Sir John William Dawson

THE IMPERFECTION OF THE GEOLOGICAL RECORD.

C

Complaints of the imperfection of the geological record are rife among those biologists who expect to find continuous series of fossils representing the gradual transmutation of species. No doubt these gaps are in some cases portentous, and unfortunately they often occur just where it is most essential to certain general conclusions that they should be filled up. Instead, however, of making vague lamentations on the subject, it is well to inquire to what causes these gaps may be due, to what extent they invalidate the completeness of geological history for scientific purposes, and how they may best be filled.

Here we may first remark that it is not so much the physical record of geology that is imperfect as the organic record. Ever since the time of Hutton and Playfair we have learned that the processes of mineral detrition and deposition are continuous, and have been so throughout geological time. The erosion of the land is constantly going on, every shower carries its tribute of earthy matter toward the sea, and every wave that strikes against a beach or cliff does some work toward the grinding of shells, pebbles or stone. Thus, everywhere around our continents there is a continuous deposition of beds of earthy matter, and it is this which, when elevated into new land, has given us our chronological series of geological formations. True, the elevating process is not continuous, but, so far as we know, intermittent; but it has been so often repeated that we have no reason to doubt that the wasting continents afford a complete series of aqueous deposits, since the time when the dry land first appeared.

In recent years the Challenger expedition and similar dredgings have informed us of still another continuity of deposition in the depths of the ocean. There, where no detritus from the land, or only a very little fine volcanic ash or pumice has ever reached, we have, going on from age to age, a deposit of the hard parts of abyssal animals and of those that swim in the open sea; so that if it were possible to bore or sink a shaft in some parts of the ocean, we should find not only a continuous bed, but a continuous series of pelagic life from the Laurentian to the present day. Thus we have continuous physical records, could we but reach or completely put them together, and eliminate the disturbing influence of merely local vicissitudes. It is when we begin to search the geological formations for fossils, that imperfection in our record first becomes painfully manifest.

In the case of many groups of marine animals, as, for example, the shell-fish and the corals, and I may add the bivalve crustaceans, so admirably worked up by my friend Prof. Rupert Jones, we have very complete series. With the and snails the case is altogether different. As stated in another paper of this series, a few species of these animals appear in the later PalÆozoic age, and after that they have no successors known to us in all the great periods covered by the Permian, the Trias, and the earlier Jurassic. A few air-breathing water-snails appear in the upper Jurassic, and true land snails are not met with again until the Tertiary. Were there no land snails in this vast lapse of time? Have we two successive creations, so to speak, of these creatures at distant intervals? Were they only diminished in numbers and distribution in the intervening time? Is the hiatus owing merely to the unlikelihood of such shells being preserved? Or is it owing to the lack of diligence and care in collecting?

In this particular case we are, no doubt, disposed to say that the series must have been continuous. But we cannot be sure of this. In whatever way a few species of land snails were so early introduced in the time of the Devonian or of the Coal formation, if from physical vicissitudes or lack of proper pabulum they became extinct, there is no reason known to us why, when circumstances again became favourable, they should not be reintroduced in the same manner as at first, whether by development from allied types or otherwise. The fact that the few Devonian and Carboniferous species are very like those that still exist, perhaps makes against this supposition, but does not exclude it. If we suppose that new forms of life of low grade are introduced from time to time in the course of the geological ages, and if we adopt the Darwinian hypothesis of evolution, we arrive, as Naegeli has so well pointed out, at the strange paradox, that the highest forms of life must be the oldest of all, since they will be the descendants of the earliest of the lower animals, whereas the animals now of low grade may have been introduced later, and may not have had time to improve. But all our attempts to reduce nature to one philosophic expression necessarily lead to such paradoxes.

On the other hand, the chances of the preservation of land snails in aqueous deposits are vastly less than those in favour of the preservation of aquatic species. The first Carboniferous species found[15] had been preserved in the very exceptional circumstances afforded by the existence of hollow trunks of SigillariÆ on the borders of the Coal formation flats, and the others subsequently found were in beds no doubt receiving the drainage of neighbouring land areas. Still it is not uncommon on the modern sea-shore, anywhere near the mouths of rivers, to find a few fresh-water shells here and there. The carbonaceous beds of the Trias, the fossil soils of the Portland series, the estuarine Wealden beds would seem to be as favourably situated as those of the coal formation for preserving land shells, though possibly the flora of the Mesozoic was less suitable for feeding such creatures than that of the Coal period, and they may consequently have become few and local. After all, perhaps more diligent collecting and more numerous collectors might succeed, and may succeed in the future, in filling this and similar gaps.

