266 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
conclusions of most authorities that there was & 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? and the beginning of the Pliocene, 
as testified by the uncomformable 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. 
Biologie and Geologic Deductions oppose the Theory of a Passage im 
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 be synonymous with the term Miocene of many previous 
authors as applied in Tropical America, especially by Gabb, Guppy, and Euro- 
pean paleontologists. 
