This brief summary of Mr. Huxley’s observations, which he was generous enough to give me the privilege of seeing in manuscript, taken in conjunction with many other facts of a like kind given in these pages, seems to lend support to the view that an excessive amorousness is commonly associated with conspicuous ornamentation, as if these stood in the relation of cause and effect. Finally, it is contended, the facts garnered during recent years show that the theory of Sexual Selection, as Darwin propounded it, especially in so far as birds are concerned, is no longer tenable: but it is not an exploded theory, it has only undergone modification. So far as the evidence goes, it would seem that the first of the series of events in the sexual cycle is performed by the already avid male, when he proceeds to secure a “territory” large enough for his needs. In insectivorous and carnivorous species this area is fairly extensive. No other male will be allowed within its confines. The perfection of this instinct is vitally important, if sufficient food for the offspring that are to be is to be assured. Where the food is inexhaustible, as with the Auk-tribe, only a ledge large enough to hold the egg is required. Only avid males will develop and respond to this stimulus. The second stage occurs with the arrival of a female in the area. She does not at once proceed to “select” her mate, passing on if he fails to provoke her admiration. Her sexual condition is apparently as yet but half awakened: to rouse this, the male supplies an aphrodisiac in some form of display to which, in the normal course of things, she responds, often also with some form of display, or indication of the desire which has been aroused. The intensity of the performance seems to vary with the intensity of the sexual passion, which appears to be greater in some species than others, and especially so with such as have conspicuously ornamental plumage. There is, indeed, a variation in the sexual appetite as there is in the ornamentation. The two are reciprocal, and are determined in degree by the stimulatory qualities of the hormones of the sexual glands. Where these have been developed in like intensity by the females, they also display. Diminution in the quality and quantity of the stimulating secretions of the ancillary sexual glands, the hormones of the pituitary and thyroid, or the primary glands—testis and ovary—decreases fertility, or induces sterility. Where these stimulants are lacking there will be no desire, no display, and no pairing, and consequently an end to this defective strain. Here then is Sexual Selection. [Illustration: Plate 26. Photo copyright by D. Seth-Smith. ANOTHER ASPECT OF THE KAGU’S “DISPLAY.”