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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/2937-h.zip b/2937-h.zip Binary files differnew file mode 100644 index 0000000..2e47a5a --- /dev/null +++ b/2937-h.zip diff --git a/2937-h/2937-h.htm b/2937-h/2937-h.htm new file mode 100644 index 0000000..24c1e33 --- /dev/null +++ b/2937-h/2937-h.htm @@ -0,0 +1,1041 @@ +<?xml version="1.0" encoding="us-ascii"?> + +<!DOCTYPE html + PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" > + +<html xmlns="http://www.w3.org/1999/xhtml" lang="en"> + <head> + <title> + Coral and Coral Reefs, by Thomas H. Huxley + </title> + <style type="text/css" xml:space="preserve"> + + body { margin:5%; background:#faebd0; text-align:justify} + P { text-indent: 1em; margin-top: .25em; margin-bottom: .25em; } + H1,H2,H3,H4,H5,H6 { text-align: center; margin-left: 15%; margin-right: 15%; } + hr { width: 50%; text-align: center;} + .foot { margin-left: 20%; margin-right: 20%; text-align: justify; text-indent: -3em; font-size: 90%; } + blockquote {font-size: 97%; font-style: italic; margin-left: 10%; margin-right: 10%;} + .mynote {background-color: #DDE; color: #000; padding: .5em; margin-left: 10%; margin-right: 10%; font-family: sans-serif; font-size: 95%;} + .toc { margin-left: 10%; margin-bottom: .75em;} + .toc2 { margin-left: 20%;} + div.fig { display:block; margin:0 auto; text-align:center; } + div.middle { margin-left: 20%; margin-right: 20%; text-align: justify; } + .figleft {float: left; margin-left: 0%; margin-right: 1%;} + .figright {float: right; margin-right: 0%; margin-left: 1%;} + .pagenum {display:inline; font-size: 70%; font-style:normal; + margin: 0; padding: 0; position: absolute; right: 1%; + text-align: right;} + pre { font-style: italic; font-size: 90%; margin-left: 10%;} + +</style> + </head> + <body> +<pre xml:space="preserve"> + +The Project Gutenberg EBook of Coral and Coral Reefs, by Thomas H. Huxley + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Coral and Coral Reefs + +Author: Thomas H. Huxley + +Release Date: January 6, 2009 [EBook #2937] +Last Updated: January 22, 2013 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK CORAL AND CORAL REEFS *** + + + + +Produced by Amy E. Zelmer, and David Widger + + + + + + +</pre> + <p> + <br /><br /> + </p> + <h1> + CORAL AND CORAL REEFS + </h1> + <p> + <br /> + </p> + <h2> + by Thomas H. Huxley + </h2> + <p> + <br /> <a href="#linknote-1" name="linknoteref-1" id="linknoteref-1"><small>[1]</small></a> + <br /> <br /> + </p> + <hr /> + <p> + <br /> <br /> + </p> + <p> + THE subject upon which I wish to address you to-night is the structure and + origin of Coral and Coral Reefs. Under the head of "coral" there are + included two very different things; one of them is that substance which I + imagine a great number of us have champed when we were very much younger + than we are now,—the common red coral, which is used so much, as you + know, for the edification and the delectation of children of tender years, + and is also employed for the purposes of ornament for those who are much + older, and as some think might know better. The other kind of coral is a + very different substance; it may for distinction's sake be called the + white coral; it is a material which most assuredly not the hardest-hearted + of baby farmers would give to a baby to chew, and it is a substance which + is to be seen only in the cabinets of curious persons, or in museums, or, + may be, over the mantelpieces of sea-faring men. But although the red + coral, as I have mentioned to you, has access to the very best society; + and although the white coral is comparatively a despised product, yet in + this, as in many other cases, the humbler thing is in reality the greater; + the amount of work which is done in the world by the white coral being + absolutely infinite compared with that effected by its delicate and + pampered namesake. Each of these substances, the white coral and the red, + however, has a relationship to the other. They are, in a zoological sense, + cousins, each of them being formed by the same kind of animals in what is + substantially the same way. Each of these bodies is, in fact, the hard + skeleton of a very curious and a very simple animal, more comparable to + the bones of such animals as ourselves than to the shells of oysters or + creatures of that kind; for it is the hardening of the internal tissue of + the creature, of its internal substance, by the deposit in the body of a + material which is exceedingly common, not only in fresh but in sea water, + and which is specially abundant in those waters which we know as "hard," + those waters, for example, which leave a "fur" upon the bottom of a + tea-kettle. This "fur" is carbonate of lime, the same sort of substance as + limestone and chalk. That material is contained in solution in sea water, + and it is out of the sea water in which these coral creatures live that + they get the lime which is needed for the forming of their hard skeleton. + </p> + <p> + But now what manner of creatures are these which form these hard + skeletons? I dare say that in these days of keeping aquaria, of locomotion + to the sea-side, most of those whom I am addressing may have seen one of + those creatures which used to be known as the "sea anemone," receiving + that name on account of its general resemblance, in a rough sort of way, + to the flower which is known as the "anemone"; but being a thing which + lives in the sea, it was qualified as the "sea anemone." Well, then, you + must suppose a body shaped like a short cylinder, the top cut off, and in + the top a hole rather oval than round. All round this aperture, which is + the mouth, imagine that there are placed a number of feelers forming a + circle. The cavity of the mouth leads into a sort of stomach, which is + very unlike those of the higher animals, in the circumstance that it opens + at the lower end into a cavity of the body, and all the digested matter, + converted into nourishment, is thus distributed through the rest of the + body. That is the general structure of one of these sea anemones. If you + touch it it contracts immediately into a heap. It looks at first quite + like a flower in the sea, but if you touch it you find that it exhibits + all the peculiarities of a living animal; and if anything which can serve + as its prey comes near its tentacles, it closes them round it and sucks + the material into its stomach and there digests it and turns it to the + account of its own body. + </p> + <p> + These creatures are very voracious, and not at all particular what they + seize; and sometimes it may be that they lay hold of a shellfish which is + far too big to be packed into that interior cavity, and, of course, in any + ordinary animal a proceeding of this kind would give rise to a very severe + fit of indigestion. But this is by no means the case in the sea anemone, + because when digestive difficulties of this kind arise he gets out of them + by splitting himself in two; and then each half builds itself up into a + fresh creature, and you have two polypes where there was previously one, + and the bone which stuck in the way lying between them! Not only can these + creatures multiply in this fashion, but they can multiply by buds. A bud + will grow out of the side of the body (I am not speaking of the common sea + anemone, but of allied creatures) just like the bud of a plant, and that + will fashion itself into a creature just like the parent. There are some + of them in which these buds remain connected together, and you will soon + see what would be the result of that. If I make a bud grow out here, and + another on the opposite side, and each fashions itself into a new polype, + the practical effect will be that before long you will see a single polype + converted into a sort of tree or bush of polypes. And these will all + remain associated together, like a kind of co-operative store, which is a + thing I believe you understand very well here,—each mouth will help + to feed the body and each part of the body help to support the + multifarious mouths. I think that is as good an example of a zoological + co-operative store as you can well have. Such are these wonderful + creatures. But they are capable not only of multiplying in this way, but + in other ways, by having a more ordinary and regular kind of offspring. + Little eggs are hatched and the young are passed out by the way of the + mouth, and they go swimming about as little oval bodies covered with a + very curious kind of hairlike processes. Each of these processes is + capable of striking water like an oar; and the consequence is that the + young creature is propelled through the water. So that you have the young + polype floating about in this fashion, covered by its 'vibratile cilia', + as these long filaments, which are capable of vibration are termed. And + thus, although the polype itself may be a fixed creature unable to move + about, it is able to spread its offspring over great areas. For these + creatures not only propel themselves, but while swimming about in the sea + for many hours, or perhaps days, it will be obvious that they must be + carried hither and thither by the currents of the sea, which not + unfrequently move at the rate of one or two miles an hour. Thus, in the + course of a few days, the offspring of this stationary creature may be + carried to a very great distance from its parent; and having been so + carried it loses these organs by which it is propelled, and settles down + upon the bottom of the sea and grows up again into the form and condition + of its parents. So that if you suppose a single polype of this kind + settled upon the bottom of the sea, it may by these various methods—that + is to say, by cutting itself in two, which we call "fission," or by + budding; or by sending out these swimming embryos,—multiply itself + to an enormous extent, and give rise to thousands, or millions, of progeny + in a comparatively short time; and these thousands, or millions, of + progeny may cover a very large surface of the sea bottom; in fact, you + will readily perceive that, give them time, and there is no limit to the + surface which they may cover. + </p> + <p> + Having understood thus far the general nature of these polypes, which are + the fabricators both of the red and white coral, let us consider a little + more particularly how the skeletons of the red coral and of the white + coral are formed. The red coral polype perches upon the sea bottom, it + then grows up into a sort of stem, and out of that stem there grow + branches, each of which has its own polypes; and thus you have a kind of + tree formed, every branch of the tree terminated by its polype. It is a + tree, but at the end of the branches there are open mouths of polypes + instead of flowers. Thus there is a common soft body connecting the whole, + and as it grows up the soft body deposits in its interior a quantity of + carbonate of lime, which acquires a beautiful red or flesh colour, and + forms a kind of stem running through the whole, and it is that stem which + is the red coral. The red coral grows principally at the bottom of the + Mediterranean Sea, at very great depths, and the coral fishers, who are + very adventurous seamen, take their drag nets, of a peculiar kind, roughly + made, but efficient for their purpose, and drag them along the bottom of + the sea to catch the branches of the red coral, which become entangled and + are thus brought up to the surface. They are then allowed to putrefy, in + order to get rid of the animal matter, and the red coral is the skeleton + that is left. + </p> + <p> + In the case of the white coral, the skeleton is more complete. In the red + coral, the skeleton belongs to the whole; in the white coral there is a + special skeleton for every one of these polypes in addition to that for + the whole body. There is a skeleton formed in the body of each of them, + like a cup divided by a number of radiating partitions towards the + outside; and that cup is formed of carbonate of lime, only not stained + red, as in the case of the red coral. And all these cups are joined + together into a common branch, the result of which is the formation of a + beautiful coral tree. This is a great mass of madrepore, and in the living + state every one of the ends of these branches was terminated by a + beautiful little polype, like a sea anemone, and all the skeleton was + covered by a soft body which united the polypes together. You must + understand that all this skeleton has been formed in the interior of the + body, to suit the branched body of the polype mass, and that it is as much + its skeleton as our own bones are our skeleton. In this next coral the + creature which has formed the skeleton has divided itself as it grew, and + consequently has formed a great expansion; but scattered all over this + surface there were polype bodies like those I previously described. Again, + when this great cup was alive, the whole surface was covered with a + beautiful body upon which were set innumerable small polype flowers, if we + may so call them, often brilliantly coloured; and the whole cup was built + up in the same fashion by the deposit of carbonate of lime in the interior + of the combined polype body, formed by budding and by fission in the way I + described. You will perceive that there is no necessary limit to this + process. There is no reason why we should not have coral three or four + times as big; and there are certain creatures of this kind that do + fabricate very large masses, or half spheres several feet in diameter. + Thus the activity of these animals in separating carbonate of lime from + the sea and building it up into definite shapes is very considerable + indeed. + </p> + <p> + Now I think I have said sufficient—as much as I can without taking + you into technical details, of the general nature of these creatures which + form coral. The animals which form coral are scattered over the seas of + all countries in the world. The red coral is comparatively limited, but + the polypes which form the white coral are widely scattered. There are + some of them which remain single, or which give rise to only small + accumulations; and the skeletons of these, as they die, accumulate upon + the bottom of the sea, but they do not come to much; they are washed about + and do not adhere together, but become mixed up with the mud of the sea. + But there are certain parts of the world in which the coral polypes which + live and grow are of a kind which remain, adhere together, and form great + masses. They differ from the ordinary polypes just in the same way as + those plants which form a peat-bog or meadow-turf differ from ordinary + plants. They have a habit of growing together in masses in the same place; + they are what we call "gregarious" things; and the consequence of