262 THE WORLD OF LIFE 



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 breeding 

 season." l 



Now the idea here suggested, of all these strange and 

 beautiful 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 have most offspring who were equally 

 or even more highly gifted ; and thus there would arise a 

 continually increasing vitality which would be partly ex- 

 pended in the further development of those ornaments and 

 plumes which are its result and outward manifestation. 

 The varying conditions of existence would determine the 

 particular part of the body at which such accessory orna- 



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

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



