ORIGIN -OF THE COLOUR-SENSE. 243 



and birds, whose structure and emotions are so similar 

 to our own, do probably receive somewhat similar im- 

 pressions of colour ; but we have no evidence to show 

 that they experience pleasurable emotions from colour 

 itself, when not associated with the satisfaction of their 

 wants or the gratification of their passions. 



The primary necessity which led to the development 

 of the sense of colour, was probably the need of dis- 

 tinguishing objects much alike in form and size, but 

 differing in important properties; such as ripe and 

 unripe, or eatabls and poisonous fruits ; flowers with 

 honey or without ; the sexes of the same or of closely 

 allied species. In most cases the strongest contrast 

 would be the most useful, especially as the colours of the 

 objects to be distinguished would form but minute spots 

 or points when compared with the broad masses of tint 

 of sky, earth, or foliage against which they would be set. 



Throughout the long epochs in which the sense of 

 sight was being gradually developed in the higher 

 animals, their visual organs would be mainly subjected to 

 two groups of rays the green from vegetation, and the 

 blue from the sky. The immense preponderance of these 

 over all other groups of rays would naturally lead the 

 eye to become specially adapted for their perception ; and 

 it is quite possible that at first these were the only kinds 

 of light-vibrations which could be perceived at all. 

 When the need for differentiation of colour arose, rays 

 of greater and of smaller wave-lengths would necessarily 

 be made use of to excite the new sensations required ; 

 and we can thus understand why green and blue form 

 the central portion of the visible spectrum, and are the 

 colours which are most agreeable to us in large surfaces ; 



K 2 



