202 W. M. DAVIS 
which some geologists have doubted the occurrence of important 
subsidences in the ocean beds during later geological times; but 
Darwin’s opinion is re-enforced by the conclusions reached by 
other geologists: for example, Schuchert has recently summarized 
the results of his study of a large oceanic region as follows: “The 
entire western half of the Pacific bottom, and especially the Aus- 
tralasian region, appears to be as mobile as any of the continents 
of the Northern Hemisphere, with the difference that the sum of 
the continental movements is upward, while that of the ocean 
bottom is downward.’ Additional re-enforcement is found in 
Crampton’s conclusions based on the study of land snails in the 
Society Islands: ‘The evidence tends to prove that the dominant 
process in the South Pacific has been one of subsidence, which 
has progressively isolated various mountain ranges previously 
connected, so that they have become separate island masses, 
which in their turn have been subsequently converted into the — 
disconnected islands of the several groups.’? In view of these 
conclusions, independently reached by researches of different kinds, 
it is to my reading unwarranted to assume “‘a long period of nearly 
perfect stability for the general ocean floor.”’ Such an assumption 
fully deserves provisional consideration as an abstract possibility, 
but it does not merit the rank of an accepted postulate from which 
a long sequence of consequences can be safely deduced. 
Darwin’s views as to the manner in which subsidence acted 
deserve attention. He first pointed out that the existence of 
many widely separated reefs is ‘‘quite inexplicable, excepting on 
the theory that the bases on which the reefs first became attached 
slowly and successively sank beneath the level of the sea, whilst 
the corals continued to grow upward” (98); but on the next page 
two variations are introduced: first, by recognizing possible 
changes of rate of subsidence in the phrase, ‘‘as the island sinks 
down, either a few feet at a time or quite insensibly,” and 
secondly, by adverting to the probable occurrence of still-stand 
«C. Schuchert, ‘“The Problem of Continental Fracturing and Disastrophism in 
Oceanica,”’ Amer. Jour. Sci., XLII (1916), g1-105; see p. 105. 
2H. E. Crampton, ‘Studies in the Variation Distribution, and Evolution of the 
Genus Partula . ... ,”’ Carnegie Institution of Washington, 1916, p. 12. 
