THE TURKEY 89 



These vegetable feeders, again, were preyed on by fell 

 monsters compared with which our lions and tigers seem 

 insignificant. The air also was agitated ceaselessly by 

 the wings of flying reptiles (pterodactyles) of all sizes, 

 flitting like bats, but sometimes with the proportions 

 of the albatross. 



This hasty glance at the two classes of beasts and 

 reptiles will enable us to appreciate the distinctness of 

 the class of birds. They are the most easily defined of 

 any class, since the single epithet " feathered " suflices to 

 characterise them. It does so because every bird has 

 feathers, while no such thing is possessed by any creature 

 which is not a bird. 



Birds, as we have already said, stand between beasts 

 and reptiles, but are widely distinct from them both. 

 All beasts possess, as we possess, warm blood, but the 

 blood of the bird is warmer still, and thus birds diflter 

 greatly from reptiles, in spite of their possessing certain 

 structural characters in common with them. For the 

 reptile the blood is hardly ever warmer than the 

 medium which may surround it, the only exception 

 known to us being that which occurs in a boa -constrictor 

 or python, when hatching its eggs, around which it twists 

 itself in a conical coil, surmounted by the ever-watchful 

 head. 



But birds differ from both beasts and reptiles, by 

 that singular uniformity of their structure before 

 adverted to. Some beasts and reptiles have but a 

 single pair of limbs. Thus the whales and porpoises 

 have but a single pair each, and some reptiles — serpents 

 and some lizards — have none, but every bird has two 

 pairs. The limbs of beasts and reptiles may be con- 

 structed variously. How different are the wings of the 

 bat from the scoop of the mole, the paddle of the 



