COLOURS OF ANIMALS. 217 



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 mois- 

 ture. It is also produced by interference of light in 

 superposed transparent lamellae, or by excessively fine 

 surface-strise. These elementary conditions for the pro- 

 duction 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 ; 

 and special markings, either for easy recognition by 

 strayed individuals, females, or young, or to direct 

 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 

 development, either where the integument or its 

 appendages undergo great extension or modification, or 

 where there is a surplus of vital energy, as in male 

 animals generally, and more especially at the breeding- 

 season. 



Colours are also more or less influenced by a variety of 

 causes, such as the nature of the food, the photographic 

 action of light, and also by some unknown local action 

 probably dependent on chemical peculiarities in the soil 

 or vegetation. 



These various causes have acted and reacted in a 

 variety of ways, and have been modified by conditions 

 dependent on age or on sex, on competition with new 



