RECORD: Darwin, C. R. 1890. On the structure and distribution of coral reefs. Introduction by Joseph W. Williams. London, Felling-on-Tyne and New York: Walter Scott. [Introduction by Williams only]
REVISION HISTORY: Photographed by John van Wyhe, transcribed (single key) by AEL Data 6.2009, further corrections by van Wyhe 11.2012. RN3
ON THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS. BY
CHARLES DARWIN. EDITED, WITH
AN INTRODUCTION, BY JOSEPH
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In the following pages, first published in 1842, as a companion volume to his Journal of Researches, Charles Darwin divided coral-reefs into three great classes, each of which is, however, formed upon the same type, and each succeeds the other in general plan of formation 'in much the same way, as childhood passes into youth, and youth into manhood. The first class, or 'fringing-reef' (Fig. 1), generally surrounds islands, or skirts
great masses of continent, and has a channel of shallow water between it and the shore and a gently sloping sea-bed on its ocean side. Examples are numerous:—The Sandwich Islands, the Seychelle Islands, the Solomon Isles, the Friendly Isles, the Navigator Isles, the New Hebrides, and Mauritius, are margined with this kind of reef; they are also common in the Red Sea, on both its African and Arabian shores, and they form a prolongation from the southern extremity of the promontory of Florida. They surround the Nicobar Islands, and skirt nearly the whole of the islands of the West Indies. The reefs on the Florida coast are extending inland, and from the rate of their encroachment upon the shore Louis Agassiz1 has tried
1 Natural History Studies.
to determine their period of growth, with a result that he considers that it would take 1,000 or 1,200 years for coral to grow upwards from a depth of seven fathoms to the surface of the sea; this estimate cannot, however, be universally applicable, since their rate of growth differs in different seas, and varies according to the species. Thus, for example, on a ship, which was sunk for twenty months in the Persian Gulf, there was an incrustation of coral to a depth of two feet on her copper bottom; while in the case of the sunken ship shannon, which was found crusted with coral, it was reckoned that, even supposing the coral to have commenced growing as soon as the ship reached the bottom, the growth could not have been more than three inches in a year. Again, some corals have been planted on the Madagascar coast, and these were observed to have grown to a height of nearly three feet in no less than six months.
The second class, or 'barrier-reef' (Fig. 2), differs from the first class in that it is situated at a greater distance from the shore, and that, as a consequence of this, the depth of the water on both its littoral and seaward sides is much greater. They occur in the middle of the Red Sea; they are common in the Pacific, where they form the great barrier-reef on the north-east coast of Australia, and extend around the Society Islands, the Fijis, and New Caledonia; and they surround islands like the Pelew Islands, and the Comora Isles in the Mozambique Channel. Some of them are very large;—that surrounding New Caledonia is four hundred miles long, and about ten miles distant from the shore; and the one off the north-east coast of Australia is from ten to ninety miles broad, about 1,250 statute miles in length, and rises from the ocean bed on its seaward side from a depth which often exceeds 1,800 feet.
This last example of reef has been compared by Jukes1 to 'a great submarine wall or terrace, fronting the whole north-east coast of Australia, resting at each end on shallow water, but rising from very great depths about the centre; its upper surface forming a plateau covered by 10 to 30 fathoms of water, but studded all over with steep-sided block-like masses which rise up to low water-level. These masses are especially numerous, and most linear along the edge of the great bank on which they rest; the passage between them being often very narrow, like regular embrasures opened here and there through the parapet wall of a fortress. These 'individual reefs' running along the outer edger protect the comparatively shallow water inside, and with the numerous inner reefs that are scattered over its space make it one great natural harbour.'
The third and last class, or 'atoll' (Fig. 3), is an elliptical, oval or roundish ring of coral, with here and there a break in its
Fig. 3.—Stewart Atoll or Sikiana (lat.8° 22' S.; long. 162° 58' E.).
