98 LECTURES ON EVOLUTION m 



have been extended to all birds ; it might have been 

 said that the absence of teeth was characteristic 

 of the class of birds; but the discovery of an 

 animal which, in every part of its skeleton, closely 

 agrees with existing birds, and yet possesses teeth, 

 shows that there were ancient birds which, in 

 respect of possessing teeth, approached reptiles 

 more nearly than any existing bird does, and, to 

 that extent, diminishes the hiatus between the two 

 classes. 



The same formation has yielded another bird 

 Ichthyornis (Fig. 5), which also possesses teeth ; 

 but the teeth are situated in distinct sockets, while 

 those of Hesperornis are not so lodged. The lat- 

 ter also has such very small, almost rudimentary 

 wings, that it must have been chiefly a swimmer 

 and a diver like a Penguin ; while Ichthyornis has 

 strong wings and no doubt possessed correspond- 

 ing powers of flight. Ichthyornis also differed in 

 the fact that its vertebrae have not the peculiar 

 characters of the vertebrae of existing and of all 

 known tertiary birds, but were concave at each 

 end. This discovery leads us to make a further 

 modification in the definition of the group of 

 birds, and to part with another of the characters 

 by which almost all existing birds are distinguished 

 from reptiles. 



Apart from the few fragmentary remains from 

 the English greensand, to which I have referred, 

 the Mesozoic rocks, older than those in which 



