ORIGIN OF BIRDS 



229 



hypotheses assign two phases to the origin of flight in birds: 



first, a primary terrestrial phase, during which the pecuUar 



characters of the hind limbs 



and feet were developed with 



their strong analogies to the 



bipedal feet of dinosaurs; 



second, a purely arboreal 



phase. It is believed by the 



adherents of both the two- 



FiG. 108. Theoretic Mode of Para- 

 chute Flight of the Primitive 

 Bird. 



Based on the four-wing theory. After 

 Beebe. 



wing and the four-wing theory 

 that following the arboreal 

 phase, in which the powers of 

 flight were fully developed, 

 there occurred among the 

 struthious birds, such as the 

 ostriches, a secondary terres- 

 trial phase in which the 

 powers of flight were secon- 

 darily lost and rapid cursorial 

 locomotion on the ground was 

 secondarily developed. This 

 interpretation of the foot and 

 limb structure associated 

 with the loss of teeth, which 

 is characteristic of all the higher birds, will explain the close 

 analogies which exist between the ostrich-like dinosaur Stru- 



FiG. 109. Restoration of the Ancient 

 Jurassic Bird, Archaopteryx. 



Capable of relatively feeble flight. After 

 Heilman. 