[15] Pupa vetusta of the Nova Scotia coal formation.

It is a great mistake to suppose that discoveries of this kind are made by chance. It is only by the careful and painstaking examination of much material that the gaps in the geological record can be filled up, and I propose in the sequel of this article to note a few instances, in a country where the range of territory is altogether out of proportion to the number of observers, and which have come within my own knowledge.

It was not altogether by accident that Sir C. Lyell and the writer discovered a few reptilian bones and a land-snail in breaking up portions of the material filling an erect Sigillaria in the South Joggins coal measures. We were engaged in a deliberate survey of the section, to ascertain as far as might be the conditions of accumulation of coal, and one point which occurred to us was to inquire as to the circumstances of preservation of stumps of forest trees in an erect position, to trace their roots into the soils on which they stood, and to ascertain the circumstances in which they had been buried, had decayed, and had been filled with mineral matter. It was in questioning these erect trees on such subjects and this not without some digging and hammering that we made the discovery referred to.

But we found such remains only in one tree, and they were very imperfect, and indicated only two species of batrachians and one land-snail. There the discovery might have rested. But I undertook to follow it up. In successive visits to the coast, a large number of trees standing in the cliff and reefs, or fallen to the shore, were broken up and examined, the result being to discover that, with one unimportant exception, the productive trees were confined to one of the beds at Coal Mine Point, that from which the original specimens had been obtained. Attention was accordingly concentrated on this, and as many as thirty trees were at different times extracted from it, of which rather more than one-half proved more or less productive. By these means bones representing about sixty specimens and twelve species were extracted, besides numerous remains of land shells, millipedes, and scorpions. In this way a very complete idea was obtained of the land life, or at least of the smaller land animals, of this portion of the coal formation of Nova Scotia. It is not too much to say that if similar repositories could be found in the succeeding formations, and properly worked when found, our record of the history of land quadrupeds might be made very complete.

When in 1855 I changed my residence from Nova Scotia to Montreal, and so was removed to some distance from the carboniferous rocks which I had been accustomed to study, I naturally felt somewhat out of place in a Cambro-Silurian district, more especially as my friend Billings had already almost exhausted its fossils. I found, however, a congenial field in the Pleistocene shell beds; more especially as I had given some attention to recent marine animals when on the sea coast. The very perfect series of Pleistocene deposits in the St. Lawrence valley locally contain marine shells from the bottom of the till or boulder clay up to the overlying sands and gravels. The assemblage was a more boreal one than that on the coast of Nova Scotia, though many of the species were the same, and both the climatal and bathymetrial conditions differed in different parts of the Pleistocene beds themselves. The gap in the record here could at that time be filled up only by collecting recent shells. In addition to what could be obtained by exchanging with naturalists who had collected in Greenland, Labrador, and Norway, I employed myself, summer after summer, in dredging both on the south and north shore of the St. Lawrence, until able at length to discover in a living state, but under different conditions as to temperature and depth, nearly every species found in the beds on the land, from the lower boulder clay to the top of the formation, and from the sea-level to the beds six hundred feet high on the hills. Not only so: I could ascertain in certain places and conditions all the peculiar varieties of the species, and the special modes of life which they indicated. Thus, in the cases of the Peter Redpath Museum, and in notes on the Post-pliocene of Canada, the gap between the Modern and the Glacial age was completely filled up in so far as Canadian marine species are concerned. The net result was, as I have elsewhere stated, that no change other than varietal had occurred.

