How the History of the Past Is Read 



higher in the scale. This is really what might 

 be expected, for the more highly organized a 

 creature the more susceptible should it be to 

 changes of any kind, although another factor 

 probably plays a part here, the fact that the 

 simpler animals as a rule move about less, and 

 live now, and did in the past, under more uni- 

 form conditions than their relatives. And 

 among animals the mammals, after they became 

 fairly established, changed the most rapidly of 

 all, so that, aside from the marsupials, there is 

 not now living a single family that dates back 

 to the Eocene. The birds of that period were 

 very much like those of to-day, while many 

 families of fishes, and genera even, go back to 

 the Cretaceous. So mammals indicate changes 

 of time and of surrounding conditions much 

 more exactly than other animals. 



Fresh-water shells, or, better yet, fresh- 

 water fishes, furnish the best testimony as to 

 former land connection between countries now 

 separated by the sea ; for, owing to their mode 

 of life, these spread but slowly, and long 

 lapses of time were necessary in order that 



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