The Foot-prints of the Creator

by Hugh Miller

So much for the fact of the existence of vertebrata in the Lower Silurian formations, and the argument founded on their presumed absence. Let me now refer—their presence being determined—to the tests of size and organization. Were these Silurian fishes of a bulk so inconsiderable as in any degree to sanction the belief that they had been developed shortly before from microscopic points? Or were they of a structure so low as to render it probable that their development was at the time incomplete? Were they, in other words, the embryos and fœtuses of their class? or did they, on the contrary, rank with the higher and larger fishes of the present time?

It is of importance that not only the direct bearing, but also the actual amount, of the evidence in this case, should be fairly stated. So far as it extends, the testimony is clear; but it does not extend far. All the vertebrate remains yet detected in the Silurian System, if we except the debris of the Upper Ludlow bone-bed, might be sent through the Post-Office in a box scarcely twice the size of a copy of the “Vestiges.” The naturalist of an exploring party, who, in crossing some unknown lake, had looked down over the side of his canoe, and seen a few fish gliding through the obscure depths of the water, would be but indifferently qualified, from what he had witnessed, to write a history of all its fish. Nor, were the some six or eight individuals of which he had caught a glimpse to be of small size, would it be legitimate for him to infer that only small-sized fish lived in the lake; though, were there to be some two or three large ones among them, he might safely affirm the contrary. Now, the evidence regarding the fishes of the Silurian formation very much resembles what that of the naturalist would be, in the supposed case, regarding the fishes of the unexplored lake; with, however, this difference, that as the deposits of the ancient system in which they occur have been examined for years in various parts of the world, and all its characteristic organisms, save the ichthyic ones, found in great abundance and fine keeping, we may conclude that the fish of the period were comparatively few. The palæontologist, so far as the question of number is involved, is in the circumstances, not of the naturalist who has only once crossed the unknown lake, but of the angler who, day after day, casts his line into some inland sea abounding in shell-fish and crustacea, and, after the lapse of months, can scarce detect a nibble, and, after the lapse of years, can reckon up all the fish which he has caught as considerably under a score. The existence of this great division of the animal kingdom, like that of the earlier reptiles during the Carboniferous period, did not form a prominent characteristic of those ages of the earth’s history in which they began to be.

The earliest discovered vertebral remains of the system—those of the Upper Ludlow rock—were found in digging the foundations of a house at Ludford, on the confines of Shropshire, and submitted, in 1838, by Sir Roderick Murchison to Agassiz, through the late Dr. Malcolmson of Madras. I used at the time to correspond on geological subjects with Dr. Malcolmson,—an accomplished geologist and a good man, too early lost to science and his friends,—and still remember the interest which attached on this occasion to his communication bearing the Paris post-mark, from which I learned for the first time that there existed ichthyic fragments greatly older than even the ichthyolites of the Lower Old Red Sandstone, and which made me acquainted with Agassiz’s earliest formed decision regarding them. Though existing in an exceedingly fragmentary condition,—for the materials of the thin dark-colored layer in which they had lain seemed as if they had been triturated in a mortar,—the ichthyologist succeeded in erecting them into six genera; though it may be very possible,—as some of these were formed for the reception of detached spines, and others for the reception of detached teeth,—that, as in the case of Dipterus and Asterolepis, the fragments of but a single genus may have been multiplied into two genera or more. And minute scale-like markings, which mingled with the general mass, and were at first regarded as the impressions of real scales, have been since recognized as of the same character with the scale-like markings of the Seraphim of Forfarshire, a huge crustacean. Even admitting, however, that a set of teeth and spines, with perhaps the shagreen points represented in [page 54], fig. 2, b, in addition, may have all belonged to but a single species of fish, there seem to be materials enough, among the remains found, for the erection of two species more. And we have evidence that at least two of the three kinds were fishes of the Placoid order, (Onchus Murchisoni and Onchus tenuistriatus,) and—as the supposed scales must be given up—no good evidence that the other kind was not. The ichthyic remains of the Silurian System next discovered were first introduced to the notice of geologists by Professor Phillips, at the meeting of the British Association in 1842.[22] They occurred, he stated, in a quarry near Hales End, at the base of the Upper Ludlow rock, immediately over the Aymestry Limestone, and were so exceedingly diminutive, that they appeared to the naked eye as mere discolored spots; but resolved under the microscope into scattered groupes of minute spines, like those of the Cheiracanthus, with what seemed to be still more minute scales, or, perhaps,—what in such circumstances could scarce be distinguished from scales,—shagreen points of the scale-like type. The next ichthyic organism detected in the Silurian rocks occurred in the Wenlock Limestone, a considerably lower and older deposit, and was first described in the “Edinburgh Review” for 1845 by a vigorous writer and masterly geologist, (generally understood to be Professor Sedgwick of Cambridge,) as “a characteristic portion of a fish undoubtedly belonging to the Cestraciont family of the Placoid order.” In the “American Journal of Science” for 1846, Professor Silliman figured, from a work of the States’ Surveyors, the defensive spine of a Placoid found in the Onondago Limestone of New York,—a rock which occurs near the base of the Upper Silurian System, as developed in the western world;[23] and in the same passage he made reference to a mutilated spine detected in a still lower American deposit,—the Oriskany Sandstone. In the Geological Journal for 1847, it was announced by Professor Sedgwick, that he had found “defences of fishes” in the Upper Llandeilo Flags, and by Sir Roderick Murchison, that the “defence of an Onchus” had been detected by the geologists of the Government survey, in the Limestone near Bala. Sir Roderick referred in the same number to the remains of a fish found by Professor Phillips in the Wenlock Shale. And such, up to the present time, is the actual amount of the evidence with which we have to deal, and the dates of its piecemeal production. Let us next consider the order of its occurrence in the geologic scale.

