“Cycle and epicycle, orb in orb.”
In the classifications of the naturalist, for instance, all species range round some central generic idea; all genera round some central idea, to which we give the name of order; all orders round some central idea of class; all classes round some central idea of division; and all divisions round the interior central idea which constitutes a kingdom. Sir Joshua Reynolds forms his theory of beauty on this principle of central ideas. “Every species of the animal, as well as of the vegetable creation,” he remarks, “may be said to have a fixed or determinate form, towards which nature is continually inclining, like various lines terminating in a centre; or it may be compared to pendulums vibrating in different directions over one central point, which they all cross, though only one of their number passes through any other point.” He instances, in illustrating his theory, the Grecian beau ideal of the human nose, as seen in the statues of the Greek deities. It formed a straight line; whereas all deformity of nose is of a convex or concave character, and occasioned by either a rising above or a sinking below this medial line of beauty. And it may be of use, as it is unquestionably of interest, to conceive, after this manner, of a certain type of skeleton, embodying, as it were, the central or primary type of all vertebral skeletons, and consisting of a double range of rings, united by the bodies of the vertebræ, as the two rings of a figure 8 are united at their point of junction; the upper ring forming the enclosure of the brain,—spinal, and cephalic; the lower that of the viscera,—respiratory, circulatory, and digestive. Such is the idea embodied in Professor Owen’s archetypal skeleton. It is a series of vertebræ composing double rings,—their brain-rings comparatively small in the vertebræ of the trunk, but of much greater size in the vertebræ of the head. But it must not be forgotten, that central ideas, however necessary to the classification of the naturalist, are not historic facts. We may safely hold, with the philosophic painter, that the outline of the typical human nose is a straight line; but it would be very unsafe to hold, as a consequence, that the first men had all straight noses. And when we find it urged by at least one eminent assertor of the development hypothesis,—Professor Oken,—that light was the main agent in developing the substance of nerve,—that the nerves, ranged in pairs, in turn developed the vertebræ, each vertebra being but “the periphery or envelope of a pair of nerves,”—and that the nerves of those four senses of smell, sight, taste, and hearing, which, according to the Professor, “make up the head,” originated the four cranial vertebræ which constitute the skull,—it becomes us to test the central idea, thus converted into a sort of historic myth, by the realities of actual history. What, then, let us inquire, is the real history of the cerebral development of the vertebrata, as recorded in the rocks of the earlier geologic periods?
Fig. 7.
Osseous points of placoid cranium.[12]
(Mag. twelve diameters)
Though the vertebrata existed in the ichthyic form throughout the vastly extended Silurian period, we find in that system no remains of the cranium: the Silurian fishes seem, as has been already said, ([page 53],) to have been exclusively Placoid, and the purely cartilaginous box formed by nature for the protection of the brain in this order has in no case been preserved. Teeth, and, in at least one or two instances, the minute jaws over which they were planted have been found, but no portion of the skull. We know, however, that in the fishes of the same order which now exist, the cranium consists of one undivided piece of a cartilaginous substance, set thickly over its outer surface with minute polygonal points of bone, (fig. 7,) composed internally of star-like rays, that radiate from the centre of ossification, and that present, in consequence, seen through a microscope, the appearance of the polygonal cells of a coral of the genus Astrea. The pattern induced is that of stars set within polygons. Along the sides or top of this unbroken cranial box, that exhibits no mark of suture, we find the perforations through which the nerves of smell, sight, taste, and hearing passed from the brain outwards, and see that they have failed to originate distinct vertebral envelopes for themselves;—they all lodge in one undivided mansion-house, and have merely separate doors. We find, further, that the homotypal ribs of the entire cranium consist, not of four, but simply of a single pair, attached to the occiput, and which serves both to suspend the jaws, upper and nether, in their place under the middle of the head, and to lend support to the hyoid and branchial framework; while the scapular ring we find existing, as in the higher vertebrata, not as a cerebral, but as a cervical or dorsal appendage. In the wide range of the animal kingdom there are scarce any two pieces of organization that less resemble one another in form than the vertebræ of the placoids resemble their skulls; and the difference is not merely external, but extends to even their internal construction. In both skull and vertebræ we detect an union of bone and cartilage; but the bone of each vertebra forms an internal continuous nucleus, round which the cartilage is arranged, whereas in the skulls it is the cartilage that is internal, and the bone is spread in granular points over it. If we dip the body of one of the dorsal vertebræ of a herring into melted wax, and then withdraw it, we will find it to represent in its crusted state the vertebral centrum of a Placoid,—soft without, and osseous within; but in order to represent the placoid skull, we would have first to mould it out of one unbroken piece of wax, and then to cover it over with a priming of bone-dust. And such is the effect of this arrangement, that, while the skull of a Placoid, exposed to a red heat, falls into dust, from the circumstance that the supporting framework on which the granular bone was arranged perishes in the fire, the vertebral centrum, whose internal framework is itself bone, and so not perishable, comes out in a state of beautiful entireness,—resembling in the thornback a squat sand-glass, elegantly fenced round by the lateral pillars, (fig. 8, b;) and in the dog-fish (a) a more elongated sand-glass, in which the lateral pillars are wanting. Such are the heads and vertebral joints of the existing Placoids; and such, reasoning from analogy, seem to have been the character and construction of the heads and vertebral joints of the Placoids of the Silurian period,—earliest-born of the Vertebrata.
Fig. 8.
a. Osseous centrum of Spinax Acanthias.