12 SCOPE OF COMPARATIVE MORPHOLOGY 



can readily be found in the vegetative and propagative parts of plants both 

 high and lower in the scale : it may affect not only the modification of 

 parts already present, but also the origin of new parts. As prominent 

 examples which will be discussed at length later, the polyphyletic origin 

 of leaves, of heterospory, and of the seed-habit may be quoted This 

 frequent occurrence of parallel development should serve as a check on 

 the too ready acceptance of conclusions based on mere formal comparison, 

 and it shows that it is necessary to be sure of the phyletic unity of a 

 series before sound conclusions can be arrived at from comparison of 

 its components. 



It may be useful to quote a specific case of fallacious reasoning based 

 on comparisons which are not within one phyletic unity. It is possible 

 to compare the sporangia of Calamostachys with those of Selaginella, of 

 Isoetes, and of the Hydropterideae, as examples of heterospory : and general 

 conclusions might be drawn from such comparison as to the progressive 

 steps of the heterosporous differentiation. But these plants are now 

 recognised as representing three (and possibly even four) distinct phyla, 

 all of which include homosporous forms. The latter fact indicates that 

 heterospory arose after the differentiation of those phyla. It is therefore 

 impossible to argue correctly from one phylum to another as to the 

 course which a common spore-differentiation has taken, since its course 

 must have been distinct in each case from the others. The most that 

 can be properly attained is an analogy between the separate progressions 

 as seen in those several phyla. 



It is plain then that organic nature is not self-explanatory, and that 

 Comparative Morphology is a study beset with pitfalls. There is uncertainty, 

 first, in the recognition of true evolutionary sequences : still more in 

 their interpretation as ascending, descending, or divergent : and again 

 in the connecting 1 of these sequences together so as to construct some 

 more or less consecutive story of descent : indeed, this can only be done 

 when liberal use is made of the imagination, in bridging over the wide 

 gaps in the series, which even the known fossils are so far from filling. 

 The details of a story thus constructed depend so largely on comparative 

 opinion, or balancing of probabilities, and in so slight a degree upon 

 positive demonstration that the history as told by competent experts in 

 Comparative Morphology may vary in material features. A little more 

 weight allowed for certain observed details, or a little less for others, 

 will be sufficient to disturb the balance of the evidence derived from a 

 wide field of observation, and consequently to distort the historical 

 picture. In the absence of more full "documentary" evidence from the 

 fossils there is in truth no finality in discussions on the genesis and 

 progress of organic life. But as long as the human mind has the power 

 of and inclination towards enquiry, so long will such discussions con 

 tinue, together with their kaleidoscopic changes of opinion. Every new 

 fact of importance will in some degree affect the weight accorded to 



