INTRODUCTION TO VOLUME II 3 



As a result of simple induction from the observed facts it may be stated 

 generally that simple hairs are a primitive feature, and that branched hairs 

 and in particular flattened scales are an indication of advance from that 

 simple state. Evidence of this advance parallel with other indications of 

 advance in other characters may be found in various phyletic sequences, and 

 this fully substantiates the general correctness of the conclusion thus stated 

 (Chapter Xl). 



(6) Comparison shows that the sorus is not a constant entity for Ferns 

 at large. It is liable to var}- in position, individuality, and constitution, and 

 the differences afford material for phyletic comparison. The marginal posi- 

 tion, more frequent in early than in late Fern-types, is shown to have passed 

 in several distinct phyletic series into the superficial as the area of the leaf- 

 blade increased. This makes it seem probable that the latter is a derivative 

 state wherever it occurs. The individuality of the sorus is lost in many 

 sequences, whether by fissions or by fusions: or the sori may be obliterated 

 by spreading the sporangia generally over the leaf-surface. These states 

 appear to be all derivative. The most important variations are, however, 

 those of constitution of the sorus. Three types have been distinguished, viz. 

 the Simple, Gradate, and Mixed types. The first is characteristic of Palaeo- 

 zoic Ferns, though it survives to the present day. The last is the type which 

 prevails at the present day but does not appear in Palaeozoic Ferns. The 

 Gradate is an intermediate type in many but not in all phyletic lines. Thus 

 the soral characters provide ample material for phyletic comparison 

 (Chapter XIl). 



(7) Palaeozoic Ferns have no indusial protection, nor have those charac- 

 teristic of modern conditions which are included in the old comprehensive 

 genera Pvlypodhnii and Acrostidmin. But between such extremes protec- 

 tion by various means has been achieved, notably by different types of in- 

 dusium. An indusiate sorus is then a later and derivative type: on the other 

 hand there is evidence that the modern non-indusiate state has often resulted 

 from the abortion of an indusium previously present. Such facts afford ample 

 material useful for phyletic seriation (Chapter XIl). 



(8) The characters of the sporangium are more important than those of 

 the sorus. The sporangium shows consistent reduction in size, with increasing 

 specialisation of the mechanism of dispersal, as we progress from the Palaeo- 

 zoic to modern types. Moreover the form, length of stalk, and the position 

 of the annulus and stomium vary in close relation to the constitution of the 

 sorus. The ends of the series, viz. the Eusporangiate and the advanced Lepto- 

 sporangiate types, are strongly dissimilar, but they grade one into the other 

 by most gentle steps, which give plentiful phyletic material (Chapter XIII). 



(9) At the back of this gently graded series are the facts of spore-output 

 from each sporangium. The gradual diminution of the spore-number from 



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