ILLUSTRATIONS 



Figure 2 in the center of the hook shows in synopsis the 

 evolution of the vertebrates in relation to a salt-water 

 (darkly shaded) and fresh-water (lightly shaded) habi- 

 tat. The irregular curve illustrates mountain-building epi- 

 sodes (geologic revolutions) which have importantly in- 

 fluenced this evolutionary history. 



The time scale is such that the Pleistocene era (one 

 million years in length) and Recent Time (about twenty- 

 five thousand years) could not be included, and these 

 are merely suggested by the heavy line at zero time. 



The entire period encompassed by documented history 

 is only about six thousand years, or one hundred-thou- 

 sandth of the interval elapsing since the opening of the 

 Paleozoic era (Cambrian period), when fossilized ani- 

 muls first begin to appear in the sedimentary rocks. 



1. Cross Section of the Earth 



2. Synopsis of Evolution of Vertebrates 



3. Simpson's Alphabet Analogy 



4. The Hypothetical Protovertebrate 



Three Types of Armored Fishes, or Ostracoderms, 



from the Devonian 

 Three Types of Emypterids from the Silurian 



5. Four Stages in the Evolution of the Vertebrate 



Nephron 



6. Evolution of the Early Fishes 



7. The Lungfish, Protopterus, in Estivation in a Block 



of Mud 



8. The Carboniferous Amphibia 



