ON CENERA AND PPECIES. 7 



species will vary according- to the evidence possessed by 

 different authors, and the views tliey take of the apparent 

 transition forms. 



In verification of what is stated above respecting species, 

 I quote the words of an eminent botanist, that " the pre- 

 liminary exact determination of what is a species no one 

 has yet succeeded in giving." It is well known that phre- 

 nogamous plants assume different forms and aspects brought 

 about by natural causes or the agency of man ; the differ- 

 ence from the original types being often so great, that if 

 evidence of the change were not on record, the botanist of 

 the present day would be justified in describing them as 

 distinct species, and originally created as such. With 

 Ferns, we possess bvit little evidence of new forms having 

 come into existence in modern times, the chief examples 

 being found in several intermediate states in the g-enera 

 Gymnogramnio and Pterls, which of late years have made 

 their appearance in gardens, some being so different fi'om 

 their supposed parents that, if they had been found wild 

 they would by some botanists be considered sufficiently 

 distinct to rank as species. If such changes do actually 

 take place, and we are to deduce from them that races of 

 intermediate forms originate in the progress of time, and 

 through the causes above alluded to, great difficulty must 

 attend any attempt to define species of Ferns. 



These observations show the principal conflicting- causes 

 that have led to the confusion in the nomenclature of Ferns. 

 This may be considered a sufficient history of species. I 

 will now treat of genera and their classification ; but to 

 make the subject well understood, I deem it best to give 

 first a general exposition of the structure of the different 

 jaarts of Ferns, on which genera are founded and classified. 



