130 THE BIRD WATCHER 
and open and shut @Mfe beak several times in suc- 
cession ; 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 ex- 
hibited, 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 sur- 
face 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 any- 
thing 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 
