ORIGIN OF BIRDS 227 



a terrestrial into a terrestrio-arboreal mode of life, probably 

 for purposes of safety. This early arboreo-terrestrial phase is 

 indicated in the most ancient known birds {Archceopteryx) by 

 the presence of claws at the ends of the bones of the wing, fit- 

 ting them for clinging to trees, it is argued, through analogy 

 to the tree-clinging habits of existing young hoatzins of South 



QUATERNARY 



CRETACEOUS 



CRETACEOUS 



PENNSYLVANIAN 



,.^_ 



RELATIVELY *■ 



., CURSORIAL, CLIMBING <l 



OF CROCODILES. PHYTOSAURS. 

 PTEROSAURS AND BIRDS 



Fig. 104. Ancestral Tree of the Birds. 



The ancestors of the birds branch off in Permian time from the same stock that gives rise 

 to the dinosaurs, adding to swift, bipedal locomotion along the ground the power of 

 tree climbing and, with their very active life, the development of a high and uniform 

 body temperature. Primitive types of birds exhibit a fore limb terminating in claws, 

 probably for grasping tree branches. The power of flight began to develop in Triassic 

 time through the conversion of scales into feathers either on the fore limbs (two-wing 

 theory) or on both fore and hind limbs (four-wing theory). From the Jurassic birds 

 { Archceopteryx), capable of only feeble flight, there arises an adaptive radiation into 

 aerial, arboreal, arboreo-terrestrial, terrestrial, and aquatic forms, the last exhibiting a 

 reversal of evolution. Diagram prepared for the author by W. K. Gregory. 



America. Ancestral tree existence is rendered still more prob- 

 able by the fact that the origin of flight was apparently sub- 

 served in the parachute function of the fore limb and perhaps 

 of both the fore and hind limbs for descent from the branches 

 of trees to the ground. 



Two theories have been advanced as to the origin of flight 

 in the stages succeeding the arboreal phase of bird evolution. 

 First, the pair-wing theory, developed from the earlier studies 

 on Archceopteryx, in which the transformation of lateral scales 



