Читать книгу: «The Atlantic Monthly, Volume 09, No. 55, May, 1862», страница 6

Well, and all this while Faith wasn't standing still; she was changing steadily, as much as ever the moon changed in the sky. I noticed it first one day when Mr. Gabriel'd caught every child in the region and given them a picnic in the woods of the Stack-Yard-Gate, and Faith was nowhere to be seen tiptoeing round every one as she used to do, but I found her at last standing at the head of the table,—Mr. Gabriel dancing here and there, seeing to it that all should be as gay as he seemed to be,—quiet and dignified as you please, and feeling every one of her inches. But it wasn't dignity really that was the matter with Faith,—it was just gloom. She'd brighten up for a moment or two and then down would fall the cloud again, she took to long fits of dreaming, and sometimes she'd burst out crying at any careless word, so that my heart fairly bled for the poor child,—for one couldn't help seeing that she'd some secret unhappiness or other; and I was as gentle and soothing to her as it's in my nature to be. She was in to our house a good deal; she kept it pretty well out of Dan's way, and I hoped she'd get over it sooner or later, and make up her mind to circumstances. And I talked to her a sight about Dan, praising him constantly before her, though I couldn't hear to do it; and finally, one very confidential evening, I told her that I'd been in love with Dan myself once a little, but I'd seen that he would marry her, and so had left off thinking about it; for, do you know, I thought it might make her set more price on him now, if she knew somebody else had ever cared for him. Well, that did answer awhile: whether she thought she ought to make it up to Dan, or whether he really did grow more in her eyes, Faith got to being very neat and domestic and praiseworthy. But still there was the change, and it didn't make her any the less lovely. Indeed, if I'd been a man, I should have cared for her more than ever: it was like turning a child into a woman: and I really think, as Dan saw her going about with such a pleasant gravity, her pretty figure moving so quietly, her pretty face so still and fair, as if she had thoughts and feelings now, he began to wonder what had come over Faith, and, if she were really as charming as this, why he hadn't felt it before; and then, you know, whether you love a woman or not, the mere fact that she's your wife, that her life is sunk in yours, that she's something for you to protect and that your honor lies in doing so, gives you a certain kindly feeling that might ripen into love any day under sunshine and a south wall.

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METHODS OF STUDY IN NATURAL HISTORY

XI

Among the astounding discoveries of modern science is that of the immense periods which have passed in the gradual formation of our earth. So vast were the cycles of time preceding even the appearance of man on the surface of our globe, that our own period seems as yesterday when compared with the epochs that have gone before it. Had we only the evidence of the deposits of rock heaped above each other in regular strata by the slow accumulation of materials, they alone would convince us of the long and slow maturing of God's work on the earth but when we add to these the successive populations of whose life this world has been the theatre, and whose remains are hidden in the rocks into which the mud or sand or soil of whatever kind on which they lived has hardened in the course of time,—or the enormous chains of mountains whose upheaval divided these periods of quiet accumulation by great convulsions,—or the changes of a different nature in the configuration of our globe, as the sinking of lands beneath the ocean, or the gradual rising of continents and islands above it,—or the wearing of great river-beds, or the filling of extensive water-basins, till marshes first and then dry land succeeded to inland seas,—or the slow growth of coral reefs, those wonderful sea-walls raised by the little ocean-architects whose own bodies furnish both the building-stones and the cement that binds them together, and who have worked so busily during the long centuries, that there are extensive countries, mountain-chains, islands, and long lines of coast consisting solely of their remains,—or the countless forests that must have grown up, flourished, died, and decayed, to fill the storehouses of coal that feed the fires of the human race to-day,—if we consider all these records of the past, the intellect fails to grasp a chronology for which our experience furnishes no data, and the time that lies behind us seems as much an eternity to our conception as the future that stretches indefinitely before us.

