How else can desire be indicated? And here is “sexual selection.” For males, mate-hungry as they might be, which resorted to no means of expressing their condition would go mateless: and the same is true, though perhaps in less extent, with the females; hence, then, it is clear display is a product of sexual selection. That sexual desire is less intense in the case of the females is to be regarded as another result of this form of selection. If they displayed the same intensity of passion the males would speedily become exhausted, for it is well known that the gratification of the sexual emotions is far more enervating in the case of the male. It may well be that polyandry has arisen from this transference to the females, or development by the females, of increased sexual hunger. The fact that birds will repeat, albeit imperfectly, the phases of the sexual display under the stimulus of fear, or anger, and when no females are present, must be regarded as an indication, for we can scarcely call it a proof, that exaggerated movements have become the normal concomitants of great excitement, at any rate during the season of reproductive activity. They are purely nervous responses to external conditions. It must not be forgotten that, at this time, fear begets other movements, equally striking, such as feigning lameness, and death, which have no part in the sexual display. Interpreted in this light one can understand that to the female not as yet sexually “ripe” or sexually “hungry,” these movements, when not interpreted as signs of fear or anger, fail to produce any response. So soon, however, as this period of “ripeness” arrives, the stimulus through the nervous system produces the desired response, begetting a complementary stimulus through the secretions of the sexual glands, by what we may call the flow of the hormones; just as the sight of food stimulates the flow of saliva, or “makes the mouth water” before we are conscious of feeling hungry. In due time hunger will assert itself without the stimulus of the nervous system through the senses. But there must in any case be some form of display, some form of communicating and stimulating desire between the sexes, to secure the consummation of the reproductive acts. How else could intimation of sex hunger be indicated and satisfied?