CONCLUSION. 161 



It is of interest to notice the geographical range of the species 

 of Rhaetic and Jurassic (Liassic and Oolitic) plants represented 

 in English floras. In attempting to illustrate the distribution 

 in space of fossil species one is confronted with the difficulty of 

 deciding to what extent species recorded from Western Europe may 

 he regarded as identical with similar forms described from distant 

 regions. The resemblance of one flora to another is usually 

 obscured by the use of different generic or specific names for plants, 

 which are either identical or represent closely allied members of 

 the same family. This diversity in nomenclature is, to some 

 extent, the result of geographical separation ; an author naturally 

 hesitates to assign the same specific name to plants from India and 

 Europe unless the evidence as to identity is convincing. On the 

 other hand, wide separation in space has often been allowed to 

 exercise a misleading influence in the determination of species. 

 Another reason for the use of different names for plants which are 

 either specifically identical or very closely allied, is to be found 

 in the individual preferences of authors in the choice of possible 

 generic designations for a particular type. 



In order to obtain a clear idea of the botanical relationship of 

 one flora to another it is essential to devise some method by 

 which distinctions, either imaginary or exaggerated, between 

 plants recorded by different writers under distinct names may be 

 eliminated. 



As Darwin wrote in the Origin of Species,^ "It is notorious on 

 what excessively slight differences many palaeontologists have 

 founded their species ; and they do this the more readily if the 

 specimens come from different sub-stages of the same formation." 



Our aim is to ascertain to what extent homotaxial floras in 

 different parts of the world were characterised by similar botanical 

 facies. The subject of the geographical distribution of plants 

 during past ages is one which has not received sufficient attention ; 

 it is trae our data are too meagre to admit of sweeping generalisa- 

 tions, but we have enough evidence at our disposal to enable us to 

 form an estimate of the comparative composition of widely separated 

 floras. If we confine ourselves to a comparison of published lists 



' Darwin (00), p. 423. 



