166 PAST AND PRESENT 
Quaternary Ice Age, from the effects of which our 
country and its fauna and flora are still in process of 
recovery. At the close of the Pliocene Period, then, 
snow began to extend on the higher grounds, and 
glaciers to fill the mountain valleys; these conditions 
were intensified until all Northern Europe, including 
the British Isles as far south as the Thames valley, 
lay under a mantle of ice. The plants which occupied 
the ground were forced southward as the ice 
advanced, or exterminated by the increasing cold. 
After long fluctuations of climate, the extent of which 
appears still in doubt, the ice at length slowly passed 
away, leaving the surface of our country greatly 
altered. The ancient soils which had been in process 
of accumulation since last the land rose above sea- 
level were swept away, the surface was strewn with . 
materials formed by the grinding down of the hills or 
the pushing up of sea-bottom material, valleys were 
choked, rivers diverted, lakes formed by dams of 
glacial detritus, or by the scooping action of the ice; 
the whole surface of the country was remodelled on 
new lines. Into this new land the plants remigrated, 
and we now view on our hills and plains the results 
of this repopulation. The difficulties of which I have 
spoken arise especially in connection with the manner 
of this recolonization. On a continental area one 
can conceive of a gradual retirement of the flora 
before the advance of the ice, and its subsequent re- 
migration northward into its old haunts as the ice 
retired. But on an insular area like Great Britain no 
such line of retreat was open. The ice-free area of 
Southern England and possibly Southern Ireland 
does not appear adequate to harbour the crowd of 
