His theory is, that though fish can undoubtedly discriminate between different shades of light and dark, they cannot distinguish one colour from another. The only conclusion that can be drawn from this theory is the conclusion at which Sir Herbert Maxwell has apparently arrived. This is, that if the same relations of light and shade be maintained in the artificial which exist in the natural fly, the colour of the imitation is quite immaterial.
The facts upon which he based this theory were (1) that during the May-fly season he used several artificial May-flies, some of which were coloured scarlet, some bright blue, and some coloured to imitate the natural fly, all of them being similarly graduated with regard to the shade of their various component parts; (2) that he caught trout with all these flies, no particular one of them being decidedly more successful than the others.
This experience of his no doubt would at first strike one as being very strongly in favour of his theory; but on going deeply into the matter, its bearing on the fish’s powers of vision is not so great as it appears.
To begin with, we must consider whether, judging from experience in the past, trout have been known to rise at things on the water which were not only unlike in colour to any flies on the water, but also unlike them in shape and gradations of shade. This we know they will sometimes do. I have on several occasions seen a trout which refused a fairly accurate imitation of the flies which were on the water rise at and take below the surface a swan’s feather. There are also many other much more extraordinary but similar cases on record. Thus, the fact that these trout took an abnormally coloured fly is not a conclusive proof that they mistook it for the natural fly, particularly as this experiment was made during the May-fly season, when the trout sometimes appear to be quite mad, but are at any rate always much less shy than at any other time of the year.
The experiment, too, was made upon a private water, and I think that there is great doubt that the same result would have occurred had it been made upon a well-fished water where the trout were more shy and better educated.
We must then consider whether, in what we know of the natural history of fish, there are any facts which point towards the probability of their being able to discriminate between different colours. Here we find that there are cases in which in certain species the males are more brilliantly coloured than the females, either at the spawning season or always. This is probably a process in evolution which tends to make them more attractive to the female. We also know that fish sometimes assume a colour similar to their surroundings. This colour is, no doubt, evolved for their protection from enemies, and surely a very large proportion of these enemies are other and larger fish. Many of the larvæ of water insects and other creatures upon which fish feed are also coloured according to their surroundings, in order to facilitate their concealment. These facts would naturally lead us to come to a conclusion opposed to that of Sir Herbert Maxwell, as the probabilities all point towards the power of fish to discern various colours.
Another very important point is the structure of the fish’s eye in comparison with that of man, who we know has the power of discriminating between colours. This power is, in the human eye, probably situated in the layer of rods and cones of the retina. Had the fish’s retina not contained this layer, as is stated by Sir Herbert Maxwell, there would certainly have been most excellent grounds for supposing that his theory was true; but this layer is contained in the fish’s eye, though it is not the same as in man. If the fish’s eye did not contain it, fish would have been totally blind.
How far this difference in the retina of the fish bears on its sense of colour is, at present, a moot point, though I believe researches are being made in this direction. At present, our knowledge is too limited with regard to it for any definite statement to be made. The probability is, that fish have the power of distinguishing colour from colour. A probability, however, is not a certainty, though one is more inclined towards it than towards an improbability.
Even should Sir Herbert Maxwell’s theory prove true, in spite of probabilities to the contrary, I do not see that we should have progressed very much further with regard to facilities in imitating the natural fly. We know that the relative values of light and shade in various colours contiguous to each other, is not actually the same as the impression conveyed to our eyes. We have an example of this always with us in the photograph, where red and blue, in relation to each other, certainly do not produce the same effects on the plate as they do on the eye; and as we have no accurate knowledge as to the effect of contiguous colours upon a normally monochromatic eye, we could hardly be certain of producing an accurate monochromatic imitation of a multi-coloured object, which would deceive that eye.
The case of a colour-blind human being is certainly not a normal case, so the shade value of the various colours to this eye could hardly be taken as a safe standard.