THE EOCENE OF EUROPE AND NORTH AMERICA 175 



creased in size. Marsh's laws '■ of the relations of brain growth to survival 

 are apparently borne out by these comparisons, namely, that the brains 

 of surviving races are upon the average larger than those of declining 

 races. On the other hand in following the many causes of extinction 

 through the entire Caenozoic we shall find that even large cerebral develop- 

 ment, as in certain rhinoceroses (Teleoceras) and elephants (Mastodon), 

 may fail to preserve a race. 



Diminished or contracted land areas. — In Europe especially the vary- 

 ing coast lines, the insular conditions, the archipelagic surfaces, are to be 

 seriously studied in connection with the extinction which overtook so many 

 characteristic Eocene mammals before the opening of the Oligocene, so 

 that the general aspect of the fauna is altogether different when the Oligo- 

 cene fairly opens. Changes of land caused by elevation or subsidence 

 operate indirectly through causing changes in all the physical conditions 

 of climate, moisture, desiccation, temperature, and so forth; also more 

 directly in facilitating the cutting off of migrations and introducing new 

 competitions. North America and Africa were the stable continents of 

 Tertiary times, which underwent slight fluctuations of land area as com- 

 pared with the highly unstable continents of Europe and the southern 

 half of South America. It must be stated, however, that the main phe- 

 nomena of extinction in unstable Europe coincide with those in stable 

 America. We have seen in group after group that the Upper Eocene mam- 

 mals of peninsular Europe are not those which in the main give rise to 

 the Oligocene fauna. 



A glance at western North America in Tertiary times as studied by 

 J. Perrin Smith - displays the important influence which must have been 

 exerted by the relations of the Arctic, Pacific, and Atlantic oceans as 

 affected by continental elevation and depression. During part of the 

 Cretaceous, Smith believes that Asia and the Alaskan peninsula were con- 

 nected across Behring Straits. Whenever the cold currents of the Arctic 

 Ocean were cut off, the western coast of America enjoyed a warm, 

 probably a subtropical climate (see p. 93). The same author believes 

 that during early Tertiary times a connection existed between the Eocene 

 seas of the Atlantic and Pacific to the south of California. By Miocene 

 times this passage appears to have been closed. The opinion of this author 

 is based upon the marine fauna. That based upon the land fauna is cited 

 elsewhere (p. 81). In the north the land appears to have risen again 

 toward the end of the Miocene, cutting off the Arctic Ocean, and giving a 

 temperate though not tropical climate to the entire North Pacific. 



Insular conditions. — The substitution of insidar for continental condi- 



' Marsh, Small Size of the Brain in Tertiary Mammals. Amer. Jour. Sci., Vol. VHI, 

 1874, pp. 66-67; also. On the Size of the Brain in Extinct Animals, Abstr. Nature, Vol. 

 XXXII, London, 1885, p. 562. 



^ Smith, J. P., Periodic Migrations between the Asiatic and the American Coasts of the 

 Pacific Ocean. Amer. Jour. Sci., Vol. XVII, Mar. 1904, pp. 217-233. 



