Alternation of Generations based on Ontogeny, n 
It will be useful in conclusion to anticipate the detailed 
application of the theory, by a very brief survey of the broad features 
of the two generations in the three great groups of homosporous 
Vascular Cryptogams in which the life-history is known, the Ferns, 
the Lycopods and the Horsetails. 
It is a remarkable fact that the general form of the prothallus 
is very uniform throughout the Ferns. This is important because 
we have in this group a wide ground of comparison, which is lacking 
in the homosporous Lycopods and in the Equisetacese. The flattened 
form with a distinction of midrib and wings, the occurrence of 
occasional dichotomous branching and less commonly of definite 
lateral lobing, and the position of the sexual organs on the under 
surface are characteristic. There is a considerable body of evidence 
from various sources, which points to the possibility of instituting a 
comparison between the fern leaf and a complete branch or 
branch-system of the fern prothallus. I shall not enter here into 
the bearing of this on the nature of the axis in the fern sporophyte, 
but it appears that it is in the comparison of the fern prothallus 
and fern leaf, that the homology between the two generations is 
most apparent in the group. The sporangia may perhaps be 
regarded as corresponding to the sexual organs on the prothallus, 
and in support of this the correspondence in position and the parallel 
in the various groups between the antheridia and sporangia in bulk 
and mode of development may be pointed out. On this view the 
sporophyte of the fern would be primitively megaphyllous and 
composed of a system of structures, each of which is comparable 
to a more or less branched fern prothallus. 
The prothalli of the Lycopods and Equisetum differ in important 
details which need not now be considered. They resemble one 
another, however, and differ from the fern prothallus in one respect 
which has hardly been given the weight it deserves. I have else¬ 
where given my reasons for regarding the green prothalli of the 
type of Lycopodium cernuum and L. inundatum as relatively primitive 
in the Lycopodiaceae. These and the prothallus of Equisetum consist 
of a main portion bearing definite lobes, which are developed in 
regular succession on the upper surface, and are not merely marginal. 
These lobes are most strikingly seen in large, well-grown prothalli. 
The sexual organs may be seated at the bases of these lobes in 
more or less regular relation to them. It is an equally striking and 
characteristic feature of the sporophyte in both these groups that 
the leaves are small in proportion to the stem and are often simple. 
A comparison of the prothallus and plant in Lycopodium was 