C, Reef Channel; F, Faule Island.
continuity, and with a central lake-like expanse of water, known as the lagoon. The outside is generally very deep, and the inside shallow; thus off the Cocos-Keeling Atoll the
1 Manual of Geology, p. 131; Voyage of H.M.S. Fly, vol. i. chap. xiii.
sounding-lead, at a distance of 2,200 yards from the reef's edge, sinks to a depth of 1,200 fathoms, while the lagoon is only from two to seven fathoms deep. 'Such a basin with its deep, clear channels through the reef—affording (as many of them do) room enough for all the navies of Christendom to ride at anchor—supplies the very perfection of harbour accommodation, even though the surrounding reefs are so low that during storms the breakers outside will dash over the massed wall of coral. In this lagoon marine animals of all sorts——including fishes, and pre-eminently sharks—swarm: but it is seldom that the wind disturbs the water with its smooth, glassy expanse, and curiously enough the openings in the reef are always on the leeward side, that is, in the one least exposed to the prevailing winds, so that while a ship has no difficulty in getting out to sea, it sometimes happens that it does not find an escape from the storm so easy.' The commonest localities for atolls are the Indian and Pacific Oceans. Stewart's atoll (Fig.3) in the Solomon Islands, and the Menschikoff Island may be taken as typical examples.
Before Darwin wrote, it was universally believed that these tolls were formed by the coral polypes growing upon submerged volcanic craters. This theory finds expression and support in the second volume of Lyell's Principles of Geology, a book which was published in 1832, and which Darwin read. It is known that he procured the first volume (just then issued) at the suggestion of Henslow previous to his setting out on the voyage of the Beagle, which started from Devonport on December 27, 1831, and probably, too, that kindly, sagacious, sympathetic friend and teacher forwarded him the second volume as soon as published. But whatever the case may be, it is evident that Darwin was acquainted with the prevailing idea, and that his acute and penetrating mind discerned at once its weaknesses, for he tells us in his Autobiography that the main features of his theory were conceived while on the voyage, and that even previous to seeing 'a true coral-reef.' 'No other work of mine,' he says,1 'was
1 Life and Letters of Charles Darwin, 1888, vol. i.p.70.
begun in so deductive a spirit as this, for the whole theory was thought out on the west coast of South America, before I had seen a true coal-reef. I had only to verify and extend my views by a careful examination of living reefs. But it should be observed that I had during the two previous years been incessantly attending to the effects on the shores of South America of the intermittent elevation of the land, together with denudation and the deposition of sediment. This necessarily led me to reflect much on the effects of subsidence, and it was easy to replace in imagination the continued deposition of sediment by the upward growth of corals. To do this was to form my theory of the formation of barrier-reefs and atolls.' Coupled with this, Darwin, as is manifested by his work, also saw that, in forming any theory of the genesis and development of coral-reefs, not only must the nature of the platform on which the corals build be taken into account, but that other factors, of as equally great importance, come into play and must be reckoned,—notably, the peculiar conditions of the life of the coral-polypes themselves, and the peculiar, and, then, inexplicable distribution of the reefs and atolls. He saw more clearly than his precursors had done the validity of the dictum of Johannes Muller, in this and indeed in all his works, that the most important truths in Natural Science are to be discovered, neither by the mere analysis of philosophical ideas, nor by simple experience, but by reflective experience, which distinguishes the essential from the accidental in the phenomena observed, and thus finds principles from which many experiences can be derived.
The conditions necessary for the proper continuance of the organic life of these polypes seem to be a temperature which is not lower than 68° F., the presence of clear water, and a depth not exceeding twenty fathoms. They also cannot survive exposure to the sun and air, and thus are unable to flourish unless the top of the reef be below the mark of the lowest tides. Thus reefs are absent from the West Coast of America because it is washed by a cold extra-tropical current; they are not found in the South Atlantic because that ocean is not within their special isotherm; and they are not present on the shores of Trinidad
or the north-eastern coast of North America, since these are in the neighbourhood of the mouths of large rivers which bring down a large amount of mud and other suspended matters from the land.
