CORAL-ISLANDS. 123 



groups of islands, or form immense rings or annular break- 

 waters round an interior lake originally occupied by land, which 

 the gradual subsidence of the bottom has long since whelmed 

 under the waters. 



As living coral-reefs do not grow above low-water mark, it 

 may well be asked how habitable islands, of which there are a 

 large number dispersed over the Pacific and Indian seas, can 

 form upon their crests. The breakers are here the agents of 

 construction. They rend fragments and blocks from the outer 

 border of the reef, and throw them upon the surface, corals 

 and shells being pulverised by their crushing grinding power, 

 and gradually consolidated into a compact mass. In this 

 manner the pile rises higher and higher, till at last even the 

 spring tides can no longer wash over it into the tranquillagoon 

 on the border of which the fine coral-sand accumulates undis- 

 turbed. The seeds which the ocean-currents carry with them 

 from distant continents find here a congenial soil, and begin to 

 deck the white chalk with an emerald carpet. Trees drifting 

 from the primeval forest, where they have been uprooted by the 

 swelling of the river on whose banks they grew, are also con- 

 veyed by the same agency to the coral shore, and bring along 

 with them small animals insects or lizards as its first in- 

 habitants. 



Before the screw-pine raises its streaming tufts, or the stately 

 palm waves its broad feathery fronds, sea-birds assemble on 

 this new resting-place, and land-birds, driven by storms from 

 their usual haunts, enjoy the shade of the rising shrubbery. 

 At last, after vegetation has completed its work, man appears 

 on the scene, and calls himself the sovereign of this little 

 world. 



Thus these wonderful coral-islands, with their plants and 

 animals, are the product of numerous agencies acting indepen- 

 dently of each other, and yet all directed to one common end. 

 The peculiar organisation of the reef-building polyps, the heav- 

 ing force of the breakers, the ocean-currents conveying seeds 

 and germs from vast distances over the surface of the sea, the 

 peculiar formation of the fruits of the cocoa and the screw- 

 pine, which enables them to remain steeped for a long time in 

 salt water and to perform immense sea voyages without losing 

 their germinating power, all were necessarv either to raise 



