2 INTRODUCTION TO VOLUME III 



possess the necessary time and specific knowledge, to work out the system 

 towards its distal evolutionary twigs. It will be found difficult enough to 

 compress within the limits of this volume what needs to be said of the main 

 groups in general terms, together with some estimate of the probable 

 relations of the genera one to another. But certain features of the scheme 

 are already familiar. For instance, the affinity of the Davallioid and the 

 Pteroid Ferns with the Dicksoniaceae is generally recognised. They may be 

 held as marginal types advanced from a gradate to a mixed state of the 



Phyletic scheme suggesting the inter-relationsof themore Primitive 

 Filicales: above are placed six main Phyla of Leptosporangiate 

 Ferns, the dotted lines indicating their probable connection respec- 

 tively with the Dicksoniaceae, Plagiogyna, and the Cyatheaceae. 



sorus. Similarly the Blechnoid and Dryopteroid Ferns may be regarded as 

 derivatives from a gradate Cyatheoid source, showing transition to a mixed 

 type of the superficial sorus. The Gymnogrammoid and Dipteroid Ferns 

 form phyla of less familiar relationship, as they stand in the current 

 systematic works. The grounds for suggesting the affinities shown in the 

 phyletic scheme will have to be set down in detail later in the text. 



As the treatment proceeds it will become clear not only that there is 

 evidence of polyphyletic progression to a mixed sorus, but also that in respect 

 of the relations of the sori one to another, as well as in the manner of their 

 protection, certain changes appear. One of these is the abortion of indusial 

 growths previously present in one phylum or another: and so there may be 