Reasoning on these facts, and fresh with the evidences of subsidence, obtained by reading and observation on the South American coast, Darwin conceived and nurtured the theory which is set forth in the succeeding pages. Briefly, this theory is as follows:—That—as the polypes cannot live below a depth of 100 feet, and are killed by exposure to sunshine and air, and could not therefore have grown upward from those vast depths to which the coral-masses extend—each atoll began as a fringing-reef, then became a barrier-reef, and at last appeared as a ring of coral with a central lagoon, owing to a slow but progressive subsidence of the site on which the polypes first began to build. If, on this view, a fringing-reef be formed round an island (Fig. 4, 1st period) between the sea level and the 20-athom line, and then the island gradually
FIG.4.—Illustrating Darwin's theory of formation of the three kinds of Coral Reefs.
First period, the Fringing-Reef; second period, the Barrier-Reef; third period, the Atoll.
sink deeper into the sea, it (i.e., the island) will have become smaller, and the channel between it and the reef wider; the fringing-reef will in time have become changed into a barrier reef (Fig. 4, 2nd period), provided that the polypes grow upward at a rate which keeps pace with the depression. Again, another gradual subsidence of the island taking place, and the coral growing upward as fast as the base sinks downward, there would at last result a more or less ring-shaped reef
with a central expanse of water (Fig. 4, 3rd period). The barrier-reef has become an atoll. On the outer margins of a reef thus formed, the waves dash and break off pieces of coral, and heap the broken masses upon its surface, so that its edge appears above the low-tide level. The majority of the polypes then die; 'but the waves continue to pile up on the reef, sand, pebbles, and broken masses of coral, some of the masses being two to three hundred cubic feet in size, and a field of rough rocks begins to appear above the waves. Next a beach is formed; and the bank of coral débris, now mostly above the salt-water, becomes planted by the waves with sea-borne seeds. Trailing shrubs spring up; and afterwards, as the soil deepens, palms and other trees rise into forests, and the coral-island or atoll comes forth finished.'
For many years geologists universally accepted the general validity of Darwin's theory. The first note of dissent seems to have been sounded in 1863 when Professor Semper published an article1 on the Pelew Islands, which are situated at the western extremity of the Caroline Archipelago, and which appeared to him to show evidence of elevation, rather than of subsidence. It is a significant fact that at the southern end of these islands, there are raised coral-reefs from 400-500 feet in height, and halos and island which is entirely destitute of reefs, while at the northern extremity, only 60 miles distant, there are true atolls. Darwin, however, in the Appendix to his second edition, published in 1874, replied that he did not think these conditions were insuperable by his theory, and that they might be explained on the supposition that the whole group had originally subsided, then was upraised,—probably at the time when the volcanic rocks to the north were erupted'—and after wards again depressed. 'The existence of atolls and barrier-reefs in close proximity is manifestly not opposed to my views. On the other hand, the presence of reefs fringing the southern islands is opposed to my views, as such reefs generally indicate that the land has either long remained stationary, or has been upraised. It must, however, be borne in mind (as remarked
1 Zeitschr. f. Wissensch. Zoologie., 1863, Bd, xiii. p. 558.
in our sixth chapter) that when the land is prolonged beneath the sea in an extremely steep slope, reefs formed there during subsidence will remain closely attached to the shore, and will remain undistinguishable from fringing-reefs. Now we know that the submarine flanks of most atolls are very steep; and if
FIG. 5.—Madrepore (Goniopora columna, Dana). Natural size.
