162), COMPARATIVE ZOOLOGY. 
The order in which the legs of quadrupeds succeed 
each other determines the various modes of progression, 
called the walk, trot, gallop, and leap. Many, as the 
Horse, have all these movements; while some only leap, 
as the Frog and Kangaroo. In leaping animals, the hind 
limbs are extraordinarily developed. In many Mam- 
mals, like the Squirrel, Cat, and Dog, the fore legs are 
used for prehension as well as locomotion, and such have 
a collar-bone. Monkeys use all four, and also the tail, for 
mm 
Fic. 198.—Feet of Hoofed Mammals: A, Elephant; B, Hippopotamus; C, Rhinoc- 
eros; D, Ox; H, Horse. 4, astragalus; cl, caleaneum, or heel; s, naviculare ; 0B, 
cuboides; ce, ci, em, cuneiform bones ; the numbers indicate the digits in use. 
locomotion and prehension, keeping a horizontal attitude ; 
while the Apes, half erect, as if they were half-quadruped, 
half-biped, go shambling along, touching the ground with 
the knuckles of one hand and then of the other. In de- 
scending the scale, from the most anthropoid Ape to the 
true quadruped, we find the centre of gravity placed in- 
creasingly higher up—that is, farther forward. Birds 
and Men are the only true bipeds; the former standing 
on their toes, the latter on the soles of the feet. Terres- 
trial Birds walk and run; while Birds of Flight usually 
