SOME ANIMAL ARCHITECTS. 139- 



coasts of South America and Africa is due to the lower tem- 

 perature which prevails in these areas, but it is possible that 

 other causes to be hereafter noted less dependent on 

 temperature or on the sea itself, may more feasibly explain 

 the non-development of coral life in certain regions. 



The condition included under the head of depth is, if 

 anything, a more important item in the maintenance of coral 

 life and growth than that of heat. If we cite evidence on 

 this point, we may ascertain that the subject of the depth at 

 which corals live received attention from more than one 

 naturalist in past days. The French explorers Quoy and 

 Gaimard, in their report of observations published in 1824, 

 were probably the first who ventilated the opinion that the 

 living reef-building corals existed in limited depths of sea. 

 Foster and the earlier navigators assumed that, as coral-reefs 

 were found in depths of literally unfathomable kind, the 

 coral-polypes grew from the abysses of ocean. But Quoy and 

 Gaimard concluded, from observations made in two voyages, 

 that a depth of from thirty to thirty-six feet represented 

 the zone of coral life. Ehrenberg set the limit from which 

 living coral was fished at six fathoms, and Mr. Stutchbury, 

 another observer, maintained that a depth of sixteen or 

 seventeen fathoms might be regarded as the furthest limit of 

 the living reef-forming corals. Mr. Darwin concludes " that 

 in ordinary cases, reef-building polypifers do not flourish at 

 greater depths than between twenty and thirty fathoms, and 

 rarely at above fifteen fathoms." And Mr. Dana remarks 

 that " there is hence little room to doubt that twenty fathoms 

 may be received as the ordinary limit in depth of reef-corals 

 in the tropics." In answer to a suggestion that " reefs may 

 possibly rise from very great depths through the means of 

 small corals first making a platform for the growth of the 

 stronger kinds," Mr. Darwin says, " this, however, is an 

 arbitrary supposition ; it is not always remembered that in 

 such cases there is an antagonistic power at work, namely,, 

 the decay of organic bodies when not protected by a cover- 

 ing of sediment or by their own rapid growth. We have," 



