Herein two birds are seen facing one another with the great head-crest fully erected. While in this mood these birds will strut up and down with mincing gait and drooping wings. This is a posture commonly assumed during momentary excitement, whereas the posture shown in plate 24 is apparently only assumed during moments of sexual excitement. Face page 154.] Instances of such impotency on the part of either sex are wanting, and we can only speculate as to how such cases would be met. Would a female who had chanced to settle in the territory of a male whose sexual impulses carried him no further than seizing territory remain with him throughout the mating season, held by an imperfectly roused, ill-defined, sexual instinct? Or, eventually becoming mate-hungry, and failing to stimulate him to perform his part, would she desert him and seek another mate? On the other hand, would a male, failing to arouse response in the female he had secured, drive her away and supplant her? In other words, are we then justified in postulating differential effects in regard to display: a minimum of intensity to ensure mating? A display of some sort is essential. It may be feeble as compared with that of another species—that of the Sparrow, for instance, compared with that of the Peacock—but it must be sufficiently good of its kind to effect its purpose, which is to “hustle” up the production of offspring. A phlegmatic but virile male, or a too feeble performer, is almost as certainly doomed to extinction as an impotent male; for his offspring will probably be eliminated by the adverse conditions of existence to which their late appearance exposed them. Where a female settles down with a male which does not attain to the standard of display characteristic of his race, it is conceivable she may sooner or later seek a mate elsewhere, deserting the phlegmatic bird as if under the impression that she had made the mistake of settling down with one of her own sex. There is no need that the female should have to “select” the best performer of a number of males displaying at the same time and place as a number of rivals.