NA TURE 



\J>J'ov. 29, I < 



tii'st. But when Darwin's own views were published, first 

 ill abstract before tlie Geological Society in 1837, and 

 subsequently more fully in his separate volume on the 

 Structure and Distribution of Coral-reefs in 1842, they 

 were soon generally accepted, and were regarded not 

 only as affording a satisfactory explanation of the whole 

 phenomena, but as comprising one of the most impressive 

 generalisations with which geology, fertile in such achieve- 

 ments, had yet astonished the world. 



The theory proposed by Darwin, now so familiar, con- 

 nected all the typei of reef together as stages of one long 

 process, every step in which could be illustrated by actual 

 examples. At the one end stood the fringing-reefs, some 

 of which might only lately have been started upon a 

 recently upraised sea-bottom. Out of this stage, by 

 continuous or intermittent subsidence, came barrier-reefs. 

 Then as depression went on and the islands encircled by 

 ihe barrier-reefs disappeared, their sites were taken by 

 atolls. Lastly, where the rate of subsidence was too 

 rapid for the upward growth of the corals, an atoll might 

 become a submerged bank. Not only was this explana- 

 tion self-consistent, but it harmonised well with the con- 

 clusion, derived from totally different evidence, that there 



may have been widespread and long-continued subsidence 

 over the ocean basins. It was moreover supported by the 

 independent testimony of competent observers, who, with 

 at least equal opportunities of studying the subject, had 

 espoused Darwin's views. Of these witnesses the most 

 important was undoubtedly Prof. Dana, who accom- 

 panied the Wilkes Exploring Expedition of 1838-42.' 

 Another powerful ally was found in Mr. Couthouy, who 

 had studied coral-growth in the Pacific and in the West 

 Indian seas." But even without the concurrent testimony 

 of eye-witnesses the theory proposed by Darwin fitted so 

 admirably into the geological theory of the day that it 

 came itself to be used as one of the most cogent proofs of 

 vast oceanic depression. And such is still the position 

 which it holds. 



By a gradually widening circle of observation, however, 

 a series of facts has been established, which were either 

 not known or only partially known to Darwin. It should 

 be borne in mind that, compared with more recent ex- 

 plorers, he did not enjoy a large opportunity of investi- 

 gating coral reefs. So far as can be judged from his 

 published works, he appears to have examined only one 

 atoll — the Keeling reef ; and one barrier reef— that of 



Scale of Fathoms 

 60 100 500 



Fri, I —Seen in of the 



■ Reef, fa>iil 



nd honzont ,1. By Mr. Murray and Lieut. Swire, K.N"., cf the Challenger 



Tahiti. The Admiralty charts, the work of previous 

 voyagers, and unpublished information communicated to 

 him, enabled him to extend his generalisation over the 

 whole of the rest of the coral-regions which he had not 

 personally explored. The deep-sea expeditions of recent 

 years have now brought so much new light to bear on the 

 whole question that we are in a much better position to 

 discuss it than he was, nearly half a century ago. Of a 

 tew of the more important investigations a brief rhunid 

 may here be given, and their bearing upon Darwin's 

 theory of coral-reefs will then be discussed. 



As far back as the year 1S51 the late L. Agassiz stated 

 that, in his opinion, the theory of subsidence could not be 

 applied in explanation of the Florida reefs ; that on the 

 contrary the southern end of Florida is built up on succes- 

 sive concentric barrier-reefs which have been gradually 

 connected and cemented into continuous dry land by the 

 a cumulation of mud-flats between them, and that this 

 process is still going on and must eventually convert the 

 present keys and reefs from Cape Florida to the Tortugas 

 into similar land.' 



In 1S63 Prof. Carl Semper published the results of his 



' Bull. Mits. Comp. Zool, vol. i. See also J 

 ■funial, x.xiii. (1857), p. 46, and E. B. Hunt, o/. cit 



Conte, Si'lliman's 

 cxv. (1863). p. 3S8. 



researches among the Pelew Islands. He found himself 

 unable, by the theory of subsidence, to account for the 

 phenomena there presented, and threw doubts on the 

 general applicability of that theory. He pointed out that 

 while the southern islands, probably once atolls, consist 

 of coral-rock, upraised to from 400 to 500 feet above the 

 sea, and are flanked by living coast-reefs, true living atolls 

 exist at the northern end of the group. He contended 

 that there is absolutely no evidence of subsidence, that 

 the association of all the different kinds of reefs within so 

 circumscribed an area seems entirely to disprove the 

 notion of subsidence, and that, at least in this group of 

 islands, Darwin's theory cannot be applied. In some 

 suggestive observations on their probable origin, he re- 

 marks that the reefs depend mainly for their form upon 

 the nature of the bottom on which they begin. Atolls are 

 formed on submarine banks. A species of Pofites takes 

 root in little colonies varying from the size of the fist to 

 masses six or eight feet in diameter. In time the central 

 portions of these growing colonies die, while the outer 



* The narrative containing Prof. Dana's observations on coral-reefs was 

 published among the Reports of the Expedition. In 1S73 he published a 

 volume on " Coral and Coral-Reefs," where he again gave the weigh! cf 

 his authority to the theory of subsidence. 



- Boston /onrn. Nat. Hist., iv, (1843-44), P- 137. 



