ANCESTRY OF HOMOIOTHERMIC ANIMALS 11 



appears to be true of the pterosaurs, or extinct flying 

 reptiles. 



Birds 



Among several groups of ancient and modern reptiles 

 the habit of standing more or less upright and walking, 

 running, or leaping with the hind legs has been developed 

 in various localities and at various periods. These bi- 

 pedal reptiles are, and were, often quite agile. They 

 commonly use their tails to help support or balance their 

 bodies, develop processes on their pelvic bones for the 

 attachment of muscles, and possess other characteristics 

 adapted to their method of locomotion. Their fore legs 

 are often small and delicate and serve more for balancing 

 than support. 



Heilmann (1926) has given a very careful account of 

 the evolution of birds from reptilian ancestors. He be- 

 lieves that birds sprang from the Pseudoseuchia, a group 

 in which the pubic bones already showed a tendency to 

 extend backward. These reptiles were also related, 

 though not directly, to the dinosaurs, pterosaurs, and 

 crocodiles. The particular group from which birds arose 

 were generally characterized by sharp-clawed first, sec- 

 ond, and third fingers, and by a tendency to reduction 

 of the fourth and fifth fingers; adaptations which indi- 

 cate arboreal, climbing habits. 



The evolution of birds apparently took the following 

 course. Certain pseudoseuchians raised the forepart of 

 the body off the ground and as they became bipedal the 

 hind foot centered along the median line. From terres- 

 trial runners or leapers these animals gradually changed 

 to arboreal climbers. They leaped from branch to 

 branch; long, parachute scales developed along the pos- 

 terior borders of the fore limbs: these later became 



