310 Prof. Dr. G. Pfoffcr on the Mutual Relations 



either case the forms most capable of resisting cold, and 

 therefore best suited to a cooler environment, would remain in 

 their old l.abitat. 



Secondly, although the separation of the surface- and 

 subsurface-water faunas in the warmer regions of the earth 

 appears to be fairly distinct, the Mediterranean forms an 

 exception. Quite half the molluscs of the western shores of 

 Norway and fully a quarter of those of the coasts of arctic 

 Norway occur in the Mediterranean ; but it is quite out of the 

 question that in the Mediterranean they live only in the deeper 

 layers of constant temperature. It is of course possible that 

 faunistic displacements occur according to the season, so that 

 Mediterranean animals of northern and arctic character live in 

 the surface-water only in winter ; on comparatively steep 

 shores the distance, for many at least, would not be too long. 

 Unfortunately I know of no data on this last point so far 

 as it affects the benthos animals. Nevertheless the state of 

 affairs in the Mediterranean confirms our conclusion that the 

 separation between surface- and subsurface-/aM/2a, whether it 

 be actual or only potential, is not of supreme importance. 



ClRCUMTEOPICITY OF THE EARLIER TERTIARY FaUNA. 



And now that nothing more stands in the way of the 

 recognition of our Early Tertiary fauna as one of tropical 

 habit, we come to the question of the development of its 

 circumtropicity. The surface-water fauna of our tropics is 

 circum tropical, and this holds true of by far the greater 

 number of genera and even of many of the species. The 

 similarity of many species from the Indo-Pacific and West- 

 Indian seas, and, on the other hand, from the eastern and 

 western shores of Central America, proves to us that the 

 modern separating conditions have not sufficed to efface 

 circumtropicity, and that, if these separations were suddenly 

 to disappear, the circumtropicity would be expressed through- 

 out the whole region to a much more perfect degree. 



Thus the surface-water fauna of our present tropics is the 

 remains of the Early Tertiary fauna shrunk back into the 

 equatorial zone ; it lives in appi'oxiraately the same thermal 

 conditions as the ancestral fauna enjoyed in our latitudes. 

 On what possible grounds, then, can it be asserted that 

 circumtropicity was less developed in the Early Tertiary 

 fauna than in the present surface-water fauna of the tropics ? 



No one doubts that the subsurface-fauna of the Early 

 Tertiary, whether it was actually or only potentially deve- 

 loped, may have been distributed over the whole area of 



