October 11, 1901.] 



SCIENCE. 



557 



that the Isthmus of Panama could not have 

 been depressed for any great length of time 

 in a recent geological period. 



CONCLUSIONS OF DR. HILL. 



These writers have not, however, consid- 

 ered the question of generic identity. To 

 this we may find a clue in the geological 

 investigations of Dr. Robert T. Hill. 



In a study of ' The Geological History of 

 thelsthmusof Panamaand Portions of Costa 

 Rica,' Dr. Hill uses the following language : 



" By elimination we have concluded that 

 the only period of time since the Mesozoic 

 within which communication between the 

 seas could have taken place is the Tertiary 

 period, and this must be restricted to the 

 Eocene and Oligocene epochs of that period. 

 The paleontologic evidence upon which such 

 an opening can be surmised at this period 

 is the occurrence of a few California Eocene 

 types in the Atlantic sides of the tropical 

 American barrier, within the ranges of lati- 

 tude between Galveston (Texas) and Colon, 

 which are similar to others found in Cali- 

 fornia. There are no known structural 

 data upon which to locate the site of this 

 passage, but we must bear in mind, how- 

 ever, that this structure has not been com- 

 pletely explored. 



" Even though it was granted that the 

 coincidence of the occurrence of a few 

 identical forms on both sides of the tropical 

 American region, out of the thousands 

 which are not common, indicates a connec- 

 tion between the two seas, there is still an 

 absence of any reason for placing this con- 

 nection at the Isthmus of Panama, and we 

 could just as well maintain that the locus 

 thereof might have been at some other 

 point in the Central 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 geo- 

 logic or a biologic standpoint for believing 

 that the oceans have ever communicated 

 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, espe- 

 cially those of Pleistocene time. 



" The numerous assertions, so frequently 

 found in literature, that the two oceans have 

 been frequently and recently connected 

 across the Isthmus, and that the low passes 

 indicative of this connection still exist, may 

 be dismissed at once and forever and rele- 

 gated to the domain of the apocryphal. A 

 few species common to the waters of both 

 oceans in a predominantly 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. Maack has erroneously 

 given the pass at 186 feet. Dr. J. W. 

 Gregory states ' that the summit of the 

 Isthmus at one locality 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 Cha- 

 gres, and the Rio Grande flowing into the 

 Pacific, and has been cut by fiuviatile action, 

 and not by marine erosion, out of a land 

 mass which has existed since Miocene time. 

 Those who attempt to establish Pleistocene 

 inter-oceanic channels through this pass on 



