16 



THE AGE OF MAMMALS 



types of hoofed animals. Another device of nature in the same direction 

 is the development of tongue-and-groove joints out of rounded or universal 

 joints, bringing about precision of movement and preventing lateral dislo- 

 cation. All these steps seem to have a distinct survival value, that is, to 

 affect mammals in the struggle for existence. Thus Kowalevsky was the 

 first to point out that certain quadrupeds with less perfected foot mechanisms 

 were gradually replaced by others with more perfected foot mechanisms. 



A lowly organized or primitive hoofed mammal is found to be full of 

 mechanical defects when its feet are applied to the severe tests of rapid 

 progression; it is condylarthrous, 'ball and socket' or loose-jointed, it is 

 plantigrade, or clumsy, or slow-gaited, it is pentadadyl, or with five fingers 



Fig. 6. — Foot proportions among rhinoceroses. A. Brachypody, short-footed, Teleoceras 

 fossiger. B. Mesatipody, medium-footed, Aphelops malacorhinus. C. Mesatipod}^, medium- 

 footed, Rhinoceros unicornis. D. Dolichopody, long-footed, Colodon longipes. 



still resting on the ground, it is often taxeopod, or with imperfectly alter- 

 nating joints in the different rows of bones in the hands and feet. As a rule 

 also it is short-footed, or brachypod. ■ 



In short, all the ancestral adaptations to ambulatory, tree, and rock- 

 climbing purposes must be abandoned and new adaptations acquired. 



Like the teeth, the feet evolve partly through changes of proportion 

 as well as through reduction, or loss of useless parts. Thus the passage 

 from slow-walking, or ambulatory types, to swift-moving, or cursorial types, 

 is almost invariably marked by elongation of the primitively short hand 

 and foot. This gives us another series of stages, as follows: 



Mesatipody, mean, or intermediate foot forms, as of the tapir. 

 Brachypody, extremely short feet, as of the elephant. 

 Dolichopody, elongate foot forms, as of the horse. 



Altogether the changes of foot structure in the hoofed animals con- 

 stitute a fascinating study and are easily comprehended. Through these 

 changes the prehensile and locomotor powers of the limbs of animals are 



