Growth^ and Habits of Coral Zoophytes. 43 



diameter, is alive only to a depth of a line or two, the grow- 

 ing polyps of the surface having progressively died at their 

 lower or inner extremity as they increased outward. 



The large domes of Astrseas, which have been stated to 

 attain sometimes a diameter of ten or twenty feet, and are 

 alive over the whole surface, owing to a symmetrical and un- 

 limited mode of budding, are nothing but lifeless coral through- 

 out the interior. Could the living portion be separated, it 

 would form a hemispherical shell of polyps, in most species 

 about half an inch thick. In some Porites of the same size, 

 the whole mass is lifeless, excepting the exterior for a sixth 

 of an inch in depth. 



With such a mode of increase, there is no necessary limit 

 to the growth of zoophytes. The rising column may grow 

 upward, until it nears the surface of the sea, when death 

 ensues simply from exposure, and not from any failure in its 

 powers of life. The huge domes may enlarge till the same 

 exposure just mentioned causes the death of the summit, and 

 leaves only the sides to grow, which may increase indefin- 

 itely. Moreover, it is evident that, if the land supporting 

 the growing coral were very gradually sinking, the upward 

 increase of the coral might still be without limit. 



There is hence sufficient means provided for the production 

 of coral material for islands, however numerous. These humble 

 ministers of creative power might, without other attributes 

 than those they now possess, have laid the foundations of 

 continents, and covered them with mountain ranges. This 

 remark requires no limitation if we allow the requisite time, 

 and connect with the power of growth such other agencies, 

 soon to be explained, as have been at work in the Pacific 

 since tlie reefs were there in progress. 



The death of the polyps about the base of a coral tree 

 would expose it seemingly to immediate wear from the waters 

 around it, and especially as the texture is usually porous. 

 But nature is not without an expedient to prevent a catas- 

 trophe that would be destructive to a large part of growing 

 zoophytes, and would prevent the indefinite increase just 

 explained. The dead surface becomes the resting place of 

 numberless small incrusting species of corals, besides Nulli- 



