GEOGRAPHIC DISTRIBUTION 261 



in the extreme elongation and eel-like mode of propulsion of 

 the body. 



A zoogeographic feature of Eocene life is the strong and in- 

 creasing evidence of migration between South America and 

 North America by means of land connection in late Cretaceous 

 or basal Eocene time, between the northern and southern 

 hemispheres, which was then interrupted for 1,000,000 or per- 

 haps 1,500,000 years until the middle of the Pliocene Epoch, 

 when the South American types again appear in North Amer- 

 ica. Another relation which has been established by recent 

 discoveries is seen in the resemblance between certain Rocky 

 Mountain primates (lemurs) and those existing at the present 

 time in the IVIalayan Peninsula. 



North America and western Europe pass alike through 

 three great phases of mammalian life in Eocene time: first, the 

 archaic phase of the Palaeocene; second, a long phase in which 

 the archaic and modern mammals of the Lower Eocene inter- 

 mingle; third, a very prolonged period from the Lower to the 

 Upper Eocene, in which Europe and North /Vmerica are widely 

 separated and each of the ancestral types of mammals undergoes 

 an independent evolution. This is followed in Oligocene time 

 by a phase in which the animal life of western Europe and 

 North America was reunited. Again in Miocene time a fur- 

 ther wave of European mammalian life sweeps over North 

 America, including the advance wave of the great order Pro- 

 boscidea embracing both mastodons and elephants which ap- 

 pear to have originated in Africa or in southern Asia. During 

 the entire Miocene and Pliocene Epochs there is more or less 

 unity of evolution between North America, Europe, and Asia, 

 but it is a very striking fact that in Middle Pliocene time, 

 when a wave of South American life enters North America, 

 certain very highly characteristic forms of North American 



