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 
