332 GENERAL REVIEW OF THE PRIMITIVE FERNS [ch. 



how the living Marattiaceae occupy a broad belt including both Eastern and 

 Western Tropics. But the fossil record demonstrates that in the Mesozoic 

 Period they extended farther northward, in a manner curiously similar to 

 the Schizaeaceae. The result of such comparisons is to show that the leading 

 types of these relatively primitive Ferns are now more restricted geographi- 

 cally than in earlier times, a fact which accords ill with the theory of "Age 

 and Area" of Willis. The geological record proves them not only to have 

 been of ancient origin, but also that they are now decadent, as indeed the 

 paucity of genera and species of many of them clearly suggests. Such 

 results are in accord with our general theoretical position. The facts of 

 Palaeontological History, present distribution, Form, Anatomy, dermal 

 appendages, soral position and structure, sporangial detail, and spore-output, 

 all converge as evidence. They all take their part in consolidating the con- 

 clusion that these Ferns, including some Simplices and most of the Gradatae, 

 are properly placed in an intermediate position between the earlier Simplices, 

 which date from the Palaeozoic Age, and the advanced Leptosporangiates 

 that are essentially the Ferns of the Present Day. The latter we assume to 

 have descended from some such original sources, and comparison shows that 

 the assumption is justified. 



In order to visualise, though not unduly to crystallise, these conclusions, 

 the primitive families of Ferns may be plotted roughly into a disconnected 

 scheme, constructed so as to convey some approximate idea of their probable 

 relations. Marginal types are placed to the left, and the superficial types to 

 the right. The Simplices naturally take their place below, where the Palaeozoic 

 types will be found: while the Mesozoic types, many of them Gradatae, are 

 ranged above. This is the length to which the detailed treatment has reached 

 in the present Volume. But it may be permitted to cast a preliminary glance 

 onwards to the great mass of the Leptosporangiate Ferns, which with their 

 multifarious forms, so rich in genera and species, constitute the chief Fern- 

 Flora of the present period. It will be found that these may be grouped 

 along natural lines, sometimes clearly marked but at others uncertain and 

 obscure, into six large phyla, which are (in the most general way, and with 

 reservation as to detail) placed in the phyletic scheme, in their probable 

 relation to the earlier and more primitive families. These phyla cannot be 

 discussed, defined, or compared at present. It must suffice to indicate, as 

 their names indeed convey, that they centre each round some well-known 

 generic type. The Davallioid Ferns centre round Davallia. They retain their 

 soral identity, and being marginal may be held as Dicksonioid derivatives. 

 The Pteroid Ferns, which centre round the type of Pteris, appear also as 

 Dicksonioid derivatives, and are in the first instance marginal : but they show 

 a strong tendency to a slide of the sorus to the surface of the sporophyll. 

 Moreover in them the sori are apt to be fused into linear sequence, which 



