FORM AND HABIT: THE WING. 17 



organs we shall direct our attention to their external form 

 rather than their internal structure. 



The Wing. Birds' wings are primarily organs of 

 locomotion, but they are also used as weapons, as musical 

 instruments, in expressing emotion, and they are some- 



FIG. 4. Young Hoatzin, showing use of hooked fingers in climbing. 



Lucas.) 



(After 



times the seat of sexual adornment. As an organ of loco- 

 motion the wing's most primitve use is doubtless for 

 climbing. Gallinules, for instance, have a small spur on 

 the wrist or " bend of the wing," and the young birds 

 use it to assist their progress among the reeds. A more 

 striking instance of this nature is shown by that singular 

 South American bird, the Hoatzin (Opisthocomus cris- 

 3 



