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TROPICAL NATURE 
VI 
ON THE ORIGIN OP THE COLOUR-SENSE 
Throughout the preceding discussion we have accepted 
the subjective phenomena of colour — that is, our perception 
of varied hues and the mental emotions excited by them — as 
ultimate facts needing no explanation. Yet they present 
certain features well worthy of attention, a brief considera- 
tion of which will form a fitting sequel to the present essay. 
The perception of colour seems, to the present writer, the 
most wonderful and the most mysterious of our sensations. 
Its extreme diversities and exquisite beauties seem out of 
proportion to the causes that are supposed to have produced 
them, or the physical needs to which they minister. If we 
look at pure tints of red, green, blue, and yellow, they appear 
so absolutely contrasted and unlike each other, that it is 
almost impossible to believe (what we nevertheless know to 
be the fact) that the rays of light producing these very dis- 
tinct sensations differ only in wave-length and rate of vibra- 
tion, and that there is from one to the other a continuous 
series and gradation of such vibrating waves. The positive 
diversity we see in them must then depend upon special 
adaptations in ourselves ; and the question arises, For what 
purpose have our visual organs and mental perceptions become 
so highly specialised in this respect ? 
When the sense of sight was first developed in the animal 
kingdom, we can hardly doubt that what was perceived was 
light only, and its more or less complete withdrawal. As the 
sense became perfected, more delicate gradations of light and 
shade would be perceived, and there seems no reason why a 
visual capacity might not have been developed as perfect as 
our own, or even more so in respect of light and shade, but 
entirely insensible to differences of colour, except in so far 
as these implied a difference in the quantity of light. The 
world would in that case appear somewhat as we see it in 
good stereoscopic photographs; and we all know how ex- 
quisitely beautiful such pictures are, and how completely 
they give us all requisite information as to form, surface- 
texture, solidity, and distance, and even to some extent as to 
colour, for almost all colours are distinguishable in a photo- 
graph by some differences of tint, and it is quite conceivable 
