INTRODUCTION. 41 
by a special articulation. The scheme, as hitherto explained, does 
not apply to the Osmundacew, Ophioglossacee, Se. 
The adoption of this mode of classification would to a great 
extent break up the groups and genera now recognised; and we 
cannot think such radical changes, in the case of plants already so 
well classified as the Ferns, at all necessary or desirable. Probably 
the necessity which has been felt for such changes has arisen from 
a concentration of the attention on matters of detail, without pausing 
to form clear generalised perceptions of important differences. It 
must, moreover, be admitted in extenuation of proposed changes, that 
under the systems now in use, difficulties and objections here and 
there arise, such as a disturbance of the ordinary development of 
the veins, or the impossibility of determining on the presence or 
absence of indusia; but these are not more important than those 
which are constantly occurring in other departments of botanical 
science, nor more insurmountable than those which would be likely 
to occur in the application of any other set of characters by the 
light of our present limited and ever-varying knowledge. The same 
difficulties, too, would still occur at a subsequent stage of inquiry, 
even if other characters were resorted to for primary distinctions. 
There is, moreover, no real physiological difference, as has been 
claimed, between the two apparently different modes of development 
which have been made the basis of this mode of classification. In 
both the axis is a stem, assuming in one case the form of a rhizome, 
in the other more or less that of a caudex or trunk—both being 
equally, forms of stem. Neither is the development of the fronds 
in the one case really terminal, though apparently so, for in the very 
nature of things, the axis must be developed before the part it sup- 
ports. The original suggestion, at first sight, appears to produce a 
natural division, in some measure equivalent to that of Exogens 
and Endogens among flowering plants, but such a contrast is, in 
reality, inadmissible, the whole race of eryptogams going to make 

