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CHAPTER IV. 
Changes in Geography and Climate. 
WHEN we discuss the origin of the British flora or 
fauna it is impossible to assume, as we can in the case 
of certain oceanic islands, that the process has been no 
more than the gradual introduction of the plants, under 
unchanging climatic conditions, into an area of limited 
and almost unvarying extent, holding unchanging relations 
with the nearest land, and till that time unoccupied by 
any other flora. Both geographical and climatic changes 
have played an essential part in shaping our flora as we 
now see it. Moreover, except in part of our country 
immediately after the retreat of the ice, each plant intro- 
duced seems to have been brought into an area already 
clothed with vegetation, though, under a changing climate, 
the native plants may have become less adapted for the 
station than were the intruders. It will be necessary, 
therefore, to trace out the changes of land and sea which 
have affected our islands since the existing plants and 
animals first made their appearance here; though, as was 
suggested in the last chapter, I greatly doubt whether in 
islands so near a continent the actual junction or isolation 
is of such great importance as has been imagined, Plants 
can certainly overleap barriers more easily than is usually 
thought. In various indirect ways, however, former geo- 
graphical changes must greatly have facilitated the dis- 
D 
