THE OPEN SEA BEACH 295 



1500 species may be found in the entire region. 

 Among the absolutely identical forms of both 

 coasts is our common Atlantic oyster (Ostrea 

 virginicd) which has also been reported from the 

 Gulf of California. No less than nine species of 

 crabs are common to the tropical shores of both, 

 and a large number of Crustaceans on one side 

 of the continent are exceeding close specifically to 

 an equal number on the other side. More than 

 seventy-five species of fishes are common to the 

 two coasts and doubtless the same approxima- 

 tions are true in other classes of ocean life. 



The question will naturally be asked, "How did 

 these marine animals get across a continent? 

 Why should so many animals in one ocean closely 

 imitate animals in another?" Obviously they 

 could never have passed from one sea to the other 

 around the north part of North America or by way 

 of the Antarctic Ocean. Neither is it possible 

 that birds or any other natural agency could have 

 carried them across the Isthmus of Panama. 

 During Miocene time there was a depression of 

 Central America to the extent of opening a sea 

 passage in the Panamic region. A mingling, 

 though not very general and complete, of the life 



