Prof, T. Fuclis on the Deep-sea Fauna. 5 



the remarkable deep sea corals from the Philippines described 



by Semper were obtained by him from a depth of about 40 



fathoms. 



In this connexion the numerous lists of local coral-faunas 

 which Studer gives, founded on the rich coral-material col- 

 lected by the l Gazelle, 1 are very interesting*. Whenever 

 he cites a locality which is below 40 fathoms, we may bt 

 sure that the coral-fauna bears the character of the deep-sea 

 Corals. 



. This long series of facts from different seas therefore indi- 



f cates very accordantly a depth of about 50 fathoms as that 



critical zone in which is situated the great turning-point that 

 separates the littoral from the deep-sea fauna; and we are 



therefore justified in regarding the line of 50 fathoms as an 

 ideal boundary between the littoral and the deep-sea fauna. 



It is very interesting to observe that this depth is pretty nearly 

 Hie same in all seas. Between the tropics, however, the sepa- 

 ration of the littoral and deep-sea faunas, on the principle 

 here adopted, seems to be not only ideal, but to a certain 

 degree real. Thus, according to the observations at present 

 extant, it would appear that within the tropics, below a depth 

 of 30 fathoms, there follows an extremely sterile region with 

 few animals, and that a more abundant fauna recurs only in 

 j proportion as, with increasing depth, true deep-sea animals 



begin to make their appearance in great variety at 80 and 90 

 fathoms. Conseqaently within the tropics the littoral fauna 

 would be separated from the deep-sea fauna by a compara- 

 tively sterile region, extending about from 30 to 90 fathoms. 



In the temperate and cold seas such an intermediate zone 

 is unknown. Here, on the contrary, the two faunas inter- 

 mingle very plentifully at their boundary-line, and thus 

 produce a very great abundance of animals precisely in this 



critical boundary region. 



In this way it is explained why, as Xordenskiold has again 

 quite recently indicated, there is in temperate latitudes a much 

 greater abundance of animals at a depth of 40, 50, and 60 

 fathoms than the tropical seas present at the same deptii. 



If, then, founding upon the preceding statements, we regard 

 the depth of 50 fathoms as the boundary between the littoral 

 and deep-sea faunas, the question next arises, by what phy- 

 sical conditions this boundary-line is determined, and what, 

 consequently, is the true conditioning cause of the appearance 



of the deep-sea fauna. 



When the study of the bathymetric distribution of organ- 



* " Verzeiehniss der auf der Weltumseereluug der i Gazelle ' ge.?animel- 

 teu Anthozoen/' Monatsber. Berlin. Akad. 1878, p. 676. 





