CREATION BY EVOLUTION 



kept warm. The heart of a frog beats nearly twice as fast 

 at 68° as at 50°. Thus if an animal is to be equally active 

 under all climatic conditions it should be able to keep its 

 body at a constant fairly high temperature. 



An animal actually warms itself by moving its muscles, 

 and it is able to keep its muscles going only by burning 

 oxygen taken from the air, or, rather, by the natural process 

 of breathing. The maintenance of a high bodily tempera- 

 ture uses up a good deal of food, and for mere economy it is 

 desirable to reduce the quantity required by providing the 

 animal with a coat that will allow its heat to escape very 

 slowly. This is the original reason for the fur that covers 

 a mammal and for the feathers that cover a bird. Probably 

 one of the first steps in converting a reptile into a bird is to 

 change its scales to feathers. Yet when we compare a wing 

 or tail feather of a bird with a scale it seems at first impos- 

 sible that the one should have come from the other; but the 

 first feathers of the chick — those which it grows while it is 

 still in the egg — consist of very short scale-like quills, whose 

 ends fray out into fine plumes. These feathers are formed 

 from the upper layers of the skin in exactly the same way 

 as the scales of lizards are formed; indeed, they differ from 

 such scales only in being longer. Between these incipient 

 feathers and those which we know as quills we find all 

 intermediate stages. 



In order to enable the bird ancestor to utilize fully the 

 increased activity made possible by its higher body tempera- 

 ture many changes of its structure were necessary. One of 

 the most important of these has to do with the heart. A 

 lizard can run very fast for a short distance, but it then col- 

 lapses, completely exhausted, whereas a mammal or bird can 

 hardly work so fast and so long that its muscles will no 

 longer contract. This difference is due to the fact that 



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