VI 
PREFACE. 
The introduction contains much new and valuable matter on the 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 contributed remarks 
of real interest on the geographical distribution of the various tribes; 
and Mr. Bevis, of the Royal Botanic Garden, supplied me with the 
ground-work 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 observation 
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 arc 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 continued and 
multiplied. To these causes I am inclined to attribute the contrariety 
of opinion which exists relative to our Ferns; and 1 may add, perhaps, 
to our Willows, Roses, and Brambles. 
