134 PHILOSOPHY OF ZOOLOGY. 
time, the latter bends in order to its being moved forward. 
While they are off the ground, the right hind-foot begins 
toextend itself, and the moment they touch the ground, 
the léft fore-foot moves forward to support the impulse of 
the right foot, which likewise moves forward. The body 
is thus supported alternately by two legs -placed in a dia- 
gonal manner. When the right fore-foot moves, in order to 
sustain the body, pushed forward by the right hind-foot, 
the motion is then called an amble. 'The body, being al- 
ternately supported by two legs on the same side, is obli- 
ged to balance itself to the right and left, in order to avoid 
falling; and it is this balancing movement which renders 
the gait so soft and agreeable to women and persons in a 
weak state of body *. 
In walking with two feet, as takes place in man and birds, 
the whole weight of the body rests on each leg alternately ; 
so that the movement is in’a zig-zag direction. ‘These un- 
dulations are rendered more conspicuous in some birds, by 
the form of their feet and toes, by which they are prevented 
from moving them ‘in the direction of the mesial line; but 
are, at each step, compelled to move the leg outwards, in a 
semicircular manner. In some quadrupeds, as the seal, 
and in many reptiles, the limbs are placed perpendicular to 
the mesial line ; and, in the progressive motion, the body is 
dragged along the ground, as the flexion and extension of 
the limbs are unable to elevate it above the surface. Such 
animals are said to cravwl. 
But, besides these modes of walking, which we have now 
stated, there are others, in which the progressive motion is 
sunilar, but the means employed to accomplish it are diffe- 
rent. The principal of these may be termed the Serpen- 
tine’ Motion. It consists im bringing up the tail towards 

ee _— 
* Cuvigr’s Comp, Anat, Lect. vii. 
