xiii THE GEOLOGICAL EVIDENCES 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 Avide, but as we go back in 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, having anatomical characteristics 

 intermediate between the living crocodiles and the most 

 ancient forms. These, forming tAvo genera Belodon and 

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

 They have characters resembling some lizards, especially the 

 remarkable Hatteria of New Zealand, and have also some 

 resemblances to the Dinosaurians — reptiles which in some 

 respects approach birds. Considering how comparatively few 

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

 aJSbrds of jDrogressive development is remarkably clear. ^ 



Among the higher animals the rhinoceros, the horse, and 

 the deer afford good eAddence of advance in organisation and 

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

 from their nearest allies. The earliest ancestral forms of the 

 rhinoceroses occur in the Middle Eocene of the United States, 

 and were to some extent intermediate betAveen the rhinoceros 

 and tapir families, haAdng like the latter four toes to the front 

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

 Upper Eocene by the genus Amynodon, in Avhich the skull 

 assumes more distinctly the rhinocerotic tyj^e. Following this 

 in the LoAA^er Miocene Ave 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 Ncav. In the former, to Avhich 

 the Aceratherium is supposed to have migrated in early 

 Miocene times, Avhen a mild climate and luxuriant A'egetation 

 prevailed far AAathin the arctic circle, it gave rise to the 

 Ceratorhiniis and the various horned rhinoceroses of late 

 Tertiary times and of those noAV living. In America a 



1 On " Sta^ronolepis Robertsoui and on the Evolution of tlie Crocodilia," in 

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