an atoll after upheaval and before the sea had eaten deeply into the land, and had formed a broad flat surface, were again to subside, the reefs which grew to the surface during the subsiding movement would still closely skirt the coast.' In this connection, Darwin's letter to the same observer is also interesting and instructive; he had received from Professor Semper the
portion of the proof-sheets of his book on Animal Life which related to corals.1 (This book was afterwards translated in the International Scientific Series.) The letter2 is dated 'Down, October 2, 1879', and runs thus:—'My dear Professor Semper, — I thank you for your extremely kind letter of the 19th, and for the proof-sheets. I believe that I understand all, excepting one or two sentences, where my imperfect knowledge of German has interfered. This is my sole and poor excuse for the mistake which I made in the second edition of my Coral book. Your account of the Pelew Islands is a fine addition to our knowledge on coral-reefs. I have very little to say on the subject, even if I had formerly read your account and seen your maps, but had known nothing of the proofs of recent elevation, and of your belief that the islands have not since subsided, I have no doubt that I should have considered them as formed during subsidence. But I should have been much troubled in my mind by the sea not being so deep as it usually is round atolls, and by the reef on one side sloping so gradually beneath the sea; for this latter fact, as far as my memory serves me, is a very unusual and almost unparalleled case. I always foresaw that a bank at the proper depth beneath the surface would give rise to a reef which could not be distinguished from an atoll formed during subsidence. I must still adhere to my opinion, that the atolls and barrier-reefs in the middle of the Pacific and Indian Oceans indicate subsidence, but I fully agree with you that such cases as that of the Pelew Islands, if of at all frequent occurrence would make my general conclusions of very little value. Future observers must decide between us. It will be a strange fact if there has not been
1 In the original edition of this book, Professor Semper refers to the subject of coral-reefs in the following words:—"Es scheint mir als ob er in der zweiten Ausgabe seines allgemein bekannten Werks über Korallenriffe einen Irrthume über meine Beobachtungen zum Opfer gefallen ist, indem er die Angaben, die ich allerding bisher immer nur sehr kurz gehalten hatte, vollstandig falsch wiedergegeben hat."
2 Life and Letters of Charles Darwin, vol. iii. p. 182.
subsidence of the beds of the great oceans, and if this has not affected the forms of the coral-reefs.' The second note of dissent was sounded in 1870, when J. J. Rein1 published some observations on the Bermuda Islands, and considered that they could be explained on the grounds of an extension upwards of accumulations of calcareous sediment from the sea-bottom; this contribution to the subject Darwin did not mention in his second edition, and, probably, it escaped him: it has, however, been contradicted by the more recent observations of Professor Rice in 1884, and Professor Heilprin 1889.
The greatest contribution to the controversy has been rendered by Dr. J. Murray, who, after his return from the Challenger Expedition (to which he acted as naturalist), read, on April 5, 1880, a paper2 before the Royal Society of Edinburgh which has entirely revolutionised the scientific concept of coral-reef formation, and modified to no small degree in the minds of thinking geologists the theory which Darwin promulgated of the polypes building reefs on areas of subsidence. He has pointed out that barrier-reefs do not by themselves prove depression, since their bases may be and are formed of a talus of their own débris produced by wave-action, and that, where such a condition obtains, they appear at first sight to consist of a solid, calcareous, coral-like substance which had been secreted by the polypes in the exact locality where they are now found, and on a bed which had undergone depression. He has further shown that those islands which are fringed by reefs do not give any evidences of gradual subsidence, and he, moreover, states that, in his opinion, were the platforms, on which the reefs are built, remnants of a pre-existent continent which has been submerged beneath the waves, then it would be expected to find traces of strata, other than volcanic, on their flanks; and this it is known is not the case, since the only rocks found are laves and tufas. Again, it is a well-known fact that volcanic action takes place on the
1 Senckenburg, Naturf. Gesellsch. Würzburg, 1869-70, p. I57.
2 Proc. Roy. Soc. Edin., 188o, p. 505; and Article "Pacific" in Encyclopedia Britannica, vol. xviii. pp. I28, I29.
sea-bed, as well as on land, and that sometimes new islands are erupted, and sometimes submarine peaks. Thus islands like Ascension, St. Paul, Amsterdam, and Reunion were, it is evident from their petrological texture, formed in this way, and, indeed, in one of them—Reunion—there are still two volcanic vents which from time to time throw out molten rock and cinders.1 Etna and Vesuvius, it is also believed, originated as submarine volcanoes on a sea-bed which was afterwards elevated;2 and in the case of the Islands of Santorin and Thracia, their structure consists of trass, scoriæ, and lava-sheets overlying marbles and schists. Indeed these two islands form the rim of a vast volcanic crater which descends for 1,278 feet below the level of the sea. They are the 'outward and visible signs' of an immense submarine volcanic peak which, geologically speaking, has been elevated during later times, since, on them, Von Fritsch has found in several places, up to an altitude of nearly 600 feet above the sea-level, marine shells belonging to species which are now living in the surrounding ocean. Fouqué, who has studied these islands more closely than any other observer, has arrived at the conclusion that 'the volcano formed at one time a large island with wooded slopes, and a somewhat civilised human population, cultivating a fertile valley in the south-western district, and that in prehistoric times the tremendous explosion occurred whereby the centre of the island was blown out.'3 Many more examples may be found in our geological text-books.4
