The Principles of Palaeontology. 195 



have at once concluded from these facts that the ibasin ^of the 

 Atlantic has not emerged since the Cretaceous period, and that 

 the fauna of that epoch had maintained itself there in part with- 

 out transformation. In other words, the bottom of the sea has 

 remained, to a certain degree, since the Cretaceous period. 



This induction is justified to a certain extent, but we must add 

 furthermore, in order to form an exact idea of the fauna of that 

 zone, that it contains numerous elements of much more recent 

 origin, and that forms which evolved on the littoral zone must 

 have migrated toward the deep sea at various successive epochs ; 

 this is especially proved by the entire absence of transition types 

 between the archaic types and the others in the region in 

 question. 



It would be natural to suppose, a priori, that the extreme 

 depths of the ocean would reveal phenomena of the same order. 

 It was expected that the zones lately explored below 3000 

 metres, would show us forms recalling the most ancient 

 types. It is not so ; not only is the abyssal fauna destitute of 

 palaeozoic elements, but even the mesozoic types have disap- 

 peared. The inhabitants of the deep abysses are representatives 

 of some of the most specialized types of diverse classes. These 

 forms, few in number, bear marks of a special adaptation. It is 

 manifestly a fauna which has migrated from a recent epoch. As 

 to the types which have persisted since the palaeozoic period, as 

 Lingula, the Areas, the AvicuLas, the siliceous sponges, the Gas- 

 teropoda diotocardia, the Nautili, etc., it is at a much shallower 

 depth, and even on coasts that they are to be sought. 



From what precedes it is evident how difiicult it will be in 

 Palaeontology to find a solid basis on which to rest an estimate 

 of the characteristics of the deep sea fauna; the middle zone, 

 from 1500 to 3000 metres has, as a principal characteristic, a 

 mixture of very ancient forms, (the littoral excepted), with recent 

 forms. One can conceive how perplexing this criterion is for 

 the older geologic epochs. 



Another and a more suggestive characteristic is furnished by 

 the especial adaptation of the organisms to the conditions of 

 light below a certain depth. It is known that the solar rays 

 do not traverse the thickness of a body of water of more 

 than 400 metres. Below this depth, however, there does 



