f if the Arctic and the Antarctic Faunas. 309 



Cretaceous period the old fauna with the liabits of our present- 

 day tropical fauna extended further northward-s than in Early 

 Tertiary times. Our studie.^* 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 Early 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, abovo all 

 other classes, the material skeleton for all palaeontological 

 and zoo-geographical studies of marine fauna) from the Early 

 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 seijaratiou 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 Early 

 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 conccrneJ, 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 tlie Early Tertiary period ; in 



