412 TROPICAL NATURE 



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 ; while at its 

 two extremities we find yellow, red, and violet colours 

 which we best appreciate in smaller masses, and when con- 

 trasted with the other two, or with light neutral tints. We 

 have here probably the foundations of a natural theory of 

 harmonious colouring, derived from the order in which our 

 colour-sensations have arisen and the nature of the emotions 

 with which the several tints have been always associated. 

 The agreeable and soothing influence of green light may be 

 in part due to the green rays having little heating power ; 

 but this can hardly be the chief cause, for the blue and violet, 

 though they contain less heat, are not generally felt to be so 

 cool and sedative. But when we consider how dependent are 

 all the higher animals on vegetation, and that man himself 

 has been developed in the closest relation to it, we shall find, 

 probably, a sufficient explanation. The green mantle with 

 which the earth is overspread caused this one colour to pre- 

 dominate over all others that meet our sight, and to be almost 

 always associated with the satisfaction of human wants. 

 Where the grass is greenest, and vegetation most abundant 

 and varied, there has man always found his most suitable 

 dwelling-place. In such spots hunger and thirst are unknown, 

 and the choicest productions of nature gratify the appetite 

 and please the eye. In the greatest heats of summer, cool- 

 ness, shade, and moisture are found in the green forest glades, 

 and we can thus understand how our visual apparatus has 

 become especially adapted to receive pleasurable and soothing 

 sensations from this class of rays. 



Supposed increase of Colour-perception within the Historical Period 

 Some writers believe that our power of distinguishing 

 colours has increased even in historical times. The subject 

 has attracted the attention of German philologists, and I have 

 been furnished by a friend with some notes from a work of 



