XXXVII] 35 



COMPARISON OF THE DAVALLIOID FERNS 



The difficulties of the systematist often provide the comparative morpho- 

 logist with his best opportunities. The Davallioid Ferns present as profuse 

 a synonymy as any other group : and this itself shows how divergent have 

 been the views of systematists in the disposal of a tribe richer in species 

 than in the materials for their ready diagnosis and classification. But from 

 the point of view of the practical student of evolution this means that, 

 whenever the necessary facts shall be to hand, there should be the opportunity 

 for tracing comparatively progressions which may finally illustrate not merely 

 detailed changes, but steps of very material advance. To guard against mere 

 comparative theorising it is necessary to keep a critical eye upon physiological 

 probability. The suggested progressions must be functionally probable as 

 well as comparatively possible. It will appear that the progressions which 

 comparative study of the Davallioid Ferns brings to light satisfy this demand 

 The changes that are traced comparatively in them are palpable amendments 

 upon the more primitive state of the Dicksonioid Ferns. 



The leading Dicksonioid characters that serve in this comparison with 

 their derivatives are such features as the creeping habit, the highly branched 

 frond, the dermal hairs not scales, the sustained individuality of the per- 

 sistently marginal and gradate sorus, the superficial origin of the two-lipped 

 indusium, the oblique annulus, and robust sporangial stalk: also internally, 

 solenostely, and an undivided leaf-trace. These features are all shared more 

 or less perfectly by those Dicksonioid derivatives, such as the Dennstaedtiinae, 

 which are held to be relatively primitive. But the comparative study of the 

 Davallioid Ferns shows how these features are liable to be gradually modified 

 and obliterated. Nevertheless their general correspondence with the main 

 type, and even the detailed similarity in some of them, point clearly to the 

 conclusion that the changes are not differences of kind, but modifications 

 often resulting from minute progressive shades of change. Thus the tran- 

 sition may be traced from the primitive Dicksonioid-Dennstaedtioid type to 

 the full Leptosporangiate character of the most advanced Davallioids. 



The modifications from a Dennstaedtioid type seen in Davallia itself 

 involve the presence of scales in place of dermal hairs : a higher disintegration 

 of the vascular system, and especially of the leaf-trace: the sorus tends 

 towards a superficial position, with the margin of the lower indusium adherent 

 to the surface of the blade, with which the upper indusium is merged: the 

 flattened receptacle bears a mass of sporangia of mixed ages, though still 

 showing traces of a basipetal sequence: the sporangia themselves are atten- 

 uated, with vertical annulus and long stalks consisting only of one row of cells. 

 All these are recognised on the basis of the general principles enunciated in 

 Vol. I as features of advance. But the lines along which they have been 



3-2 



