CHAPTER XXIV 



SUB-PHYLUM IV. CRANIATA 



DIVISION II. GNATHOSTOMATA 



SUB-DIVISION II. AMNIOTA 



Class IV. AVES 



IT is probable that if the first child one met were asked to 

 describe a bird, he would say that birds were animals 

 which were covered with feathers and had wings to 

 fly with. Though it often happens that the marks 

 by which the ordinary person distinguishes one animal from another 

 are not those which seem most important to a zoologist, yet in this 

 case the zoologist could not find more important features to serve 

 as the basis of a definition of the class Aves or Birds. 



Birds then are vertebrate animals in which the fore-limb is 

 modified into a wing or flying organ and in which the body is 

 covered with feathers. Bats likewise have the fore-limb con- 

 verted into a wing, but they are covered with hair, not feathers, 

 and their wing is not constructed on the same plan as that of the 

 bird. 



. Birds are sometimes classed along with the Hep tiles asSaurop- 

 sida, since they have a good many features in common with them, 

 and are thus contrasted with the Mammalia, or ordinary quadrupeds. 

 This, however, gives a wrong view of the relationships of the three 

 groups. Both Birds and Mammals are believed to be descended 

 from Reptilian-like ancestors, and it is an open question whether 

 the changes which Birds have undergone are not at least as im- 

 portant as those which have taken place in Mammals in the process 

 of their evolution from ancestor^ which, had they lived now, would 

 have been termed Reptiles. 



Birds agree with Reptiles in that they lay large eggs from which 

 the young are hatched in a form closely resembling the parent; 

 they are like Reptiles also in the structure of their jaws the lower 

 jaw consisting of five bones and articulating with a quadrate 



