Part 15

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,—

“Each on his _back_, a slender store, His forty days’ provision bore, As ancient statutes tell.”

Around that terminal part of the creature’s body traversed by the caudal portion of the vertebral column, which commences in the salmon immediately behind the ventrals, there runs at this period, and for the ensuing five weeks in which it does not feed, a membranous fringe or fin, which exactly resembles that of the tadpole, and which, existing simply as an expansion of the skin, exhibits no mark or rays. In the place of the true caudal fin, however, we may detect with the assistance of a lens, an internal framework with two well-marked lobes, and ascertain, further, that this tail is set on awry,—the effect of a slight upward bend in the creature’s body. And when viewed in a strong light as a transparency, we perceive that the spinal cord takes the same upward bend, and, as in the sturgeon, passes in an exceedingly attenuated form into the upper lobe. What may be regarded as the _design_ of the arrangement is probably to be found in the peculiar form given to the little creature by the protuberan bag in front. A wise instinct teaches it, from the moment of its exclusion from the egg, to avoid its enemies. In the instant the human shadow falls upon its pool, we see it darting into some recess at the side or bottom, with singular alacrity; and in order to enable it to do so, and to steer itself aright,—as, like an ill-trimmed vessel, deep in the water ahead, the balance of its body is imperfect,—there is, if I may so express myself, a heterocercal peculiarity of helm required. It has got an irregularly-developed tail to balance an irregularly-developed body, as skiffs _lean_ on the one beam and _full_ on the other require, in rowing, a cast of the rudder to keep them straight in their course.

Sinking altogether, however, the final cause of the peculiarity, and regarding it simply as a _fœtal_ one, that indicates a certain stage of imperfection in the creature in which it occurs, on what principle, I ask, are we to infer that what is a sign of immaturity in the young of one set of animals, is a mark of inferior organization in the adult forms of another set? The want of eyes in any of the animal families, or the want of organs of progression, or a fixed and sedentary condition, like that of the oyster, are all marks of great inferiority. And yet, if we admit the principle, that what are evidences of immaturity in the young members of one family are signs of inferior organization in the fully-grown members of another, it could easily be shown that eyes and legs are defects, and that the unmoving oyster stands higher in the scale than the ever-restless fish or bird. The immature _Tubularia_ possess locomotive powers, whereas in their fully developed state they remain fixed to one spot in their convoluted tubes. The immature _Lepas_ is furnished with members well adapted for swimming, and with which it swims freely; as it rises towards maturity, these become blighted and weak; and, when fully grown,—fixed by its fleshy pedicle to the rock or floating log to which it attached itself in its transition state,—it is no longer able to swim. The immature _Balanus_ is furnished with two eyes: in its state of maturity these are extinguished, and it passes its period of full development in darkness. Further, it is not generally held that in the human family a white skin is a decided mark of degradation, but rather the reverse; and yet nothing can be more certain than that the Negro fœtus has a white skin. Since eyes, and organs of progression, and a power of moving freely, and a white skin, are mere embryonic peculiarities in the _Balanus_, the _Lepas_, the _Tubularia_, and the Negro, and yet are in themselves, when found in the mature animal, evidences of a high, not of a low standing, on what principle, I ask are we to infer that the peculiarity of a heterocercal tail, embryonic in the salmon, is, when found in the mature Placoid, an evidence, not of a high standing, but of a low? Every true analogy in the case favors an exactly opposite view. In the heterocercal or one-sided tail, the vertebral joints gradually diminish, as in the tails of the _Sauria_ and _Ophidia_, till they terminate in a point; whereas the homocercal tail common to the osseous fishes exhibits no true analogy with the tails of the higher orders. Its abruptly terminating vertebral column, immensely developed posterior processes, and broadly expanded osseous rays, seem to be simply a few of the many marks of decline and degradation which fishes, the oldest of the vertebrata, exhibit in this late age of the world, and which, in at least the earlier geologic periods, when they were greatly younger as a class, they did not betray.

