CHAPTER 24 



A History of Vertebrates: 

 Birds and Mammals 



The reptiles made two significant improvements upon the terrestrial 

 adaptations of amphibians: the evohition of the cleidoic egg and the 

 development of a means of conserving body water. Birds (class Aves) and 

 mammals (class Mammalia) evolved from reptiles, and both groups have 

 further improved upon the adaptations of reptiles by developing mech- 

 anisms for the maintenance of fairly high and constant body tempera- 

 tures. They are said to be homoiothermic, or warm-blooded, animals. 

 Their metabolic processes can proceed at an optimal rate despite the 

 wide range in external temperatures common in the terrestrial environ- 

 ment, and they are typically very active creatures. 



Higher metabolic rates require higher rates of exchange of materials 

 with the environment and rapid distribution of these materials within 

 the body. Birds and mammals have met these requirements in somewhat 

 similar ways; their adaptations for increased activity provide interesting 

 examples of convergent evolution, although in other respects they are 

 quite different. Birds evolved from early bipedal archosaurs (Fig. 22.2) 

 and have undergone specializations for flight; mammals evolved from 

 a stock of mammal-like reptiles and have become specialized for ter- 

 restrial life. 



207. Principles of Flight 



A group of extinct reptiles, the pterosaurs, and a group of mammals, 

 the bats, have evolved true flight, but neither group has been as suc- 

 cessful fliers as have birds. Bird wings are modified pectoral appendages 

 and the flying surfaces are covered with feathers. A bird's wings must, 

 of course, provide a lift force at least equal to the weight of the bird. 

 The wing is shaped so that it is slightly concave on the under surface 

 and convex on the upper surface, and its angle of attack is such that its 

 anterior edge is slightly higher than the posterior edge (Fig. 24.1). As 

 the airstream flows across the wing, it is deflected in such a way that it 

 reduces the pressure above the wing and increases the pressure on the 

 lower surface. These two forces, especially the reduced pressure on 

 the upper surface, provide the lift. The lift force can be increased by an 

 increase in the speed of the airstream across the wing, and by raising 



468 



