252 ■ HENRY F. OS BORN 



accept in full his correlation of the mammal-bearing horizons on the 

 continent, together with his subdivisions of geologic time. 



Similarly in America there are the pioneer correlations of Leidy 

 of the American formations with each other and with those of Europe, 

 followed with increasing precision by those of Cope, Marsh, Scott, 

 Clarke, Dall, and Osborn. In 1899 Matthew published A Provisional 

 Classification of the Freshwater Tertiary of the West, and this together 

 with his Faunal Lists of the Tertiary Mammalia of the West, published 

 in 1909, afforded the American bases for Osborn's Cenozoic Mammal 

 Horizons of Western North America, published in 1909, in which for 

 the first time the succession of the mammalian life of the New and 

 Old Worlds is closely compared. 



In the meantime increasingly accurate field methods, especially 

 in the horizontal recording of levels after methods introduced by 

 Osborn, Hatcher, and Wortman, have resulted in the subdivision of 

 the old "formations" of Leidy, Cope, and Marsh into successive Life- 

 zones similar to those long in use in invertebrate paleontology. 

 These life-zones are obviously as important in questions of time as 

 they are in questions of phylogeny or descent; they narrow down the 

 old correlation standard of the comparison of similar specific and 

 generic stages to different levels; they add greatly to the possibilities 

 of precise comparison in respect to the newer data of correlation, such 

 as detailed evolution of related forms, the simultaneous introduction 

 of new forms by migration, the predominance or abundance of certain 

 forms, the convergence and divergence of American and European 

 faunas. 



Putting together all these facts of various kinds, the first result 

 is the proof that the mammalian life of Eurasia and America in 

 Tertiary times passed through a series of grand phases of union, 

 of divergence, of reunion, and perhaps again of divergence. There 

 are seven of these phases. 



In the first, in Basal Eocene times, we find North America, Europe, 

 and possibly South America peopled with archaic mammals of 

 Mesozoic ancestry. 



In the second faunal phase, of Lower Eocene times, we observe the 

 first modernization, which occurs simultaneously in Europe and 

 North America, by the invasion of many modern families of mammals, 



