Thirdly, and lastly, as regards such serial homology as is exemplified by the backbone of man, there are also several objections to Mr. Spencer's mechanical explanation.
On the theory of evolution most in favour, the first Vertebrata were aquatic. Now, as natation is generally effected by repeated and vigorous lateral flexions of the body, we ought to find the segmentation much more complete laterally than on the dorsal and ventral aspects of the spinal column. Nevertheless, in those species which, taken together, constitute a series of more and more distinctly segmented forms, the segmentation gradually increases all round the central part of the spinal column.
Mr. Spencer[[173]] thinks it probable that the sturgeon has retained the notochordal (that is, the primitive, unsegmented) structure
because it is sluggish. But Dr. Günther informs me that the sluggishness of the common tope (Galeus vulgaris) is much like that of the sturgeon, and yet the bodies of its vertebræ are distinct and well-ossified. Moreover, the great salamander of Japan is much more inert and sluggish than either, and yet it has a well-developed, bony spine.
I can learn nothing of the habits of the sharks Hexanchus, Heptanchus, and Echinorhinus, but Müller describes them as possessing a persistent chorda dorsalis.[[174]] It may be they have the habits of the tope, but other sharks are amongst the very swiftest and most active of fishes.
In the bony pike (lepidosteus), the rigidity of the bony scales by which it is completely enclosed must prevent any excessive flexion of the body, and yet its vertebral column presents a degree of ossification and vertebral completeness greater than that found in any other fish whatever.
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,[[175]] is probably formed of a number of coalesced segments, of some of which the trabeculæ 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[[176]] 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.