THE GREAT ICE AGE 167 
refugees throughout the cold period. There is good 
evidence that the time of maximum glaciation was 
also one of elevation of the land, and possibly this 
persisted for a while after the passing away of the 
ice. If this were so, some relief from the congestion 
might have been afforded to the refugees during the 
cold period, and an opportunity might have existed 
when the ice passed away for recolonization across a 
land surface from the east, since a comparatively 
small elevation would connect the British Islands 
with the Continent. But that such an elevation con- 
tinued for long after the passing of the ice is by no 
means certain. On the whole, the evidence of general 
glaciation of our islands as interpreted by geologists 
almost postulates the extinction within our area of 
the whole existing flora and fauna, and consequently 
its reconstruction by immigration when a temperate 
climate returned. But there is a body of evidence to 
be drawn from the present and past distribution of the 
existing plants and animals which is of great im- 
portance in this connection. Is this biological testi- 
mony in favour of the theory of the immigration of 
our flora and fauna during the relatively short period 
which has elapsed since the passing of the ice? To 
this question different observers have given very 
different answers. In order to form an idea of the 
nature of the problem—it is possible here to deal 
only with the case of the plants—we need to study 
briefly the composition of the present flora, from the 
point of view of its origin. 
In the first place, it must be recalled that the British 
Isles are situated on a broad shelf which extends into 
the Atlantic on the western edge of Europe. In com- 