this is, + that as they die and leave their skeletons, those skeletons form a + considerable solid aggregation at the bottom of the sea, and other polypes + perch upon them, and begin building upon them, and so by degrees a great + mass is formed. And just as we know there are some ancient cities in which + you have a British city, and over that the foundations of a Roman city; + and over that a Saxon city, and over that again a modern city, so in these + localities of which I am speaking, you have the accumulations of the + foundations of the houses, if I may use the term, of nation after nation + of these coral polypes; and these accumulations may cover a very + considerable space, and may rise in the course of time from the bottom to + the surface of the sea. + </p> + <p> + Mariners have a name which they apply to all sorts of obstacles consisting + of hard and rocky matter which comes in their way in the course of their + navigation; they call such obstacles "reefs," and they have long been in + the habit of calling the particular kind of reef, which is formed by the + accumulation of the skeletons of dead corals, by the name of "coral + reefs," therefore, those parts of the world in which these accumulations + occur have been termed by them "coral reef areas," or regions in which + coral reefs are found. There is a very notable example of a simple coral + reef about the island of Mauritius, which I dare say you all know, lies in + the middle of the Indian Ocean. It is a very considerable and beautiful + island, and is surrounded on all sides by a mass of coral, which has been + formed in the way I have described; so that if you could get upon the top + of one of the peaks of the island, and look down upon the Indian Ocean, + you would see that the beach round the Island was continued outward by a + kind of shallow terrace, which is covered by the sea, and where the sea is + quite shallow; and at a distance varying from three-quarters of a mile to + a mile and a half from the proper beach, you would see a line of foam or + surf which looks most beautiful in contrast with the bright green water in + the inside, and the deep blue of the sea beyond. That line of surf + indicates the point at which the waters of the ocean are breaking upon the + coral reef which surrounds the island. You see it sweep round the island + upon all sides, except where a river may chance to come down, and that + always makes a gap in the shore. + </p> + <p> + There are two or three points which I wish to bring clearly before your + notice about such a reef as this. In the first place, you perceive it + forms a kind of fringe round the island, and is therefore called a + "fringing reef." In the next place, if you go out in a boat, and take + soundings at the edge of the reef, you find that the depth of the water is + not more than from 20 to 25 fathoms—that is about 120 to 150 feet. + Outside that point you come to the natural sea bottom; but all inside that + depth is coral, built up from the bottom by the accumulation of the + skeletons of innumerable generations of coral polypes. So that you see the + coral forms a very considerable rampart round the island. What the exact + circumference may be I do not remember, but it cannot be less than 100 + miles, and the outward height of this wall of coral rock nowhere amounts + to less than about 100 or 150 feet. + </p> + <p> + When the outward face of the reef is examined, you find that the upper + edge, which is exposed to the wash of the sea, and all the seaward face, + is covered with those living plant-like flowers which I have described to + you. They are the coral polypes which grow, flourish, and add to the mass + of calcareous matter which already forms the reef. But towards the lower + part of the reef, at a depth of about 120 feet, these creatures are less + active, and fewer of them at work; and at greater depths than that you + find no living coral polype at all; and it may be laid down as a rule, + derived from very extensive observation, that these reef-building corals + cannot live in a greater depth of water than about 120 to 150 feet. I beg + you to recollect that fact, because it is one I shall have to come back to + by and by, and to show to what very curious consequences that rule leads. + Well then, coming back to the margin of the reef, you find that part of it + which lies just within the surf to be coated by a very curious plant, a + sort of seaweed, which contains in its substance a very great deal of + carbonate of lime, and looks almost like rock; this is what is called the + nulli pore. More towards the land, we come to the shallow water upon the + inside of the reef, which has a particular name, derived from the Spanish + or the Portuguese—it is called a "lagoon," or lake. In this lagoon + there is comparatively little living coral; the bottom of it is formed of + coral mud. If we pounded this coral in water, it would be converted into + calcareous mud, and the waves during storms do for the coral skeletons + exactly what we might do for this coral in a mortar; the waves tear off + great fragments and crush them with prodigious force, until they are + ground into the merest powder, and that powder is washed into the interior + of the lagoon, and forms a muddy coating at the bottom. Beside that there + are a great many animals that prey upon the coral—fishes, worms, and + creatures of that kind, and all these, by their digestive processes, + reduce the coral to the same state, and contribute a very important + element to this fine mud. The living coral found in the lagoon, is not the + reef building coral; it does not give rise to the same massive skeletons. + As you go in a boat over these shallow pools, you see these beautiful + things, coloured red, blue, green, and all colours, building their houses; + but these are mere tenements, and not to be compared in magnitude and + importance to the masses which are built by the reef-builders themselves. + Now such a structure as this is what is termed a "fringing reef." You meet + with fringing reefs of this kind not only in the Mauritius, but in a + number of other parts of the world. If these were the only reefs to be + seen anywhere, the problem of the formation of coral reefs would never + have been a difficult one. Nothing can be easier than to understand how + there must have been a time when the coral polypes came and settled on the + shores of this island, everywhere within the 20 to 25 fathom line, and + how, having perched there, they gradually grew until they built up the + reef. + </p> + <p> + But these are by no means the only sort of coral reefs in the world; on + the contrary, there are very large areas, not only of the Indian ocean, + but of the Pacific, in which many many thousands of square miles are + covered either with a peculiar kind of reef, which is called the + "encircling reef," or by a still more curious reef which goes by the name + of the "atoll." There is a very good picture, which Professor Roscoe has + been kind enough to prepare for me, of one of these atolls, which will + enable you to form a notion of it as a landscape. You have in the + foreground the waters of the Pacific. You must fancy yourself in the + middle of the great ocean, and you will perceive that there is an almost + circular island, with a low beach, which is formed entirely of coral sand; + growing upon that beach you have vegetation, which takes, of course, the + shape of the circular land; and then, in the interior of the circle, there + is a pool of water, which is not very deep—probably in this case not + more than eight or nine fathoms—and which forms a strange and + beautiful contrast to the deep blue water outside. This circular island, + or atoll, with a lagoon in the middle, is not a complete circle; upon one + side of it there is a break, exactly like the entrance into a dock; and, + as a matter of course, these circular islets, or atolls, form most + efficient break-waters, for if you can only get inside your ship is in + perfect safety, with admirable anchorage in the interior. If the ship were + lying within a mile of that beach, the water would be one or two thousand + feet deep; therefore, a section of that atoll, with the soundings as deep + as this all round, would give you the notion of a great cone, cut off at + the top, and with a shallow cup in the middle of it. Now, what a very + singular fact this is, that we should have rising from the bottom of the + deep ocean a great pyramid, beside which all human pyramids sink into the + most utter insignificance! These singular coral limestone structures are + very beautiful, especially when crowned with cocoa-nut trees. There you + see the long line of land, covered with vegetation—cocoa-nut trees—and + you have the sea upon the inner and outer sides, with a vessel very + comfortably riding at anchor. That is one of the remarkable forms of reef + in the Pacific. Another is a sort of half-way house, between the atoll and + the fringing reef; it is what is called an "encircling reef." In this case + you see an Island rising out of the sea, and at two or three miles + distance, or more, and separated by a deep channel, which may be eight to + twelve fathoms deep, there is a reef, which encircles it like a great + girdle; and outside that again the water is one or two thousand feet deep. + I spent three or four years of my life in cruising about a modification of + one of these encircling reefs, called a "barrier reef," upon the east + coast of Australia—one of the most wonderful accumulations of coral + rock in the world. It is about 1,100 miles long, and varies in width from + one or two to many miles. It is separated from the coast of Australia by a + channel of about 25 fathoms deep; while outside, looking toward America, + the water is two or three thousand feet deep at a mile from the edge of + the reef. This is an accumulation of limestone rock, built up by corals, + to which we have no parallel anywhere else. Imagine to yourself a heap of + this material more than one thousand miles long, and several miles wide. + That is a barrier reef; but a barrier reef is merely as it were a fragment + of an encircling reef running parallel to the coast of a great continent. + </p> + <p> + I told you that the polypes which built these reefs were not able to live + at a greater depth than 20 to 25 fathoms of water; and that is the reason + why the fringing reef goes no farther from the land than it does. And for + the same reason, if the Pacific could be laid bare we should have a most + singular spectacle. There would be a number of mountains with truncated + tops scattered over it, and those mountains would have an appearance just + the very reverse of that presented by the mountains we see on shore. You + know that the mountains on shore are covered with vegetation at their + bases, while their tops are barren or covered with snow; but these + mountains would be perfectly bare at their bases, and all round their tops + they would be covered with a beautiful vegetation of coral polypes. And + not only would this be the case, but we should find that for a + considerable distance down, all the material of these atoll and encircling + reefs was built up of precisely the same coral rock as the fringing reef. + That is to say, you have an enormous mass of coral rock at a depth below + the surface of the water where we know perfectly well that the coral + animals could not have lived to form it. When those two facts were first + put together, naturalists were quite as much puzzled as I daresay you are, + at present, to understand how these two seeming contradictions could be + reconciled; and all sorts of odd hypotheses were resorted to. It was + supposed that the coral did not extend so far down, but that there was a + great chain of submarine mountains stretching through the Pacific, and + that the coral had grown upon them. But only fancy what supposition that + was, for you would have to imagine that there was a chain of mountains a + thousand miles or more long, and that the top of every mountain came + within 20 fathoms of the surface of the sea, and neither rose above nor + sunk beneath that level. That is highly improbable: such a chain of + mountains was never known. Then how can you possibly account for the + curious circular form of the atolls by any supposition of this kind? I + believe there was some one who imagined that all these mountains were + volcanoes, and that the reefs had grown round the tops of the craters, so + we all stuck fast. I may say "we," though it was rather before my time. + And when we all stick fast, it is just the use of a man of genius that he + comes and shows us the meaning of the thing. He generally gives an + explanation which is so ridiculously simple that everybody is ashamed that + he did not find it out before; and the way such a discoverer is often + rewarded is by finding out that some one had made the discovery before + him! I do not mean to say that it was so in this particular instance, + because the great man who played the part of Columbus and the egg on this + occasion had, I believe, always had the full credit which he so well + deserves. The discoverer of the key to these problems was a man whose name + you know very well in connection with other matters, and I should not + wonder if some of you have heard it said that he was a superficial kind of + person who did not know much about the subject on which he writes. He was + Mr. Darwin, and this brilliant discovery of his was made public thirty + years ago, long before he became the celebrated man he now is; and it was + one of the most singular instances of that astonishing sagacity which he + possesses of drawing consequences by way of deduction from simple + principles of natural science—a power which has served him in good + stead on other occasions. Well, Mr. Darwin, looking at these curious + difficulties and having that sort of knowledge of natural phenomena in + general, without which he could not have made a step towards the solution + of the problem, said to himself—"It is perfectly clear that the + coral which forms the base of the atolls and fringing reefs could not + possibly have been formed there if the level of the sea has always been + exactly where it is now, for we know for certain that these polypes cannot + build at a greater depth than 20 to 25 fathoms, and here we find them at + 50 to 100 fathoms." + </p> + <p> + That was the first point to make clear. The second point to deal with was—if + the polypes cannot have built there while the level of the sea has + remained stationary, then one of two things must have happened—either + the sea has gone up, or the land has gone down. + </p> + <p> + There is no escape from one of these two alternatives. Now the objections + to the notion of the sea having gone up are very considerable indeed; for + you will readily perceive that the sea could not possibly have risen a + thousand feet in the Pacific without rising pretty much the same distance + everywhere else; and if it had risen that height everywhere else since the + reefs began to be formed, the geography of the world in general must have + been very different indeed, at that time, from what it is now. And we have + very good means of knowing that any such rise as this certainly has not + taken place in the level of the sea since the time that the corals have + been building their houses. And so the only other alternative was to + suppose that the land had gone down, and at so slow a rate that the corals + were able to grow upward as fast as it went downward. You will see at once + that this is the solution of the mystery, and nothing can be simpler or + more obvious when you come to think about it. Suppose we start with a + coral sea and put in the middle of it an island such as the Mauritius. Now + let the coral polypes come and perch on the shore and build a fringing + reef, which will stop when they come to 20 or 25 fathoms, and you will + have a fringing reef like that round the island in the illustration. So + long as the land remains stationary, so long as it does not descend so + long will that reef be unable to get any further out, because the moment + the polype embryos try to get below they die. But now suppose that the + land sinks very gradually indeed. Let it subside by slow degrees, until + the mountain peak, which we have in the middle of it, alone projects + beyond the sea level. The fringing reef would be carried down also; but we + suppose that the sinking is so slow that the coral polypes are able to + grow up as fast as the land is carried down; consequently they will add + layer upon layer until they form a deep cup, because the inner part of the + reef grows much more slowly than the outer part. Thus you have the reef + forming a bed thicker upon the flanks of the island; but the edge of the + reef will be very much further out from the land, and the lagoon will be + many times deeper; in short, your fringing reef will be converted into an + encircling reef. And if, instead of this being an island, it were a great + continent like Australia, then you will have the phenomenon of a barrier + reef which I have described. The barrier reef of Australia was originally + a fringing reef; the land has gone slowly down; the consequence is the + lagoon has deepened until its depth is now 25 fathoms and the corals have + grown up at the outer edge until you have that prodigious accumulation + which forms the barrier reef at present. Now let this process go on + further still; let us take the land a further step down, so as to submerge + even the peak. The coral, still growing up, will cover the surface of the + land, and you will have an atoll reef; that is to say, a more or less + circular or oval ring of coral rock with a lagoon in the middle. Thus you + see that every peculiarity and phenomenon of these different forms of + coral reef was explained at once by the simplest of all possible + suppositions, namely, by supposing that the land has gone down at a rate + not greater than that at which the coral polypes have grown up. You + explain a Fringing Reef as a reef which is formed round land comparatively + stationary; an Encircling Reef as one which is formed round land going + down; and an Atoll as a reef formed upon land gone down; and the thing is + so simple that a child may understand it when it is once explained. + </p> + <p> + But this would by no means satisfy the conditions of a scientific + hypothesis. No man who is cautious would dream of trusting to an + explanation of this kind simply because it explained one particular set of + facts. Before you can possibly be safe in dealing with Nature—who is + very properly made of the feminine gender, on account of the astonishing + tricks which she plays upon her admirers!—I say before you can be + safe in dealing with Nature, you must get two or three kinds of cross + proofs, so as to make sure not only that your hypothesis fits that + particular set of facts, but that it is not contradicted by some other set + of facts which is just as clear and certain. And it so happens, that in + this case Mr. Darwin supplied the cross proofs as well as the immediate + evidence. You have all heard of volcanoes, those wonderful vents in the + surface of the earth out of which pour masses of lava, cinders and ashes, + and the like. Now, it is a matter of observation and experience that all + volcanoes are placed in areas in which the surface of the earth is + undergoing elevation, or at any rate is stationary; they are not placed in + parts of the world in which the level of the land is being lowered. They + are all indications of a great subterranean activity, of a something being + pushed up, and therefore naturally the land either gives way and lets it + come through, or else is raised up by its violence. And so Mr. Darwin, + being desirous not to merely put out a flashy hypothesis, but to get at + the truth of the matter, said to himself, "If my notion of this matter is + right, then atolls and encircling reefs, inasmuch as they are dependent + upon subsidence, ought not to be found in company with volcanoes; and, + 'vice versa', volcanoes ought not to be found in company with atolls, but + they ought to be found in company with fringing reefs." And if you turn to + Mr. Darwin's great work upon the coral reefs, you will see a very + beautiful chart of the world, which he prepared with great pains and + labour, showing the distribution on the one hand of the reefs, and on the + other of the volcanoes; you will find that in no case does the atoll + accompany the volcano, or the volcano burst up among the atolls. It is + most instructive to look at the great area of the Pacific on the map, and + see the great masses of atolls forming in one region of it a most enormous + belt, running from north-west to south-east; while the volcanoes, which + are very numerous in that region, go round the margin, so that we can + picture the Pacific to ourselves a section of a kind of very shallow basin—shallow + in proportion to its width, with the atolls rising from the bottom of it, + and at the margins the volcanoes. It is exactly as if you had taken a flat + mass and lifted up the edges of it; the subterranean force which lifted up + the edges shows itself in volcanoes, and as the edges have been raised, + the middle part of the mass has gone down. In other words, the facts of + physical geography precisely and exactly correspond with the hypothesis + which accounts for the infinite varieties of coral reefs. + </p> + <p> + One other point, before I conclude, about this matter. These reefs, as you + have just perceived, are in a most singular and unexpected manner + indications of physical changes of elevations and depressions going on + upon the surface of the globe. I dare say it may have surprised you to + hear me talk in this familiar sort of way of land going up and down; but + it is one of the universal lessons of geology that the land is going down + and going up, and has been going up and down, in all sorts of places and + to all sorts of distances, through all recorded time. Geologists would be + quite right in maintaining the seeming paradox that the stable thing in + the world is the fluid sea and the shifting thing is the solid land. That + may sound a very hard saying at first, but the more you look into geology, + the more you will see ground for believing that it is not a mere paradox. + </p> + <p> + In an unexpected manner, again, these reefs afford us not only an + indication of change of place, but they afford an indication of lapse of + time. The reef is a timekeeper of a very curious character; and you can + easily understand why. The coral polype, like everything else, takes a + certain time to grow to its full size; it does not do it in a minute; just + as a child takes a certain time to grow into a man so does the embryo + polype take time to grow into a perfect polype and form its skeleton. + Consequently every particle of coral limestone is an expression of time. + It must have taken a certain time to separate the lime from the sea water. + It is not possible to arrive at an accurate computation of the time it + must have taken to form these coral islands, because we lack the necessary + data; but we can form a rough calculation, which leads to very curious and + striking results. The computations of the rate at which corals grow are so + exceedingly variable, that we must allow the widest possible margin for + error; and it is better in this case to make the allowance upon the side + of excess. I think that anybody who knows anything about the matter will + tell you that I am making a computation far in excess of what is probable, + if I say that an inch of coral limestone may be added to one of these + reefs in the course of a year. I think most naturalists would be inclined + to laugh at me for making such an assumption, and would put the growth at + certainly not more than half that amount. But supposing it is so, what a + very curious notion of the antiquity of some of these great living + pyramids comes out by a very simple calculation. There is no doubt + whatever that the sea faces of some of them are fully a thousand feet + high, and if you take the reckoning of an inch a year, that will give you + 12,000 years for the age of that particular pyramid or cone of coral + limestone; 12,000 long years have these creatures been labouring in + conditions which must have been substantially the same as they are now, + otherwise the polypes could not have continued their work. But I believe I + very much understate both the height of some of these masses, and + overstate the amount which these animals can form in the course of a year; + so that you might very safely double the period as the time during which + the Pacific Ocean, the general state of the climate, and the sea, and the + temperature has been substantially what it is now; and yet that state of + things which now obtains in the Pacific Ocean is the yesterday of the + history of the life of the globe. Those pyramids of coral rock are built + upon a foundation which is itself formed by the deposits which the + geologist has to deal with. If we go back in time and search through the + series of the rocks, we find at every age of the world's history which has + yet been examined, accumulations of limestone, many of which have + certainly been built up in just the same way as those coral reefs which + are now forming the bottom of the Pacific Ocean. And even if we turn to + the oldest periods of geologic history, although the nature of the + materials is changed, although we cannot apply to them the same reasonings + that we can to the existing corals, yet still there are vast masses of + limestone formed of nothing else than the accumulations of the skeletons + of similar animals, and testifying that even in those remote periods of + the world's history, as now, the order of things implies that the earth + had already endured for a period of which our ordinary standards of + chronology give us not the slightest conception. In other words, the + history of these coral reefs, traced out honestly and carefully, and with + the same sort of reasoning that you would use in the ordinary affairs of + life, testifies, like every fact that I know of, to the prodigious + antiquity of the earth since it existed in a condition in the main similar + to that in which it now is. + </p> + <p> + <br /> <br /> + </p> + <hr /> + <p> + <br /> <br /> <a name="linknote-1" id="linknote-1"> + <!-- Note --></a> + </p> + <p class="foot"> + 1 (<a href="#linknoteref-1">return</a>)<br /> [ A Lecture delivered in + Manchester, November 4th, 1870.] + </p> + <p> + <br /><br /><br /><br /> + </p> +<pre xml:space="preserve"> + + + + + +End of Project Gutenberg's Coral and Coral Reefs, by Thomas H. 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Huxley + +Posting Date: January 6, 2009 [EBook #2937] +Release Date: November, 2001 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK CORAL AND CORAL REEFS *** + + + + +Produced by Amy E. Zelmer + + + + + +CORAL AND CORAL REEFS + +by Thomas H. Huxley + +[1] + + +THE subject upon which I wish to address you to-night is the structure +and origin of Coral and Coral Reefs. Under the head of "coral" there are +included two very different things; one of them is that substance which +I imagine a great number of us have champed when we were very much +younger than we are now,--the common red coral, which is used so much, +as you know, for the edification and the delectation of children of +tender years, and is also employed for the purposes of ornament for +those who are much older, and as some think might know better. The other +kind of coral is a very different substance; it may for distinction's +sake be called the white coral; it is a material which most assuredly +not the hardest-hearted of baby farmers would give to a baby to chew, +and it is a substance which is to be seen only in the cabinets of +curious persons, or in museums, or, may be, over the mantelpieces of +sea-faring men. But although the red coral, as I have mentioned to you, +has access to the very best society; and although the white coral is +comparatively a despised product, yet in this, as in many other cases, +the humbler thing is in reality the greater; the amount of work which is +done in the world by the white coral being absolutely infinite compared +with that effected by its delicate and pampered namesake. Each of these +substances, the white coral and the red, however, has a relationship to +the other. They are, in a zoological sense, cousins, each of them being +formed by the same kind of animals in what is substantially the same +way. Each of these bodies is, in fact, the hard skeleton of a very +curious and a very simple animal, more comparable to the bones of such +animals as ourselves than to the shells of oysters or creatures of that +kind; for it is the hardening of the internal tissue of the creature, of +its internal substance, by the deposit in the body of a material which +is exceedingly common, not only in fresh but in sea water, and which +is specially abundant in those waters which we know as "hard," +those waters, for example, which leave a "fur" upon the bottom of a +tea-kettle. This "fur" is carbonate of lime, the same sort of substance +as limestone and chalk. That material is contained in solution in sea +water, and it is out of the sea water in which these coral creatures +live that they get the lime which is needed for the forming of their +hard skeleton. + +But now what manner of creatures are these which form these hard +skeletons? I dare say that in these days of keeping aquaria, of +locomotion to the sea-side, most