130 VARIATIONS IN NUMBER OF SPORANGIA 
thus came to be that sporangia are bodies essentially indefinite in number 
and variable in place. The habit of interpreting lower forms in terms of 
the higher, which was also prevalent in earlier times, further encouraged 
this view: the common occurrence of indefinite stamens in Angiosperms. 
made unduly familiar the idea of the sporangium-bearing parts, as well 
as sporangia themselves, being irregular in number and in position. But 
now there is good reason to believe that both the Angiosperms and the 
Polypodiaceous Ferns were of relatively late origin, and in no_ sense 
primitive forms. Also that the interpolation of sporangia, or of sporangium- 
bearing parts, which is found in them both, arose secondarily as a con- 
comitant of the increase of the sporangia in either type to high numbers. 
If this be so, then there is no sufficient reason to look upon the sporangia 
of earlier and more definite types as in any way affected by the presence 
of this secondary indefiniteness in number and position. This principle 
finds its application as follows: it may be held that the Lycopods were 
independent of the Ferns in phyletic origin: hence the question of origin 
of their sporangia may be discussed without reference to interpolation at 
all, since it is not a characteristic of that family nor of its relatives. 
Similarly with the Marattiaceous Ferns, interpolation is not seen in the sori 
of any living representative of that family, nor is it demonstrated in their 
fossil prototypes: their sporangia are simultaneous in origin; the fact that 
the relatively recent Leptosporangiate Ferns show frequent interpolation of 
successive sporangia should not affect our view of those Fern-types which 
certainly preceded them in time. In point of fact, interpolation of spor- 
angia is an occasional and not a general phenomenon: it is restricted to. 
certain groups of plants, and is probably of secondary origin: in other 
groups where it has not been observed the sporangia are often seen to 
be stable in number and also in position. Accordingly the problem of 
evolutionary origin becomes more simple in those cases where interpolation 
may be left out of consideration altogether. 
A second case in point, where it is necessary to clear the mind of 
old habits of thought before entering on evolutionary questions, is that of 
synangial sporangia, which are so frequent in the Eusporangiate Pterido- 
phytes. ‘The assumption was formerly common that where this condition 
is seen it is the result of fusion of sporangia previously in the race 
distinct. This assumption is again to be traced to the old habit of 
regarding the Polypodiaceous Ferns as the primitive stock of Pterido- 
phytes: the sporangia in them are all separate; consequently it was held 
that where in other Ferns they are united, this must be a secondary 
condition. But the synangial condition may just as well have resulted 
from septation as from fusion: the question which is the correct view in 
any individual case is difficult to decide, but evidence of some value may 
be obtained by examination of the structure and development, as well as 
by general comparison of allied forms. In approaching such matters 
opinion must be unbiassed and open. Both fusion and septation have 
