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 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 