The physical as well as the human history of the world has its mythical age, lying dim and vague in the morning mists of creation, like that of the heroes and demigods in the early traditions of man, defying all our ordinary dates and measures. But if the succession of periods that prepared the earth for the coming of man, and the animals and plants that accompany him on earth, baffles our finite attempts to estimate its duration, have we any means of determining even approximately the length of the period to which we ourselves belong? If so, it may furnish us with some data for the further solution of these wonderful mysteries of time, and it is besides of especial importance with reference to the question of permanence of Species. Those who maintain the mutability of Species, and account for all the variety of life on earth by the gradual changes wrought by time and circumstances, do not accept historical evidence as affecting the question at all. The monuments of those oldest nations, all whose history is preserved in monumental records, do not indicate the slightest variation of organic types from that day to this. The animals that were preserved within their tombs or carved upon their walls by the ancient Egyptians were the same as those that have their home in the valley of the Nile today; the negro, whose peculiar features are unmistakable even in their rude artistic attempts to represent them, was the same woolly-haired, thick-lipped, flat-nosed, dark-skinned being in the days of the Rameses that he is now. The Apis, the Ibis, the Crocodiles, the sacred Beetles, have brought down to us unchanged all the characters that superstition hallowed in those early days. The stony face of the Sphinx is not more true to its past, nor the massive architecture of the Pyramids more unchanged, than they are. But the advocates of the mutability of Species say truly enough that the most ancient traditions are but as yesterday in the world's history, and that what six thousand years could not do sixty thousand years might effect. Leaving aside, then, all historical chronology, how far back can we trace our own geological period, and the Species belonging to it? By what means can we determine its duration? Within what limits, by what standard, may it be measured? Shall hundreds, or thousands, or hundreds of thousands, or millions of years be the unit from which we start?

I will begin this inquiry with a series of facts which I myself have had an opportunity of investigating with especial care respecting the formation and growth of the Coral Reefs of Florida. But first a few words on Coral Reefs in general. They are living limestone walls that are built up from certain depths in the ocean by the natural growth of a variety of animals, but limited by the level of high-water, beyond which they cannot rise, since the little beings that compose them die as soon as they are removed from the vitalizing influence of the pure sea-water. These walls have a variety of outlines: they may be straight, circular, semicircular, oblong, according to the form of the coast along which the little Reef-Builders establish themselves; and their height is, of course, determined by the depth of the bottom on which they rest. If they settle about an island on all sides of which the conditions for their growth are equally favorable, they will raise a wall all around it, thus encircling it with a ring of Coral growth. The Athols in the Pacific Ocean, those circular islands inclosing sometimes a fresh-water lake in mid-ocean, are Coral walls of this kind, that have formed a ring around a central island. This is easily understood, if we remember that the bottom of the Pacific Ocean is by no means a stable foundation for such a structure. On the contrary, over a certain area, which has already been surveyed with some accuracy by Professor Dana, during the United States Exploring Expedition, it is subsiding; and if an island upon which the Reef-Builders have established themselves be situated in that area of subsidence, it will, of course, sink with the floor on which it rests, carrying down also the Coral wall to a greater depth in the sea. In such instances, if the rate of subsidence be more rapid than the rate of growth in the Corals, the island and the wall itself will disappear beneath the ocean. But whenever, on the contrary, the rate of increase in the wall is greater than that of subsidence in the island, while the latter gradually sinks below the surface, the former rises in proportion, and by the time it has completed its growth the central island has vanished, and there remains only a ring of Coral Reef, with here and there a break, perhaps, at some spot where the more prosperous growth of the Corals has been checked. If, however, as sometimes happens, there is no such break, and the wall is perfectly uninterrupted, the sheet of sea-water so inclosed may be changed to fresh water by the rains that are poured into it. Such a water-basin will remain salt, it is true, in its lower part, and the fact that it is affected by the rise and fall of the tides shows that it is not entirely secluded from communication with the ocean outside; but the salt water, being heavier, sinks, while the lighter rain-water remains above, and it is to all appearance actually changed into a fresh-water lake.

