That animals utter musical notes is familiar to every one, as we may daily hear in the singing of birds. It is a more remarkable fact that an ape, one of the Gibbons, produces an exact octave of musical sounds, ascending and descending the scale by half-tones; so that this monkey, “alone of brute mammals, may be said to sing.” From this fact, and from the analogy of other animals, I have been led to infer that the progenitors of man probably uttered musical tones before they had acquired the power of articulate speech; and that, consequently, when the voice is used under any strong emotion, it tends to assume, through the principle of association, a musical character. ERECTION OF THE HAIR. [Page 96.] The enraged lion erects his mane. The bristling of the hair along the neck and back of the dog, and over the whole body of the cat, especially on the tail, is familiar to every one. With the cat it apparently occurs only under fear; with the dog, under anger and fear; but not, as far as I have observed, under abject fear, as when a dog is going to be flogged by a severe gamekeeper. If, however, the dog shows fight, as sometimes happens, up goes his hair. I have often noticed that the hair of a dog is particularly liable to rise if he is half angry and half afraid, as on beholding some object only indistinctly seen in the dusk. * * * * * [Page 97.] Birds belonging to all the chief orders ruffle their feathers when angry or frightened. Every one must have seen two cocks, even quite young birds, preparing to fight with erected neck-hackles; nor can these feathers when erected serve as a means of defense, for cock-fighters have found by experience that it is advantageous to trim them. The male Ruff (_Machetes pugnax_) likewise erects its collar of feathers when fighting. When a dog approaches a common hen with her chickens, she spreads out her wings, raises her tail, ruffles all her feathers, and, looking as ferocious as possible, dashes at the intruder. * * * * * [Page 105.] Several kinds of snakes inflate themselves when irritated. The puff-adder (_Clotho arietans_) is remarkable in this respect; but, I believe, after carefully watching these animals, that they do not act thus for the sake of increasing their apparent bulk, but simply for inhaling a large supply of air, so as to produce their surprisingly loud, harsh, and prolonged hissing sound. ERECTION OF THE EARS. [Page 111.]