Part 7

So far as is yet known, all the fish of the earliest fossiliferous system belonged to the placoid or “_broad plated_” order,—a great division of fishes, represented in the existing seas by the Sharks and Rays,—animals that to an internal skeleton of cartilage unite a dermal covering of points, plates, or spines of enamelled bone, and have their gills fixed. The dermal or cuticular bones of this order vary greatly in form, according to the species or family: in some cases they even vary, according to their place, on the same individual. Those button-like tubercles, for instance, with an enamelled thorn, bent like a hook, growing out of the centre of each, which run down the back and tail, and stud the pectorals of the thorn-back, (_Raja clavata_,) differ very much from the smaller thorns, with star-formed bases, which roughen the other parts of the creature’s body; and the bony points which mottle the back and sides of the sharks are, in most of the known species, considerably more elongated and prickly than the points which cover their fins, belly, and snout. The extreme forms, however, of the shagreen tubercle or plate seem to be those of the upright prickle or spine on the one hand, and of the slant-laid, rhomboidal, scale-shaped plate on the other. The minuter thorns of the ray (fig. 2, _a_) exemplify the extreme of the prickly type; the fins, abdomen, and anterior part of the head of the spotted dog-fish (_Scyllium stellare_) are covered by lozenge-shaped little plates, which glisten with enamel, and are so thickly set that they cover the entire surface of the skin, (fig. 3, _b_,)—and these seem equally illustrative of the scale-like form. They are shagreen points passing into osseous scales, without, however, becoming really such; though they approach them so nearly in the shape and disposition of their upper disks, that the true scales, also osseous, of the _Acanthodes sulcatus_, (fig. 3, _a_,) a Ganoid of the Coal Measures, can scarce be distinguished from them, even when microscopically examined. It is only when seen in section that the distinctive difference appears. The true scale of the Acanthodes, though considerably elevated in the centre, seems to have been planted on the skin; whereas the scale-like shagreen of the dog-fish is elevated over it on an osseous pedicle or footstalk (fig. 5, _a_) as a mushroom is elevated over the sward on its stem; and the base of the stalk is found to resemble in its stellate character that of a shagreen point of the prickly type. The apparent scale is, we find, a bony prickle bent at right angles a little over its base, and flattened into a rhomboidal disk atop.

In small fragments of shagreen, (fig. 2 _b_) which have been detected in the bone-bed of the Upper Ludlow Rocks, (Upper Silurian,) and constitute the most ancient portions of this substance known to the palæontologist, the osseous tubercles are, as in the minuter spikes of the ray, of the upright thorn-like type; they merely serve to show that the placoids of the first period possessed, like those of the existing seas, an ability of secreting solid bone on their cuticular surfaces; and that, though at least such of them as have bequeathed to us specimens of their dermal armature possessed it in the form farthest removed from that of their immediate successors the ganoid fishes, they resembled them not less in the substance of which their dermoskeletal, than in that of which their endoskeletal, parts were composed. For the internal skeleton in both orders, during these early ages, seems to have been equally cartilaginous, and the cuticular skeleton equally osseous. In the ichthyolitic formation immediately over the Silurians,—that of the Lower Old Red Sandstone,—the Ganoids first appear; and the members of at least one of the families of the deposit, the Acanths,—a family rich in genera and species,—seem to have formed connecting links between this second order and their placoid predecessors. They were covered with true scales (fig. 4, _a_,) and their free gills were protected by gill-covers; and so they must be regarded as real Ganoids but as the shagreen of the spotted dog-fish nearly approaches, in form and character, to ganoid scales, without being really such, the scales of this family, on the other hand, approached equally near, without changing their nature, to the shagreen of the Placoids, especially to that of the spiked dogfish, (_Spinax Acanthias_.) (Fig. 4, _b_.) We even find on their under surfaces what seems to be an approximation to the characteristic footstalk. They so considerably thicken in the middle from their edges inwards, (fig. 5, _c_,) as to terminate in their centres in obtuse points. With these shagreen-like scales, the heads, bodies, and fins of all the species of at least two of the Acanth genera,—_Cheiracanthus_ and _Diplacanthus_,—were as thickly covered as the heads, bodies, and fins of the sharks are with their shagreen; and so slight was the degree of imbrication, that the portion of each scale overlaid by the two scales in immediate advance of it did not exceed the one twelfth part of its entire area. In the scale of the _Cheiracanthus_ we find the covered portion indicated by a smooth, narrow band, that ran along its anterior edges, and which the furrows that fretted the exposed surface did not traverse. It may be added, that both genera had the anterior edge of their fins armed with strong spines,—a characteristic of several of the Placoid families.

