268 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Jenkins, for a comparison of the fishes. From the study of much 
larger collections than those available to Dr. Gunther, Jordan reduced 
the number of species common to the two faunas to six per cent, and 
declared that they were so * substantially distinct" that no recent con- 
nection between the seas had existed. Evermann and Jenkins consider 
the Pacific fish fauna as composed of 1,307 species, and, excluding 16 
species of such wide distribution that their evidence does not count, 
they estimate that only 4.3 per cent are common to the two shores. 
Dr. Dall has also summarized some facts upon this subject." 2 
One of the strongest, arguments against the Isthmian marine passage 
in Pleistocene time, as alleged by some, or a profound subsidence of the 
whole region, as has еп argued by Spencer, is the testimony of the 
vertebrate remains of Nicaragua. According to Professor Cope, these 
remains show a well defined mingling of South American Pampean 
and North American faunas, the former being represented by Hydro- 
chorus and Toxodon, the latter by Bos, Equus, and Elephas. Surely this 
mixture of the two continental faunas of the Pleistocene epoch premises 
a land barrier which could not have existed if there had been any 
extensive submergence in this epoch. 
CONCLUSION. 
In the foregoing pages I have endeavored to present all the facts 
known upon the geological history of the Isthmian region. We have 
shown its antiquity by the interpretation of its topography. This 
antiquity is stamped upon every form. We have shown that as far 
back as Cretaceous time there is evidence of nucleal rocks, and that 
there was a striking dissimilarity in the sedimente and life of the 
adjacent oceans to that existing to-day. We have shown that the igneous 
rocks both eruptive and intrusive were buried and are now exposed by 
erosion. The volcanic fires, which still persist eastward iu the Andean 
heights and to the westward in the Central American plateau, have long 
ceased to exist throughout the Isthmian region. Since the Pliocene 
Tertiary, at least, the region has been one of volcanic quiescence. 
1 B. W. Evermann and О.Р. Jenkins, “ Report upon a Collection of Fishes made 
at Guaymas, Sonora, Mexico, with Descriptions of New Species," Proc. U, 8; 
Nat. Mus., Vol. XIV. No. 846, p. 126 (1891). i 
2 Bull. U. S. Geological Survey, No. 84, 1892, Wm. H. Dall, pp. 151, 152. 
3 "This opinion by Professor Cope, already stated in his writings, was reaffirmed 
by him in a personal conversation, the substance of which was written down and 
approved by him a few weeks previous to hie death. 
