THE MIGRATIONS OF MARINE ANIMALS 283 



connection between the two Americas, which 

 allowed at that epoch alone exchanges in both 

 directions of terrestrial animals, the Mastodons and 

 Horses emigrating to the South, whilst by an 

 inverse migration the Edentata were introduced 

 into North America. 



The exact history of these modifications in the 

 forms of seas and continents in each geological 

 epoch is, therefore, a necessary element of and a 

 solid basis for the comprehension of the migrations 

 of fossil beings. Since this path has been opened 

 up by the brilliant attempt of Neumayr at a geo- 

 graphy of Jurassic times, palseographical studies 

 have taken an increasingly important place in 

 the researches of geologists. We possess, at the 

 present time, a series of geographical sketches by 

 various authors, Lapparent, Freeh, Osborn, Matthew, 

 etc., which attempt to retrace the relative posi- 

 tions of lands and seas from the latest to the 

 earliest geological epochs. These sketches differ 

 somewhat according to the interpretation of geo- 

 logical facts, and principally from the degree of 

 importance attributed by the authors to the phe- 

 nomena of erosion and the denudation of early 

 Marine formations. One map, for example, sup- 

 poses, as does Neumayr, the central plateau of France 

 to have been entirely covered by the Jurassic seas, 

 while another represents this same plateau as an 

 island of greater or smaller dimensions. The agree- 

 ment of the various authors is generally more com- 

 pletely established the nearer we get to our own 

 times. Thus the maps of the different Tertiary stages 

 offer, at this moment, a basis of argument much 



