62 A NATURALIST ON DESERT ISLANDS. 



these animals secrete all this limestone in much 

 the same way that a bee manufactures its comb ; that 

 unconsciously they are "working" to produce tons of 

 calcium carbonate which will one day go to form the 

 foundations of a tropical island ; that countless millions 

 of generations fall fainting by the way and are trodden 

 underfoot, pushed do^vn and built upon, by as many 

 more living millions of generations which carry on, for 

 ever upwards, the colossal fabric. 



How grievously the processes of coral growth and coral 

 life generally, in addition to the processes of formation of 

 coral atolls or islands, have been thus misconceived, it 

 would take too long to indicate here. Suffice it to say, 

 that if it had entirely depended on the efforts of the coral 

 polypes, alone and unaided, no coral island would ever 

 have been possible. 



It is the sea-waves, which by smashing, pounding, and 

 pulverising their colonies to pieces and then piling up the 

 disintegrated fragments one on top of the other, put the 

 crowning finish to a process, which commences (as in the 

 case of Swan Island itself) by the elevation of submarine 

 eminences to a point within a comparatively short distance 

 from the surface. Reef -building corals cannot grow in 

 deep water because of the sediment which would fall upon 

 them in such a position. It is, therefore, only when they 

 have a foundation to grow on which is swept by currents 

 of clear water, and so is above the level where sedimenta- 

 tion can take place, that their growth is possible. If we 

 eliminate wave-action, it is obvious that the corals 

 would simply grow to a uniform level immediately 

 beneath the surface, and there all growth would 

 cease. 



Among corals a rough division can be made between 

 those which secrete calcium carbonate or coral, and are 

 thus chiefly instrumental in the formation of reefs and 

 islands ; and those in which the secretion formed consists 

 of some horny material, plus a few spicules of limestone, 

 which play but a very inconspicuous part in reef pro- 



