Genesis of Species
Mr. Spencer supports his argument by the non-segmentation of the anterior end of the skeletal axis, i.e. by the non-segmentation of the skull. But in fact the skull is segmented, and, according to the quasi-vertebral theory of the skull put forward by Professor Huxley, is probably formed of a number of coalesced segments, of some of which the trabeculae cranii and the mandibular and hyoidean arches are indications. What is, perhaps, most remarkable however is, that the segmentation of the skull—its separation into the three occipital, parietal, and frontal elements—is most complete and distinct in the highest class, and this can have nothing, however remotely, to do with the cause suggested by Mr. Spencer.
Thus, then, there is something to be said in opposition to both the aggregational and the mechanical explanations of serial homology. The explanations suggested are very ingenious, yet repose upon a very small basis of fact. Not but that the process of vertebral segmentation may have been sometimes assisted by the mechanical action suggested.
It remains now to consider what are the evidences in support of the existence of an internal power, by the action of which these homological manifestations are evolved. It is here contended that there is good evidence of the existence of some such special internal power, and that not only from facts of comparative anatomy, but also from those of teratology and pathology. These facts appear to show, not only that there are homological internal relations, but that they are so strong and energetic as to re-assert and re-exhibit themselves in creatures which, on the Darwinian theory, are the descendants of others in which they were much less marked. They are, in fact, sometimes even more plain and distinct in animals of the highest types than in inferior forms, and, moreover, this deep-seated tendency acts even in diseased and abnormal conditions.
Mr. Darwin recognizes these homological relations, and does “not doubt that they may be mastered more or less completely by Natural Selection.” He does not, however, give any explanation of these phenomena other than the imposition on them of the name “laws of correlation;” and indeed he says, “The nature of the bond of correlation is frequently quite obscure.” Now, it is surely more desirable to make use, if possible, of one conception than to imagine a number of, to all appearance, separate and independent “laws of correlation” between different parts of each animal.
But even some of these alleged laws hardly appear well founded. Thus Mr. Darwin, in support of such a law of concomitant variation as regards hair and teeth, brings forward the case of Julia Pastrana, and a man of the Burmese Court, and adds, “These cases and those of the hairless dogs forcibly call to mind the fact that the two orders of mammals, namely, the Edentata and Cetacea, which are the most abnormal in their dermal covering, are likewise the most abnormal either by deficiency or redundancy of teeth.” The assertion with regard to these orders is certainly true, but it should be borne in mind at the same time that the armadillos, which are much more abnormal than are the American anteaters as regards their dermal covering, in their dentition are less so. The Cape ant-eater, on the other hand, the Aard-vark (Orycteropus), has teeth formed on a type quite different from that existing in any other mammal; yet its hairy coat is not known to exhibit any such strange peculiarity. Again, those remarkable scaly ant-eaters of the Old World—the pangolins (Manis)—stand alone amongst mammals as regards their dermal covering; having been classed with lizards by early naturalists on account of their clothing of scales, yet their mouth is like that of the hairy ant-eaters of the New World. On the other hand, the duck-billed platypus of Australia (Ornithorhynchus) is the only mammal which has teeth formed of horn, yet its furry coat is normal and ordinary. Again, the Dugong and Manatee are dermally alike, yet extremely different as regards the structure and number of their teeth. The porcupine also, in spite of its enormous armature of quills, is furnished with as good a supply of teeth as are the hairy members of the same family, but not with a better one; and in spite of the deficiency of teeth in the hairless dogs, no converse redundancy of teeth has, it is believed, been remarked in Angora cats and rabbits. To say the least, then, this law of correlation presents numerous and remarkable exceptions.
To return, however, to the subject of homological relations: it is surely inconceivable that indefinite variation with survival of the fittest can ever have built up these serial, bilateral, and vertical homologies, without the action of some special innate power or tendency so to build up, possessed by the organism itself in each case. By “special tendency” is meant one the laws and conditions of which are as yet unknown, but which is analogous to the innate power and tendency possessed by crystals similarly, to build up certain peculiar and very definite forms.
