Part 12

It has been confidently asserted, not merely that it _may_, but that it _does_. Ever since the publication, in 1839, of Sir Roderick Murchison’s great work on the Silurian System, it had been known that the remains of fishes occur in a bed of the “Ludlow Rock,”—one of the most modern deposits of the _Upper_ Silurian division; and subsequent discoveries both in England and America, had shown that even the _base_ of this division has its ichthyic organisms. But for year after year, the lower half of the system,—a division more than three thousand feet in thickness,—had failed, though there were hands and eyes busy among its deposits, to yield any vertebrate remains. During the earlier half of the first great period of organic existence, though the polyparia, radiata, articulata, and mollusca, existed, as their remains testified, by myriads, fish had, it was held, not yet entered upon the scene; and the assertors of the development theory founded largely on the presumed fact of their absence. “It is still customary,” says the author of the “Vestiges of Creation,” in his volume of “Explanations,” “to speak of the earliest fauna as one of an elevated kind. When rigidly examined, it is not found to be so. IN THE FIRST PLACE, IT CONTAINS NO FISH. There were seas supporting crustacean and molluscan life, but _utterly devoid of a class of tenants who seem able to live in every example of that element which supports meaner creatures_. This single fact, that only invertebrated animals now lived, is surely in itself a strong proof that, in the course of nature, _time_ was necessary for the creation of the superior creatures. And if so, it undoubtedly is a powerful evidence of such a theory of development as that which I have presented. If not, let me hear an equally plausible reason for the great and amazing fact, that seas were for numberless ages destitute of fish. I fix my opponents down to the consideration of this fact, so that no diversion respecting high molluscs shall avail them.” And how is this bold challenge to be met?

Most directly, and after a fashion that at once discomfits the challenger.

It might be rationally enough argued in the case, that the author of the “Vestiges” was building greatly more on a piece of purely negative evidence,—the presumed absence of fish from the Lower Silurian formations,—than purely negative evidence is, from its nature as such, suited to bear; that only a very few years had passed since it was known that vertebrate remains occurred in the _Upper_ Silurian, and only a few more since they had been detected in the Old Red Sandstone; nay, that within the present century their frequent occurrence in even the Coal Measures was scarce suspected; and that, as his argument, had it been founded twelve years ago on the supposed absence of fishes from the Upper Silurian, or twenty years ago on the supposed absence of fishes from the Old Red Sandstone, would have been quite as plausible in reference to its negative data then as in reference to its negative data now, so it might now be quite as erroneous as it assuredly would have been then. Or it might be urged, that the fact of the absence of fish from the Lower Silurians, even were it really a fact, would be in no degree less reconcilable with the theory of creation by direct act, than with the hypothesis of gradual development. The fact that Adam did not exist during the first, second, third, fourth, and fifth days of the introductory week of Scripture narrative, furnishes no argument whatever against the fact of his creation on the sixth day. And the remark would of course equally apply to the non-existence of fishes during the Lower Silurian period, had they been really non-existent at the time, and to their sudden appearance in that of the Upper. But the objection admits of a greatly more conclusive answer. “I fix my opponents down,” says the author of the “Vestiges,” “to the consideration of this fact,” _i. e._ that of the absence of fishes from the earliest fossiliferous formations. And I, in turn, fix you down, I reply, to the consideration of the antagonist fact, not negative, but positive, and now, in the course of geological discovery, fully established, that fishes were _not_ absent from the earliest fossiliferous formations. From none of the great geological formations were fishes absent,—not even from the formations of the Cambrian division. “The Lower Silurian,” says Sir Roderick Murchison, in a communication with which, in 1847, he honored the writer of these chapters, “is no longer to be viewed as an invertebrate period; for the _Onchus_ (species not yet decided) has been found in the Llandeilo Flags and in the Lower Silurian rocks of Bala. In one respect I am gratified by the discovery; for the form is so very like that of the _Onchus Murchisoni_ of the Upper Ludlow rock, that it is clear the Silurian system is one great natural-history series, as is proved, indeed, by all its other organic remains.” It may be mentioned further, in addition to this interesting statement, that the Bala spine was detected in its calcareous matrix by the geologists of the Government Survey, and described to Sir Roderick as that of an _Onchus_, by a very competent authority in such matters,—Professor Edward Forbes, and that the annunciation of the existence of spines of fishes in the Llandeilo Flags we owe to one of the most cautious and practised geologists of the present age,—Professor Sedgwick of Cambridge.

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_.

{ +-----+ { Upper | | Fish, 1838, { Ludlow. | 1 | (Murchison.) { | | Fish, 1842, { +-----+ (Phillips.) { Aymestry | | { Limestone. | 2 | { +-----+ UPPER SILURIAN ROCKS. { Lower | | { Ludlow. | 3 | { +-----+ { Wenlock | | Fish, 1845, { Limestone. | 4 | (Sedgwick.) { | | Fish, 1846, { +-----+ (Silliman.) { Wenlock | 5 | Fish, 1847. { Shale. | | (Phillips.) +-----+ ----- { +-----+ { Caradoc | | { Sandstone, | 6 | { &c. | | LOWER SILURIAN ROCKS. { +-----+ { Llandeilo | | Fish, 1847, { Flags, &c. | 7 | (Sedgwick.) { +-----+ ----- { +-----+ { Plynlimmon | | { Group. | _a_ | { +-----+ { Bala | | Fish, 1847, CAMBRIAN ROCKS. { Limestone. | _b_ | (Geologists of { | | Government { +-----+ Survey.) { Snowdon | | { Group. | _c_ | Fucoids. { | | { +-----+

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.