ON GENERA AND SPECIES, ` 61 
= 9.— ON THE GENERA AND THEIR CLASSIFICATION, AS FOUNDED ON 
THEIR MODES OF GROWTH AND HABIT. 
In the preceding pages I have given a general view of 
the principal writers on Ferns, and their systems of classi- 
fication. I will now proceed to explain the reasons that 
have induced me to attempt to draw up a natural arrange- 
ment of Ferns, reviewing as I proceed the data upon 
which the genera are founded. 
In my first general arrangement in 1841, I characterised 
the whole of the circinate Ferns under divisions and tribes, 
as shown in the abstract of my arrangement, given at 
page 33, the characters of which are in general obvious, so 
that any Fern with perfect fructification can readily be ` 
referred to its proper tribe. A system of classification so 
based is therefore practically useful, but it must be viewed 
as purely artificial, for, although many of the genera in 
any one tribe are naturally allied in general habit, yet 
others of totally different habit, but possessing the technical 
character of the tribe, are found associated with them; 
while on the other hand genera closely allied in natural 
habit and organisation are placed in separate tribes. For 
example, the genus Goniopleris has naked sori, while its ` 
congener Nephrodium has indusiate sori, they are conse- - 
qtently placed apart in any arrangement which is based on 
. the absence or presence of indusia. In both these genera - 
the species are numerous, and in many cases are 80 alike 
that in the absence of the indusium, specimens of Nephro- 
dium are referred to Goniopteris. The same applies to the 
extensive genera Lastrea and Phegopteris, both. of which 
contain closely allied species. 
When the indusium is absent in normal indusiate gener, ` 
is in general impossible, even by the most careful micro- ` 
