FORM AND HABIT: THE WING. 



17 



organs we shall direct our attention to their external form 

 rather than their internal structure. 



The Vising. — Birds' wings are primarily organs of 

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

 mstrmiients, in expressing emotion, and they are some- 



Fio. 4. — Young Iloatzin, showing use of hooked fingers in climbing. (After 

 Lucas.) 



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 oris- 



