The life-history already described at the opening of this article holds for ferns generally, so that the grouping and natural classi fication of the class must depend upon differences of detail other than the life-history itself. A general comparison of them led long ago to the recognition of eight main families, which may here be placed in the reverse order to that first given by Mettenius (1856) :— I. Ophioglossaceae. V. Gleicheniaceae.
II. Marattiaceae. VI. Hymenophyllaceae.
III. Osmundaceae. VII. Cyatheaceae.
IV Schizaeaceae, VIII. Polypodiaceae.
This grouping in linear sequence places the more robust types first, and the more delicate last, while the rest take middle posi tions_ The former have been styled by Von Goebel the Euspo rangiate ferns, in which the sporangium is from the first a massive body, in the formation of which many cells co-operate; in the latter each sporangium arises from a single cell, and those ferns in which this is so were styled Leptosporangiate. Intermediate states exist, and these suggest that the whole series constitutes an evolutionary progression. If this be true the question arises which is the more primitive, and which the more advanced state? The importance of this question is enhanced by the fact that the sporangium is a mere index of a general difference of organization of the two contrasted types. In point of fact the Eusporangiate ferns are relatively robust in their general constitution, while the Leptosporangiate are relatively delicate. Thus the question is whether there has been in the course of evolution a progression from a robust to a delicate state, or the reverse Since the Eusporangiate ferns find their correlatives in the fossils of Palaeo zoic time and are relatively few to-day, while the specialized Leptosporangiate ferns are absent from the Palaeozoic rocks and comprise the bulk of living ferns, it is concluded that the general progression has been from a more robust ancestry towards a more delicate and precise constitution.
Having perceived this general scheme of progressive refinement, it cannot be assumed that the r so genera, and 6,000 species have formed a simple sequence. In testing the question of their re lationships it will become necessary to revise the methods in use by systematists, whose aims were primarily classification. They worked as a rule upon few criteria of comparison, drawn almost exclusively from the sporophyte generation. A more exact com parison will be necessary not only as regards external form, but also of internal structure and development, both of the vegetative and the propagative organs, and it must be extended to both generations. The larger the number of the criteria used in com parispn the more trustworthy will be the conclusions drawn from them. The criteria currently used in the comparison of ferns
are these: I. The external morphology of the shoot.
2. The initial constitution of the plant-body as indicated by segmentation.
3. The architecture and venation of the leaf.
4. The vascular system of the shoot.
5. The dermal appendages.
6. The position and structure of the sorus.
7. The indusial protections.
8. The characters of the sporangium, and the form and markings of the spores.
9. The spore-output.
1o. The morphology of the prothallus.
The position and structure of the sexual organs. 12. The embryology of the sporophyte.
By the combined use of these criteria it has been possible to revise the natural groupings of ferns, sometimes amending but more often upholding the decisions of the earlier systematists. The main conclusions may be stated as follows: The Euspo rangiatae include the living Ophioglossaceae, Marattiaceae and Osmundaceae, together with the fossil Coenopteridaceae (see PALAEOBOTANY). All these are Palaeozoic types, though they overlapped into the Mesozoic, and some representatives have even survived to the present day. With them are to be associated also the Schizaeaceae, Gleicheniaceae and Matoniaceae, all of which figured prominently in the Mesozoic, and are well repre sented among living ferns. In addition to many archaic features of the vegetative system they all possess relatively massive sporangia, which originate simultaneously, being produced either singly or in small numbers in the sori. They are collectively styled Simplices. Each sporangium has a relatively large spore-output.
The Ophioglossaceae and Marattiaceae appear to have ended blindly and left no further derivatives. But derivative phyla may be traced by comparison from each of the Schizaeaceae, Osmun daceae, and Gleicheniaceae, while to the Matoniaceae so closely allied to the Dipterids, another phylum may be ascribed. The Schizaeaceae with their solitary marginal sporangia lead to the marginal Dicksoniaceae. The Osmundaceae have many features in common with Plagiogyria, while the superficial Gleicheniaceae link on to the Cyatheaceae, these being distinguished by their superficial sori from the marginal Dicksoniaceae. In both of these last-named families the sorus has become "gradate," i.e., li after the distal sporangia have been formed on the receptacle, a sequence of further sporangia follows in basipetal sequence, the effect of which is that the drain of nutrition is spread over a longer period of time. This may well have been a real factor in the success of these families.