Fishes, according to the classification of a preponderating majority of the ichthyologists that have flourished from the earliest times down to those of Agassiz, have been divided into two great series, the Ordinary or osseous, and the Chondropterygii or cartilaginous. And these two divisions of the class, instead of being ranged consecutively in a continuous line, the one in advance of the other, have been ranged in two parallel lines, the one directly abreast of the other. There is this further peculiarity in the arrangement, that the line of the cartilaginous series, from the circumstance that some of its families rise higher and some sink lower in the scale than any of the ordinary fishes, outflanks the array of the osseous series at both ends. The front which it presents contains fewer genera and species than that of the osseous division; but, like the front of an army drawn out in single file, it extends along a greater length of ground. And to this long-fronted series of the cartilaginous, or, according to Cuvier, chondropterygian fishes, the Placoid families of Agassiz belong,—among the rest, the Placoids of the Silurian formations, Upper and Lower. But though all the Placoids of this latter naturalist be cartilaginous fishes, all cartilaginous fishes are not Placoids. The Sturionidæ are cartilaginous, and are, as such, ranked by Cuvier among the Chondropterygii, whereas Agassiz places them in his Ganoid order. Many of the extinct fishes, too, such as the Acanthodei, Dipteridæ, Cephalaspidæ, were, as we have seen, cartilaginous in their internal framework, and yet true Ganoids notwithstanding. The principle of Agassiz’s classification wholly differs from that of Cuvier and the older ichthyologists; for it is a classification founded, not on the character of the internal but on that of the cuticular or dermal skeleton. And while to the geologist it possesses great and obvious advantages over every other,—for of the earlier fishes very little more than the cuticular skeleton survives,—it has this further recommendation to the naturalist, that, (in so far at least as its author has been true to his own principles,) instead of anomalously uniting the highest and lowest specimens of their class,—the fishes that most nearly approximate to the reptiles on the one hand, and the fishes that sink furthest towards the worms on the other,—it gathers into one consistent order all the individuals of the higher type, distinguished above their fellows by their development of brain, the extensive range of their instincts, and the perfection of their generative systems. Further, the history of animal existences, as recorded in the sedimentary rocks of our planet, reads a recommendation of this scheme of classification which it extends to no other. We find that in the progress of creation the fishes began to be by groupes and septs, arranged according to the principle on which it erects its orders. The Placoids came first, the Ganoids succeeded them, and the Ctenoids and Cycloids brought up the rear. The march has been marshalled according to an appointed programme, the order of which it is peculiarly the merit of Agassiz to have ascertained.
Now, may I request the reader to mark, in the first place that what we have specially to deal with at the present stage of the argument are the Placoid fishes of the Silurian formations, Upper and Lower. May I ask him to take note, in the second, that the long-fronted chondropterygian series of Cuvier, though it includes, as has already been said, the Placoid order of Agassiz,—just as the red-blooded division of animals includes the bimana and quadrumana,—is no more to be regarded as identical with the Placoids, than the red-blooded animals are to be regarded as identical with the apes or with the human family. It simply includes them in the character of one of the three great divisions into which it has been separated,—the division ranged, if I may so express myself, on the extreme right of the line; its middle portion, or main body, being composed of the Sturiones, a family on the general level of the osseous fishes; while, ranged on the extreme left, we find the low division of the Suctorii, i. e. Cyclostomi, or Lampreys. But with the middle and lower divisions we have at present nothing to do; for of neither of them, whether Sturiones or Suctorii, does the Silurian System exhibit a trace. Further be it remarked, that the scheme of classification which gives an abstract standing to the Chondropterygii, is in itself merely a certain perception of resemblance which existed in certain minds, having cartilage for its general idea; just as another certain perception of resemblance in one other certain mind had cuticular skeleton for its general idea, and as yet another perception of resemblance in yet other certain minds had red blood for its general idea. As shown by the disparities which obtain among the section which the scheme serves to separate from the others, it no more determines rank or standing than that greatly more ancient scheme of classification into “ring-streaked and spotted,” which served to distinguish the flocks of the patriarch Jacob from those of Laban his father-in-law, but which did not distinguish goats from sheep, nor sheep from cattle.
The effect of introducing, after this manner, generalizations made altogether irrespective of rank, and avowedly without reference to it, into what are inherently and specifically questions of rank, admits of a simple illustration.
