PELAGIC FAUNA. 119 



guishes the zone immediately underlying the twenty fathom line ? 

 This scarcely appears possible. The extended range of a very 

 large proportion of the animal forms entering into the composition 

 of the littoral fauna, and the extreme rarity of instances in which 

 limitation is so marked as to render most effective the difference 

 between light and darkness, argue strongly against the notion of 

 the all- paramount influence of light as affecting distribution. This 

 objection to the views advanced by Professor Puchs, as opposed to 

 the doctrine of thermal limitation, is further strengthened by the 

 fact which has been noted in the case of many animal groups («. g., 

 the Brachiopoda) that the vertical range of surface forms is on an 

 average greater in the boreal and hyperboreal regions — i: e., where 

 the temperature of the. water is more nearly uniform — than in the 

 more centrally located regions, where a much broader variation in 

 the temperature of the water manifests itself. It is true, as has 

 been urged by Professor Fuchs, that the littoral fauna is largely 

 dependent for its development upon the existence of coral reefs 

 and coast-binding shell-banks ; but in how far this association is 

 connected with the presence or absence of light, still remains to 

 be determined. On the whole, while it may be assumed that we 

 are still largely ignorant of the fundamental facts underlying dis- 

 tribution, it appears more than likely that not a single cause, but 

 a combination of causes, is operative in bringing about the general 

 result. That the deep-sea fauna is a fauna of darkness must be 

 admitted ; but this is so from the necessity of the case rather than 

 a matter of choice resting with the animals composing it. 



A singular correspondence has been noted as existing between 

 the pelagic (surface) oceanic fauna and the fauna of the oceanic 

 bottom (abyssal) — a correspondence that has likewise been attrib- 

 uted by some to a condition of darkness by which the different 

 organisms are supposed to be governed. To what exact degree the 

 members of this animal assemblage are nocturnal in their habits, or 

 constitute true animals of darkness, the observations are not suffi- 

 ciently far advanced to permit of a general conclusion being ar- 

 rived at. 



Pelagic Fauna. — Under the designation "pelagic" may be in- 

 cluded those forms of life which habitually pass their existence on 

 the free expanse of the ocean, and which only on accidental occa- 

 sions, if at all, visit the continental borders, or descend to the floor 



