Ill] THE SEARCH FOR NEW CRITERIA 59 



The Criteria of Comparison 



The Sporophyte, or Fern-Plant, is the most prominent phase of the Hfe- 

 cycle, and it provides the most marked features which are used in comparison. 

 But the alternate sexual phase, or Prothallus,must also be considered. Certain 

 characters which it shows will be brought into the comparative treatment, 

 though these are less reliable than those of the sporophyte. Theoretically 

 all possible features should be regarded as criteria of comparison, and used 

 in drawing conclusions as to relationship and descent. But the relative 

 importance to be attached to each will vary according to its constancy, and 

 to the directness of its bearing on the physiological success of the organism. 

 Such weighing of evidence from the widest possible sources forms the proper 

 basis for classification according to a Natural System. If due value be allowed 

 to the various lines of evidence, the result should be the disposition of the 

 members of a family or group according to their relation by descent. Con- 

 venience of arrangement should only find its place in such a method in default 

 of sufficient evidence from comparison. Where the knowledge of detail is 

 sufficient it should be possible to place not only groups and families, but 

 genera and even species according to their natural affinities. This is the 

 theoretical end of a Natural Classification. 



The first step in the present case will be to ascertain for the Filicales 

 what are the criteria upon which comparison may be based. It is desirable 

 to use those available from both of the alternating generations. In the main 

 the progressions traced should run substantially parallel in respect of the 

 several criteria if their recognition be valid and true. The more nearly this 

 proves to be the case the more solid and reliable will be the conclusions drawn 

 from them. Any apparent discrepancy may have arisen from one or another 

 of various sources. Of these the most important are (i) that the facts may 

 have been erroneously observed: (ii) that they may have been wrongly 

 interpreted : (iii) that the advance may not have been synchronous in the 

 evolution of the plants compared in respect of the various criteria: (iv) that 

 the suggested relationship may have been too wide to constitute part of a 

 real sequence : or (v) that the facts are themselves deficient. Such difficulties 

 are liable to arise in all sequences, but most frequently in the case of the 

 fossils. It cannot then be anticipated that the phyletic conclusions arrived 

 at by such comparison will in all cases be definite or final. To attain finality 

 presumes a completeness of knowledge greater than we can reasonably expect 

 at present. The limitation of fact and the uncertainty of interpretation make 

 it impossible to advance such conclusions beyond the point of reasonable 

 probability. But though a final grouping based on comparison, which is the 

 theoretical end, may not be actually attained at the moment, the method 



