64 FERNS I BRITISH AND FOEEIGN. 



of their association with a host of species of quite dis- 

 tinct characters. It also leads to investigation, and, 

 accordingly, to a better knowledge of the structure of 

 Ferns. 



With regard to the characters that define the limits 

 of species, as much uncertainty prevails amongst 

 authors as with genera. This is owing to several 

 causes ; such as many species being normally hetero- 

 morphous, presenting at the same time different forms, 

 which again vary at another period of growth ; and in 

 many instances authors have described the different 

 states as distinct species ; and in some cases different 

 fronds of the same plant, and even portions of the 

 same frond, have been placed under separate genera. 

 It also frequently happens that two or more presumed 

 species present so many intermediate gradations of 

 form, that only the most extreme states appear suffi- 

 ciently distinct to warrant their adoption as species, 

 the numerous intermediate forms seeming to set 

 specific distinctions at defiance. It therefore becomes 

 a question what is the limit of form or of structure that 

 constitutes a species. Generally understood, a species 

 is an organized structure endowed with an essence or 

 quality peculiar to itself, and possessing the power of 

 multiplying and transmitting its type to new genera- 

 tions without change, ad infinitum. Admitting this 

 definition as correct, it seems to be beyond human 

 power to ascertain whether the serial gradations of 

 form are genuine descendants of original creations, or 

 only deviations from one original, brought into exist- 

 ence during the lapse of ages by the different climatic 

 and local influences they have been subjected to. It 

 is well known that phsenogamous plants assume differ- 



