It seems clear, then, that the evolution of colour is not the stimulant to display, for this is present where conspicuous colours are wanting. Yet it can readily be understood how the association of ideas in regard to colour and display arose, for there are cases where this interpretation seems inevitable. Such are afforded by certain sea-birds like the Kittiwake, Guillemot, Fulmar and Cormorant, wherein the inside of the mouth is of a lurid orange-red in the case of the first-mentioned, and of flaming gamboge yellow in that of the others. During moments of sexual ecstasy the mouth is widely opened, as if to charm the beholder with its gaudy hue. Both sexes have the same colouring, and both behave alike. But it is doubtful whether either is conscious that its own mouth is like that exposed to its gaze: the action is sympathetic. No doubt it may play its part in stimulating desire, but we cannot contend from this that it has been evolved by sexual selection, that is to say, that the hues have undergone a process of gradual intensification owing to the deliberate rejection of the less gaily-coloured suitors. The tendency to develop colour in the mouth would appear to be latent in all birds. [Illustration: Plate 23. _From a drawing by H. Grönvold._ THE DISPLAY OF THE GRASSHOPPER WARBLER. The behaviour of this bird under the stimulus of sexual excitement is precisely similar to that of the Sun-bittern and the Kagu, yet it has no brilliant colours to exhibit by such actions. Face page 142.] [Illustration: Plate 24. THE DISPLAY OF THE SUN-BITTERN. Quite inconspicuous in repose, this bird, in its moments of exaltation, becomes banded and blotched with vivid colours, revealed by spreading the wings and tail.] [Illustration: _Photos copyright, D. Seth-Smith._ THE KAGU IN DISPLAY. What is true of the Sun-Bittern is true also of the Kagu.] It is significant that whenever bright colours appear, they do so first in the males, the females and young retaining the dress common, up to this time, to the species at all ages. In the majority of instances, at any rate, it would seem that this accession of colour appears with the seasonal re-awakening of the reproductive activities: it forms a “nuptial” dress, and is discarded after the breeding season is over for a livery indistinguishable from that of the female, this forming the so-called “winter plumage.” But if all the available facts are taken into consideration there seems good reason to believe that the nuptial plumage tends to be assumed earlier and to be retained later, as this disposition to develop ornament gathers force, till finally only the head and neck go into “eclipse,” as in the case of the Black-cock, Jungle-fowl and Partridge.