Читать книгу: «The Atlantic Monthly, Volume 11, No. 66, April, 1863», страница 9

THE SILURIAN BEACH

With what interest do we look upon any relic of early human history! The monument that tells of a civilization whose hieroglyphic records we cannot even decipher, the slightest trace of a nation that vanished and left no sign of its life except the rough tools and utensils buried in the old site of its towns or villages, arouses our imagination and excites our curiosity. Men gaze with awe at the inscription on an ancient Egyptian or Assyrian stone; they hold with reverential touch the yellow parchment-roll whose dim, defaced characters record the meagre learning of a buried nationality; and the announcement, that for centuries the tropical forests of Central America have hidden within their tangled growth the ruined homes and temples of a past race, stirs the civilized world with a strange, deep wonder.

To me it seems that to look on the first land that was ever lifted above the waste of waters, to follow the shore where the earliest animals and plants were created when the thought of God first expressed itself in organic forms, to hold in one's hand a bit of stone from an old sea-beach, hardened into rock thousands of centuries ago, and studded with the beings that once crept upon its surface or were stranded there by some retreating wave, is even of deeper interest to men than the relics of their own race, for these things tell more directly of the thoughts and creative acts of God.

Standing in the neighborhood of Whitehall, near Lake George, one may look along such a sea-shore, and see it stretching westward and sloping gently southward as far as the eye can reach. It must have had a very gradual slope, and the waters must have been very shallow; for at that time no great mountains had been uplifted, and deep oceans are always the concomitants of lofty heights. We do not, however, judge of this by inference merely; we have an evidence of the shallowness of the sea in those days in the character of the shells found in the Silurian deposits, which shows that they belonged in shoal waters.

Indeed, the fossil remains of all times tell us almost as much of the physical condition of the world at different epochs as they do of its animal and vegetable population. When Robinson Crusoe first caught sight of the footprint on the sand, he saw in it more than the mere footprint, for it spoke to him of the presence of men on his desert island. We walk on the old geological shores, like Crusoe along his beach, and the footprints we find there tell us, too, more than we actually see in them. The crust of our earth is a great cemetery where the rocks are tombstones on which the buried dead have written their own epitaphs. They tell us not only who they were and when and where they lived, but much also of the circumstances under which they lived. We ascertain the prevalence of certain physical conditions at special epochs by the presence of animals and plants whose existence and maintenance required such a state of things, more than by any positive knowledge respecting it. Where we find the remains of quadrupeds corresponding to our ruminating animals, we infer not only land, but grassy meadows also, and an extensive vegetation; where we find none but marine animals, we know the ocean must have covered the earth; the remains of large reptiles, representing, though in gigantic size, the half aquatic, half terrestrial reptiles of our own period, indicate to us the existence of spreading marshes still soaked by the retreating waters; while the traces of such animals as live now in sand and shoal waters, or in mud, speak to us of shelving sandy beaches and of mud-flats. The eye of the Trilobite tells us that the sun shone on the old beach where he lived; for there is nothing in Nature without a purpose, and when so complicated an organ was made to receive the light, there must have been light to enter it. The immense vegetable deposits in the Carboniferous period announce the introduction of an extensive terrestrial vegetation; and the impressions left by the wood and leaves of the trees show that these first forests must have grown in a damp soil and a moist atmosphere. In short, all the remains of animals and plants hidden in the rocks have something to tell of the climatic conditions and the general circumstances under which they lived, and the study of fossils is to the naturalist a thermometer by which he reads the variations of temperature in past times, a plummet by which he sounds the depths of the ancient oceans,—a register, in fact, of all the important physical changes the earth has undergone.

But although the animals of the early geological deposits indicate shallow seas by their similarity to our shoal-water animals, it must not be supposed that they are by any means the same. On the contrary, the old shells, crustacea, corals, etc., represent types which have existed in all times with the same essential structural elements, but under different specific forms in the several geological periods. And here it may not be amiss to say something of what are called by naturalists representative types.

The statement that different sets of animals and plants have characterized the successive epochs is often understood as indicating a difference of another kind than that which distinguishes animals now living in different parts of the world. This is a mistake. There are so-called representative types all over the globe, united to each other by structural relations and separated by specific differences of the same kind as those that unite and separate animals of different geological periods. Take, for instance, mud-flats or sandy shores in the same latitudes of Europe and America; we find living on each animals of the same structural character and of the same general appearance, but with certain specific differences, as of color, size, external appendages, etc. They represent each other on the two continents. The American wolves, foxes, bears, rabbits, are not the same as the European, but those of one continent are as true to their respective types as those of the other; under a somewhat different aspect they represent the same groups of animals. In certain latitudes, or under conditions of nearer proximity, these differences may be less marked. It is well known that there is a great monotony of type, not only among animals and plants, but in the human races also, throughout the Arctic regions; and the animals characteristic of the high North reappear under such identical forms in the neighborhood of the snow-fields in lofty mountains, that to trace the difference between the ptarmigans, rabbits, and other gnawing animals of the Alps, for instance, and those of the Arctics, is among the most difficult problems of modern science.

