88 CORALS 



tion of the calcareous substance of which the reefs are com- 

 posed than any other group of corals. They are, however, 

 of comparatively recent origin, as the}^ do not seem to have 

 attained to any degree of importance as reef-builders until 

 the later Tertiary times, when they overtook and replaced 

 to a great extent the Astraeidae and other groups of imper- 

 forate corals in the struggle for existence on the reefs. The 

 cause of this change of dominance may be due partlv to 

 the rapidity of growth and partly to the extraordinary 

 plasticity in form of the Madrepores as compared with other 

 corals. 



The construction of a perforated corallum requiring the 

 secretion of a relatively small amount of calcium carbonate 

 for a given surface of support for the polyps and the pro- 

 vision of an elaborate system of coenosarcal canals, usualh' 

 crowded with active zooxanthellae, are characters which un- 

 doubtedly assist phj-siologically in rapid growth. 



The complex of conditions which renders some coral reefs 

 of the tropical seas more favourable for the growth of 

 Madrepores and others less so is so intricate that it will 

 prove to be a very difficult tangle to unravel. The mean 

 temperature of the water, the violence of the breakers on 

 the shore, the abundance and character of the food supply of 

 microscopic organisms, and the chemical constitution of the 

 sea-water are factors which, in varying combinations, deter- 

 mine whether a particular kind of coral shall predominate 

 on a reef. On one reef may be found an abundance of 

 Madrepores, on another Heliopores and Astraeids, on another 

 little more than a carpet of Lithothamnion or some other 

 form of calcareous Alga ; but taking the reefs of the world 

 as a whole, there can be little doubt that the three genera, 

 Madrepora, Porites, and Montipora, do maintain the premier 

 position in abundance and in luxuriance of growth on the 

 coral reefs of the present day. 



It is in this family also that we find, in a more exaggerated 

 form, perhaps, than in any other, the difticulty of dividing 

 up the genera into specific groups. 



The careful study of a single large colony or of a collection 

 of specimens from the same locality reveals so much variet}' 



