The Foot-prints of the Creator

by Hugh Miller

“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.

Fig. 48.

a. Tail of Spinax Acanthias.

b. Tail of Ichthyosaurus Tenuirostris, (Buckland.)

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.