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CHAPTER VI. 



Former Distribution of British Plants. 



I HAVE set down in this chapter what is known of the 

 past history of our British Plants ; but the species about 

 which we have as yet been able to learn anything amount 

 only to about one-sixth of the flora, though constant 

 additions are being made to the number. Under these 

 circumstances, and in face of the imperfection of the record 

 in Pliocene times, I doubt whether it would be of much use 

 to attempt any minute analysis of the list ; all that can be 

 done with advantage, is to draw attention to the leading 

 changes in geographical distribution that have already 

 been proved. 



Variations caused by climatic changes were spoken of 

 in Chapter IV. In the course of time, however, there have 

 been other changes in distribution ; for it is obvious that a 

 flora driven south by a cold wave, on its return when the 

 climate has again become genial is not likely to consist 

 of exactly the same species. The chances of dispersal 

 cannot be twice alike. When the mammals and birds 

 change, the relative power of spreading possessed by the 

 different plants must change also; when England is con- 

 nected with the Continent, and the Rhine flows to Norfolk, 

 heavy seeds must have easier travelling than when Britain 

 becomes an island. Other differences in geographical dis- 

 tribution seem to be the result of accident — one plant has 

 accidentally been introduced and has had time to spread, 



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