194 ACTINOZOA. 



Atolls and Barrier-reefs attest that subsidence has 

 taken place. 



That such slow subsidence does occur over 

 many parts of the extensive area occupied by 

 Coral-reefs is proved by a number of considera- 

 tions ; some of which were forcibly suggested by 

 Sir Charles Lyell, several years before Mr. Darwin's 

 theory was promulgated. 



Observations and experiments made with a 

 view to ascertain the rate of growth of the reef- 

 building Corals have not hitherto yielded a suffi- 

 cient number of accurately recorded results. 

 That, in certain instances, their growth is rapid, 

 varying, however, mth the species, may be re- 

 garded as proven ; but it is also far from impro- 

 bable that there are many species which have the 

 same average rate of increase. It may likewise 

 be conceded that the growth of the same species 

 varies at different periods and under difterent 

 external conditions. In future investigations on 

 this subject the particular form of the corallum, 

 and its mode of fission or budding, should in each 

 case receive attention. For it is obvious that the 

 same amount of calcareous deposit, if appropriated 

 respectively by a massive and by a dendroid 

 species, would give rise to apparently dissimilar 

 quantitative products. 



The Coral-polypes are, however, powerless to 

 raise their structures higher than the line of low- 

 water. To effect this, various agencies, but chiefly 

 the forces of wind and ocean, acting upon masses 

 detached from the reef and other marine debris, 

 are brought into play. But the mode of operation 

 of these agencies, the general theory of elevation 

 and subsidence, the conversion of the irregular 



