TEETH AND THEIR VARIATIONS. 211 



they are subjected in the process of chewing the cud, 

 and thus reducing the fibres of grass to a fine pulp. 

 Probably also this increased height of teeth enables 

 their owners to enjoy a longer duration of life than 

 was possible to the Tertiary ancestors of the Euminants. 

 Then, again, the consolidation of the bones of the 

 lower part of the leg into a single shaft is far better 

 adapted to withstand the strain of supporting long- 

 limbed and large-bodied animals when galloping at 

 a rapid pace than would be a limb with the separate 

 bones of the Pig. Indeed, the most specialised 

 Ruminants, such as the beautiful Antelopes of South 

 Africa, seem to be so admirably adapted to their mode 

 of life that it is impossible to imagine how this type 

 of animal life could be improved upon. 



When we turn to the second great group of 

 Ungulates, or that which contains the Tapirs, Pthino- 

 ceroses, and Horses, we shall find that a precisely 

 analogous series of modifications has taken place in 

 the structure of the teeth and feet, although it has 

 followed a different plan. All these animals are 

 characterised by the middle toe in each foot being 

 symmetrical in itself; and whereas all the ancient 

 forms, as well as the living Tapirs and Rhinoceroses, 

 have at least three toes to each foot, the highly 

 specialised Horse, as we all know, has but a single 

 very large toe to each limb. This reduction of the 

 number of toes and the accompanying lengthening of 

 the whole limb, has resulted in a speed which is out 

 of all comparison with that which can be attained by 



