XLViii] DIFFICULTIES IN PHYLETIC GROUPING 263 



of a phyletic classification lies in that standardisation of structure of the 

 more modern types which naturally follows on homoplastic adaptation. 

 It is this which has deprived the archegonium and antheridium, as well as 

 the sporangium and even the sorus itself, of their comparative value in 

 the advanced Leptosporangiate Ferns, though these provide more ample 

 material for comparison in the ancestral forms. In a measure it is the same 

 with the vegetative system of the sporophyte, and in still greater degree of 

 the gametophyte. 



But if the deficiency of differentiated detail is felt in the comparative 

 study of Ferns, what shall we say of the Mosses, or of the Fungi, with their 

 multitudinous genera and species, so highly standardised in so many re- 

 spects, and with a marked deficiency in their palaeontological record? Or 

 finally of the Angiospermic Plants, in which the ovule and pollen-sac, or 

 the pollen-grain and embryo-sac, are severally so highly standardised, while 

 their vegetative system is so open to homoplastic adaptation? The student 

 of phyletic classification who devotes himself to the Filicales may, even at 

 the close of a Chapter on ''genera incertae sedis" consider himself fortunate 

 by comparison. He may be pardoned for believing that his chosen field is 

 perhaps the most favourable of all Classes of Plants for the pursuit of his 

 distant "El Dorado" — a Natural Classification that shall include all known 

 types. 



