INTEODUCTION. XV 



Myrrhidifolium, E. Ruta-muraria, R. Tunbridgense, R 



Phegopteris, R. Scolopendrium, R. tUliginosum, N. 



Radicans, R. Septentrionale, R. *'Uiiilaterale, H. 



Regalis, R. Spicant, R. Viride, R. 



Rigidum, H. Spinosum, N. Vulgatum, R. 



Robertianum, S. Thelypteris, R. Vulgare, R. 



t Rutaceum, R. Trichomanes, R. 



There is scarcely anything so difficult to define as a species. 

 We all agree that it has an existence in Nature, but we are at 

 a loss for terms of definition, that shall be at once sufficiently 

 restrictive and sufficiently comprehensive. And another diffi- 

 culty exists against which it is next to impossible to contend, 

 and that is, the different modes in which different minds view 

 the same object. No plant ever displayed this difference more 

 prominently than Foenisecii : some minds look on this as the 

 most distinct of ferns ; others, myself for instance, regard it 

 as taking an ordinary station as a species, Hke lanceolatum, 

 Trichomanes, viride or marinum; others, again, as the learned 

 authors of the sixth edition of the ' British Flora,' not only 

 omit it from their list, but feel themselves called on to devote 

 fifty-six lines of their smallest type to explanations, as though 

 it must be argued away at any cost of space and trouble. I 

 will not say that either of these is wrong ; but I do say that 

 such a discrepancy of opinion on what appears a very simple 

 question, shows the simplicity is one of seeming only. 



Amongst the ferns I have described, there are certain inde- 

 scribable grades of rank. Those which I regard of the highest 

 rank, stand in the preceding list without any prefix. A grade 

 lower than these, are others to which I prefix an asterisk : — 

 Woodsia *alpina, Cystopteris *Dickieana, Ophioglossum *lusi- 

 tanicum, and Hymenophyllum *unilaterale ; but all these stand 

 as established species in the text, without any mark of doubt : 