In studying the fossil plants of the Carboniferous, so abundant in the fine exposures of the coal formation in Nova Scotia, two defects struck me painfully. One was the fragmentary and imperfect state of the specimens procurable. Another was the question, What preceded these plants in the older rocks? The first of these was to be met only by thorough exploration. When a fragment of a plant was disclosed it was necessary to inquire if more existed in the same bed, and to dig, or blast away or break up the rock, until some remaining portions were disclosed. In this way it has been possible to obtain entire specimens of many trees of the Carboniferous; and to such an extent has the laborious and somewhat costly process been effectual, that more species of carboniferous trees are probably known in their entire forms from the Coal formations of Nova Scotia than from any other part of the world. I have been amused to find that so little are experiences of this kind known to some of my confrÈres abroad, that they are disposed to look with scepticism on the information obtained by this laborious but certain process, and to suppose that they are being presented with imaginary "restorations." I think it right here to copy a remark of a German botanist, who has felt himself called to criticise my work: "Dawson's description of the genus (Psilophyton) rests chiefly on the impression made on him in his repeated researches," etc. "He puts us off with an account of the general idea which he has drawn from the study of them." This is the remark of a closet naturalist, with reference to the kind of work above referred to, which, of course, cannot be represented in its entirety in figures or hand specimens.[16]

As to the precursors of the Carboniferous flora, in default of information already acquired, I proceeded to question the Erian or Devonian rocks of Canada, in which Sir William Logan had already found remains of plants which had not, however, been studied or described. Laboriously coasting along the cliffs of GaspÉ and the Baie des Chaleurs, digging into the sandstones of Eastern Maine, and studying the plants collected by the New York Survey, I began to find that there was a rich Devonian flora, and that, like that of the Carboniferous, it presented different stages from the base to the summit of the formation. But here a great advance was made in a somewhat unexpected way. My then young friends, the late Prof. Hartt and Mr. Matthew, of St. John, had found a few remains of plants in the Devonian, or at least pre-Carboniferous beds of St. John, which were placed in my hands for description. They were so novel and curious that inquiry was stimulated, and these gentlemen, with some friends of similar tastes, explored the shales exposed in the reefs near St. John, and when they found the more productive beds, broke them up by actual quarrying operations in such a way that they soon obtained the richest Devonian plant collections ever known. I think I may truly say that these young and enthusiastic explorers worked the St. John plant-beds in a manner previously unexampled in the world. Their researches were not only thus rewarded, but incidentally they discovered the first known Devonian insects, which could not have been found by a less painstaking process, and one of them discovered what I believe to be the oldest known land shell. Still more, their studies led to the separation from the Devonian beds of the Underlying Cambrian slates, previously confounded with them; and this, followed up by the able and earnest work of Mr. Matthew, has carried back our knowledge of the older rocks in Canada several stages, or as far as the earliest Cambrian previously known in Europe, but not before fully recognised in America, and has discovered in these old rocks the precursors of many forms of life not previously traced so far back.

The moral of these statements of fact is that the imperfections of the record will yield only to patient and painstaking work, and that much is in the power of local amateurs. I would enforce this last statement by a reference to a little research, in which I have happened to take part at a summer resort on the Lower St. Lawrence, at which I have from time to time spent a few restful vacation weeks. Little Metis is on the Quebec Group of Sir William Logan, that peculiar local representative of the lower part of the Cambro-Silurian and Upper Cambrian formations which stretches along the south side of the St. Lawrence all the way from Quebec to Cape Rosier, near GaspÉ, a distance of five hundred miles. This great series of rocks is a jumble of deposits belonging at that early time to the marginal area of what is now the American continent, and indicating the action not merely of ordinary causes of aqueous deposit, but of violent volcanic ejections, accompanied perhaps by earthquake waves, and not improbably by the action of heavy coast ice. The result is that mud rocks now in the form of black, grey, and red shales and slates alternate with thick and irregular beds of hard sandstone, sometimes so coarse that it resembles the angular dÉbris of the first treatment of quartz in a crusher. With these sandstones are thick and still more irregular conglomerates formed of pebbles and boulders of all sizes, up to several feet in diameter, some of which are of older limestones containing Cambrian fossils, while others are of quartzite or of igneous or volcanic rocks.

The whole formation, as presented at Metis, is of the most unpromising character as regards fossils, and after visiting the place for ten years, and taking many long walks along the shore and into the interior, and scrutinising every exposure, I had found nothing more interesting than a few fragments of graptolites, little zoophytes, ancient representatives of our sea mosses, and which are quite characteristic of several portions of the Quebec Group. With these were some marks of fucoids and tracks or burrows of worms. The explorers of the Geological Survey had been equally unsuccessful.

