THE EVOLUTION OF THE BIRD
become separated into two groups, one of which fuses with the shin bone and the other becomes part of a single bone that corresponds with the bones of the arch of the foot and supports the three large toes. Thus the bird gradually comes to have a simple and very strong foot.
All animals that run very fast run on their toes, and the bird ancestor, in order to run more conveniently, permanently raised its heel from the ground. This change enabled the arch of its foot to be lengthened, so that with each stride it covered more ground.
Our bird ancestor has thus become a creature with long legs and a tail, capable of running very rapidly on its hind legs alone, the fore legs and hands being carried in the air. Such an animal, if it makes full use of its speed, must be able to use its hands for capturing prey and for carrying food. In order that it may do this its fore legs should remain rather short and the fingers should be provided with claws for holding the prey securely. But a clawed foot that is used for holding a struggling animal is an encumbrance if it retains all five fingers, and we find in many reptiles, even nascently in a crocodile, that the fourth and fifth fingers become smaller and smaller and finally disappear. A hand that is used for handling food must be capable of being turned about, and the arm that supports it must be freely movable. This stage of bird evolution is represented by many of the extinct reptiles that are called dinosaurs. These animals are not the direct ancestors of the birds, but they are close relatives, which went part of the way with them but, probably because they never developed feathers, were unable to make the last great step and begin to fly.
As the bird ancestors became able to run faster and faster they soon reached a stage when, like kangaroos, they travelled in a series of great leaps. They must then have
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