GRAND CANYON 



I I r I I 11 I I 



500 



PALEOZOIC 



I I I !"l 



FIGURE 2. Synopsis of the Evolution of VertehrsLtes— showing 

 the evolution of the vertebrates in relation to a salt-water 

 (darkly sJiaded) and fresh-water (lightly shaded) habitat. The 

 irregular curve illustrates mountain-building episodes (geologic 

 revolutions) which Jiave importantly influenced this evolution- 

 ary history. 



The time scale is such that the Pleistocene era (one million 

 years in length) and Recent Time (about twenty-five thousand 



