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 mammalian 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 or (A) THE Tarir; (B), THE RHINOCEROS 
AND (C) THE HORSE 
(After FLowEr.) 
Ulna. 
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, z.¢., the other Perissodactyla and Artiodactyla respectively, 
arrange themselves in two series, thus :— 
Perissodactyla— Artiodactyla— 
‘Tapir. Pig. : 
Rhinoceros. Chevrotain. 
Horse. 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 
