168 PAST AND PRESENT 
parison with the depth of the adjoining ocean, this 
shelf is but little below sea-level, and a slight eleva- 
tion of the land—much smaller than those which have 
occurred over and over again in recent geological 
times—would join our islands to Germany, Holland, 
Belgium, and France. The British Isles are geo- 
graphically and biologically by no means a separate 
area, and they have derived their population, both 
plant and animal, by immigration at various periods of 
time from the great land area to the eastward. Our 
present flora proves the truth of this as a general 
assertion; a study of its constituents shows that it 
is essentially a reduced continental European flora. 
As we step from France across to England we lose a 
number of plants familiar on the French side. As we 
' step again from England into Ireland a further num- 
ber of plants disappears; and these losses are no 
doubt due either to an unsuitability of climate on the 
insular areas, especially the absence of a hot summer, 
or to the inability of the plants to cross the barriers 
of sea which have now existed for some time. If the 
whole of the flora fitted in with this idea of mere 
reduction of the Continental flora by elimination, the 
problem would be much simplified. But there are 
other elements in it which do not harmonize with this 
conception of simply a general western migration, 
and which give rise to very interesting problems. 
Let us first consider the main mass of our flora, 
which is closely akin to that of the adjoining parts 
of the Continent. When we say that it represents a 
reduced Continental flora we do not imply that it is 
therefore uniform in its composition throughout the 
British Isles, We know, on the contrary, by every- 
