THE EVOLUTION OF THE BIRD
tures of an animal are derived from ancestors to which they were useful, and that they have not yet been lost or fully utilized.
I purpose here to sketch the mode of origin of a single kind of animal—the bird—though perhaps I may not be able to do so very well for all my readers, because many of them may not have much knowledge of anatomy and physiology, and it is to these sciences that I shall turn for my evidence.
All zoölogists now believe that the birds, which in the flight of an eagle or an albatross show a special mode of life in its highest perfection, have arisen by a long process of change from reptiles—that is, from creatures similar in their structure and appearance to lizards and crocodiles. The actual reptilian ancestors of the birds are no longer living, but we know several animals that are closely related to these ancestors. By examining the structure of these related animals we can see what changes are necessary to convert a reptile into a bird, and we can show that by postulating such conversion many of the anomalous details of the structure of a bird may be explained.
Everyone who has watched lizards knows that soon after the sun rises they come out of the holes in the ground in which they sleep and gradually become more and more active as the day goes on. This increase in their vigor depends entirely on increase in temperature. The birds and mammals have the means of keeping their bodies warm at a uniform temperature, but the reptiles take their temperature from the air about them. After a lizard has been basking long in sunlight it may be almost uncomfortably hot to touch, and during a cold night its temperature may fall almost to the freezing point. Just as most chemical combinations go on faster at a high rather than at a low temperature, so all the parts of an animal work better when they are
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