The Foot-prints of the Creator

by Hugh Miller

THE PLACOID BRAIN.
EMBRYONIC CHARACTERISTICS NOT NECESSARILY OF A LOW ORDER.

That special substance, according to whose mass and degree of development all the creatures of this world take rank in the scale of creation, is not bone, but brain. Were animals to be ranged according to the solidity of their bones, the class of birds would be assigned the first place; the family of the Felidæ, including the tiger and lion, the second; and the other terrestrial carnivora the third. Man and the herbivorous animals, though tolerably low in the scale, would be in advance of at least the reptiles. Most of these, however, would take precedence of the sagacious Delphinidæ; the osseous fishes would come next in order; the true Placoids would follow, succeeded by the Sturiones; and the Suctorii, i. e. Cyclostomi or Lampreys, would bring up the rear. There would be evidently no order here: the utter confusion of such an arrangement, like that of the bits of a dissected map flung carelessly out of its box by a child, would of itself demonstrate the inadequacy and erroneousness of the regulating principle. But how very different the appearance presented, when for solidity of bone we substitute development of brain! Man takes his proper place at the head of creation; the lower mammalia follow,—each species in due order, according to its modicum of intelligence; the birds succeed the mammalia; the reptiles succeed the birds; the fishes succeed the reptiles; next in the long procession come the invertebrate animals; and these, too, take rank, if not according to their development of brain proper, at least according to their development of the substance of brain. The occipital nervous ganglion of the scorpion greatly exceeds in size that of the earthworm; and the occipital nervous ring of the lobster, that of the intestinal Ascaris. At length, when we reach the lowest or acrite division of the animal kingdom, the substance of brain altogether disappears. It has been calculated by naturalists, that in the vertebrata, the brain in the class of fishes bears an average proportion to the spinal cord of about two to one; in the class of reptiles, of about two and a half to one; in the class of birds, of about three to one; in the class of mammals, of about four to one; and in the high-placed, sceptre-bearing human family, a proportion of not less than twenty-three to one. It is palpably according to development of brain, not development of bone, that we are to determine points of precedence among the animals,—a fact of which no one can be more thoroughly aware than the author of the “Vestiges” himself. Of this let me adduce a striking instance, of which I shall make further use anon.

“All life,” says Oken, “is from the sea; none from the continent. Man also is a child of the warm and shallow parts of the sea in the neighborhood of the land.” Such also was the hypothesis of Lamarck and Maillet. In following up the view of his masters, the author of the “Vestiges” fixes on the Delphinidæ as the sea-inhabiting progenitors of the simial family, and, through the simial family, of man For that highest order of the mammalia to which the Simiadæ (monkeys) belong, “there remains,” he says, “a basis in the Delphinidæ, the last and smallest of the cetacean tribes. This affiliation has a special support in the brain of the dolphin family, which is distinctly allowed to be, in proportion to general bulk, the greatest among mammalia next to the orang-outang and man. We learn from Tiedemann, that each of the cerebral hemispheres is composed, as in man and the monkey tribe, of three lobes,—an anterior, a middle, and a posterior; and these hemispheres present much more numerous circumvolutions and grooves than those of any other animal. Here it might be rash to found any thing upon the ancient accounts of the dolphin,—its familiarity with man, and its helping him in shipwreck and various marine disasters; although it is difficult to believe these stories to be altogether without some basis in fact. There is no doubt, however, that the dolphin evinces a predilection for human society, and charms the mariner by the gambols which it performs beside his vessel.”

Here, then, the author of the “Vestiges” palpably founds on a large development of brain in the dolphin, and on the manifestation of a correspondingly high order of instincts,—and this altogether irrespective of the structure or composition of the creature’s internal skeleton. The substance to which he looks as all-important in the case is brain, not bone. For were he to estimate the standing of the dolphin, not by its brain, but by its skeleton, he would have to assign to it a place, not only not in advance of its brethren the mammalia of the sea, but even in the rear of the reptiles of the sea, the marine tortoises, or turtles,—and scarce more than abreast of the osseous fishes. “Fishes,” says Professor Owen, in his “Lectures on the Vertebrate Animals,” “have the least proportion of earthy matter in their bones; birds the largest. The mammalia, especially the active, predatory species, have more earth, or harder bones, than reptiles. In each class, however, there are differences in the density of bone among its several members. For example, in the fresh-water fishes, the bones are lighter, and retain more animal matter, than in those which swim in the denser sea. And in the dolphin, a warm-blooded marine animal, they differ little in this respect from those of the sea-fish.” Such being the fact, it is surely but fair to inquire of the author of the “Vestiges,” why he should determine the rank and standing of the Delphinidæ according to one set of principles, and the rank and standing of the Placoids according to another and entirely different set? If the Delphinidæ are to be placed high in the scale, notwithstanding the softness of their skeletons, simply because their brains are large, why are the Placoids to be placed low in the scale, notwithstanding the largeness of their brains, simply because their skeletons are soft? It is not too much to demand, that on the principle which he himself recognizes as just, he should either degrade the dolphin or elevate the Placoid. For it is altogether inadmissible that he should reason on one set of laws when the exigencies of his hypothesis require that creatures with soft skeletons should be raised in the scale, and on another and entirely different set when its necessities demand that they should be depressed.

