84 THE LESSON OF EVOLUTION 



or flying reptiles, lived in the Jurassic and Cretaceous 

 periods. They stand apart from other reptiles, show- 

 ing only very slight relations to the Anomodontia. 

 They have some bird -like characters such as a 

 keeled sternum and hollow bones but these are only 

 adaptive, and essential for flight ; they show no real 

 relationship to birds. The skull also looks something 

 like that of a bird, but it is really more like that of a 

 lizard. 



All the Dinosaurians lived on land, and some of 

 them were the largest of land animals, being over a 

 hundred feet in length. They differed greatly in 

 appearance ; for, while the earlier forms had the shape 

 of crocodiles, to which they were closely related, others 

 resembled in shape the rhinoceros, and others again 

 the kangaroo ; for they walked on their hind feet, 

 which had only three toes, and their fore-legs were 

 sometimes small. In some of these, the bones of the 

 pelvis make a considerable approach to that of birds ; 

 and Professor Marsh has described, from the American 

 Jurassic rocks, some small Dinosaurians, apparently 

 closely related to birds, which he thinks may have 

 been arboreal in their habits. " The difference," he 

 says, "between them and the birds that lived with 

 them may have been at first mainly one of feathers." x 

 The early Crocodiles are difficult to distinguish from Dino- 

 saurians, but afterwards they become more specialised ; 

 and there is abundant evidence to show that the 

 modern crocodiles, alligators, and gavials are derived 

 from the early generalised type. At first the centra 

 of the vertebrae were hollowed at both ends, then they 

 became nearly flat, and then hollow in front and 

 1 American Journal of Scieim, vol. xxii. (1881), p. 340. 



