242 TROPICAL NATURE, AND OTHER ESSAYS. 



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 

 exquisitely beautiful such pictures are, and how com- 

 pletely 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 dis- 

 tinguishable in a photograph by some differences of tint, 

 and it is quite conceivable that visual organs might exist 

 which would differentiate what we term colour by deli- 

 cate gradations of some one characteristic neutral tint. 

 Now such a capacity of vision would be simple as 

 compared with that which we actually possess ; which, 

 besides distinguishing infinite gradations of the quantity 

 of light, distinguishes also, by a totally distinct set of 

 sensations, gradations of quality, as determined by 

 differences of wave-lengths or rate of vibration. At 

 what grade in animal development this new and more 

 complex sense first began to appear we have no means 

 of determining. The fact that the higher vertebrates, 

 and even some insects, distinguish what are to us 

 diversities of colour, by no means proves that their 

 sensations of colour bear any resemblance whatever to 

 ours. An insect's capacity to distinguish red from blue 

 or yellow may be (and probably is) due to perceptions 

 of a totally distinct nature, and quite unaccompanied by 

 any of that sense of enjoyment or even of radical dis- 

 tinctness which pure colours excite in us. Mammalia 



