588 DISTRIBUTION OF PLANTS IN BRITAIN. 



" The breaking up or submergence of the first barrier led to the 

 destruction 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 Forbes seeks among fossil plants 

 for a starting-point. This he gets 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 of the mammaliferous crag with the third. The date of the fourth 

 is too evident to be questioned ; and he regards the glacial region in 

 which it flourished as a local climate, of which no true traces as far 

 as animal life is concerned exist southwards of his second and third 

 barriers. This was the newer Pliocene epoch. The period of the 

 fifth flora was that of the post-tertiary, when the present aspect of 

 things was organised. 



" Adopting such a view of the relations of these floras in time, 

 he thinks that the greatest difficulties in the way of changes of the 

 earth's surface and destruction of barriers deep sea being found 

 where land (probably high land) was are removed when we find that 

 those greater changes must have happened during the epoch imme- 

 diately subsequent to the Miocene period; for we have undoubted 

 evidence that elsewhere, during that epoch, the Miocene sea-bed was 

 raised 6000 feet in the chain of Taurus, and the barriers forming the 

 westward boundary of the Asiatic Eocene lakes so completely annihi- 

 lated, that a sea several hundred fathoms deep now takes then- probable 

 place. The changes required for the events which he would connect 

 with the peculiar distribution of the British flora are not greater than 

 these. Professor Forbes thinks that the peculiar distribution of en- 

 demic animals especially that of the terrestrial mollusca bears him 

 out in these views." 



1166. The observations of Watson and Forbes lead to the conclusion 

 that, with the exception of Eriocaulon septangulare,the British islands do 

 not contain a single plant which is not found on the continent of Europe. 

 These islands, therefore, cannot be considered as a centre of vegetation, 

 but as having been colonized by successive vegetable migrations. Their 

 opinion as to the origin of British plants, as condensed by Martins, is, that 

 these islands have been peopled by many colonies successively leaving 

 the continent of Europe, from the epoch of the middle tertiary formation 

 up to our own. When a vast continent extended from the Mediter- 