of those whom I am addressing may have +seen one of those creatures which used to be known as the "sea anemone," +receiving that name on account of its general resemblance, in a rough +sort of way, to the flower which is known as the "anemone"; but being +a thing which lives in the sea, it was qualified as the "sea anemone." +Well, then, you must suppose a body shaped like a short cylinder, the +top cut off, and in the top a hole rather oval than round. All round +this aperture, which is the mouth, imagine that there are placed a +number of feelers forming a circle. The cavity of the mouth leads into +a sort of stomach, which is very unlike those of the higher animals, +in the circumstance that it opens at the lower end into a cavity of the +body, and all the digested matter, converted into nourishment, is thus +distributed through the rest of the body. That is the general structure +of one of these sea anemones. If you touch it it contracts immediately +into a heap. It looks at first quite like a flower in the sea, but if +you touch it you find that it exhibits all the peculiarities of a living +animal; and if anything which can serve as its prey comes near its +tentacles, it closes them round it and sucks the material into its +stomach and there digests it and turns it to the account of its own +body. + +These creatures are very voracious, and not at all particular what they +seize; and sometimes it may be that they lay hold of a shellfish which +is far too big to be packed into that interior cavity, and, of course, +in any ordinary animal a proceeding of this kind would give rise to a +very severe fit of indigestion. But this is by no means the case in the +sea anemone, because when digestive difficulties of this kind arise he +gets out of them by splitting himself in two; and then each half builds +itself up into a fresh creature, and you have two polypes where there +was previously one, and the bone which stuck in the way lying between +them! Not only can these creatures multiply in this fashion, but they +can multiply by buds. A bud will grow out of the side of the body (I am +not speaking of the common sea anemone, but of allied creatures) just +like the bud of a plant, and that will fashion itself into a creature +just like the parent. There are some of them in which these buds remain +connected together, and you will soon see what would be the result of +that. If I make a bud grow out here, and another on the opposite side, +and each fashions itself into a new polype, the practical effect will be +that before long you will see a single polype converted into a sort of +tree or bush of polypes. And these will all remain associated together, +like a kind of co-operative store, which is a thing I believe you +understand very well here,--each mouth will help to feed the body and +each part of the body help to support the multifarious mouths. I think +that is as good an example of a zoological co-operative store as you can +well have. Such are these wonderful creatures. But they are capable not +only of multiplying in this way, but in other ways, by having a more +ordinary and regular kind of offspring. Little eggs are hatched and the +young are passed out by the way of the mouth, and they go swimming +about as little oval bodies covered with a very curious kind of hairlike +processes. Each of these processes is capable of striking water like an +oar; and the consequence is that the young creature is propelled through +the water. So that you have the young polype floating about in this +fashion, covered by its 'vibratile cilia', as these long filaments, +which are capable of vibration are termed. And thus, although the polype +itself may be a fixed creature unable to move about, it is able to +spread its offspring over great areas. For these creatures not only +propel themselves, but while swimming about in the sea for many hours, +or perhaps days, it will be obvious that they must be carried hither and +thither by the currents of the sea, which not unfrequently move at the +rate of one or two miles an hour. Thus, in the course of a few days, +the offspring of this stationary creature may be carried to a very great +distance from its parent; and having been so carried it loses these +organs by which it is propelled, and settles down upon the bottom of the +sea and grows up again into the form and condition of its parents. So +that if you suppose a single polype of this kind settled upon the bottom +of the sea, it may by these various methods--that is to say, by cutting +itself in two, which we call "fission," or by budding; or by sending out +these swimming embryos,--multiply itself to an enormous extent, and +give rise to thousands, or millions, of progeny in a comparatively short +time; and these thousands, or millions, of progeny may cover a very +large surface of the sea bottom; in fact, you will readily perceive +that, give them time, and there is no limit to the surface which they +may cover. + +Having understood thus far the general nature of these polypes, which +are the fabricators both of the red and white coral, let us consider a +little more particularly how the skeletons of the red coral and of +the white coral are formed. The red coral polype perches upon the sea +bottom, it then grows up into a sort of stem, and out of that stem there +grow branches, each of which has its own polypes; and thus you have a +kind of tree formed, every branch of the tree terminated by its polype. +It is a tree, but at the end of the branches there are open mouths of +polypes instead of flowers. Thus there is a common soft body connecting +the whole, and as it grows up the soft body deposits in its interior a +quantity of carbonate of lime, which acquires a beautiful red or flesh +colour, and forms a kind of stem running through the whole, and it is +that stem which is the red coral. The red coral grows principally at +the bottom of the Mediterranean Sea, at very great depths, and the coral +fishers, who are very adventurous seamen, take their drag nets, of a +peculiar kind, roughly made, but efficient for their purpose, and drag +them along the bottom of the sea to catch the branches of the red coral, +which become entangled and are thus brought up to the surface. They are +then allowed to putrefy, in order to get rid of the animal matter, and +the red coral is the skeleton that is left. + +In the case of the white coral, the skeleton is more complete. In the +red coral, the skeleton belongs to the whole; in the white coral there +is a special skeleton for every one of these polypes in addition to that +for the whole body. There is a skeleton formed in the body of each of +them, like a cup divided by a number of radiating partitions towards the +outside; and that cup is formed of carbonate of lime, only not stained +red, as in the case of the red coral. And all these cups are joined +together into a common branch, the result of which is the formation of +a beautiful coral tree. This is a great mass of madrepore, and in the +living state every one of the ends of these branches was terminated by +a beautiful little polype, like a sea anemone, and all the skeleton +was covered by a soft body which united the polypes together. You must +understand that all this skeleton has been formed in the interior of the +body, to suit the branched body of the polype mass, and that it is as +much its skeleton as our own bones are our skeleton. In this next coral +the creature which has formed the skeleton has divided itself as it +grew, and consequently has formed a great expansion; but scattered +all over this surface there were polype bodies like those I previously +described. Again, when this great cup was alive, the whole surface was +covered with a beautiful body upon which were set innumerable small +polype flowers, if we may so call them, often brilliantly coloured; +and the whole cup was built up in the same fashion by the deposit of +carbonate of lime in the interior of the combined polype body, formed +by budding and by fission in the way I described. You will perceive that +there is no necessary limit to this process. There is no reason why we +should not have coral three or four times as big; and there are certain +creatures of this kind that do fabricate very large masses, or half +spheres several feet in diameter. Thus the activity of these animals +in separating carbonate of lime from the sea and building it up into +definite shapes is very considerable indeed. + +Now I think I have said sufficient--as much as I can without taking you +into technical details, of the general nature of these creatures which +form coral. The animals which form coral are scattered over the seas of +all countries in the world. The red coral is comparatively limited, but +the polypes which form the white coral are widely scattered. There +are some of them which remain single, or which give rise to only small +accumulations; and the skeletons of these, as they die, accumulate upon +the bottom of the sea, but they do not come to much; they are washed +about and do not adhere together, but become mixed up with the mud of +the sea. But there are certain parts of the world in which the coral +polypes which live and grow are of a kind which remain, adhere together, +and form great masses. They differ from the ordinary polypes just in +the same way as those plants which form a peat-bog or meadow-turf differ +from ordinary plants. They have a habit of growing together in masses +in the same place; they are what we call "gregarious" things; and the +consequence of this is, that as they die and leave their skeletons, +those skeletons form a considerable solid aggregation at the bottom +of the sea, and other polypes perch upon them, and begin building upon +them, and so by degrees a great mass is formed. And just as we know +there are some ancient cities in which you have a British city, and over +that the foundations of a Roman city; and over that a Saxon city, and +over that again a modern city, so in these localities of which I am +speaking, you have the accumulations of the foundations of the houses, +if I may use the term, of nation after nation of these coral polypes; +and these accumulations may cover a very considerable space, and may +rise in the course of time from the bottom to the surface of the sea. + +Mariners have a name which they apply to all sorts of obstacles +consisting of hard and rocky matter which comes in their way in the +course of their navigation; they call such obstacles "reefs," and they +have long been in the habit of calling the particular kind of reef, +which is formed by the accumulation of the skeletons of dead corals, by +the name of "coral reefs," therefore, those parts of the world in which +these accumulations occur have been termed by them "coral reef areas," +or regions in which coral reefs are found. There is a very notable +example of a simple coral reef about the island of Mauritius, which I +dare say you all know, lies in the middle of the Indian Ocean. It is a +very considerable and beautiful island, and is surrounded on all sides +by a mass of coral, which has been formed in the way I have described; +so that if you could get upon the top of one of the peaks of the island, +and look down upon the Indian Ocean, you would see that the beach round +the Island was continued outward by a kind of shallow terrace, which +is covered by the sea, and where the sea is quite shallow; and at a +distance varying from three-quarters of a mile to a mile and a half from +the proper beach, you would see a line of foam or surf which looks most +beautiful in contrast with the bright green water in the inside, and the +deep blue of the sea beyond. That line of surf indicates the point at +which the waters of the ocean are breaking upon the coral reef which +surrounds the island. You see it sweep round the island upon all sides, +except where a river may chance to come down, and that always makes a +gap in the shore. + +There are two or three points which I wish to bring clearly before your +notice about such a reef as this. In the first place, you perceive +it forms a kind of fringe round the island, and is therefore called a +"fringing reef." In the next place, if you go out in a boat, and take +soundings at the edge of the reef, you find that the depth of the water +is not more than from 20 to 25 fathoms--that is about 120 to 150 feet. +Outside that point you come to the natural sea bottom; but all inside +that depth is coral, built up from the bottom by the accumulation of the +skeletons of innumerable generations of coral polypes. So that you see +the coral forms a very considerable rampart round the island. What the +exact circumference may be I do not remember, but it cannot be less than +100 miles, and the outward height of this wall of coral rock nowhere +amounts to less than about 100 or 150 feet. + +When the outward face of the reef is examined, you find that the upper +edge, which is exposed to the wash of the sea, and all the seaward face, +is covered with those living plant-like flowers which I have described +to you. They are the coral polypes which grow, flourish, and add to the +mass of calcareous matter which already forms the reef. But towards the +lower part of the reef, at a depth of about 120 feet, these creatures +are less active, and fewer of them at work; and at greater depths than +that you find no living coral polype at all; and it may be laid down +as a rule, derived from very extensive observation, that these +reef-building corals cannot live in a greater depth of water than about +120 to 150 feet. I beg you to recollect that fact, because it is one I +shall have to come back to by and by, and to show to what very curious +consequences that rule leads. Well then, coming back to the margin of +the reef, you find that part of it which lies just within the surf to be +coated by a very curious plant, a sort of seaweed, which contains in its +substance a very great deal of carbonate of lime, and looks almost like +rock; this is what is called the nulli pore. More towards the land, +we come to the shallow water upon the inside of the reef, which has +a particular name, derived from the Spanish or the Portuguese--it is +called a "lagoon," or lake. In this lagoon there is comparatively little +living coral; the bottom of it is formed of coral mud. If we pounded +this coral in water, it would be converted into calcareous mud, and the +waves during storms do for the coral skeletons exactly what we might do +for this coral in a mortar; the waves tear off great fragments and +crush them with prodigious force, until they are ground into the merest +powder, and that powder is washed into the interior of the lagoon, and +forms a muddy coating at the bottom. Beside that there are a great many +animals that prey upon the coral--fishes, worms, and creatures of that +kind, and all these, by their digestive processes, reduce