PDF - Wallace Online

412 TROPICAL NATUREthese were the only kinds of light-vibrations which could beperceivedat all. When the need for differentiation of colourarose, rays of greater and of smaller wave-lengths wouldnecessarily be made use of to excite the new sensations required,and we can thus understand why green and blue form thecentral portion of the visible spectrum, and are the colourswhich are most agreeable to us in large surfaces ;while at itstwo extremities we find yellow, red, and violet colourswhich we best appreciatein smaller masses, and when contrastedwith the other two, or with light neutral tints. Wehave here probably the foundations of a natural theory ofharmonious colouring, derived from the order in which ourcolour-sensations have arisen and the nature of the emotionswith which the several tints have been always associated.The agreeable and soothing influence of green light may bein 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 socool and sedative. But when we consider how dependent areall the higher animals on vegetation, and that man himselfhas been developed in the closest relation to it,we shall find,probably, a sufficient explanation. The green mantle withwhich the earth is overspread caused this one colour to predominateover all others that meet our sight, and to be almostalways associated with the satisfaction of human wants.Where the grass is greenest, and vegetation most abundantand varied, there has man always found his most suitabledwelling-place. In such spots hunger and thirst are unknown,and the choicest productions of nature gratify the appetiteand please the eye. In the greatest heats of summer, coolness,shade, and moisture are found in the green forest glades,and we can thus understand how our visual apparatus hasbecome especially adapted to receive pleasurable and soothingsensations from this class of rays.Supposed increase of Colour-perception within the Historical PeriodSome writers believe that our power of distinguishingcolours has increased even in historical times. The subjecthas attracted the attention of German philologists, and I havebeen furnished by a friend with some notes from a work of