578 DR. G. H. FOWLER OK THE [June 21, 



sense of the word ; but it is not a closed sea like the Mediterranean 

 or Gulf of California, in which high temperatures are maintained to 

 such a depth thafrthere is practically no thermal limit to the descent 

 of a surface organism. It is a closed sea on one side only, open to 

 the Arctic Ocean on the North-east, with the isothermobath of 

 35 F. at about 250 fathoms, and in many places with a tempe- 

 rature of 30 F. at 500 fathoms. One is far from land nowhere in 

 the Faeroe Channel ; the single station of 1897 (Sta. 20) being only 

 about a hundred miles from Cape Wrath, but far enough to be 

 beyond the range of continental influence, in a case where the 

 continental slope (100 to 500 fathoms) is steep, and no rivers 

 discharge into the sea. The water at these depths is directly 

 derived from the open Arctic ocean, and is practically unaffected 

 by continental influence. 



I would urge therefore, as against Prof. Agassiz, that planktonic 

 animals can and do flourish at greater depths than 200 fathoms, 

 even under oceanic and not neritic conditions : that they apparently 

 flourish in utter darkness, at a temperature of 30 to 32 P., and at 

 a depth of at least 500 to 400 fathoms. 



The animals captured in the mid-water appear to fall into at least 

 five categories : (1) Organisms which range indifferently over all 

 depths (eurybathic) ; of these, at any rate so far as the Faeroe 

 Channel is concerned, Galanus finmarchicus may be taken as an 

 example (p. 544 ante) : (2) those which live habitually at great 

 depths, and rarely or never appear at the surface, if at all, generally 

 at night ; of these characteristically mesoplanktonic animals, the 

 Tuscarorida of the ' Challenger' Expedition, the deep-sea Schizopoda 

 of Prof. Chun, Sagitta whartoni and Conchoecia maxima of the 

 1 Research ' collections 1 may be cited : (3) those which spend their 

 earlier life at or near the surface, but of which adults are almost 

 or quite confined to deep water, such as Nyctiphanes norveyica : 



(4) those which when adult inhabit the surface, but spend their 

 larval life at considerable depths, such as Chun's Ctenophora : 



(5) the corpses of any of the foregoing classes, and of purely epi- 

 plunktonic animals, such as Temora longicornis (p. 546, table, ante). 



With regard to this latter class, it will no doubt be urged by some 

 naturalists that the capture of organisms in the Mesoplankton 

 points, not necessarily to the fact of their living at great depths, 

 but to their having been killed at the surface by unfavourable 

 physical conditions and their subsequently sinking through the 

 deeper strata towards the bottom. In many cases this is no doubt 

 the true explanation of their presence in deep water : I have sug- 

 gested this as the explanation of a particular haul of Doliolum (p. 583 

 infra), and of the presence of six species of Copepoda (pp. 548-9, 

 supra) in the ' Research ' collections from the Mesoplankton. 



(1) In cases where numerous observations on successive days in 

 the same district show numerous specimens of a species in the 

 upper strata, but only a few specimens are rarely, not constantly, 

 taken in the lower zones, this explanation probably holds good, 

 especially in a Frontier district (p. 545) such as the Faeroe 

 1 Proc. Zool. Soc. 1896, p. 992 ; 1897, p. 523. 