It is on such platforms as these that Dr. Murray would have
1 Drasche in Bericht der K. K. Geol. Reichsanstadt, 1875-1876; also Vélain, Les Volcans, 1884.
2 Sartorius von Waltershausen and A. von Lasaulx, Der Aetna, 4to. Leipzig, 1880, vol. ii. p. 327.
3 Compare on the subject: Fritsch, Z. Deutsch. Geol. Ges., xxiii., 187I, pp. 125-213; Fouqué's Santorin et ses Eruptions, Paris, 1880; Geikie, Text-Book of Geology, London, 1885, p. 235.
4 Such as Jukes-Browne's Physical Geology, Prestwich's Manual of Geology, Geikie's Text-Book of Geology, and Philips' Manual of Geology.
us believe that coral-reefs are built. 'Whether built up sufficiently high to rise above the surface of the sea and thus form islands, or brought up only to varying heights below the sea-level, these volcanic eminences tend to become platforms on which coral-reefs may be formed.' Thus he conceives that if the volcanic peak be above the surface, it will be brought down to the lower limit of breaker action by the force of the waves,—as
FIG. 6.—Section of Red Coral showing the polypes.
was the case, for example, with Graham's Island in the Mediterranean1 which arose on July 18th, 1831, as an active volcanic crater, about thirty miles off the south-western shores of Sicily, but which was soon demolished by the waves, until a shoal of
1 Phil. Trans., 1832; Prévost, Mém. Soc. Géol. France, ii. p. 9I; and Ann. des Sci. Nat., vol. xxiv.
scoriæ alone remained to mark its former site—or if submarine and more than I00 feet below the surface, then it would tend to reach the bathymetrical zone at which the polypes live by accumulation on its summit of the dead shells of forminifera, molluscs and other testaceous organisms. Then, on such a peak, it is evident that the coral-polypes, growing upwards, would assume the shape of an atoll. The windward side of a reef thus formed grows faster than the lagoon-side, because it is on that side that the currents bring food to the polypes; and as the atoll grows outward so the lagoon enlarges, owing to its water containing carbonic acid, derived from the decay of the polypes and the sea-weed brought in by the tides, which dissolves the dead coral and removes in solution the calcium carbonate of which it consists. The size of the lagoon can then be taken as a general index of the age of the reef. Similarly a fringing-reef may be formed round an island which has not undergone, or is not undergoing, subsidence and became converted by extension outwards, on a talus of its own débris, into a barrier-reef; provided that, pari passu with the outward growth, the littoral side of the reef has its channel widened by the solvent action of the carbonic acid in the water obtained by the disintegration of the dead polypes. Darwin did not live to bring out a third edition of his book,and was therefore unable to criticise this theory which had been advanced by Murray. In fact, his only contribution to the controversy was a letter1 which, as it shows in a great measure that either his theory had been misrepresented or his work had not been given the justice which it deserved, must be reproduced here. The letter is as follows:—'You will have seen Mr. Murray's views on the formation of atolls and barrier-reefs. Before publishing my book, I thought long over the same view, but only as far as ordinary marine organisms are concerned, for at that time little was known of the multitude of minute oceanic organisms. I rejected this view, as from the few dredgings made in the Beagle, in the south temperate
