178 Darwin, and after Darwin. 
understood, that although in the case of many other 
mammalia some of these bones may be dwindled or 
altogether absent, while others may be greatly ex- 
aggerated as to relative size, in no case do any 
additional bones appear. 
On looking, then, at the skeleton of a bear (Fig. 74), 
the first thing to observe is that there is a perfect serial 
homology between the bones of the hind legs and of 
the fore legs. The thigh-bone, or femur, corresponds 
to the shoulder-bone, or humerus; the two shank 
bones (tibia and fibula) correspond to the two arm-bones 
(radius and ulna); the many little ankle-bones (tarsals) 
correspond to the many little wrist-bones (carpals) ; 
the foot-bones (meta-tarsals) correspond to the hand- 
bones (meta-carpals) ; and, lastly, the bones of each 
of the toes correspond to those of each of the fingers. 
The next thing to observe is, that the disposition of 
bones in the case of the bear is such that the animal 
walks in the way that has been called plantigrade. 
That is to say, all the bones of the fingers, as well as 
those of the toes, feet, and ankles, rest upon the ground, 
or help to constitute the “soles.” Our own feet are 
constructed ona closely similar pattern. But in the 
majority of living mammalian forms this is not the 
case. For the majority of mammals are what has 
been called digitigrade. That is to say, the bones of 
the limb are so disposed that both the foot and hand 
bones, and therefore also the ankle and wrist, are 
removed from the ground altogether, so that the 
animal walks exclusively upon its toes and fingers—as 
in the case of this skeleton (Fig. 75), which is the skele- 
ton of a lion. The next figures display a series of 
limbs, showing the progressive passage of a completely 
