XIII THE GEOLOGICAL EVIDEXCES OF EVOLUTION 383 



Another illustration of the filling up of gaps between 

 existing groups is afforded by Professor Huxley's researches 

 on fossil crocodiles. The gap between the existing crocodiles 

 and the lizards is very wide, but as we go back ^n geological 

 time we meet with fossil forms which are to some extent 

 intermediate and form a connected series. The three living 

 genera — Crocodilus, Alligator, and Gavialis — are found in the 

 Eocene formation, and allied forms of another genus, Holops, 

 in the Chalk. From the Chalk backward to the Lias another 

 group of genera occurs, ha\dng anatomical characteristics 

 intermediate between the living crocodiles and the most 

 ancient forms. These, forming two genera Belodon and 

 Stagonolepis, are found in a still older formation, the Trias. 

 They have characters resembling some lizards, especially the 

 remarkable Hatteria of Xew Zealand, and have also some 

 resemblances to the Dinosaimans — re2:)tiles which in some 

 respects apjjroach birds. Considering how comparatively few 

 are the remains of this group of animals, the evidence which it 

 affords of progressive development is remarkably clear. ^ 



Among the higher animals the rhinoceros, the horse, and 

 the deer afford good e^ddence of advance in organisation and 

 of the filling up of the gaps which separate the living forms 

 from their nearest allies. The earliest ancestral forms of the 

 rhinoceroses occur in the Middle Eocene of the L^nited States, 

 and were to some extent intermediate between the rhinoceros 

 and tapir families, ha^ang like the latter foiu' toes to the front 

 feet, and three to those behind. These are followed in the 

 L^pper Eocene by the genus Amynodon, in Avhich the skull 

 assumes more distinctly the rhinocerotic type. FolloAnng this 

 in the Lower Miocene we have the Aceratherium, like the last 

 in its feet, but still more decidedly a rhinoceros in its general 

 structure. From this there are two diverging lines — one in 

 the Old World, the other in the Xew. In the former, to which 

 the Aceratherium is supposed to have migrated in early 

 Miocene times, when a mild climate and luxuriant vegetation 

 prevailed far ^Wthin the arctic circle, it gave rise to the 

 Ceratorhinus and the various horned rhinoceroses of late 

 Tertiary times and of those now living. Li America a 



^ On " Stagonolepis Robertson! and on the Evolution of the Crocodilia." in 

 Q. J. of Geological Society, 1875 ; and abstract in Naixwe^ vol. xii. p. 38. 



