DARWIN. C7 



distribution of coral reefs, he remarks that " the study of 

 the terrestrial and better-known half of the world must 

 convince every one that no station capable of supporting 

 life is lost nay more, that there is a struggle for each 

 station between the different orders of nature." He 

 describes the large fishes and the trepangs (holothuria) 

 preying upon the coral-polyps, and shows how complex 

 ase the conditions which determine the formation of 

 reefs on any shore. Perhaps no part of his work is 

 more important than that in which he collects the 

 evidence proving how rapidly coral masses grow, and that 

 they for the most part cannot flourish in a greater depth 

 of water than fifteen fathoms. 



Reasoning upon the facts observed by himself and others 

 Darwin now proceeded to upset the received theory that 

 atolls were based upon submarine volcanic craters, and to 

 substitute for it the view that there has been a prolonged 

 and gradual subsidence of the areas upon which the atolls 

 are based, and a corresponding upward growth of the reef- 

 building corals. Thus fringing-reefs in time become 

 barrier-reefs ; and barrier-reefs, when they encircle islands, 

 are converted into atolls, or lagoon islands, as soon as the 

 last pinnacle of land sinks beneath the surface of the 

 ocean. The whole matter is summed up thus : "A magni- 

 ficent and harmonious picture of the movements which the 

 crust of the earth has within a late period undergone is 

 presented to us. We see vast areas rising, with volcanic 

 matter every now and then bursting forth through the 

 vents or fissures with which they are traversed. We see 

 other wide spaces slowly sinking without any volcanic 

 outbursts ; and we may feel sure that this sinking must 



