and open and shut the beak several times in succession; and sometimes they hold it wide open for several seconds together. Each time, as the jaws gape, a splendid surface of bright gamboge yellow is exhibited, which the human eye, at any rate, has to admire, and which exactly matches with the naked yellow skin at the base of the two mandibles on either side, where they become lost in what may be called the bird's cheek. This exterior brightly-tinted surface is continuous with the interior and much larger one, and my view is that the colour of the latter represents an extension of that of the former, by a similar process of sexual selection. There is no doubt whatever that this outward adornment largely adds to the handsome appearance of the shag, and probably those naturalists who believe in sexual selection at all will think it as much due to that agency as the crest and the sheeny green plumage. But if the closely similar colouring of the adjacent interior region is to be looked on as merely fortuitous (we escape here, thank heaven, from the all-pervading protective theory), why should the other be thought to be anything more? If the shag had not this habit of opening its mouth and thus displaying what is, in itself, so very striking, it would be difficult, I think, to accept sexual selection as an explanation even of the facial adornment, since, if the one effect were non-significant, so might the other be. As it is, I can see no reason why it should not have brought about both.
I have often watched shags thus throwing up their
