GENERA OP FERNS AND THEIE CLASSIFICATION. 57 



An examination of the works of Linnaeus shows that 

 he was acquainted with about one hundred and eighty 

 species, and these he classed under eleven genera (viz., 

 Osmunda, Onoclea, Acrostichum, Hemionitis, Poly podium, 

 Asplenium, Pteris, Blechnum, Lonchitis, Adiantwn, and 

 Trichomanes) , which were founded upon purely artificial 

 characters, derived solely from the shape and position 

 of the fructification. This system was amply sufficient 

 for the limited number of species then known indeed ? 

 the proportion of genera to species was much larger 

 in Linnaeus's days than in our own; but when the 

 number of species had been greatly augmented, it 

 became obvious that, in order to avoid genera of 

 unwieldy dimensions, if not for other reasons, addi- 

 tional characters must be sought for ; and these have 

 gradually been introduced. It is a remarkable fact, 

 however, that although the number of species now 

 known exceeds by about twenty-fold that known to 

 Linnaeus, it is quite possible to arrange them all under 

 the eleven genera established by that author. 



After the time of Linnaeus, the first additional organ 

 relied upon for generic characters was the indusium, 

 which was employed by Sir J. E. Smith and Pro- 

 fessor Roth, and afterwards more fully by Swartz, 

 who divided the twenty-five genera known to him 

 into " naked " and ' ' indusiate." Linnaeus noticed 

 the fact of the sori following the course of the veins in 

 his character of Hemionitis, but, in 1810, Kobert Brown 

 first specially employed characters taken from the 

 position of the sori upon the veins. This was the next 

 important step in advance. The same learned botanist 

 was also before anybody else to point out the im- 

 portance of venation as an aid to classification, but 