the coral to +the same state, and contribute a very important element to this fine +mud. The living coral found in the lagoon, is not the reef building +coral; it does not give rise to the same massive skeletons. As you go +in a boat over these shallow pools, you see these beautiful things, +coloured red, blue, green, and all colours, building their houses; +but these are mere tenements, and not to be compared in magnitude +and importance to the masses which are built by the reef-builders +themselves. Now such a structure as this is what is termed a "fringing +reef." You meet with fringing reefs of this kind not only in the +Mauritius, but in a number of other parts of the world. If these were +the only reefs to be seen anywhere, the problem of the formation of +coral reefs would never have been a difficult one. Nothing can be +easier than to understand how there must have been a time when the coral +polypes came and settled on the shores of this island, everywhere within +the 20 to 25 fathom line, and how, having perched there, they gradually +grew until they built up the reef. + +But these are by no means the only sort of coral reefs in the world; on +the contrary, there are very large areas, not only of the Indian ocean, +but of the Pacific, in which many many thousands of square miles +are covered either with a peculiar kind of reef, which is called the +"encircling reef," or by a still more curious reef which goes by the +name of the "atoll." There is a very good picture, which Professor +Roscoe has been kind enough to prepare for me, of one of these atolls, +which will enable you to form a notion of it as a landscape. You have in +the foreground the waters of the Pacific. You must fancy yourself in the +middle of the great ocean, and you will perceive that there is an almost +circular island, with a low beach, which is formed entirely of coral +sand; growing upon that beach you have vegetation, which takes, of +course, the shape of the circular land; and then, in the interior of the +circle, there is a pool of water, which is not very deep--probably in +this case not more than eight or nine fathoms--and which forms a strange +and beautiful contrast to the deep blue water outside. This circular +island, or atoll, with a lagoon in the middle, is not a complete circle; +upon one side of it there is a break, exactly like the entrance into a +dock; and, as a matter of course, these circular islets, or atolls, form +most efficient break-waters, for if you can only get inside your ship is +in perfect safety, with admirable anchorage in the interior. If the ship +were lying within a mile of that beach, the water would be one or +two thousand feet deep; therefore, a section of that atoll, with the +soundings as deep as this all round, would give you the notion of a +great cone, cut off at the top, and with a shallow cup in the middle of +it. Now, what a very singular fact this is, that we should have rising +from the bottom of the deep ocean a great pyramid, beside which all +human pyramids sink into the most utter insignificance! These singular +coral limestone structures are very beautiful, especially when crowned +with cocoa-nut trees. There you see the long line of land, covered with +vegetation--cocoa-nut trees--and you have the sea upon the inner and +outer sides, with a vessel very comfortably riding at anchor. That is +one of the remarkable forms of reef in the Pacific. Another is a sort of +half-way house, between the atoll and the fringing reef; it is what is +called an "encircling reef." In this case you see an Island rising out +of the sea, and at two or three miles distance, or more, and separated +by a deep channel, which may be eight to twelve fathoms deep, there is a +reef, which encircles it like a great girdle; and outside that again the +water is one or two thousand feet deep. I spent three or four years +of my life in cruising about a modification of one of these encircling +reefs, called a "barrier reef," upon the east coast of Australia--one of +the most wonderful accumulations of coral rock in the world. It is about +1,100 miles long, and varies in width from one or two to many miles. +It is separated from the coast of Australia by a channel of about 25 +fathoms deep; while outside, looking toward America, the water is two or +three thousand feet deep at a mile from the edge of the reef. This is an +accumulation of limestone rock, built up by corals, to which we have no +parallel anywhere else. Imagine to yourself a heap of this material more +than one thousand miles long, and several miles wide. That is a +barrier reef; but a barrier reef is merely as it were a fragment of an +encircling reef running parallel to the coast of a great continent. + +I told you that the polypes which built these reefs were not able to +live at a greater depth than 20 to 25 fathoms of water; and that is the +reason why the fringing reef goes no farther from the land than it does. +And for the same reason, if the Pacific could be laid bare we should +have a most singular spectacle. There would be a number of mountains +with truncated tops scattered over it, and those mountains would have an +appearance just the very reverse of that presented by the mountains +we see on shore. You know that the mountains on shore are covered with +vegetation at their bases, while their tops are barren or covered with +snow; but these mountains would be perfectly bare at their bases, and +all round their tops they would be covered with a beautiful vegetation +of coral polypes. And not only would this be the case, but we should +find that for a considerable distance down, all the material of these +atoll and encircling reefs was built up of precisely the same coral rock +as the fringing reef. That is to say, you have an enormous mass of coral +rock at a depth below the surface of the water where we know perfectly +well that the coral animals could not have lived to form it. When +those two facts were first put together, naturalists were quite as much +puzzled as I daresay you are, at present, to understand how these +two seeming contradictions could be reconciled; and all sorts of odd +hypotheses were resorted to. It was supposed that the coral did not +extend so far down, but that there was a great chain of submarine +mountains stretching through the Pacific, and that the coral had grown +upon them. But only fancy what supposition that was, for you would have +to imagine that there was a chain of mountains a thousand miles or more +long, and that the top of every mountain came within 20 fathoms of the +surface of the sea, and neither rose above nor sunk beneath that level. +That is highly improbable: such a chain of mountains was never known. +Then how can you possibly account for the curious circular form of the +atolls by any supposition of this kind? I believe there was some one who +imagined that all these mountains were volcanoes, and that the reefs +had grown round the tops of the craters, so we all stuck fast. I may say +"we," though it was rather before my time. And when we all stick fast, +it is just the use of a man of genius that he comes and shows us the +meaning of the thing. He generally gives an explanation which is so +ridiculously simple that everybody is ashamed that he did not find +it out before; and the way such a discoverer is often rewarded is by +finding out that some one had made the discovery before him! I do not +mean to say that it was so in this particular instance, because the +great man who played the part of Columbus and the egg on this occasion +had, I believe, always had the full credit which he so well deserves. +The discoverer of the key to these problems was a man whose name you +know very well in connection with other matters, and I should not wonder +if some of you have heard it said that he was a superficial kind of +person who did not know much about the subject on which he writes. He +was Mr. Darwin, and this brilliant discovery of his was made public +thirty years ago, long before he became the celebrated man he now +is; and it was one of the most singular instances of that astonishing +sagacity which he possesses of drawing consequences by way of deduction +from simple principles of natural science--a power which has served him +in good stead on other occasions. Well, Mr. Darwin, looking at these +curious difficulties and having that sort of knowledge of natural +phenomena in general, without which he could not have made a step +towards the solution of the problem, said to himself--"It is perfectly +clear that the coral which forms the base of the atolls and fringing +reefs could not possibly have been formed there if the level of the sea +has always been exactly where it is now, for we know for certain that +these polypes cannot build at a greater depth than 20 to 25 fathoms, and +here we find them at 50 to 100 fathoms." + +That was the first point to make clear. The second point to deal with +was--if the polypes cannot have built there while the level of the +sea has remained stationary, then one of two things must have +happened--either the sea has gone up, or the land has gone down. + +There is no escape from one of these two alternatives. Now the +objections to the notion of the sea having gone up are very considerable +indeed; for you will readily perceive that the sea could not possibly +have risen a thousand feet in the Pacific without rising pretty much +the same distance everywhere else; and if it had risen that height +everywhere else since the reefs began to be formed, the geography of +the world in general must have been very different indeed, at that time, +from what it is now. And we have very good means of knowing that any +such rise as this certainly has not taken place in the level of the sea +since the time that the corals have been building their houses. And so +the only other alternative was to suppose that the land had gone down, +and at so slow a rate that the corals were able to grow upward as fast +as it went downward. You will see at once that this is the solution of +the mystery, and nothing can be simpler or more obvious when you come to +think about it. Suppose we start with a coral sea and put in the middle +of it an island such as the Mauritius. Now let the coral polypes come +and perch on the shore and build a fringing reef, which will stop when +they come to 20 or 25 fathoms, and you will have a fringing reef like +that round the island in the illustration. So long as the land remains +stationary, so long as it does not descend so long will that reef be +unable to get any further out, because the moment the polype embryos +try to get below they die. But now suppose that the land sinks very +gradually indeed. Let it subside by slow degrees, until the mountain +peak, which we have in the middle of it, alone projects beyond the sea +level. The fringing reef would be carried down also; but we suppose that +the sinking is so slow that the coral polypes are able to grow up as +fast as the land is carried down; consequently they will add layer upon +layer until they form a deep cup, because the inner part of the reef +grows much more slowly than the outer part. Thus you have the reef +forming a bed thicker upon the flanks of the island; but the edge of the +reef will be very much further out from the land, and the lagoon will be +many times deeper; in short, your fringing reef will be converted into +an encircling reef. And if, instead of this being an island, it were a +great continent like Australia, then you will have the phenomenon of a +barrier reef which I have described. The barrier reef of Australia +was originally a fringing reef; the land has gone slowly down; the +consequence is the lagoon has deepened until its depth is now 25 fathoms +and the corals have grown up at the outer edge until you have that +prodigious accumulation which forms the barrier reef at present. Now let +this process go on further still; let us take the land a further step +down, so as to submerge even the peak. The coral, still growing up, will +cover the surface of the land, and you will have an atoll reef; that is +to say, a more or less circular or oval ring of coral rock with a lagoon +in the middle. Thus you see that every peculiarity and phenomenon +of these different forms of coral reef was explained at once by the +simplest of all possible suppositions, namely, by supposing that the +land has gone down at a rate not greater than that at which the coral +polypes have grown up. You explain a Fringing Reef as a reef which is +formed round land comparatively stationary; an Encircling Reef as one +which is formed round land going down; and an Atoll as a reef formed +upon land gone down; and the thing is so simple that a child may +understand it when it is once explained. + +But this would by no means satisfy the conditions of a scientific +hypothesis. No man who is cautious would dream of trusting to an +explanation of this kind simply because it explained one particular set +of facts. Before you can possibly be safe in dealing with Nature--who is +very properly made of the feminine gender, on account of the astonishing +tricks which she plays upon her admirers!