The most highly developed of the recent cephalopods exhibit a true internal skeleton, in the form of a strong oblong bone, on which the body is hung. In this respect they occupy a sort of intermediate place between the lower molluscs on the one hand, and the lower fishes on the other. Theirs is not a vertebral column, but rather, as it were, a foreshadowing of it; not, however, as a link in some process of self-development from mollusc to fish, for these higher cephalopods do not appear to have been created until fishes and reptiles had lived for ages. The vertebrate type has been traced well-nigh as far back into the past as we have yet been able to penetrate. Once introduced, it has never ceased to exist, but in the successive geological ages has been ever receiving newer and higher modifications, reaching its perfection at length in man. The vertebrate form of structure fulfils the highest adaptations of which terrestrial beings seem capable. We can hardly conceive of corporeal existence reaching a more elevated stage of development, save in thereby becoming less material, and receiving an impartation of some higher element. The vertebrate animals display not merely the most complexly organized structures, but manifest in their habits the workings of the higher instincts and affections. Among the invertebrate tribes the propagation of the species is, in the vast majority of cases, a mere mechanical function, like that of feeding or respiration, and the eggs once deposited, the parent has no further care of her young. But among the vertebrated animals, on the other hand, the perpetuation of the race forms the central pillar round which the natural affections are entwined. It parcels out every species into pairs, in each of which the mates are bound together by the strongest ties of attachment. It gives birth, too, to that noble instinct which leads the mother to expose her own life rather than suffer harm to come to her offspring. It produces, at least in man, that reciprocal attachment of offspring to parent, from which springs no small part of all that is holiest and best in this world. These attributes, to a greater or less extent, belong to all the vertebrate animals, from the fish up to man. In looking over the relics of animal life in the earlier geological formations, we are apt, as we gaze on the massive jaws and teeth, the strong bony armour, and the sharp, barbed spines, to think only of a time of war and carnage, when the larger forms preyed upon the smaller, or ruthlessly sought to exterminate each other. Yet should we not remember, that with all these weapons and instincts of self-preservation there were linked attributes of a nobler kind; that the earliest vertebrate remains point to the introduction—though perhaps in but a rudimentary form—of self-sacrificing love into our planet? The march of creation from the first dawn of life has ever been an onward one, as regards the development not only of organic structure but of the social relations; and if it be true that physical organization finds its archetype in man, it is assuredly no less so that in him too we meet with the highest manifestation of those instincts which, by linking individual to individual, have ever marked out the vertebrate tribes of animals from the more machine-like characteristics of the invertebrate.
We pass now to the vertebrate animals, and shall look for a little into the general grade and organization of the fishes that characterized the carboniferous rivers and seas.
A collection of the ichthyolites of the carboniferous rocks presents almost every variety in the mode of preservation. The smaller species are frequently found entire, and show their shining scales still regularly imbricated as when the creatures were alive. The larger forms seldom occur in other than a very fragmentary condition. The limestones yield dark-brown or black, oblong, leech-like teeth, which are found on examination to be those of an ancient family of sharks. The shales are often sprinkled over with glittering scales and enamelled bones. Some of the coals and ironstones yield in abundance long sculptured spines, huge jaws bristling with sharp conical teeth, and detached tusks, sometimes five or six inches long. In short, the naturalist who would decipher the ichthyology of the Coal formation, finds before him, in the rocks, not a suite of correctly arranged, and carefully preserved skeletons, but a set of disjointed, unconnected bones; here a tooth, there a scale, now a jaw, now a dermal plate, all mingled at random. And yet, though the evidence lie in this fragmentary state, our knowledge of these ancient fishes is far from being correspondingly meagre. To such precision has the science of comparative anatomy arrived, that a mere scale or tooth is often enough to indicate the nature and functions of the individual to which it belonged, and to establish the existence in former times of a particular class or order of animals. Thus the smooth rounded teeth of the mountain limestone are found to present both externally and internally a close resemblance to the hinder flat teeth of the sole living cestraciont (C. Philippi); and we hence learn that a family of sharks, now all but extinct, abounded in the palæozoic seas. The occurrence of a set of dark, rounded little objects, which by the unpractised eye would be apt to be mistaken for pebbles, is in this way sufficient at once to augment our knowledge of the various animals of the Carboniferous period, and to establish an important fact in the history of creation.
Of the four great Orders into which Agassiz[41] subdivided the class Pisces, the Placoids and Ganoids, agreeing on the whole with the cartilaginous fishes of Cuvier, occur abundantly in the palæozoic rocks, while the Cycloids and Ctenoids, answering to Cuvier's osseous fishes, began in the Secondary formations, and are found in all subsequent deposits. The two former reached their maximum in the earlier geological ages, and have been gradually dwindling down ever since, till now they are represented by comparatively few genera; the two latter are emphatically modern orders; they have been constantly increasing in numbers since their creation, and swarm in every sea at the present day. The carboniferous ichthyolites belong, of course, only to the two first-mentioned orders the placoids and ganoids.
