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. 
