DEEP-SEA FISHES. 297 



the fish seized, nearly as large or strong as the attacking fisli, 

 carries the latter out of its depth into a higher stratum, 

 where the diminished pressure causes such an expansion of 

 gases as to make the destroyer with its victim rise with in- 

 creasing rapidity towards the surface, which they reach dead 

 or in a dying condition. Specimens in this condition are not 

 rarely picked up ; and as, of course, comparatively few can 

 by accident fall into the hands of naturalists, occurrences of 

 the kind related must happen very often. 



Thus, the existence of fishes peculiarly adapted for the 

 deep sea has been a fact maintained and admitted for some 

 time in Ichthyology ; and as the same genera and species were 

 found at very distant parts of the ocean, it was further stated 

 that those Deep-sea fishes were not limited in their range, and 

 that, consequently, the physical conditions of the depths of the 

 ocean must be the same or nearly the same over the whole 

 globe. That Deep-sea fishes were not of a peculiar order, but 

 chiefly modified forms of surface types, was another conclusion 

 arrived at from the sporadic evidence collected during the 

 period which preceded systematic deep-sea dredging. 



However, nothing was positively known as to the exact 

 depths inhabited by those fishes until observations were made 

 during the voyage of H.M.S. " Challenger." The results 

 obtained by this expedition afforded a surer and more extended 

 basis for our knowledge of Deep-sea fishes. 



The physical conditions of the deep sea, which must 

 affect the organisation and distribution of fishes, are the 

 following : — 



1. Absence of sunlight. Probably the rays of the sun do 

 not penetrate to, and certainly do not extend beyond, a depth 

 of 200 fathoms, therefore we may consider this to be the 

 depth where the Deep-sea fauna commences. Absence of 

 light is, of necessity, accompanied by modifications of the 

 organs of vision and by simplification of colours. 



