44 DESCEIPTION OE SPECIMENS. 



specific separation too artificial. In cases where we have a large 

 number of forms constituting a series, and the extreme types 

 exhibit marked distinctive features, it has been found convenient 

 to use a specific term in a comprehensive sense, and to append 

 a second name as indicative of a ' form ' or variety. The species 

 Sagenopteris PMlUpsi (Brongn.), as used in this sense, includes 

 leaves or leaflets differing considerably from one another in size 

 and shape ; but from the analogy of recent plants, and from the 

 occurrence of more or less connecting Knks between the extreme 

 types, it seems preferable to include aU under one term, and to 

 refer to the more distinct forms by varietal names, which in some 

 cases may have been previously used as specific designations. The 

 small form of Sagenopteris named by Lindley & Hutton S. euneata 

 may be spoken of as S. PMllipsi, var. euneata, while a few 

 examples of iznusuaUy large leaves are referred to as S. PMllipsi, 

 var. major. 



My tendency has been to diminish the number of specific names 

 in cases where the data afford insufficient evidence of important 

 differences. It would reduce specific distinctions to an absui-dity 

 to designate by a special name the various forms of cycadean or 

 fern leaves which may be grouped around a weU-marked type. At 

 best the material is insufficient for accurate diagnosis and deter- 

 mination; and while drawing attention to such forms as afford 

 valuable evidence in the recognition of geological horizons, our 

 chief aim should be to deal with the fossil specimens on the same 

 principles as are applied to recent plants, and to interpret the 

 botanical records in a manner best calculated to render them useful 

 as indices of plant development and distribution. 



In a letter to Lyell in 1860, Darwin wrote : " How far to lump 

 and split species is, indeed, a hopeless problem. It must in the 

 end, I think, be determined by mere convenience." ' 



In the description of each species a definition is given after the 

 list of synonyms, and where possible the loeale of the type-specimen 

 has been mentioned. Comparisons of the fossil types with recent 

 plants are, for the sake of uniformity, usually discussed at the end 

 of the remarks on the synonymy or history of each species. 



1 I am iudetted to my friend Mr. Francis Darwin for permission to quote this 

 passage from an unpublished letter. 



