ii THE VOYAGE OF H.M.S. CHALLENGER. 



One of the instructions given to me by the late Sir Wyville Thomson was, to include 

 in my Report not only the species collected by the Challenger, but also those which from 

 other sources are known to inhabit the deep sea. He agreed with me in the conclusion 1 

 arrived at from the materials then available, that a depth of 300 or 350 fathoms should be 

 considered the boundary between the surface and deep-sea fishes, the fishes above that level 

 belonwiuo- principally to littoral genera, whilst those specially organized for bathybial 

 life appeared at or below that depth. Accordingly, almost all fishes captured by the 

 Challeno-er at a less depth than 350 fathoms were included in my Report on the Shore 

 Fishes. 



However, the subsequent Norwegian and North American explorations brought to 

 light instances of fishes with an unmistakably bathybial organisation occurring at a much 

 shallower depth than the forms discovered by the Challenger, or, on the other hand, 

 showed that ceitain littoral forms descend not only to 100, but even to beyond 300 

 fathoms. 



It consequently seemed advisable to abandon the intention of limiting this Report to 

 fishes occurring below 300 fathoms, and to adopt instead the 100-fathom line as the 

 boundary at which, with the extinction of sun-light, the bathybial fauna commences, 

 sporadically at first and largely mixed with surface forms. This line does not express 

 a sharply defined boundary any more than any other depth, but it is chosen for the 

 purposes of the present Report, in which a certain upper limit of the deep-sea fauna 

 had to be fixed. In employing it I intend only to express the fact, that no fish not 

 known at present to have occun-ed beyond the 100-fathom hne, is admitted in the 

 present Report ; and, further, that no truly bathybial fish is known to live habitually 

 above that line. 



