1 
W. H. Lang. 
In the first place we have the antithetic theory, the essential 
feature of which lies in its recognition of the spore-producing 
generation as an interpolated phase in the life-history, originating 
in post-sexual complications, and in its not regarding this generation 
as derived by the modification of a generation resembling the 
sexual one. On this theory the spore-producing generation has 
had a history on lines of its own, which are suggested by such 
a series of forms as Oedogonium, Coleochaete , Riccia, the more 
complicated sporogonia of the Bryophyta, and the sporophyte of 
Pteridophyta. These are recognised by the advocates of the 
antithetic theory as not phyletically related. 
The alternative view that the spore-producing generation in 
Bryophyta and Pteridophyta is strictly homologous with the sexual 
generation, and that the alternation in these groups differs only in 
its regularity from the succession of sexual and asexual forms in 
many Thallophyta, was advanced by Pringsheim, and, as mentioned 
above, restated by Scott. The homologous theory in this form was 
not applied in detail to the origin of the sporophyte, as we find it in 
Bryophytes and Pteridophytes, nor to the tracing of particular 
homologies between the two generations. 
The theories of Potonie, Lignier and Tansley, while differing 
widely among themselves, agree in asserting the homology of the 
two generations. In speculating on the origin of the sporophyte of 
Pteridophyta they make use of wide comparisons with the 
organisation of the gametophyte in other groups, especially in the 
Brown Algae and Liverworts. 
The three classes of theories which have been distinguished 
above may all be termed purely phylogenetic. In all of them 
little or no account is taken of the relations between the two 
generations in the ontogeny, and the comparisons with the gameto¬ 
phyte which are made ( e.g . by Tansley and Lignier) in order to 
throw light on the morphology of the sporophyte frequently omit 
any consideration of the gametophyte of the particular form in 
question, and certainly attach no special importance to this. 
A comparison between the two generations in Lycopodium 
cernuum was made by Treub in 1884, and is referred to by Scott in 
his address. A provisional and rather crude hypothesis of the 
origin of a homologous spore-bearing generation in the Ferns, 
advanced by the author in 1898, was based on comparison of the 
two generations in that group in the light of the facts of apogamy. 
Recent work indicates that the special significance of comparisons 
between the two generations in the same group is being more 
