102 



THE THEORY OF EVOLUTION 



mud. A small, nodular rudiment of the fifth toe is 

 also present. The horses of the middle and lower 



FIG. 7. Four stages in the evolution of the equine skull. A, Eohippus, 

 lower Eocene. B, Mesohippus, lower Oligocene. C, Protohippua, 

 upper Miocene. D, Equus, Pleistocene and Recent. 



Miocene display the transition between the high- 

 crowned, cement-covered grinding teeth and those 



