THE EOCENE PERIOD. 223 



deformative movement which closed the Cretaceous period and inau- 

 gurated the Eocene quite certainly made man}^ new land connections, 

 and furnished the conditions for a migratory invasion, if, in any of the 

 previous areas, a mammalian stock of the requisite potentialities was 

 awaiting the opportunity. 



Some of the hypotheses of the place of origin of the placentals look 

 to relatively isolated areas within the northern hemisphere. Some 

 special fitness may be assigned to one of these, the old lands of north- 

 eastern North America, the area in which the angiosperms probably 

 originated. During the larger part of the Cretaceous period this was 

 isolated from the western portion of the continent by the great " mediter- 

 ranean" sea of the Great plains region. In such intervals as there 

 may have been between the actual sea occupancy of this tract, it was 

 the site of extensive lowlands interrupted by lakes, swamps, and plex- 

 uses of streams, more inviting to reptiles than to upland mammals. 

 Unfortunately, the Cretaceous record of the old northeastern lands is 

 almost entirely wanting. We have already noted that the deploy- 

 ment of the angiosperms in that region invited a biological revolution 

 which did not seem to be registered during the Cretaceous. The 

 hypothesis that the placentals were evolving there during the Creta- 

 ceous responds to this obvious fitness of things. A dispersion from 

 this area, when the deformative movement at the close of the Creta- 

 ceous made the requisite land connections, is not inconsistent with the 

 fact that the earliest American deployment of placentals is recorded 

 in the Puerco beds of Colorado and New Mexico, and the earliest Euro- 

 pean in the Cernaysian formation of France, and that these were fol- 

 lowed by the more pronounced and cosmopolitan dispersion which 

 took place in the Wasatch epoch, when "'the correspondence between 

 the mammals of Europe and North America was never closer/' l 



As an alternative view, an originating tract for the placental mam- 

 mals has been postulated in the high northern latitudes, partly on the 

 theoretical presumption that the oncoming of cool climates would 

 earliest affect that region, and partly because not a few of the migra- 

 tory paths of the Tertiary mammals seem to have trended southward. 

 As, however, the land connections between Eurasia and North America 

 seem to have been wholly in high latitudes, it is difficult to distinguish 

 between what might have been migrants from a neighboring continent, 

 1 Scott, Introduction to Geology, p. 505. 



