the Lusitanian flora now existing 

 in the country. This may be true, but the explanation 

 is beset with difficulties. Though the south of England 

 was not actually under ice, conditions must have been 

 very severe there, and it is hard to understand how such 

 plants could have found anywhere so near the ice-sheets 

 a retreat sufficiently mild to preserve their existence. 

 Some of them would have been killed by a climate only 

 slightly more severe than they experience to-day — e.g., 

 Arbutus Unedo. Towards the close of the Glacial Period 

 it is probable that the first great step towards the com- 

 plete severance of Great Britain from the Continent took 



