INTRODUCTION. 
XV 
rature of 70 ° Falir. it will grow with vast rapidity ; and 
with a lower temperature, with artificial protection, it also 
answers very well. 
Adopting this plan of cultivation, I have possessed the 
opportunity of observing the changes that took place, and 
of watching, as it were, the progress of variation. I have 
pressed fronds from the same root for three successive 
years, and have found variations abundantly adequate to 
the establishment of species quite as distinct as many of 
those in the English Flora ; and I consider all that culti- 
vation, as I have explained it, can accomplish for any 
plant, is to hasten or delay those changes to which that 
plant is by nature liable : it cannot increase or diminish 
the number of actual species. In those species liable to 
great extremes in the cutting of their fronds, I have ob- 
served that a soil composed of decaying wood, abundantly 
supplied, and completely covering the roots, hastens a de- 
velopment of the most divided form which they can possi- 
bly assume, while a mixture of sand and stones, and a 
deficiency even of these, retards the development, and 
not unfrequently causes the plant to return to a more 
simple form. 
Besides the British Ferns, all the species indigenous to 
the northern regions of America, Europe, and Asia, may 
be grown in the open air, and without protection, excepting 
from severe irost, when they should he covered with straw, 
matting, or dried tan, thus supplying that warm clothing 
of snow which protects them from extreme cold in their 
native habitats. But if we advance one step, and restrain 
