33 



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 



