[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