This order includes all of the hoofed animals native to the United States. These are the mammals which are ordinarily spoken of as the “cloven-hoofed animals.” An odd-toed group (_Perissodactyla_), which includes the so-called wild horses and burros, cannot properly be included as natives since these animals date back only to the time of the Spanish conquest of our Southwest. In earlier geologic ages horses ranged this continent, but in more primitive forms than those now found in other parts of the world. Through a study of fossil forms it has been determined that our present hoofed animals evolved from creatures which lived on the edges of the great tropical swamps that once covered large areas of our present land masses. They were long-legged and splay-footed, well adapted to an environment of deep mud and lush vegetation. As the waters gradually disappeared and the animals were forced to take to dry land, their strange feet underwent a slow transformation. Because they had become accustomed to walking on the tips of their toes to stay up out of the mud, the first toe did not touch solid ground at all in this new environment. Since it was of no use it soon vanished entirely or became vestigial. Some species developed a divided foot in which the second and third toes and the fourth and fifth toes combined respectively to bear the animal’s weight. Eventually the third and fourth toes assumed this responsibility alone, and the second and fifth toes became dew claws. These are the cloven-hoofed animals of today. In other species the third toe was developed to bear the weight, and this resulted in a single-toed group of which the horse is an example. In all cases an enormous modification of the nails or claws with which most animals are equipped has resulted in that protective covering called the hoof. The under surface of the foot is somewhat softer and corresponds to the heavy pad that protects the bottom of a dog’s toe. This brief explanation refers only in the broadest sense to the order as represented in the United States. The feet of the various species have become so specialized to their separate ways of life that an individual can usually be easily identified by its tracks alone. It is quite possible that many species are still undergoing subtle changes in this respect.