316 Prof. Dr. G. PtefFer on the Matual Relations 



Deep-Sea Fauna. 



Eeasons of a tlieovetical kind, which I have elsewhere 

 analyzed, make it probable that the peopling of the deep .sea 

 with living creatures first took place from the polar zone 

 in Mesozoic times. Observation shows us that, even now, 

 animals from higher latitudes — by no means all, but very 

 many — descend to the deep sea. The peopling of the deep 

 sea from the polar zone has thus been an uninterrupted pro- 

 cess from the ]\Iesozoic age till now. Therefore we find in 

 the deep sea a mingling of either archaic or highly adapted — 

 1. e. certainly very old — forms with those of the same habit 

 as our present polar animals. Of an EryonAW^o, Crustacean 

 or a Salenia I can say with certainty that it belongs to the 

 old immigrants, and with probability I can say the same of 

 those quite peculiarly adapted deep-sea fishes of the families 

 of the Ophidiidai, ^lacrurida}, Mur£enida3, and so on. But I 

 cannot afiirm it of a Leda or Neceruj for these genera date 

 from the Palaeozoic or Mesozoic age and are still living; 

 the species in question or their ancestors may belong to the 

 oldest or most recent migrants to the deep sea. 



If I find a species in the deep sea in the northern hemi- 

 sphere which still lives in the surface-water of the arctic or 

 boreal zone and there only, I can say that the immigration is 

 of comparatively recent date ; but if the species is already 

 known from the Mid-Tertiary, I am forced to say — and with 

 the greatest probability — that the immigration dates from the 

 middle of the Tertiary period ; for there is no reason why a 

 species which descends to the deep sea to-day should not have 

 so descended at any period of its existence. The probability 

 that the deep-sea species of arctic origin did not migrate in 

 the present-day period is increased by the fact that now, 

 by suboceanic upheavals, the polar zone in the Pacific Ocean 

 is absolutely, and in the Atlantic almost entirely, shut off 

 from the deep sea of the temperate zone. 



The age of the great majority of marine species dates 

 back to the Tertiary, perhaps even to the Mid-Tertiary 

 period. We may therefore assume, even in the case o£ 

 species whose palseontological age we do not know, that 

 tiie process of their migration into the deep sea occurred in 

 Tertiary times, and that this process has certainly gone on 

 in the south uninterruptedly to the present day, while in the 

 north it has now become considerably restricted. 



Kow, for J7«'c?- Tertiary times the similarity of species in our 

 latitudes and in South Australia is established by palseontolo- 

 gical research. If we find one of these species in the deep sea 