In illustration of this view, I would fain recommend to the reader a simple experiment. Let him procure the tail of a common dog-fish, (fig. 48, _a_,) and cutting it across about half an inch above where the caudal fin begins, let him boil it smartly for about half an hour. He will first see it swell and then burst, all around those thinner parts of the fin that are traversed by the caudal rays,—wholly mucoidal, as shown by this test, in their texture, and which yield to the boiling water, as if formed of isinglass. They finally dissolve, and drop away, with the surrounding cuticular integument; and then there only remains, as the insoluble framework of the whole, the bodies of the vertebræ, with their neural and hœmal processes. The tail has now lost much of its ichthyic character, and has acquired, instead, a considerable degree of resemblance to the reptilian tail, as exemplified in the saurians. I have introduced into the wood-cut, for the purpose of comparison, the tail of the ichthyosaurus, (_b_.) It consists, like the other, of a series of gradually diminishing vertebræ, and must have also supported, says Professor Owen, a propelling fin, placed vertically, as in the shark, which, however, from its perishable nature, has in every instance disappeared in the earth, as that of the dog-fish disappears in the boiling water. It will be seen that its processes are comparatively smaller than those of the fish, and that the bodies of its vertebræ are shorter and bulkier; but there is at least a general correspondence of the parts; and were the tail of the crocodile, of which the vertebral bodies are slender and the processes large, to be substituted for that of the enaliosaur here, the correspondence would be more marked still. After thus _developing_ the tail of the reptile out of that of the fish,—as the cauldron-bearing Irish magician of the tale developed young ladies out of old women,—simply by _boiling_, let the reader proceed to a second stage of the experiment, and see whether he may not be able still further to develope the reptilian tail so obtained, into that of the mammal, by _burning_. Let him spread it out on a piece of iron hoop, and thrust it into the fire; and then, after exposure for some time to a red heat has consumed and dissipated its merely cartilaginous portions, such as the neural and hœmal processes, with the little pieces which form the sides of the neural arch, and left only the whitened bodies of the vertebræ, let him say whether the bony portion which remains does not present a more exact resemblance to the mammiferous tail—that of the dog, for example—than any thing else he ever saw. The Lamarckians may well deem it an unlucky circumstance, that one special portion of their theory should demand the depreciation of the heterocercal tail, seeing that it might be represented with excellent effect in another, as not merely a connecting link in the upward march of progression between the tail of the true fish and that of the true reptile, but as actually containing in itself—as the caterpillar contains the future pupa and butterfly—the elements of the reptilian and mammiferous tail. If there be any virtue in analogy, the heterocercal tail is, I repeat, of a decidedly higher type than the homocercal one. It furnishes the first example in the vertebrata of the coccygeal vertebræ diminishing to a point, which characterizes not only all the higher reptiles, but also all the higher mammals, and which we find represented by the _Os coccygis_ in man himself. But to this special point I shall again refer.

With regard to that rudimentary state of the _occipital_ framework of the Placoids to which the author of the “Vestiges” refers, it may be but necessary to say that, notwithstanding the simplicity of their box-like skulls, they bear in their character, as cases for the protection of the brain, at least as close an analogy to the skulls of the higher animals, as those of the osseous fishes, which consist usually of the extraordinary number of from sixty to eighty bones,—a mark—the author of the “Vestiges” himself being judge in the case—rather of inferiority than the reverse. “Elevation is marked in the scale,” we find him saying, “by an animal exchanging a multiplicity of parts serving one end, for a smaller number.” The skull of a cod consists of about thrice as many separate bones as that of a man. But I do not well see that in this case the fact either of _simplicity_ in excess or of _multiplicity_ in excess can be insisted upon in either direction, as a proper basis for argument. Nearly the same remark applies to the maxillaries as to the skull. The under jaw in man consists of a single bone; that of the thornback—if we do not include the two suspending _ribs_, which belong equally to the upper jaw—of two bones, (the number in all the mammiferous quadrupeds:) that of the cod of four bones, and, if we include the suspending _ribs_, of twelve. On what principle are we to hold, with _one_ as the representative number of the highest type of jaw, that _two_ indicates a lower standing than _four_, or _four_ than _twelve_? In reference to the further statement, that in many of the ancient fishes “traces can be observed of the muscles having been attached to the external plates, strikingly indicating their low grade as vertebrate animals,” it may be answer enough to state, that the peculiarity in question was not a characteristic of the _most_ ancient fishes,—the Placoids of the Silurian system,—but of some Ganoids of the succeeding systems. The reader may remember, as a case in point, the example furnished by the nail-like bone of _Asterolepis_, figured in page 111, in which there exists depressions resembling that of the round ligament in the head of the quadrupedal thigh-bone. And as for the remark that the opening of the mouth of the Placoid, “on the under side of the head,” is indicative of a low embryonic condition, it might be almost sufficient to remark, in turn, that the lowest family of fishes—that to which the supposed worms of Linnæus belong—have the mouth not under, but at the anterior termination of the head,—in itself an evidence that the position of the mouth at the extremity of the muzzle, common to the greater number of the osseous fishes, can be no very high character, seeing that the humblest of the _Suctorii_ possess it; and that many osseous fishes, whose mouths open, not on the under, but the upper side of the snout, as in the distorted and asymmetrical genus _Platessa_, are not only in no degree superior to their bony neighbors, and far inferior to the placoid ones, but bear, in direct consequence of the arrangement, an expression of unmistakable stupidity. The objection, however, admits of a greatly more conclusive reply.