--I say before you can be safe +in dealing with Nature, you must get two or three kinds of cross proofs, +so as to make sure not only that your hypothesis fits that particular +set of facts, but that it is not contradicted by some other set of facts +which is just as clear and certain. And it so happens, that in this case +Mr. Darwin supplied the cross proofs as well as the immediate evidence. +You have all heard of volcanoes, those wonderful vents in the surface of +the earth out of which pour masses of lava, cinders and ashes, and +the like. Now, it is a matter of observation and experience that all +volcanoes are placed in areas in which the surface of the earth is +undergoing elevation, or at any rate is stationary; they are not placed +in parts of the world in which the level of the land is being lowered. +They are all indications of a great subterranean activity, of a +something being pushed up, and therefore naturally the land either gives +way and lets it come through, or else is raised up by its violence. And +so Mr. Darwin, being desirous not to merely put out a flashy hypothesis, +but to get at the truth of the matter, said to himself, "If my notion of +this matter is right, then atolls and encircling reefs, inasmuch as they +are dependent upon subsidence, ought not to be found in company with +volcanoes; and, 'vice versa', volcanoes ought not to be found in company +with atolls, but they ought to be found in company with fringing reefs." +And if you turn to Mr. Darwin's great work upon the coral reefs, you +will see a very beautiful chart of the world, which he prepared with +great pains and labour, showing the distribution on the one hand of the +reefs, and on the other of the volcanoes; you will find that in no case +does the atoll accompany the volcano, or the volcano burst up among the +atolls. It is most instructive to look at the great area of the Pacific +on the map, and see the great masses of atolls forming in one region of +it a most enormous belt, running from north-west to south-east; while +the volcanoes, which are very numerous in that region, go round the +margin, so that we can picture the Pacific to ourselves a section of a +kind of very shallow basin--shallow in proportion to its width, with the +atolls rising from the bottom of it, and at the margins the volcanoes. +It is exactly as if you had taken a flat mass and lifted up the edges +of it; the subterranean force which lifted up the edges shows itself +in volcanoes, and as the edges have been raised, the middle part of +the mass has gone down. In other words, the facts of physical geography +precisely and exactly correspond with the hypothesis which accounts for +the infinite varieties of coral reefs. + +One other point, before I conclude, about this matter. These reefs, as +you have just perceived, are in a most singular and unexpected manner +indications of physical changes of elevations and depressions going on +upon the surface of the globe. I dare say it may have surprised you to +hear me talk in this familiar sort of way of land going up and down; +but it is one of the universal lessons of geology that the land is +going down and going up, and has been going up and down, in all sorts +of places and to all sorts of distances, through all recorded time. +Geologists would be quite right in maintaining the seeming paradox that +the stable thing in the world is the fluid sea and the shifting thing is +the solid land. That may sound a very hard saying at first, but the more +you look into geology, the more you will see ground for believing that +it is not a mere paradox. + +In an unexpected manner, again, these reefs afford us not only an +indication of change of place, but they afford an indication of lapse of +time. The reef is a timekeeper of a very curious character; and you can +easily understand why. The coral polype, like everything else, takes a +certain time to grow to its full size; it does not do it in a minute; +just as a child takes a certain time to grow into a man so does the +embryo polype take time to grow into a perfect polype and form +its skeleton. Consequently every particle of coral limestone is an +expression of time. It must have taken a certain time to separate the +lime from the sea water. It is not possible to arrive at an accurate +computation of the time it must have taken to form these coral islands, +because we lack the necessary data; but we can form a rough calculation, +which leads to very curious and striking results. The computations of +the rate at which corals grow are so exceedingly variable, that we must +allow the widest possible margin for error; and it is better in this +case to make the allowance upon the side of excess. I think that anybody +who knows anything about the matter will tell you that I am making a +computation far in excess of what is probable, if I say that an inch of +coral limestone may be added to one of these reefs in the course of +a year. I think most naturalists would be inclined to laugh at me for +making such an assumption, and would put the growth at certainly not +more than half that amount. But supposing it is so, what a very curious +notion of the antiquity of some of these great living pyramids comes out +by a very simple calculation. There is no doubt whatever that the sea +faces of some of them are fully a thousand feet high, and if you take +the reckoning of an inch a year, that will give you 12,000 years for the +age of that particular pyramid or cone of coral limestone; 12,000 long +years have these creatures been labouring in conditions which must have +been substantially the same as they are now, otherwise the polypes could +not have continued their work. But I believe I very much understate both +the height of some of these masses, and overstate the amount which these +animals can form in the course of a year; so that you might very safely +double the period as the time during which the Pacific Ocean, the +general state of the climate, and the sea, and the temperature has been +substantially what it is now; and yet that state of things which now +obtains in the Pacific Ocean is the yesterday of the history of the life +of the globe. Those pyramids of coral rock are built upon a foundation +which is itself formed by the deposits which the geologist has to deal +with. If we go back in time and search through the series of the rocks, +we find at every age of the world's history which has yet been examined, +accumulations of limestone, many of which have certainly been built +up in just the same way as those coral reefs which are now forming the +bottom of the Pacific Ocean. And even if we turn to the oldest periods +of geologic history, although the nature of the materials is changed, +although we cannot apply to them the same reasonings that we can to the +existing corals, yet still there are vast masses of limestone formed of +nothing else than the accumulations of the skeletons of similar animals, +and testifying that even in those remote periods of the world's history, +as now, the order of things implies that the earth had already endured +for a period of which our ordinary standards of chronology give us not +the slightest conception. In other words, the history of these coral +reefs, traced out honestly and carefully, and with the same sort of +reasoning that you would use in the ordinary affairs of life, testifies, +like every fact that I know of, to the prodigious antiquity of the earth +since it existed in a condition in the main similar to that in which it +now is. + + + +[Footnote 1: A Lecture delivered in Manchester, November 4th, 1870.] + + + + + +End of Project Gutenberg's Coral and Coral Reefs, by Thomas H. Huxley + +*** END OF THIS PROJECT GUTENBERG EBOOK CORAL AND CORAL REEFS *** + +***** This file should be named 2937.txt or 2937.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/2/9/3/2937/ + +Produced by Amy E. 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Huxley + + + + +*[Foonote] A Lecture delivered in Manchester, November 4th, 1970. + +THE subject upon which I wish to address you to-night is the structure +and origin of Coral and Coral Reefs. Under the head of "coral" there +are included two very different things; one of them is that substance +which I imagine a great number of us have champed when we were very +much younger than we are now,--the common red coral, which is used so +much, as you know, for the edification and the delectation of children +of tender years, and is also employed for the purposes of ornament for +those who are much older, and as some think might know better. The +other kind of coral is a very different substance; it may for +distinction's sake be called the white coral; it is a material which +most assuredly not the hardest-hearted of baby farmers would give to a +baby to chew, and it is a substance which is to be seen only in the +cabinets of curious persons, or in museums, or, may be, over the +mantelpieces of sea-faring men. But although the red coral, as I have +mentioned to you, has access to the very best society; and although the +white coral is comparatively a despised product, yet in this, as in +many other cases, the humbler thing is in reality the greater; the +amount of work which is done in the world by the white coral being +absolutely infinite compared with that effected by its delicate and +pampered namesake. Each of these substances, the white coral and the +red, however, has a relationship to the other. They are, in a +zoological sense, cousins, each of them being formed by the same kind +of animals in what is substantially the same way. Each of these bodies +is, in fact, the hard skeleton of a very curious and a very simple +animal, more comparable to the bones of such animals as ourselves than +to the shells of oysters or creatures of that kind; for it is the +hardening of the internal tissue of the creature, of its internal +substance, by the deposit in the body of a material which is exceedingly +common, not only in fresh but in sea water, and which is specially +abundant in those waters which we know as "hard," those waters, for +example, which leave a "fur" upon the bottom of a tea-kettle. This +"fur" is carbonate of lime, the same sort of substance as limestone and +chalk. That material is contained in solution in sea water, and it is +out of the sea water in which these coral creatures live that they get +the lime which is needed for the forming of their hard skeleton. + +But now what manner of creatures are these which form these hard +skeletons? I dare say that in these days of keeping aquaria, of +locomotion to the sea-side, most of those whom I am addressing may have +seen one of those creatures which used to be known as the "sea +anemone," receiving that name on account of its general resemblance, in +a rough sort of way, to the flower which is known as the "anemone"; but +being a thing which lives in the sea, it was qualified as the "sea +anemone." Well, then, you must suppose a body shaped like a short +cylinder, the top cut off, and in the top a hole rather oval than round. +All round this aperture, which is the mouth, imagine that there are +placed a number of feelers forming a circle. The cavity of the mouth +leads into a sort of stomach, which is very unlike those of the higher +animals, in the circumstance that it opens at the lower end into a +cavity of the body, and all the digested matter, converted into +nourishment, is thus distributed through the rest of the body. That is +the general structure of one of these sea anemones. If you touch it it +contracts immediately into a heap. It looks at first quite like a +flower in the sea, but if you touch it you find that it exhibits all +the peculiarities of a living animal; and if anything which can serve as +its prey comes near its tentacles, it closes them round it and sucks +the material into its stomach and there digests it and turns it to the +account of its own body. + +These creatures are very voracious, and not at all particular what they +seize; and sometimes it may be that they lay hold of a shellfish which +is far too big to be packed into that interior cavity, and, of course, +in any ordinary animal a proceeding of this kind would give rise to a +very severe fit of indigestion. But this is by no means the case in the +sea anemone, because when digestive difficulties of this kind arise he +gets out of them by splitting himself in two; and then each half builds +itself up into a fresh creature, and you have two polypes where there +was previously one, and the bone which stuck in the way lying between +them! Not only can these creatures multiply in this fashion, but they +can multiply by buds. A bud will grow out of the side of the body (I +am not speaking of the common sea anemone, but of allied creatures) +just like the bud of a plant, and that will fashion itself into a +creature just like the parent. There are some of them in which these +buds remain connected together, and you will soon see what would be the +result of that. If I make a bud grow out here, and another on the +opposite side, and each fashions itself into a new polype, the +practical effect will be that before long you will see a single polype +converted into a sort of tree or bush of polypes. And these will all +remain associated together, like a kind of co-operative store, which is +a thing I believe you understand very well here,--each mouth will help +to feed the body and each part of the body help to support the +multifarious mouths. I think that is as good an example of a +zoological co-operative store as you can well have. Such are these +wonderful creatures. But they are capable not only of multiplying in +this way, but in other ways, by having a more ordinary and regular kind +of offspring. Little eggs are hatched and the young are passed out by +the way of the mouth, and they go swimming about as little oval bodies +covered with a very curious kind of hairlike processes. Each of these +processes is capable of striking water like an oar; and the consequence +is that the young creature is propelled through the water. So that you +have the young polype floating about in this fashion, covered by its +'vibratile cilia', as these long filaments, which are capable of +vibration are termed. And thus, although the polype itself may be a +fixed creature unable to move about, it is able to spread its offspring +over great areas. For these creatures not only propel themselves, but +while swimming about in the sea for many hours, or perhaps days, it +will be obvious that they must be carried hither and thither by the +currents of the sea, which not unfrequently move at the rate of one or +two miles an hour. Thus, in the course of a few days, the offspring of +this stationary creature may be carried to a very great distance from +its parent; and having been so carried it loses these organs by which +it is propelled, and settles down upon the bottom of the sea and grows +up again into the form and condition of its parents. So that if you +suppose a single polype of this kind settled upon the bottom of the +sea, it may by these various methods--that is to say, by cutting itself +in two, which we call "fission," or by budding; or by sending out these +swimming embryos,--multiply itself to an enormous extent, and give rise +to thousands, or millions, of progeny in a comparatively short time; +and these thousands, or millions, of progeny may cover a very large +surface of the sea bottom; in fact, you will readily perceive that, give +them time, and there is no limit to the surface which they may cover. + +Having understood thus far the general nature of these polypes, which +are the fabricators both of the red and white coral, let us consider a +little more particularly how the skeletons of the red coral and of the +white coral are formed. The red coral polype perches upon the sea +bottom, it then grows up into a sort of stem, and out of that stem there +grow branches, each of which has its own polypes; and thus you have a +kind of tree formed, every branch of the tree terminated by its +polype. It is a tree, but at the end of the branches there are open +mouths of polypes instead of flowers. Thus there is a common soft body +connecting the whole, and as it grows up the soft body deposits in its +interior a quantity of carbonate of lime, which acquires a beautiful +red or flesh colour, and forms a kind of stem running through the whole, +and it is that stem which is the red coral. The red coral grows +principally at the bottom of the Mediterranean Sea, at very great +depths, and the coral fishers, who are very adventurous seamen, take +their drag