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 zoologists 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 ani- 

 mals we can see what changes are necessary to convert a rep- 

 tile 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 tempera- 

 ture from the air about them. After a lizard has been bask- 

 ing 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 com- 

 binations go on faster at a high rather than at a low tempera- 

 ture, so all the parts of an animal work better when they are 



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