118 CORALS AND CORAL ISLANDS. 



harbor, on the same island, there was no coral, with a depth 

 of fifteen fathoms. 



Similar results were obtained about all the islands surveyed, 

 as the charts satisfactorily show. 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. 



It may however be much less, possibly not over half this, on 

 the colder border of the coral-reef seas, as, for example, at the 

 Hawaian Islands and the atolls northwest of that group. It 

 is natural that regions so little favorable for corals on account 

 of the temperature should differ in this respect from those in 

 the warmer tropics. 



It may be here remarked, that soundings with reference to 

 this subject are liable to be incorrectly reported, by persons 

 who have not particularly studied living zoophytes. It is of 

 the utmost importance, in order that an observation supposed 

 to prove the occurrence of living coral should be of any value, 

 that fragments should be brought up for examination, in order 

 that it may be unequivocally determined whether the corals 

 are living or not. Dead corals may make impressions on a 

 lead as perfectly as living ones. 



As to the origin of this small range in depth — about 120 

 feet — temperature must be admitted as one cause, it having 

 been proved to be predominant with regard to distribution of 

 life throughout the extent and depths of the ocean. Yet it can 

 hardly in this case be the only cause. The range of tempera- 

 ture 85° to 74° gives sufficient heat for the development of the 

 greater part of coral-reef species ; and yet the temperature at 

 the 100 foot plane in the middle Pacific is mostly above 74.°. 



