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 Liltacew 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, a9 it 
saves & 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 conforme than Acrostichum (Elaphoglossum) con- | 
forme, or Gymnogramma calomelanos than Gymnogramma — 
 (Hugymnogramma) 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- ` 
