ON GENERA AND SPECIES. 71 



law of nature to mark where a genus begins or where it 

 ends. On this point, in my " Arrangement of the Genera 

 of Ferns," published in 1841, I have said "A gradual 

 transition of form is seen to pervade through the whole, 

 not in a lineal, but in a complex reticulated manner, it 

 becomes very difficult to determine in which genus the 

 extreme or transition species of groups should be placed." 



On considering that in many cases habit is not excluded 

 in assisting to define the genera of flowering plants, and 

 in numerous instances it is allowed by eminent botanists 

 to constitute the chief distinction between allied genera. 

 For example, the order Liliacece is represented by nearly 

 1,700 species, arranged under about 150 genera, which all 

 agree as regards their parts of fructification, the slight 

 variations of which are made use of for defining genera ; 

 and, indeed, if it was not for the difference in habit, the 

 number of genera are capable of being greatly reduced. 

 This and other instances that might be quoted favour my 

 view, that in order to constitute natural genera of Ferns, 

 habit must play an important part ; and so long as any 

 number of species agree in habit, and possessing other 

 characters in common, it matters not whether such groups 

 are called genera, sub-genera, or sections of genera. For 

 my own part, I prefer regarding them as genera, as it 

 saves a great deal of unnecessary trouble in speaking or 

 writing about Ferns, it being much easier to say Drynaria 

 coronans than Polypodium (Drynaria) coronans, or Elapho- 

 glossum conforms than Acrosticlium (Elaplioglossum) con- 

 forme, or Gymnogramma calomelanos than Gymnogramma 

 (Eugymnogramma) calomelanos; also by studying the cha- 

 racter of the smaller groups individually, and treating 

 them as genera, their nature at once becomes fixed upon 

 the mind, without having to think of them being asso- 



