MAMMALIA. 



521 



ancestor which had five toes, was plantigrade, and had other primitive 

 characters ? The proofs are several. 



Firstly, we note that the horse has splint bones or vestiges of meta- 

 podials, 2 and 4, and that the ox has two complete though small 

 vestigial digits, the second and fifth, making four in all. As the 

 pentadactyle limb is the only type from which all niammalian limbs 

 can be derived by a supposition of fusions and reductions having taken 

 place in the course of evolution, it is legitimate to infer that these forms 

 have degraded from this type and lost four and three functional toes 

 respectively. 



Fig. 360.— The Manus of (A) the Tapir ; (B), the Rhinoceros 

 AND (C) THE Horse 



(After Flower.) 



Ulna. 



2. 



Sn 



Note the alternate carpal bones and the predominant third digit in each, but the 

 gradual reduction in the other digits. 



Secondly, the types which are most kindred in structure to the horse 

 and the ox, i.e.^ the other Perissodadyla and Artiodactyla respectively, 

 arrange themselves in two series, thus : — 



Perissodadyla— Artiodactyla— 



Tapir. Pig. 



Rhinoceros. 

 Horse. 



Chevrotain. 

 Ox. 



In all three of the first series the third toe is the largest and strongest, 

 but whilst the tapir has four toes touching the ground (in the fore 



