VI 
PREFACE. 
organic structure of certain species, which the laborious researches, 
and the acumen of Messrs. Henderson, Valentine, &c., have enabled 
me to avail myself of. Mr. II. C. Watson also has kindly contri- 
buted remarks of real interest on the geographical distribution of the 
various tribes ; and Mr. Bevis, of the Royal Botanic Garden, supplied 
me with the groundwork of the Appendix, my own cultivation of the 
class having been very limited. The habitats have been arranged 
with more regard to latitude, but they have not been materially 
increased. That portion of the work devoted to scientific description 
has, no less than the other parts, been carefully corrected, yet the 
alterations will be found very few and unimportant. 
My opinion of the identity and distinctness of certain species re- 
mains unaltered ; yet I am aware that some writers differ from me, 
and would unite two of the Polypodiums — Asplenium alternifolium, 
and Ruta muraria, &c., founding their opinion upon specimens pre- 
served in herbaria. I venture to protest against a too general reliance 
being placed upon this source of information ; not but that reference 
to celebrated herbaria is most valuable, in addition to research ; but 
this is all — a knowledge of plants can never be acquired by such 
means, because herbaria do not testify the circumstances of aspect, 
season, altitude, moisture, soil, and other influences, to which plants 
are subject. I make this remark chiefly in reference to the Cysteas, 
Woodsias, some of the Aspidiums, &c., about which botanists differ 
more than about any other genera. It is true that numerous fronds 
may be found of an intermediate character between two species ; 
still, unless circumstances of growth be corresponding to all, this 
does not prove identity between the remote examples. This observa- 
tion particularly applies to Ferns, because the diagnoses of the 
species rest mainly upon the shape of the frond, and this is the part 
which is alone subject to variation. The herbaria of even the most 
celebrated botanists are not always to be depended upon for accuracy. 
Plants are often received named — the name, without examination, is 
almost as frequently considered correct, and that specimen forms 
henceforth a criterion whereby to name others ; thus error is con- 
tinued and multiplied. To these causes I am inclined to attribute 
tbe contrariety of opinion which exists relative to our Ferns ; and l 
may add, perhaps to our Willows, Roses, and Brambles. 
It may, perhaps, be expected that l should have said something 
