MUSEUM OF COMPARATIVE ZOOLOGY. 139 



that there was any proof of subsidence, or that the causes cited by the 

 opponents of Darwin's theory were not amply sufl&cient to account for 

 all the phenomena he observed there. Mr. Forbes, who spent more 

 than a month in its study, felt inclined to believe that the Keeling 

 Eeef foundation has been formed as suggested by Murray, Agassiz, and 

 Semper, and that the islets have been the result of the combined action 

 of storms and the slow elevation of the volcanically upheaved ocean 

 floor on which the reef is built. 



Semper,* who visited the Pelew Archipelago in 1863, was among the 

 first to come to the conclusion that the presence of barrier reefs, atolls, 

 and fringing reefs in one district could not be explained by the theory 

 of subsidence, and he looked to natural and simpler causes to explain 

 the reefs of the Pelew Islands. He was one of the first, after the general 

 adoption of Darwin's theory of the formation of coral reefs, to visit an 

 atoll district in the Pacific, and he was the first also to point out for 

 that region a condition of things which seemed to him incompatible 

 with the accepted view. He found at the Pelew Islands, within a com- 

 paratively restricted area, atolls, barrier reefs, and fringing reefs. He 

 speaks of the channels eaten away between the coast and the barrier 

 reef, distant three to six miles from shore, and forming a labyrinth of 

 channels, which he considers as due to the action of currents, and in 

 which the flow of brackish water prevents the ready growth of corals, 

 while in the case of the barrier reefs less than half a mile or so from the 

 shore the action of the currents is reduced to a minimum and the chan- 

 nels scarcely marked. He speaks of elevated coral reefs of two hundred 

 and fifty feet in height, and comes to the conclusion that the presence 

 of atolls, barrier reefs, and fringing reefs in an area where there had 

 been elevation, and which had remained stationary for a long period, 

 does not indicate that they have been formed during a period of subsid- 

 ence, while their simultaneous existence would seem to preclude such a 

 conclusion. 



Semper is inclined to attribute to the action of currents mainly the 

 great irregularities existing in reefs, which may form even closed atolls, 

 and are in great degree dependent for their ultimate shape upon the 

 configuration of the underlying base. On steep shores barrier reefs, 

 according to him, could not flourish ; only fringing reefs closely hugging 



1 Semper, Carl, Die Philippinen und ihre Bewohner, pp. 100-108, Wiirzburg, 

 1869. A reprint, with additions, of Semper's article in Zeits. f. Wiss. Zool., XIII. 

 p. 558, 1868. Also, Die Natiirlichen Existenzbedingungen der Thiere, Leipzig, 

 1880, Zweiter Theil, p. 39. 



