INTRODUCTION TO VOLUME III 3 



brought into existence from several distinct sources examples of that 

 unprotected state which was held as characteristic of the old comprehensive 

 genus, Polypodmm. Another change consists in the merging together of sori 

 in linear series. This may happen either in the marginal sequence, giving 

 the leading feature of the Pteroid Ferns: or in the superficial sequence, 

 giving the leading feature of the Blechnoid Ferns, Again, in several distinct 

 phyla the identity of the sori may be lost, by the spreading of the production 

 of sporangia generally over the surface of the sporophyll. This gives the 

 condition characteristic of the old genus Acrostichum. Such changes when 

 proved to be polyphyletic can no longer be accepted as giving satisfactory 

 generic characters, if the classification is to be one reflecting evolutionary 

 results. In fact these old genera will have to be broken up in any phyletic 

 classification. It will have to be realised, as indeed it already is in some 

 quarters, that such old comprehensive "genera" as Polypodiitniox Acrostichicm 

 are not phyletic unities at all. The soral characters upon which they were 

 based really represent states or conditions arrived at by evolution along a 

 plurality of converging lines. Consequently an increased difficulty arises in 

 the segregation of the distal evolutionary twigs. It is well thus to visualise 

 some of these difficulties at the outset, so as to avoid misunderstanding as 

 the treatment of the subject-matter proceeds. The fact is that a natural, or 

 phyletic. Classification is becoming increasingly difficult, and is of necessity 

 complex. For it is not always sufficient to observe the physical characters 

 presented; we also require to have some reasonable view or knowledge of 

 how the characters of the individual observed were arrived at in its Descent. 



No attempt will be made to place all Ferns in their natural, that is their 

 phyletic, relation to one another. There are not a few genera of so problematic 

 a nature that they will- have to be left under the designation "incertae sedis" 

 (Chapter XLVlll). Nor will the treatment run into ultimate detail, except in 

 some few instances where specific comparison appears to throw light upon 

 the relations of genera, or even of families. What is undertaken is, on a 

 basis of a wider comparison than has been generally practised hitherto, to 

 lead from below upwards along reasonably safe and probable lines. But the 

 work is offered as a "tentamen" only, not a finished task: and as such it is 

 submitted to the judgment of those who shall come after. 



Further, the treatment of the groupings will be as far as possible con- 

 servative. The author has much too high a respect for that intuition or 

 special sense for afifinity that is exemplified so often in the work of the 

 great systematists, to allow him to neglect or needlessly to disturb the 

 natural relations that they have already defined. These will be generally 

 adopted, and changes will only be suggested where facts and comparisons, 

 which had previously been unknown or undervalued, clearly dictate some 

 other grouping. At the same time the relationships, as traced by previous 



