ACTS OF WALKING AND RUNNING. 
495 
(fig. 240), the wing of a Bat (fig. 251), and the fore-paw of a 
Squirrel or a Mole, we find the same bones as in the arm of 
Man (fig. 223). And even in the fore-legs of the Buminant 
Quadrupeds, and in those of the Solidungula , or single-toed 
animals (such as the Horse), we can usually perceive traces of 
the existence of three or four toes, whose bones are more or 
less completely united. 
659. From what has already been stated as to the influence 
of the length of the levers on the quickness of the movement 
of the extremities (§. 614), it is easy to see that animals 
which have the most rapid progression must necessarily have 
long members ; since, the quickness with which the extensor 
muscles act remaining the same, the change of place in the 
free extremity of the lever will be greater, in proportion as 
that extremity is more distant from the point of insertion of 
the muscles that move it, and from the fulcrum on which the 
lever works. But in proportion to the elongation of this arm 
of the lever, must be the increase in the power of the muscles 
that move it, in order to overcome the same resistance ; 
according to the general principle that what is gained in 
velocity is lost in power. Hence, in order to endow an animal 
with great agility, it is only necessary to lengthen its limbs, 
and to render its muscles capable of exerting a proportional 
power. 
660. We have seen that in walking , the body is sustained 
upon one limb (in quadrupeds , upon one pair of limbs), 
whilst it is pushed onwards by the other ; so that it never 
ceases to bear upon the ground. In running , however, the 
body of Man momentarily quits its support at intervals ; the 
foot in advance not being planted on the ground by the time 
that the hinder one quits it. In this action, the Ostrich and 
its allies probably surpass all other animals ; as they can out¬ 
strip the fleetest horse at full gallop, or the swiftest greyhound 
at its greatest speed. The awhle of Quadrupeds is a pace 
which resembles the walk or run of bipeds, the two legs on 
one side being moved together, whilst the body rests upon the 
other. This pace is peculiar to the Giraffe, and to horses 
which have been trained to execute it. The trot , however, is 
a step of a different and much more secure nature. The fore¬ 
foot of one side is raised and advanced with the hind foot on 
the other side ; and when these are set down, the other fore 
