of the Arctic and the Antarctic Faunas. 309 
Cretaceous period the old fauna with the habits of our present- 
day tropical fauna extended further northwards than in Early 
Tertiary times. Our studies have already shown us that 
we cannot regard such a condition as a purely local one, and 
so we arrive at the theory that in the middle of the Cretaceous 
period a climate of tropical warmth must have prevailed over 
the whole region of the present temperate zones. 
SURFACE- AND SUBSURFACE-FAUNA IN TERTIARY TIMES. 
Up to this point we have characterized the Karly Tertiary 
fauna quite generally as one of tropical habit; this brief 
designation now requires further analysis. If we make a 
table of the genera of molluscs (the molluscs form, above all 
other classes, the material skeleton for all paleontological 
and zoo-geographical studies of marine fauna) from the Harly 
Tertiary in our latitudes, and note their distribution in the 
present surface-water, we find among them genera which now 
occur only in the surface-water of the tropics; but beside 
these are components of subtropical habit, of the habit of our 
North Sea forms, and, finally, also boreal and even arctic 
genera which never occur in the surface-water of warmer 
regions. But the Early Tertiary fauna cannot be compared 
with the surface-water fauna of the tropics; it corresponds 
rather to the surface-water fauna plus the subsurface-water 
fauna. If, nevertheless, we still characterize it as a fauna of 
tropical habit, we are justified by the consideration that in 
the tropics, and nowhere else on earth, warm-water, cool. 
water, and cold-water animals may occur quite close together, 
may, indeed, be disposed vertically under one another. 
It is a question whether the strict separation between the 
surface- and the subsurface-fauna obtaining in the tropical 
fauna of the present day already existed in the fauna of the 
Early Tertiary ; there seems much to be said on both sides. 
We may, however, leave this question open until exact 
statistics regarding it are compiled, and this for two reasons. 
First, supposing that the subsurface-fauna of the Harly 
Tertiary extended into the region of the surface-water and 
mixed with its fauna, the change of climate during the 
Tertiary period would have brought about exactly the same 
consequences so far as the present-day fauna is concerned, as 
if the separation of the faunas into surface- and subsurface- 
faunas had taken place before the change of climate. So far 
as our present study is concerned, it is quite indifferent 
whether the subsurface-water fauna was actually or only 
potentially in existence in the Karly Tertiary period; in 
