(From A First Book in Geology.)
By N.S. SHALER, S.D.[1]
The geologist cannot find his way back in the record of the great stone book, to the far-off day when life began. The various changes that come over rocks from the action of heat, of water, and of pressure, have slowly modified these ancient beds, so that they no longer preserve the frames of the animals that were buried in them.
These old rocks, which are so changed that we cannot any longer make sure that any animals lived in them, are called the "archæan," which is Greek for ancient. They were probably mud and sand and limestone when first made, but they have been changed to mica schists, gneiss, granite, marble, and other crystalline rocks. When any rock becomes crystalline, the fossils dissolve and disappear, as coins lose their stamp and form when they are melted in the jeweller's gold-pot.
These ancient rocks that lie deepest in the earth are very thick, and must have taken a great time in building; great continents must have been worn down by rain and waves in order to supply the waste out of which they were made. It is tolerably certain that they took as much time during their making as has been required for all the other times since they were formed. During the vast ages of this archæan the life of our earth began to be. We first find many certain evidences of life in the rocks which lie on top of the archæan rock, and are known as the Cambriani and Silurian periods. There we have creatures akin to our corals and crabs and worms, and others that are the distant kindred of the cuttle-fishes and of our lamp-shells. There were no backboned animals, that is to say, no land mammals, reptiles, or fishes at this stage of the earth's history. It is not likely that there was any land life except of plants and those forms like the lowest ferns, and probably mosses. Nor is it likely that there were any large continents as at the present time, but rather a host of islands lying where the great lands now are, the budding tops of the continents just appearing above the sea.
Although the life of this time was far simpler than at the present day, it had about as great variety as we would find on our present sea-floors. There were as many different species living at the same time on a given surface.
The Cambrian and Silurian time—the time before the coming of the fishes—must have endured for many million years without any great change in the world. Hosts of species lived and died; half a dozen times or more the life of the earth was greatly changed. New species came much like those that had gone before, and only a little gain here and there was perceptible at any time. Still, at the end of the Silurian, the life of the world had climbed some steps higher in structure and in intelligence.
The next set of periods is known as the Devonian. It is marked by the rapid extension of the fishes; for, although the fishes began in the uppermost Silurian, they first became abundant in this time. These, the first strong-jawed tyrants of the sea, came all at once, like a rush of the old Norman pirates into the peaceful seas of Great Britain. They made a lively time among the sluggish beings of that olden sea. Creatures that were able to meet feebler enemies were swept away or compelled to undergo great changes, and all the life of the oceans seems to have a spur given to it by these quicker-formed and quicker-willed animals. In this Devonian section of our rocks we have proofs that the lands were extensively covered with forests of low fern trees, and we find the first trace of air-breathing animals in certain insects akin to our dragon-flies. In this stage of the earth's history the fishes grew constantly more plentiful, and the seas had a great abundance of corals and crinoids. Except for the fishes, there were no very great changes in the character of the life from that which existed in the earlier time of the Cambrian and Silurian. The animals are constantly changing, but the general nature of the life remains the same as in the earlier time.
In the Carboniferous or coal-bearing age, we have the second great change in the character of the life on the earth. Of the earlier times, we have preserved only the rocks formed in the seas. But rarely do we find any trace of the land life or even of the life that lived along the shores. In this Carboniferous time, however, we have very extensive sheets of rocks which were formed in swamps in the way shown in the earlier part of this book. They constitute our coal-beds, which, though much worn away by rain and sea, still cover a large part of the land surface. These beds of coal grew in the air, and, although the swamps where they were formed had very little animal life in them, we find some fossils which tell us that the life of the land was making great progress; there are new insects, including beetles, cockroaches, spiders, and scorpions, and, what is far more important, there are some air-breathing, back-boned animals, akin to the salamanders and water-dogs of the present day. These were nearly as large as alligators, and of much the same shape, but they were probably born from the egg in the shape of tadpoles and lived for a time in the water as our young frogs, toads, and salamanders do. This is the first step upwards from the fishes to land vertebrates; and we may well be interested in it, for it makes one most important advance in creatures through whose lives our own existence became possible. Still, these ancient woods of the coal period must have had little of the life we now associate with the forests; there were still no birds, no serpents, no true lizards, no suck-giving animals, no flowers, and no fruits. These coal-period forests were sombre wastes of shade, with no sound save those of the wind, the thunder, and the volcano, or of the running streams and the waves on the shores.
