214 MARINE ANIMALS 



reef building. Rapid uniform elevation, or interrupted or slow eleva- 

 tion, similarly produce varied results. Every coral reef is an individual 

 formation, the product of a whole series of factors, and is understand- 

 able only from its own special history. The uniformity of the whole 

 process postulated by Darwin is not valid in detail, but Davis 73 in a 

 mature review of the whole problem in 1928 decided that in general 

 Darwin's theory still represents the known facts better than any other 

 proposed to date. To us the original theory appears oversimplified 

 and needs extension to cover the evidence that water levels have 

 varied through other processes than simple submergence. Thus the 

 so-called geological control theory 55 takes cognizance of the Pleistocene 

 ice age during which much water was piled up as ice on the land and 



Fig. 43. — Pontes sp. After Vaughan. 



the ocean level was lowered sufficiently so that submarine platforms 

 could be wave eroded. Such platforms have since been covered by 

 water released from glaciers to about the depth actually supporting 

 coral reefs in certain regions. 74 



The order of the corals in the reef is not uniform. At its upper edge, 

 in the surf-zone, Pontes (Fig. 43) and Millepora are the dominant 

 genera. "These alone seem able to withstand the force of the waves 

 at their outer border, and they form with the nullipores a breakwater 

 which prevents fragmentation by the waves" (Darwin) . Astraea may 

 also grow in shallow water and can be exposed at low tide without 

 injury; it accordingly extends as a rule about 30 cm. above Mean- 

 drina.™ The branched madrepores have a different habit of growth 

 at different depths, more and more loosely branched as the depth 

 increases, often within the same species, as for example in Madrepora 

 muricata with its three forms palmata, prolifera, and cervicornis (Fig. 

 44). M. scherzeriana of the Red Sea varies similarly, as do some 

 Porites. The madrepores are not true rock builders. "The cornerstones 



