484 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



time the differences between the respective faunas of these three sub- 

 realms evidently have been on the whole less than in early Paleozoic times. 

 The comparative uniformity in bottom faunas then established in the 

 Atlantic doubtless is attributable chiefly to the length of time in which 

 adventitious physical circumstances may have assisted geminate, ortho- 

 genetic, parallel, and convergent evolution of long separated faunas in 

 promoting such uniformity. At the same time the process probably was 

 augumented by occasional land or shallow water connections between the 



Figure 19. — Map of Portion of Land Hemisphere 



Showing Paleozoic land connections and paths of marine faunal migrations. The 

 vertically lined areas indicate occasional land connections between parts of North Amer- 

 ica and Europe in the north and with the Antilles in the south. 



American continents on the west and Africa and Europe on the east side. 

 It is not readily conceivable how the often close agreements between 

 Paleozoic faunas in western Europe and eastern America could have been 

 brought about except by such means. Conditions favoring littoral migra- 

 tion between England and the Gulf of Saint Lawrence are indicated in 

 the middle Cambrian, the early Canadian, and rather commonly during the 



