308 Prof. Dr. G. Pfeffer on the Mutual Rdatlons 



faunistic variations of the Later Tertiary also. And if 

 paleontology tcaclics us that towards the close of the Early 

 Tertiary in our latitudes the components of the Early Tertiary 

 fauna of tropical liabit disappear, that in the Middle Tertiary 

 in our latitudes a fauna is found which resembles the present 

 Mediterranean fauna in habit, and that, finally, in the more 

 recent Tertiary the character of the fauna approaches more 

 and more closely to that of our present-day fauna, then we 

 may, indeed we must, assume that corresponding climatic 

 changes underlie these faunistic variations. 



A gradual shrinking-back of the tropical climate from its 

 former wider domain must have brought about a zonally- 

 disposed separation of the Early Tertiary fauna, inasmuch as 

 only those members of the old fauna as were able to endure 

 the lowering of the temperature could remain behind. The 

 zonal disposition of the marine benthos-fauna of the present 

 day is quite distinctly marked over the whole earth, although 

 the definiteness of this is influenced by the development of 

 local faunas. Then Ave have every reason to argue retro- 

 spectively and to assume that all the zonally disposed faunas 

 of the earth have had the same cause, and that they have all 

 originated through a zonal separating-out of the Early 

 Tertiary fauna. 



This theory becomes a certainty when we consider circum- 

 boreality. There is a large number of species which occur 

 both in the North Atlantic and the North Pacific Oceans, 

 without, however, extending into the arctic or torrid zones; 

 indeed, there are similar boreal-European, East American, 

 West American, and North Japanese species on the one hand, 

 and, on the other, similar South-European and Japanese 

 species. Now it cannot be seriously maintained that in recent 

 Tertiary or still later times there may have existed in the 

 boreal or warmer temperate zone a connexion between the 

 Atlantic and Pacific through America or Asia ; but the theory 

 that similar relics of the Early Tertiary fauna must have 

 remained at places of similar climate at once explains every 

 peculiarity in the paleeontological data, and it is quite in- 

 diflerent whether at the time of the separation of the faunas the 

 different boreal regions of the Atlantic and Pacific side were 

 ■wholly and impassably separated from each other or not. If 

 •we have thoroughly grasped the historical conception of the 

 evolution of faunas, particular cases of notal cir cum polarity, 

 such as we find developed at the southern extremities of the 

 continents, at once become intelligible. 



Finally, we have to go a step lurther, and assume, on the 

 authority of palseontological observations, that in the later 



