1872.] 



' Shearwater 3 Scientific Researches. 



589 



to Animal life ; and that it is a matter of great Scientific importance, espe- 

 ciallyin relation to Geological inquiry, that these conditions should becarefully 

 examined. The Red Sea would probably be found to present a striking con- 

 trast, in many particulars, both to the Mediterranean and to the open Ocean* 

 Its Thermal condition, as I have already shown, is altogether peculiar ; for 

 while its surface-temperature rises as high as that of any Intertropical portion 

 of the Ocean, that temperature seems to be maintained with very little 

 reduction even to its greatest depths. Now it seems to be the universal 

 opinion of those who have most carefully studied the existing Coral For- 

 mations, that the Reef-building Corals do not live and grow at a greater 

 depth than 20 fathoms ; and as it is affirmed by Mr. Dana, as a deduction 

 from the Distribution of Coral Formations, that the existence of the reef- 

 builders is geographically limited by the isocrymal line of 68° *, I cannot 

 but suspect that the bathymetrical limit may be essentially a thermal one. 

 For all we know of the relation of Temperature to Depth (§§ 3, 8) would 

 indicate that even within the Intertropical area of the open Ocean, the 

 temperature at 20 fathoms may not be above 68° ; and that in the next 10 

 fathoms it suffers considerable reduction. Now if the temperature of the 

 Red Sea nowhere falls below 71°, it is obviously a most interesting question 

 to determine whether the reef-building Corals are, or are not, to be found 

 in that Sea at a greater depth than in the Oceanic area ; and if so, what is 

 the greatest depth at which they there exist. For since our own inquiries 

 show that Stony Corals similar in physiological characters to the reef- 

 building types can live and grow at the depth of many hundred fathoms, 

 there seems to me no a priori reason why the latter should not thrive at 

 like depths, if the Temperature be congenial to them. 



83. The Red Sea further differs essentially from the Mediterranean, in 

 not being the recipient of any great Rivers bringing down detritus from 

 the land. This, of course, will affect the condition of the bottom ; on which 

 we should not expect to find the abundant sedimentary deposit that is every- 

 where settling down on the abyssal depths of the Mediterranean. It will 

 also leave the bottom-water clear ; and in this respect the condition of the 

 bed of the Red Sea will be more favourable to Animal life than that of the 

 Mediterranean. But the absence of organic sediment, if the views pre- 

 viously advanced be correct, will constitute a still more important difference 

 between the conditions of the two Seas, in relation to Animal life : for 

 while its progressive decomposition, in the abyssal water of the Mediterra- 

 nean, consumes its Oxygen and imparts to it Carbonic acid, at a greater 



* On Corals and Coral Formations, p, 108. — It is a very significant fact that the 

 cold current which comes up from the south on the Western coast of South America, 

 and which I regard as the indraught of the Pacific Equatorial Current (as the similar 

 current on the Western coast of South Africa is of the Atlantic Equatorial Current, § 120), 

 pushes the southern Isocryme of 68° — the Coral-Sea boundary — to the north of the 

 Equator, between the South American coast and tho Galapagos, which, though under 

 the Equator, lie outside of that boundary. 