(The single scales mag. two diameters;—the others nat. size.)]

In the Dipterian genera _Osteolepis_ and _Diplopterus_ the scales were more unequivocally such than in the Acanths, and more removed from shagreen. The under surface of each was traversed longitudinally by a raised bar, which attached it to the skin, and which, in the transverse section, serves to remind one of the shagreen footstalk. They are, besides, of a rhomboidal form; and, when seen in the finer specimens, lying in their proper places on what had been once the creature’s body, they seem merely laid down side by side in line, like those rows of glazed tiles that pave a cathedral floor; but on more careful examination, we find that each little tile was deeply grooved on its higher side and end, (for it lay diagonally in relation to the head,) like the flags of a stone roof, (fig. 6, _a_,)—that its lateral and anterior neighbors impinged upon it along these grooves to the extent of about one third its area,—and that it impinged, in turn, to the same extent on the scales that bordered on it posteriorly and latero-posteriorly. Now, in the Cœlacanth family, (and on this special point the foregoing remarks are intended to bear,) the scales, which were generally of a round or irregularly oval form, (fig. 6, _b_,) overlapped each other to as great an extent as in any of the existing fishes of the Cycloid or Ctenoid orders,—to as great an extent, for instance, as in the carp, salmon, or herring. In a slated roof there is no part on which the slates do not lie double, and along the lower edge of each tier they lie triple;—there is more of slate covered than of slate seen: whereas in a tile-roof, the covered portion is restricted to a small strip running along the top and one of the edges of each tile, and the tiles do not lie double in more than the same degree in which the slates lie triple. The scaly cover of the two genera of Dipterians to which I have referred was a cover on the _tile_-roof principle; and this is an exceedingly common characteristic of the scales of the Ganoids. The scaly cover of the Cœlacanths, on the other hand, was a cover on the _slate_-roof principle;—there was in some of their genera about one third more of each scale covered than exposed; and this is so rare a ganoidal mode of arrangement, that, with the exception of the _Dipterus_,—a genus which, though it gives its name to the Dipterian sept, differed greatly from every other Dipterian,—I know not, beyond the limits of the ancient Cœlacanth family, a single Ganoid that possessed it. The bony covering of the Cœlacanths was _farthest_ removed in character from shagreen, as that of their contemporaries the Acanths approximated to it most nearly; they were, in this respect, the two extremes of their order; and did we find the Cœlacanths in but the later geological formations, while the Acanths were restricted to the earlier, it might be argued by assertors of the development hypothesis, that the amply imbricated, slate-like scale of the latter had been developed in the lapse of ages from the shagreen tubercle, by passing in its downward course—broadening and expanding as it descended—through the minute, scarcely imbricated disks of the Acanths, and the more amply imbricated tile-like rhombs of the Dipterians and Palæonisci, until it had reached its full extent of imbrication in the familiar modern type exemplified in both the Cœlacanths and the ordinary fishes. But such is not the order which nature has observed;—the two extremes of the ganoid scale appear together in the same early formation: both become extinct at a period geologically remote; and the ganoid scales of the existing state of things which most nearly resemble those of ancient time are scales formed on the intermediate or tile-roof principle.