I need not dwell here on the further history of such a Coral island, or follow it through the changes by which the summit of its circular wall becomes covered with a fertile soil, a tropical vegetation springs up on it, and it is at last perhaps inhabited by man. There is something very attractive in the idea of these green rings inclosing sheltered harbors and quiet lakes in mid-ocean, and the subject has lost none of its fascination since the mystery of their existence has been solved by the investigations of several contemporary naturalists who have enabled us to trace the whole story of their structure. I would refer all who wish for a more detailed account of them to Charles Darwin's charming little volume on "Coral Reefs," where their mode of formation is fully described, and also to James D. Dana's "Geological Report of the United States Exploring Expedition."

Coral Reefs are found only in tropical regions: although Polyps, animals of the same class as those chiefly instrumental in their formation, are found in all parts of the globe, yet the Reef-Building Polyps are limited to the Tropics. We are too apt to forget that the homes of animals are as definitely limited in the water as on the land. Indeed, the subject of the geographical distribution of animals according to laws that are established by altitude, by latitude and longitude, by pressure of atmosphere or pressure of water, already alluded to in a previous article, is exceedingly interesting, and presents a most important field of investigation. The climatic effect of different degrees of altitude upon the growth of animals and plants is the same as that of different degrees of latitude; and the slope of a high mountain in the Tropics, from base to summit, presents, in a condensed form, an epitome, as it were, of the same kind of gradation in vegetable growth that may be observed from the Tropics to the Arctics. At the base of such a mountain we have all the luxuriance of growth characteristic of the tropical forest,—the Palms, the Bananas, the Bread-trees, the Mimosas; higher up, these give way to a different kind of growth, corresponding to our Oaks, Chestnuts, Maples, etc.; as these wane, on the loftier slopes comes in the Pine forest, fading gradually, as it ascends, into a dwarfish growth of the same kind; and this at last gives way to the low creeping Mosses and Lichens of the greater heights, till even these find a foothold no longer, and the summit of the mountain is clothed in perpetual snow and ice. What have we here but the same series of changes through which we pass, if, travelling northward from the Tropics, we leave Palms and Pomegranates and Bananas behind, where the Live-Oaks and Cypresses, the Orange-trees and Myrtles of the warmer Temperate Zone come in, and these die out as we reach the Oaks, Chestnuts, Maples, Elms, Nut-trees, Beeches, and Birches of the colder Temperate Zone, these again waning as we enter the Pine forests of the Arctic borders, till, passing out of these, nothing but a dwarf vegetation, a carpet of Moss and Lichen, fit food for the Reindeer and the Esquimaux, greets us, and beyond that lies the region of the snow and ice fields, impenetrable to all but the daring Arctic voyager?

I have thus far spoken of the changes in the vegetable growth alone as influenced by altitude and latitude, but the same is equally true of animals. Every zone of the earth's surface has its own animals, suited to the conditions under which they are meant to live; and with the exception of those that accompany man in all his pilgrimages, and are subject to the same modifying influences by which he adapts his home and himself to all climates, animals are absolutely bound by the laws of their nature within the range assigned to them. Nor is this the case only on land, where river-banks, lake-shores, and mountain-ranges might be supposed to form the impassable boundaries that keep animals within certain limits; but the ocean as well as the land has its faunae and florae bound within their respective zoölogical and botanical provinces; and a wall of granite is not more impassable to a marine animal than that ocean-line, fluid and flowing and ever-changing though it be, on which is written for him, "Hitherto shalt thou come, but no farther." One word as to the effect of pressure on animals will explain this.

