212 BRITISH 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 into 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 (Smilaa), 
magnolia, bamboo, swamp-cypress (T'avodiwm), 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 
Troland, 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 
