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 srptation 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 