And so is it also with the animated world of past ages; in similar deposits of sand, mud, or lime, in adjoining regions of the same geological age, identical remains of animals and plants may be found, while at greater distances, but under similar circumstances, representative species may occur. In very remote regions, however, whether the circumstances be similar or dissimilar, the general aspect of the organic world differs greatly, remoteness in space being thus in some measure an indication of the degree of affinity between different faunae. In deposits of different geological periods immediately following each other we sometimes find remains of animals and plants so closely allied to those of earlier or later periods that at first sight the specific differences are hardly discernible. The difficulty of solving these questions, and of appreciating correctly the differences and similarities between such closely allied organisms, explains the antagonistic views of many naturalists respecting the range of existence of animals, during longer or shorter geological periods; and the superficial way in which discussions concerning the transition of species are carried on is mainly owing to an ignorance of the conditions above alluded to. My own personal observation and experience in these matters have led me to the conviction that every geological period has had its own representatives, and that no single species has been repeated in successive ages.

The laws regulating the geographical distribution of animals and their combination into distinct or zoological provinces called faunae with definite limits are very imperfectly understood as yet; but so closely are all things linked together from the beginning till to-day that I am convinced we shall never find the clue to their meaning till we carry on our investigations in the past and the present simultaneously. The same principle according to which animal and vegetable life is distributed over the surface of the earth now prevailed in the earliest geological periods. The geological deposits of all times have had their characteristic faunae under various zones, their zoological provinces presenting special combinations of animal and vegetable life over certain regions, and their representative types reproducing in different countries, but under similar latitudes, the same groups with specific differences.

Of course, the nearer we approach the beginning of organic life, the less marked do we find the differences to be, and for a very obvious reason. The inequalities of the earth's surface, her mountain-barriers protecting whole continents from the Arctic winds, her open plains exposing others to the full force of the polar blasts, her snug valleys and her lofty heights, her table-lands and rolling prairies, her river-systems and her dry deserts, her cold ocean-currents pouring down from the high North on some of her shores, while warm ones from tropical seas carry their softer influence to others,—in short, all the contrasts in the external configuration of the globe, with the physical conditions attendant upon them, are naturally accompanied by a corresponding variety in animal and vegetable life.

But in the Silurian age, when there were no elevations higher than the Canadian hills, when water covered the face of the earth with the exception of a few isolated portions lifted above the almost universal ocean, how monotonous must have been the conditions of life! And what should we expect to find on those first shores? If we are walking on a sea-beach to-day, we do not look for animals that haunt the forests or roam over the open plains, or for those that live in sheltered valleys or in inland regions or on mountain-heights. We look for Shells, for Mussels and Barnacles, for Crabs, for Shrimps, for Marine Worms, for Star-Fishes and Sea-Urchins, and we may find here and there a fish stranded on the sand or tangled in the sea-weed. Let us remember, then, that, in the Silurian period, the world, so far as it was raised above the ocean, was a beach, and let us seek there for such creatures as God has made to live on sea-shores, and not belittle the Creative work, or say that He first scattered the seeds of life in meagre or stinted measure, because we do not find air-breathing animals when there was no fitting atmosphere to feed their lungs, insects with no terrestrial plants to live upon, reptiles without marshes, birds without trees, cattle without grass, all things, in short, without the essential conditions for their existence.

What we do find—and these, as I shall endeavor to show my readers, in such profusion that it would seem as if God, in the joy of creation, had compensated Himself for a less variety of forms in the greater richness of the early types—is an immense number of beings belonging to the four primary divisions of the Animal Kingdom, but only to those classes whose representatives are marine, whose home then, as now, was either in the sea or along its shores. In other words, the first organic creation expressed in its totality the structural conception since carried out in such wonderful variety of details, and purposely limited then, because the world, which was to be the home of the higher animals, was not yet made ready to receive them.

