170 CLASS REPTILIA. 



When, by the assistance of an alternate motion of the feet, 

 these animals desire to transport their body from one solid 

 place to another, different phenomena take place, according 

 as their feet are of similar length, or of unequal dimensions. 

 But it never happens, in this kind of progressive motion, 

 that the body is entirely suspended above the ground on 

 which the limbs are placed. 



In all such cases the entire body of the animal may be 

 compared to a spring with two branches, one of which leans 

 against a resisting object. If these branches, after having 

 been drawn together by any force, are restored to their pri- 

 mitive liberty, the power of their elasticity will tend to sepa- 

 rate them widely ; but the branch resting against the obstacle, 

 not being able to overcome it, transmits the motion, which 

 takes place entirely in the opposite direction, so that the 

 centre of gravity of the spring is removed from the obstacle 

 with a greater or less degree of velocity. 



In these animals, as in the mammifera, the flexor muscles 

 constitutes the force which compresses the spring ; the ex- 

 tensors represent the elasticity which separates its branches, 

 and the resistance of the ground or water, is the obstacle. 

 All the saurians whose feet are nearly equal in length, walk 

 with great vivacity. The lizards, one species of which has 

 received the epithet of velox, and another that of agilis, the 

 ameivas, the monitors, the agamae, the anolis, and many 

 others are in this predicament. In them, in the most natural 

 mode of walking, the body is in equilibrio on one of the 

 fore-feet and on the opposite hind-foot, so that the centre of 

 gravity does not move, following a right line, but advances 

 between two parallels, in the intervals of which it describes 

 oblique lines, which go from one to the other, forming zig- 

 zags. The impulses communicated to the trunk counter- 

 balance each other reciprocally, and the trunk itself moves 



