A third motive of display is the need of finding an outlet for overflowing nervous energy and excitement. To this Mr. Wallace refers as follows: “At pairing time the male is in a state of excitement and full of exuberant energy. Even unornamented birds flutter their wings or spread them out, erect their tails or crests, and thus give vent to the nervous excitability with which they are overcharged.” “It is not improbable,” he continues,—and this suggests a fourth use of display—"that crests and other erectile feathers may be primarily of use in _frightening away enemies_, since they are generally erected when angry or during combat." A fifth motive of display is suggested by an analogy furnished by human butterflies and birds of Paradise. Among animals where the sexes differ, it is commonly the male who is adorned the most. With us it is the women. But woman’s fineries are not intended to charm the eyes of men, but to excite one another’s rivalry and envy. Now it seems that male birds, with whose plumes our heartless women are so fond of decking themselves, are guilty of an analogous weakness. They will sometimes display their ornaments, says Darwin, “when not in the presence of the females, as occasionally occurs with grouse at their holy places, and as may be noticed with the peacock; this latter bird, however, evidently wishes for a spectator of some kind, and, as I have often seen, will show off his finery before poultry or even pigs. All naturalists who have closely attended to the habits of birds, whether in a state of nature or under confinement, are unanimously of opinion that the males take delight in displaying their beauty.” And, once more, “with birds of Paradise a dozen or more full-plumaged males congregate in a tree to hold a _dancing-party_, as it is called by the natives; and here they fly about, raise their wings, elevate their exquisite plumes, and make them vibrate; and the whole tree seems, as Mr. Wallace remarks, to be filled with waving plumes.” But if it be the unanimous opinion of naturalists who have closely studied the habits of birds, “that the males take delight in displaying their beauty,” why should not the females also take pleasure in witnessing this display? Perhaps they do, sometimes; for even Mr. Wallace admits that “the display of the various ornamental appendages of the male during courtship may be attractive” to the female. But there is a world-wide difference between this assertion and the doctrine that the females are so greatly and so constantly influenced by their æsthetic taste that they always prefer among males those that are slightly more beautiful than the others, thus increasing their personal beauty by transmission. This is an assumption unsupported by facts, and rendered unnecessary because Natural Selection accounts for all the phenomena in question.