The specialization of the air-sacs, that is to say their transformation to perform new functions subservient to the ends of sexual activities, is not exclusively confined to display. In at least one instance an air-sac has been specially developed to act as a voice resonator. This is furnished by the Emu, wherein the wind-pipe, near the middle of its length and on its anterior aspect, has a number of incomplete rings forming a long slit. The lining of the windpipe escapes from this slit in a hernia-like pouch, and takes up a position beneath the skin. Even when inflated this pouch gives no very obvious sign of its existence, but it serves to produce a curious hollow, drumming sound, like the boom of a big drum softly beaten. But why it should have been developed, when the Ostrich and the Cassowary produce similar but louder “music” without any special apparatus whatever, is a mystery. At least one species of Cassowary can emit a roar which would do credit to a lion. In the males of all healthy animals the periodic stimulus to reproduction finds expression in more or less striking eccentricity of conduct. Sometimes, as the foregoing instances have shown, this has been exaggerated by the development of long, resplendent plumes: sometimes by brilliant coloration, displayed either by the plumage or by bare areas of skin, or by both, while in not a few cases attitudes, to our eyes grotesque and made still more so by the aid of inflatable pouches, are the outward and visible sign of the raging fires within. For the completion of this chapter yet other instances of this kind must be cited, instances which reveal a further elaboration of some of the more striking of these tricks of posturing; or which concern the growth of the aggressive instincts, which are proclaimed by the development of armature often of a very formidable character. As the sequel shows, however, there are no hard and fast dividing lines between these several modes of expression. That remarkable bird, the Ruff (_Machetes pugnax_), now, alas! no longer to be met with in our fens, exhibits a curiously composite character in the phases of its love display.