Part 17

Now, in tracing the geologic history of the ichthyic tail, we find these several steps or gradations from the heterocercal to the homocercal, represented by periods and formations. The Siluran periods may be regarded as representative of that true heterocercal tail of the Placoids, exemplified in _Spinax_, (page 172, fig. 48,) and _Cestracion_, (page 177, fig. 49.) The whole caudal portion of this latter animal, commencing immediately behind the ventrals, is, as becomes a true tail, slim, when compared with its trunk; the vertebræ are of very considerable solidity; the rays mucoidal; and where the spinal column runs into the terminal fin, it takes such an upward turn as that which the horse-jockey imparts, by the process of _nicking_, to the tails of the hunter and the racehorse. And with the heterocercal tail, so true in its homologies to the tails of the higher vertebrata, we find associated, as has been shown, the true homological position of the fore limbs. With the commencement of the Old Red Sandstone the ganoidal tail first presents itself; and we become sensible of a change in the structure of the attached fin, similar to that exemplified in the caudal rays of the sturgeon. As shown by the irregularly-angular patch of scales which in all the true Cœlacanths, and almost all the Dipterians,[29] runs through the _upper_ lobe of the fin, and terminates in a point, (see fig. 50,) it must have possessed the gradually diminishing vertebræ, or a diminishing spinal cord, their analogue; but the rays, fairly _set_, as their state of keeping in the rocks certify, exist as narrow oblong plates of solid bone; and their anterior edges are strengthened by a line of osseous defences, that pass from scales into rays. And in harmonious accompaniment with this fairly _stereotyped_ edition of the ichthyic tail, we find, in the fishes in which it appears, the first instance of displacement of _limb_,—the bases of the pectorals being removed from their original position, and stuck on to the nape of the neck. It may be remarked, in passing, that in the tails of two ganoidal genera of this period,—the _Coccosteus_ and _Pterichthys_,—the analogies traceable lie rather in the direction of the tails of the Rays than in those of the Sharks; and that one of these, the _Coccosteus_, seems, as has been already intimated, to have had no pectorals, while it is doubtful whether in the _Pterichthys_ the pectorals were not attached to the shoulder, instead on the head. In the Carboniferous and Permian systems there occur, especially among the numerous species of the genus _Palæoniscus_, tails of the type exemplified by the internal angle of the tail of the sturgeon: the lozenge-shaped scales run in acutely angular patches through their upper lobes; but such is their extreme flatness, as shown by the disposition of the enamelled covering, that it appears exceedingly doubtful whether any vertebral column ran beneath;—they seem but to have covered greatly diminished prolongations of the spinal cord. In the base of the Secondary division,—another long stage towards the existing state of things,—we find, with the homocercal tail, which now appears for the first time, numerous tails like that of the _Lepidosteus_, (fig. 51,) of an intermediate type;—they are rather tails set on awry than truly heterocercal. The diminished cord has disappeared from among the fin rays. In the numerous Lepidoid genus, and the genera _Semionotus_ and _Tetra gonolepis_,—all ganoidal fishes of the Secondary period—this intermediate style is very marked; while in their contemporaries of the genera _Uræus_, _Microdon_, and _Pycnodus_, we find the earliest examples of true homocercal tails. And in the Ctenoids and Cycloids of the Chalk the homocercal tail receives its fullest development. It finds bases for its rays in broad non-homological processes, that spread out behind abruptly-terminating vertebræ, (fig. 52,) in the same period in which, by a strange process of degradation, the four ichthyic limbs are first gathered into a cluster, and hung about the neck.[30]