In the seas of the Carboniferous time, we notice that the ancient life of the earth is passing away. Many creatures, such as the trilobites, die out, and many other forms such as the crinoids or sea lilies become fewer in kind and of less importance. These marks of decay in the marine life continue into the beds just after the Carboniferous, known as the Permian, which are really the last stages of the coal-bearing period.
When with the changing time we pass to the beds known as the Triassic, which were made just after the close of the Carboniferous time, we find the earth undergoing swift changes in its life. The moist climate and low lands that caused the swamps to grow so rapidly have ceased to be, and in their place we appear to have warm, dry air, and higher lands.
On these lands of the Triassic time the air-breathing life made very rapid advances. The plants are seen to undergo considerable changes. The ferns no longer make up all the forests, but trees more like the pines began to abound, and insects became more plentiful and more varied.
FIG. 3. CYCAS CIRCINALIS, AKIN TO HIGHEST PLANTS OF COAL TIME.
Hitherto the only land back-boned animal was akin to our salamanders. Now we have true lizards in abundance, many of them of large size. Some of them were probably plant-eaters, but most were flesh-eaters; some seem to have been tenants of the early swamps, and some dwelt in the forests.
The creatures related to the salamanders have increased in the variety of their forms to a wonderful extent. We know them best by the tracks which they have left on the mud stones formed on the borders of lakes or the edge of the sea. In some places these footprints are found in amazing numbers and perfection. The best place for them is in the Connecticut Valley, near Turner's Falls, Mass. At this point the red sandstone and shale beds, which are composed of thin layers having a total thickness of several hundred feet, are often stamped over by these footprints like the mud of a barnyard. From the little we can determine from these footprints, the creatures seem to have been somewhat related to our frogs, but they generally had tails, and, though provided with four legs, were in the habit of walking on the hind ones alone like the kangaroo. A few of these tracks are shown in the figure on this page.
These strange creatures were of many different species. Some of them must have been six or seven feet high, for their steps are as much as three feet apart, and seem to imply a creature weighing several hundred pounds. Others were not bigger than robins. Strangely enough, we have never found their bones nor the creatures on which they fed, and but for the formation of a little patch of rocks here and there we should not have had even these footprints to prove to us that such creatures had lived in the Connecticut Valley in this far-off time.
FIG. 5. FOOT-PRINT, TURNER'S FALLS.
But these wonderful forms are less interesting than two or three little fossil jaw-bones that prove to us that in this Triassic time the earth now bore another animal more akin to ourselves, in the shape of a little creature that gave suck to its young. Once more life takes a long upward step in this little opossum-like animal, perhaps the first creature whose young was born alive. These little creatures called Microlestes or Dromatherium, of which only one or two different but related species have been found in England and in North Carolina, appear to have been insect-eaters of about the size and shape of the Australian creature shown in Fig. 7. So far we know it in but few specimens,—altogether only an ounce or two of bones,—but they are very precious monuments of the past.
In this Triassic time the climate appears to have been rather dry, for in it we have many extensive deposits of salt formed by the evaporation of closed lakes, of seas, such as are now forming on the bottom of the Dead Sea, and the Great Salt Lake of Utah, and a hundred or more other similar basins of the present day.
In the sea animals of this time we find many changes. Already some of the giant lizard-like animals, which first took shape on the land, are becoming swimming-animals. They changed their feet to paddles, which, with the help of a flattened tail, force them through the water.
The fishes on which these great swimming lizards preyed are more like the fishes of our present day than they were before. The trilobites are gone, and of the crinoids only a remnant is left. Most of the corals of the earlier days have disappeared, but the mollusks have not changed more than they did at several different times in the earliest stages of the earth's history.
