412 TROPICAL NATURE vI 
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 