But do the Placoids possess in reality a large development of brain? I have examined the brains of almost all the common fish of our coast, both osseous and cartilaginous, not, I fear, with the skill of a Tiedemann, but all the more intelligently in consequence of what Tiedemann had previously done and written: and so I can speak with some little confidence on the subject, so far at least as my modicum of experience, thus acquired, extends. Of all the common fish of the Scottish seas, the spotted or lesser dog-fish bears, in proportion to its size, the largest brain; the gray or picked dog-fish ranks next in its degree of development; the Rays, in their various species, follow after; and the osseous fishes compose at least the great body of the rear; while still further behind, there lags a hapless class—the Suctorii, one of which, the glutinous hag, has scarce any brain, and one, the Amphioxus or lancelet, wants brain altogether. I have compared the brain of the spotted dog-fish with that of a young alligator, and have found that in scarce any perceptible degree was it inferior, in point of bulk, and very slightly indeed in point of organization, to the brain of the reptile. And the instincts of this Placoid family,—one of the truest existing representatives of the Placoids of the Silurian System[25] to which we can appeal,—correspond, we invariably find, with their superior cerebral development. I have seen the common dog-fish, Spinax Acanthias, hovering in packs in the Moray Frith, some one or two fathoms away from the side of the herring boat from which, when the fishermen were engaged in hauling their nets, I have watched them, and have admired the caution which, with all their ferocity of disposition, they rarely failed to manifest;—how they kept aloof from the net, even more warily than the cetacea themselves,—though both dog-fish and cetacea are occasionally entangled;—and how, when a few herrings were shaken loose from the meshes they at once darted upon them, exhibiting for a moment, through the green depths, the pale gleam of their abdomens, as they turned upon their sides to seize the desired morsels,—a motion rendered necessary by the position of the mouth in this family; and how next, their object accomplished, they fell back into their old position, and waited on as before. And I have been assured by intelligent fishermen, that at the deep-sea white-fishing, in which baited hooks, not nets, are employed, the degree of shrewd caution exercised by these creatures seems more extraordinary still. The hatred which the fisher bears to them arises not more from the actual amount of mischief which they do him, than from the circumstance that in most cases they persist in doing it with complete impunity to themselves. I have seen, said an observant Cromarty fisherman to the writer of these chapters, a pack of dog-fish watching beside our boat, as we were hauling our lines, and severing the hooked fish, as they passed them, at a bite, just a little above the vent, so that they themselves escaped the swallowed hook; and I have frequently lost, in this way, no inconsiderable portion of a fishing. I have observed, however, he continued, that when a fresh pack of hungry dog-fish came up, and joined the pack that had been robbing us so coolly, and at their leisure, a sudden rashness would seize the whole,—the united packs would become a mere heedless mob, and, rushing forward, they would swallow our fish entire, and be caught themselves by the score and the hundred. We may see something very similar to this taking place among even the shrewder mammalia. When pig refuses to take his food, his mistress straightway calls upon the cat, and, quickened by the dread of the coming rival, he gobbles up his rations at once. With the comparatively large development of brain, and the corresponding manifestations of instinct, which the true Placoids exhibit, we find other unequivocal marks of a general superiority to their class. In their reproductive organs they rank not with the common fishes, nor even with the lower reptiles, but with the Chelonians and the Sauria. Among the Rays, as among the higher animals, there are individual attachments formed between male and female: their eggs unlike the mere spawn of the osseous fishes, or of even the Batrachians, are, like those of the tortoise and the crocodile, comparatively few in number, and of considerable size: their young, too, like the young of birds and of the higher reptiles, pass through no such metamorphosis as those of the toad and frog, or of the amphibia generally. And some of their number—the common dog-fish for instance—are ovoviviparous, bringing forth their young, like the common viper and the viviparous lizard, alive and fully formed.