After the Trias comes a long succession of ages in which the life of the world is steadily advancing to higher and higher planes; but for a long time there is no such startling change as that which came in the passage from the coal series of rocks to the Trias. This long set of periods is known to geologists as the age of reptiles. It is well named, for the kindred of the lizards then had the control of the land. There were then none of our large fish to dispute their control, so they shaped themselves to suit all the occupations that could give them a chance for a living. Some remained beasts of prey like our alligators, but grew to larger size; some took to eating the plants, and came to walk on their four legs as our ordinary beasts do, no longer dragging themselves on their bellies as do the lizard and alligator, their lower kindred. Others became flying creatures like our bats, only vastly larger, often with a spread of wing of fifteen or twenty feet. Yet others, even as strangely shaped, dwelt with the sharks in the sea.
In this time of the earth's history we have the first bird-like forms. They were feathered creatures, with bills carrying true teeth, and with strong wings; but they were reptiles in many features, having long, pointed tails such as none of our existing birds have. They show us that the birds are the descendants of reptiles, coming off from them as a branch does from the parent tree. The tortoises began in this series of rocks. At first they are marine or swimming forms, the box-turtles coming later. Here too begin many of the higher insects. Creatures like moths and bees appear, and the forests are enlivened with all the important kinds of insects, though the species were very different from those now living.
In the age of reptiles the plants have made a considerable advance. Palms are plenty; forms akin to our pines and firs abound, and the old flowerless group of ferns begins to shrink in size, and no longer spreads its feathery foliage over all the land as before. Still there were none of our common broad-leaved trees; the world had not yet known the oaks, birches, maples, or any of our hard-wood trees that lose their leaves in autumn; nor were the flowering plants, those with gay blossoms, yet on the earth. The woods and fields were doubtless fresh and green, but they wanted the grace of blossoms, plants, and singing-birds. None of the animals could have had the social qualities or the finer instincts that are so common among animals of the present day. There were probably no social animals like our ants and bees, no merry singing creatures; probably no forms that went in herds. Life was a dull round of uncared-for birth, cruel self-seeking, and of death. The animals at best were clumsy, poorly-endowed creatures, with hardly more intelligence than our alligators.
The little thread of higher life begun in the Microlestes and Dromatherium, the little insect-eating mammals of the forest, is visible all through this time. It held in its warm blood the powers of the time to come, but it was an insignificant thing among the mighty cold-blooded reptiles of these ancient lands. There are several species of them, but they are all small, and have no chance to make headway against the older masters of the earth.
The Jurassic or first part of the reptilian time shades insensibly into the second part, called the Cretaceous, which immediately follows it. During this period the lands were undergoing perpetual changes; rather deep seas came to cover much of the land surfaces, and there is some reason to believe that the climate of the earth became much colder than it had been, at least in those regions where the great reptiles had flourished. It may be that it is due to a colder climate that we owe the rapid passing away of this gigantic reptilian life of the previous age. The reptiles, being cold-blooded, cannot stand even a moderate winter cold, save when they are so small that they can crawl deep into crevices in the rocks to sleep the winter away, guarded from the cold by the warmth of the earth. At any rate these gigantic animals rapidly ceased to be, so that by the middle of the Cretaceous period they were almost all gone, except those that inhabited the sea; and at the end of this time they had shrunk to lizards in size. The birds continue to increase and to become more like those of our day; their tails shrink away, their long bills lose their teeth; they are mostly water-birds of large size, and there are none of our songsters yet; still they are for the first time perfect birds, and no longer half-lizard in their nature.
The greatest change in the plants is found in the coming of the broad-leaved trees belonging to the families of our oaks, maples, etc. Now for the first time our woods take on their aspect of to-day; pines and other cone-bearers mingle with the more varied foliage of nut-bearing or large-seeded trees. Curiously enough, we lose sight of the little mammals of the earlier time. This is probably because there is very little in the way of land animals of this period preserved to us. There are hardly any mines or quarries in the beds of this age to bring these fossils to light. In the most of the other rocks there is more to tempt man to explore them for coal ores or building stones.