“This fish, to speak in the technical language of Agassiz,” says the Edinburgh Reviewer, in reference to the ancient ichthyolite of the Wenlock Shale, “undoubtedly belongs to the Cestraciont family of the Placoid order,—proving to demonstration that the oldest known fossil fish [1845] belongs to the highest type of that division of the vertebrata.” I may add, that the character and family of this ancient specimen was determined by our highest British authority in fossil ichthyology, Sir Philip Egerton. And it is in depreciation of Professor Sedgwick’s statement regarding its high standing that the author of the “Vestiges” refers to the supposed inferiority indicated by a mouth opening, not at the extremity of the muzzle, but under the head. Let us, then, fully grant, for the argument’s sake, that the occurrence of the mouth in the muzzle _is_ a sign of superiority, and its occurrence under the head a mark of great inferiority, and then ascertain how the fact stands with regard to the _Cestracion_. “The Cestracion sub-genus,” says Mr. James Wilson, in his admirable treatise on fishes, which forms the article ICHTHYOLOGY in the “Encyclopædia Britannica,” “has the temporal aperture, the anal fin, and rounded teeth, of _Squalus Mustelus_; _but the mouth is TERMINAL, or AT THE EXTREMITY OF THE POINTED MUZZLE_.” The accompanying figure, (fig. 49,) taken from a specimen of _Cestracion_ in the collection of Professor John Fleming, may be recorded as of some little interest, both from its direct bearing on the point in question, and from the circumstance that it represents, not inadequately for its size, the sole surviving species (_Cestracion Phillippi_) of the oldest vertebrate family of creation. With this family, so far as is yet known, ichthyic existence first began. It does not appear that on the globe which we inhabit there was ever an ocean tenanted by living creatures at all that had not its _Cestracion_,—a statement which could not be made regarding any other vertebrate family. In Agassiz’s “Tabular View of the Genealogy of Fishes,” the Cestracionts, and they only, sweep across the entire geologic scale. And, as shown in the figure, the mouth in this ancient family, instead of opening, as in the ordinary sharks, under the middle of the head, to expose them to the suspicion of being creatures of low and embryonic character, opened in a broad, honest-looking muzzle, very much resembling that of the hog. The mouths of the most ancient Placoids of which we know any thing, _did not_, I reiterate, _open under their heads_.

But why introduce the element of embryonic progress into this question at all? It is not a question of embryonic progress. The very legerdemain of the sophist—the juggling by which he substitutes his white balls for black, or converts his pigeons into crows—consists in the art of attaching the conclusions founded on the facts or conditions of one subject, to some other subject essentially distinct in its nature. Gestation is not creation. The history of the young of animals in their embryonic state is simply the history of the fœtal young; just as the history of insect transformation, in which it has been held by good men, but weak reasoners, that there exists direct evidence of the doctrine of the resurrection, is the history of insect transformation, and of nothing else. True, the human mind is so constituted that it converts all nature into a storehouse of comparisons and analogies; and this fact of the metamorphosis of the creeping caterpillar, after first passing through an intermediate period of apparent death as an inert aurelia, into a winged image, seemed to have seized on the human fancy at a very early age, as wonderfully illustrative of life, death, and the future state. The Egyptians wrapped up the bodies of their dead in the chrysalis form, so that a mummy, in their apprehension, was simply a human pupa, waiting the period of its enlargement; and the Greeks had but one word in their language for butterfly and the soul. But not the less true is it, notwithstanding, that the facts of insect transformation furnish no legitimate key to the totally distinct facts of a resurrection of the body, and of a life after death. And on what principle, then, are we to trace the origin of past dynasties in the changes of the fœtus if not the rise of the future dynasty in the transformations of the caterpillar? “These [embryonic] characters [that of the heterocercal tail, and of the mouth of the ordinary shark type] are essential and important,” remarks the author of the “Vestiges,” “whatever the Edinburgh Reviewer may say to the contrary;—they are the characters which, above all, I am chiefly concerned in looking to, for they are the features of embryonic progress, and embryonic progress is the grand key to the theory of development.” Yes; the grand key to the theory of _fœtal_ development; for embryonic progress _is_ fœtal development. But on what is the assertion based that they form a key to the history of creation? Aurelia are not human bodies laid out for the sepulchre, nor are butterflies human souls; as certainly gestation is not creation, nor a life of months in the uterus a succession of races for millions of ages outside of it. On what grounds, then, is the assertion made? Does it embody the result of a discovery or announce the message of a revelation? Did the author of the “Vestiges” find it out for himself, or did an angel from heaven tell it him? If it be a discovery, show us, we ask, the steps through which you have been conducted to it; if a revolution produce, for our satisfaction, the evidence on which it rests. For we are not to accept as data, in a question of science, idle comparisons or vague analogies, whether produced through the intentional juggling of the sophist, or involuntarily conjured up in the dreamy delirium of an excited fancy.

It is one of the difficulties incident to the task of replying to any dogmatic statement of error, that every mere annunciation of a false fact or false principle must be met by elaborate counter-statement or carefully constructed argument and that prolixity is thus unavoidably entailed on the controversialist who labors to set right what his antagonist has set wrong. The promulgator of error may be lively and entertaining, whereas his pains-taking confutator runs no small risk of being tedious and dull. May I, however, solicit the forbearance of the reader, if, after already spending much time in skirmishing on ground taken up by the enemy,—one of the disadvantages incident to the mere defendant in a controversy of this nature,—I spend a little more in indicating what I deem the proper ground on which the standing of the earlier vertebrata should be decided. To the test of _brain_ I have already referred, as all-important in the question: I would now refer to the test of what may be termed _homological symmetry of organization_.

THE PROGRESS OF DEGRADATION. ITS HISTORY.