THE EOCENE OF EUROPE AND NORTH AMERICA 113 



great land mass to the north, of warm to temperate climate favorable to 

 the evolution of these higher forms of mammalian life; in fact, there is 

 every reason to beheve that this northerly region was throughout the 

 whole pre-Pleistocene Caenozoic period highly favorable to the evolution 

 and migration of the higher forms of the Mammalia. This, as seen in a 

 north-polar view of the earth, was the area of the great migrating routes 

 and must have enjoyed a favorable climate, otherwise the faunal con- 

 tinuity between Europe and western America could not have been so fre- 

 quently renewed or sustained by intermigration. As detailed on p. G6, 

 this hypothesis of a northerly or circumpolar center has been advocated 

 by. Wortman and others. It must be remembered, however, that the 

 actual center from which these modernized mammals suddenly spread into 

 Europe and North America is still hj^iothetical and will not be determined 

 until the Basal Eocene fossil mammal beds in the unknown portions of 

 America and Asia shall have been discovered. 



Placing in contrast the archaic and modern orders in North America 

 during the Second Faunal Phase, they appear somewhat as follows: 



Archaic Orders Modern Orders 



Crcodonta, creodonts Carnivora, fissipede carnivores 



Iiisectivora, insectivores Rodentia, rodents 



Tillodontia, tillodonts Perissodactyla, odd-toed ungulates 



Tseniodonta, ganodonts Artiodactyla, even-toed ungulates 



Condylarthra, phenacodonts Primates, lemuroids or monkeys 

 Amblypoda, coryphodonts 



As noted above, the division is very arbitrary; the archaic or modern 

 columns will be swollen or diminished by the respective transfer of the 

 prmiitive Insectivora to the modern column, or of the primitive Lemuroidea 

 to the archaic column. 



Lower Eocene Life of Europe 



As compared with that of America the Lower Eocene of Europe is 

 more precisely subdivided at the present time by Deperet ^ and others 

 through the alternation of marine and terrestrial formations, upon which 

 the European palaeontologist relies, while his American confreres are de- 

 pendent entirely upon the freshwater phases of the Rocky Mountain 

 basins. After it has been possible to make very close comparisons between 

 the evolution stages of a large number of related mammals on the two 

 continents, closer correlations may be made than are at present prac- 

 ticable. It is therefore best to treat Europe and America separately, first 

 noting the broad parallelism of stages, as follows: 



1 Deperet, L'cvolution des Mammifferes tertiaires, etc. (Eocene), 1905. 

 I 



