103 



than from 50 to 100 fathoms deep. The shallow vertical range 

 of the reef-building Polyps has been already explained. Certain 

 species are, however, chiefly found in particular parts of the reef 

 itself, Astrceidoe and Seriatoporidce choosing its more submerged 

 portions below the outer exposed edge, upon which Porites and 

 its allies flourish. On the surface of the reef, Astrceidoe and 

 Fungidm may readily be distinguished, and among these the 

 globtilar masses of the Brainstone Coral, Meand/rina, are often 

 conspicuous. 



The reef-building Actinozoa have Hkewise a limited range 

 in space, and certain specific forms appear to characterise the 

 various shores and oceanic regions in which they are found. 

 The existence of natural barriers, whether of land or deep water, 

 of thermal or frigid currents, exercise a marked influence on 

 their distribution; hence we find that the species hving in the 

 West Indies, and in the seas of the Antilles, are special to 

 those regions, and none of them are identical with the forms 

 existing in the East Indies and in the Pacific, the central region 

 of that ocean having likewise been found to possess its own 

 species. Although these oceans appear to possess similar 

 physical conditions of depth and temperature, still we find 

 specific Polyp forms limited to certain areal regions of the same, 

 and obeying the great law of their being — ''hitherto shalt thou 

 come, but no farther." For of 306 species collected in the East 

 Indies and Pacific, 27 only are common to the two oceans, and 

 there are none between the Great Ocean and the Atlantic. 



The proximity of volcanic land, owing to the Hme generally 

 evolved from it, has been thought to be favourable to the growth 

 of Coral reefs; but Daewin* has well remarked, "there is not 

 much foundation for this view, for nowhere are Coral reefs 

 more extensive than on the shores of New Caledonia, and of 

 North-eastern Austraha, which consist of primary formations ; 

 and in the largest group of Atolls, namely Moldiva, Chagos, 

 Marshall, Gilbert, and the Low Archipelagoes, there is no volcanic 

 or other kind of rock, excepting that formed of Coral." 



• On Coral Keefs, p. 61. 



