The Differentiation of Populations i 239 



Fig. 10.13 I Distribution of seas in tlie northern Western Hemisphere 

 during four geologic periods. Black, oceanic areas; dark gray, most 

 persistent seaways on the continental platform; light gray, areas submerged 

 during part of period only; dotted light gray, areas of temporary flooding; 

 white, persistent land areas. ( From Moore, 1949, Introduction to Historical 

 Geology, McGraw-Hill. ) 