We all live under the pressure of the atmosphere. Now thirty-two feet under the sea doubles that pressure, since a column of water of that height is equal in weight to the pressure of one atmosphere. At the depth of thirty-two feet, then, any marine animal is under the pressure of two atmospheres,—that of the air which surrounds our globe, and of a weight of water equal to it; at sixty-four feet he is under the pressure of three atmospheres, and so on,—the weight of one atmosphere being always added for every thirty-two feet of depth. There is a great difference in the sensitiveness of animals to this pressure. Some fishes live at a great depth and find the weight of water genial to them, while others would be killed at once by the same pressure, and the latter naturally seek the shallow waters. Every fisherman knows that he must throw a long line for a Halibut, while with a common fishing-rod he will catch plenty of Perch from the rocks near the shore; and the differently colored bands of sea-weed revealed by low tide, from the green line of the Ulvas through the brown zone of the common Fucas to the rosy and purple hued sea-weeds of the deeper water show that the florae as well as the faunae of the ocean have their precise boundaries. This wider or narrower range of marine animals is in direct relation to their structure, which enables them to bear a greater or less pressure of water. All fishes, and, indeed, all animals having a wide range of distribution in ocean-depths, have a special apparatus of water-pores, so that the surrounding element penetrates their structure, thus equalizing the pressure of the weight, which is diminished from without in proportion to the quantity of water they can admit into their bodies. Marine animals differ in their ability to sustain this pressure, just as land animals differ in their power of enduring great variations of climate and of atmospheric pressure.

Of all air-breathing animals, none exhibits a more surprising power of adapting itself to great and rapid changes of external influences than the Condor. It may be seen feeding on the sea-shore under a burning tropical sun, and then, rising from its repast, it floats up among the highest summits of the Andes and is lost to sight beyond them, miles above the line of perpetual snow, where the temperature must be lower than that of the Arctics. But even the Condor, sweeping at one flight from tropic heat to arctic cold, although it passes through greater changes of temperature, does not undergo such changes of pressure as a fish that rises from a depth of sixty-four feet to the surface of the sea; for the former remains within the air that surrounds our globe, and therefore the increase or diminution of pressure to which it is subjected must be confined within the limits of one atmosphere, while the latter, at a depth of sixty-four feet, is under a weight equal to that of three such atmospheres, which is reduced to one when it reaches the sea-level. The change is even much greater for those fishes that come from a depth of several hundred feet. These laws of limitation in space explain many facts in the growth of Coral Reefs that would be otherwise inexplicable, and which I will endeavor to make clear to my readers.

For a long time it was supposed that the Coral animals inhabited very deep waters, for they were sometimes brought up on sounding-lines from a depth of many hundreds or even thousands of feet, and it was taken for granted that they must have had their home where they were found; but the facts recently ascertained respecting the subsidence of ocean-bottoms have shown that the foundation of a Coral wall may have sunk far below the place where it was laid, and it is now proved beyond a doubt that no Reef-Building Coral can thrive at a depth of more than fifteen fathoms, though Corals of other kinds occur far lower, and that the dead Reef-Corals sometimes brought to the surface from much greater depths are only broken fragments of some Reef that has subsided with the bottom on which it was growing. But though fifteen fathoms is the maximum depth at which any Reef-Builder can prosper, there are many which will not sustain even that degree of pressure, and this fact has, as we shall see, an important influence on the structure of the Reef.

