Holt and Calderwood — Report on the Rarer Fishes. 365 



also to be met with nearer the Equator, but only in the somewhat deeper regions 

 in these more southern areas. The subsequent experience of ichthyologists has 

 not only confirmed tliis, but has also, as we have seen, established the truth of an 

 exactly converse phenomenon. Either condition renders it somewhat difficult to 

 formulate the relationships of structure to vertical habitat in the case of fish 

 which inhabit or cross the border-land dividing the deep-sea and littoral areas ; 

 but the coincidence of both phenomena throws great obstacles in the way of 

 a satisfactory explanation of either. 



The increase in pressure which accompanies increase in depth of water 

 seems to have impressed Day with the necessity for some special structural 

 provision in the case of physoklistous fishes, and he puts forward a conjecture 

 that the larval physostomous condition may persist throughout life in such 

 examples of a species, normally physoklistous and normally littoral, as are found 

 inhabiting great depths.* As a matter of fact, such provision (which has, 

 moreover, no existence) is quite uncalled for, since we know by analogy that 

 the objection is far from formidable. Thus, to take a common instance, every 

 fisherman knows how readily the air-bladder of the ling {Molva vulgaris) is 

 distended by even such a slight diminution of pressure as is brought about by 

 hauling the fish up from a depth of only thirty fathoms, or thereabouts. Now, 

 the vertical range of the ling is about 0*2 to 150 fathoms ; and while we learn 

 from CoUett that young examples are rarely met with on the Norwegian coast at 

 less than one hundred fathoms, a young fish has been observed by one of us, at 

 St. Andrews, contentedly swimming in a shallow tidal pool. 



It follows, therefore, that however much a sudden alteration of outside 

 pressure may affect a physoklistous fish, the latter is quite capable of adapting 

 itself under natural conditions to a very considerable vertical range. Indeed, had 

 the enormous vertical range of some of the deep-sea fishes been present to Day's 

 mind, he jn'obably would not have ventured upon such a fantastic suggestion. 



M. Vaillant, in the introduction to his discription of the ichthyological results 

 of the voyages of the "Talisman" and " Travailleur," draws attention to the 

 resemblance which exists between the bathybial fish-fauna of the regions explored 

 by those vessels and the fish-fauna of the Polar waters. His remark, no doubt, 

 refers rather to a resemblance of structure in the fishes inhabiting these two areas 

 than to any general identity of the species composing the two faunas. We may 

 so expand this proposition as to imply that the same type of specialization is 

 associated indifferently with either an Arctic or a bathybial habitat, an implication 

 which will be found to be borne out by the facts of the case.f 



* " Fish. Great Brit.," i., pi. IxxxYiii. 



•j- We refer to type of general confirmation, and not to sucli characters as the reduction of the eyes, or 

 the development of luminous organs. 



