[CHAPTER V.]
The Colour Sense.
T
THE previous considerations lead us, naturally, to enquire in what manner the sense of colour is perceived.
In thinking over this obscure subject, the opinion has steadily gathered strength that form and colour are closely allied; for form is essential to pattern; and colour without pattern, that is to say, colour indefinitely marked, or distributed, is hardly decoration at all, in the sense we are using the term. That many animals possess the power of discriminating form is certain. Deformed or monstrous forms are driven from the herds and packs of such social animals as cattle, deer, and hogs, and maimed individuals are destroyed. Similar facts have been noticed in the case of birds. This shows a power of recognising any departure from the standard of form, just as the remorseless destruction of abnormally coloured birds, such as white or piebald rooks and blackbirds, by their fellows, is proof of the recognition and dislike of a departure from normal colouring. Authentic anecdotes of dogs recognising their masters' portraits are on record; and in West Suffolk, of late years, a zinc, homely representation of a cat has been found useful in protecting garden produce from the ravages of birds. In this latter case the birds soon found out the innocent nature of the fraud, for we have noticed, after a fortnight, the amusing sight of sparrows cleaning their beaks on the whilom object of terror. Many fish are deceived with artificial bait, as the pike, with silvered minnows; the salmon, and trout, with artificial flies; the glitter of the spoon-bait is often most attractive; and mackerel take greedily to bits of red flannel. Bees sometimes mistake artificial for real flowers; and both they and butterflies have been known to seek vainly for nourishment from the gaudy painted flowers on cottage wall-papers. Sir John Lubbock has demonstrated the existence of a colour sense in bees, wasps, and ants; and the great fact that flowers are lures for insects proves beyond the power of doubt that these creatures have a very strong faculty for perceiving colour.
The pale yellows and white of night-flowering plants render them conspicuous to the flower-haunting moths; and no one who has ever used an entomologist's lantern, or watched a daddy-long-legs (Tipula) dancing madly round a candle, can fail to see that intense excitement is caused by the flame. In the dim shades of night the faint light of the flowers tells the insects of the land of plenty, and the stimulus thus excited is multiplied into a frenzy by the glow of a lamp, which, doubtless, seems to insect eyes the promise of a feast that shall transcend that of ordinary flowers, as a Lord Mayor's feast transcends a homely crust of bread and cheese.
We take it, then, as proven that the colour sense does exist, at least, in all creatures possessing eyes. But there are myriads of animals revelling in bright tints; such as the jelly-fishes and anemones, and even lower organisms, in which eyes are either entirely wanting or are mere eye-specks, as will be explained in the sequel. How these behave with regard to colour is a question that may, with propriety, be asked of science, but to which, at present, we can give no very definite reply. Still, certain modern researches open to us a prospect of being able, eventually, to decide even this obscure problem.
The question, however, is not a simple one, but involves two distinct principles; firstly, as to how colour affects the animal coloured, and, secondly, how it affects other animals. In other words, How does colour affect the sensibility of its possessor? and how does it affect the sense organs of others?
To endeavour to answer the first question we must start with the lowest forms of life, and their receptivity to the action of light; for, as colour is only a differentiation of ordinary so-called white light, we might a priori expect that animals would show sensibility to light as distinguished from darkness, before they had the power of discriminating between different kinds of light.