and these animals, with whom running and springing have become a constant habit, have come to use only their toes always; the heel no longer touches the ground except when the animal is crouching.
Now, in the foot of the crocodile (Fig. 1) it is seen that all the toes are not of the same length. If animals with such feet began to walk on their toes, some would not touch the ground. This, indeed, is what we find in many animals, especially in those cases where running is all that is required. In the feet of the dog, for instance,
| Fig. 1. | Fig. 2. | Fig. 3. | Fig. 4. |
only four toes touch the ground; the fifth toe, having long been less used than the others, has become very small. In the hind feet, which are used exclusively for running, the remains of the fifth toe can be felt as a small projection under the skin, some way up the back of the leg. In the fore foot of the dog, however, it is a distinct toe, with a nail, which is still used for digging.
In cats and dogs, as in the bears, claws are well developed. In the dogs they are short and hard but not very sharp, and are used chiefly for digging. The catlike animals, on the contrary, have very sharp, hooklike claws, which are of great use in seizing and holding prey.
It is obvious that if an animal, changing still further its method of walking, took to running on the tips of its nails or claws instead of on its toes, it would have a still greater length of leg, and therefore would be still better fitted for running swiftly. This remarkable manner of running is actually found in most of the swiftest four-footed animals, such as the horses and deer, and also in the cows, the sheep, and the pigs, which are therefore called ungulates, ungula meaning a nail. All these animals feed chiefly on grass, of which they need great quantities to nourish their usually large bodies. To obtain constant supplies of grass, in a wild condition, they have continually to roam from place to place. Their feet are admirably suited for these roaming habits, and also for very swift running, which is their best chance of escape from their enemies, the flesh-eating wolves, tigers, etc.
The foot of the bison (Fig. 4), in which the tips of only two toes
