Nov., 1890. 
PLANT MARCHES. 
258 
latitude the great ice sheet descended there must be taken 
also into account the effect of such an accumulation of ice, 
which must have abstracted heat from regions extending 
into Asia and Africa. There are irresistible proofs that once 
upon a time a temperate climate, if not indeed a climate of 
sub-tropical sweetness and warmth, extended as far north as 
Greenland and the borders of the present Polar Seas, and 
continued through many geological periods; this is evident 
from the fact that rich and luxuriant vegetation flourished 
there. These favourable conditions certainly existed in the 
Eocene age; then commenced a gradual falling, which 
continued until the close of the Pliocene, compelling the 
migration of a rich and interesting native flora. 
A little later, and the earliest snows and ice of the forth¬ 
coming GlaciaL age put in an appearance, and were 
peremptory in their notices to quit, when the more tender 
species migrated or perished. The severity of the increasing 
cold drove the flora before the ever increasing glacier, 
and in turn the lands of Scandinavia were passed ; the North 
sea area—then land—was crossed, England, France and 
Switzerland were traversed; the Alps, Pyrenees, and 
Apennines were occupied, and the chilly influence of the vast 
icy region pushed forward the northern plants southward, 
som e even as far as the Atlas Mountains, and the Himalaya. 
Corresponding events happened upon the American Continent. 
From the Polar Sea the plants were driven over the lands of 
the Hudson Bay Territory, across Washington Territory, 
through Canada, and far along the range of the Rocky 
Mountains. The same history also belongs to Northern Asia, 
Russia, and Siberia. 
When at last the force and energy of this ice invasion had 
expended itself, the returning warmth began gradually to 
melt the accumulated ice, and the retreat of the glaciers 
commenced. The hardy plants which had accustomed them¬ 
selves to the severity of glacial conditions, and had learnt to 
flourish on the borders and even amid the depths of the ice 
fields, followed closely the retreating ice, and found their way 
once more northward. 
It was now their turn to make a struggle for existence. 
Many perished from the attacks made upon them from their 
approaching southern enemies, now strengthened and en¬ 
couraged by the increasing warmth, others recreated to bracing 
altitudes in mountainous chains, and were cut off from the 
main army advancing northwards. Colonies were left here 
and there, on the Alps, the Pyrenees, the Carpathians, the 
Caucasus, the Apennines, on the Scotch and Welsh mountains, 
