The Origin of the Pteridophyta. 31 
Lignier has evidently been impressed with the separateness of 
the Lycopods from the other Pteridophytes, for while he relates 
the Equisetales and Sphenophyllales to the Ferns, he supposes that 
the Lycopods had an altogether distinct origin, from what he calls the 
“ prohepatic type,” 1 and that their leaves are not modified thallus- 
branches as in the case of the other groups, but elaborated scales 
of a nature similar to the “amphigastria ” of Liverworts. So bold a 
morphological assumption would require a great deal of evidence 
before it could be seriously entertained, and evidence of a kind of 
which none is forthcoming—I mean the evidence of transitional 
forms. There may, perhaps, be another way out of the difficulty, 
but before considering it let us return to the hypothetical Proto- 
pteridophyte, towards which the prevalence of the dichotomous 
habit in both stem and leaf in most of the more primitive 
known forms appears to point. 
In all known Pteridophytes the leaf and stem are of course 
very sharply differentiated from one another, as assimilating and 
spore-bearing, and as supporting and conducting organs respectively, 
but both in Algae and Liverworts there is the clearest evidence in 
certain cases of transition from undifferentiated thallus-branches 
to definitely specialised, expanded, assimilating organs, borne by 
sympodia of stem-like thallus-segments, and there appears to be no 
theoretical objection whatever to supposing that such a transition 
occurred in the ancestors of the Ferns. Of course such ancestors 
could not have been Liverworts, but specialisation of the kind 
described is just as likely to have occurred in a sporophyte as in a 
gametophyte. We are now face to face with the old, much 
debated problem of the origin of alternation of generations. 
The existence of a form like Dictyota, in which two mor¬ 
phologically identical generations, the one bearing sexual organs 
and the other bearing tetrasporangia, follow one another in 
regular alternation, is sufficient to demonstrate the possibility of 
“ homologous” alternation of generations arising within the Algae. 
If we imagine such a form to become sub-terrestrial, its spores 
becoming adapted to aerial dispersion, its thallus-branches becoming 
specialised into stem and leaf, while its sexual generation is reduced 
in vegetative development, we have a practicable ancestor of the 
Pteridophytes, so far as the main features of the plant-body and 
of the life-history are concerned. Such an. ancestor, however, 
cannot be conceived of as also giving rise to the Bryophyta without 
1 Op. cit. 
