116 GEOGRAPHICAL DISTRIBUTION. 



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That it is not one governed by conditions of temperature alone, 

 or in principal part, as has very generally been conceived, is made 

 manifest by an examination of the bathymetric distribution which 

 particular animal groups affect. Thus, the reef-building corals, 

 which for their proper development require an average tempera- 

 ture of 70 to 75 Fahr., and a temperature never falling below 68 

 Fahr. , are confined to a superficial zone of twenty fathoms ; yet 

 at most parts of the oceanic surface inhabited by these animals a 

 suitable temperature would be found at depths fully five times as 

 great, and in some quarters even very much greater. Over the 

 tropical Pacific, for example, a temperature of 77 Fahr. prevails to 

 a depth of eighty fathoms, and of 70 Fahr. down to one hundred 

 fathoms, so that, as far as temperature alone is concerned, the 

 coral animal might just as well have found a congenial home in 

 those greater depths as in the shallower one of ten to twenty fath- 

 oms. Indeed, in the Red Sea the coral isotherm would still be 

 found at the very bottom, or in a depth of water of six hundred 

 fathoms ; but here, as elsewhere, the limiting line is found at twenty 

 fathoms. With reference to the vertical distribution of these ani- 

 mals, therefore, the matter of temperature would seem to be but 

 little involved. 



What is true of the corals doubtless applies in considerable 

 part to many other animal groups ; but it must be confessed that 

 our knowledge respecting the thermal conditions necessary for the 

 existence of most marine organisms is so limited that we can hardly 

 premise at the present day upon any safe deduction being based 

 upon it. Professor Fuchs 46 has quite recently emphasised the fact, 

 however, as tending to prove the non-influence of temperature in 

 determining distribution, that over the entire world almost all 

 the important types of the deep-sea fauna are already represented 

 at the comparatively insignificant depth of ninety to one hundred 

 fathoms, and consequently inhabit a zone the extremes of whose av- 

 erage temperature may be separated by fully thirty to forty degrees. 

 Thus, it is pointed out that on the Pourtales Plateau, off the coast 

 of Florida, which begins at ninety fathoms, and descends to three 

 hundred fathoms without showing any essential modification in its 

 inhabiting fauna, deep-sea forms are very plentiful, especially corals, 

 siliceous sponges, and echinoderms ; and the same is the case with 

 the famous Barbadoes grounds. A well-marked deep-sea fauna has 



