124 Association of American Geologists and Naturalists. 



s 



as great as that given by Quoy and Gaimard as the result of their in- 

 vestigations, which is twenty-five to thirty feet for the Astrseas, the on- 

 ly genus forming a solid surface of any considerable extent. More- 

 over, since our arrival in these seas, I have repeatedly observed this class 

 of corals in the greatest abundance on the outer plateaux, at a depth of 

 from seven to ten fathoms. How far below the estimated temperature 

 here obtained, the Polypes continue to build, can only be determined 

 by many experiments of a more accurate nature than any I have been 

 able to make. Probably (judging from the temperature at the islands 

 already visited) they thrive best between 80° and 76° or 77°, and grad- 

 ually decrease as the water falls below this last. On some of the Pau- 

 motus, I saw branching corals growing on the surface reef where the 

 water was as hot as 85° or 86°, and not more than eighteen inches deep, 

 with here and there among them a few thin encrusting ones ; but they 

 had not the vigorous healthy look of those on the margin within wash 

 of the surf, where the temperature was not over 78°. The Astneas 

 especially, seem in such exposed places, washed by the cool breakers, 

 to find their most congenial climate." 



By these extracts it will be seen that while I had nearly fixed in my 

 own mind upon an upward limiting temperature, and suspected that we 

 should find the corals gradually decreasing below the temperatures of 

 76° or 77°, I made no pretense of determining their downward limit. 

 I think I may venture to affirm that had there been opportunity, I should 

 have established this point, but such it was not my good fortune to enjoy. 



The two theories or principles therefore which I claim as originating 

 from my own observation and reflection, entirely independent of any 

 communications or suggestions from other sources, are these, expressed 

 in my article in the Boston Journal of Natural History :* — " It is my 

 belief, that (to a certain extent) the corals are limited in their range of 

 growth by temperature, rather than depth, and that wherever this is not 

 below 76° Fahr., there, ceteris paribus, they will be found to flourish 

 as in the Polynesian seas" — or, as the idea is perhaps more clearly ex- 

 pressed in my remarks on Vincennes Island, U I am convinced of one 

 thing, that this matter of temperature has (to a certain extent) quite as 

 much eflect on the growth of corals as depth — that is, they will groio 

 abundantly in ten or twelve fathoms as here, if the temperature is the 

 same, say 77° or thereabouts to 82° or 83° Fahr ., token if it teas below 

 that, even at only half the depth, they tvould not flourish so well" 

 The second principle which I claim as original with myself, is a natural 

 deduction from the first. " I am persuaded after a careful examination 

 of the facts, that the absence of coral on the other side of the continent, 



Vol. IV, p. 76. 