I am fully aware that the intimate relations between the organic and physical world are interpreted by many as indicating the absence, rather than the presence, of an intelligent Creator. They argue, that the dependence of animals on material laws gives us the clue to their origin as well as to their maintenance. Were this influence as absolute and unvarying as the purely mechanical action of physical circumstances must necessarily be, this inference might have some pretence to logical probability,—though it seems to me unnecessary, under any circumstances, to resort to climatic influences or the action of any physical laws to explain the thoughtful distribution of the organic and inorganic world, so evidently intended to secure for all beings what best suits their nature and their needs. But the truth is, that, while these harmonious relations underlie the whole creation in such a manner as to indicate a great central plan, of which all things are a part, there is at the same time a freedom, an arbitrary element in the mode of carrying it out, which seems to point to the exercise of an individual will; for, side by side with facts, apparently the direct result of physical laws, are other facts, the nature of which shows a complete independence of external influences.

Take, for instance, the similarity above alluded to between the fauna of the Arctics and that of the Alps, certainly showing a direct relation between climatic conditions and animal and vegetable life. Yet even there, where the shades of specific difference between many animals and plants of the same class are so slight as to battle the keenest investigators, we have representative types both in the Animal and Vegetable Kingdoms as distinct and peculiar as those of widely removed and strongly contrasted climatic conditions. Shall we attribute the similarities and the differences alike to physical causes? Compare, for example, the Reindeer of the Arctics with the Ibex and the Chamois, representing the same group in the Alps. Even on mountain-heights of similar altitudes, where not only climate, but other physical conditions would suggest a recurrence of identical animals, we do not find the same, but representative types. The Ibex of the Alps differs, for instance, from that of the Pyrenees, that of the Pyrenees from those of the Caucasus and Himalayas, these again from each other and from that of the Altai.

But perhaps the most conclusive proof that we must seek for the origin of organic life outside of physical causes consists in the permanence of the fundamental types, while the species representing these types have differed in every geological period. Now what we call typical features of structure are in themselves no more stable or permanent than specific features. If physical causes, such as light, heat, moisture, food, habits of life, etc., acting upon individuals, have gradually in successive generations changed the character of the species to which they belong, why not that of the class and the branch also? If we judge this question from the material side at all, we must, in order to judge it fairly, look at it wholly from that point of view. If these specific changes are brought about in this way, it is because external causes have positive permanent effects upon the substances of which animals are built: they have power to change their hair, to change their skin, to change certain external appendages or ornamentations, and any other of those ultimate features which naturalists call specific characters. Now I would ask what there is in the substances out of which class characters are built that would make them less susceptible to such external influences than these specific characters. In many instances the former are more delicate, more sensitive, far more fragile and transient in their material nature than the latter. And yet never, in all the chances and changes of time, have we seen any alteration in the mode of respiration, of reproduction, of circulation, or in any of the systems of organs which characterize the more comprehensive groups of the Animal Kingdom, although they are quite as much under the immediate influence of physical causes as those structural features which have been constantly changing.

The woody fibre of the Pine-trees has had the same structure from the Carboniferous age to this day, while their mode of branching and the forms of their cones and leaves have been different in each period according to their respective species. The combination of rings, the structure of the wings, and the articulations of the legs are the same in the Cockroaches of the Carboniferous age as in those which infest our ships and our dwellings to-day, while the proportion of their parts is on quite another scale. The tissue of the Corals in the Silurian age is identical in chemical combination and organic structure with that of the Corals of our modern reefs, and yet the extensive researches upon this class for which we are indebted to Milne Edwards and Haime have not revealed a single species extending through successive geological ages, but show us, on the contrary, that every age has had its own kinds, differing among themselves in the same way as those of the Gulf of Mexico differ now from those of the Indian Ocean and the Pacific. The scales of the oldest known fishes in the Silurian beds have the same microscopic structure as those of their representative types today, and yet I have never seen a single fossil fish presenting the same specific characters in the successive geological epochs. The teeth of the oldest Sharks show the same microscopic structure as those of the present time, and we do not lack opportunities for comparison, since the former are as common in the mountain-limestone of Ireland as are those of the living Sharks on any beach where our fishermen boil them for the sake of their oil, and yet the Sharks appear under different generic and specific forms in each geological age.

