140 BULLETIN OF THE 



the shores, would thrive in such a position, and he lays great stress also 

 on the difference to be traced in the conditions of the two sides of the 

 same islands, where the one side is exposed to the action of the open 

 sea, while on the other side the long periods of calms are most favor- 

 able to the growth of corals. Although Semper does not deny that 

 subsidence may have accompanied in some cases the formation of atolls 

 and of barrier reefs, yet the explanation of the existing conditions of the 

 reefs of the Pelew Islands seems to him more plausible by the theory of 

 currents than by that of subsidence. 



Semper has more fully developed these views in his " Natiirliche Ex- 

 istenzbedingungen der Thiere." He calls attention to the difference be- 

 tween the theories of Darwin and Dana, while Dana agrees with Darwin 

 that atolls and barrier reefs can only be formed in regions of subsid- 

 ence, he differs from Darwin in claiming that fringing reefs indicate a 

 greater amount of subsidence than either of the other types of reefs. 

 He looks upon the steep coast line of many volcanic islands as a proof that 

 there has been great subsidence, and of course upon such steep shores, 

 often with a vertical cliff of more than one hundred and fifty feet, there 

 is no possibility of the formation of a fringing reef. We must admit, 

 with Dana, that in the volcanic regions of the Pacific, for which Darwin 

 claims a general subsidence, there have been local phenomena of eleva- 

 tion, and also that in regions of elevation a slight subsidence may also 

 have taken place. But if we have to depend upon either elevation or 

 subsidence to account for the structure of the reefs, there seems to be 

 no possible application of a general law regulating the shape of the reefs. 

 Darwin's map of districts of elevation and of subsidence shows that he 

 considered a region of elevation as one where fringing reefs alone could 

 be formed. ' 



The basis of the whole of Semper's objections lies in the presence 

 of barrier reefs, atolls, and fringing reefs in the same region, and he 

 has attempted to prove that he can explain their presence and peculiar 

 conformation by the action of currents upon growing reefs in a region 

 which has been assumed, according to Darwin's theory, to be one of sub- 

 sidence. Semper and Rein were among the first to see the importance 

 of the discovery by Pourtales of the formation of great limestone plateaus 

 at considerable depth, far below that at which corals can grow, and the 

 possibility of having thus many extensive plateaus gi-owing gradually up 

 to the depth at which corals can flourish. The close connection of ele- 

 vated and growing reefs are strong proofs against subsidence. To estab- 

 lish this view, we are obliged to prove that the peculiar shape of the 



