102 DIRECTION OF THE TRACK. 



footpaths, if there is nothing to confine them to a 

 particular direction. So that the formation of the 

 path is not a matter of reasoning and judgment at 

 all, but purely mechanical. When an animal, how- 

 ever small or large, walks, it must always so move 

 as that its centre of gravity is supported, otherwise 

 it would fall. Now that is obtained partly by the 

 motions of the legs, and partly by those of the 

 upper portions of the body. The more legs there 

 are, the centre of gravity has the more props, and 

 so there is less need for counterbalancing motions 

 in the body above. Ants have six legs, quadrupeds 

 four, and man only two ; so that man needs more 

 exertion of his body to balance himself than the 

 quadruped, and the quadruped more than the ant. 

 The man too is upright, and even the quadruped is 

 higher in proportion to its breadth than the ant. 

 Thus the centre of gravity swings by the longest 

 lever in the man in proportion to his whole weight, 

 and by the shortest in the insect; and thus the man 

 is more affected by the position of the surface on 

 which he walks than the quadruped ; and the quad- 

 ruped is more affected in that way than the insect. 

 Where the ground is perfectly level, the man's path 

 is nearly straight ; but if the ground rises to the one 

 hand, the path always takes a twist to the other, 

 because the foot which is on the high ground throws 

 the centre of gravity the other way, and the other 

 foot is advanced towards the low side, in order to 

 support the centre of gravity, and keep the body 

 steady ; and as long as the one side of the ground 

 continues higher than the other, the track continues 

 bending towards the low side. If the ground again 

 becomes level, the path, if not counteracted by ob- 

 servation and design, goes on in the last direction 

 of the body, how different soever that may be to 

 the direction of the path on former level ground ; 

 and if the ground begins to slope the other way, a 

 bend in the other direction takes place, uuless where 



