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Such symbolism will harmonize with the peculiar reproduction, before mentioned, of heads in the body of certain annelids, with the facts of serial homology, as well as those of bilateral and vertical symmetry.
It is not improbable that, could we arrive at the causes conditioning all the complex inter-relations between the several parts of one animal, we should at the same time obtain the key to unlock the secrets of specific origination. The facts to be considered are the phenomena of "homology," and especially of serial, bilateral, and vertical homology.
I must preface this with a few words upon the homology of the roots of the Rhizocephala, i.e. the tubules which penetrate from its point of adhesion into the body of the host, ramify amongst the viscera of the latter, and terminate in caecal branchlets. I have therefore supposed these appendages to be the rudiments of the future roots.
This homology goes so far within one class, particularly within the class of mammalia, that, for instance, the hands and feet of man, the hands of the ape, the paws of the beast of prey, the hoof of the horse and of the ox, the paws of the mole, the fins of the seal and of the whale, the wing-membranes of the flying-squirrel, correspond to one another in their smallest parts and ossicles, and can all be registered with the same numbers and letters; i.e., they are homologous to one another even to the minutest detail.
Bay Lankester has proposed the term "homoplasy." "Serial homology" is a relation of resemblance existing between two or more parts placed in series one behind the other in the same individual. Examples of such homologues are the ribs, or joints of the backbone of a horse, or the limbs of a centipede. The latter animal is a striking example of serial homology.
The word "homology" indicates such a relation between two parts that they may be said in some sense to be "the same," or at least "of similar nature." This similarity, however, does not relate to the use to which parts are put, but only to their relative position with regard to other parts, or to their mode of origin.
Spencer would explain it as the coalescence of organisms of a lower degree of aggregation in one longitudinal series, through survival of the fittest aggregations. This may be so. It is certainly an ingenious speculation, but facts have not yet been brought forward which demonstrate it. Had they been so, this kind of serial homology might be termed "homogenetic."
Yet these animals are far from being the root-forms from which all the Vertebrata have diverged, as is evidenced from the degree of specialization which their structure presents. If they have descended from such primitive forms as Professor Gegenbaur imagines, then they have built up a secondary serial homology a repetition of similar modifications fully as remarkable as if it were primary.
The pursuit itself is all absorbing, all exacting, and when at last the purpose is attained, and out of darkness flashes the light of some novel law, the knowledge of some new connecting link, some simple explanation of a range of facts or phenomena, or even the discovery of a fresh analogy or homology, or of an undescribed fossil being, the purity of the pleasure which they win is something which to be understood must have been felt.
The fact that living organisms form an ascending series from the less perfect to the more perfect; the further fact that they also form a series according as they display more or less homology of structure and are formed according to similar types; and, lastly, that the fossil remains of organisms found in the various strata of the earth's surface likewise represent an ascending series from the simple to the more complex these three facts suggested to naturalists the thought that living organisms were not always as we find them to-day, but that the more perfect had developed from simpler forms through a series of modifications.
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