PROGRESSIVE MOTION IN QU.VDKUPEDS. 437 



served. It is an elementary proposition in me- 

 chanics tliat all that is necessary for ensuring the 

 support of a body on any given base, is that the ver- 

 tical line drawn from the centre of gravity shall fall 

 within that base. When the animal is standing, 

 the feet form a quadrilateral base, and the centre of 

 gravity is in a vertical line passing either through 

 the centre of the base, or, as, for the reasons already 

 mentioned, more frequently happens, through a 

 point a little in front of the exact centre. At the 

 time when the hind foot which began the action is 

 raised from the ground, the centre of gravity, having 

 been, by that action, impelled forwards, still re- 

 mains above the base formed by the other three 

 feet, and which is now reduced to a triangle. That 

 hind foot being set down, while the corresponding- 

 fore foot is raised, a new triangular base is formed 

 by the same hind foot, together with the two of the 

 other side, which have not yet been raised. The 

 centre of gravity is still situated above this new tri- 

 angle, and the body is consequently still supported 

 on these three feet. The fore foot may now be ad- 

 vanced, without endangering the stability of the 

 body ; and by the time this foot is set down, and 

 has thereby formed a new quadrilateral base with 

 the other feet, the centre of gravity has arrived 

 above the centre of this new base. But at this 

 moment the centre of gravity is again urged for- 

 wards by the other hind foot, which now comes 

 into action, and repeats on the other side the same 

 succession of actions, which are attended with the 

 same consequences as before. Thus, during its 

 whole progress, the animal is never for an instant 

 in danger of falling ; for whichever of the feet may 