In passing from the Cretaceous to the Tertiary, we enter upon the threshold of our modern world. We leave behind all the great wonders of the old world, the gigantic reptiles, the forests of tree ferns, the seas full of ammonites and belemnites, and come among the no less wonderful but more familiar modern forms. We come at once into lands and seas where the back-boned animals are the ruling beings. The reptiles have shrunk to a few low forms,—the small lizards, the crocodiles and alligators, the tortoises and turtles, and, as if to mark more clearly the banishment of this group from their old empire, the serpents, which are peculiarly degraded forms of reptiles which have lost the legs they once had, came to be the commonest reptiles of the earth.
The first mammals that have no pouches now appear. In earlier times, the suck-giving animals all belonged to the group that contains our opossums, kangaroos, etc. These creatures are much lower and feebler than the mammals that have no pouches. Although they have probably been on the earth two or three times as long as the higher mammals, they have never attained any eminent success whatever; they cannot endure cold climates; none of them are fitted for swimming as are the seals and whales, or for flying as the bats, or for burrowing as the moles; they are dull, weak things, which are not able to contend with their stronger, better-organized, higher kindred. They seem not only weak, but unable to fit themselves to many different kinds of existence.
In the lower part of the Tertiary rocks, we find at once a great variety of large beasts that gave suck to their young. It is likely that these creatures had come into existence in a somewhat earlier time in other lands, where we have not been able to study the fossils; for to make their wonderful forms slowly, as we believe them to have been made, would require a very long time. It is probable that during the Cretaceous time, in some land where we have not yet had a chance to study the rocks, these creatures grew to their varied forms, and that in the beginning of the Tertiary time, they spread into the regions where we find their bones.
Beginning with the Tertiary time, we find these lower kinsmen of man, through whom man came to be. The mammals were marked by much greater simplicity and likeness to each other than they now have. There were probably no monkeys, no horses, no bulls, no sheep, no goats, no seals, no whales, and no bats. All these animals had many-fingered feet. There were no cloven feet like those of our bulls, and no solid feet as our horses have. Their brains, which by their size give us a general idea of the intelligence of the creature, are small; hence we conclude that these early mammals were less intelligent than those of our day.
It would require volumes to trace the history of the growth of these early mammals, and show how they, step by step, came to their present higher state. We will take only one of the simplest of these changes, which happens to be also the one which we know best. This is the change that led to the making of our common horses, which seem to have been brought into life on the continent of North America. The most singular thing about our horses is that the feet have but one large toe or finger, the hoof, the hard covering of which is the nail of that extremity. Now it seems hard to turn the weak, five-fingered feet of the animals of the lower Tertiary—feet which seem to be better fitted for tree-climbing than anything else—into feet such as we find in the horse. Yet the change is brought about by easy stages that lead the successive creatures from the weak and loose-jointed foot of the ancient forms to the solid, single-fingered horse's hoof, which is wonderfully well-fitted for carrying a large beast at a swift speed, and is so strong a weapon of defence that an active donkey can kill a lion with a well-delivered kick.
The oldest of these creatures that lead to the horses is called Eohippus or beginning horse. This fellow had on the forefeet four large toes, each with a small hoof and fifth imperfect one, which answered to the thumb. The hind feet had gone further in the change, for they each had but three toes, each with hoofs, the middle-toed hoof larger and longer than the others. A little later toward our day we find another advance in the Orohippus, when the little imperfect thumb has disappeared, and there are only four toes on the forefeet and three on the hind.
Yet later we have the Mesohippus or half-way horse. There are still three toes on the hind foot, but one more of the fingers of the forefeet has disappeared. This time it is the little finger that goes, leaving only a small bone to show that its going was by a slow shrinking. The creature now has three little hoofs on each of its feet.
Still nearer our own time comes the Miohippus, which shows the two side hoofs on each foot shrinking up so that they do not touch the ground, but they still bear little hoofs. Lastly, about the time of man's coming on the earth, appears his faithful servant, the horse, in which those little side hoofs have disappeared, leaving only two little "splint" bones to mark the place where these side hoofs belong. Thus, step by step, our horses' feet were built up; while these parts were changing, the other parts of the animals were also slowly altering. They were at first smaller than our horses,—some of them not as large as an ordinary Newfoundland dog; others as small as foxes.
