308 Prof. Dr. G: Pfeffer on the Mutual Relations 
faunistic variations of the Later Tertiary also. And if 
paleontology teaches us that towards the close of the Early 
Tertiary in our latitudes the components of the Early Tertiary 
fauna of tropical habit 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 ef 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 
Jocal faunas. Then we 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 cireum- 
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-Kuropean 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 paleontological data, and it is quite in- 
different 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 circumpolarity, 
such as we find developed at the southern extremities of the 
continents, at once become intelligible. 
Finally, we have to go a step turther, and assume, on the 
authority of paleontological observations, that in the later 
