266 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



conclusions of most authorities that there was a land barrier between 

 the oceans at the close of the Miocene period is in perfect harmony with 

 the geological deductions of this paper. The mountain making epoch 

 of the Isthmus, as shown on previous pages, which was but a part of a 

 great orogenic revolution that affected the entire perimeter of the Great 

 Antilles and the Caribbean, can be definitely fixed in that interval of time 

 between the close of the Oligocene a and the beginning of the Pliocene, 

 as testified by the uucomformable deposition of the sub-horizontal Plio- 

 cene rocks against the folded and upturned Oligocene. The vast extent 

 and visible effects of this orogenic movement throughout the Caribbean 

 and Antillean region, and the possibility of the future demonstration of 

 its continuity as far north as the Pacific coast of California, and even 

 Alaska, will lead to a complete readjustment of many current geographic 

 conceptions. In any case, if a passage existed between the seas previous 

 to this time such an orogenic evolution would theoretically furnish every 

 cause for its closure during this epoch. 



This testimony of the geologic structure is in harmony with the ma- 

 jority of biologic opinions that the Isthmian barrier established during 

 the close of the Tertiary persisted as a permanent land until the present 

 time. 



Biologic and Geologic Deductions oppose the Theory of a Passage in 

 Pliocene or Pleistocene Time. — All the authentic biologic and geologic 

 evidences are entirely opposed to the possibility of a communication 

 between the two oceans across the Isthmian or Tropical American 

 region in Pliocene or Pleistocene time. 



The possibility of such a connection has never been seriously main- 

 tained on biologic grounds. The deductions from the study of all spe- 

 cialists of the life of the adjacent shores are distinctly opposed to this 

 conclusion. 



One of the strongest arguments against the existence of an inter- 

 oceanic passage during this epoch is the testimony of the present life 

 of the bordering seas. It is a well known fact that the Pleistocene 

 faunas differ so little from the recent that they are almost similar, — 

 the number of identical species being far greater than the distinguish- 



1 The use of the term Oligocene throughout this report, upon the authority of 

 Dr. Dall, is somewhat of an innovation in American usage of Tertiary nomencla- 

 ture. It includes the beds equivalent to the Vicksburgian or upper part of the 

 Eocene and the lower part of the Miocene of older usage. Hence in many in- 

 stances its use may he synonymous with the term Miocene of many previous 

 authors as applied in Tropical America, especially by Gabb, Guppy, and Euro- 

 pean paleontologists. 



