PREFA TOR Y NOTE. xi 



with a central expanse of water (Fig. 4, 3rd period). The 

 barrier-reef has become an atoll. On the outer margins of a 

 reef thus formed, the waves dash and break off pieces of coral, 

 and heap the broken masses upon its surface, so that its edge 

 appears above the low-tide level. The majority of the polypes 

 then die; 'but the waves continue to pile up on the reef, sand, 

 pebbles, and broken masses of coral, some of the masses being 

 two to three hundred cubic feet in size, and a field of rough 

 rocks begins to appear above the waves. Next a beach is 

 formed; and the bank of coral debris, now mostly above the 

 salt-water, becomes planted by the waves with sea-borne seeds. 

 Trailing shrubs spring up; and afterwards, as the soil deepens, 

 palms and other trees rise into forests, and the coral-island 

 or atoll comes forth finished.' 



For many years geologists universally accepted the general 

 validity of Darwin's theory. The first note of dissent seems to 

 have been sounded in 1863 when Professor Semper published an 

 article 1 on the Pelew Islands, which are situated at the western 

 extremity of the Caroline Archipelago, and which appeared to 

 him to show evidence of elevation, rather than of subsidence. 

 It is a significant fact that at the southern end of these islands, 

 there are raised coral-reefs from 400-500 feet in height, and 

 also an island which is entirely destitute of reefs, while at the 

 northern extremity, only 60 miles distant, there are true 

 atolls. Darwin, however, in the Appendix to his second 

 edition, published in 1874, replied that he did not think 

 these conditions were insuperable by his theory, and that 

 they might be explained on the supposition that the whole 

 group had originally subsided, then was upraised, — ' probably 

 at the time when the volcanic rocks to the north were erupted' 

 — and afterwards again depressed. ' The existence of atolls and 

 barrier-reefs in close proximity is manifestly not opposed to my 

 views. On the other hand, the presence of reefs fringing the 

 southern islands is opposed to my views, as such reefs generally 

 indicate that the land has either long remained stationary, or has 

 been upraised. It must, however, be borne in mind (as remarked 

 1 Zeitschr. f, Wissensch, Zoologie., 1863, Bd. xiii. p. 558. 



