26 ON GENERA AND SPECIES. 



their fructification, yet a great many differ so widely in 

 general habit and mode of growth, that any systematic 

 arrangement, founded on genera so constituted, must be 

 received as purely artificial, and only admissible on the 

 principle of its bringing together under a few brief cha- 

 racters a number of species possessing such characters in 

 common. This being the case, modern Pteridologists have 

 found it necessary to seek for other characters in order to 

 classify the various groups in accordance with their natural 

 relationship to one another, as exhibited by their general 

 appearance in habit and mode of growth. This became 

 the more necessary on account of the great increase in the 

 number of species from time to time brought into notice 

 by botanical collectors as previously explained. Robert 

 Brown was the first to point out and use an additional 

 character for defining genera. In 1810, in characterising 

 the genera Cyathea, Hemitelia, and Alsophila, he took into 

 account the difference of the position of the sori on the 

 veins, and this he again brought into special notice in the 

 character of his genera Matonia and Hypoderris, first de- 

 scribed in Wallich's " Plantae Asiatics Rariores." In 

 Horsfield's " Plantae Javanicse Rariores," published in 

 1838, he enters more fully upon the importance of employ- 

 ing venation as affording auxiliary generic data. He there 

 sketches out a method for dividing the large genus Polypo- 

 dium into groups of species naturally allied in general 

 habit, on characters derived from the structure of the 

 venation and position of the sori on the veins. 



About this period several botanists on the Continent 

 directed their attention to the study of the venation of 

 Perns. The first worthy of notice is Ferdinand Schott, 

 Director of the Imperial Gardens at Vienna, who, in 1834, 

 commenced a work entitled " Genera Filicum." This 



