210 THE STORY OF BIRD-LIFE. 



trees or ia crawling about over the ground. 

 That, in short, progression was quadrupedal 

 and not bipedal. That the wing as an organ 

 of flight was useless : it served as a parachute 

 and nothing more. 



As a matter of fact, this claw was probably- 

 functional during the nestling period in the life 

 of the bird, enabling it to climb before its 

 feathers grew long enough to serve the purposes 

 of flight, and also during the process of moultins: 

 after the bird had reached maturity. At this 

 time it is possible that all the feathers of the 

 wing were cast off" at the same time, and until 

 the new ones were sufficiently strong, flight 

 would be impossible. The ducks to-day moult 

 in this way, and for a season swimming and 

 walking are their only means of locomotion. 

 The nestling opisthocomus, a strictly arboreal 

 bird — as was archceOjpteryx — uses its wing for 

 climbing purposes, and is aided in this by claws, 

 which however are absorbed as soon as the wing 

 is capable of sustaining the bird by flight. 



The disappearance of these claws is instructive. 

 It is a link in the chain of evidence upon which 

 the recapitulation theory is founded. In a general 

 way, this means that an animal in the course of 

 its development from the germ upwards repeats 

 the main phases of the development of the race 

 to which it belongs. Structures which no longer 

 appear in the adult or young after birth put in 

 an appearance for a longer or shorter time during 

 embryonic growth. Thus oxen and sheep which 

 have no cutting, or incisor, teeth in the upper 

 jaw, have tiny ones which never cut the gum in. 



