386 X. GENERAL REMARKS. 



fragment of the European flora, repeated in Britain, with 

 very few additions from any other source. The inference 

 seems plausible enough, and scarcely disputable on first 

 thought, that the whole flora (less, those few exceptions, 

 apparently from other sources) has been derived from the 

 European continent. But this is by no means a certain 

 fact, howsoever probable or plausible the opinion may at 

 first appear to be ; there being as yet no other evidence 

 of such origin, beyond the one fact of species-identity. 

 It is likely enough that many species have reached the 

 present surface of Britain by a migration from east to 

 west, independently of those which have been carried in 

 the same direction by human operations. But in admit- 

 ting this likelihood, it is not to be hastily assumed also, 

 that the migration of British plants has been exclusively 

 in the one longitudinal course. Other species may have 

 originally spread in a contrary course, that is, from west 

 to east. Looking to the present distribution of the 

 European flora, a gradual migration eastward seems as 

 well supported by facts, as is a like migration westward. 

 And no sufficient reason has yet been adduced, for 

 holding Britain to be an exceptional portion of Europe 

 in this respect. There are western species which are 

 quite absent from the eastern side of Britain. There are 

 other species which are now much more prevalent or 

 plentiful on the western side of the island, though com- 

 mon to both sides. And there are many species found 

 in Britain and western Europe, which have early limits 

 eastward on the continent. These and other facts suggest 

 ideas of a diffusion from west to east ; while various 

 other facts of converse character equally suggest a diffu- 

 sion from east to west. 



In addition, the northern and southern tendencies are 

 much more obvious than those in relation to longitude. 



