Colouration in Animals and Plants

by Alfred Tylor

First, as to the nature of light and colour. Colour is essentially the effect of different kinds of vibrations upon certain nerves. Without such nerves, light can produce no luminous effect whatever; and to a world of blind creatures, there would be neither light nor colour, for as we have said, light and colour are not material things, but are the peculiar results or effects of vibrations of different size and velocity.

These effects are due to the impact of minute undulations or waves, which stream from luminous objects, the chief of which is the sun. These waves are of extreme smallness, the longest being only 226 ten-millionths of an inch from crest to crest. The tiny billows roll outwards and onwards from their source at inconceivable velocities, their mean speed being 185,000 miles in a second. Could we see these light billows themselves and count them as they rolled by, 450 billions (450,000,000,000,000) would pass in a single second, and as the last ranged alongside us, the first would be 185,000 miles away. We are not able, however, to see the waves themselves, for the ocean whose vibrations they are, is composed of matter infinitely more transparent than air, and infinitely less dense. Light, then, be it clearly understood, is not the ethereal billows or waves themselves, but only the effect they produce on falling upon a peculiar kind of matter called the optic nerve. When the same vibrations fall upon a photographic sensitive film, another effect—chemical action—is produced: when they fall upon other matter, heat is the result. Thus heat, light and chemical action are but phases, expressions, effects or results of the different influences of waves upon different kinds of matter. The same waves or billows will affect the eye itself as light, the ordinary nerves as warmth, and the skin as chemical action, in tanning it.

Though we cannot see these waves with the material eye, they are visible indeed to the mental eye; and are as amenable to experimental research as the mightiest waves of the sea. Still, to render this subject clearer, we will use the analogy of sound. A musical note, we all know, is the effect upon our ears of regularly recurring vibrations. A pianoforte wire emits a given note, or in other words, vibrates at a certain and constant rate. These vibrations are taken up by the air, and by it communicated to the ear, and the sensation of sound is produced. Here we see the wire impressing its motion on the air, and the air communicating its motion to the ear; but if another wire similar in all respects is near, it will also be set in motion, and emit its note; and so will any other body that can vibrate in unison. Further, the note of the pianoforte string is not a simple tone, but superposed, as it were, upon the fundamental note, are a series of higher tones, called harmonics, which give richness. Now, a ray of sun-light may be likened to such a note; it consists not of waves all of a certain length or velocity, but of numbers of waves of different lengths and speed. When all these fall upon the eye, the sensation of white light is produced, white light being the compound effect, like the richness of the tone of the wire and its harmonies; or we may look upon it as a luminous chord. When light strikes on any body, part or all is reflected to the eye. If all the waves are thus reflected equally, the result is whiteness. If only a part is reflected, the effect is colour, the tint depending upon the particular waves reflected. If none of the waves are reflected, the result is blackness.

Colour, then, depends upon the nature of the body reflecting light. The exact nature of the action of the body upon the light is not known, but depends most probably upon the molecular condition of the surface. Bodies which allow the light to pass through them, are in like manner coloured according to the waves they allow to pass.

We find in nature, however, a somewhat different class of colour, namely, the iridescent tints, like mother of pearl or shot silk, which give splendour to such butterflies, as some Morphos and the Purple Emperor. These are called diffraction colours, and are caused by minute lines upon the reflecting surface, or by thin transparent films. These lines or films must be so minute that the tiny light waves are broken up among them, and are hence reflected irregularly to the eye.

Dr. Hagen has divided the colours of insects into two classes, the epidermal and hypodermal. The epidermal colours are produced in the external layer or epidermis which is comparatively dry, and are persistent, and do not alter after death. Of this nature are the metallic tints of blue, green, bronze, gold and silver, and the dead blacks and browns, and some of the reds. The hypodermal colours are formed in the moister cells underlying the epidermis, and on the drying up of the specimen fade, as might be expected. They show through the epidermis, which is more or less transparent. These colours are often brighter and lighter in hue than the epidermal; and such are most of the blues, and greens, and yellow, milk white, orange, and the numerous intermediate shades. These colours are sometimes changeable by voluntary act, and the varying tints of the chameleon and many fishes are of this character.

In this connection, Dr. Hagen remarks, that probably all mimetic colours are hypodermal. The importance of this suggestion will be seen at once, for it necessitates a certain consciousness or knowledge on the part of the mimicker, which we have shown, seems to be an essential factor in the theory of colouration.

This author further says, that "the pattern is not the product of an accidental circumstance, but apparently the product of a certain law, or rather the consequence of certain actions or wants in the interior of the animal and in its development."

This remarkable paper, to which our attention was called after our work was nearly completed, is the only record we have been able to find which recognises a law of colouration.

From what has been said of the nature of light, and the physical origin of colour, we see that to produce any distinct tint such as red, yellow, green, or blue, a definite physical structure must be formed, capable of reflecting certain rays of the same nature and absorbing others. Hence, whenever we see any distinct colour, we may be sure that a very considerable development in a certain direction has taken place. This is a most important conclusion, though not very obvious at first sight. Still, when we bear in mind the numbers of light waves of different lengths, and know that if these are reflected irregularly, we get only mixed tints such as indefinite browns; we can at once see how, in the case of such objects as tree trunks, and, still more, in inanimate things like rocks and soils, these, so-to-say, undifferentiated hues are just what we might expect to prevail, and that when definite colours are produced, it of necessity implies an effort of some sort. Now, if this be true of such tints as red and blue, how much more must it be the case with black and white, in which all the rays are absorbed or all reflected? These imply an even stronger effort, and a priori reasoning would suggest that where they occur, they have been developed for important purposes by what may be termed a supreme effort. Consequently, we find them far less common than the others; and it is a most singular fact that in mimetic insects, these are the colours that are most frequently made use of. It would almost seem as if a double struggle had gone on: first, the efforts which resulted in the protective colouring of the mimicked species, and then a more severe, because necessarily more rapid, struggle on the part of the mimicker.