COKAL EEEFS. 379 



of growth : including such knowledge as we possess regard- 

 ing the species and habits of the polypifers, or coral animals, 

 which have produced them. On the subject of these wonder- 

 ful zoophytes, the details given are not perhaps as distinct as 

 might have been desired for general information. It is Mr. 

 Darwin's main object, in reference to his theory, to determine 

 the rate as to time, and the depth below the surface of the 

 sea, at which the workings of living coral can go on ; and we 

 do not find in his volume any consecutive description of the 

 aspect, species, and habitudes of these active tenants of the 

 deep. It must be admitted, indeed, that our knowledge on 

 these points is very deficient. Exact observations are not 

 easy where the animal works either below the surface of the 

 water, or amidst the heaviest surf and breakers on the edge 

 of the reef. Here, where all besides perishes, the zone of 

 coralline life exists in its greatest activity ; but hardly more 

 accessible to observation than are the dead corals brought 

 up by dredging ; in such variety of species as to leave it 

 uncertain which are the true artificers of the reef, and which 

 contribute to it solely by being agglutinated, with broken 

 coral, sand, shells and other casual materials, into the common 

 mass. Ehrenberg has described more than a hundred species 

 of coral which he found in the reefs of the Eed Sea. Some 

 observers have believed that the genus Astraea is that most' 

 efficient in the formation of coral rocks. But the Madrepore, 

 Millepore, Porites, Meandrina, Caryophyllia, and various 

 other genera contribute largely to these great and mysterious 

 works ; in what proportion, and under what laws or instincts 

 of combination, we shall probably never be able fully to 

 comprehend. 



Other problems equally difficult embarrass this subject; 

 and notably in the first place the question, why coral reefs 

 should be so vast and numerous in certain tracts of ocean, 

 while others are wholly destitute of them ? The limitation 



