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-
