52 ON CORAL REEFS AND ISLANDS. 



size, were it not for a peculiar mode of growth which character- 

 izes most coral zoophytes. 



Life and death are here in concurrent or parallel progress, a 

 condition favored by the existence of coral secretions. In some 

 instances, a simple polyp, while growing at top arid constantly 

 lengthening itself upward, is dying at its lower extremity, leav- 

 ing the base of the coral bare, and destitute of any living tissues. 

 The polyp thus continues rising in height, and death progresses 

 below at the same rate, till at last the live polyp may be at the 

 extremity of a coral stem many times its own length. This 

 process is illustrated by figures on pages 62 and 78 of the Report 

 on Zoophytes. 



In species which bud and form large groups, the same opera- 

 tion takes place. In some instances the summit polyp or polyps 

 bud and grow, while at a certain distance below the summit, the 

 work of death is going on and polyps are gradually disappearing. 

 There is thus a certain interval of life, the length of which in- 

 terval is different for different species. There are zoophytes 

 which grow to a height of several feet, and still only the upper 

 one or two inches are living. The recent polyps at the top of 

 the column are active with life and vigorous in reproduction, 

 while the more aged below, having reached the fixed limits of 

 their existence, are disappearing. The enduring coral remains, 

 and constitutes the basement or stage of action for future genera- 

 tions of polyps. 



But this death is not in progress alone at the base of the col- 

 umn or branch. Generally the whole interior of a corallum is 

 dead, a result of the same process with that just explained. Thus, 

 a Madrepora, although the branch may be an inch in diameter, is 

 alive only to a depth of a line or two, the growing polyps of the 

 surface having progressively died at their lower or inner extrem- 

 ity as they increased outward. 



The large domes of Astraeas, 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 unlimited mode 

 of budding, are nothing but lifeless coral throughout the interior. 

 Could the living portion be separated, it would form a hemis- 

 pherical shell of polyps, in most species about half an inch thick. 

 In some Porites of the same size, the whole mass is lifeless, ex- 

 cepting 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 up- 

 ward, 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 indefinitely. Moreover, it is 



