726 PERISSODACTYLS xxx. 3- 



direct line of evolution to Equus. Abundant though the material is, 

 we have not, therefore, anything like a complete series of fossils to 

 show every shade and grade of change of the populations throughout 

 the 50 million years or so of their evolution. Our knowledge is based 

 on a small sample of individuals, preserved at random at scattered 

 intervals. The remains often suggest evolutionary sequences and many 

 accounts speak confidently of changes and trends. We shall try, even 



CERA TOMORPH A 



HIPPOMORPHA 



Fig. 480. Chart of the evolution of the perissodactyls. 



in this brief account, to describe the actual discoveries and to indicate 

 clearly what evidence is available for evolutionary speculation. With 

 all the mass of information we possess it must yet be realized that the 

 study of the details of perissodactyl evolution has hardly been begun, 

 for example we have little quantitative information about the vari- 

 ability of the characters concerned. 



The earliest perissodactyls had departed but little from condylarth 

 conditions. *Hyracotherium{= *Eohippns) from the Eocene of Europe 

 and North America (Fig. 479) was the size of a dog and resembled the 

 condylarth *Phenacodus. The tooth row was complete, with square 

 bunodont molars (Fig. 467). In addition to the four main upper molar 

 cusps an anterior protoconule and posterior metaconule are suggested, 

 and between these and their neighbours the dentine is partly built up 

 to form two transverse ridges. The premolars were tritubercular. The 



