706 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



STRATUM 3 









\LATE ISLAND\ 



LATE 







HOPPERS | IMMIGRANTS 



STRATUM 2 



\0LD ISLAND 







, HOPPERS 





STRATUM 1 ANCIENT 







IMMIGRANTSX 





NORTH t SOUTH 





1 ' ^~~ 



AMERICA -«v^ 

 CONNECTED ^*sj 









ISLAND CHAINS 













NORTH t SOUTH 

 AMERICA DISCON- 

 NECTED 













PLEISTOCENE 



CRETACEOUS 



PA LEO CENE 



EOCENE 



OLIGOCENE 



MIOCENE 



PLIOCENE 



+ RECENT 



Fig. 43.7. Times of migrations between North and South America. Copied from lantern slide of G. W. 

 Simpson, 1950 Sigma Xi lecture. 



grants from North America. Some of the peculiar forms from South 

 America also made their way into North America. 



It is possible that the Antillean arc from latest Cretaceous time on 

 could have been a land connection as well as the Costa Rica-Panama 

 isthmus, and that their histories may not have run exactly parallel. With 

 the possibility of two bridges between continents, ocean-to-ocean migra- 

 tion may have been delayed for a while in the mediterranean between 



bridges, in the manner of a ship negotiating locks in a canal. Also, if the 

 history of the two bridges did not run parallel, then opportunities for ex- 

 change of land animals would be more frequent than if only one bridge 

 had recurring emergences and submergences. The paleontologic record 

 of the Greater and Lesser Antilles, however, does not indicate that the 

 eastern orogenic belt was of importance at any time as a land bridge be- 

 tween the continents. 



