Coloration of the Mouths and Erjgs of Birds. 291 



for the fitter mates, and as one of several " tonic " factors 

 that have themselves, perhaps, been selected for their bur- 

 nishing effect on the specific and sexual characters that are 

 actually useful in the struggle for existence and their accen- 

 tuation of the general vigour and vitality of the species. 

 The relationship thus suggested for vitality and ornamenta- 

 tion is one of common eftect, not cause and effect. Other 

 tonic factors, such as combat and persecution of the atypical, 

 will, in many cases (as, apparently, in the Warblers), replace 

 sexual selection wholly or in part, and the latter's complete 

 or partial absence in these and other cases by no means 

 proves its invalidity elsewhere. Elimination, again, may be 

 indirect as well as direct. A female (or male) attracted now 

 may already, by failing to be excited on former occasions, have 

 relegated several potential mates to the greater likelihood of 

 a poor or sterile match that will tend to result from delay ; 

 and discriminative coyness could produce selection of this 

 less direct kind even where the sexes are equal in numbers. 



This all brings us down to the view that display in court- 

 ship, though in many cases it has come to be modified and 

 elaborated in special relation to courtship, is, in its essence, 

 an exhibition of prowess or fitness in the various qualities — 

 including distinctive coloration — that make for success in 

 the everyday life of the species. That, in its origin, it had 

 nothing to do with courtship, is suggested by the fact that 

 the plumage-display, or mouth-display, of an animal at bay 

 is often nearly identical with that of an animal courting, 

 though without the added elaboration. 



One such (perhaps unconscious) claim to fitness, in a 

 character useful "in real life'^ mainly in relation to enemies, 

 is probably represented by the mouth-display I have referred 

 to just above. The fact that the coloration of the mouths 

 was dull in the Hornbills, brilliant in the sea-birds (yet the 

 same in both sexes), and somewhat different in the sexes of 

 the Drongos, is in line with the fact that bright and dull 

 plumage, plumage common to the sexes and plumage that is 

 not so, is equally displayed in courtship ; and both facts are 



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