MIXTAE A HETEROGENEOUS GROUP 623 
these conclusions have been shown to be supported by the facts of anatomy. 
It must, therefore, be allowed that those Ferns which are associated under 
the. general heading of “Mixtae” are relatively late derivative forms, 
and that they do not constitute a natural group, any more than do those 
plants which are heterosporous or those which produce seeds. It would 
then seem desirable to proceed at once to divide this heterogenous group 
into true phyletic sequences. But to do this requires much greater com- 
mand of facts, and especially of those of anatomy, than is at present 
available. Here it must suffice to recognise the unsatisfactoriness of the 
present position, and at the same time to give a very few general indications 
of the form the future system may take. 
It would appear probable that the main bulk of the Mixtae have been 
derived along a line where the sori were marginal, with the Dennstaedtiinae, 
the Davalliinae, and Pterideae of Prantl as early representatives of it. This 
was accompanied by transition through the solenostelic to the dictyostelic 
structure of the stem. A gradual shifting of the sorus to the under surface 
of the leaf also occurred, till the condition was reached as seen in the 
Aspidiinae and Aspleniinae of Prantl. Certain forms allied to these, losing 
their indusium altogether, constituted one section of the old comprehensive 
genus Folypodium. All through the more advanced members of this 
sequence the dictyostelic structure of the stem was maintained. Another 
contingent, with very similar final result, probably arose from forms with 
superficial sori and basal indusium, allied to the Cyatheae: in this also 
the dictyostelic structure.is seen. A third series, also with superficial sori, 
is represented by the Gleichenia-Matonia-Difteris line: it is true that 
Dipteris is at present the only recognised representative of this sequence 
which has attained to the rank of the Mixtae, and it has consequently 
been removed from its old position in Folypodium. It seems, however, 
not improbable that future investigations may add fresh contingents from 
the ranks of Polypodium, and possibly from some other genera, and one 
useful criterion will be found in the stem-structure, for in the recognised 
-forms it is persistently solenostelic. 
The attempt will not be made at present to assort all the remaining 
forms of Polypodiaceous Ferns into probable phyletic sequences: they are 
left to be dealt with as knowledge increases. Meanwhile the general view 
of them will be as of a brush of diverging phyletic lines, which have proved 
blind. In fact, the ultimate fulness of development of the Homosporous 
Ferns is that which is before us to-day. 
