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 
Giinther’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. ce. are considered by Jordan to be common 
to both the Atlantic and Pacific coasts. He therefore concludes that “ the 
two faunz 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 marsh during the rainy 
season. 
In determining the genetic relationship between two faunze 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 Panamian Fishes considered conspecific 
by Dr. Giinther are at any rate closely allied. This degree of divergence 
between the faunz 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 
faunze 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,} 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., VIII. 394, 1885. Cf. also Evermann and Jenkins, Proc. U. 8. Nat. Mus., 
XIV. 123 ef seqq., 1891. 
+ See Proc. Amer. Philosoph. Soc., XII. 571, 1872. 