Imagine now a sloping shore on some tropical coast descending gradually below the surface of the sea. Upon that slope, at a depth of from ten to twelve or fifteen fathoms, and two or three or more miles from the main-land, according to the shelving of the shore, we will suppose that one of those little Coral animals to whom a home in such deep waters is genial has established itself. How it happens that such a being, which we know is immovably attached to the ground and forms the foundation of a solid wall, was ever able to swim freely about in the water till it found a suitable resting-place, I shall explain hereafter, when I say something of the mode of reproduction of these animals. Accept, for the moment, my unsustained assertion, and plant our little Coral on this sloping shore some twelve or fifteen fathoms below the surface of the sea. The internal structure of such a Coral corresponds to that of the Sea-Anemone: the body is divided by vertical partitions from top to bottom, leaving open chambers between, while in the centre hangs the digestive cavity connecting by an opening in the bottom with all these chambers; at the top is an aperture which serves as a mouth, surrounded by a wreath of hollow tentacles, each one connecting at its base with one of the chambers, so that all parts of the animal communicate freely with each other. But though the structure of the Coral is identical in all its parts with that of the Sea-Anemone, it nevertheless presents one important difference. The body of the Sea-Anemone is soft, while that of the Coral is hard. It is well known that all animals and plants have the power of appropriating to themselves and assimilating the materials they need, each selecting from the surrounding elements whatever contributes to its well-being. The plant takes carbon, the animal takes oxygen, each rejecting what the other requires. We ourselves build our bones with the lime that we find unconsciously in the world around us; much of our nourishment supplies us with it, and the very vegetables we eat have, perhaps, themselves been fed from some old lime strata deposited centuries ago. We all represent materials that have contributed to construct our bodies. Now Corals possess, in an extraordinary degree, the power of assimilating to themselves the lime contained in the salt water around them; and as soon as our little Coral is established on a firm foundation, a lime deposit begins to form in all the walls of its body, so that its base, its partitions, and its outer wall, which in the Sea-Anemone remain always soft, become perfectly solid in the Polyp Coral and form a frame as hard as bone. It may naturally be asked where the lime comes from in the sea which the Corals absorb in such quantities. As far as the living Corals are concerned the answer is easy, for an immense deal of lime is brought down to the ocean by rivers that wear away the lime deposits through which they pass. The Mississippi, whose course lies through extensive lime regions, brings down yearly lime enough to supply all the animals living in the Gulf of Mexico. But behind this lies a question not so easily settled, as to the origin of the extensive deposits of limestone found at the very beginning of life upon earth. This problem brings us to the threshold of astronomy, for limestone is metallic in character, susceptible therefore of fusion, and may have formed a part of the materials of our earth, even in an incandescent state, when the worlds were forming. But though this investigation as to the origin of lime does not belong either to the naturalist or the geologist, its suggestion reminds us that the time has come when all the sciences and their results are so intimately connected that no one can be carried on independently of the others. Since the study of the rocks has revealed a crowded life whose records are hoarded within them, the work of the geologist and the naturalist has become one and the same, and at that border-land where the first crust of the earth condensed out of the igneous mass of materials which formed its earliest condition their investigation mingles with that of the astronomer, and we cannot trace the limestone in a little Coral without going back to the creation of our solar system, when the worlds that compose it were thrown off from a central mass in a gaseous condition.

When the Coral has become in this way permeated with lime, all parts of the body are rigid, with the exception of the upper margin, the stomach, and the tentacles. The tentacles are soft and waving, projected or drawn in at will, and they retain their flexible character through life, and decompose when the animal dies. For this reason the dried specimens of Corals preserved in museums do not give us the least idea of the living Corals, in which every one of the millions of beings composing such a community is crowned by a waving wreath of white or green or rose-colored tentacles.

As soon as the little Coral is fairly established and solidly attached to the ground, it begins to bud. This may take place in a variety of ways, dividing at the top or budding from the base or from the sides, till the primitive animal is surrounded by a number of individuals like itself, of which it forms the nucleus, and which now begin to bud in their turn, each one surrounding itself with a numerous progeny, all remaining, however, attached to the parent. Such a community increases till its individuals are numbered by millions; and I have myself counted no less than fourteen millions of individuals in a Coral mass measuring not more than twelve feet in diameter. These are the so-called Coral heads which form the foundation of a Coral wall, and their massive character and regular form seem to be especially adapted to give a strong, solid base to the whole structure. They are known in our classifications as the Astraeans, so named on account of the star-shaped form of the little pits that are crowded upon the surface, each one marking the place of a single individual in such a community.

Thus firmly and strongly is the foundation of the reef laid by the Astraeans; but we have seen that for their prosperous growth they require a certain depth and pressure of water, and when they have brought the wall so high that they have not more than six fathoms of water above them, this kind of Coral ceases to grow. They have, however, prepared a fitting surface for different kinds of Corals that could not live in the depths from which the Astraeans have come, but find their genial home nearer the surface; such a home being made ready for them by their predecessors, they now establish themselves on the top of the Coral wall and continue its growth for a certain time. These are the Mandrinas, or the so-called Brain-Corals, and the Porites. The Mandrinas differ from the Astraeans by their less compact and definite pits. In the Astraeans the place occupied by the animal in the community is marked by a little star-shaped spot, in the centre of which all the partition-walls meet. But in the Mandrinas, although all the partitions converge toward the central opening, as in the Astraeans, these central openings elongate, run into each other, and form waving furrows all over the surface, instead of the small round pits so characteristic of the Astraeans. The Porites resemble the Astraeans, but the pits are smaller, with fewer partitions and fewer tentacles, and their whole substance is more porous.

