v COLOURS OF ANIMALS 391 



however, that this would apply to white-coloured butterflies ; 

 and this may be a reason why the effect of an insular habitat 

 is more marked in these insects than in birds or mammals. 1 



It is even possible that this relation of sense -acuteness 

 with colour may have had some influence on the development 

 of the higher human races. If light tints of the skin were 

 generally accompanied by some deficiency in the senses of 

 smell, hearing, and vision, the white could never compete 

 with the darker races so long as man was in a very low or 

 savage condition, and wholly dependent for existence on the 

 acuteness of his senses. But as the mental faculties became 

 more fully developed and more important to his welfare than 

 mere sense-acuteness, the lighter tints of skin and hair and 

 eyes would cease to be disadvantageous whenever they were 

 accompanied by superior brain-power. Such variations would 

 then be preserved ; and thus may have arisen the Xantho- 

 chroic race of mankind, in which we find a high development 

 of intellect accompanied by a slight deficiency in the acuteness 

 of the senses as compared with the darker forms. 



Summary on Colour-development in Animals 

 Let us now sum up the conclusions at which we have 

 arrived as to the various modes in which colour is produced 

 or modified in the animal kingdom. 



The various causes of colour in the animal world are, 

 molecular and chemical change of the substance of their 

 integuments, or the action on it of heat, light, or moisture. 

 It is also produced by interference of light in superposed 

 transparent lamellae, or by excessively fine surf ace -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 as 

 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 j and special 

 markings, either for easy recognition by strayed individuals, 

 females, or young, or to divert attack from a vital part, as in 



1 In Darwinism, pp. 229, 230, 1 have suggested an explanation of most of 

 the facts of colour in islands as due to the lesser need of protection. 



