97 
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, 
H 
