WHY DO ANIMALS BECOME EXTINCT? 179 



slow changes in the level of portions of the earth's 

 crust, with their far-reaching effects on temperature, 

 climate, and vegetation. Animals that were what is 

 termed plastic kept pace with the altering conditions 

 about them and became modified, too, while those that 

 could not adapt themselves to their surroundings died 

 out. 



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 natural inter-oceanic canal. And yet the alterations 

 wrought by this were, so to speak, superficial, 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 con- 

 tinent 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-bottom 

 in a relatively short time exterminated a large propor- 

 tion of the less active moUusks. And in this instance 

 ' ' relatively short ' ' means positively long ; for, compared 

 to the rise of a continent from the ocean's bed, the fiow 

 of a glacier is the rapid rush of a mountain torrent. 



Then, too, while a tendency to vary seems to be in- 

 herent in animals, some appear to be vastly more sus- 

 ceptible than others to outside influences, to respond 

 much more readily to any change in the world about 

 them. In fact. Professor Cook has recently suggested 

 that the inborn tendency to variation is sufficient in 

 itself to account for evolution, this tendency being 



