MESOZOIC ERA 187 



were warm-blooded; it is difficult to see how a really cold-blooded animal 

 could maintain the activity necessary for flight. 



We may well note at this point that comparison of ichthyosaurs with 

 fishes and dolphins (Fig. 3.5, p. 30), and of pterosaurs with birds and bats, 

 presents some of the most beautiful examples available of that convergent 

 evolution discussed in an earlier chapter (p. 29). It would be difficult to 

 find a more fascinating aspect of evolution than that afforded by study of 

 the variety of ways in which a given problem (e.g., flight) has been solved 

 independently by differing groups of animals. Some solutions are better 

 than others. Thus, it has been pointed out that the pterosaur wing, con- 

 sisting of an unbroken expanse of membrane supported only along its outer 

 edge, would not lend itself to agile maneuvering in flight. Also, a tear in it 

 would be more disastrous than would a tear in a bat's wing, since the 

 latter is supported by four elongated fingers instead of only one (Fig. 3.1, 

 p. 22). As compared to the attainments of birds and bats, only partial 

 success crowned pterosaur invasion of the air. 



ORIGIN OF BIRDS 



The thecodonts have claimed our attention as ances- 

 tors of the two orders of dinosaurs, and of pterosaurs. Birds also arose dur- 

 ing the Mesozoic from this same bipedal stock (Fig. 9.3). Indeed, the 

 term "glorified reptiles" frequently applied to birds suggests the fact that 

 they are similar to reptiles in many ways. 



The principal distinguishing characteristic of birds is possession of 

 feathers. But the structure and development of feathers reveal that they 

 are modified reptilian scales. Birds are warm-blooded, a condition, as men- 

 tioned above, which is really necessary if an animal is to be capable of 

 sustained flight. Unlike pterosaurs, birds have an insulating body covering 

 of feathers; this aids greatly in prevention of loss of heat from the body 

 surface. Modern birds, hke pterosaurs, have the light construction afforded 

 by hollow bones. In flying birds the sternum or breastbone is greatly en- 

 larged to provide anchorage for muscles operating the wings. We have 

 seen that pterosaurs also showed development of this kind. Both birds and 

 pterosaurs have, or had, "eye brains" — brains showing pronounced domi- 

 nance of visual areas, with reduction of the portions connected with the 

 sense of smell. Birds have a system of air sacs connected to the lungs. 

 Mayr has suggested that these serve principally as an internal ventilating 

 system, dissipating the heat generated by the vigorous metabolic activity 

 necessary to flight. Birds have well-developed legs, with structure similar 



