DIFFICULTIES OF ANALYSIS 129 
is apparent externally, for in the absence of vestigial parts there would 
be no trace of what had happened. 
The methods of change thus enumerated are, then, the known factors 
which affect the morphological problem of origin and present condition 
of the polysporangiate state. Any one of these, or more, may have been 
represented in the history of descent of any polysporangiate sporophyte 
as it is seen to-day. The examples quoted show that the methods 
enumerated are seen to have been actually operative in definite, living 
instances. Possibly these heads do not exhaust the methods of change 
of the present day nor of the past, and the list is open to additions. 
We are justified in assuming that (subject to the possibility of other 
factors having been operative of which we are yet unaware) the condition 
of any polysporangiate sporophyte as we see it is the resultant of modifi- 
cations such as these operative during its descent. The problem will 
therefore be in each case to assign its proper place in the history to any 
or each of these factors. But in each case the physiological probability 
of any modification which the structure would suggest should be con- 
sidered before it is admitted as part of the evolutionary story. Especially 
is this desirable in determining the probable relative prevalence of modifica- 
tions of increase as against those of decrease. It is only in this way that any 
apparent morphological series can be put upon a convincing footing as 
an evolutionary sequence. In complex cases, however, it may be a matter 
of difficulty to analyse a progressive change, and to decide which of the 
factors enumerated have actually been involved. 
It will be obvious that a complete account, in any given case, of the 
steps which have led to its present polysporangiate state involves a full 
knowledge of its evolutionary history—a knowledge which is far beyond 
present powers. ‘The advantage which an attempt to analyse the factors 
of sporangial modification brings, however imperfectly it may be carried 
out, is to simplify the problem in certain definite cases. For instance, 
if in a whole phylum of living plants a certain mode of sporangial 
increase be unrepresented, and if the related fossils show a similar absence 
of it, then it seems just to hold that that mode of increase may be 
dismissed from consideration in the probable evolutionary history: of that 
phylum. The case of interpolation already alluded to is one in point: 
in connection with this it is necessary to reconsider and examine certain 
old habits of thought which have too long dominated such discussions 
as the present. About the middle of the nineteenth century it was habitually 
maintained that the Polypodiaceous Ferns were primitive forms, and the 
probable progenitors of all other Pteridophytes. So long as this view 
was held interpolation of new sporangia between older ones, which is so 
prominent in them, was regarded as a general phenomenon which might 
appear anywhere among the derivative forms. The fundamental idea 
I 