But without multiplying examples, which might be adduced ad infinitum, to show permanence of type combined with repeated changes of species, suffice it to say, that, while the general features in the framework of the organic world and the materials of which that framework is built, though quite as subject to the influence of physical external circumstances as any so-called specific-features, have remained perfectly intact from the beginning of Creation till now, so that not the smallest difference is to be discerned in these respects between the oldest representatives of the oldest types in the oldest Silurian rocks and their successors through all the geological ages up to the present day, the species have been different in each epoch. It is surely a fair question to ask the advocates of the transmutation theory, whether they attribute to physical laws the discernment that would lead them to change the specific features, but to respect all those characters by which the higher structural combinations of the Animal Kingdom are preserved without alteration,—in other words, to maintain the organic plan, while constantly diversifying the mode of expressing it. If so, it would perhaps be as well to call them by another name, since they show all the comprehensive wisdom of an intelligent Creator. Until they can tell us why certain features of animals and plants are permanent under conditions which, according to their view, have power to change certain other features no more perishable or transient in themselves, the supporters of the development theory will have failed to substantiate their peculiar scientific doctrine.

But this discussion has led us far away from our starting point, and interrupted our walk along the Silurian beach; let us return to gather a few specimens there, and compare them with the more familiar ones of our own shores. I have said that the beach was a shelving one, and covered of course with shoal waters; but as I have no desire to mislead my readers, or to present truths as generally accepted which are still subject to dispute, I would state here that the parallel ridges across the State of New York, considered by some geologists as the successive shores of a receding ocean, are believed by others to be the inequalities on the bottom of a shallow sea. Not only, however, does the general character of these successive terraces suggest the idea that they must have been shores, but the ripple-marks upon them are as distinct as upon any modern beach. The regular rise and fall of the water is registered there in waving, undulating lines as clearly as on the sand-beaches of Newport or Nahant; and we can see on any one of those ancient shores the track left by the waves as they rippled back at ebb of the tide thousands of centuries ago. One can often see where some obstacle interrupted the course of the water, causing it to break around it; and such an indentation even retains the soft, muddy, plastic look that we observe on the present beaches, where the resistance made by any pebble or shell to the retreating wave has given it greater force at that point, so that the sand around the spot is soaked and loosened. There is still another sign, equally familiar to those who have watched the action of water on a beach. Where a shore is very shelving and flat, so that the waves do not recede in ripples from it, but in one unbroken sheet, the sand and small pebbles are dragged and form lines which diverge whenever the water meets an obstacle, thus forming sharp angles on the sand. Such marks are as distinct on the oldest Silurian rocks as if they had been made yesterday. Nor are these the only indications of the same fact. There are certain animals living always upon sandy or muddy shores, which require for their well-being that the beach should be left dry a part of the day. These animals, moving about in the sand or mud from which the water has retreated, leave their tracks there; and if, at such a time, the wind is blowing dust over the beach, and the sun is hot enough to bake it upon the impressions so formed, they are left in a kind of mould. Such trails and furrows, made by small Shells or Crustacea, are also found in plenty on the oldest deposits.

Admitting it, then, to be a beach, let us begin with the lowest type of the Animal Kingdom, and see what Radiates are to be found there. There are plenty of Corals, but they are not the same kinds of Corals as those that build up our reefs and islands now. The modern Coral animals are chiefly Polyps, but the prevailing Corals of the Silurian age were Acalephian Hydroids, animals which indeed resemble Polyps in certain external features, and have been mistaken for them, but which are nevertheless Acalephs by their internal structure; for, instead of having the vertical partitions dividing the body into chambers, so characteristic of the Polyps, they are divided by tubes corresponding to the radiating tubes of the Acalephs proper, these tubes being themselves divided at regular distances by horizontal floors, so that they never run uninterruptedly from top to bottom of the body. I subjoin a woodcut of a Silurian Coral, which does not, however, show the peculiar internal structure, but gives some idea of the general appearance of the old Hydroid Corals. We have but one Acalephian Coral now living, the Millepore; and it was by comparing that with these ancient ones that I first detected their relation to the Acalephs. For the true Acalephs or Jelly-Fishes we shall look in vain; but the presence of the Acalephian Corals establishes the existence of the type, and we cannot expect to find those kinds preserved which are wholly destitute of hard parts. I do not attempt any description of the Polyps proper, because the early Corals of that class are comparatively few, and do not present features sufficiently characteristic to attract the notice of the casual observer.

Of the Echinoderms, the class of Radiates represented now by our Star-Fishes and Sea-Urchins, we may gather any quantity, though the old fashioned forms are very different from the living ones. I have dwelt at such length in a former article⁸ on the wonderful beauty and variety of the Crinoids, or "Stone Lilies," as they have been called, from their resemblance to flowers, that I will only briefly allude to them here. The subjoined wood-cut represents one with a closed cup; but the number of their different patterns is hardly to be counted, and I would invite any one who questions the abundant expression of life in those days to look at some slabs of ancient limestone in the Zoölogical Museum at Cambridge, where the stems of the Crinoids are tangled together as thickly as sea-weed on the shore. Indeed, some of our rock-deposits consist chiefly of the fragments of their remains.

