268 SUMMARY OF RESULTS [ch. 



successful Class, the most successful in fact of all those with homosporous 

 propagation. Probably their conservatism in point of their method of fertilisa- 

 tion restricted them, as confirmed amphibians, to those habitats where 

 syngamy through external water could be efficiently carried out. A special 

 interest which they present is then in seeing how, subject to this restriction, 

 they have been able in a long evolutionary history to make the best of an 

 obsolete method. 



In drawing to a close this preliminary sketch of the relation of the two 

 generations to the environment, it is best to admit at once that the weight 

 of comparison has necessarily fallen upon the sporophyte. This is partly due 

 to its higher complexity providing a more ample field of fact, showing greater 

 phyletic inertia, and with progressive sequences standardised relatively late: 

 but it also arises from the fact that the simpler gametophyte is highly plastic 

 in its vegetative structure, while the sexual organs, and particularly the 

 archegonium, were standardised relatively early. Partly, however, or even 

 mainly it is due to the deficiency of specific fact : for not only is the knowledge 

 of the gametophyte of the fossils practically a blank, but the same may be 

 said for many of the advanced Leptosporangiates, owing to the difficulty or 

 neglect of cultivation; but more particularly to the lack of accurately deter- 

 mined material collected in the field. This weakness may be overcome by 

 time and care, but none the less it exists now. 



On the other hand, if the summary of phyletic results that follows is to be 

 concise, much of the varied detail on which conclusions have been based 

 must needs be omitted, and attention focused on the most consistent com- 

 parative features. These are now, as they have always been, those of the 

 sori and sporangia. That such evidence as they afford does not stand alone 

 may readily be seen in any specific case by reference to the text of Vols. II 

 and III : and this will frequently be aided by citations of Chapter or Page. 



Summary of Phyletic Grouping. 

 There are two ways in which we may attain some conception of the primal 

 representatives of the Filicales : either by direct observation of early fossils, 

 or by the comparative study of those living Ferns which we have reason to 

 hold as themselves primitive. The best results may, however, be expected to 

 follow from a combination of both. This was attempted in Vol. I, Chapter XVII, 

 where, on the basis of a wide comparison, an archetype of the sporophyte for 

 the Class of Ferns was sketched as a word-picture. It would consist of a simple 

 upright shoot of radial symmetry, possibly rootless,dichotomising if it branched 

 at all, and with the distinction of axis and leaf ill-defined. The leaf, where 

 recognisable as such, long-stalked with distal dichotomy, tending in advanced 

 forms towards sympodial dichotomy. All the limbs of the dichotomy would be 

 narrow, and separate from one another. The whole plant would be relatively 



