25 



water on the surface of our globe liass becu iu so many instances 

 modified from time to time in the course of ages. In this way 

 the bridge, as it were, by which these particular plants arrived 

 at their present localities having been broken down, they have 

 necessarily become isolated from the rest of their species. 



It would be foreign to the subject of this lecture to speak at 

 all in reference to the arguments which have been brought 

 forward either in support of this theory or the contrary ; I will 

 therefore simply state his general view respecting the original 

 sources of the great mass of our British plants, founded on this 

 theory, assuming it to be correct. 



He remarks that the vegetation of the British Islands may 

 be said to consist of five Floras : — " 1st. A West Pyrenean, 

 confined to the west of Ireland, and mostly to the mountains 

 of that district," the plants of which " have the nearest portion 

 of their specific centres in the north-west of Spaiij." He considers 

 this as " the oldest of the Floras ; its very southern character, 

 its limitation, and its extreme isolation, being evidences of its 

 antiquity, and pointing to a period when a great mountain 

 barrier extended across the mouth of the Bay of Biscay, from 

 Spain to Ireland." " 2ndly. A Flora related to that of the 

 south-west of France, extending from the Channel Isles, across 

 Devon and Cornwall, to the south-east and part of the south-west 

 of Ireland. This is the next in point of probable date, its 

 distribution having depended on the extension of a barrier, the 

 traces of which (he says) still remain, from the west of France to 

 the south-west of Britain, and thence to Ireland." " 3rdly. A 

 Flora coinmon to the north of France and south-east of England, 

 and especially developed in the chalk districts. Its distribution 

 depended on the connexion, generally allowed by geologists, of 

 the coasts of France and England towards the eastern part of 

 the Channel.'' " 4thly. An Alpine Flora, developed in the 

 mountains of Wales, north of England, arid Scotland ; its 

 distribution having been effected during the glacial period, when 