Quite accidentally a new light broke upon these unpromising rocks. My friend, Dr. Harrington, strolling one day on the shore, sat down to rest on a stone, and picked up a piece of black slate lying at his feet. He noticed on it some faintly traced lines which seemed peculiar. He put it in his pocket and showed it to me. On examination with a lens it proved to have on it a few spicules of a hexactinellid sponge—little crosses forming a sort of mesh or lattice-work similar to that which Salter had many years before found in the Cambrian rocks of Wales, and had named Protospongia—the first sponge. The discovery seemed worth following up, and we took an early opportunity of proceeding to the place, where, after some search, we succeeded in tracing the loose pieces to a ledge of shale on the beach, where there was a little band, only about an inch thick, stored with remains of sponges, a small bivalve shell and a slender branching seaweed. This was one small layer in reefs of slate more than one hundred feet thick. We subsequently found two other thin layers, but less productive. Tools and workmen were procured, and we proceeded to quarry in the reef, taking out at low tide as large slabs as possible of the most productive layer, and carefully splitting these up. The results, as published in the Transactions of the Royal Society of Canada,[17] show more than twelve species of siliceous sponges belonging to six genera, besides fragments indicating other species, and all of these living at one time on a very limited space of what is practically a single surface of muddy sea-bottom.[18] The specimens show the parts of these ancient sponges much more perfectly than they were previously known, and indeed, enable many of them to be perfectly restored. They for the first time connect the modern siliceous sponges of the deep sea with those that flourished on the old sea-bottom of the early Cambro-Silurian, and thus bridge over a great, gap in the history of this low form of life, showing that the principles of construction embodied in the remarkable and beautiful siliceous sponges, like Euplectella, the "Venus flower-basket," now dredged from the deep sea, were already perfectly carried out in this far-back beginning of life. This little discovery further indicates that portions of the older PalÆozoic sea-bottoms were as well stored with a varied sponge life as those of any part of the modern ocean. I figure[19] a number of species, remains of all of which may be gathered from a few yards of a single surface at Little Metis. The multitude of interesting details embodied in all this it is impossible to enter into here, but may be judged of from the forms reproduced. These examples tend to show that the imperfection of the record may not depend on the record itself, but on the incompleteness of our work. We must make large allowance for imperfect collecting, and especially for the too prevalent habit of remaining content with few and incomplete specimens, and of grudging the time and labour necessary to explore thoroughly the contents of special beds, and to work out all the parts of forms found more or less in fragments.

The point of all this at present is that patient work is needed to fill up the breaks in our record. A collector passing along the shore at Metis might have picked up a fragment of a fossil sponge, and recorded it as a fossil, or possibly described the fragment. This fact alone would have been valuable, but to make it bear its full fruit it was necessary to trace the fragment to its source, and then to spend time and labour in extracting from the stubborn rock the story it had to tell. Instances of this kind crowd on my memory as coming within my own experience and observation. It is hopeful to think that the record is daily becoming less imperfect; it is stimulating to know that so much is only waiting for investigation. The history never can be absolutely complete. Practically, to us it is infinite. Yet every series of facts known may be complete in itself for certain purposes, however many gaps there may be in the story. Even if we cannot find a continuous series between the snails of the Coal formation or the sponges of the Quebec Group and their successors to-day, we can at least see that they are identical in plan and structure, and can note the differences of detail which fitted them for their places in the ancient or the modern world. Nor need we be too discontented if the order of succession, such as it is, does not exactly square with some theories we may have formed. Perhaps it may in the end lead us to greater and better truths.

Another subject which merits attention here is the evidence which mere markings or other indications may sometimes give as to the existence of unknown creatures, and thus may be as important to us as the footprints of Friday to Robinson Crusoe. As I have been taking Canadian examples, I may borrow one here from Mr. Matthew, of St. John, New Brunswick.

