STRUCTURAL ADAPTATIONS 



177 



reduced to mere vestiges, save only in the remarkable Hoatzin 

 {Opistkocouius cristatus). In this bird, as we have already 

 pointed out, they are during the nestling period of considerable 

 size, but later they become absorbed. Ducks and Accipitres re- 

 tain vestiges of these claws throughout life ; in other birds they 

 have either become entirely suppressed on one or both digits, or 

 they appear only during embryonic life. In the young Ostrich, 

 however, a vestigal claw has been found on the third digit. 



This reduction in the number of the digits, and the welding 

 together which they have undergone, has resulted in the forma- 

 tion of what may be described as a jointed rod, eminently adapted 





III. 38. — Wing of a "Wood-Owl," Showing the Rounded Type 



for the support of the elastic ribbons which we call the "flight 

 feathers," but so specialised as to be entirely useless as a 

 support for the body — save only in a few cases where the wings 

 are used for climbing purposes during the nestling period. 



From the earliest known bird then to the present day the 

 only changes which have taken place in the skeleton of the 

 wing — cases of degeneration apart — have been in the shorten- 

 ing or lengthening of the separate segments of the limb. The 

 most striking instances of these are to be found, curiously 

 enough, in the two birds most renowned for their remarkable 

 powers of flight — the Albatross and the Swift. In the former 