I am aware that by some very distinguished comparative anatomists, among the rest Professor Owen, the attachment, so common among fishes, of the scapular arch and the fore limbs to the occipital bone, is regarded, not as a displacement, but as a normal and primary condition of the parts. Recognizing in the scapular bones the _ribs_ of the occipital _centrum_, the anatomists of this school of course consider them, when found articulated to the occiput, as in their proper and original place, and as in a state of natural dislocation when removed, as in all the reptiles, birds, and mammals, farther down. We find Professor Oken borrowing support to his hypothesis from this view. The limbs, he tells us, are simply ribs, that in the course of ages have been set free, and have become by development what they now are. And it is unquestionably a curious and interesting fact, that there are certain animals, such as the crocodile, in which every centrum of the vertebral column, and of every _vertebra_ of the head, has its ribs or rib-like appendages, with the exception of the occipital _centrum_. And it is another equally curious fact, that there is another certain class of animals, such as the osseous horn-covered fishes, with the Sturionidæ, Salamandroidei, and at least one genus among the Placoids, (the Chimæroidei,) in which this occipital centrum bears as its _ribs_ the scapular bones, with their appendages the fore limbs. It is the _centrum_ without _ribs_ that is selected in these animals as the centrum to which the scapular _ribs_ should be attached. Be it remembered, however, that while it is unquestionably the part of the comparative anatomist to determine the relations and homologies of those parts of which all animals are composed, and to interpret the significancy in the scale of being of the various modes and forms in which they exist, it is as unquestionably the part of the geologist to declare their history, and the order of their succession _in time_. The questions which fall to be determined by the geologist and anatomist are entirely different. It is the function of the anatomist to decide regarding the high and the low, the typical and the aberrant; and so, beginning at what is lowest or highest in the scale, or least or most symmetrical in type, he passes through the intermediate forms to the opposite extreme: and such is the order natural and proper to his science. It is the vocation of the geologist, on the other hand, to decide regarding the early and the late. It is with _time_, not with _rank_, that he has to deal. Nor is it in the least surprising that he should seem at issue with the comparative anatomist, when, in classifying his groupes of organized being according to the periods of their appearance, there is an order of arrangement forced upon him, different from that which, on an entirely different principle, the anatomist pursues. Nor can there be a better illustration of a collision of this kind, than the one furnished by the case in point. That peculiarity of structure which, as the lowest in the vertebral skeleton, is to the comparative anatomist the primary and original one, and which, as such, furnishes him with his starting point, is to the geologist not primary, but secondary, simply because it was not primary, but secondary, in the order of its occurrence. It belongs, so far as we yet know, not to the _first_ period of vertebrate existence, but to the _second_; and appears in geologic history as does that savage state which certain philosophers have deemed the original condition of the human species, in the history of civilization, when read by the light of the Revealed Record, under the shadow of those gigantic ruins of the East that date only a few centuries after the Flood. It is found to be a _degradation_ first introduced during the lapse of an intermediate age,—not the normal condition which obtained during the long cycles of the primal one. It indicates, not the starting point from which the race of creation began, but the stage of retrogradation beyond it at which the pilgrims who set out in a direction opposite to that of the goal first arrived.[31]

This fact of degradation, strangely indicated in geologic history, with reference to all the greater divisions of the animal kingdom, has often appeared to me a surpassingly wonderful one. We can see but imperfectly, in those twilight depths to which all such subjects necessarily belong; and yet at times enough does appear to show us what a very superficial thing infidelity may be. The general advance in creation has been incalculably great. The lower divisions of the vertebrata preceded the higher;—the fish preceded the reptile, the reptile preceded the bird, the bird preceded the mammiferous quadruped, and the mammiferous quadruped preceded man. And yet, is there one of these great divisions in which, in at least some prominent feature, the present, through this mysterious element of degradation, is not inferior to the past? There was a time in which the ichthyic form constituted the highest example of life; but the seas during that period did not swarm with fish of the degraded type. There was, in like manner, a time when all the carnivora and all the herbivorous quadrupeds were represented by reptiles; but there are no such magnificent reptiles on the earth now as reigned over it then. There was an after time, when birds seem to have been the sole representatives of the warm-blooded animals; but we find, from the prints of their feet left in sandstone, that the tallest men might have

“Walked under their huge legs, and peeped about.”

Further, there was an age when the quadrupedal mammals were the magnates of creation; but it was an age in which the sagacious elephant, now extinct, save in the comparatively small Asiatic and African circles, and restricted to two species, was the inhabitant of every country of the Old World, from its southern extremity to the frozen shores of the northern ocean; and when vast herds of a closely allied and equally colossal genus occupied its place in the New. And now, in the times of the high-placed human dynasty,—of those formally delegated monarchs of creation, whose nature it is to look behind them upon the past, and before them, with mingled fear and hope, upon the future,—do we not as certainly see the elements of a state of ever-sinking degradation, which is to exist for ever, as of a state of ever-increasing perfectibility, to which there is to be no end? Nay, of a higher race, of which we know but little, this much we at least know, that they long since separated into two great classes,—that of the “elect angels,” and of “angels, that kept not their first estate.”

EVIDENCE OF THE SILURIAN MOLLUSCS—OF THE FOSSIL FLORA. ANCIENT TREE.

After dwelling at such length on the earlier fishes, it may seem scarce necessary to advert to their lower contemporaries the mollusca,—that great division of the animal kingdom which Cuvier places second in the descending order, in his survey of the entire series, and first among the invertebrates; and which Oken regards as the division out of which the immediately preceding class of the vertebral animals have been developed. “The fish,” he says, “is to be viewed as a mussel, from between whose shells a monstrous abdomen has grown out.” There is, however, a peculiarity in the molluscan group of the Silurian system, to which I must be permitted briefly to refer, as, to employ the figure of Sterne, it presents “two handles” of an essentially different kind, and as in all such two-handled cases, the mere special pleader is sure to avail himself of only the handle which best suits his purpose for the time.

