2 CHARACTERS OF BIRDS. 



The characters that distinguish birds from mammals 

 on the one hand, and from reptiles on the other, are more 

 apparent than real. Thus flight, the most striking of a 

 bird's gifts, is shared by bats among mammals. Egg-lay- 

 ing is the habit of most reptiles and of three mammals 

 (the Australian duckbill and the echidnas). But incuba- 

 tion by one or both of the parents is peculiar to birds, 

 though the python is said to coil on its eggs. 



Birds breathe more rapidly than either mammals or 

 reptiles, and their pneumaticity, or power of inflating 

 numerous air-sacs and even certain bones, is unique. 



The temperature of birds ranges from 100° to 112°, 

 while in mammals it reaches 98° to 100°, and in the com- 

 paratively cold-blooded reptiles it averages only 40°. 



The skull in mammals articulates with the last verte- 

 bra (atlas) by two condyles or balls ; in birds and reptiles 

 by only one. In mammals and birds the heart has four 

 chambers ; in reptiles it has but three. 



Mammals and reptiles both have teeth, a character 

 possessed by no existing bird ; but fossil birds appar- 

 ently prove that early in the development of the class 

 all birds had teeth. 



Thus we might continue the comparison, finding that 

 birds have no universal peculiarities of structure which 

 are not present in some degree in either mammals or 

 reptiles, until we come to their external covering. The 

 reptile is scaled, and so is the fish ; the mammal is haired, 

 and so are some insects ; but birds alone possess feathers. 

 They are worn by every bird — a fit clothing for a body 

 which is a marvelous combination of beauty, lightness, 

 and strength. 



There is good evidence for the belief that birds have 

 descended from reptilian ancestors. This evidence con- 

 sists of the remains of fossil birds, some of which show 

 marked reptilian characters and, as just said, are toothed. 



