8 BRITISH EOCENE ELORA. 



difference between deposition in flowing and in still water has, it will be seen, greatly 

 influenced the relative characteristics of the floras. 



Unfortunately authors when describing fossil plants have seldom given indications of the 

 reasons which have led them to adopt one genus in preference to others with similar fruit 

 or foliage ; and they do not state why they have unhesitatingly attached generic and specific 

 names to the most ordinary forms of leaf, whose bases or tops are wanting. If, for 

 example, the suspicion communicated to me by a very distinguished botanist is well 

 founded, that the oldest known dicotyledonous leaf is not Populus, and that the leaves 

 determined as Vitis, from near the Arctic Circle, are not vines, all the theories built on their 

 presence are valueless. The practice of identifying plants from diff'erent localities, by 

 comparing them with lithographs, also very often leads to false assumptions ; for, 

 apart from inaccuracies introduced by artists and engravers, the finer variations, and, 

 above all, the texture, are seldom represented j and single leaves especially can seldom 

 be definitely identified without a comparison between actual specimens. 



Although there are these difficulties, very valuable knowledge may be derived from the 

 study, when the indefinite is omitted, and the same methods are used as in other branches of 

 palaeontology. It is fortunate that there are some fossil plants among those of the Tertiary 

 Period which require no special ability to identify ; others, however, require very careful 

 consideration ; and there are a few about which, in the present state of our knowledge, it 

 would be unsafe to say anything, though it may be useful to record their existence. 

 The relative value of these determinations should be made quite apparent and the 

 indeterminable forms omitted entirely from all speculations. 



I now pass on to a review of the groups of vegetable forms revealed in the British 

 Eocene deposits. 



