290 Mr. C. F. M. Swyiinerton on the 



I feel that this unconscious display of the mouth-colours 

 is equivalent to the final display of their concealed bright 

 colours by so many insects when cornered and unable to 

 escape. It is the last appeal to the enemy's memory, and 

 the colour-groups I have referred to above are in some 

 cases, I believe, in the nature of mimicry and "• common 

 warning colours." 



Actually, there are three occasions on which a bird opens 

 its mouth to an enemy — when a nestling, when at bay, and 

 when mobbing. Even under the latter circumstances the 

 display may conceivably be useful. But adult birds also 

 sometimes show their mouths in ordinary intercourse and in 

 courtship. I have seen this myself in Drongos and Horn- 

 bills, and it has been recorded for various sea-birds. 

 Again, the female's mouth sometimes differs from the 

 male's. 



These two facts at once suggest, for the complete explana- 

 tion of mouth-colours, the discussion of factors I have not 

 yet touched on. They are best discussed with any real 

 fullness under adult plumage and in connection with my 

 detailed observational and experimental results from adult 

 birds ; but the brief discussion of one of them, and a short 

 general statement of view, will be in place here, and the 

 latter will help to preface my ^remarks on the coloration 

 of eggs. 



There is nothing new in the view that such sexual selec- 

 tion as would seem to take place is based, not necessarily on 

 an admiration of the brightest suitors, but on a tendency to 

 be attracted instinctively by masculine males and feminine 

 females — according to the species, general standard of mas- 

 culinity and femininity (which may or may not include 

 brightness), and to fail to be thus attracted by atypical 

 members of the species or of the opposite sex. But the 

 instinct would be based, in turn, on the fact that atypical 

 individuals and those showing the characters of the opposite 

 sex are commonly specifically or sexually inefficient. Sexual 

 selection might, therefore, be regarded both as a time-saver 



