100 LAND MAMMALS IN THE WESTERN HEMISPHERE 



In my judgment, however, the balance of advantage is in favour 

 of giving to this so-called Basal Eocene a rank equivalent to 

 that of the four other universally recognized and admitted 

 epochs of the Tertiary period. No marine rocks of Paleocene 

 date have yet been found in North America, which indicates 

 that the continent' was at least as extensive as it is now. The 

 very scanty development of deposits representing this epoch 

 in Europe renders the comparison with the fossils of the Old 

 World unsatisfactory and hence leads to uncertainty, when it 

 is attempted to determine the land-connections of the time. 

 During the Mesozoic era the shallow Bering Sea had repeatedly 

 been elevated into a land joining North America with Asia 

 and had as often been depressed, so as to separate the conti- 

 nents and allow the waters of the Arctic Ocean to mingle with 

 those of the Pacific. A like alternation of junction and separa- 

 tion went on during the Tertiary and Quaternary periods and, 

 by a comparison of the fossil mammals of Europe and America 

 for any particular division of geological time, it is almost 

 always feasible to say whether the two continents were con- 

 nected, or altogether separated. This statement does not 

 imply that the proportion of common elements in the two 

 faunas during epochs of continental connection was a con- 

 stant one at all times, for that was by no means true. Mere 

 land-connections or separations are not the only factors which 

 limit the spread of terrestrial animals ; if they were, the com- 

 munity of forms between North and South America would 

 be much greater than it actually is. Climatic barriers are of 

 almost equal importance in determining animal distribution, 

 and changes of climate may greatly alter the conditions of 

 migration between connected continents. As the connections 

 between North America and the Old World were probably in 

 high latitudes, where the seas are narrow, changes of climate 

 produced a greater effect upon migration than they could have 

 done had the land-bridges been in the tropical or warm tem- 

 perate zones. That these vicissitudes of climate really did 