Let us suppose that it was not with the standing of the Silurian Placoids that we had to deal, but with that of the mammals of the recent period, including the quadrumana, and even the bimana, and that we had ventured to describe them, in the words of the Edinburgh Reviewer, as “the very highest types of their class.” What would be thought of the reasoner who, in challenging the justice of the estimate, would argue that these creatures, men as well as monkeys, belonged simply to that division of red-blooded animals which includes, with the bimana and quadrumana, the frog, the gudgeon, and the earthworm?—a division, he might add, “which, when details of organization are regarded, stretches farther, both downward and upward,” than that division of the white-blooded animals to which the crab, the spider, the cuttle-fish, and the dragon fly belong; “so that, looking at one extremity, any one is as much entitled to call the red-blooded animals the lowest division, as any other, looking at another extremity, is to call them the highest division, of animals.” What, it might well be asked in reply, has the earthworm, with its red-blood to do in a question respecting the place and standing of the bimana? Or what, in the parallel case, have the Suctorii—the worms of Linnæus—to do in a question respecting the place and standing of the real Placoids? True it is that, according to one principle of classification, now grown somewhat obsolete, men and earthworms are equally red-blooded animals; true it is that, according to another principle of classification, the Placoids of Agassiz and the cartilaginous worms of Linnæus are equally Chondropterygii. The bimana and the earthworm have their red blood in common; the glutinous hag and the true Placoids have as certainly their internal cartilage in common; and if the fact of the red blood of the worm lowers in no degree the rank of the bimana, then, on the same principle, the fact of the internal cartilage of the glutinous hag cannot possibly detract from the standing of the true Placoid. In both cases they are creatures that entirely differ,—the earthworms from the bimana, and the cartilaginous worms from the Placoids; and the classification which tags them together, whether it be that of Aristotle or that of Cuvier, cannot be converted into a sort of minus quantity, of force enough to detract from the value and standing of the bimana in the one case, or of the true Placoids in the other. It is in no degree derogatory to the human family that earthworms possess red blood; it is in no degree derogatory to the true Placoids that the Suctorii possess cartilaginous skeletons.
Let the reader now mark the use which has been made, by the author of the “Vestiges,” of the name and authority of Linnæus. “Linnæus,” he states, “was so impressed by the low character of many of this order, (the Chondropterygii,) that he actually ranked them with worms.” Now, what is the fact here? Simply that Linnæus had no such general order as the Chondropterygii in his eye at all. Though chiefly remarkable as a naturalist for the artificialness of his classifications, his estimate of the cartilaginous fishes was remarkable—though carried too far in its extremes, and in some degree founded in error—for an opposite quality. It was an estimate formed, in the main, on a natural basis. Instead of taking their cartilaginous skeleton into account, he looked chiefly at their standing as animals; and, struck with that extent of front which they present, and with both their superiority on the extreme right, and their inferiority on the extreme left, to the ordinary fishes, he erected them into two separate orders, the one lower and the other higher than the members of the osseous line. And so far was he from regarding the true Placoids—those Chondropterygii which to an internal skeleton of cartilage add external plates, points, or spines of bone—as low in the scale, that he actually raised them above fishes altogether, by erecting them into an order of reptiles,—the older Amphibia Nantes. Surely, if the name of Linnæus was to be introduced into this controversy at all, it ought to have been in connection with this special fact; seeing that the point to be determined in the question under discussion is simply the place and standing of that very order which the naturalist rated so high,—not the place and standing of the order which he degraded. It so happens that there is one of the Chondropterygii which, so far from being a true Placoid, does not possess a single osseous plate, point, or spine: it is a worm like creature, without eyes, without movable jaws, without vertebral joints, without scales, always enveloped in slime, and greatly abhorred by our Scotch boatmen of the Moray Frith, who hold that it burrows, like the grave-worm, in the decaying bodies of the dead. And this creature, “the glutinous hag,” or, according to north-country fishermen, the “ramper-eel,” or “poison-ramper,” was regarded by Linnæus as belonging, not to the class of fishes, but to the Vermes. Now, this is the special fact with which, in the development controversy, the author of the “Vestiges” connects the name of the Swedish naturalist! All the fish of the Silurian System belonged to that true Placoid order which Linnæus, impressed by its high standing, erected into an order, not of worms, but of reptiles. He elevated A, the true Placoid, while he degraded B, the glutinous hag. But it was necessary to the argument of the author of the “Vestiges” that the earliest existing fish should be represented as fish low in the scale; and so he has cited the name and authority of Linnæus in its bearing against the glutinous hag B, as if it had borne against the standing of the true Placoid A. The Patagonians are the tallest and bulkiest men in the world, whereas their neighbors, the Fuegians are a slim and diminutive race. And if, in some controversy raised regarding the real size of the more gigantic tribe, they were to be described as the “very tallest types of their class,” any statement in reply, to the effect that some trustworthy voyager had examined certain races of the extreme south of America, and had found that they were both short and thin, would be neither relevant in its facts nor legitimate in its bearing. But if the controversialist who thus strove to strengthen his case by the voyager’s authority, was at the same time fully aware that the voyager had seen not only the diminutive Fuegians, but also the gigantic Patagonians, and that he had described these last as very gigantic indeed, the introduction of the statement regarding the smaller race, when he wholly sank the statement regarding the larger, would be not merely very irrelevant in the circumstances, but also very unfair. Such, however, is the style of statement to which the author of the “Vestiges” has (I trust inadvertently) resorted in this controversy.
