18 LAND MAMMALS IN THE WESTERN HEMISPHERE 



This is a representative series of the wide-spread and manifold 

 non-marine Tertiary deposits of the Great Plains, but a much 

 more extensive and subdivided scheme would be needed to 

 show with any degree of fullness the wonderfully complete 

 record of that portion of the continent during the Tertiary 

 period. A much more elaborate table will be found in Pro- 

 fessor Osborn's "Age of Mammals," p. 41. There are 

 some differences of practice among geologists as to this 

 scheme of classification, though the differences are not 

 those of principle. No question arises concerning the reality 

 of the divisions, or their order of succession in time, 

 but merely as to the rank or relative importance which 

 should be attributed to some of them, and that is a very 

 minor consideration. 



Much greater difficulty and, consequently, much more radical 

 differences of interpretation arise when the attempt is made to 

 correlate or synchronize the smaller subdivisions, as found in 

 the various continents, with one another, because of the 

 geographical differences in contemporary life. Between Eu- 

 rope and North America there has always been a certain pro- 

 portion of mammalian forms in common, a proportion that 

 was at one time greater, at another less, and this community 

 renders the correlation of the larger divisions of the Tertiary 

 in the two continents comparatively easy, and even in the minor 

 subdivisions very satisfactory progress has been made, so 

 that it is possible to trace in some detail the migrations of 

 mammals from the eastern to the western hemisphere and 

 vice versa. Such intermigrations were made possible by the 

 land-bridges connecting America with Europe across the 

 Atlantic, perhaps on the line of Greenland and Iceland, and 

 with Asia where now is Bering Strait. These connections were 

 repeatedly made and repeatedly broken during the Mesozoic 

 and Cenozoic eras down to the latest epoch, the Pleistocene. 

 By comparing the fossil mammals of Europe with those of 

 North America for any particular division of geological time, 



