Isthmus Barriers Separating Fish Faunas 279 



masses which have been removed by the general levelling of 

 the surface by erosion." 



In conclusion, Dr. Hill asserts that "there is considerable 

 evidence that a land barrier in the tropical region separated 

 the two oceans as far back in geologic history as Jurassic time, 



FIG. 182. Xenocys jessice Jordan and Bollman. Galapagos Islands. 

 Family Lutianidce. 



and that that barrier continued throughout the Cretaceous 

 period. The geological structure of the Isthmus and Central 

 American regions, so far as investigated, when considered aside 

 from the paleontology, presents .no evidence by which the 

 former existence of a free communication of oceanic waters 

 across the present tropical land barriers can be established. The- 

 paleontologic evidence indicates the ephemeral existence of a 

 passage at the close of the Eocene period. All lines of inquiry 

 geologic, paleontologic, and biologic give evidence that no 

 connection has existed between the two oceans since the close 

 of the Oligocene. This structural geology is decidedly opposed 

 to any hypothesis by which the waters of the two oceans could 

 have been connected across the regions in Miocene, Pliocene, 

 Pleistocene, or recent times." 



Final Hypothesis as to Panama. If we assume the correct- 

 ness of Dr. Hill's conclusions, they may accord in a remarkable 

 degree with the actual facts of the distribution of the fishes 

 about the Isthmus. To account for the remarkable identity 

 of genera and divergence of species I may suggest the following 

 hypothesis : 



