52 THE LESSON OF EVOLUTION 



diles, alligators, and ga vials 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 

 convex behind. The inner opening of the nostrils 

 has also gradually moved backwards, from the 

 middle of the roof of the mouth in the early forms, 

 to the base of the skull in living forms. 



Archeopteryx, from the upper Jurassic of Bavaria, 

 is the earliest known bird. At first it was thought 

 to be a reptile, because it has three free fingers with 

 claws on each hand, biconcave vertebrae, a long 

 lizard-like tail, abdominal ribs, and teeth in its jaws. 

 Also the bones of the pelvis are not united together, 

 and even the metatarsi of the leg seem to be but 

 imperfectly joined ; all of which are reptilian charac- 

 ters. Nevertheless, the presence of feathers on the 

 wings and tail, the structure of the foot, and the 

 fact that all the bones of the skull are fused into one, 

 are such truly avian characters that all naturalists 

 now agree that it should be considered as a bird. 

 Only two specimens are known. In the middle 

 Cretaceous of North America the remains of other 

 birds have been found, all of which had teeth in their 

 jaws. There were two distinct types, one of which 

 Ichthyornis had the teeth in separate sockets 

 and biconcave vertebrae, while the other Hesperor- 

 nis had the teeth in a single groove in each jaw, 

 and the centra of the vertebrae were saddle-shaped, 

 as in ordinary birds. Hesperornis was a swimming 

 bird, which had lost the power of flying, and its 

 wings were in a very degenerate state ; but Ich- 



