710 GEOGRAPHICAL BOTANY. 



Forbes endeavours to prove that the specific identity, to any 

 extent, of the plants of one area with those of another, depends on 

 both areas forming, or having formed, part of the same specific centre, 

 or on their having derived their vegetable population by transmission, 

 through migration, over continuous or closely contiguous land, aided, 

 in the case of alpine floras, by transportation on floating masses of 

 ice. According to him, " the oldest of the floras now composing the 

 vegetation of the British isles, is that of the mountains of the west 

 of Ireland. Though an alpine flora, it is southernmost in character, 

 and is quite distinct as a system from the floras of the Scottish and 

 Welsh Alps. Its very southern character, its limitation, and its 

 extreme isolation, are evidences of its antiquity, pointing to a period 

 when a great mountain barrier extended across the Atlantic from 

 Ireland to Spain. The distribution of the second flora, next in point 

 of probable date, depended on the extension of a barrier, the traces of 

 which stUl remain, from the west of Prance to the south-east of 

 Britain, and thence to Ireland. The distribution of the third flora 

 depended on the connection of the coast of Prance and England 

 towards the eastern part of the channel. Of the former existence of 

 this union no geologist doubts. The distribution of the fourth, or 

 alpine flora ojF Scotland and Wales, was efiected during the glacial 

 period, when the mountain summits of Britain were low islands, or 

 members of chains of islands, extending to the area of Norway 

 through a glacial sea, and clothed with an arctic vegetation, which in 

 the gradual upheaval of those islands and consequent change of 

 climate, became limited to the summits of the new-formed and still 

 existing mountains. The distribution of the fifth, or Germanic fiora, 

 depended on the upheaval of the bed of the glacial sea, and the con- 

 sequent connection of Ireland with England, and of England with 

 Grermany, by great plains, the fragments of which stUl exist, and 

 upon which lived the great elk, and other quadrupeds now extinct. 

 The breaking up or submergence of the first barrier led to the destruc- 

 tion of the second ; that of the second to that of the third ; but the 

 well-marked epoch of migration of the Germanic flora indicates the 

 subsequent formation of the straits of Dover and of the Irish Sea, as 

 now existing. 



" To determine the probable geological epoch of the first or west- 

 Irish flora — a fragment, perhaps with that of north-western Spain, of 

 a vegetation of the true Atlantic — we must seek among fossil plants 

 for a starting-point. This we get in the flora of the London clay, or 

 Eocene, which is tropical in character, and far anterior to the oldest 

 of the existing floras. The geographical relations of the Miocene sea, 

 indicated by the fossils of the crag, give an after-date certainly to the 

 second and third of the above floras, if not to the first. The epoch of 

 the red or middle crag was probably coeval with the second flora ; that 