1 Life and Letters of Charles Darwin, vol. iii. p. 183.
regions, I concluded that shells, the smaller corals, etc., decayed, and were dissolved, when not protected by the deposition of sediment, and sediment could not accumulate in the open ocean. Certainly, shells, etc., were in several places completely rotten, and crumbled into mud between my fingers; but you will know well whether this is in any degree common. I have expressly said that a bank at the proper depth would give rise to an atoll, which could not be distinguished from one formed during subsidence. I can, however, hardly believe in the former presence of as many banks (there having been no subsidence) as there are atolls in the great oceans, within a reasonable depth, on which minute oceanic organisms could have accumulated to the thickness of many hundred feet …. Pray forgive me for troubling you at such length, but it has occurred [to me] that you might be disposed to give, after your wide experience, your judgment. If I am wrong, the sooner I am knocked on the head and annihilated so much the better. It still seems to me a marvellous thing that there should not have been much, and long continued subsidence in the beds of the great oceans. I wish that some doubly rich millionaire would take it into his head to have borings made in some of the Pacific and Indian atolls, and bring home cores for slicing from a depth of 500 or 600 feet.'
Stimulated, perhaps, by this letter from Darwin, Agassiz went to work on the Florida reefs, and, in the next year, published a paper1 which contained the gist of his researches. In this paper, he considered that these reefs could not be explained by the theory of subsidence; but that the polypes have grown, under the most favourable conditions of food, temperature, and oceanic currents, on banks which have been brought into their bathymetrical zone by the accumulation of calcareous detritus. 'This explanation,' he says, 'tested as it has been by penetrating into the thickness of the beds underlying the coral-reefs,
1 Mem. Amer. Acad. Arts and Sci., vol. xi. p. 107; see also the "Three Cruises of the Blake" (Bull. Mus. Comp. Zool. Harvard Univ., vol. xiv. 1888).
seems a more natural one, for many of the phenomena at least, than that of the subsidence of the foundation to which the great vertical thickness of barrier-reefs has been hitherto referred.' He, however, acknowledges that it is 'difficult to account for the great depth of some of the lagoons—forty fathoms—on any other theory than that of subsidence.' This explanation, however, appears to be negatived in some measure by the observations of some American geologists, among whom the names of W. H. Dall and A. Heilprin may be specially mentioned. Thus, the former states1 that 'the coral formation observed by Agassiz in the region in the keys must be of very limited scope, as it has not been identified from the mainland of Florida by any modern geologist;' and the latter also notes2 that 'no observed facts sustain the coral theory of formation as propounded by Agassiz. They prove, on the contrary, that the coral tract of Florida is confined to a border region on the south and south-east, and there are no tertiary reefs whatever.'
More recent still than the observations of Agassiz are those of Guppy,3 who has spent several years among the islands of the Solomon Archipelago. These observations, as far as they concern the theories in question, may be briefly summarised. The islands, by the masses of coral limestone which have been found on them, indicate elevation; these upheaved reefs are situated upon a basis of volcanic mud having the same character as that dredged up by the Challenger Expedition from around volcanic islands; and this mud envelops 'anciently submerged volcanic peaks.' He also states that corals thrive best in the breaker-wash, and do not flourish in the 'break of the tide-swell;' that the detached reefs, which are submerged round these islands, represent the earliest stage in reef-formation, and that when they have in their growth upward reached a height of from 4 to 8 fathoms distant from the surface,
1 Amer. Journ. Sci., 3rd ser., xxxiv. p. 161.
2 Trans. Wagner. Inst. Sci., May, 1887.
3 Trans. Roy. Soc. Edinb., xxxii. p. 545; Proc. Roy. Soc. Edinb., xiii. p. 857.
they are unable to extend higher, without the help of elevation; that the islands north of St. Christoval, called the "Three Sisters," commenced their growth as two flat-topped and submerged reefs; and that coral-reefs may grow from a depth greater than 25 fathoms,—the conditions necessary being the state of the water, and more particularly as to whether it carries suspended mud, which is often fatal to the life of the polypes. And in a recent letter to Dr. Murray, which has been published in Nature (vol. xxxix. p. 236), the same observer states that, in his opinion, many features of importance were overlooked by Darwin when examining the Keeling atoll, and that these give no support whatever to the theory of subsidence.
