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410 TROPICAL NATUREON THE ORIGINOF THE COLOUR-SENSEThroughout the preceding discussion we have acceptedthe subjective phenomena of colour that is, our perceptionof varied hues and the mental emotions excited by them asultimate facts needing no explanation. Yet they presentcertain features well worthy of attention, a brief considerationof which will form a fitting sequel to the present essay.The perception of colour seems, to the present writer, themost wonderful and the most mysterious of our sensations.Its extreme diversities and exquisite beauties seem out ofproportion to the causes that are supposed to have producedthem, or the physical needs to which they minister. If welook at pure tints of red, green, blue, and yellow, they appearso absolutely contrasted and unlike each other, that it isalmost impossible to believe (what we nevertheless know tobe the fact) that the rays of light producing these very distinctsensations differ only in wave-length and rate of vibration,and that there is from one to the other a continuousseries and gradation of such vibrating waves. The positivediversity we see in them must then depend upon specialadaptations in ourselves ;and the question arises, For whatpurpose have our visual organs and mental perceptions becomeso highly specialised in this respect?When the sense of sight was first developed in the animalkingdom, we can hardly doubt that what was perceived waslight only, and its more or less complete withdrawal. As thesense became perfected, more delicate gradations of light andshade would be perceived, and thereseems no reason why avisual capacity might not have been developed as perfect asour own, or even more so in respect of light and shade, butentirely insensible to differences of colour, except in so faras these implied a difference in the quantity of light.Theworld would in that case appear somewhat as we see it ingood stereoscopic photographs ;and we all know how exquisitelybeautiful such pictures are, and how completelythey give us all requisite information as to form, surfacetexture.,solidity, and distance, and even to some extent as tocolour, for almost all colours are distinguishable in a photographby some differences of tint, and it is quite conceivable