THE DEVELOPMENT OF THE ANIMAL WORLD 81 



in temperature and rainfall would not only reduce the 

 rich swamp vegetation on which the large herbivorous 

 monsters lived, but would of itself drive them either to 

 retreat into hot and moist regions or to adapt themselves 

 to the new conditions to advance a step in organisa- 

 tion. Most of the reptiles perished, a few orders 

 retreated to the south, and what is most interesting 

 two distinct types adapted themselves (or evolved 

 adaptation) to the new conditions, and became the 

 ancestors of the bird and the mammal. To do this they 

 needed to develop their scales into either feathers or fur, 

 to improve the heart so as to become warm-blooded 

 animals, and to evolve structures for developing their 

 young inside the mother's body or by hatching. 



The evolution of the bird from the reptile is plain, as 

 several of the " missing links " have been discovered. In 

 some German Jurassic limestone complete skeletons, with 

 feathers, have been found of a creature (Archceopteryx} 

 that combines features of the reptile and the bird. Its 

 jaws are lined with teeth, it has claws on its wing limb 

 as well as foot, and its long tail is that of a lizard with 

 feathers growing out of each vertebra. Even later, fossil 

 toothed birds (Hesperornis, etc.) have been found in the 

 Cretaceous. With these links it is inevitable to connect 

 the bird with the reptile. Some of the smaller Dinosaurs 

 had hollow bones, and only four toes, and may be of 

 a common ancestor with the bird. Probably they 

 originated from one of the smaller leaping lizards with 

 broader scales on its feet. An expansion of the scales 

 would tend to give a fan-like action on the air and 

 lengthen the leap, and in time the fore-limb would evolve 

 into a wing. The claws would then cease to be of use 

 and die away. 



To follow the development of the Archaeopteryx into 

 the wide-branching family of the modern birds would be 



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