DARWIN'S THEORY 



93 



tides is once more encountered, and growth again 

 arrested. This process of submergence and upward 

 growth may, of course, be repeated indefinitely, and by 

 the time the island has descended 50 fathoms below its 

 original position, the reef will have acquired a corre- 

 sponding thickness. In such a case the unfavourable 

 conditions to coral growth which prevail on the inner 

 side of the reef, together with the retreating slope of the 

 flanks of the island, will have led to the formation of 

 a channel of sea-water between the reef and the shore 

 (Fig. 21, second stage). Finally, let the submergence of 



ENCIRCLING Rer 



PL.ANJ 



FIG. 21. Second Stage. 



the island continue till it is completely swallowed up by 

 the sea, not a vestige of its summit remaining to mark 

 its place ; the upward growth of the corals, constantly 

 proceeding, will bring them once more to the level of 

 low tides, and the result will be the formation of a ring- 

 shaped reef surrounding a central lagoon, or, in other 

 words, of an atoll (Fig. 22, third stage). 



If this hypothetical scheme of the progress of events 

 correspond to the facts, we may expect to find its various 

 stages still represented among the numerous islands of 

 the Pacific ; and this, as Darwin endeavoured to show 