Cuvier’s first and highest class of the molluscs is formed of what are termed the Cephalopods,—a class of creatures possessed of great freedom of motion: they can walk, swim, and seize their prey; they have what even the lowest fishes such as the lancelet, want,—a brain enclosed in a cartilaginous cavity in the head, and perfectly formed organs of sight; they possess, too, what is found in no other mollusc,—organs of hearing; and in sagacity and activity they prove more than matches for the smaller fishes, many of which they overmaster and devour. With this highest class there contrasts an exceedingly low molluscous class at the bottom of the scale, or, at least, at what is now the bottom of the scale; for they constitute Cuvier’s _fifth_ class; while his _sixth_ and last, the Cirrhopodes, has been since withdrawn from the molluscs altogether, and placed in a different division of the animal kingdom. And this low class, the Brachiopods, are creatures that, living in bivalve shells, unfurnished with spring hinges to throw them open, and always fast anchored to the same spot, can but thrust forth, through the interstitial chinks of their prison-houses, spiral arms, covered with cilia, and winnow the water for a living. Now, it so happens that the molluscan group of the Silurian system is composed chiefly of these two extreme classes. It contains some of the other forms; but they are few in number, and give no character to the rocks in which they occur. There was nothing by which I was more impressed, in a visit to a Silurian region, than that in its ancient graveyards, as in those of the present day, though in a different sense, the high and the low should so invariably meet together. It is, however, not impossible that, in even the present state of things, a similar union of the extreme forms of the marine mollusca may be taking place in deep-sea deposits. Most of the intermediate forms provided with shells capable of preservation, such as the shelled Gasteropoda and the Conchifers, are either littoral, or restricted to comparatively small depths; whereas the Brachiopoda are deep-sea shells; and the Cephalopoda may be found voyaging far from land, in the upper strata of the sea above them. Even in the seas that surround our own island, the Brachiopodous molluscs—terebratula and crania—have been found, ever since deep-sea dredging became common, to be not very rare shells; and in the Mediterranean, where they are less rare still, fleets of Argonauts, the representatives of a highly organized family of the Cephalopods, to which it is now believed the Bellerophon of the Palæozoic rocks belonged, may be seen skimming along the surface, with sail and oar, high over the profound depths in which they lie. And, of course, when death comes, that comes to high and low, the remains of both Argonauts and Brachiopods must lie together at the bottom, in beds almost totally devoid of the intermediate forms.

Now, the author of the “Vestiges,” in maintaining his hypothesis, suspends it on the handle furnished him by the immense abundance of the Silurian Brachiopods. The Silurian period, he says, exhibits “a scanty and most defective development of life; so much so, that Mr. Lyell calls it, _par excellence_, the age of Brachiopods, with reference to the by no means exalted bivalve shell-fish which forms its predominant class. Such being the actual state of the case, I must persist in describing even the fauna of this age, which we now know was not the first, as, generally speaking, such a humble exhibition of the animal kingdom as we might expect, upon the development theory, to find at an early stage of the history of organization.” The reader will at once discern the fallacy here. The Silurian period was peculiarly an age of Brachiopods, for in no other period were Brachiopods so numerous, specifically or individually, or of such size or importance; whereas it was not _so peculiarly_ an age of Cephalopods, for these we find introduced in still greater numbers during the Liasic and Oolitic periods. In 1848, when Professor Edward Forbes edited the Palæontological map of Britain and Ireland, which forms one of the very admirable series of “Johnstone’s Physical Atlas,” the Cephalopods of the Silurian rocks of England and Wales were estimated at forty-eight species, and the Brachiopods at one hundred and fifty; whereas at the same date there were two hundred and five Cephalopods of the Oolitic formations enumerated, and but fifty-four Brachiopods. It is the molluscs of the inferior, not those of the superior class, that constitute (with their contemporaries the Trilobites) the characteristic fossils of the Silurian rocks; and hence the propriety of the distinctive name suggested by Sir Charles Lyell. But in the development question, what we have specially to consider is, not the _numbers_ of the low, but the _standing_ of the high. A country may be distinctively a country of flocks and herds, or a country of the carnivorous mammalia, or, like New South Wales or the Galapagos, a country of marsupial animals or of reptiles. Its human inhabitants may be merely a few hunters or shepherds, too inconsiderable in numbers, and too much like their brethren elsewhere, to give it any peculiar standing as a home of men. But in estimating the highest point in the scale to which the animal kingdom has attained within its limits, it is of its few men, not of its many beasts, that we must take note. And the point to be specially decided regarding the organisms of the Silurian system, in this question, is, not the proportion in _number_ which the lower forms bore to the higher, but the exact _rank_ which the higher bore in the scale of existence. Did the system furnish but a single Cephalopod or a single fish, we would yet have as certainly to determine that the chain of being reached as high as the Cephalopod or the fish, as if the remains of these creatures constituted its most abundant fossils. The chain of animal life reached quite as high on the evening of the sixth day of creation, when the human family was restricted to a single pair, as it does now, when our statists reckon up by millions the inhabitants of the greater capitals of the world; and the special pleader who, in asserting the contrary, would insist on determining the point, not by the _rank_ of the men of Eden, but by the _number_ of minnows or sticklebacks that swarmed in its rivers, might be perhaps deemed ingenious in his expedients, but certainly not very judicious in the use of them. It is worthy of remark, however, that the Brachiopods of those Palæozoic periods in which the group occupied such large space in creation, consisted of greatly larger and more important animals than any which it contains in the present day. It has yielded to what geological history shows to be the common fate, and sunk into a state of degradation and decline.