[41] The classification of Agassiz, which is certainly not a little arbitrary and artificial, has been altered by Müller, a distinguished German anatomist, whose arrangement has been modified again by Professor Owen. See Owen's Lectures on Comparative Anatomy, vol. ii. p. 47. There is far from anything like unanimity on the subject. Every naturalist thinks himself at liberty to modify and restrict the groupings of his predecessors or contemporaries, sometimes without condescending to give synonyms or any clue by which one may compare the rival classifications. The geological student cannot engage in a more sickening task than that of ranging through these various arrangements, and he must possess some self-command who can refrain from throwing up the search in disgust. The best way of progressing is to select some standard work and keep to it, until the characteristics of the genera and families have been mastered, and as far as possible, verified from actual observation. After such preliminary training, the student will be more able to grope his way through the "chaos and dark night" of synonyms and systems.
The Placoid, or Plagiostome fishes, are familiar to us all as exemplified in the common thornback and skate of our markets. They are covered with a tough skin, which either supports a set of tuberculed plates as in the thornback, or a thick crop of small rounded bony points or plates, as in the shagreen of the sharks. The head consists of a single cartilaginous box. The spinal column is likewise formed of cartilage, built up in the higher genera of partially ossified vertebræ. The tail is heterocercal or unequally lobed, inasmuch as the spinal column, instead of ending off abruptly as it does in the herring, trout, and all our commoner fishes, passes on to the extreme point of the upper half of the tail. This is a noticeable feature, for it has been found to characterize all the fishes that lived in the earlier geological periods. The fins are often strengthened by strong spines of bone, which stand up in front of them and serve the double purpose of organs of progression and weapons of defence. The teeth vary a good deal in form. In the larger number of existing placoids they are of a sharp cutting shape, often with saw-like edges. Among the sharks they run along the jaws in numerous rows, of which, however, only the outer one is used, those behind lying in reserve to fill up the successive gaps in the front rank. The teeth do not sink into the jaw, as in the ganoids, but are merely bound together by the tough integument which forms the lips. Another form of tooth, abundant among the ancient placoids, and visible on some of those at the present day, shows a smooth rounded surface, the teeth being closely grouped together into a sort of tessellated pavement which, in the recent species, runs round the inner part of the jaws, while a row of conical teeth guards the entrance of the mouth.
Fig. 28.—Ctenacanthus hybodoides. (Edgerton.)
The animals which possess these characteristics include the various tribes of the sharks and rays, and form the highest group of fishes. They are all active and predaceous, frequenting every part of the ocean where their prey is to be found. The formidable spines and hideous "chasm of teeth" belonging to the bulkier forms, render them more than a match for any other denizens of the deep, and thus they reign in undisputed supremacy—the scourge of their congeners, and a terror to man.
The seas of the Carboniferous era abounded with similar predaceous fishes, some of which must have been of enormous size. An entire specimen has never been obtained; nor, from the destructible nature of the animal framework, can we expect to meet with one. But the hard bony parts of the animals, those capable in short of preservation in mineral accumulations, are of common occurrence in the mountain limestone beds and even among the coal seams. The dorsal spines or ichthyodorulites, are especially conspicuous ([Fig. 28]). They stood up along the creature's back like masts, the fin which was attached to the hinder margin of each, representing the sail. The spine could be raised or depressed at pleasure, its movements regulating those of the fin, much as the raising or lowering of the mast in a boat influences the lug-sail that is attached to it. The general form of these spines was long, tapering, and more or less rounded. But they assumed many varieties of surface ornament. Some species were ribbed longitudinally, and had along their posterior concave side a set of little hooks somewhat like the thorns of a rose. Others seem to have been quite smooth, and of a flattened shape, with a thick-set row of sharp hooks down both of the edges, like the spine on the tail of the sting-ray of the Mediterranean. Such weapons have considerable resemblance to the barbed spear-heads of savage tribes, and it is certain they were intended to act in a similar way, as at once offensive and defensive arms. The toothed spines of the sting-rays are still used in some parts of the world to point the warrior's spear and arrow. Is there not something suggestive in the fact that these stings, after having accomplished their appointed purpose as weapons of war in the great deep, should come to be employed over again in a like capacity on the land; and that an instrument, which was designed by the Creator as a means of protecting its possessor, should be turned by man into an implement for gratifying his cupidity and satiating his revenge? Other ichthyodorulites are elegantly ornamented by long rows of tuberculed lines arranged in a zig-zag fashion, or in straight rows tapering from base to point. In all there was a blunt unornamented base, which sank into the back and served as a point of attachment for the muscles employed in raising or depressing the spine. In some specimens the outer point appears rounded and worn, the characteristic ornament being effaced for some distance—a circumstance which probably indicates that these fishes frequented the more rocky parts of the sea.[42]