CH. Ill] A PHYLETIC CLASSIFICATION 53 



form a considerable part of the undergrowth below the forest-canopy. But 

 their habitat is variable. Some specialised types are actually aquatic, while 

 others are able to withstand conditions of moderate, some even of extreme 

 drought. 



The Ferns are much richer in genera, species, and individuals than any 

 other living Pteridophytes. They probably present the climax of successful 

 development in Homosporous Vascular Plants. They show also a high degree 

 of variety both in their vegetative and in their propagative characters. These 

 characters provide good diagnostic features upon which their Classification 

 may be based. They have a full and long palaeontological history, which 

 may be traced through successive horizons backwards to Palaeozoic times. 

 The characters of the fossils have been proved to be so far comparable with 

 those of certain of the living Ferns that their relationship cannot be held in 

 doubt. The geological record can therefore be used as a valid check upon 

 such conclusions as to relative antiquity as may be drawn from the com- 

 parison of living types of Ferns. There is in fact no group of living plants 

 which offers so favourable an opportunity for phyletic classification: for none 

 is there so long, rich, and consecutive a geological record, combined with 

 so great a variety of living types, possessed of diagnostic characters so 

 marked, and with progressions so susceptible of reasonable physiological 

 interpretation. This gives the Filicales an interest peculiarly their own. But 

 the classification of such a group need not have as its end merely the reduc- 

 tion of the group itself to phyletic order. It may be made a means of demon- 

 strating methods of comparison applicable to other groups. This is the way 

 in which it is proposed to treat them in the present work, which will thus 

 illustrate phyletic method as seen in practice in a natural alliance of plants 

 specially favourable for 'giving definite conclusions. 



Current systematic arrangements of the Filicales 

 Before entering on the detailed examination of the facts which should 

 thus supply the basis for a natural system of the Filicales it will be well to 

 summarise the current classification of them, as set forth in standard works 

 of the present time. Naturally it is to Christensen's Index Filiciim (1906) 

 that one looks first(23). In that great work, — which is primarily a catalogue of 

 the accepted generic and specific names of Ferns, together with all the 

 synonyms that have been used by various writers, — a general grouping of 

 the genera and families is given at the outset. This corresponds in essentials 

 with the scheme laid out in Die Naturlichen PflanzeJif ami lien, by Engler 

 and Prantl (i902)(24), which is the most comprehensive systematic treatise on 

 Ferns of recent decades. The sequence of Families and their Orders as 

 given by Christensen is as follow^s, a few explanatory and descriptive words 

 being- added after each: — 