nets, of a peculiar kind, roughly made, but efficient for +their purpose, and drag them along the bottom of the sea to catch the +branches of the red coral, which become entangled and are thus brought +up to the surface. They are then allowed to putrefy, in order to get +rid of the animal matter, and the red coral is the skeleton that is +left. + +In the case of the white coral, the skeleton is more complete. In the +red coral, the skeleton belongs to the whole; in the white coral there +is a special skeleton for every one of these polypes in addition to +that for the whole body. There is a skeleton formed in the body of +each of them, like a cup divided by a number of radiating partitions +towards the outside; and that cup is formed of carbonate of lime, only +not stained red, as in the case of the red coral. And all these cups +are joined together into a common branch, the result of which is the +formation of a beautiful coral tree. This is a great mass of +madrepore, and in the living state every one of the ends of these +branches was terminated by a beautiful little polype, like a sea +anemone, and all the skeleton was covered by a soft body which united +the polypes together. You must understand that all this skeleton has +been formed in the interior of the body, to suit the branched body of +the polype mass, and that it is as much its skeleton as our own bones +are our skeleton. In this next coral the creature which has formed the +skeleton has divided itself as it grew, and consequently has formed a +great expansion; but scattered all over this surface there were polype +bodies like those I previously described. Again, when this great cup +was alive, the whole surface was covered with a beautiful body upon +which were set innumerable small polype flowers, if we may so call +them, often brilliantly coloured; and the whole cup was built up in the +same fashion by the deposit of carbonate of lime in the interior of the +combined polype body, formed by budding and by fission in the way I +described. You will perceive that there is no necessary limit to this +process. There is no reason why we should not have coral three or four +times as big; and there are certain creatures of this kind that do +fabricate very large masses, or half spheres several feet in diameter. +Thus the activity of these animals in separating carbonate of lime from +the sea and building it up into definite shapes is very considerable +indeed. + +Now I think I have said sufficient--as much as I can without taking you +into technical details, of the general nature of these creatures which +form coral. The animals which form coral are scattered over the seas +of all countries in the world. The red coral is comparatively limited, +but the polypes which form the white coral are widely scattered. There +are some of them which remain single, or which give rise to only small +accumulations; and the skeletons of these, as they die, accumulate upon +the bottom of the sea, but they do not come to much; they are washed +about and do not adhere together, but become mixed up with the mud of +the sea. But there are certain parts of the world in which the coral +polypes which live and grow are of a kind which remain, adhere +together, and form great masses. They differ from the ordinary polypes +just in the same way as those plants which form a peat-bog or +meadow-turf differ from ordinary plants. They have a habit of growing +together in masses in the same place; they are what we call +"gregarious" things; and the consequence of this is, that as they die +and leave their skeletons, those skeletons form a considerable solid +aggregation at the bottom of the sea, and other polypes perch upon +them, and begin building upon them, and so by degrees a great mass is +formed. And just as we know there are some ancient cities in which you +have a British city, and over that the foundations of a Roman city; and +over that a Saxon city, and over that again a modern city, so in these +localities of which I am speaking, you have the accumulations of the +foundations of the houses, if I may use the term, of nation after nation +of these coral polypes; and these accumulations may cover a very +considerable space, and may rise in the course of time from the bottom +to the surface of the sea. + +Mariners have a name which they apply to all sorts of obstacles +consisting of hard and rocky matter which comes in their way in the +course of their navigation; they call such obstacles "reefs," and they +have long been in the habit of calling the particular kind of reef, +which is formed by the accumulation of the skeletons of dead corals, by +the name of "coral reefs," therefore, those parts of the world in which +these accumulations occur have been termed by them "coral reef areas," +or regions in which coral reefs are found. There is a very notable +example of a simple coral reef about the island of Mauritius, which I +dare say you all know, lies in the middle of the Indian Ocean. It is a +very considerable and beautiful island, and is surrounded on all sides +by a mass of coral, which has been formed in the way I have described; +so that if you could get upon the top of one of the peaks of the island, +and look down upon the Indian Ocean, you would see that the beach round +the Island was continued outward by a kind of shallow terrace, which is +covered by the sea, and where the sea is quite shallow; and at a +distance varying from three-quarters of a mile to a mile and a half from +the proper beach, you would see a line of foam or surf which looks most +beautiful in contrast with the bright green water in the inside, and +the deep blue of the sea beyond. That line of surf indicates the point +at which the waters of the ocean are breaking upon the coral reef which +surrounds the island. You see it sweep round the island upon all +sides, except where a river may chance to come down, and that always +makes a gap in the shore. + +There are two or three points which I wish to bring clearly before your +notice about such a reef as this. In the first place, you perceive it +forms a kind of fringe round the island, and is therefore called a +"fringing reef." In the next place, if you go out in a boat, and take +soundings at the edge of the reef, you find that the depth of the water +is not more than from 20 to 25 fathoms--that is about 120 to 150 feet. +Outside that point you come to the natural sea bottom; but all inside +that depth is coral, built up from the bottom by the accumulation of the +skeletons of innumerable generations of coral polypes. So that you see +the coral forms a very considerable rampart round the island. What the +exact circumference may be I do not remember, but it cannot be less +than 100 miles, and the outward height of this wall of coral rock +nowhere amounts to less than about 100 or 150 feet. + +When the outward face of the reef is examined, you find that the upper +edge, which is exposed to the wash of the sea, and all the seaward +face, is covered with those living plant-like flowers which I have +described to you. They are the coral polypes which grow, flourish, and +add to the mass of calcareous matter which already forms the reef. But +towards the lower part of the reef, at a depth of about 120 feet, these +creatures are less active, and fewer of them at work; and at greater +depths than that you find no living coral polype at all; and it may be +laid down as a rule, derived from very extensive observation, that +these reef-building corals cannot live in a greater depth of water than +about 120 to 150 feet. I beg you to recollect that fact, because it is +one I shall have to come back to by and by, and to show to what very +curious consequences that rule leads. Well then, coming back to the +margin of the reef, you find that part of it which lies just within the +surf to be coated by a very curious plant, a sort of seaweed, which +contains in its substance a very great deal of carbonate of lime, and +looks almost like rock; this is what is called the nulli pore. More +towards the land, we come to the shallow water upon the inside of the +reef, which has a particular name, derived from the Spanish or the +Portuguese--it is called a "lagoon," or lake. In this lagoon there is +comparatively little living coral; the bottom of it is formed of coral +mud. If we pounded this coral in water, it would be converted into +calcareous mud, and the waves during storms do for the coral skeletons +exactly what we might do for this coral in a mortar; the waves tear off +great fragments and crush them with prodigious force, until they are +ground into the merest powder, and that powder is washed into the +interior of the lagoon, and forms a muddy coating at the bottom. Beside +that there are a great many animals that prey upon the coral--fishes, +worms, and creatures of that kind, and all these, by their digestive +processes, reduce the coral to the same state, and contribute a very +important element to this fine mud. The living coral found in the +lagoon, is not the reef building coral; it does not give rise to the +same massive skeletons. As you go in a boat over these shallow pools, +you see these beautiful things, coloured red, blue, green, and all +colours, building their houses; but these are mere tenements, and not to +be compared in magnitude and importance to the masses which are built +by the reef-builders themselves. Now such a structure as this is what +is termed a "fringing reef." You meet with fringing reefs of this kind +not only in the Mauritius, but in a number of other parts of the world. +If these were the only reefs to be seen anywhere, the problem of the +formation of coral reefs would never have been a difficult one. Nothing +can be easier than to understand how there must have been a time when +the coral polypes came and settled on the shores of this island, +everywhere within the 20 to 25 fathom line, and how, having perched +there, they gradually grew until they built up the reef. + +But these are by no means the only sort of coral reefs in the world; on +the contrary, there are very large areas, not only of the Indian ocean, +but of the Pacific, in which many many thousands of square miles are +covered either with a peculiar kind of reef, which is called the +"encircling reef," or by a still more curious reef which goes by the +name of the "atoll." There is a very good picture, which Professor +Roscoe has been kind enough to prepare for me, of one of these atolls, +which will enable you to form a notion of it as a landscape. You have +in the foreground the waters of the Pacific. You must fancy yourself +in the middle of the great ocean, and you will perceive that there is +an almost circular island, with a low beach, which is formed entirely +of coral sand; growing upon that beach you have vegetation, which takes, +of course, the shape of the circular land; and then, in the interior of +the circle, there is a pool of water, which is not very deep--probably +in this case not more than eight or nine fathoms--and which forms a +strange and beautiful contrast to the deep blue water outside. This +circular island, or atoll, with a lagoon in the middle, is not a +complete circle; upon one side of it there is a break, exactly like the +entrance into a dock; and, as a matter of course, these circular +islets, or atolls, form most efficient break-waters, for if you can only +get inside your ship is in perfect safety, with admirable anchorage in +the interior. If the ship were lying within a mile of that beach, the +water would be one or two thousand feet deep; therefore, a section of +that atoll, with the soundings as deep as this all round, would give +you the notion of a great cone, cut off at the top, and with a shallow +cup in the middle of it. Now, what a very singular fact this is, that +we should have rising from the bottom of the deep ocean a great pyramid, +beside which all human pyramids sink into the most utter +insignificance! These singular coral limestone structures are very +beautiful, especially when crowned with cocoa-nut trees. There you see +the long line of land, covered with vegetation--cocoa-nut trees--and you +have the sea upon the inner and outer sides, with a vessel very +comfortably riding at anchor. That is one of the remarkable forms of +reef in the Pacific. Another is a sort of half-way house, between the +atoll and the fringing reef; it is what is called an "encircling reef." +In this case you see an Island rising out of the sea, and at two or +three miles distance, or more, and separated by a deep channel, which +may be eight to twelve fathoms deep, there is a reef, which encircles +it like a great girdle; and outside that again the water is one or two +thousand feet deep. I spent three or four years of my life in cruising +about a modification of one of these encircling reefs, called a +"barrier reef," upon the east coast of Australia--one of the most +wonderful accumulations of coral rock in the world. It is about 1,100 +miles long, and varies in width from one or two to many miles. It is +separated from the coast of Australia by a channel of about 25 fathoms +deep; while outside, looking toward America, the water is two or three +thousand feet deep at a mile from the edge of the reef. This is an +accumulation of limestone rock, built up by corals, to which we have no +parallel anywhere else. Imagine to yourself a heap of this material +more than one thousand miles long, and several miles wide. That is a +barrier reef; but a barrier reef is merely as it were a fragment of an +encircling reef running parallel to the coast of a great continent. + +I told you that the polypes which built these reefs were not able to +live at a greater depth than 20 to 25 fathoms of water; and that is the +reason why the fringing reef goes no farther from the land than it +does. And for the same reason, if the Pacific could be laid bare we +should have a most singular spectacle. There would be a number of +mountains with truncated tops scattered over it, and those mountains +would have an appearance just the very reverse of that presented by the +mountains we see on shore. You know that the mountains on shore are +covered with vegetation at their bases, while their tops are barren or +covered with snow; but these mountains would be perfectly bare at their +bases, and all round their tops they would be covered with a beautiful +vegetation of coral polypes. And not only would this be the case, but +we should find that for a considerable distance down, all the material +of these atoll and encircling reefs was built up of precisely the same +coral rock as the fringing reef. That is to say, you have an enormous +mass of coral rock at a depth below the surface of the water where we +know perfectly well that the coral animals could not have lived to form +it. When those two facts were first put together, naturalists were +quite as much puzzled