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 on a 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 



