558 



SCIENCE. 



[N. S. Vol. XIV. No. 354. 



account of its present low altitude, must 

 not omit from their calculations the restora- 

 tion of former rock masses, which have been 

 removed by the general leveling of the sur- 

 face by erosion." 



SUMMARY OF DR. HILL. 



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, and that that barrier con- 

 tinued throughout the Cretaceous period. 

 The geological structure of the Isthmus and 

 Central American regions, so far as inves- 

 tigated, 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 pale- 

 ontologic evidence indicates the ephemeral 

 existence of a passage at the close of the 

 Eocene period. All lines of inquiry — geo- 

 logic, paleontologic and biologic — give evi- 

 dence that no connection has existed be- 

 tween the two oceans since the close of the 

 Oligocene. This structural geology is de- 

 cidedly opposed to any hypothesis by which 

 the waters of the two oceans could have 

 been connected across the regions in Mio- 

 cene, Pliocene, Pleistocene, or recent times." 



FINAL HYPOTHESIS AS TO PANAMA. 



If we assume the correctness of Dr. Hill's 

 conclusionSjthey may accord in a remarkable 

 degree with the actual facts of the distribu- 

 tion of the fishes about the Isthmus. To 

 account for the remarkable identity of gen- 

 era and divergence of species I may suggest 

 the following hypothesis : 



During the lifetime of most of the pres- 

 ent species, the Isthmus has not been de- 

 pressed. It was depressed in or before Mio- 

 cene time, during the lifetime of most of 

 the present genera. The channel formed 

 was relatively shallow, excluding forms in- 

 habiting rocky bottoms at considerable 



depths. It was wide enough to permit the 

 infiltration from the Caribbean Sea of nu- 

 merous species, especially of shore fishes of 

 sandy bays, tide pools and brackish estu- 

 aries. The currents set chiefly to the west- 

 ward, favoring the transfer of Atlantic 

 rather than Pacific types. 



Since the date of the closing of this chan- 

 nel, the species left on the two sides have 

 been altered in varying degrees by the pro- 

 cesses of natural selection and isolation. 

 The cases of actual specific identity are 

 few, and the date of the establishment as 

 species, of the existing forms, is subsequent 

 to the date of the last depression of the isth- 

 mus. 



While local oscillations, involving changes 

 in coast-lines, have doubtless frequently 

 taken place and are still going on, our 

 knowledge of the distribution of fishes 

 should render impossible the speculations 

 on the dance of continents, which certain 

 geologists and certain biologists have, at 

 one time or another, used as a convenient 

 means of accounting for glacial phenomena, 

 or for anomalies in distribution. We may 

 be also certain that none of the common 

 genera ever found their way around Cape 

 Horn. Most of them disappear to the 

 southward, along the coasts of Brazil and 

 Peru. 



Further, it goes without saying, that we 

 have no knowledge of the period of time 

 necessary to work specific changes in a 

 body of species isolated in an alien sea. 

 Nor have we any data as to the e£fect on a 

 given fish fauna of the infiltration of many 

 species and genera belonging to another. 

 All such forces and results must be matters 

 of inference. 



LAWS GOVERNING DISTRIBUTION OP 

 ANIMALS. 



I have elsewhere* had occasion to say 

 that the laws governing the distribution of 



* 'Footnotes to Evolution.' 



