IV. _ ^ 



BnTT^flPrCT? 



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s 



I 



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 assumption of complete identity to be erroneous in 30 out of 

 GUnthcr'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 17| p. c. are considered by Jordan to be common 

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

 two faunae 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 oi 

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

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



] ; 



season. 



In determining the genetic relationship between two fauno3 one must 

 take into account not merely the species that arc absohitcly 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 a,nd other recent ichthyologists have very much 

 increased the percentage ol representative species from the two coasts or 

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

 assumed that the Caribbean and Panamian Fishes considered conspecific 

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

 between the fauna) of the two coasts is only what one might expect to find^ 

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

 probable, since the early Miocene. 



The belief that the resemblance between the Panamian and Caribbean 



jfauna^ is due to the intercommunication of the tropical Atlantic and Pacihc 



, 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.f who spent 



eologist 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, wine i 

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

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

 bean waters since the Miocene period. 



* Proc. U. S. Nat. Mas., VIII. 394, 1885. Cf. also Evcrmann aad Jculdus, Troc. U. ft. Nat. Mus., 



XIV. 123 d seqq., 1 801. 



+ Sec Proc. Aiiier. Plnlosoi)li. Soc, XH. 571, 1873. 



three years in the exploration of San Domingo. This g 



now 



t 



