CHAPTER III.

INTRODUCTORY SKETCH.

Natural science has shown us how the existing colouration of an animal or plant can be laid hold of and modified in almost infinite ways under the influence of natural or artificial evolution.

It shows us, for example, how the early pink leaf-buds have been modified into attractive flowers to ensure fertilisation; and it has tracked this action through many of its details. It has explained the rich hue of the bracts of _Bougainvillea_, in which the flowers themselves are inconspicuous, and the coloured flower-stems in other plants, as efforts to attract notice of the flower-frequenting insects. It has explained how a blaze of colour is attained in some plants, as in roses and lilies by large single flowers; how the same effect is produced by a number of small flowers brought to the same plane by gradually increasing flower-stalks, as in the elderberry, or by still smaller flowers clustered into a head, as in daisies and sunflowers.

It teaches us again how fruits have become highly coloured to lure fruit-eating birds and mammals, and how many flowers are striped as guides to the honey-bearing nectary.

Entering more into detail, we are enabled to see how the weird walking-stick and leaf-insects have attained their remarkable protective resemblances, and how the East Indian leaf-butterflies are enabled to deceive alike the birds that would fain devour them, and the naturalist who would study them. Even the still more remarkable cases of protective mimicry, in which one animal so closely mimics another as to derive all the benefits that accrue to its protector, are made clear.

All these and many other points have been deeply investigated, and are now the common property of naturalists.

But up to the present no one has attempted systematically to find out the principles or laws which govern the distribution of colouration; laws which underlie natural selection, and by which alone it can work. Natural selection can show, for instance, how the lion has become almost uniform in colour, while the leopard is spotted, and the tiger striped. The lion living on the plains in open country is thus rendered less conspicuous to his prey, the leopard delighting in forest glades is hardly distinguishable among the changing lights and shadows that flicker through the leaves, and the tiger lurking amid the jungle simulates the banded shades of the cane-brake in his striped mantle.

Beyond this, science has not yet gone; and it is our object to carry the study of natural colouration still further: to show that the lion's simple coat, the leopard's spots, and the tiger's stripes, are but modifications of a deeper principle.

Let us, as an easy and familiar example, study carefully the colouration of a common tabby cat. First, we notice, it is darker on the back than beneath, and this is an almost universal law. It would, indeed, be quite universal among mammals but for some curious exceptions among monkeys and a few other creatures of arboreal habits, which delight in hanging from the branches in such a way as to expose their ventral surface to the light. These apparent exceptions thus lead us to the first general law, namely, that colouration is invariably most intense upon that surface upon which the light falls.

As in most cases the back of the animal is the most exposed, that is the seat of intensest colour. But whenever any modification of position exists, as for instance in the side-swimming fishes like the sole, the upper side is dark and the lower light.

The next point to notice in the cat is that from the neck, along the back to the tail, is a dark stripe. This stripe is generally continued, but slighter in character across the top of the skull; but it will be seen clearly that at the neck the pattern changes, and the skull-pattern is quite distinct from that on the body.

From the central, or what we may call the back-bone stripe, bands pass at a strong but varying angle, which we may call rib-stripes.

Now examine the body carefully, and the pattern will be seen to change at the shoulders and thighs, and also at each limb-joint. In fact, if the cat be attentively remarked, it will clearly be seen that the colouration or pattern is _regional_, and dependent upon the structure of the cat.

Now a cat is a vertebrate or backboned animal, possessing four limbs, and if we had to describe its parts roughly, we should specify the head, trunk, limbs and tail. Each of these regions has its own pattern or decoration. The head is marked by a central line, on each side of which are other irregular lines, or more frequently convoluted or twisted spots. The trunk has its central axial backbone stripe and its lateral rib-lines. The tail is ringed; the limbs have each particular stripes and patches. Moreover, the limb-marks are largest at the shoulder and hip-girdles, and decrease downwards, being smallest, or even wanting, on the feet; and the changes take place at the joints.

