1 88 THE ORIGIN AND EVOLUTION OF LIFE 



The most remarkable types in this complex reptilian society 

 of Permian Texas are the giant fin-backed lizards, Clepsydrops, 

 Dimetrodon, Edaphosaiirus, of Cope, probably terrestrial and 

 carnivorous in habit. In these animals the neural spines of 

 the dorsal vertebrae are vertically elongated to support a power- 

 ful median membranous fin, the spines of which are sometimes 



Fig. 68. Theoretic World Environment in Middle Permian Tuvle. 



Great extension of the Baltic Sea and of the Eurasiatic Mediterranean Tethys. Rise of 

 the Appalachian, Northern European Alps, and many other mountains. Modified 

 after Schuchert. 



smooth (Dimetrodon), sometimes provided with transverse rods 

 {Edaphosaurus cruciger). These structures may have devel- 

 oped through social or racial competition and selection within 

 this reptile family rather than as offensive or defensive organs 

 in relation to other reptile families. 



We now glance at the Permian life of another great zoologic 

 region. Africa has been throughout all geologic time the 

 most stable of the continents, especially since the begin- 