It is not uninstructive to mark how slowly and gradually the naturalists have been groping their way to a right classification in the ichthyic department of their science, and how it has been that identical perception of resemblance, having cartilage for its general idea, to which the author of the “Vestiges” attaches so much importance, that has served mainly to retard their progress. Not a few of the more distinguished among their number deemed it too important a distinction to be regarded as merely secondary; and so long as it was retained as a primary characteristic, the fishes failed to range themselves in the natural order;—dissimilar tribes were brought into close neighborhood, while tribes nearly allied were widely separated. It failed, as has been shown, to influence Linnæus; and though he no doubt pressed his peculiar views too far when he degraded the glutinous hag into a worm, and elevated the Sharks and Rays into reptiles, it is certainly worthy of remark, that, in the scheme of classification which is now regarded as the most natural,—that of Professor Muller, modified by Professor Owen,—the ichthyic worms of the Swede are placed in the first and lowest order of fishes,—the Dermopteri,—and the greater part of his ichthyic reptiles, in the eleventh and highest,—the Plagiostomi. Cuvier yielded, as has been shown, to the idea of resemblance founded on the material of the ichthyic framework, and so ranged his fishes into two parallel lines. Professor Oken, after first enunciating as law that “the characteristic organ of fishes is the osseous system,” confessed the “great difficulty” which attaches to the question of skeletal “texture or substance,” and finally gave up the distinction founded on it as obstinately irreducible to the purposes of a natural classification. “The cartilaginous fishes,” he says, “appear to belong to each other, and are also usually arranged together; yet amongst them we find those species, such as the Lampreys, which obviously occupy the lowest grade of all fishes, while the Sharks and Rays remind us of the Reptilia.” And so, sinking the consideration of texture altogether, he placed the family of the Lamprey, including the glutinous hag, at the bottom of the scale, and the Sharks and Rays at the top. Agassiz’s system, peculiarly his own, has had the rare merit, as I have shown, of furnishing a key to the history of the fish in its several dynasties, which we may in vain seek in any other. His divisions,—if, retaining his strongly-marked Placoids and Ganoids, as orders stamped in the mint of nature, we throw his perhaps less obviously divisible Ctenoids and Cycloids into one order,—the corneous or horn-covered,—are scarcely less representative of periods than those great classes of the vertebrata, mammals, birds, reptiles, and fishes, which we find not less regularly ranged in their order of succession in the geologic record than in the “Animal Kingdom” of Cuvier,—a shrewd corroboration, in both cases, I am disposed to think, of the rectitude of the arrangement. What seems to be the special defect of his system is, that having erected his four orders, and then finding a certain number of residuary families that, on his principle of cuticular character, stubbornly refused to fall into any determinate place, he distributed them among the others, with reference chiefly to the totally distinct principle of Cuvier. Thus the Suctorii, soft, smooth, slimy-skinned fishes, that do not possess a single placoid character, and are not true Placoids, he has yet placed in his Placoid order, influenced, apparently, by the “perception of resemblance that has cartilage for its central idea;” and the effect has been a massing into one anomalous and entangled group the fishes of the first period of geologic history, with fishes of which we do not find a trace save in the existing scene of things, and of the highest families of their class with families that occupy the lowest place. But we live in an age in which even the benefactors of the world of mind cannot make false steps with impunity; and so, while Agassiz’s three ichthyic orders will continue to be recognized by the palæontologist as the orders of three great geologic periods, the Suctorii have already been struck from off his higher fishes by the classification of Muller and Owen, and carried to that lowest point in the scale (indicated by Linnæus and Oken) which their inferior standing renders so obviously the natural one. Some of my readers may perhaps remember how finely Bacon, in his “Wisdom of the Ancients,” interprets the old mythologic story of Prometheus. Prometheus, says the philosopher, had conferred inestimable favors on men, by moulding their forms into shape, and bringing them fire from heaven; and yet they complained of him and his teachings to Jupiter. And the god, instead of censuring their ingratitude, was pleased with the complaint, and rewarded them with gifts. In putting nature to the question, it is eminently wholesome to be doubting, cross-examining, complaining; ever demanding of our masters and benefactors the philosophers, that they should reign over us, not arbitrarily and despotically,
“Like the old kings, with high exacting looks,
Sceptred and globed,”
but like our modern constitutional monarchs, who govern by law; and, further, that an appeal from their decisions on all subjects within the jurisdiction of Nature should for ever be open to Nature herself. The seeming ingratitude of such a course, if the “complaints” be made in a right spirit and on proper grounds, Jupiter always rewards with gifts.
Let us now see for ourselves, in this spirit, whether there may not be something absolutely derogatory, in the existence of a cartilaginous skeleton, to the creatures possessing it; or whether a deficit of internal bone may not be greatly more than neutralized, as it assuredly must have been in the view of Linnæus, Muller, and Owen, by a larger than ordinary share of a vastly more important substance.