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 trav- 

 elled in a series of great leaps. They must then have 



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