Theory of Antithetic Alternation of Generations. T 21 
is merely a difference of the direction and degree of development 
of the two branches, and we can pass from one to the other with 
great readiness. A comparison of two such forms as Fucus and 
Halidrys shows that a passage from an apparently more primitive 
equivalently branched “dichotomous” thallus to an apparently 
monopodial system, in which some of the lateral branches are 
differentiated, has actually occurred, and it is such a passage which 
is hypothecated by the theory under discussion. Direct evidence 
of such a change in the ancestors of the Pteridophytes is not 
available because such ancestors are unknown. The differentiation 
between the specifically assimilating and specifically supporting 
members is already fully established in the most primitive members 
of the Pteridophytic stocks. The “ test of fact ” is no more 
applicable to such a theory than to Professor Bower’s “ enation ” 
theory of the origin of the leaf. If it were, the controversy would 
be at an end. 
It may be admitted, however, that it is unnecessary to burden 
the theory of the origin of the leaf and stem from branches of 
originally equivalent morphological nature with any hypothesis of 
the primitiveness of “ dichotomy,” and Professor Bower’s criticisms 
certainly help to bring this conclusion into relief. The real weak¬ 
ness of the theory of primitive dichotomy appears to the present 
writer to be the absence of evidence that is primitive among the 
Green Algae, where clear monopodial branching of the thallus is 
extremely common, and the elaborateness of the hypotheses 
required to support a theory of the origin of the whole plant-body 
from a dichotomous thallus in the microphyllous Pteridophytee. 
To these considerations must be added Professor Bower’s point 
about the primitively radial construction of the young sporophyte, 
even in Ferns. 
If the primitive sporophyte were originally free-living it is 
conceivable that it was originally dorsiventral in structure and that 
the almost universal radial structure of the young sporophyte of 
the present day has been impressed upon it by its emprisonment 
within the archegonium. To such a view it may, however, be fairly 
objected that the hypothesis is gratuitous, without stronger evidence 
of primitive dorsiventrality than we actually possess. 
We may more easily derive the microphyllous forms with 
whorled or spiral leaves from a radially organised thallus with 
monopodial branching, or with mixed monopodial and dicho¬ 
tomous branching. The exact stage at which the specialised 
assimilating leaf-structures were separated from the supporting 
axial structures it is, of course, impossible to determine. But it 
may be relevantly pointed out that among the Algae there exist an 
abundance of types of branched thallus from which the various forms 
of. morphological construction actually found among the Pteri¬ 
dophytes can be theoretically derived without any series of 
gratuitous assumptions. The fact that these types of thallus 
actually bear sexual organs as well as spores is not, of course, any 
obstacle from the standpoint of believers in the “ homologous ” 
theory of the origin of alternation. 
From this somewhat lengthy discussion of the theory “that 
the axis and leaf are the result of differentiation of an indifferent 
branch-system of which the limbs were originally all alike,” (a view 
which the reviewer still considers as the least open to objection if it 
is freed from the further hypothesis that this primitive branch- 
