90 LECTURE VII. 



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 high- 

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

 of food, are thus constantly brought within the sphere of the ten- 

 tacula of the hungry polypes. Their reproductive gemmules are 

 rapidly and extensively dispersed amongst the crevices of the cal- 

 careous 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 becomes the seat of attachment of the young of the neigh- 

 bouring corals, the successive generations of which, by the rapid 

 growth and develoj^ment of their calcareous skeleton, 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 Sumatra or Java, 600 

 miles distant. Turtles likewise 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 length 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 sup- 

 plies himself with that necessary of life fresh water ? This is ob- 

 tained in a very simple and unexpected manner from 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 fabricated by the action of the feeble, 

 gelatinous polypes, and a wild and almost boundless waste of waters 

 is enlivened by oases which navigators have described as earthly pa- 

 radises. 



A Barrier Reef {fig. 49. r% r) is essentially similar to the Atoll or 

 Coral-island. It runs parallel with the shores of some larger island 

 or continent ; separated, however, from the land;, by a broad and 

 deep lagoon channel (ii, ?i), and having the outer side as deep and 



