Another fact which seems very difficult to reconcile with the hypothesis of an “ideal typical vertebra,” is the not infrequent presence of some of the typical elements in duplicate. Not only, as we have seen, may they severally be absent, but they may severally be present in greater number than they should be. When we see, in the ideal diagram, one centrum, two neurapophyses, two pleurapophyses, two hæmapophyses, one neural spine, and one hæmal spine, we naturally expect to find them always bearing to each other these numerical relations. Though we may not be greatly surprised by the absence of some of them, we are hardly prepared to find others multiplied. Yet such cases are common. Thus the neural spine “is double in the anterior vertebræ of some fishes” (p. 98). Again, in the abdominal region of extinct saurians, and in crocodiles, “the freely-suspended hæmapophyses are compounded of two or more overlapping bony pieces” (p. 100). Yet again, at p. 99, we read—“I have observed some of the expanded pleurapophyses in the great Testudo elephantopus ossified from two centres, and the resulting divisions continuing distinct, but united by suture.” Once more “the neurapophyses, which do not advance beyond the cartilaginous stage in the sturgeon, consist in that fish of two distinct pieces of cartilage; and the anterior pleurapophyses also consist of two or more cartilages, set end on end” (p. 91). And elsewhere referring to this structure, he says:—
“Vegetative repetition of perivertebral parts not only manifests itself in the composite neurapophyses and pleurapophyses, but in a small accessory (interneural) cartilage, at the fore and back part of the base of the neurapophysis; and by a similar (interhæmal) one at the fore and back part of most of the parapophyses” (p. 87).
Thus the neural and hæmal spines, the neurapophyses, the pleurapophyses, the hæmapophyses, may severally consist of two or more pieces. This is not all: the like is true even of the centrums.
“In Heptanchus (Squalus cinereus) the vertebral centres are feebly and vegetatively marked out by numerous slender rings of hard cartilage in the notochordal capsule, the number of vertebræ being more definitely indicated by the neurapophyses and parapophyses.... In the piked dog-fish (Acanthias) and the spotted dog-fish (Scyllium) the vertebral centres coincide in number with the neural arches” (p. 87).
Is it not strange that the pattern-vertebra should be so little adhered to, that each of its single typical pieces may be transformed into two or three?
But there are still more startling departures from the alleged type. The numerical relations of the elements vary not only in this way, but in the opposite way. A given part may be present not only in greater number than it should be, but also in less. In the tails of homocercal fishes, the centrums “are rendered by centripetal shortening and bony confluence fewer in number than the persistent, neural, and hæmal arches of that part”—that is, there is only a fraction of a centrum to each vertebra. Nay, even this is not the most heteroclite structure. Paradoxical as it may seem, there are cases in which the same vertebral element is, considered under different aspects, at once in excess and defect. Speaking of the hæmal spine, Professor Owen says:—
“The horizontal extension of this vertebral element is sometimes accompanied by a median division, or in other words, it is ossified from two lateral centres; this is seen in the development of parts of the human sternum; the same vegetative character is constant in the broader thoracic hæmal spines of birds; though, sometimes, as e.g., in the struthionidæ, ossification extends from the same lateral centre lengthwise—i.e., forwards and backwards, calcifying the connate cartilaginous homologues of halves of four or five hæmal spines, before these finally coalesce with their fellows at the median line” (p. 101).
So that the sternum of the ostrich, which according to the hypothesis, should, in its cartilaginous stage, have consisted of four or five transverse pieces, answering to the vertebral segments, and should have been ossified from four or five centres, one to each cartilaginous piece, shows not a trace of this structure; but instead, consists of two longitudinal pieces of cartilage, each ossified from one centre, and finally coalescing on the median line. These four or five hæmal spines have at the same time doubled their individualities transversely, and entirely lost them longitudinally!
There still remains to be considered the test of relative position. It might be held that, spite of all the foregoing anomalies, if the typical parts of the vertebræ always stood towards each other in the same relations—always preserved the same connexions, something like a case would be made out. Doubtless, relative position is an important point; and it is one on which Professor Owen manifestly places great dependence. In his discussion of “moot cases of special homology,” it is the general test to which he appeals. The typical natures of the alisphenoid, the mastoid, the orbito-sphenoid, the prefrontal, the malar, the squamosal, &c. he determines almost wholly by reference to the adjacent nerve-perforations and the articulations with neighbouring bones (see pp. 19 to 72): the general form of the argument being—This bone is to be classed as such or such, because it is connected thus and thus with these others, which are so and so. Moreover, by putting forth an “ideal typical vertebra,” consisting of a number of elements standing towards each other in certain definite arrangement, this persistency of relative position is manifestly alleged. The essential attribute of this group of bones, considered as a typical group, is the constancy in the connexions of its parts: change the connexions, and the type is changed. But the constancy of relative position thus tacitly asserted, and appealed to as a conclusive test in “moot cases of special homology,” is clearly negatived by Professor Owen’s own facts. For instance, in the “ideal typical vertebra,” the hæmal arch is represented as formed by the two hæmapophyses and the hæmal spine; but at p. 91 we are told that