As if to remind us of his old shape, our horses now and then, but rarely, have, in place of the little splint bones above the hoof, two smaller hoofs, just like the foot of Miohippus. Sometimes these are about the size of a silver dollar, on the part that receives the shoe when horses are shod.
In this way, by slow-made changes, the early mammals pass into the higher. Out of one original part are made limbs as different as the feet of the horse, the wing of a bat, the paddle of a whale, and the hand of man. So with all the parts of the body the forms change to meet the different uses to which they are put.
At the end of this long promise, which was written in the very first animals, comes man himself, in form closely akin to the lower animals, but in mind immeasurably apart from them. We can find every part of man's body in a little different shape in the monkeys, but his mind is of a very different quality. While his lower kindred cannot be made to advance in intelligence any more than man himself can grow a horse's foot or a bat's wing, he is constantly going higher and higher in his mental and moral growth.
So far we have found but few traces of man that lead us to suppose that he has been for a long geological time on the earth, yet there is good evidence that he has been here for a hundred thousand years or more. It seems pretty clear that he has changed little in his body in all these thousands of generations. The earliest remains show us a large-brained creature, who used tools and probably had already made a servant of fire, which so admirably aids him in his work.
Besides the development of this wonderful series of animals, that we may call in a certain way our kindred, there have been several other remarkable advances in this Tertiary time, this age of crowning wonders in the earth's history. The birds have gone forward very rapidly; it is likely that there were no songsters at the first part of this period, but these singing birds have developed very rapidly in later times. Among the insects the most remarkable growth is among the ants, the bees, and their kindred. These creatures have very wonderful habits; they combine together for the making of what we may call states, they care for their young, they wage great battles, they keep slaves, they domesticate other insects, and in many ways their acts resemble the doings of man. Coming at about the same time as man, these intellectual insects help to mark this later stage of the earth as the intellectual period in its history. Now for the first time creatures are on the earth which can form societies and help each other in the difficult work of living.
Among the mollusks, the most important change is in the creation of the great, strong swimming squids, the most remarkable creatures of the sea. Some of these have arms that can stretch for fifty feet from tip to tip.
Among the plants, the most important change has been in the growth of flowering plants, which have been constantly becoming more plenty, and the plants which bear fruits have also become more numerous. The broad-leaved trees seem to be constantly gaining on the forests of narrow-leaved cone-bearers, which had in an earlier day replaced the forests of ferns.
In these Tertiary ages, as in the preceding times of the earth, the lands and seas were much changed in their shape. It seems that in the earlier ages the land had been mostly in the shape of large islands grouped close together where the continents now are. In this time, these islands grew together to form the united lands of Europe, Asia, Africa, Australia, and the twin American continents; so that, as life rose higher, the earth was better fitted for it. Still there were great troubles that it had to undergo. There were at least two different times during the Tertiary age termed glacial periods, times when the ice covered a large part of the northern continents, compelling life of all sorts to abandon great regions, and to find new places in more southern lands. Many kinds of animals and plants seem to have been destroyed in these journeys; but these times of trial, by removing the weaker and less competent creatures, made room for new forms to rise in their places. All advance in nature makes death necessary, and this must come to races as well as to individuals if the life of the world is to go onward and upward.
Looking back into the darkened past, of which we yet know but little compared with what we would like to know, we can see the great armies of living beings led onward from victory to victory toward the higher life of our own time. Each age sees some advance, though death overtakes all its creatures. Those that escape their actual enemies or accident, fall a prey to old age: volcanoes, earthquakes, glacial periods, and a host of other violent accidents sweep away the life of wide regions, yet the host moves on under a control that lies beyond the knowledge of science. Man finds himself here as the crowning victory of this long war. For him all this life appears to have striven. In his hands lies the profit of all its toil and pain. Surely this should make us feel that our duty to all these living things, that have shared in the struggle that has given man his elevation, is great, but above all, great is our duty to the powers that have been placed in our bodies and our minds.