The geological history of the vegetable, like that of the animal kingdom, has been pressed into the service of the development hypothesis; and certainly their respective courses, both in actual arrangement and in their relation to human knowledge, seem wonderfully alike. It is not much more than twenty years since it was held that no exogenous plant existed during the Carboniferous period. The frequent occurrence of Coniferæ in the Secondary deposits had been conclusively determined from numerous specimens; but, founding on what seemed a large amount of negative evidence, it was concluded that, previous to the Liasic age, nature had failed to achieve a tree, and that the rich vegetation of the Coal Measures had been exclusively composed of magnificent immaturities of the vegetable kingdom,—of gigantic ferns and club-mosses, that attained to the size of forest trees, and of thickets of the swamp-loving horsetail family of plants, that well nigh rivalled in height those forests of masts which darken the rivers of our great commercial cities. Such was the view promulgated by M. Adolphe Brongniart; and it may be well to remark that, so far as the evidence on which it was based was positive, the view was sound. It _is_ a fact, that inferior orders of plants were developed in those ages in a style which, in their present state of degradation, they never exemplify: they took their place, not, as now, among the pigmies and abortions of creation, but among its tallest and goodliest productions. It is, however, _not_ a fact that they were the highest vegetable forms of their time. True exogenous trees also existed in great numbers and of vast size. In various localities in the coal fields of both England and Scotland,—such as Lennel Braes and Allan Bank in Berwickshire, High-Heworth, Fellon, Gateshead, and Wideopen near Newcastle-upon-Tyne, and in quarries to the west of the city of Durham,—the most abundant fossils of the system are its true woods. In the quarry of Craigleith, near Edinburgh, three huge trunks have been laid open during the last twenty years, within the space of about a hundred and fifty yards, and two equally massy trunks, within half that space, in the neighboring quarry of Granton, all low in the Coal Measures. They lie diagonally athwart the strata,—at an angle of about thirty,—with the nether and weightier portion of their boles below, like snags in the Mississippi; and we infer, from their general direction, that the stream to which they reclined must have flowed from nearly north-east to south-west. The current was probably that of a noble river, which reflected on its broad bosom the shadow of many a stately tree. With the exception of one of the Granton specimens, which still retains its strong-kneed roots, they are all mere portions of trees, rounded at both ends as if by attrition or decay; and yet one of these portions measures about six feet in diameter by sixty-one feet in length; another four feet in diameter by seventy feet in length; and the others, of various thickness, but all bulky enough to equal the masts of large vessels, range in length from thirty-six to forty-seven feet. It seems strange to one who derives his supply of domestic fuel from the Dalkeith and Falkirk coal-fields, that the Carboniferous flora could ever have been described as devoid of trees. I can scarce take up a piece of coal from beside my study fire, without detecting in it fragments of carbonized wood, which almost always exhibit the characteristic longitudinal fibres, and not unfrequently the medullary rays. Even the trap-rocks of the district enclose, in some instances, their masses of lignite, which present in their transverse sections, when cut by the lapidary, the net-like reticulations of the coniferæ. The fossil botanist, who devoted himself chiefly to the study of microscopic structure, would have to decide, from the facts of the case, not that trees were absent during the Carboniferous period, but that, in consequence of their having been present in amazing numbers, their remains had entered more palpably and extensively into the composition of coal than those of any other vegetable.[32] So far as is yet known, they all belonged to the two great divisions of the coniferous family, araucarians and pines. The huge trees of Craigleith and Granton were of the former tribe, and approximate more nearly to _Altingia excelsa_, the Norfolk-Island pine,—a noble araucarian, that rears its proud head from a hundred and sixty to two hundred feet over the soil, and exhibits a green and luxuriant breadth of foliage rare among the Coniferæ,—than any other living tree.