MUTUAL AFFINITIES AND PHYLOGENY OF PTERIDOPHYTA. Beyond the characteristics which they possess in common, and which have caused them to be collected into one class, there are no special points of contact between the three sub-classes of the Pteridophyta. The classes have each its own well-marked habit ; and it therefore appears probable that they are to be regarded as equivalent branches of the Pteridophyta, springing from a common origin, in each of which a differentiation of homosporous and heterosporous forms has taken place. The relation of the homosporous to the heterosporous forms in each class is a point of interest. There is little or nothing to be said on this subject as regards the Equisetinse. But in the Ftlieinx it appears probable that the simpler heterosporous forms have sprung from the simpler homosporous forms, that is, the Salviniace,se from the Hymenophyllaeex ; and in the Lyeopodinx that Selaginella has sprung from Lycopodium. Assuming that Isoetes belongs to the LycopodMre, its nearest ally would appear to be Phylloglossum ; but this point must remain undecided until the whole life-history, especially the embryogeny, of Phylloglossum is known, and its true systematic position thereby fixed.

Various attempts have been made to trace back the sporophyte of the Pleridophyta to that of the Museimew, the points of contact being the Ilymenophyllaem on the one hand and Auth,oceros on the other ; but these attempts cannot be considered to have been successful. The differences between the known forms of sporophyte in the two classes are too great to be explained away on any hypothesis. The case is different with regard to the gametophyte ; there are, in fact, many points of resemblance between the prothallium of the Pleridophyta and the gametophyte of the Museinex. As Goebel and Bower have pointed out, the prothallium of some species of Trichomanes is a branched filamentous structure, closely resembling Moss protonema ; and in most ferns, as also in Equisetune and Lyeopodium, the prothallium is at first filamentous. The filamentous prothallium gives rise to one or more flattened expansions, on which the sexual organs, or at least the archegonia, are developed (arehegoniophores). Thus there are indications in the gametophyte of many Pteridophyta of that distinction of protonema and sexual shoot which is so characteristic of the gametophyte of the Arimciflex. But this remarkable correspondence in the gametophyte does not warrant the conclusion that the Pteridophyta have sprung from the Ai-ascii/ex. It rather indicates that they are groups having a common origin in the Confervoid Chlorophyeefe, but diverging from the first in the relation of the sporophyte to the gametophyte, so that, whereas in the Micseimets the sporophyte remains attached to the gametophyte throughout its whole existence, in the Pteridophyta the sporophyte develops as an independent organism. There can be little doubt that in the primitive Pteridophyta the two generations resembled each other, and that, as evolution proceeded, they became more and more widely dissimilar, the gametophyte losing, the sporophyte gaining, in both morphological and histological differentiation. An indication of this is still afforded by the general resemblance of the sporophyte of Phylloglossum to the gametophyte of Lyeopodium eernnum as described by Treub, and of the sporophyte of Lycopodium Phlegmaria to its gametophyte (Treub). It will probably be found, when material is obtained for investigating it, that the sporophyte and the gametophyte of Phylloglossum are very similar.

The phylogeny of the Pteridophyta, as suggested in the foregoing remarks, is indicated in the following table : -