194 THE ORIGIN AND EVOLUTION OF LIFE 



ture specialization. Five other great branches, namely, the 

 ichthyosaurs, plesiosaurs, two great branches of the dinosaurs, 

 and the pterosaurs, were destined to dominate the waters, 

 the earth, and the air during the Mesozoic Era, i. e., the Tri- 

 assic, Jurassic, and Cretaceous Epochs. Thus altogether thir- 

 teen great branches of the reptilian stock became extinct either 

 before or near the close of the Age of Reptiles. Out of the 

 total of eighteen reptilian branches only five were destined to 

 survive into Tertiary time, namely, the orders which include 

 the existing turtles, tuateras, lizards, snakes, and crocodiles. 



Geologic Blanks and Vistas of Reptilian Evolution 



As pointed out in the introduction of this chapter, the rep- 

 tile ancestor of these eighteen branches of the class Reptilia — 

 a class with an adaptive radiation which represents the mechan- 

 ical conquest of every one of the great life zones, from the aerial 

 to the deep sea — will some day be discovered as a small, lizard- 

 like, cold-blooded, egg-laying, four-limbed, long-tailed terres- 

 trial form, with a solid skull roof, of carnivorous or more prob- 

 ably insectivorous habit, which lived somewhere on the land 

 surfaces of Carboniferous time. Such undoubtedly was the 

 reptilian protot>q3e from which evolved every one of the 

 marvellous mechanical types which we may now briefly re- 

 view. By methods first clearly enunciated by Huxley in 1880 

 several of the ideal vertebrate prototypes have been theoreti- 

 cally reconstructed, and in more than one instance discovery 

 has confirmed these hypothetical reconstructions. 



The early geologic vistas of this entire radiation are seen 

 in the reptilian life of the Permian Epoch of North America, 

 Europe, and Africa just described, consisting exclusively of ter- 

 restrial and terrestrio-aquatic forms. In the Triassic we obtain 

 succeeding vistas of the terrestrial and fluviatile life of North 