The better marked sub-divisions of the Silurian System, as described in the great work specially devoted to it, may be regarded as seven in number. An eight has since been added, by the transference of the Tilestones from the lower part of the Old Red Sandstone group, to the upper part of the Silurian group underneath; but in order the better to show how ichthyic discovery has in its slow course penetrated into the depths, I shall retain the divisions recognized as those of the system when that course began. The highest or most modern Silurian deposit, then, (No. 1 of the accompanying diagram,) is the Upper Ludlow Rock; and it is in the superior strata of this division that the bone-bed discovered in 1838 occurs; while the exceedingly minute vertebrate remains described by Professor Phillips in 1842 occur in its base. The division next in the descending order is the Aymestry Limestone, (No. 2;) the next (No. 3.) the Lower Ludlow rock; then (No. 4.) the Wenlock or Dudley Limestone occurs; and then, last and oldest deposit of the Upper Silurian formation, the Wenlock shale, (No. 5.) It is in the fourth, or Wenlock Limestone division, that the defensive spine described in the “Edinburgh Review” for 1845 as the oldest vertebrate organism known at the time, was found;[24] while the vertebrate organism found by Professor Phillips belongs to the fifth, or base deposit of the Upper Silurian. Further, the American spines of Onondago and Oriskany, described in 1846, occurred in rocks deemed contemporary with those of the Wenlock division. We next cross the line which separates the base of the Upper from the top of the Lower Silurian deposits, and find a great arenaceous formation, (No. 6,) known as the Caradoc Sandstones; while the Llandeilo Flags, (No. 7,) the formation upon which the sandstones rest, compose, according to the sections of Sir Roderick, published in 1839, the lowest deposit of the Lower Silurian rocks. And it is in the upper part of this lowest member of the system that the ichthyic defences, announced in 1847 by Professor Sedgwick, occur. Vertebrate remains have now been detected in the same relative position in the seventh and most ancient member of the system, that they were found to occupy in its first and most modern member ten years ago. But this is not all. Beneath the Lower Silurian division there occur vast fossiliferous deposits, to which the name “Cambrian System” was given, merely provisionally, by Sir Roderick, but which Professor Sedgwick still retains as representative of a distinct geologic period; and it is in these, greatly below the Lower Silurian base line, as drawn in 1839, that the Bala Limestones occur. The Plynlimmon rocks (a)—a series of conglomerate, grauwacke, and slate beds, several thousand yards in thickness—intervene between the Llandeilo Flags and the Limestones of Bala, (b.) And, of consequence, the defensive spine of the Onchus, announced in 1847 as detected in these limestones by the geologists of the Government Survey, must have formed part of a fish that perished many ages ere the oldest of the Lower Silurian formations began to be deposited.

Let us now, after this survey of both the amount of our materials, and the order and time of their occurrence, pass on to the question of size, as already stated. Did the ichthyic remains of the Silurian System, hitherto examined and described, belong to large or to small fishes? The question cannot be altogether so conclusively answered as in the case of those Ganoids of the Lower Old Red Sandstone whose dermal skeletons indicate their original dimensions and form. In fishes of the Placoid order, such as the Sharks and Rays, the dermal skeleton is greatly less continuous and persistent than in such Ganoids as the Dipterians and Cœlacanths; and when their remains occur in the fossil state, we can reason, in most instances, regarding the bulk of the individuals of which they formed part, merely from that of detached teeth or spines, whose proportion to the entire size of the animals that bore them cannot be strictly determined. We can, indeed, do little more than infer, that though a large Placoid may have been armed with but small spines or teeth, a small Placoid could not have borne very large ones. And to this Placoid order all the Silurian fish, from the Aymestry Limestone to the Cambrian deposits of Bala inclusive, unequivocally belong. Nor, as has been already said, is there sufficient evidence to show that any of the ichthyic remains of the Upper Ludlow rocks do not belong to it. It is peculiarly the order of the system. The Ludlow bone-bed contains not only defensive spines, but also teeth, fragments of jaws, and shagreen points; whereas, in all the inferior deposits which yield any trace of the vertebrata, the remains are those of defensive spines exclusively. Let us, then, take the defensive spine as the part on which to found our comparison.

