6 Panama\Shells. INTRODUCTION. 



coast of North America, a polar current appears to extend 

 the Arctic Fauna of marine shells to 41 N. lat., and the cold 

 current along the shores of the Middle and Southern States 

 seems to limit the tropical Fauna to about 26 or 28 N. lat. 

 on the coast, although on the east side of the Gulf Stream it 

 extends to the Bermuda Islands in latitude 32 N. 



It will be seen also in the following pages, that a large 

 number of the species which occur at Panama were collected 

 by Mr. Cuming at the Gallapago Islands. But if any of the 

 species occur in the Polynesian Islands, the number does not 

 exceed three or four, and in these cases the specific identity 

 of the shells is very doubtful. The western boundary of the 

 Panama province may therefore be made to include only the 

 Grallapago Islands. 



Thus, in the language of one hypothesis, climate prevents 

 the dispersion of the species to the north or south ; and the 

 Antarctic current setting over from the coast of Peru to the 

 Gallapagos, has favored their dispersion to those islands, and 

 the broad sea beyond has restrained them from any farther 

 dispersion westward. Perhaps this is as favorable an illus- 

 tration as can be found of the doctrine of physical causes ac- 

 counting for the distribution of the individuals of each species 

 rom a single centre. Yet this doctrine is obviously a mere 

 hypothesis. If it be granted that these mollusks can travel 

 coastwise until they find the waters too cold for them, it 

 would still remain to be proved that they actually do so. 



But the supposed dispersion of the species over the sea to 

 the Gallapagos, presents more difficulties than at first appear. 

 The shells of Cirrhopods which adhere to floating wood, 

 have been often found far out at sea ; perhaps, too, some 

 species of Pholas may be distributed in this manner. But 

 the only way in which such dispersion of the species generally 

 can be accounted for is by an imaginary voyage of their spawn 

 across the sea. The species, in those stages of growth which 

 have been observed, are mostly restricted to narrow, vertical 