First, with regard to comparative anatomy. The correspondence between the thoracic and pelvic limbs is notorious. Professor Gegenbaur has lately endeavoured to explain this resemblance by the derivation of each limb from a primitive form of fin. This fin is supposed to have had a marginal external (radial) series of cartilages, each of which supported a series of secondary cartilages, starting from the inner (ulnar) side of the distal part of the supporting marginal piece. The root marginal piece would become the humerus or femur, as the case might be: the second marginal piece, with the piece attached to the inner side of the distal end of the root marginal piece, would together form either the radius and ulna or the tibia and fibula, and so on.
Now there is little doubt (from a priori considerations) but that the special differentiation of the limb bones of the higher Vertebrates has been evolved from anterior conditions existing in some fish-like form or other. But the particular view advocated by the learned Professor is open to criticism. Thus, it may be objected against this view, first, that it takes no account of the radial ossicle which becomes so enormous in the mole; secondly, that it does not explain the extra series of ossicles which are formed on the outer (radial or marginal) side of the paddle in the Ichthyosaurus; and thirdly, and most importantly, that even if this had been the way in which the limbs had been differentiated, it would not be at all inconsistent with the possession of an innate power of producing, and an innate tendency to produce similar and symmetrical homological resemblances. It would not be so because resemblances of the kind are found to exist, which, on the Darwinian theory, must be subsequent and secondary, not primitive and ancestral. Thus we find in animals of the eft kind (certain amphibians), in which the tarsus is cartilaginous, that the carpus is cartilaginous likewise. And we shall see in cases of disease and of malformation what a tendency there is to a similar affection of homologous parts. In efts, as Professor Gegenbaur himself has pointed out, there is a striking correspondence between the bones or cartilages supporting the arm, wrist, and fingers, and those sustaining the leg, ankle, and toes, with the exception that the toes exceed the fingers in number by one.
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 Plesiosauria—those extinct marine reptiles of the Secondary period, with long necks, small heads, and paddle-like limbs—are of yet higher organization than are the efts and other Amphibia. Nevertheless they present us with a similarity of structure between the fore and hind limb, which is so great as almost to be identity. But the Amphibia and Plesiosauria, though not themselves primitive vertebrate types, may be thought by some to have derived their limb-structure by direct descent from such. Tortoises, however, must be admitted to be not only highly differentiated organisms, but to be far indeed removed from primeval vertebrate structure. Yet certain tortoises (notably Chelydra Temminckii) exhibit such a remarkable uniformity in fore and hind limb structure (extending even up to the proximal ends of the humerus and femur) that it is impossible to doubt its independent development in these forms.
Again in the Potto (Perodicticus) there is an extra bone in the foot, situated in the transverse ligament enclosing the flexor tendons. It is noteworthy that in the hand of the same animal a serially homologous structure should also be developed. In the allied form called the slow lemur (Nycticebus) we have certain arrangements of the muscles and tendons of the hand which reproduce in great measure those of the foot and _vice versa_. And in the Hyrax another myological resemblance appears. It is, however, needless to multiply instances which can easily be produced in large numbers if required.
Secondly, with regard to teratology, it is notorious that similar abnormalities are often found to co-exist in both the pelvic and thoracic limbs.
M. Isidore Geoffroy St. Hilaire remarks, “L’anomalie se repete d’un membre thoracique au membre abdominal du meme cote.” And he afterwards quotes from Weitbrecht, who had “observe dans un cas l’absence simultanee aux deux mains et aux deux pieds, de quelques doigts, de quelques metacarpiens et metatarsiens, enfin de quelques os du carpe et du tarse.”
Professor Burt G. Wilder, in his paper on extra digits, has recorded no less than twenty-four cases where such excess coexisted in both little fingers; also one case in which the right little finger and little toe were so affected; six in which it was both the little fingers and both the little toes; and twenty-two other cases more or less the same, but in which the details were not accurately to be obtained.