REPTILES, BIRDS, AND MAMMALS 



455 



Though Archaeopteryx could probably fly, it certainly could not have 

 flown well and may have used its wings chiefly for gliding. The fossils 

 give no clue as to how birds first acquired feathered wings. One theory is 

 that their ancestors were tree dwellers with projecting scale fringes on the 

 edges of the limbs and tail; that, jumping from branch to branch, they 

 spread their arms as parachutes to break their fall, and thus in time pro- 

 gressed to gliding flight. A second theory is that they were light-bodied 

 runners, and that the fringed arms and tail helped, by acting as planes, to 

 increase speed in running by lifting the animal somewhat off the ground. 



Fig. 28.20. Penguins have lost the power of flight; the wings have become swimming 

 paddles. A group of the Snares Island crested penguin (Eudyples atratus); one of the birds 

 is feeding its young. (Courtesy American Museum of Natural History.) 



Unlikely as either hypothesis may seem, flight itself is unlikely, and yet it 

 did somehow come about. 



Other characters in which modern birds differ from reptiles are : posses- 

 sion of warm blood, a four-chambered heart and double circulation, and 

 greatly increased development of the sight, balance, and coordination 

 centers of the brain — all these being features related to the demands of 

 flight, incubation of the eggs, and increased complexity of behavior, 

 including such phenomena as song, migration, social habits, etc. Archae- 

 opteryx was probably more reptile than bird in most of these respects. 



Later evolution of the birds. Bird fossils are rare. Two birds known 

 from Cretaceous time represent an intermediate stage between Archae- 

 opteryx and modern birds. They still had toothed jaws but in other 

 respects were much more birdlike than reptilelike. One, Ichthyornis, was 

 a small form with well-developed wings; the other, Hesperornis, was a 

 large marine diving bird without external traces of wings; only a reduced 

 shoulder girdle and rudimentary humerus remained. By the beginning 



