THE EVOLUTION OF MAN 



the structure of the skeleton, the resemblance to 

 reptiles exceeds that to birds. The duckbill, the 

 contemporary of saurians, seems to lead directly 

 to the saurians, without touching the birds. 



The straight succession of our system misleads 

 us in this instance. Birds represent a subsequent 

 and one-sided branch line of reptiles, and have 

 evidently nothing at all to do with the develop- 

 ment of mammals. It is true that the birds have 

 also permanently warm blood like mammals, and 

 owing to this similarity they have been placed 

 side by side in the system. A bird has often 

 warmer blood than a mammal. But this again 

 is one of those qualities which, though indicating 

 a higher stage, were nevertheless acquired inde- 

 pendently in widely different periods. In this 

 connection we might point to the fact that the 

 representatives of other dissimilar groups of 

 animals have acquired the faculty of flying inde- 

 pendently and at far distant stages. This is the 

 case, for instance, with flies, bees, dragon-flies, 

 butterflies, flying-fish, frogs, such as the flying 

 frog of the Sunda Islands which flies by means of 

 a skin between its separate toes, and lizards, such 

 as the Australian flying lizard. There are, fur- 

 thermore, the birds, and among mammals, the 

 bats and the flying squirrels. There can be no 



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