XXxiv THE VOYAGE OF H.il.S. CHALLENGER. 



approach of the dredge than bottom fishes, which are generally of a more sluggish 

 nature, and which, seeking safety by burying themselves in the ooze, would be 

 readily enclosed in the dredge. 



So far as the observations go at present, no distinct bathymetrical zones, characterised 

 by peculiar forms, can be defined. As the transition fi'om the organisation of the t}^ical 

 surface fish to that of the most highly specialised deep-sea form is gradual, so the passage 

 from the littoral to the abyssal fauna is continuous and not marked by a sudden change. 

 The limits between the two faunae are still more obscm-ed by the remarkable manner in 

 which they overlap each other owing to the wide bathymetrical range of certain species 

 of either fixuna. I need only mention such common littoral forms as Lo2:)Jiius j^iscatorius, 

 Merluccius vulgaris, and Pleuronectes cynocjlossus, which descend to depths of from 

 300 to 700 fathoms, thus living in the same areas which arc inhabited by the most 

 highly specialised abyssal forms like Chiasmodus, Trachyptcrus, and Alepocephaliis. 

 Also the temperature of the water clearly interferes with a uniform bathymetrical dis- 

 tribution of fishes, as many species, which in low latitudes are found at considerable depths, 

 often ascend into the cold surface strata of high latitudes.^ Further, nocturnal pelagic 

 surface fishes seek, during the day-time, the darkness of greater depths. The instances of 

 the wide bathymetrical range of deep-sea species ai-e numerous : some may range fi-om 

 a depth of some 300 fathoms down to one of 2000 fathoms ; or, in other words, a fish 

 which has once attained in its organisation to that modification b)^ which it is enabled to 

 exist under the pressure of half a ton, can easily accommodate itself to one of two tons or 

 more. 



But if there are serious obstacles to a division of the deep-sea fish-fauna into vertical 

 zones characterised by the presence of peculiar forms, there are less objections to an 

 attempt to show to what limits the families of fishes descend which are represented in 

 the deep-sea by surface-forms or their modifications. As far as our present knowledge 

 goes, we find — 



1. That at a depth of 400 fathoms the Gobiidaj, Trachinidae (with the exception of 

 Bathydraco), Blenniidae, Percida;, Scorpa5nida;, Trichiuridaj, Cyttidaj, Cataphracti, 

 Batliytliri.ssidae cease to exist. 



2. At a depth of 500 fathoms two important types, viz., the Sharks and Rays, and 

 the Flat-fishes cease to flourish, only one isolated example of each descending beyond that 

 limit. 



3. At 700 fathoms several other families, which are characteristic forms of the littoral 

 fauna of the cold and temperate zones disappear, viz., the Cottidaj, Discoboli, and 

 Lycodidaj (with one exception); there is also no evidence of the Trachypterida.^ and 

 Myxine extending beyond this depth. 



4 The depth of 1200 fathoms seems to be the limit of the Holocephali. 



1 This fact was first expressed in Introduction to the Study of Fishes, p. 30.5. 