Lastly, Mr. G. C. Bourne has printed a very interesting paper1 on the Chagos group, in which he arrives at the conclusion that the majority of the reefs in the Indian Ocean show evidences of elevation 'rather than of rest'; and that 'certainly they are not evidences of subsidence.' He challenges the two chief features in Murray's theory—the shape and character of lagoons depending on the more vigorous growth of the polypes on the periphery of the reef owing to ocean-currents, and the solution of its interior by the carbonic acid in the water; and states that it must be realised 'that the laws governing the formation of coral-reefs are exceedingly complex, and that many circumstances have to be taken into account before any perfect explanation of their structure can be obtained.' 'That sea-water exercises a solvent action upon carbonate of lime does not admit of a doubt, and that the scour of tides, combined with the solvent action of the water, does affect the extent and depth of a lagoon is obvious. But I challenge the statement that the destructive agencies within an atoll or a submerged bank are in excess of the constructive. It would be nearer the mark to say that they nearly balance one another. In the first place, the carbonate of lime held in solution by sea-water is deposited as crystalline limestone in the interstices of dead corals or coral débris. Any one who is acquainted with the structure of coralline rock, knows how such a porous mass
1 Proc. Roy. Soc., xliii. p. 440, 1888.
as a mæandrina head becomes perfectly solid by the deposition of lime within its mass. This deposition can only be effected by the infiltration of sea-water. In reckoning the solvent action of sea-water, therefore, account must be taken of the fact that a not inconsiderable proportion of the carbonate of lime held in solution is re-deposited in the form of crystalline limestone. Of this, it seems, Mr. Murray has not taken sufficient account, and has, therefore, overstated the destructive agency of the sea. Secondly, the growth of corals, and the consequent formation of coral-rock within the lagoon, is generally overlooked.
'Whilst diving for corals at Diego Garcia, I had abundant opportunities of studying the formation of coral-rock within the lagoon, in depths under 2 fathoms. The layers of tolerably compact rock thus formed are of no mean extent or thickness; they soon become covered with sand, and are thus protected from the solvent action of the water. I have found it impossible to reconcile Mr. Murray's views with what I saw of coral growth within a lagoon. Not only do the more delicate branching species of the madreporaria flourish in considerable numbers, but true reef-building species, porites, mæandrina, pocillopora, and various stout species of madrepora are found there. It is a mistake to suppose that certain species of corals are restricted to the external shores, others to the lagoon. My collections proved that many of the species growing in the lagoon at distances of five miles and upwards from its outlet are identical with those growing on the outer reef. In addition to them are numerous species, such as Seriatopora stricta, Mussa corymbosa, Favia lobata, Fungia dentata, and many others that are not found on the outside. The reason is that the last-named are either free forms such as fungia, or are attached by such slender and fragile stems to their supports that they could not possibly obtain a foothold and maintain themselves among the powerful currents and waves of the open ocean.
'These various species, numbers of which grow close together, form knolls and patches within the lagoon, and it cannot be doubted that their tendency is to fill it up. Again, in reefs
which do not rise above the surface, or are awash for the greater part of their extent at low tides, great quantities of débris, torn from the outer slopes, are constantly carried over the rim of the reef and tend to fill it up. Hence it follows that in a lagoon entirely surrounded by dry land, or nearly so, as is the case at Diego Garcia, the tendency to the accumulation of material within the lagoon would be less than in submerged or incomplete atolls, for débris cannot be swept over into the lagoon, and the only constructive agency is the growth of coral. If the power of solution of sea-water is so great, it must be supposed that in complete or nearly complete atolls the lagoon would be deepening rather than shallowing; yet at Diego Garcia the lagoon is obviously shallowing in many places, and has nowhere increased in depth since Captain Moresby's survey in 1837. Indeed, the southern part seemed to have shoaled a fathom since that time, and this is the more remarkable, since the S.E. trade-winds are by far the most constant and strongest winds there, and tend to accumulate material at the northern rather than the southern end. The fact is, that these winds sweep the sand out of the southern part, and thus leave an area particularly favourably situated for the growth of corals. Mr. Murray points out that larger atolls generally have deeper lagoons than small atolls, and urges this fact in support of his theory; but here again the facts in the Chagos group are against him. Victory Bank is a submerged atoll, the Solomons is an atoll with a large extent of dry land; in each the lagoon attains a depth of 17-18 fathoms, and in Diego Garcia the lagoon, although far larger, does not attain a greater depth. Peros Banhos is far smaller than the Great Chagos Bank, yet in both the lagoons attain nearly the same maximum depth, viz., 41 fathoms for Peros Banhos, 44 fathoms for the Great Chagos Bank. Speaker's Bank is very little larger than Peros Banhos; it lagoon is far shallower, having a maximum depth of 24 fathoms…. Corals grow best in places where a moderate current flows constantly over them. They are killed in still water by the deposition of sediment, and they will not grow in places where a strong current sets directly
