What exactly are the factors which govern the evolution of resplendent plumage is not known. But they would seem to be more complex than was supposed. That the primary sexual glands play an important part, through the juices or “hormones” which they liberate, there can be no doubt but these are only partial factors. The “hormones” of the pituitary and thyroid glands are also necessary contributors, controlling as they do both fertility and the more superficial characters, such as colour and ornament. Evidence, indeed, is slowly accumulating to show that the problem of the behaviour of animals during the period of sexual activity, as well as the peculiarities of structure and coloration which they develop at this time, are all largely governed by the action of these secretions. These, in their turn, are undoubtedly inhibited, or increased, by the control of the nervous system, though this control is of course involuntary. This much seems clear from the fact that birds will display when under the excitement of fear, though the character of that display is never the same as that in moments of sexual exaltation. If the nervous system, through the eye, by “suggestion,” played no part, there could be no use for display, but it is equally certain that for the realization of the sexual activities a number of other factors have to contribute. The existence of this nexus of conditions is commonly overlooked, but it is extremely important. Normally, not only among birds, but other animals higher and lower in the scale of life, “suggestion” does not suggest until the “hormones” concerned with the sexual activities have, as it were, saturated the system and rendered it, so to speak, highly inflammable. Even then it commonly happens that, with the male at any rate, this inflammable state bursts into flame of its own accord. But for this, indeed, how could the consummation—of the period of sexual activity ever be realized? In many cases the sexes are sundered far apart. What, but the merest accident, could bring them together if it were not for this consuming fire of desire which impels each sex to seek out the other? This stage is manifested in the case of the Deer, where, we have seen, the stag wanders far and wide bellowing to advertise his errand and listening for a response to his call. He is possessed by a “male-hunger” which eventually attains to a state of frenzy. Here no “suggestion” is needed, but the necessity for this stimulus, for some form of aphrodisiac, occurs with him after the first relief of his pent-up state has been attained. This stimulus is applied, both through the eye and the sense of smell, by the females of his herd. The same conditions apply in the case of the birds. But it is to be noted that with the females, as in the case of mammals, sexual desire is commonly less intense than in the males, and hence, in their case the need for “suggestion” by display of some sort. But apart from this, a “display” of some kind is necessary.