WHY DO ANIMALS BECOME EXTINCT? 227 



How slowly changes may take place is 

 shown by the occurrence of a depression in the 

 Isthmus of Panama, in comparatively recent 

 geologic time, permitting free communication 

 between the Atlantic and Pacific, a sort of nat- 

 ural inter-oceanic canal. And yet the altera- 

 tions wrought by this were, so to speak, super- 

 ficial, affecting only some species of shore fishes 

 and invertebrates, having no influence on the 

 animals of the deeper waters. Again, on the 

 Pacific coast are now found a number of shells 

 that, as we learn from fossils, were in Pliocene 

 time common on both coasts of the United 

 States, and Mr. Dall interprets this to mean 

 that when this continent was rising, the steeper 

 shore on the Pacific side permitted the shell-fish 

 to move downward and adapt themselves to 

 the ever changing shore, while on the Atlantic 

 side the drying of a wide strip of level sea-bot- 

 tom in a relatively short time exterminated a 

 large proportion of the less active mollusks. 

 And in this instance " relatively short " means 

 positively long ; for, compared to the rise of a 

 continent from the ocean's bed, the flow of a 

 glacier is the rapid rush of a mountain torrent. 



