GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 19 
or ten degrees farther south on the coast of Africa, and all 
the species of these ancient deposits partake of what is now 
a more southern character. * 
During the Tertiary period there was a gradual but very ex- 
tensive elevation of the northern part of the continents. It 
was during this period that the Alps and the Pyrenees were 
raised to their present level. The lifting at the north of 
such masses of land into the cooler regions of the atmos- 
phere would have had a powerful influence in reducing the 
temperature of the neighboring seas. As the waters became 
slowly cooled, the species best adapted to migrate gradually 
extended their limits southward; on the north, the species 
were destroyed by the advancing cold, and all those species 
with little power of migrating, and those easily affected by 
changes of temperature or other physical causes were wholly 
exterminated. And thus, on the shores of Africa, still exist 
the remnants of the ancient Tertiary fauna of the southern 
European seas, driven from their former home by the ad- 
vancing cold, but living on through all the changes, even of 
a Glacial epoch. 
In North America, the land climate during the early and 
middle Tertiary was warmer than now, as is indicated by the 
plants of the lignite beds, and the marine climate undoubt- 
edly corresponded with that of Europe and with that of the 
land. In the northern parts of the country no fossil records 
of the later marine Tertiary are known, but the land faune 
of the period, the upheaval of the northern parts of both 
countries, and the changes in the European seas show very 
clearly that there were similar changes on the American 
shores. 
The arctic marine fauna of the earlier Tertiary, while 
much more land than now was submerged at the north, must _ 
have been circumpolar in character, and the retreating of 
species southward from this common point accounts for the 
occurrence of the same species on the northern coasts of both 

* Forbes, Natural History of the European Seas. 
