DIFFICULTIES OF ANALYSIS 129 



is apparent externally, for in this 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 be.en 

 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 js 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 



