278 Isthmus Barriers Separating Fish Faunas 



locus thereof might have been at some other point in the Cen- 

 tral American region. 



"The reported fossil and living species common to both 

 oceans are littoral forms, which indicate that if a passage existed 

 it must have been of a shallow and ephemeral character. 



"There is no evidence from either a geologic or a biologic 

 standpoint for believing that the oceans have ever communi- 

 cated across the Isthmian regions since Tertiary time. In 

 other words, there is no evidence for these later passages which 

 have been established upon hypothetical data, especially those 

 of Pleistocene time. 



"The numerous assertions, so frequently found in litera- 

 ture, that the two oceans have been frequently and recently 

 connected across the Isthmus, and that the low passes indic- 

 ative of this connection still exist, may be dismissed at once 

 and forever and relegated to the domain of the apocryphal. A 

 few species common to the waters of both oceans in a predomi- 

 nantly Caribbean fauna of the age of the Claiborne epoch of 

 the Eocene Tertiary is the only paleontologic evidence in any 

 time upon which such a connection may be hypothesized. 



"There has been a tendency in literature to underestimate 

 the true altitude of the isthmian passes, which, while probably 

 not intentional, has given encouragement to those who think 

 that this Pleistocene passage may have existed, ilaack has 

 erroneously given the pass at i86 feet. Dr. J. W. Gregory 

 states 'that the summit of the Isthmus at one locahty is 154 

 feet and in another 287 feet in height.' The lowest isthmian 

 pass, which is not a summit, but a drainage col, is 287-295 

 feet above the ocean. 



' ' If we could lower the isthmian region 300 feet at present, 

 the waters of the two oceans would certainly commingle through 

 the narrow Culebra Pass. But the Culebra Pass is clearly 

 the headwater col of two streams, the Obispo flowing into the 

 Chagres, and the Rio Grande flowing into the Pacific, and has 

 been cut by fluviatile action, and not by marine erosion, out 

 of a land mass which has existed since Miocene time. Those 

 who attempt to establish Pleistocene interoceanic channels 

 through this pass on account of its present low altitude must 

 not omit from their calculations the restoration of former rock 



