154 BOTANICAL GAZETTE [SEPTEMBER 
form of its gametophyte body, a third to the structure of its 
sporophyte body. Any attempt to relate these to one another 
upon the basis of a single structure, even so important a one as 
' the sporogonium, is essentially misleading. But when we con- 
sider the totality of structure, we are led to the opinion that 
these lines possibly diverged from an archetypal plexus in which 
there were gametophyte bodies as simple as that of Aneura, and 
sporophyte bodies as simple as that of Riccia. Another illus- 
tration is the recent attempt of Arnoldi to associate Isoetes with 
Selaginella largely upon the basis of endosperm development, 
without regard to great diversities in habit and anatomical 
details. The association may be perfectly proper, but the 
reason given for it is inadequate. 
In dealing with problems of phylogeny it is also important 
to remember that the origin of a prominent group of living 
forms from another group of living forms is extremely improb- 
able. We can point out resemblances in structures which we 
have come to regard as essential, but this is not likely ta mean 
the origin of the one group from the other. It may mean that 
the two groups can be traced to one, probably now extinct, 
which combined the characters now differentiated. Most living 
groups are best regarded as divergent rather than consecutive 
series. 
But even this apparently sure ground has become very 
uncertain from the fact, becoming more and more apparent, that 
similar changes in structure, even very important ones, may have 
appeared independently in different lines. The response of 
organisms in structure to their environment is deeper seated 
than we were once inclined to believe, and testimony from the 
similarity of certain structures, when contradicted by the major- 
ity of other structures, argues feebly for recent community of 
origin. Such similarities in structure argue more for physiolog- 
ical conditions than for phylogeny. For instance, from the 
standpoint of evolution, the appearance of heterospory among 
the pteridophytes is one of the most important contributions to 
plant progress made by the group, but it is impossible to escape 
