INTEODUCTION^ TO MAMMALIAN PALEONTOLOGY 



29 



crops grass with its lips and front teeth. In all 

 the rhinoceroses cropping front teeth are atrophied, 

 the four pairs of incisors and the canines being 

 reduced to a single large pair on either side and being 

 thus analogous to those of certain titanotheres. 



From these comparisons we deduce the structure 

 of the mouth parts in the titanotheres as restored by 

 Gregory. (See p. 704.) We also deduce the various 

 adaptations to the browsing and grazing habit re- 

 spectively in the different genera of titanotheres, for 

 undoubtedly some were purely browsers and others 



of the face, with a relatively short skull, and with a 

 very powerful neck, a feature that is also especially 

 characteristic of the titanotheres. 



Thus there is a general resemblance between the 

 side profile of Brontotherium platyceras and that of the 

 Indian rhinoceros, which is due to analogous mechan- 

 ical evolution, through the principles known as homo- 

 plasy, parallelism, or convergence. The titanotheres 

 pass through a long lower and middle Eocene phase of 

 tapir-like analogies, but when, in middle Eocene time, 

 horns begin to appear the head region develops 



Figure 21. — Phyletio divergence in the evolution of new proportions in horses and in titanotheres 



Lower Eocene ancestral horse Eohippus (A) and lower Eocene ancestral titanothere Eotilaijops (C) (both with the orbit in the same relative 

 position on the skull) compared with a modern horse (B) with face extended in front of the orbit and a titanothere of the latest stage (D) with 

 slvuU extended behind the orbit. Thus two very similar heads (A, C) become increasingly dissimilar (B, D). Scales various. 



tended toward grazing. Thus the orbits, the face, 

 the grinding teeth, the front teeth, the lips, and the 

 bones supporting these structures are respectively 

 transformed in adaptation to the function of prehen- 

 sion and to browsing or grazing habits. The front 

 part of the skull of the rhinoceros, with its terminal 

 dermal horn, is comparable to that of the large-horned 

 titanotheres, with their terminal bony horns. It will 

 be observed that the entire front part of the head of 

 the rhinoceros, in adaptation to the great strain of 

 the horn used as a weapon of offense and defense, is 

 correlated with a flat or a concave line along the top 



rhinoceros-like analogies. Similar analogous phases 

 also occur to a greater or less extent in the feet of the 

 rhinoceros and the titanothere. 



On comparing the heads of the types of perisso- 

 dactyls, ancient and modern, we observe that different 

 modes of feeding and of offense and defense guide the 

 dominant adaptations in evolution. The evolution 

 operates under the principles of anatomical correla- 

 tion and compensation, gain or loss in one part being 

 mechanically balanced by gains and losses in every 

 other part. This process includes the principle of 

 physiologic compensation, whereby loss of function in 



