THE EOCENE PERIOD 785 



Cretaceous species were replaced by new ones; (2) among land 

 plants so many species lived over as to make the plane of division 

 between the Mesozoic and Cenozoic very difficult of location ; (3) the 

 great saurians almost disappeared, and most other reptiles showed 

 profound changes; and (4) placental mammals appeared in force, 

 and promptly took a leading place. The combination is unique, 

 in that, while half the land life joined with the sea life in under- 

 going a profound transformation, the other half did not participate 

 notably in the revolution. In explanation of profound transform- 

 ations of epicontinental marine life, appeal has been made repeat- 

 edly to the withdrawal of the sea, to the extension of the land, and 

 to climatic changes incident to deformative movements; and this 

 appeal may be renewed so far as the change in the sea life is con- 

 cerned. The withdrawal of the sea after the Cretaceous period 

 seems peculiarly well fitted to explain the great change in epicon- 

 tinental sea life, because of the great restriction of the area of shallow 

 water. The increase of the land and the establishment of new land 

 connections at the same time might well have caused the existing 

 vegetation to spread and flourish, if the climate remained congenial; 

 but why did the land faunas not respond in like manner? Closely 

 connected with this problem is the sudden rise of the mammals. 



It is an open question whether the placental mammals of North 

 America and Eurasia arose from non-placental mammals which 

 lived in these continents earlier, or whether they were immigrants. 

 No satisfactory evidence of a transition from non-placental mam- 

 mals has been produced, and the strength and suddenness of the 

 placental development suggest invasion from some quarter where 

 their evolution had reached an advanced stage. The deformative 

 movements which closed the Cretaceous period and inaugurated 

 the Eocene quite certainly made many new land connections, and 

 furnished the conditions for an invasion, if mammals in sufficient 

 variety and numbers were awaiting the opportunity. 



The rise of placentals may have caused the downfall of the 

 reptiles, though this cannot be affirmed. The habit of bringing 

 forth relatively mature offspring, and of nourishing and protecting 

 them, gave the mammals an immense advantage. To this ad- 

 vantage were added superior agility and higher brain power. It 



