IO 
W H. Lang. 
specific cell. Whether the change in the specific cell is manifested 
in one or both generations, will depend on the conditions necessary 
for this being present. These conditions may be fulfilled in the one 
ontogeny and not in the other. The equivalence of the two germ- 
cells and the homology of the two generations in each species, does 
not preclude the occurrence of changes in the form and structure of 
the sporophyte, while the corresponding gametophyte remains 
apparently unaltered. 
The hypothesis which has now been outlined may be termed 
an ontogenetic theory of alternation to distinguish it from other theories, 
since it specially depends upon a critical consideration of the 
ontogeny of both generations of the organism. In its phylogenetic 
application it is a theory of homologous alternation, though not 
inconsistent with some of the facts associated with the antithetic 
theory. The theory has, for the sake of clearness, been stated with 
as little qualification as possible and without reference to special 
cases. The assumptions that have been made of the equivalence of 
the specific cell in the two generations, of the importance of the 
changed conditions of development of the fertilised egg, brought 
about by its relation to the maternal body, and the probability of 
such an association having taken place once or oftener in an early 
stage of the evolution of land forms, are none of them forced. If 
granted they appear to place the relation of the two generations in 
a new light. 
The test of any hypothesis of alternation lies in the degree to 
which it fits naturally with the detailed facts and explains them. I 
postpone to later papers any full discussion of the application of this 
ontogenetic theory to the great groups of the Bryophyta and Pteri- 
dophyta. These in the first place must be considered severally on 
the assumption that they may have been distinct in origin. This 
will be generally granted as probable for the Bryophyta and Pteri- 
dophyta, and the possibility of independent origin must be extended 
to the main phyla within these great groups. 
In applying the hypothesis outlined above, it should be noted 
that the really important comparisons are between the gametophyte 
and sporophyte of the same plant, or within the same group. Such 
wide comparisons between the two generations as have been made, 
e.g., between the sporophyte of Ferns and the gametophyte of 
Liverworts, may sometimes afford helpful analogies, but are as likely 
to be misleading. The objection to them is not that they are 
between sporophyte and gametophyte, but that they are between 
distant, and probably unrelated, forms, 
