INTRODUCTION. 23 
this diversity of practice. So far from following 
them I have never taken any author as a safe 
guide throughout, but have invariably given to each 
Species the earliest name that I know it to have 
received: there is one seeming exception to this 
rule, and that is in the name “cristatum,” which 
I have purposely omitted, not from any objection to 
the name, but because it originally included seven, 
_ and perhaps eight, of my species, Nos. 20 to 28; 
and I know not to which species I ought to apply 
it: in this case, therefore, I adopt in every instance 
the earliest name given to each species after it was 
: ascertained that Linneus had comprised more than 
: one, under the comprehensive name of cristatum: 
a like number of species were subsequently com: 
prised under the name of “ Dilatatum,” and again 
_ under that of “Spinulosum.” Of these eight ferns 
: it may truly be said that opinions will always differ; 
some authors divide them into three species; others 
: into four; and there is no occasion whatever for any 
hostile feeling towards those who in this respect 
differ from ourselves: the only point absolutely re- 
- quisite is to learn, thoroughly and perfectly, the object 