as I daresay you are, at present, to understand +how these two seeming contradictions could be reconciled; and all sorts +of odd hypotheses were resorted to. It was supposed that the coral did +not extend so far down, but that there was a great chain of submarine +mountains stretching through the Pacific, and that the coral had grown +upon them. But only fancy what supposition that was, for you would +have to imagine that there was a chain of mountains a thousand miles or +more long, and that the top of every mountain came within 20 fathoms of +the surface of the sea, and neither rose above nor sunk beneath that +level. That is highly improbable: such a chain of mountains was never +known. Then how can you possibly account for the curious circular form +of the atolls by any supposition of this kind? I believe there was +some one who imagined that all these mountains were volcanoes, and that +the reefs had grown round the tops of the craters, so we all stuck +fast. I may say "we," though it was rather before my time. And when we +all stick fast, it is just the use of a man of genius that he comes and +shows us the meaning of the thing. He generally gives an explanation +which is so ridiculously simple that everybody is ashamed that he did +not find it out before; and the way such a discoverer is often rewarded +is by finding out that some one had made the discovery before him! I +do not mean to say that it was so in this particular instance, because +the great man who played the part of Columbus and the egg on this +occasion had, I believe, always had the full credit which he so well +deserves. The discoverer of the key to these problems was a man whose +name you know very well in connection with other matters, and I should +not wonder if some of you have heard it said that he was a superficial +kind of person who did not know much about the subject on which he +writes. He was Mr. Darwin, and this brilliant discovery of his was made +public thirty years ago, long before he became the celebrated man he +now is; and it was one of the most singular instances of that +astonishing sagacity which he possesses of drawing consequences by way +of deduction from simple principles of natural science--a power which +has served him in good stead on other occasions. Well, Mr. Darwin, +looking at these curious difficulties and having that sort of knowledge +of natural phenomena in general, without which he could not have made a +step towards the solution of the problem, said to himself--"It is +perfectly clear that the coral which forms the base of the atolls and +fringing reefs could not possibly have been formed there if the level +of the sea has always been exactly where it is now, for we know for +certain that these polypes cannot build at a greater depth than 20 to +25 fathoms, and here we find them at 50 to 100 fathoms." + +That was the first point to make clear. The second point to deal with +was--if the polypes cannot have built there while the level of the sea +has remained stationary, then one of two things must have +happened--either the sea has gone up, or the land has gone down. + +There is no escape from one of these two alternatives. Now the +objections to the notion of the sea having gone up are very +considerable indeed; for you will readily perceive that the sea could +not possibly have risen a thousand feet in the Pacific without rising +pretty much the same distance everywhere else; and if it had risen that +height everywhere else since the reefs began to be formed, the +geography of the world in general must have been very different indeed, +at that time, from what it is now. And we have very good means of +knowing that any such rise as this certainly has not taken place in the +level of the sea since the time that the corals have been building +their houses. And so the only other alternative was to suppose that +the land had gone down, and at so slow a rate that the corals were able +to grow upward as fast as it went downward. You will see at once that +this is the solution of the mystery, and nothing can be simpler or more +obvious when you come to think about it. Suppose we start with a coral +sea and put in the middle of it an island such as the Mauritius. Now +let the coral polypes come and perch on the shore and build a fringing +reef, which will stop when they come to 20 or 25 fathoms, and you will +have a fringing reef like that round the island in the illustration. So +long as the land remains stationary, so long as it does not descend so +long will that reef be unable to get any further out, because the +moment the polype embryos try to get below they die. But now suppose +that the land sinks very gradually indeed. Let it subside by slow +degrees, until the mountain peak, which we have in the middle of it, +alone projects beyond the sea level. The fringing reef would be +carried down also; but we suppose that the sinking is so slow that the +coral polypes are able to grow up as fast as the land is carried down; +consequently they will add layer upon layer until they form a deep cup, +because the inner part of the reef grows much more slowly than the +outer part. Thus you have the reef forming a bed thicker upon the +flanks of the island; but the edge of the reef will be very much further +out from the land, and the lagoon will be many times deeper; in short, +your fringing reef will be converted into an encircling reef. And if, +instead of this being an island, it were a great continent like +Australia, then you will have the phenomenon of a barrier reef which I +have described. The barrier reef of Australia was originally a +fringing reef; the land has gone slowly down; the consequence is the +lagoon has deepened until its depth is now 25 fathoms and the corals +have grown up at the outer edge until you have that prodigious +accumulation which forms the barrier reef at present. Now let this +process go on further still; let us take the land a further step down, +so as to submerge even the peak. The coral, still growing up, will +cover the surface of the land, and you will have an atoll reef; that is +to say, a more or less circular or oval ring of coral rock with a +lagoon in the middle. Thus you see that every peculiarity and +phenomenon of these different forms of coral reef was explained at once +by the simplest of all possible suppositions, namely, by supposing that +the land has gone down at a rate not greater than that at which the +coral polypes have grown up. You explain a Fringing Reef as a reef +which is formed round land comparatively stationary; an Encircling Reef +as one which is formed round land going down; and an Atoll as a reef +formed upon land gone down; and the thing is so simple that a child may +understand it when it is once explained. + +But this would by no means satisfy the conditions of a scientific +hypothesis. No man who is cautious would dream of trusting to an +explanation of this kind simply because it explained one particular set +of facts. Before you can possibly be safe in dealing with Nature--who +is very properly made of the feminine gender, on account of the +astonishing tricks which she plays upon her admirers!--I say before you +can be safe in dealing with Nature, you must get two or three kinds of +cross proofs, so as to make sure not only that your hypothesis fits +that particular set of facts, but that it is not contradicted by some +other set of facts which is just as clear and certain. And it so +happens, that in this case Mr. Darwin supplied the cross proofs as well +as the immediate evidence. You have all heard of volcanoes, those +wonderful vents in the surface of the earth out of which pour masses of +lava, cinders and ashes, and the like. Now, it is a matter of +observation and experience that all volcanoes are placed in areas in +which the surface of the earth is undergoing elevation, or at any rate +is stationary; they are not placed in parts of the world in which the +level of the land is being lowered. They are all indications of a +great subterranean activity, of a something being pushed up, and +therefore naturally the land either gives way and lets it come through, +or else is raised up by its violence. And so Mr. Darwin, being +desirous not to merely put out a flashy hypothesis, but to get at the +truth of the matter, said to himself, "If my notion of this matter is +right, then atolls and encircling reefs, inasmuch as they are dependent +upon subsidence, ought not to be found in company with volcanoes; and, +'vice versa', volcanoes ought not to be found in company with atolls, +but they ought to be found in company with fringing reefs." And if you +turn to Mr. Darwin's great work upon the coral reefs, you will see a +very beautiful chart of the world, which he prepared with great pains +and labour, showing the distribution on the one hand of the reefs, and +on the other of the volcanoes; you will find that in no case does the +atoll accompany the volcano, or the volcano burst up among the atolls. +It is most instructive to look at the great area of the Pacific on the +map, and see the great masses of atolls forming in one region of it a +most enormous belt, running from north-west to south-east; while the +volcanoes, which are very numerous in that region, go round the margin, +so that we can picture the Pacific to ourselves a section of a kind of +very shallow basin--shallow in proportion to its width, with the atolls +rising from the bottom of it, and at the margins the volcanoes. It is +exactly as if you had taken a flat mass and lifted up the edges of it; +the subterranean force which lifted up the edges shows itself in +volcanoes, and as the edges have been raised, the middle part of the +mass has gone down. In other words, the facts of physical geography +precisely and exactly correspond with the hypothesis which accounts for +the infinite varieties of coral reefs. + +One other point, before I conclude, about this matter. These reefs, as +you have just perceived, are in a most singular and unexpected manner +indications of physical changes of elevations and depressions going on +upon the surface of the globe. I dare say it may have surprised you to +hear me talk in this familiar sort of way of land going up and down; but +it is one of the universal lessons of geology that the land is going +down and going up, and has been going up and down, in all sorts of +places and to all sorts of distances, through all recorded time. +Geologists would be quite right in maintaining the seeming paradox that +the stable thing in the world is the fluid sea and the shifting thing +is the solid land. That may sound a very hard saying at first, but the +more you look into geology, the more you will see ground for believing +that it is not a mere paradox. + +In an unexpected manner, again, these reefs afford us not only an +indication of change of place, but they afford an indication of lapse +of time. The reef is a timekeeper of a very curious character; and you +can easily understand why. The coral polype, like everything else, +takes a certain time to grow to its full size; it does not do it in a +minute; just as a child takes a certain time to grow into a man so does +the embryo polype take time to grow into a perfect polype and form its +skeleton. Consequently every particle of coral limestone is an +expression of time. It must have taken a certain time to separate the +lime from the sea water. It is not possible to arrive at an accurate +computation of the time it must have taken to form these coral islands, +because we lack the necessary data; but we can form a rough calculation, +which leads to very curious and striking results. The computations of +the rate at which corals grow are so exceedingly variable, that we must +allow the widest possible margin for error; and it is better in this +case to make the allowance upon the side of excess. I think that +anybody who knows anything about the matter will tell you that I am +making a computation far in excess of what is probable, if I say that +an inch of coral limestone may be added to one of these reefs in the +course of a year. I think most naturalists would be inclined to laugh +at me for making such an assumption, and would put the growth at +certainly not more than half that amount. But supposing it is so, what +a very curious notion of the antiquity of some of these great living +pyramids comes out by a very simple calculation. There is no doubt +whatever that the sea faces of some of them are fully a thousand feet +high, and if you take the reckoning of an inch a year, that will give +you 12,000 years for the age of that particular pyramid or cone of +coral limestone; 12,000 long years have these creatures been labouring +in conditions which must have been substantially the same as they are +now, otherwise the polypes could not have continued their work. But I +believe I very much understate both the height of some of these masses, +and overstate the amount which these animals can form in the course of a +year; so that you might very safely double the period as the time +during which the Pacific Ocean, the general state of the climate, and +the sea, and the temperature has been substantially what it is now; and +yet that state of things which now obtains in the Pacific Ocean is the +yesterday of the history of the life of the globe. Those pyramids of +coral rock are built upon a foundation which is itself formed by the +deposits which the geologist has to deal with. If we go back in time +and search through the series of the rocks, we find at every age of the +world's history which has yet been examined, accumulations of +limestone, many of which have certainly been built up in just the same +way as those coral reefs which are now forming the bottom of the +Pacific Ocean. And even if we turn to the oldest periods of geologic +history, although the nature of the materials is changed, although we +cannot apply to them the same reasonings that we can to the existing +corals, yet still there are vast masses of limestone formed of nothing +else than the accumulations of the skeletons of similar animals, and +testifying that even in those remote periods of the world's history, as +now, the order of things implies that the earth had already endured for +a period of which our ordinary standards of chronology give us not the +slightest conception. In other words, the history of these coral reefs, +traced out honestly and carefully, and with the same sort of reasoning +that you would use in the ordinary affairs of life, testifies, like +every fact that I know of, to the prodigious antiquity of the earth +since it existed in a condition in the main similar to that in which it +now is. + + + + + +End of The Project Gutenberg Etext of Coral and Coral Reefs + diff --git a/old/thx1710.zip b/old/thx1710.zip Binary files differnew file mode 100644 index 0000000..8f71df8 --- /dev/null +++ b/old/thx1710.zip |