354 



NA TURE 



\Sept. 3, 1874 



these corals belong to reef-building genera, still as species 

 they are not those which grow on flourishing reefs. The 

 reef-builders evidently grow with great rapidity, and their 

 struggle against the tide and currents and waves neces- 

 sitates a constant process of reparation or of growth to 

 replace fractured branches. They flourish in the warm, 

 highly aerated, rushing water, which is full of living things 

 —their proper food. Beyond the reach and influence of 

 these conditions other species and genera exist, which 

 add to the bulk of the coral mass, but which of themselves 

 would never build up a reef, and it is some of these which 

 have been dredged up from considerable depths. The 

 simple corals and the branching forms without a cellular 

 exotheca to hold them together have an enormous bathy- 

 metrical range, and can live in water of 76° F. close to 

 the surface, and also at a depth of more than 1,000 

 fathoms in a temperature of less than 32". But the true 

 reef-builder requires a high temperature, and it therefore 

 becomes very important to discover, as has been sug- 

 gested by Dr. Carpenter, whether the vast sub-zone of 

 cold water which underlies the superficial and heated 

 water has not much to do with this restriction of certain 

 species to definite depths. We must wait for the results 

 of systematic dredging at great depths in the Pacific before 

 this question can be for ever settled, but at present all 

 our knowledge tends to prove that this deep stratum of 

 cold water would prevent reef-builders from living at any 

 considerable depth, and therefore that they never could 

 have risen by growth from the ocean floor itself. Growing, 

 therefore, on submerged rocks, the reef-builders must 

 have their foundation slowly subsiding, if they are to 

 attain a great thickness and to assume the bulk and the 

 characters of atolls. The direct proofs of subsidence 

 advanced by Mr. Darwin were noticed especially in 

 Keeling atoll. " Appearances indicating a slight encroach- 

 ment of the water on the level are plainer within the 

 lagoon : I noticed in several places, both on its wind- 

 ward and leeward shores, old cocoa-nut trees falling 

 with their roots undermined and the rotten stumps 

 of others on the beach, where the inhabitants 

 assured us the cocoa-nut would not grow. Capt. Fit?.- 

 roy pointed out to me near the settlement the founda- 

 tion-posts of a shed, now washed by every tide, but 

 which the inhabitants stated had seven years ago stood 

 above high-water mark." " From these considerations I 

 inferred that probably the atoll had subsided to a small 

 amount : and this inference was strengthened by the cir- 

 cumstance that in 1834, two years before our visit, the 

 island had been shaken by a severe earthquake, and by 

 two slighter ones during the ten previous years." The 

 observations of such authorities as Williams, Kotzebue 

 and Stutchbury, respecting the encroachment of the sea on, 

 and the destruction of parts or the whole of islands, were 

 noticed by Darwin in his early edition, and comparisons 

 were made, as in the case of Whitsunday Island, between 

 old and new charts, in support of the evidence of subsi- 

 dence. The existence of submerged or dead reefs is very 

 properly advanced as an indirect proof of subsidence, and 

 the condition of the Great Chagos bank was considered 

 to explain the effects of a rapid subsidence which killed 

 the corals. But the principal and most interesting evi- 

 dence is afforded by the relative positions of active 

 volcanic vents and barrier reefs and atolls. Darwin 



noticed the absence of active volcanoes in the presumed 

 areas of subsidence, and their frequent presence in areas 

 of elevation, the exceptions being very few. In acknow- 

 ledging Dana's suggestive criticism that he had not laid 

 sufficient weight on the mean temperature of the sea in 

 determining the' distribution of coral reefs, Darwin very 

 properly urges that some other cause must account for 

 the absence of coral growth in localities where the surface 

 temperature of the sea is sufficient, and he refers especially 

 to the islands which rise up from the abyssal sea in the 

 Atlantic ; but he indicates that temperature evidently has 

 much to do with the absence of reefs on the west coast of 

 Tropical America, the cold current reducing the mean 

 emperature of the sea there below 68^\ 



Although investigations made subsequently to those of 

 Darwin add almost invariably to the proofs of his theory 

 of atoll formation, and it is received as correct by every 

 teacher, still there have been one or two able criticisms of 

 its general applicability. For instance, Semper, in his 

 description of the Pelew Islands, doubted the evidence of 

 subsidence. His opponent, with his usual justice and 

 candour, gives Semper's objections the most careful con- 

 sideration, and indeed they deserved this treatment. " He 

 (Semper) states that the southern islands consist of coral 

 rock upraised to the height of from 400 to 500 feet ; and 

 some of them before their upheaval appear to have existed 

 as atolls. They are now merely fringed by living reefs. 

 The northern islands are volcanic, deeply indented by 

 bays, and are fronted by barrier reefs. To the north there 

 are three true atolls. Prof. Semper doubts whether the 

 whole group has subsided, partly from the fact of the 

 southern islands being formed of upraised coral rock ; 

 but there seems to me no improbability in their having 

 originally subsided, then having been upraised (probably 

 at the time when the volcanic rocks to the north were 

 emptied), and again having subsided. 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 remained stationary or has been upraised. It 

 must, however, be borne in mind 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 be undistinguishablc from 

 fringing reefs. Now, the submarine flanks of most atolls 

 are very steep ; and if 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." The appendix, which con- 

 tains a detailed description of the reefs and islands in the 

 well-known coloured map, is of the greatest value to the 

 physical geographer, and _it includes notices of nearly 

 every known coral tract. 



After reading and pondering over this long-prized work, 

 there comes the feehng that Mr. Darwin should at some 

 future time enlarge its scope and deal with the distribution 

 of coral species, and trace back in time the reefs of old. 

 Who would not be glad to be taught from the vigorous 

 pen of the man whose theory has lasted more than thirty 

 years, and will last as long as science, what was the con- 

 dition of the vast Pacific area prior to the age of reefs 



