Colouration in Animals and Plants

by Alfred Tylor

To this general statement there is a curious and most telling exception. In a great many protozoa there exists a curious pulsating cell-like body, called the contractile vesicle, which seems to be a rudimentary organ, whose function is unknown. Here, then, if anywhere, traces of colouring should be found, and here it is accordingly found, for, though generally clear and colourless, it sometimes assumes a pale roseate hue. This may be deemed the first attempt at decoration in the animal kingdom, and it is directly applied to the only part which can be said to possess structure. Beautiful examples are plentiful in Leidy's magnificent volume on Freshwater Rhizopods.

Cœlenterata. These animals fall into two groups, the Hydrozoa, of which the hydra and jelly-fishes are types, and the Actinozoa, of which the sea-anemonies and corals are types. Most of the cœlenterata are transparent animals, but it is amongst them we first come across opaque colouring.

Of the lowest forms, the hydras, nothing need be said here, as they are so much like the protozoa in their simplicity of structure.

The Corynida, familiar to many of our sea-side visitors by their horny brown tubes (Tubularia), attached to shells and stones, are next in point of complexity. Within the tube is found a semi-fluid mass of protoplasm, giving rise at the orifice to the polypite, which possesses a double series of tentacles. These important organs are generally of a vivid red colour, thus emphasizing their importance in the strongest manner. Other members of the order are white, with pink stripes.

In the larval stage many of the animals belonging to the above and allied orders, are very like the true jelly-fishes. These free swimming larvæ, or gonophores, possess four radiating canals, passing from the digestive sac to the margins of the bell, and these are often the seat of colour. In these creatures, too, we find the earliest trace of sense organs, and consequently, the first highly differentiated organs, and they appear as richly coloured spots on the margins of the bell. The true oceanic Hydrozoa again afford us fine examples of structural colouration. The beautiful translucent blue-purple Velella, which is sometimes driven on to our shores, is a case in point; and its delicate structure lines are all emphasized in deeper hues. The true jelly-fishes (Medusidæ) with their crystal bells and radiating canals, frequently show brilliant colour, and it is applied to the canals, and also to the rudimentary eye-specks, which are frequently richly tinted, and in all cases strongly marked. In the so-called "hidden-eyed" Medusæ we find the same arrangement of colour, the same emphasized eye-specks, and the reproductive organs generally appear as a vivid coloured cross, showing through the translucent bell.

Turning now to the Actinozoa, of which the sea anemonies and corals are types, we are brought first into contact with general decorative, more or less opaque colour, applied to the surface of the animal. In the preceding cases the animals have been almost universally transparent or translucent, and the colouration is often applied to the internal organs, and shows through. In the sea-anemonies we find a nearer approach to opacity, in the dense muscular body, though even this is often translucent, and the tentacles generally so, often looking like clouded chalcedony. The wealth of colour to be found in these animals gives us a very important opportunity of studying decoration, where it first appears in profusion.

One of the first points that strikes even a casual observer is that amongst the sea-anemonies the colouration is extremely variable, even in the same species and in the same locality. This is in strong contrast to what we generally find amongst the higher organisms, such as insects and birds; for though considerable variation is found in them, it does not run riot as in the anemonies. It would almost appear as if the actual colour itself was of minor importance, and only the pattern essential; the precise hue is not fixed, is not important, but the necessity of colour of some sort properly arranged is the object to be attained. Whether this idea has a germ of truth in it or not, it is hard to say, but when we take the fact in connection with its occurrence just where opacity begins, connecting this with the transparency of the lower organisms, and the application of vivid colour to their internal organs, one seems to associate the instability of the anemony's colouring with the transference of colour from the interior to the exterior. Certain it is, that vivid colour never exists in the interior of opaque animals; it is always developed under the influence of light. The white bones, nerves and cartilages, and the uniform red of mammalian muscles, are not cases of true decorative colouring in our sense of the term, for all bodies must have some colour. All bone is practically white, all mammalian muscle red, but for these colours to be truly decorative, it would be necessary for muscles of apparently the same character often to be differently tinted, just as the apparently similar hairs on a mammal, and scales on an insect, are variously painted. This we do not find, for the shaft-bones and plate-bones, and even such odd bones as the hyoid are all one colour; and no one would undertake to tell, by its hue, a piece of striped from a piece of unstriped muscle. Decorative colouring must be external in an opaque animal; it may be internal in a transparent one.

The connection thus shown between decoration and transparency seems to suggest that hypodermal colour is the original, and epidermal the newer scheme: that the latter was derived from the former. This agrees with Haagen's shrewd hint that all mimetic colour was originally hypodermal. Certain it is that the protective colour that is still under personal control, as in the chameleon, &c., is always hypodermal.

The common crass (Bunodes crassicornis) is so extremely variable, that all one can say of it is, that it is coloured red and green. But this colour is distributed in accordance with structure. The base, or crawling surface, not being exposed to the light, is uncoloured. The column, or stem, is irregularly spotted, and striped in accordance with the somewhat undifferentiated character of its tissue, but the important organs, the tentacles, are most definitely ornamented, the colour varying, but the pattern being constant. This pattern is heart-shaped, with the apex towards the point of the tentacle; that is to say, the narrow part of the pattern points to the narrow part of the tentacle.

In the common Actinea mesembryanthemum, which is often blood red, the marginal bodies, probably sense-organs, are of the most exquisite turquoise blue colour, and the ruby disc thus beaded is as perfect an example of simple structural decoration as could be desired. A zone of similar blue runs round the base of the body.