He remarks in one of his papers the manner in which the Trilobites of the early Cambrian are protected with defensive spines, and asks against what enemies they were intended to guard. That there were enemies is further proved by the occurrence of Coprolites or masses of excrement, oval or cylindrical in form, and containing fragments of shells of Trilobites, of Pteropods (Hyolithes) and of Lingula. There must therefore have been marine animals of considerable size, which preyed on Trilobites. Dr. Hunt and myself have recorded similar facts from the Upper Cambrian and Cambro-Silurian of the Province of Quebec. No remains, however, are known of animals which could have produced such coprolites, except, indeed, some of the larger worms of the period, and they seem scarcely large enough. In these circumstances Mr. Matthew falls back on certain curious marks or scratches with which large surfaces of these old rocks are covered, and which he names Ctenichnites or "Comb tracks." These markings seem to indicate the rapid motion of some animal touching the bottom with fins or other organs; and as we know no fishes in these old rocks, the question recurs, What could it have been? From the form and character of the markings Mr. Matthew infers (1) That these animals lived in "schools," or were social in their habits; (2) That they had a rapid, direct, darting motion; (3) That they had three or four (at least) flexible arms; (4) That these arms were furnished with hooks or spines; (5) That the creatures swam with an easy motion, so that sometimes the arms of one side touched the bottom, sometimes those of the other. These indications point to animals allied to the modern squids or cuttle-fishes, and as these animals may have had no hard parts capable of preservation, except their horny beaks, nothing might remain to indicate their presence except these marks on the bottom. Mr. Matthew therefore conjectures that there may have been large cuttle-fishes in the Cambrian. Since, however, these are animals of very high rank in their class, and are not certainly known to us till a very much later period, their occurrence in these old rocks would be a very remarkable and unexpected fact.

A discovery made by Walcott in the Western States since Mr. Matthew's paper was written, throws fresh light on the question. Remains of fishes have been found by the former in the Cambro Silurian rocks nearly as far back as Mr. Matthew's comb-tracks. Besides this, Pander in Russia has found in these old rocks curious teeth, which he refers conjecturally to fishes (Conodonts). Why may there not have been in the Cambrian large fishes having, like the modern sharks, cartilage or gristle instead of bone—perhaps destitute of scales, and with small teeth which have not yet been detected. The fin rays of such fishes may have left the comb tracks, and in support of this I may say that there are in the Lower Carboniferous of Horton Bluff, in Nova Scotia, very similar tracks in beds holding many remains of fishes. Whichever view we adopt we see good evidence that there were in the early Cambrian animals of higher grade than we have yet dreamt of. Observe, however, that if we could complete the record in this point it would only give us higher forms of life at an earlier time, and so push farther back their possible development from lower forms. I fear, indeed, that I can hold out little hopes to the evolutionists that a more complete geological record would help them in any way. It would possibly only render their position more difficult.

But the saddest of all the possible defects of the geological record is that it may want the beginning, and be like the Bible of some of the German historical critics, from which they eliminate as mythical everything before the time of the later Hebrew kings. Our attention is forcibly called to this by the condition of the fauna of the earliest Cambrian rocks. The discoveries in these in Wales, in Norway, and in America show us that the seas of this early period swarmed with animals representing all the great types of invertebrate marine life. We have here highly organized Crustaceans, Worms, Mollusks and other creatures which show us that in that early age all these distinct forms of life were as well separated from each other as in later times, that eyes of different types, jointed limbs with nerves and muscles, and a vast variety of anatomical contrivances were as highly developed as at any subsequent time.[20] To a Darwinian evolutionist this means nothing less than that these creatures must have existed through countless ages of development from their imagined simple ancestral form or forms how long it is impossible to guess, since, unless change was more speedy in the infancy of the earth, the term of ages required must have far exceeded that from the Cambrian to the Modern. Yet, to represent all this we have absolutely nothing except Eozoon in its solitary grandeur, and a few other forms, possibly of Protozoa and worms. An imaginary phylogeny of animal life from Monads to Trilobites would be something as long as the whole geological history. Yet it would be almost wholly imaginary, for the record of the rocks tells little or nothing. In face of such an imperfection as this, geologists should surely be humble, and make confession of ignorance to any extent that may be desired. Yet we may at least, with all humility and self-abasement, ask our critics how they know that this great blank really exists, and whether it may not be possible that the swarming life of the early Cambrian may, after all, have appeared suddenly on the stage in some way as yet unknown to us and to them.

References:—"Fossil Sponges from the Quebec Group of Little Metis, Lower St. Lawrence": Transactions Royal Society of Canada, 1890. "RÈsumÈ of the Carboniferous Land Shells of North America": American Journal of Science, 1880. "Burrows and Tracks of Invertebrate Animals": Journal Geological Society of London, 1890. "Notes on the Pleistocene of Canada": Canadian Naturalist, 1876. "Air-breathers of the Coal Period ": Ibid., 1863.

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