ORIGIN OF THE BRITISH FLORA 209 
to spread into new territories. Most endemic forms are 
merely the survivals of nearly extinct races ; new forms, 
if successful, are virile and aggressive, and spread very 
quickly. 
Now, the British Isles form a group of recently detached 
continental islands. The separation from the mainland 
of Europe began when the Ice Age was passing away and 
the great ice-sheets began their retreat along the low- 
lands, now occupied by the North Sea, northwards 
towards the mountains of Scandinavia. It is probable 
that the passage of the Straits of Dover was forced when 
the Rhine was dammed back by the great glaciers block- 
ing the outlet towards the north at the close of the 
Glacial Epoch. Sufficient land-connections, however, 
must have remained, interrupted by no greater barrier 
than rivers, long enough after the Ice Age to allow of the 
establishment of the present flora in these islands in all 
its essential details before their final separation from the 
Continent by several miles of sea. It is probable, how- 
ever, that before the colonization was complete these 
islands had already become isolated, and that the last 
arrivals came in the form of light seeds driven by the 
wind over the narrow straits, or carried on the feet of birds 
over the intervening sea. 
We are thus prepared for the fact that the flora of 
Great Britain is practically identical with that of Europe, 
though somewhat reduced. The connection with the 
Continent is, geologically, so recent that we should 
hardly expect to find a single flower in Great Britain 
which is not also found in similar situations on the Con- 
tinent, and, as a matter of fact, not a single truly in- 
digenous species peculiar to our flora and unknown on 
the Continent is to be found throughout the land. Among 
our rarer plants, however, a few varieties have been 
described which are said not to occur elsewhere, but it is 
extremely doubtful whether these are really distinct races, 
or, if they are, whether they are really absent from the 
European flora. Among these plants are : Helianthemum 
Breweri, a variety of the spotted rock-rose, found only in 
Anglesea and two or three localities in Ireland ; Hnanthe 
fluviatilis, a floating aquatic closely allied to @. Phellan- 
drium, the.great water-dropwort ; and Hieraciwm iricum, 
a mountain hawkweed. 
14 
