32 
A. G. Tansley. 
contradicting the theory of antithetic alternation as applied to that 
group. I confess that there seems to me the plainest possible 
evidence for the antithetic origin of the Bryophytic sporogonium, and 
I cannot conceive of it as a reduced thallus, unless indeed the 
reduction took place in the algal ancestors, which seems an 
unnecessary assumption. We are therefore driven to the belief 
that the alternation of generations in the two groups had a 
perfectly distinct origin, and that the sporogonium of Bryophytes 
is not homologous with the sporophyte of the Pteridophyta. The 
only difficulty involved in such a theory appears to lie in the 
existence of that well-defined organ the archegonium, and of the 
tetrad-development of the aerial spores, in both groups. It is 
possible that the archegonium is homoplastic, but it is also quite 
possible to assume algal ancestors of the more immediate 
predecessor of the Pteridophytes which possessed archegonia as 
their female organs, and zoosporangia or aplanosporangia as 
asexual organs, with no regular alternation of generations, but with 
the power of bearing sporangia or sexual organs according to 
temporary conditions, just as is the case in so many green algae. 
From such ancestors there might have arisen on the one hand the 
sub-terrestrial Protopteridophytes by definite restriction of the 
production of sporangia to the alternate generations, leading to 
“ homologous alternation ” as in Dictyota, and on the other the 
Protobryophytes by a similar restriction of spore-production to the 
fertilised egg, as in Sphceroplea. The zoospores formed by the 
thallus and by the fertilised egg in such forms as CEdogonium and 
Coleochcete are to all appearance identical, and if the one set could 
be transformed into aerial spores under the influence of life in dry 
air, there seems no reason at all why the other set should not 
undergo a similar transformation. According to this view the 
spores of Bryophytes are not directly homologous with those of 
Pteridophytes, but both are homologous with zoospores of Algae, 
and the divergence between the two series of terrestrial plants 
took place before the aquatic life was abandoned. 
Though the existence of our hypothetical archegoniate alga is 
of course the purest speculation, it is interesting to see exactly 
where we are led by a chain of considerations based on the attempt 
to consider the salient facts without prejudice and with a minimum 
of morphological saltation. 
Having now obtained our Protopteridophyte we suppose 
that certain branch systems of the dichotomously branching 
1 cf. Scott, Presidential Address to Section K, Brit. Assoc., 1894. 
