INTRODUCTION. XV 



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 abund- 

 antly adequate to the establishment of species quite 

 as distinct as many of those in the English Flora ; 

 and I consider all that cultivation, 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 observed that a soil 

 composed of decaying wood, abundantly supplied, 

 and completely covering the roots, hastens a develop- 

 ment of the most divided form which they can pos- 

 sibly assume ; while a mixture of sand and stones, 

 and a deficiency even of these, retards the develop- 

 ment, 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 pro- 

 tection, excepting from severe frost, when they 

 should be 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 

 the free communication with the outer air, then there 

 seems to be no limit to the species we may introduce 

 the beautiful productions of the tropics may be 

 brought to our doors. 