The Mollusks were also represented then, as now, by their three classes,—Acephala, Gasteropoda, and Cephalopoda. The Acephala or Bivalves we shall find in great numbers, but of a very different pattern from the Oysters, Clams, and Mussels of recent times. The annexed wood-cut represents one of these Brachiopods, which form a very characteristic type of the Silurian deposits. The square cut of the upper edge, where the two valves meet along the back and are united by a hinge, is altogether old-fashioned, and unknown among our modern Bivalves. The wood-cut does not show the inequality of the two valves, also a very characteristic feature of this group,—one valve being flat and fitting closely into the other, which is more spreading and much fuller. These, also, were represented by a great variety of species, and we find them crowded together as closely in the ancient rocks as Oysters or Clams or Mussels on any of our modern shores. Besides these, there were the Bryozoa, a small kind of Mollusk allied to the Clams, and very busy then in the ancient Coral work. They grew in communities, and the separate individuals are so minute that a Bryozoan stock looks like some delicate moss. They still have their place among the Reef-Building Corals, but play an insignificant part in comparison with that of their predecessors.

Of the Silurian Univalves or Gasteropods there is not much to tell, for their spiral shells were so brittle that scarcely any perfect specimens are known, though their broken remains are found in such quantities as to show that this class also was very fully represented in the earliest creation. But the highest class of Mollusks, the Cephalopods or Chambered Shells, or Cuttle-Fishes, as they are called when the animal is unprotected by a shell, are, on the contrary, very well preserved, and they are very numerous. Of these I will speak somewhat more in detail, because their geological history is a very curious one.

The Chambered Nautilus is familiar to all, since, from the exquisite beauty of its shell, it is especially sought for by conchologists; but it is nevertheless not so common in our days as the Squids and Cuttle-Fishes, which are the most numerous modern representatives of the class. In the earliest geological days, on the contrary, those with a shell predominated, differing from the later ones, however, in having the shell perfectly straight instead of curved, though its internal structure was the same as it is now and has ever been. Then, as now, the animal shut himself out from his last year's home, building his annual wall behind him, till his whole shell was divided into successive chambers, all of which were connected by a siphon. Some of the shells of this kind belonging to the Silurian deposits are enormous: giants of the sea they must have been in those days. They have been found fifteen feet long, and as large round as a man's body. One can imagine that the Cuttle-Fish inhabiting such a shell must have been a formidable animal. These straight-chambered shells of the Silurian and Devonian seas are called Orthoceratites (see wood-cut below). We shall meet them again hereafter, under another name and with a different form; for, as they advance in the geological ages, they not only assume the curved outline with ever closer whorls till it culminates in the compact coil of the Ammonites of the middle periods, but the partitions, which are perfectly plain walls in these earlier forms, become scalloped and involuted along the edges in the later ones, making the most delicate and exquisite tracery on the surface of the shell.

Of Articulates we find only two classes, Worms and Crustacea. Insects there were none,—for, as we have seen, this early world was wholly marine. There is little to be said of the Worms, for their soft bodies, unprotected by any hard covering, could hardly be preserved; but, like the marine Worms of our own times, they were in the habit of constructing envelopes for themselves, built of sand, or sometimes from a secretion of their own bodies, and these cases we find in the earliest deposits, giving us assurance that the Worms were represented there. I should add, however, that many impressions described as produced by Worms are more likely to have been the tracks of Crustacea.

But by far the most characteristic class of Articulates in ancient times were the Crustaceans. The Trilobites stand in the same relation to the modern Crustacea as the Crinoids do to the modern Echinoderms. They were then the sole representatives of the class, and the variety and richness of the type are most extraordinary. They were of nearly equal breadth for the whole length of the body, and rounded at the two ends, so as to form an oval outline. To give any adequate idea of the number and variety of species would fill a volume, but I may enumerate some of the more striking differences: as, for instance, the greater or less prominence of the anterior shield,—the preponderance of the posterior end in some, while in others the two ends are nearly equal,—the presence or absence of prongs on the shield and of spines along the sides of the body,—appendages on the head in some species, of which others are entirely destitute,—and the smooth outline of some, while in others the surface is broken by a variety of external ornamentation. Such are a few of the more prominent differences among them. But the general structural features are the same in all. The middle region of the body is always divided in uniform rings, lobed in the middle so as to make a ridge along the back with a slight depression on either side of it. It is from this three-lobed division that they receive their name. The subjoined wood-cut represents a characteristic Silurian Trilobite.