REPORT ON THE DEEP-SEA FISHES. XXXV 



5. Beyond 2000 fatlioms neither Gadidse nor Salmonidse nor Notacantlii have been 

 found. 



6. The families which descend to the greatest depth at which fishes have been 

 obtained, viz., 2900 fathoms, are Berycidse, Pediculati, Ophidiidse, Macruridse, Sternopty- 

 chidge, Scopehdae, Stomiatidte, Murseuidse ; also two fomilies which have no representa- 

 tives in the surface-fauna, viz., the Alepocephalidse and Halosauridje, extend to the same 

 enormous depth ; and there is no reason why all those deep-sea forms, wliich are known 

 to live at a depth of nearly 3000 fathoms, should not occur in the greatest abyss of the 

 ocean. 



The abundance and variety of fish life decreases with the depth, as is obvious from 

 the following table, which expresses our present knowledge of the subject. There have 

 been found 



between 100- 300 fathoms 232 species. 



300- 500 „ 142 



500- 700 „ 76 



700-1500 „ 56 



1500-2000 ,, 24 „ 



2000-2900 „ 23 



Of those found between 100 and 300 fathoms 108 also occur above the 100-fathoms 

 limit. 



This decrease in the number of species, as shown in this table, is no doubt partly 

 owing to the difficulty of capturing fishes at great depths, a difficulty which increases in 

 proportion to the depth at which the dredge is worked. But it must also be regarded as 

 evidence of the actually diminished variety of fishes. Fishes may Ije, and no doubt are, 

 locally abundant at the bottom of the ocean ; but this abundance is probably one of 

 individuals rather than of distinct forms. We may safely assume, that as the majority 

 of deep-sea fishes are modifications and derivatives of surface forms, they are fewer in 

 number than the latter, especially when we consider that the physical conditions of the 

 abyss are of a very uniform character, and therefore cannot have given rise to the 

 development of numerous specific and generic forms. 



This uniformity of the physical character of the lowermost strata of the ocean is also 

 the cause of the almost unlimited horizontal distribution of deep-sea fishes. Pelagic 

 surface-fishes have already a wide range, but are more or less influenced in their 

 distribution by climatic conditions. Deep-sea fishes are no longer subject to this cause 

 of limitation when they have reached a deptli of 500 fathoms, where the temperature of 

 the water is as low as 40° F., and perfectly independent of that of the surface water. 

 Therefore, the instances already known of the same genera, and even of the same 

 species occurring in the depths of the eastern and western, northern and southern 



