68 PHYSIOGNOMY OF PLANTS. 



free and detached polypifers, and those which form wall-like 

 structures and rocks. 



If we are struck with the great accumulation of building 

 polypifers in some regions of the globe, it is not less sur- 

 prising to remark the entire absence of their structures in 

 other and often nearly adjoining regions. These differences 

 must be determined by causes which have not yet been tho- 

 roughly investigated ; such as currents, local temperature of 

 the water, and abundance or deficiency of appropriate food. 

 That certain thin-branched corals, with less deposit of lime 

 on the side opposite to the opening of the mouth, prefer the 

 repose of the interior of the lagoon, is not to be denied , 

 but this preference for the unagitated water must not, as 

 has too often been done (Annales des Sciences Naturelles, 

 1825, T. vi. p. 277), be regarded as a property belonging 

 to the entire class. According to Ehrenberg's experience 

 in the Red Sea, that of Chamisso in the Atolls of the Mar- 

 shall Islands east of the Caroline group, the observations of 

 Captain Bird Allen in the West Indies, and those of Capt. 

 Moresby in the Maldives, living Madrepores, Millepores, 

 and species of Astraea and of Meandrina, can support the 

 most violent action of the waves, -" a tremendous surf," 

 (Darwin, Coral Reefs, pp. 63-65), and even appear to 

 prefer the most stormy exposure. The living organic 

 forces or powers regulating the cellular structure, which 

 with age acquires the hardness of rock, resist with wonder- 

 ful success the mechanical forces acting in the shock of the 

 agitated water. 



In the Pacific, the Galapagos Islands, and the whole 