One of the best marked Placoids of the Upper Ludlow bone-bed is that Onchus Murchisoni to which the distinguished geologist whose name it bears refers, in his communication, as so nearly resembling the oldest Placoid yet known,—that of the Bala Limestone. And the living fishes with which the Onchus Murchisoni must be compared, says Agassiz, though “the affinity,” he adds, “may be rather distant,” are those of the genera “Cestracion, Centrina, and Spinax.” I have placed before me a specimen of recent Spinax, of a species well known to all my readers on the sea-coast, the Spinax Acanthias, or common dog-fish, so little a favorite with our fishermen. It measures exactly two feet three inches in length; and of the defensive spines of its two dorsals,—these spear-like thorns on the creature’s back immediately in advance of the fins, which so frequently wound the fisher’s hand,—the anterior and smaller measures, from base to point, an inch and a half, and the posterior and larger, two inches. I have also placed before me a specimen of Cestracion Phillippi, (the Port Jackson Shark,) a fish now recognized as the truest existing analogue of the Silurian Placoids. It measures twenty-two three fourth inches in length, and is furnished, like Spinax, with two dorsal spines, of which the anterior and larger measures from base to point one one half inch, and the posterior and smaller, one one fifth inch. But the defensive spine of the Onchus Murchisoni, as exhibited in one of the Ludlow specimens, measures, though mutilated at both ends, three inches and five eighth parts in length. Even though existing but as a fragment, it is as such nearly twice the length of the largest spine of the dog-fish, unmutilated and entire, and considerably more than twice the length of the largest spine of the Port Jackson Shark. The spines detected by Professor Phillips, in an inferior stratum of the same upper deposit, were, as has been shown, of microscopic minuteness; and when they seemed to rest on the extreme horizon of ichthyic existence as the most ancient remains of their kind, the author of the “Vestiges” availed himself of the fact. He regarded the little creatures to which they had belonged is the fœtal embryos of their class, or—to employ the language of the Edinburgh Reviewer—as “the tokens of Nature’s first and half-abortive efforts to make fish out of the lower animals.” From the latter editions of his work, the paragraph to which the Reviewer refers has, I find, been expunged; for the horizon has greatly extended, and what seemed to be its line of extreme distance has travelled into the middle of the prospect. But that the passage should have at all existed is a not uninstructive circumstance, and shows how unsafe it is, in more than external nature, to regard the line at which, for the time, the landscape closes, and heaven and earth seem to meet, as in reality the world’s end. The Wenlock spine, though certainly not microscopic, is, I am informed by Sir Philip Egerton, of but small size; whereas the contemporary spine of the Onondago Limestone, though comparatively more a fragment than the spine of the Upper Ludlow Onchus,—for it measures only three inches in length,—is at least five times as bulky as the largest spine of Spinax Acanthias. Representing one of the massier fishes disporting amid the some four or five small ones, of which in my illustration, the naturalist catches a glimpse in fording the unknown lake, it at least serves to show that all the Silurian ichthyolites must not be described as small, seeing that not only might many of its undetected fish have been large, but that some of those which have been detected were actually so. Another American spine, of nearly the same formation,—for it occurs in a limestone, varying from twenty to seventy feet in thickness, which immediately overlies that of the Onondago deposit, though still more fragmentary than the first, for its length is only two three eighth inches,—maintains throughout a nearly equal thickness,—a circumstance in itself indicative of considerable size; and in positive bulk it almost rivals the Onondago one. Of the Lower Silurian and Bala fishes no descriptions or figures have yet appeared. And such, up to the present time, is the testimony derived from this department of Geology, so far as I have been able to determine it, regarding the size of the ancient Silurian vertebrata. “No organism,” says Professor Oken, “is, nor ever has one been, created, which is not microscopic.” The Professor’s pupils and abettors, the assertors of the development hypothesis, appeal to the geological evidence as altogether on their side in the case; and straightway a few witnesses enter court. But, lo! among the expected dwarfs, there appear individuals of more than the average bulk and stature.

Fig. 47.