Sometimes the differences between these apparently related forms, as we find them in the rocks, are very great; but everything goes to show that this is because there are missing pages, so to speak, in the great stone book. When you remember how long it takes to make one of these layers of stone, and what they go through in cracking and twisting and wearing down on their way back to dust and the sea, and how quickly the remains of big animals--to say nothing of plants and insects--are destroyed, you must agree that the wonder is that we have any records at all. Yet so enormous has been the number of plants and animals that have died in the course of the world's history that there have been found hundreds and thousands of these remains and imprints between the layers of stone. In all cases the fashions in form change from age to age; and the longer the time, as shown by the thickness of the rock, the greater the change. THE RABBIT THAT TURNED INTO A HORSE The horse, which has been such a faithful carrier for man since man and horse arrived from the lower ranges of life, also brought with him on the way up one of the most complete of these strange autobiographies that our brother animals have recorded with their bones. The most of this story of the horse was found in the rocks of our Western States, but the first chapter of it saw the light about forty years ago in England. When the bones were found in the rock deposits of that country known as London Clay they looked so unhorselike that a famous paleontologist (as the students of these ancient anatomies are called) gave it a name which means "rabbit-like beast." But in rock of the same age in Wyoming they afterward found the bones of an animal that looked a little more like a horse, but plainly a close relation of the rabbit-like beast. They went on finding different forms, through thirteen successive stages of rock history, and with each new period the form kept getting larger and more horselike until they came to a horse with three toes; and finally to one with the single big toe which we call a hoof. Instead of the other two toes there were those two little lumps that you can feel in any horse's foot just above the hoof. These are the ends of two small splintlike bones that are all there is left of the other two toes.