against them. I noted at Diego Garcia in many places, but particularly at the east end of East Islet, that a strong and direct ocean current is most unfavourable to coral growth, and that the reef is barren and suffering rapid erosion at such spots as allow the whole force of the current to fall directly upon them. As the current parts and flows round the obstacle, one meets with a reef covered with débris, but barren of live coral; further on, as the current moderates in force, one finds a few growing heads of coral; and, finally, at the further end of the reef, where the current has abated its force considerably, there is a luxuriant bed of living corals and Alcyonaria. This can be seen in perfection on the southern reef of East Islet. Dr. Hickson tells me that he has observed the same facts at Celebes, that direct and strong currents are unfavourable to coral growth, that moderate tangential currents are extremely favourable, and sluggish or still water again unfavourable. This view, which both of us can support by many observations, is much at variance with the old accepted saying that corals grow best where the breakers are the heaviest. It appeared to me that heavy breakers are not favourable to coral growth, because of the quantity of shingle which they dash against the soft-bodied polypes. Some massive forms might withstand the force of breakers and violent currents if the polypes could be sufficiently protected from the shingle, but the branching madrepores are soon broken off and swept away, and even the more massive mæandrina soon follows, for whilst the surface of the colony grows the base is dead, is soon riddled by boring sponges, serpulæ, etc., and is no longer able to bear the strain put upon it. The great mass then breaks off, and is rolled along the reef, pounding other corals in its course.'
The whole question is still under consideration, and the reader must judge for himself which of these theories he will accept. But it may be stated that Professor Dana1—a distinguished authority and student of corals—has advanced his opinion that 'all the hypotheses of objection to Darwin's
1 Am. Journ. Sci., 1885, p. 190.
theory are alike weak, for all have made these processes (i.e., solution and abrasion) their chief reliance, whether appealing to a calcareous, a volcanic, or a mountain-peak basement for the structure. The subsidence which the Darwinian theory requires has not been opposed by the mention of any fact at variance with it, nor by setting aside Darwin's arguments in its favour; and it has found new support in the soundings off Tahiti that have been put in array against it, and strong corroboration in the facts from the West Indies.' And if at such an early stage another impartial opinion may be expressed, it is this one:—that many of Darwin's critics have not carefully read his work, and that, so far as the controversy has, as yet, advanced, the theory of subsidence accounts for the majority, if not all, of the features of coral-reef formation. On no other theory, indeed, can the African element in the Indian fauna be explained than on the supposition that land once stretched between Mozambique and the Malabar coast which has become depressed, and which is now alone represented by the Chagos Bank, the Saya de Malha, and the Laccadive and Maldive Islands. Darwin died on April 19, 1882, and, if we may judge by his letters, he remained convinced to the last of the general truthfulness of his theory. But, no matter how future observations may decide, had he written no other work and simply rested on his laurels, this book alone by its very inductive reasoning and patient marshalling of facts would have remained as an everlasting monument of scientific acumen, and would have placed him in the front rank of investigators.
In bringing this book, as far as possible, up to the date of our present information on the subject, the Editor has made free use of Professor Bonney's Appendix to the third edition of Darwin's Coral Reefs (Smith, Elder, & Co.), and of professor Geikie's Address to the Royal Physical Society of Edinburgh (Proceedings, vol. viii. p. I). He has also to thank his friend, Mr. A. Paling, for several valuable suggestions, and for kindly revising the sheets during the time they were passing through the press.
JOSEPH W. WILLIAMS.
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
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