The simplest fish possessing a skeleton, has a cranium composed of cartilage that is not segmented at all! Besides being inconsistent with the leading truths of Embryology and Comparative Morphology, the hypothesis of Goethe and Oken is inconsistent with itself. The facts brought forward to show that there exists an archetypal vertebra, and that the vertebrate animal is composed of archetypal vertebræ arranged in a series, and severally modified to fit their positions--these facts, I say, so far from proving as much, suffice, when impartially considered, to disprove it. No assigned, nor any conceivable, attribute of the supposed archetypal vertebra is uniformly maintained. The parts composing it are constant neither in their number, nor in their relative positions, nor in their modes of ossification, nor in the separateness of their several individualities when present. There is no fixity of any one element, or connexion, or mode of development, which justifies even a suspicion that vertebræ are modelled after an ideal pattern. To substantiate these assertions here would require too much space, and an amount of technical detail wearisome to the general reader. The warrant for them will be found in a criticism on the osteological works of Prof. Owen, originally published in the _British and Foreign Medico-Chirurgical Review_ for Oct. 1858. This criticism I add in the Appendices, for the convenience of those who may wish to study the question more fully. (See Appendix B.) Everything, then, goes to show that the segmental composition which characterises the apparatus of external relation in most _Vertebrata_, is not primordial or genetic, but functionally determined or adaptive. Our inference must be that the vertebrate animal is an aggregate of the second order, in which a relatively superficial segmentation has been produced by mechanical intercourse with the environment. We shall hereafter see that this conception leads us to a consistent interpretation of the facts--shows us why there has arisen such unity in variety as exists in every vertebral column, and why this unity in variety is displayed under countless modifications in different skeletons.[31]