140 LECTURE VII. 



calcareous skeletons are often cemented over with a layer of the 

 marine vegetable called Nullipora, which can better bear exposure 

 to the air. 



This strong barrier is well fitted to receive the first shock of the 

 heavy waves of the fathomless ocean without ; and, what at first 

 appears surprising, instead of wearing away at its outer edge, it is 

 here only that the solid reef increases. The coral animals thrive 

 best in the surf occasioned by the breakers. Through this agitation 

 an ever-changing and aerated body of sea water washes over their 

 surface, and their imperfect respiration is maintained at the highest 

 state of activity. Abundant animalcules, and the like objects of 

 food, are thus constantly brought within the sphere of the tentacula 

 of the hungry polypes. Their reproductive gemmules are rapidly 

 and extensively dispersed amongst the crevices of the calcareous mass. 

 By the force of unusual storms this outer reef is occasionally 

 breached, and huge masses are torn off" and driven towards the lagoon, 

 where they form an inner barrier or reef. The broken surface be- 

 comes the seat of attachment of the young of the neighbouring corals, 

 the successive generations of which, by the rapid growth and de- 

 velopment of their calcareous skeletons, soon repair the damage of 

 the storm. The masses of broken coral thus driven inward towards 

 the lagoon, accumulate in time to the height of some feet above high 

 water. These fragments are mixed with sand and shells, and form a 

 favourable soil for the development and growth of vegetables, as 

 cocoa palms, the large nuts of which may be borne hither by currents 

 of the ocean, from Sumatx*a or Java, 600 miles distant. Turtles like- 

 wise float to the nascent island, browse on the sea weeds which grow 

 in the lagoon, and breed there. Numerous species of fish and shell- 

 fish flourish in the same still water, which abounds with animal life. 

 Man comes at last and takes possession of the island ; and the cocoa- 

 nut, the turtle, and the fish afford him abundant and wholesome food. 

 But you will ask how he supplies himself with that necessary of 

 life, fresh water ? This is obtained in a very simple and unexpected 

 manner fi-om shallow wells, dug in the calcareous sand, which ebb 

 and flow with the tides, yet are almost wholly free from the saline 

 particles of the ocean. Some have supposed that the sea water lost 

 its peculiar salts by infiltration through the calcareous mass. Mr. 

 Darwin thinks that it is derived from the rain water, which, being 

 specifically lighter than the salt, keeps floating on its surface, and is 

 subject to the same movements : howsoever this may be, the fact is 

 certain. A fit and convenient abode for the human species is fabri- 

 cated by the action of the feeble, gelatinous polypes, and a wild and 

 almost boundless waste of waters is enlivened by oases which navi- 

 gators have descx'ibed as earthly paradises. 



