ARTICLES 44i 



place during stationary periods, as Darwin clearly stated, and 

 upward reef growth will be favoured during and for some time 

 ; after movements of subsidence, as he stated more clearly still ; 

 movements of elevation will interrupt reef growth, as he also 

 recognised. Movements of subsidence have naturally given 

 name to Darwin's theory, for only through their aid can the 

 transformation of fringing reefs into unconformable barrier 

 reefs, and of barrier reefs into atolls, be satisfactorily accounted 

 for ; but his theory explicitly recognised the occurrence of 

 stationary periods and of movements of elevation as well as 

 movements of subsidence : indeed, the emphasis that he gave 

 to the alternation of long stationary periods with brief move- 

 ments of subsidence suggests that his scheme should be called 

 the theory of intermittent subsidence. 



Only under the diverse conditions of this elastic theory 

 can the diversity of the structural features of various reef- 

 encircled islands and reef-bordered coasts be adequately ex- 

 plained. The main postulate of the theory, namely, the in- 

 stability of reef foundations, is well grounded ; for every island 

 group in the Pacific, the geological history of which has been 

 well worked out, is shown to have suffered various changes of 

 level which clearly demand the occurrence of local movements, 

 differing in date and amount from the local movements of other 

 groups. The opinion reached by Schuchert in a careful study 

 of the geological changes of land and sea in Oceanica deserves 

 quotation in this connection : " The entire western half of the 

 Pacific bottom, and especially the Australian region, appears 

 to be as mobile as any of the continents of the northern hemi- 

 sphere, with the difference that the sum of the continental 

 movements is upwards, while that of the ocean bottoms is 

 downwards." x The distribution of animals on oceanic islands 

 also calls for changes, and particularly for subsidence, in the 

 ocean floor ; land snails are especially significant in this 

 respect, as shown by the studies of Pilsbry 2 and Crampton. 3 

 In view of all this, it would seem as if the burden of proof 



1 C. Schuchert, "The Problem of Continental Fracturing and Diastrophism in 

 Oceanica," Amer. Journ. Sci. xlii. 1916, 91-105 ; see p. 105. 



1 H. A. Pilsbry, "Mid-Pacific Land Snail Faunas,'' Proc. Nat. Acad. Sci. ii. 

 1916, 429-433. This is a concise statement ; the subject is elaborately treated in 

 his Manual of Conchology. 



3 H. E. Crampton, Studies in the Variation, Distribution, and Evolution of 

 the Genus Partula, Carnegie Inst., Washington, 1916; see p. 12. 

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