239 



America. The second test is that of the fossil mammals of 

 North America. It was pointed out that, as no trace of typi- 

 cally South American species occur in North American Oligo- 

 cene or Miocene deposits, the two continents must have been 

 separated by a sea during the period in which these beds were 

 laid down. Let us examine these evidences more closely. 



A good many writers have discussed the problem of the 

 former junction of the Pacific and Atlantic Oceans across 

 Central America, from a purely biological point of view. Pro- 

 fessor Gregory,* who last summarised the subject, came to the 

 conclusion that the waterway across Central America was 

 finally closed in the Lower Miocene, or possibly even in the 

 Upper Oligocene. Among others he alluded to the researches 

 of the two Agassiz, father and son, stating that, from a study 

 of the sea-urchins, they proposed to date the junction of the 

 two oceans much further back. These zoologists referred the 

 separation of the two oceans and the formation of the Gulf 

 Stream, to the period at the close of the Cretaceous, that is to 

 say, to the end of the Secondary Era. Professor Verrill f 

 finds that no species of corals are identical on the two sides 

 of Central America, while even the genera and families show 

 remarkable contrasts. The numerous genera and families of 

 reef-building corals, so abundant on the Atlantic side, are 

 wholly wanting on the Pacific, with the exception of Porites, 

 which is represented by three or four small species. 



Mr. Belt J maintained that the marine mollusks on the two 

 coasts separated by the narrow isthmus of Darien were almost 

 entirely distinct. So remarkably distinct are the two faunas, 

 he says, that most zoologists consider that there has been no 

 communication in the tropics between the Pacific and Atlantic 

 Oceans since the close of the Miocene Period. An apparently 

 rather remote junction of the two oceans is suggested by 

 all these authors, but none of them express any very strong 

 convictions on the subject. Dr. Ortmann, on the other hand, 

 states that the affinities of the Decapod fauna of the Atlantic 

 and Pacific are unmistakable, and that we have ample and con- 

 vincing evidence of a former connection between these oceans. 



* Gregory, J. W., " Palaeontology of the West Indies," pp. 304 305. 

 t Verrill, A. E., " Comparison of Coral Faunae," p. 500. 

 J Belt, Th., " Naturalist in Nicaragua," p. 264. 



