The Foot-prints of the Creator

by Hugh Miller

[25] The Silurian Placoids are most adequately represented by the Cestracion of the southern hemisphere; but I know not that of the peculiar character and instincts of this interesting Placoid,—the last of its race,—there is any thing known. For its form and general appearance see fig. 49, [page 177].

[26] Such as the dog-fishes, picked and spotted.

[27] The twelfth in Spinax Acanthius, and the fourteenth in Scyllium Stellare.

[28] It will scarce be urged against the degradation theory, that those races which, tried by the tests of defect or misplacement of parts, we deem degraded, are not less fitted for carrying on what in their own little spheres is the proper business of life, than the non-degraded orders and families. The objection is, however, a possible one, and one which a single remark may serve to obviate. It is certainly true that the degraded families are thoroughly fitted for the performance of all the work given them to do. They greatly increase when placed in favorable circumstances, and, when vigorous and thriving, enjoy existence. But then the same may be said of all animals, without reference to their place in the scale;—the mollusc is as thoroughly adapted to its circumstances and as fitted to accomplish the end proper to its being, as the mammiferous quadruped, and the mammiferous quadruped as man himself; but the fact of perfect adaptation in no degree invalidates the other not less certain fact of difference of rank, nor proves that the mollusc is equal to the quadruped, or the quadruped to man. And, of course, the remark equally bears on the reduced as on the unelevated,—on lowness of place when a result of degradation in races pertaining to a higher division of animals, as on lowness of place when a result of the humble standing of the division to which the races belong.

[29] The vertebral column in the genus Diplopterus ran, as in the placoid genus Scyllium, nearly through the middle of the caudal fin.

[30] In the following diagram a few simple lines serve to exhibit the progress of degradation. Fig. a represents the symmetrical Placoids of the Silurian period, consisting of head, neck, body, tail, fore limbs and hinder limbs; fig. b represents those heterocercal Ganoids of the Old Red Sandstone, Coal Measures, and Permian System, in which the neck is extinguished, and the fore limbs stuck on to the occiput; fig. c, those homocercal Ganoids of the Trias Lias, Oolite, and Wealden, whose tails spread out into broad terminal processes, without homologue in the higher animals; fig. d, those Acanthopterygii of the Chalk that, in addition to the non-homological processes, have both fore limbs and hinder limbs stuck round the head; while fig. e represents the asymmetrical Platessa, of the same period, with one of its eyes in the middle of its head, and the other thrust out to the side.

[31] I would, however, respectfully suggest, that that theory of cerebral vertebræ, on which, in this question, the comparative anatomists proceed as their principle, and which finds as little support in the geologic record from the actual history of the fore limbs as from the actual history of the bones of the cranium, may be more ingenious than sound. It is a shrewd circumstance, that the rocks refuse to testify in its favor. Agassiz, I find, decides against it on other than geological grounds; and his conclusion is certainly rendered not the less worthy of careful consideration by the fact that, yielding to the force of evidence, his views on the subject underwent a thorough change. He had first held, and then rejected it. “I have shared,” he says, “with a multitude of other naturalists, the opinion which regards the cranium as composed of vertebræ; and I am consequently in some degree called upon to point out the motives which have induced me to reject it.”

“M. Oken,” he continues, “was the first to assign this signification to the bones of the cranium. The new doctrine he expounded was received in Germany with great enthusiasm by the school of the philosophers of nature. The author conceived the cranium to consist of three vertebræ, and the basal occipital, the sphenoid, and the ethmoid, were regarded as the central parts of these cranial vertebræ. On these alleged bodies of vertebræ, the arches enveloping the central parts of the nervous system were raised, while on the opposite side were attached the inferior pieces, which went to form the vegetative arch destined to embrace the intestinal canal and the large vessels. It would be too tedious to enumerate in this place the changes which each author introduced, in order to modify this matter so as to make it suit his own views. Some went the length of affirming that the vertebræ of the head were as complete as those of the trunk; and, by means of various dismemberments, separations, and combinations, all the forms of the cranium were referred to the vertebræ, by admitting that the number of pieces was invariably fixed in every head, and that all the vertebrata, whatever might be their organization in other respects, had in their heads the same number of points of ossification. At a later period, what was erroneous in this manner of regarding the subject was detected; but the idea of the vertebral composition of the head was still retained. It was admitted as a general law, that the cranium was composed of three primitive vertebræ, as the embryo is of three blastodermic leaflets; but that these vertebræ, like the leaflets, existed only ideally, and that their presence, although easily demonstrated in certain cases, could only be slightly traced, and with the greatest difficulty, in other instances. The notion thus laid down of the virtual existence of cranial vertebræ did not encounter very great opposition; it could not be denied that there was a certain general resemblance between the osseous case of the brain and the rachidian canal; the occipital, in particular, had all the characteristic features of a vertebra. But whenever an attempt was made to push the analogy further, and to determine rigorously the anterior vertebræ of the cranium, the observer found himself arrested by insurmountable obstacles, and he was obliged always to revert to the virtual existence.

“In order to explain my idea clearly, let me have recourse to an example. It is certain that organized bodies are sometimes endowed with virtual qualities, which, at a certain period of the being’s life, elude dissection, and all our means of investigation. It is thus that at the moment of their origin, the eggs of all animals have such a resemblance to each other, that it would be impossible to distinguish, even by the aid of the most powerful microscope, the ovarial egg of a craw-fish, for example, from that of true fish. And yet who would deny that beings in every respect different from each other exist in these eggs? It is precisely because the difference manifests itself at a later period, in proportion as the embryo develops itself, that we are authorized to conclude, that, even from the earliest period, the eggs were different,—that each had virtual qualities proper to itself, although they could not be discovered by our senses. If, on the contrary, any one should find two eggs perfectly alike, and should observe two beings perfectly identical issue from them, he would greatly err if he ascribed to these eggs different virtual qualities. It is therefore necessary, in order to be in a condition to suppose that virtual properties peculiar to it are concealed in an animal, that these properties should manifest themselves once, in some phase or other of its development. Now, applying this principle to the theory of cranial vertebræ, we should say, that if these vertebræ virtually exist in the adult, they must needs show themselves in reality, at a certain period of development. If, on the contrary, they are found neither in the embryo nor the adult, I am of opinion that we are entitled likewise to dispute their virtual existence.