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IV 



The Antiquity of the Deep-Sea Fish-Fauna 



IT has long been generally admitted that among marine organisms 

 the keenest struggle for existence and the most favourable condi- 

 tions for the processes of evolution, are to be met with along the 

 shore-line. Those out-of-date forms of life which can no longer 

 compete with the vigorous shore-dwelling races, are compelled 

 to retreat to the freshwaters on the one hand, or to the deep sea on 

 the other. Among fishes the antique mud-fishes (Dipnoi) have thus 

 survived only in the freshwaters of parts of Africa, Australia, and 

 South America ; the fringe-finned ganoids, so abundant in the seas 

 and estuaries of Palaeozoic times, are now confined to African rivers ; 

 while the higher bony ganoids of Mesozoic age, related to Amia and 

 Lepidosteus, seem to have become exterminated in the seas of the 

 early Eocene, thence retreating to the freshwaters both of Europe 

 and North America, and at present surviving only on the latter con- 

 tinent. Some of the primitive forms of the higher fishes — the 

 so-called Teleostei — which first appear (so far as known) in the seas 

 of the Cretaceous period, have also survived to the present day 

 solely or chiefly in freshwaters ; but many of these races have 

 migrated instead to the dark and gloomy abyss of the deep ocean. 



It is interesting to note that, so far as the present state of 

 knowledge permits the expression of an opinion, there is no evidence 

 in the geological record of the last-mentioned refuge before the 

 latter part of the Cretaceous period. There may have been seas of 

 great depth, and there may have been organisms living at the 

 bottom of them ; but, if so, none of these pre-Cretaceous sea-beds 

 have hitherto come under the observation of geologists in any part 

 of the world. The chalk and associated strata, however, must have 

 been deposited at a depth sufficiently great to accommodate a fauna 

 of an essentially deep-sea type. It is thus to these formations that 

 we turn for possible forerunners of the inhabitants of the depths of 

 the sea at the present day. 



It is, of course, difficult to decide from the fossils of any deep- 

 water deposit, which of the organisms represented have fallen from 

 the surface and which have actually lived at or near the bottom. 

 Moreover, it is quite likely that some of the animals, which inhabit 

 great depths at present, lived in shallower waters so long ago as 



