282 THE WOKLD OE LIEE 



extent of ornament, are strikingly similar to those of birds, 

 the conception of a deliberate aesthetic choice, by the females, 

 of the details of colour marking, and shape of wings, seemed 

 almost unthinkable, and was supported by even less evidence 

 than in the case of birds. 



After long consideration of the question in all its bearings, 

 and taking account of the various suggestions that had been 

 made by competent observers, I arrived at certain conclu- 

 sions which I stated as follows : 



" The various causes of colour in the animal world are, molecular 

 and chemical change of the substance of their integuments, or the 

 action upon it of heat, light, or moisture. Colour is also produced 

 by the interference of light in superposed transparent lamellae or by 

 excessively fine surface striae. These elementary conditions for the 

 production of colour are found everywhere in the surface-structures 

 of animals, so that its presence must be looked upon as normal, its 

 absence exceptional. 



" Colours are fixed or modified in animals by natural selection 

 for various purposes : obscure or imitative colours for concealment ; 

 gaudy colours as a warning; and special markings either for easy 

 recognition by strayed individuals or by young, or to divert attack 

 from a vital part, as in the large brilliantly marked wings of some 

 butterflies and moths. 



" Colours are produced or intensified by processes of develop- 

 ment, either where the integument or its appendages undergo great 

 extension or change of form, or where there is a surplus of vital 

 energy, as in male animals generally, more especially at the breed- 

 ing season." ^ 



ISTow the idea here suggested, of all these strange and beau- 

 tiful developments of plumage, of ornaments, or of colour 

 being primarily due to surplus vitality and growth-power in 

 dominant species, and especially in the males, seems a fairly 

 adequate solution of the problem. For the individuals which 

 possessed it in the highest degree would survive longest, would 



1 Natural Selection and Tropical Nature (new ed., 1895), pp. 391-392. 

 For full details see Darwinism, chap. x. (1901). 



