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 temperature 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 impossible 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 temperature 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 collapses, 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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