CHAPTER IV 



THE FLIGHT OF BIRDS 



§ I. Origin of Flight. § 2. Movements in Flight. § 3. Gliding 

 Flight. § 4. Ordinary Flight. § 5. Sailing Flight. § 6. Velocity 

 of Flight. § 7. General Biological Significance of Flight. 



§ I. Origin of Flight 



It is a reasonable hypothesis that birds ran before they 

 flew, that they took flying jumps which sometimes landed 

 them on trees, and that they practised parachuting for 

 ages before they were able to do much in the way of true 

 flight. Baron Nopcsa pictures a long-legged, long-tailed 

 biped running along the ground and swinging its arms, 

 which had acquired exaggerated scales — the beginnings of 

 feathers — on their posterior margins. It is not inconsistent 

 with this picture to suppose a subsequent arboreal apprentice- 

 ship, which is pointed to, for instance, by the gripping 

 arrangements in the toes of birds — the tendons being kept 

 from slipping by a roughened under-surface which catches 

 on transverse ridges in the tendon sheath. The very 

 primitive hoatzin climbs about when very young, using its 

 clawed hands as well as its feet, and this may be a survival 

 of ancient habit. 



The flight of Pterodactyls depended on the possession 

 of a patagium or fold of skin ; so it is with bats ; there is a 

 little of this in front of the wing in birds, and there may 

 have been more in ancestral forms, but everything depends 

 on the feathers. These light, flexible, readily replaceable 

 structures are, as we have seen, so closely coherent in their 

 numerous component parts that they form an unsurpassed 

 vane for striking the air. It is of great importance that 



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