BATHYMETRIC RANGE. 233 



were taken in the trawl, the same genus was taken in the vertical haul 

 as in the trawl, — a fact indicating of how little value a "trawl" record 

 is in locating these intermediate organisms ; and a still more important 

 fact connected with these records is that not a single species was taken in 

 hauls from below 300 fathoms, which was not also taken in other hauls 

 between 300 fathoms and the surface, although the majority of the genera 

 of Medusae as yet known to belong to the intermediate fauna were taken 

 during the Expedition, and several of them in considerable abundance. 



These positive results show beyond question that in the Eastern Tropical 

 Pacific the intermediate Medusae are by no means characteristic of abyssal 

 depths, but attain a high degree of development in the comparatively shal- 

 low zone between 300 fathoms and the surface, — a result already men- 

 tioned by the leader of the expedition for the intermediate fauna in general 

 (A. Agassiz, :06). Furthermore, considering how much more important 

 is this considerable body of positive results, which might be enlarged by 

 adding thereto the records of the "Albatross" expedition of 1891 and the 

 shallow hauls of the " Valdivia," " Siboga," and " Research " expeditions, 

 than any amount of negative evidence of the kind afforded by the contents 

 of open nets towed from abyssal depths to the surface at isolated stations, 

 and in view of the uniformity in physical conditions below the upper 50- 

 100 fathoms in all oceans, there is good reason to extend this generalization 

 to all tropical oceanic areas removed from the disturbing influence of neigh- 

 boring coast lines. The question of bathymetric range in such enclosed 

 seas as the Mediterranean is quite a different question, and one outside the 

 scope of the present discussion. 



I do not, however, mean to imply that the intermediate fauna is re- 

 stricted to this zone. On the contrary, while I believe that there is good 

 evidence of the existence of a limit to its upward dispersal, there is appar- 

 ently no such barrier to its downward movement. Therefore intermediate 

 organisms may be expected to occur, though according to my view in much 

 less abundance, even to near the ocean floor in very great depths. 



With regard to the limit to their upward migration the questions of 

 light and temperature are, no doubt, of prime importance, most investigators 

 now agreeing that in the case of such organisms as Medusae pressure cannot 

 be supposed to be an active factor. While but few experiments have been 

 made upon the penetration of light in sea water, it is probably safe to assume 

 that the last of the sun's rays are dissipated at about 500 meters, somewhat 



