THE OF ANIMALS Colours play such a large part in human life that the question whether animals also possess a sense of colour is of extreme inter est to us. In speaking of the colour-sense of animals it behoves us to make it perfectly clear at the outset that it is quite impossible for us to study the sensations of animals. We cannot even say anything about the sensations experienced by our fellow men. On the other hand, we can decide objectively whether an animal is able to distinguish different colours.
The investigation of the colour-sense of animals has followed a remarkably zig-zag course. In the time of Darwin man, almost of species have several kinds of chromatophores of different colours. Crangon, the species which up to the present has been studied most, has white, yellow, red and black. Stimulation takes place through the eyes ; blind crustaceans are no longer able to mimic their background. The light stimuli which reach the eye are communicated to the brain, and by this are transmitted to the pigment cells in a way which we do not yet understand more exactly. Koller was able to prove that the chromatophores which expand are always those which conform in colour to the back ground. On a grey ground the black cells expand, on a yellow ground, the yellow cells, and on a red ground the red cells. The intensity of the light plays as unimportant a part as in the case of bees.
With the simple inquiry whether a given animal is able to see or distinguish between colours the problem of the colour-sense of animals is by no means exhausted. It can be proved that every organism, including man himself, responds differently to light and its colours, according to the conditions under which it finds itself. According to Schlieper's investigations, in the optomotor reactions about to be described, all the animals studied hitherto act as if they were colour-blind, even those in which a sense of colour has been proved beyond all doubt, either by the "training method" or in some other way. The best-known instance of such a re action occurring in everyday life is what is called "railroad nys tagmus." If we watch a person opposite us in a railway carriage who is looking out of the window, we notice that his eyes do not remain still. They seek to follow the images of the rapidly passing landscape, and move in the direc tion opposite to that in which the train is going. They are di
rected forwards again in a par ticular rhythm. When studying animals we can represent the rail way by a rotating cylinder, in the middle of which is placed the animal under observation. The inside of the rotating cylinder has vertical strips of paper stuck on it, of which half are coloured and half grey (fig. 19). Every animal must be studied in relation to a whole series of such cylinders, which differ from one another in the shades of grey, while the coloured strips are the same in all of them. The individual animals react differently to the rotation. Some move the head, some the feelers, some the eyes. In every species, however, a particular combination of grey and colour can be found to which no movement-reaction takes place. This means that, in this instance, the animal perceives no difference between the grey and the coloured strips; it sees a uniform grey, and is thus, in this experiment, certainly as blind to colours as in the previously described experiments it was aware of them. The re action to the other "grey and coloured" combinations is only to the different degrees of brightness of the strips, not to perception of the colour contrast. The following little table may make this clear ; it is taken from the results of experiments with the common white butterfly, Pieris brassicae, using the colour "blue." Grey is the lightest, 12 is the darkest ; the measured angle is that described by the feelers of the captive insect.
From these results emerges the startling fact, which has not yet been explained, that the same animal reacts sometimes as if perceiving colours, at others as if colour-blind.
Another important result can be obtained by the methods used by Schlieper. By comparison with the grey papers, of known numerical degrees of brightness, we can calculate the relative brightness which different colours possess for the individual animals. We then reach the remarkable result that this value for the whole of the animal kingdom corresponds to that which pre vails in totally colour-blind human beings. This had already been affirmed by Hess, but not sufficient heed was paid to it.