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 wide, 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 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 New Zealand, and have also some 
resemblances to the Dinosaurians—reptiles which in some 
resjjects approach birds. Considering how comparatively few 
are the remains of this group of animals, the evidence which it 
affords of progressive development is remarkably clear. 1 
Among the higher animals the rhinoceros, the horse, and 
the deer afford good evidence 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 United States, 
and were to some extent intermediate between the rhinoceros 
and tapir families, having 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 which the skull 
assumes more distinctly the rhinocerotic type. Following 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 New. 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 within the arctic circle, it gave rise to the 
Ceratorhinus and the various horned rhinoceroses of late 
Tertiary times and of those now living. In America a 
1 On “ Stagonolepis Robertsoni and on the Evolution of the Crocodilia.” in 
Q. J. of Geological Society, 1875 ; aud abstract in Nature, vol. xii. p. 38. 
