BONES OF THE FEET 
The artiodactyls differ, then, from the perissodactyls most conspicu- 
ously in the form of the foot. Both have lost the plantigrade walk of their 
Tertiary ancestors, and now step on the tips of their toes. This has been 
gradually gained as an adaptation to the increase of dry land and the for- 
mation of grassy plains, which we know went on more and more as time 
advanced, especially 
through the last third 
of the Tertiary period. 
The short massive 
legs and spreading 
five-toed plantigrade 
feet, useful in sus- 
taining an animal’s 
weight in marshes, 
were slowly changed 
to longer, more slen- 
der limbs and a 
digitigrade walk, as 
greater speed and 
nimbleness were re- 
quired in making 
their way over wide 
pastures and to and 1, Pig. 2,Ox. 3, Tapir. 4, Horse. 
from watering places, Bones: &, radius; U, ulna; A, scaphoid : &, semi-lunar; 
or in escaping the C, cuneiform; D, trapezium; £, trapezoid; 7, magnum; 
G, unciform; P, pisiform; G, centrale carpi; //, meta- 
carpus (metapodials). The digits are numbered. 
SKELETON OF FORE FEET OF UNGULATES, 
beasts of prey which 
were themselves 
steadily becoming swifter and more active in jumping by a similar evolu- 
tion. The carnivores kept pace with them in every sense of the word. 
This useful alteration in limb structure, following changing habits, reached 
its utmost development in the two groups we are now considering, and 
brought about interesting alterations in the skeleton. The instep and palm 
bones (metapodials) were greatly lengthened, and the bonelets of the wrist 
and ankle (carpals and tarsals) were changed in form and rearranged. 
These changes proceeded from the earliest beginnings along two lines. 
In the first the third and fourth toes of the original five (practically the cen- 
tral ones, as the first, being useless, was lost almost at the start) were con- 
tinually forced to bear the weight and make the push as each step was taken, 
and consequently grew at the expense of the others, and equally; and this 
formed the two-toed, cloven, or artiodactyl style of foot. The outside 
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