472 GNETALES [CH. LII 



the venation-characters of these and other fossil leaves may lead 

 to the discovery of criteria which may enable us to separate the 

 leaves of Gnetum from similar Dicotyledonous foliage. 



It is with a keen sense of the incompleteness of my task that 

 Volume IV. is concluded without any attempt to deal with the 

 abundant if, in very many cases, undecipherable records of Angio- 

 sperms. The omission of this branch of Palaeobotany in what 

 purports to be a general text-book calls for a word of explanation. 

 A mere summary of conclusions so far pubUshed with regard to 

 the geological history of Flowering plants would not yield results 

 commensurate \vith the labour involved. What is needed is a 

 critical examination, as far as possible, of the actual specimens 

 and a careful scrutiny of the evidence on which determinations 

 are based. It is undoubtedly the fact that a large number of 

 leaf-impressions are practically valueless as trustworthy data, and 

 I venture to think that it is only with the cooperation of trained 

 systematists that any satisfactory estimate can be formed as to 

 the value of the fragmentary documents preserved in Cretaceous 

 and Tertiary strata. It is preferable to omit, at least for the pre- 

 sent, this part of the subject than for the sake of completeness — 

 in a treatise that is very far from complete in its treatment of the 

 groups that have been considered — to essay a task for which the 

 author recognises that he is very inadequately equipped. 



