162 



CHAPTERS ON EVOLUTION. 



already remarked, in a common groove, strongly remind one of the 

 manner in which the teeth of certain lizards are fixed in their jaws. 

 Some of the teeth of this curious bird also exhibit the manner in which 

 one series of teeth was replaced by another for, as most readers 

 know, reptiles and fishes possess an unlimited supply and a continual 

 succession of teeth. The old teeth are ousted from their sockets 

 by new teeth which are developed at their bases ; and in the jaws 

 of Hesperornis such a manner of tooth-formation, exactly imitating 

 a common reptilian mode of renewal, is to be plainly seen. The tail 

 of this great diver of the Chalk seas was, lastly, like that of the 

 Archseopteryx of the Oolite epoch, and exhibited a very different 

 structure from the caudal appendage of existing and of other fossil 

 birds. At its middle and under parts the joints of the tail present 

 long projections of flattened shape, which strongly suggest the idea 

 of the tail having been a rigid unyielding member in so far as a side 

 movement was concerned, but, like that of the beaver, being probably 

 mobile in a vertical direction, and being thus of use in the diving 

 movements of its possessor. The last joints of the tail were massed 

 together, but in a fashion different from that in which the " plough- 

 share-bone " of living birds is formed. 



In so far as the birds themselves have rendered an account of 

 their past history, it is clearly seen that 

 their affinities to reptiles become very 

 strongly marked in various directions, 

 especially in the structure of the spine, 

 and in the possession of true teeth. 

 Ichthyornis, in the matter of its hol- 

 lowed spine-bones (Fig. 82, B, c\ and 

 in that of its socket-implanted teeth, is 

 a more modified and more truly reptile- 

 like bird than Hesperornis. This latter 

 again, approaches much nearer reptiles 

 than Odontopteryx (Fig. 83) of the 

 London Clay, which latter, as becomes 

 its nearer approach to the existing order 

 of affairs, presents a less marked rela- 

 tionship with " the dragons of the 

 prime." 



But what evidence, we may lastly 

 FJ G- 84. ask, do the reptiles afford on their side 



RESTORATION OF COMPSOGNATHUS. f ' , , ,1 , > 



of any tendency towards the bird type ? 



Have the reptiles remained as passive to advance and evolution, as 

 they would appear at first sight to remain to-day ; or does their history 

 but repeat the changes and variations exhibited by their bird-neigh- 

 bours ? Let the history of the reptile class in the past answer these 



