Nov. 29, 1883 



NATURE 



109 



parts flourish and gradually build up a ring of coral. This 

 ring, which may be circular or elongated in form, is some- 

 times continuous, but more commonly is traversed by one 

 or more channels. The interior portions are scoured out 

 and deepened by the tidal currents. Or if the form of 

 the bottom and other conditions be suitable, a great many 

 individual masses of coral gradually grow into a more or 

 less continuous reef, through which the strong ebb and 

 flow of the tides serve to keep open some channels. Thus 

 fringing-reefs, through the scour of the sea, become 

 barrier-reefs, which retreat from the adjacent coast in 

 proportion to the gentleness of the slope on which they 

 are built. On a steeply shelving sea-bottom the reefs 

 must obviously remain fringing-reefs. 



Dr. Semper admitted that possibly many atolls 

 and barrier-reefs were formed during subsidence, and 

 even that the downward movement may in many cases 

 have furnished the conditions for starting them into 

 e.xistence. The solution of the problem ought in each 

 case, he thought, to be determined by actual detailed ob- 

 servation. Uut that the alternate currents of the tides are 

 the main agents in the building of coral-reefs could be 

 proved, he maintained, by many cases which, on the 

 theory of subsidence, must be regarded as exceptional of 

 inexplicable, such as the occurrence of true atolls in the 

 mid?t of areas of elevation. 1 



In the second edition of his "Coral Islands," published 

 in 1874, Darwin briefly referred to these observations. 

 Me thought it not improbable that the Pelew Islands 

 originally subsided, were afterwards upraised, and again 

 subsided, but admitted that the proximity of fringing-reefs 

 was opposed to his views. He suggested that if the sub- 

 marine slope were steep reefs which began as fringing- 

 reefs would continue to be of that form, even during 

 tub-idence. There is, however, no admission that any 

 valid objection had been made to his theory, or that true 

 atolls and barrier-reefs might be formed in many places 

 without subsidence. 



In 1868 Prof Semper reiterated his dissent from the 

 prevailing theory of coral-reefs." Next year he reprinted 

 his original paper (which seemed to him to have remained 

 unknown to most naturalists) in a general account of the 

 Philippine Islands, wherein he appended some additional 

 notes.' In one of these he refers to the observations of 

 Pourtales and others on a submarine calcareous deposit 

 which in some regions is slowly being upraised to serve 

 as a foundation for coral-reefs. To the objection that if 

 atolls and barrier-reefs could be formed during a period 

 of elevation, they ought to be found not merely at, or only 

 slightly above sea-level, he replies that they are not in 

 fact confined to that limited zone, but that even if they 

 were, this would not invalidate his conclusion that the 

 reefs are due to a complex cooperation of coral-growth 

 with the waves and currents of the sea, and not to the 

 one cause — the subsidence of entire regions — invoked by 

 Darwin. 



In the following year another contribution to the anti- 

 subsidence literature was made by Dr. J. J. Rein, who, in 

 an interesting me 1 oir on the physical geography of 

 Bermuda, offered some observations on the coral-reefs of 

 those islands.-" He suggested that the Bermuda group 

 might originally have been a submarine mountain or bank 

 on which colonies of deep-water corals took root, and 

 where other organisms flourished in such abundance as 

 gradually to raise the top of the submerged ground to the 

 zone in which reef-building corals could flourish. He 

 .adduced no evidence in support of this suggestion further 

 than that there is no proof in Bermuda of subsidence, 



' Zeitsch. IVisaensch. Zoohgie ,1863. xiii. p. 558. Reprinted in 1869 in 

 '■ Die Philippinen und ihre Bewohner," with additional notes. 

 ' Vtrhandl. Physik-med. GeselUch. Wurzburg; Sitzungsier., February i. 



3 " Die Philippinen und ihre Bewohner." Wiirzburg, i86q, pp Joo-iog. 

