No. 381.]}] A HALF-CENTURY OF EVOLUTION. 671 
bats, which appear in the Eocene, nature’s experiment with 
these mammalian aéronauts succeeded to the extent that they 
still exist in small numbers. Late in the Cretaceous or very 
early in the Eocene, competition apparently forced some 
unknown carnivorous type to take up an aquatic life, and the 
great success of the incoming cetacean type, resulting in the 
Eocene zeuglodonts and Miocene squalodon, may have had 
an influence on the final extinction of the colossal marine 
reptiles. 
6. The Quaternary Period. — Coming now to the glacial 
epoch of the Quaternary period, we plainly see that under the 
extreme conditions to which life in the Northern hemisphere 
was exposed as never before, how intimate are the relations of 
geology and biology. 
The rise of land at the beginning of the Quaternary, which 
carried the land and the life on it up into a cooler zone, with a 
mean temperature so low that the snows remained from century 
to century unmelted, forming continental glaciers, excited an 
immediate influence on the life. There were very soon devel- 
oped a circumpolar flora and fauna, originating from the few 
Pliocene forms, which became adapted to climatic conditions 
more extreme than ever before known in the world’s history. 
While a few forms thus survived, some must have perished, 
though the bulk of them migrated southward. 
The story told by the Port Kennedy hole, in Pennsylvania, 
just south of the limits of the ice sheet, is a most striking one. 
In that assemblage where are intermingled the bones of mam- 
mals of the Appalachian subprovince, with certain extinct 
forms, and those of the tapir and peccary and colossal sloths, 
adapted to the warmth of the Pliocene, and of the present 
Central American region, we can realize as never before the 
immediate effect of a simple though very decided change of 
climate on organic life. 
As a result of’the submergence of the land in the North 
Atlantic and Arctic regions during the Leda or Champlain 
epoch succeeding, and the consequent amelioration of the 
climate, there was a return of a portion of the Pliocene species 
to the vast area thus freed from the presence of land ice. 