But these also have their bounds within the sea: they in their turn reach the limit beyond which they are forbidden by the laws of their nature to pass, and there they also pause. But the Coral wall continues its steady progress; for here the lighter kinds set in,—the Madrepores, the Millepores, and a great variety of Sea-Fans and Corallines, and the reef is crowned at last with a many-colored shrubbery of low feathery growth. These are all branching in form, and many of them are simple calciferous plants, though most of them are true animals, resembling, however, delicate Algae more than any marine animals; but, on examination of the latter, one finds them to be covered with myriads of minute dots, each representing one of the little beings out of which the whole is built.

I would add here one word on the true nature of the Millepores, long misunderstood by naturalists, because it throws light not only on some interesting facts respecting Coral Reefs, especially the ancient ones, but also because it tells us something of the early inhabitants of the globe, and shows us that a class of Radiates supposed to be missing in that primitive creation had its representatives then as now. In the diagram of the geological periods introduced in a previous article, I have represented all the three classes of Radiates, Polyps, Acalephs, and Echinoderms, as present on the first floor of our globe that was inhabited at all. But it is only recently that positive proofs have been found of the existence of Acalephs or Jelly-Fishes, as they are called, at that early period. Their very name indicates their delicate structure; and were there no remains preserved in the rocks of these soft, transparent creatures, it would yet be no evidence that they did not exist. Fragile as they are, however, they have left here and there some faint record of themselves, and in the Museum at Carlsruhe, on a slab from Solenhofen, I have seen a very perfect outline of one which remains undescribed to this day. This, however, does not carry them farther back than the Jurassic period, and it is only lately that I have satisfied myself that they not only existed, but were among the most numerous animals in the first representation of organic life.

The earliest Corals correspond in certain features of their structure to the Millepores. They differ from them as all early animals differ from the succeeding ones, every geological period having its special set of representatives. But still they are always true to their class, and have a certain general correspondence with animals of like kind that follow them in later periods. In this sense the Millepores are in our epoch the representatives of those early Corals called by naturalists Tabulata and Rugosa,—distinguished from the Polyp Corals by the horizontal floors, waving in some, straight in others, which divide the body transversely at successive heights through its whole length, and also by the absence of the vertical partitions, extending from top to bottom of each animal, so characteristic of the true Polyps. As I have said, they were for a long time supposed, notwithstanding these differences, to be Polyps, and I had shared in this opinion, till, during the winter of 1857, while pursuing my investigations on the Coral Reefs of Florida, one of these Millepores revealed itself to me in its true character of Acaleph. It is by its soft parts alone—those parts which are seen only in its living state, and when the animal is fully open—that its Acalephian character can be perceived, and this accounts for its being so long accepted as a Polyp, when studied in the dry Coral stock. Nothing could exceed my astonishment when for the first time I saw such an animal fully expanded, and found it to be a true Acaleph. It is exceedingly difficult to obtain a view of them in this state, for, at any approach, they draw themselves in, and remain closed to all investigation. Only once, for a short hour, I had this opportunity; during that time one of these little creatures revealed to me its whole structure, as if to tell me, once for all, the story of its existence through all the successive epochs from the dawn of Creation till now, and then withdrew. With my most patient watching, I have never been able to see one of them open again. But to establish the fact that one of the Corals represented from the earliest period till now, and indeed far more numerous in the beginning than any other, was in truth no Polyp, but an Acaleph, the glimpse I had was all-sufficient. It came out as if to bear witness of its class,—as if to say, "We, too, were among the hosts of living beings with which God first peopled His earth."