GENERA OP FERNS AND THEIR 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., 
Osrrmnda, Onoclea, Acrosticlmm, Hemionitis, Polypodium, 
Asplenium, Pteris, Blechnum, Pouchitis, Adiantum, 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, Robert 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 
