June 24, 1921] 



SCIENCE 



561 



to be alterations from the original form of 

 the islands. Hence there is no warrant what- 

 ever for thinking that Darwin's theory de- 

 mands the growth of reefs on unaltered vol- 

 canic slopes. 



The particular kind of alteration caused by 

 the abrasion of eireum-insular platforms was 

 very properly not shown in his type diagram, 

 because, so far as Darwin's observation and 

 reading went, no barrier reefs were known 

 to have grown up from foundations of that 

 kind. He knew full well, however, that plat- 

 forms might be abraded and that reefs might 

 grow upon them; but he believed that, unless 

 subsidence occurred, such reefs would differ 

 from ordinary barrier reefs in having shallow 

 lagoons behind them, as will be shown below. 



Various passages in bis book make it 

 clear enough that no particular form of reef 

 foundation was regarded as essential. Any- 

 thing on which a reef might begin its growth 

 would suffice. For example, Darwin wrote: 

 " If the rim of a [non-subsiding] crater af- 

 forded a basis at the proper depth, I am far 

 from denying that a reef like a perfectly char- 

 acterized atoll might not be formed; some 

 such, perhaps, exist; but I can not believe in 

 the possibility of the greater number having 

 thus originated" (89). And again: "A bank 

 either of rock or of hardened sediment, level 

 with the surface of the sea, and fringed with 

 living coral, would ... by subsidence be con- 

 verted immediately into an atoll, without pass- 

 ing, as in the case of a reef fringing the shore 

 of an island, through the intermediate form 

 of a barrier reef " (101). Evidently, the prime 

 element in Darwin's theory of barrier reefs 

 and atolls was subsidence; no particular form 

 of the foundation on which reef growth be- 

 gins was assumed, except for purposes of 

 graphic illustration. Such illustration always 

 involves definite profiles; but the more gen- 

 eral statements of the text show that definite 

 profiles are not required. 



Moreover, a careful reading of Darwin's 

 book will discover that he clearly conceived 

 the possibility of a reef growing up from the 

 outer margin of an abraded platform, as now 

 appears to have been actually the case on 



Tutuila ; and that he gave this possibility little 

 consideration, not because such a reef would 

 not grow upward into a true barrier if the 

 platform subsided, but only because he found 

 no examples of it. He wrote: 



It will, perhaps, occur to some, that the actual 

 reefs formed of coral are not of great thickness, 

 ibut that before their first grofwih, the coasts 

 of these en(Hrcled [non-subsiding] islands were 

 deeply eaten into, and a broad but shallow sub- 

 marine ledge thus left, on the edge of which the 

 coral grew; but if this had been the case, the 

 shore would have been invariably bounded by lofty 

 cliffs, and not have sloped down to the lagoon 

 channel, as it does in many instances (49). 



Certain volcanic islands that Darwin had 

 seen in the Atlantic, before he was concerned 

 with the origin of coral reefs, had made him 

 familiar with the visible occurrence of sea- 

 cut cliffs ; and the " broad but shallow sub- 

 marine ledge " that must extend forward from 

 the base of the cliffs was apparently familiar 

 by inference. Thus he described St. Helena 

 as surrounded by " enormous cliffs, in many 

 parts between 1,000 and 2,000 feet in height," 

 and added that " the swell of the Atlantic 

 ocean has obviously been the active power in 

 forming these cliffs." In various other reef- 

 free islands he recognized " the prodigious 

 amount of degradation, by the slow action of 

 the sea, which their originally sloping coasts 

 must have suffered, when they are worn back, 

 as is so often the case, into grand precipices." ^ 

 He does not explicitly announce the contrast 

 between the " grand precipices " of volcanic 

 islands that are not defended by encircling 

 reefs, and the moderate slopes that lead " down 

 to the lagoon channel " in nearly all reef- 

 encircled islands; but he knew and correctly 

 described both classes of islands. 



In view of all this it is manifest enough 

 that, if Darwin had at hand the facts now 

 known about Tutuila, he would have said, in 

 effect : 



Tutuila is an actual island which must formerly 

 have been ' ' deeply eaten into ' ' by the sea, and 

 which must then have been surrounded by a 

 "broad but shallow submarine ledge" backed by 



2 "Geological Observations," 1844, 91, 128. 