“But such features,” says the author of the “Vestiges,” referring chiefly to certain provisions connected with the reproductory system in the Placoids, “are partly partaken of by families in inferior sub-kingdoms, showing that they cannot truly be regarded as marks of grade in their own class.” Nay, single features do here and there occur in the inferior sub-kingdoms, which very nearly resemble single features in the placoid character and organization, which even very nearly resemble single features in the human character and organization; but is there any of the inferior sub-kingdoms in which there occurs such a collocation of features? or does such a collocation occur in any class of animals—setting the Placoids wholly out of view—which is not a high class? Nay, further, does there occur in any of the inferior sub-kingdoms—existing even as a single feature—that most prominent, leading characteristic of this series of fishes,—a large brain?

But is not the “cartilaginous structure” of the Placoids analogous to the embryonic state of vertebrated animals in general? Do not the other placoid peculiarities to which the author of the “Vestiges” refers,—such as the heterocercal or one-sided tail, the position of the mouth on the under side of the head, and the rudimental state of the maxillaries and intermaxillaries,—bear further analogies with the embryonic state of the higher animals? And is not “embryonic progress the grand key to the theory of development?” Let us examine this matter. “These are the characters,” says this ingenious writer, “which, above all, I am chiefly concerned in looking to; for they are features of embryonic progress, and embryonic progress is the grand key to the theory of development.” Bold assertion, certainly; but, then, assertion is not argument! The statement is not a reason for the faith that is in the author of the “Vestiges,” but simply an avowal of it; it is simply a confession, not a defence, of the Lamarckian creed; and, instead of being admitted as embodying a first principle, it must be put stringently to the question, in order to determine whether it contain a principle at all.

In the first place, let us remark, that the cartilaginous structure of the Placoids bears no very striking analogy to the cartilaginous structure of the higher vertebrata in the embryonic state. In the case of the Delphinidæ, with their soft skeletons, the analogy is greatly more close. Bone consists of animal matter, chiefly gelatinous, hardened by a diffusion of inorganic earth. In the bones of young and fœtal mammalia, inhabitants of the land, the gelatinous prevails; in the old and middle-aged there is a preponderance of the earth. Now, in the bones of the dolphin there is comparatively little earth. The analogies of its internal skeleton bear, not on the skeletons of its brethren the mature full-grown mammals of the land, but on the skeletons of their immature or fœtal offspring. But in the case of the true Placoids that analogy is faint indeed. Their skeletons contain true bone;—the vertebral joints of the Sharks and Rays possess each, as has been shown, an osseous nucleus, which retains, when subjected to the heat of a common fire, the complete form of the joint; and their cranial framework has its surface always covered over with hard osseous points. But though their skeletons possess thus their modicum of bone, unlike those of embryonic birds or mammals, they contain, in what is properly their cartilage, no gelatine. The analogy signally fails in the very point in which it has been deemed specially to exist. The cartilage of the Chondropterygii is a substance so essentially different from that of young or embryonic birds and mammals, and so unique in the animal kingdom, that the heated water in which the one readily dissolves has no effect whatever upon the other. It is, however, a curious circumstance, exemplified in some of the Shark family,[26] though it merely serves, in its exceptive character, to establish the general fact, that while the rays of the double fins, which answer to the phalanges, are all formed of this indissoluble cartilage, those rays which constitute their outer framework, with the rays which constitute the framework of all the single fins, are composed of a mucoidal cartilage, which boils into glue. At certain definite lines a change occurs in the texture of the skeleton; and it is certainly suggestive of thought, that the difference of substance which the change involves distinguishes that part of the skeleton which is homologically representative of the skeletons of the higher vertebrata, from that part of it which is peculiar to the creature as a fish, viz. the dorsal and caudal rays, and the extremities of the double fins. These emphatically ichthyic portions of the animal may be dissipated by boiling, whereas what Linnæus would perhaps term its reptilian portion abides the heat without reduction.

But is not the one-sided tail, so characteristic of the sharks, and of almost all the ancient Ganoids, also a characteristic of the young salmon just burst from the egg? Yes, assuredly; and, so far as research on the subject has yet extended, of not only the salmon, but of all the other osseous fishes in their fœtal state. The salmon, on its escape from the egg, is a little monster of about three quarters of an inch in length, with a huge heart-shaped bag, as bulky as all the rest of its body, depending from its abdomen. In this bag provident nature has packed up for it, in lieu of a nurse, food for five weeks; and, moving about every where in its shallow pool, with its provision knapsack slung fast to it, it reminds one disposed to be fanciful, save that its burden is on the wrong side, of Scottish soldiers of the olden time summoned to attend their king in war,—