WHEELS VS. LEGS
The success of the automobile depends upon the character of the roads it has to traverse. Wheels provide the best form of transportation over a smooth, hard road, but if the road is soft, the wheel will sink into the surface and will be greatly impeded. In traveling over mud or sand runners are preferable to wheels because they have a broader and longer bearing surface. In the snowy regions of the north and the sandy regions of Africa and Asia Minor, sledges are used in place of wheeled vehicles. Sliding friction is less than rolling friction under such conditions. In fact, only where man has constructed special roads is it possible to use wheeled locomotion. It is because in nature we must contend with all sorts of surface conditions, soft and hard and seldom smooth, that the rolling principle of locomotion is not to be found in any species of animal. The legs of an animal are levers, just as a wheel is a system of levers, but in the former case the levers can be folded or extended to adapt themselves to all the unevennesses of the ground. No animal can begin to run as fast as an automobile on a good road, but on the other hand we have yet to build a machine which will run as fast as a horse on soft and uneven ground.