178 THE MEANING OF EVOLUTION 



simple forms of the life of lowly creatures, as well 

 as the simple character of the legs and feet in the 

 salamander class, make the explanation not so un- 

 likely as would at first sight appear. Suffice it to say 

 that the scientist now believes that out of the lung- 

 fish of the Devonian came the amphibians of the Car- 

 boniferous period. 



At the end of the coal period came the greatest 

 change the face of the globe had seen for many mil- 

 lions of years. Slowly the continent rose on both 

 sides of the old interior sea. A great plateau formed 

 in the region of the Alleghenies and another in the 

 western district, though this latter uplift was to be 

 completely washed away, and later to rise again into 

 the Rocky Mountains and the Sierras. With the up- 

 lift at the edges of the continent came a steady rise of 

 the internal marshes, until what had previously been 

 swamp land became progressively first dry land and, 

 in the western part, even desert, in that respect being 

 somewhat like what it is now. 



The amphibians of to-day (animals like the sala- 

 mander and frog) all lay their eggs in the water 

 and their young have a tadpole stage. This doubtless 

 was true of the amphibians of the coal period. With 

 the beginning of the Mesozoic, or "middle life" 

 period, a change and a progression comes over the 

 animal world. The tadpole life of the frog is a rather 



