316 Prof. Dr. G. Pteffer on the Mutual Relations 
DEEP-SEA FAUNA. 
Reasons of a theoretical 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 Mesozoic age till now. ‘Therefore we find in 
the deep sea a mingling of either archaic or highly adapted 
7, e. certainly very old—forms with those of the same habit 
as our present polar animals. Of an Hryon-like 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 Ophidiidw, Macruridx, Mureenidx, and so on. But I 
cannot affirm it of a Leda or Neera, for these genera date 
from the Paleozoic 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, [ am forced to say—and with 
the greatest probability—that the immigration dates from the 
middle of the ‘lertiary 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 of 
species whose palxontological age we do not know, that 
the 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. 
Now, for Méd-Tertiary times the similarity of species in our 
latitudes and in South Australia is established by paleontolo- 
gical research. If we find one of these species in the deep sea 
