268 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Jenkins, 1 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 been argued by Spencer, is the testimony of the 

 vertebrate remains of Nicaragua. According to Professor Cope, 3 these 

 remains show a well defined mingling of South American Pampean 

 and North American faunas, the former being represented by Hydro- 

 choerus 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 sediments 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 in 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 O. P. Jenkins, " Report upon a Collection of Fishes made 

 at Guaymas, Sonora, Mexico, with Descriptions of New Species," Proc. U.. S. 

 Nat. Mus., Vol. XIV. No. 846, p. 12G (1891). 



- Bull. TJ. S. Geological Survey, No. 84, 1892, Wm. H. Dall, pp. 151, 152. 



8 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 his death. 