 A brief account of the coral-reefs of the PhiHprine Islands will be found at 

 PP- I9-33- 



< Bencht. Senckcnberg. Kuliir/orsc/i Gcselhch., 1869-70, p. 157. 



which, however, as Darwin had so cogently shown, from 

 the very fact of the movement beirig downward, is in 

 most cases not to be looked for. 



.An important memoir, marking a totally new departure 

 in coral-reef literature, appeared in 1880 containing an 

 abstract of observations made by Mr. Murray during the 

 great voyage of the Chal/enger} The chief features of 

 this contribution may be thus briefly summarised ; — With 

 hardly an exception the oceanic islands are of volcanic 

 origin, and it is therefore to be presumed that the sub- 

 marine ridges and peaks, which rise to within various dis- 

 tances from the surface, arelikew'ise due to the protrusion 

 of volcanic materials. There is thus no actual evidence of 

 the still unsubmerged portions of any extensive continent 

 or mass of land such as Darwin's theory requires. 

 Whether built up above the sea-level into islands, or 

 brought up to varying heights below that level, the vol- 

 canic eminences of the ocean may conceivably be brought 

 into the condition of platforms for reef-builders by two 

 causes. In the first place the erosive force of waves 

 and tidal scour must tend to reduce all prominent oceanic 

 summits to the lower limit of breaker-action, and there- 

 by to produce truncated cones or flattened domes and 

 ridges on which coral-reefs, if not already established, 

 might spring up. In the second place, submarine emin- 

 ences may have been brought up to within the zone of 

 the reef-builders by the deposit of organic detritus upon 

 them. One of the most remarkable results, of recent 

 deep-sea exploration has been the accumulated evidence 

 of the extraordinary profusion of pelagic life in the 

 tropical surface waters. From experiments made during 

 the cruise of the Challenge)-, Mr. Murray estimated that, 

 if the organisms are as numerous down to a depth of 100 

 fathoms as they were found to be in the track of the tow- 

 net, there must be more than sixteen tons of carbonate of 

 lime in the form of calcareous shells in the uppermost 

 hundred fathoms of every square mile of ocean. The 

 shells and skeletons of these organisms fall in a constant 

 rain to the bottom, where they supply some of the food 

 needed by the fauna which there subsists upon the mud. 

 By the accumulation partly of these superficial exuviae, 

 partly of the remains of the creatures living at the 

 bottom, an organic deposit is growing over the sea- 

 floor in the tropical regions wherein coral-reefs flourish. 

 Owing probably to the greater solvent action of the 

 increased proportion of carbonic acid in sea-water 

 at great depths, or to the greater mass of water through 

 which they must sink, the shells of the upper waters seem 

 never to reach the bottom or at least soon disappear 

 from it, for they are seldom met with in deep dredgings. 

 But in shallower portions of the ocean they abound. 

 Consequently it may be legitimately inferred that the rate 

 of growth of the calcareous organic deposit on the sea- 

 bottom must be more rapid in the shallower waters. The 

 tops of submarine peaks and banks, being constantly 

 heightened from this cause, will in course of time be 

 brought up to a depth at which sponges, hydroids, deep- 

 sea corals, annelids, alcyonarians, mollusks, polyzoa, 

 echinoderms, and other organisms can flourish abundantly. 

 When this has taken place, the upward growth of the 

 calcareous formation will be accelerated by the accumula- 

 tion of the remains of this abundant fauna as it lives and 

 dies on the bottom. At last the zone of reef-building 

 corals will be reached, and thereafter a growth of coral- 

 rock will bring the sea-floor up to the level of low water. 

 That coral-reefs undistinguishable from barrier-reefs 

 and even atolls might be formed upon banks of sediment 

 in a deep sea was admitted by Darwin.- But the assurnp- 

 tion of so many submerged banks as this explanation 

 would require, seemed to him so improbable that he dis- 

 missed it from further consideration. He was not aware, 

 however, of the enormous abundance of minute cal- 



' Proc. Roy. Sac. Edin. (1879.80). x. p. 305. 

 2 "Ccral Island.s," ii.d edit. p. 118. 



