LITTORAL FAUNA OF THE PANAMA PROVINCE. 239 



scarcely any exception identical on the two sides of Central America, and 

 that one half of the species are common to both coasts. D. S. Jordan * con- 

 siders the assnmption of complete identity to be erroneous in 30 out of 

 Glinthec's 59 cases, so reducing the percentage to 15. Of 407 species of 

 Fishes known in 1885 to inhabit the Pacific coast between Cape St. Lucas and 

 Panama, only 71 species or 17i p. c. are considered by Jordan to be common 

 to botli the Atlantic and Pacific coasts. He therefore concludes that " the 

 two faume show no greater resemblances than the similarity of physical con- 

 ditions on the two sides would lead us to expect" without resorting to the 

 hypothesis of a recent communication between the two oceans. Many of 

 the species found on both coasts according to Jordan often ascend rivers and 

 may have been diffused by crossing from marsh to marsli during the rainy 

 season. 



In determining the genetic relationship between two faunce one must 

 take into account not merely the species that are absolutely indistinguish- 

 able to the discriminating eye of a modern systematist but also the number 

 of common genera and the number of closely allied or representative species. 

 The observations of Jordan and other recent ichthyologists have very much 

 increased the percentage of representative species from the two coasts of 

 Central America, at the expense of the identical ones. For it may be 

 assumed that the Caribbean and Pananiian Fishes considered conspecific 

 by Dr. Gunther are at any rate closely allied. This degree of divergence 

 between the fauure of the two coasts is only what one might expect to find, 

 if the passage through the Istlnnus of Panama has been closed, as seems 

 probable, since the early Miocene. 



The belief that the reseniljlance between tlie Panamian and Caribbean 

 faunae is due to the intercommunication of the tropical Atlantic and Pacific 

 within comparatively late geological times does not rest upon a hypothetical 

 basis, if we can rely upon the observations of the late W. M. Gabb,t who spent 

 three years in the exploration of San Domingo. This geologist found in the 

 San Domingo Miocene 217 extinct, and 97 still living species of Mollusca, 

 the still surviving forms existing on both sides of Central America, which 

 barrier is capped by Miocene rocks. Fifteen of the 97 surviving species are 

 now restricted to the Panama Province, having disappeared from the Carib- 

 bean waters since the Miocene period. 



* Proc. U. S. Nat. Mus., Vril. .'i94, 188.5. Cf. also Evprmann and Jenkins, Proc. U. S. Nat. Mus., 

 XIV. 123 et seqq., 1891. 



+ Sec Proc. Anicr. Pliilosopli. Scio., XII. 571, 1S72. 



