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- 
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4 : 
XN 
Fra. 4.—Young Hoatzin, showing te of pooked fingers in climbing. (After 
ucas, 
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- 