The scales of the Cœlacanths were, in almost all the genera which compose the family, of great size; in some species, of the greatest size to which this kind of integument ever attained. Of a Cœlacanth of the Coal Measures, the _Holoptychius Hibberti_, the scales in the larger specimens were occasionally from five to six inches in diameter. Even in the _Holoptychius nobilissimus_, in an individual scarcely exceeding two and a half feet in length, they measured from an inch and a half to an inch and three quarters each way. In the splendid specimen of this last species, in the British Museum, there occur but fourteen scales between the ventrals, though these lie low on the creature’s body, and the head; and in a specimen of a smaller species,—the _Holoptychius Andersoni_,—but about seventeen. The exposed portion of the scale was in most species of the family curiously fretted by intermingled ridges and furrows, pits and tubercles, which were either boldly relieved, as in the _Holoptychius_, or existed, as in the _Glyptolepis_, as slim, delicately chiselled threads, lines, and dots. The head was covered by strong plates, which were roughened with tubercles either confluent or detached, or hollowed, as in the _Bothriolepis_, into shallow pits. The jaws were thickly set with an outer range of true fish teeth, and more thinly with an inner range of what seem _reptile_ teeth, that stood up, tall and bulky, behind the others, like officers on horseback seen over the heads of their foot-soldiers in front. The _double_ fins,—pectorals and ventrals,—were characterized each by a thick, angular, scale-covered centre, fringed by the rays; and they must have borne externally somewhat the form of the sweeping paddles of the Ichthyosaurian genus,—a peculiarity shared also by the double fins of the _Dipterus_. The _single_ fins, in all the members of the family of which specimens have been found sufficiently entire to indicate the fact, were four in number,—an anal, a caudal, and two dorsal fins; and, with the exception of the anterior dorsal, which was comparatively small, and bent downwards along the back, as if its rays had been distorted when young,[11] they were all of large size. They crowded thickly on the posterior portion of the body,—the anterior dorsal opposite the ventrals, and the posterior dorsal opposite the anal fin. The fin-rays of the various members of the family, and such of their spinous processes as have been detected, were hollow tubular bones; or rather, like the larger pieces in the framework of the Placoids, they were cartilaginous within, and covered externally by a thin osseous crust or shell, which alone survives; and to this peculiarity they owe their family name, Cœlacanth, or “hollow-spine.” The internal hollow, _i. e._ cartilaginous centre, was, however, equally a characteristic of the spinous processes of the _Coccosteus_. In their general proportions, the Cœlacanths, if we perhaps except one species,—the _Glyptolepis microlepidotus_,—were all squat, robust, strongly-built fishes, of the Dirk Hatterick or Balfour-of-Burley type; and not only in the larger specimens gigantic in their proportions, but remarkable for the strength and weight of their armor, even when of but moderate stature. The specimen of _Holoptychius nobilissimus_ in the British Museum could have measured little more than three feet from snout to tail when most entire; but it must have been nearly a foot in breadth, and a bullet would have rebounded flattened from its scales. And such was that ancient Cœlacanth family, of which the oldest of our Scotch Ganoids,—the _Asterolepis_ of Stromness,—formed one of the members, and which for untold ages has had no living representative.

Let us now enter on our proposed inquiry regarding the cerebral development of the earlier vertebrata, and see whether we cannot ascertain after what manner the first true brains were lodged, and what those modifications were which their protecting box, the cranium, received in the subsequent periods. Independently of its own special interest, the inquiry will be found to have a direct bearing on our general subject.

CEREBRAL DEVELOPMENT OF THE EARLIER VERTEBRATA. ITS APPARENT PRINCIPLE.

It is held by a class of naturalists, some of them of the highest standing, that the skulls of the vertebrata consist, like the columns to which they are attached, of vertebral joints, composed each, in the more typical forms of head, as they are in the trunk, of five parts or elements,—the centrum or body, the two spinous processes which enclose the spinal cord, and the two ribs. These cranial vertebræ, four in number, correspond, it is said, to the four senses that have their seat in the head: there is the nasal vertebra, the centrum of which is the vomer, its spinal processes the nasal and ethmoid bones, and its ribs the _upper_ jaws; there is the ocular vertebra, the centrum of which is the anterior portion of the sphenoid bone, its spinal processes the frontals, and its ribs the _under_ jaws; there is the lingual vertebra, the centrum of which is the posterior sphenoid bone, its spinal processes the parietals, and its ribs the hyoid and branchial bones,—portions of the skeleton largely developed in fishes; and, lastly, there is the auditory vertebra, the centrum of which is the base of the occipital bone, and its spinal processes the occipital crest, and which in the osseous fishes bears attached to it, as its ribs, the bones of the scapular ring. And the cerebral segments thus constructed we find represented in typical diagrams of the skull, as real vertebræ. Professor Owen, in his lately published treatise on “The Nature of Limbs,”—work charged with valuable fact, and instinct with philosophy,—figures in his draught of the archetypal skeleton of the vertebrata, the four vertebræ of the head, in a form as unequivocally such as any of the vertebræ of the neck or body.

Now, for certain purposes of generalization, I doubt not that the conception may have its value. There are in all nature and in all philosophy certain central ideas of general bearing, round which, at distances less or more remote, the subordinate and particular ideas arrange themselves,

“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?

(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.