All this seems to have some general relation to the internal structure of the animal. Such we believe to be the case; and this brings us to the second great law of colouration, namely, that it is dependent upon the anatomy of the animal. We may enunciate these two laws as follows:--

I. THE LAW OF EXPOSURE. Colouration is primarily dependent upon the direct action of light, being always most intense upon that surface upon which the light falls most directly.

II. THE LAW OF STRUCTURE. Colouration, especially where diversified, follows the chief lines of structure, and changes at points, such as the joints, where function changes.

It is the enunciation and illustration of these two laws that form the subject of the present treatise.

In the sequel we shall treat, in more or less detail, of each point as it arises; but in order to render the argument clearer, this chapter is devoted to a general sketch of my views.

Of the first great law but little need be said here, as it is almost self-evident, and has never been disputed. It is true not only of the upper and under-sides of animals, but also of the covered and uncovered parts or organs.

For example, birds possess four kinds of feathers, of which one only, the contour feathers, occur upon the surface and are exposed to the light. It is in these alone that we find the tints and patterns that render birds so strikingly beautiful, the underlying feathers being invariably of a sober grey. Still further, many of the contour feathers overlap, and the parts so overlapped, being removed from the light are grey also, although the exposed part may be resplendent with the most vivid metallic hues. A similar illustration can be found in most butterflies and moths. The upper wing slightly overlaps the lower along the lower margin, and although the entire surface of the upper wing is covered with coloured scales, and the underwing apparently so as well, it will be found that the thin unexposed margin is of an uniform grey, and quite devoid of any pattern.

The law of structure, on the other hand, is an entirely new idea, and demands more detailed explanation. Speaking in the broadest sense, and confining ourselves to the animal kingdom, animals fall naturally into two great sections, or sub-kingdoms, marked by the possession or absence of an internal bony skeleton. Those which possess this structure are known as _Vertebrata_, or backboned animals, because the vertebral-column or backbone is always present. The other section is called the _Invertebrata_, or backboneless animals.

Now, if we take the Vertebrata, we shall find that the system of colouration, however modified, exhibits an unmistakably strong tendency to assume a vertebral or axial character. Common observation confirms this; and the dark stripes down the backs of horses, asses, cattle, goats, etc., are familiar illustrations. The only great exception to this law is in the case of birds, but here, again, the exception is more apparent than real, as will be abundantly shown in the sequel. This axial stripe is seen equally well in fishes and reptiles.

For our present purpose we may again divide the vertebrates into limbed and limbless. Wherever we find limbless animals, such as snakes, the dorsal stripe is prominent, and has a strong tendency to break up into vertebra-like markings. In the limbed animals, on the other hand, we find the limbs strongly marked by pattern, and thus, in the higher forms the system of colouration becomes axial and appendicular.

As a striking test of the universality of this law we may take the cephalopoda, as illustrated in the cuttle-fishes. These creatures are generally considered to stand at the head of the Mollusca, and are placed, in systems of classification, nearest to the Vertebrata; indeed, they have even been considered to be the lowest type of Vertebrates. This is owing to the possession of a hard axial organ, occupying much the position of the backbone, and is the well-known cuttle-bone. Now, these animals are peculiar amongst their class, from possessing, very frequently, an axial stripe. We thus see clearly that the dorsal stripe is directly related to the internal axial skeleton.

Turning now to the invertebrata, we are at once struck with the entire absence of the peculiar vertebrate plan of decoration; and find ourselves face to face with several distinct plans.

From a colouration point of view, we might readily divide the animal kingdom into two classes, marked by the presence or absence of distinct organs. The first of these includes all the animals except the Protozoa--the lowest members of the animal kingdom--which are simply masses of jelly-like protoplasm, without any distinct organs.

Now, on our view, that colouration follows structure, we ought to find an absence of decoration in this structureless group. This is what we actually do find. The lowest Protozoa are entirely without any system of colouring; being merely of uniform tint, generally of brown colour. As if to place this fact beyond doubt, we find in the higher members a tendency to organization in a pulsating vesicle, which constantly retains the same position, and may, hence, be deemed an incipient organ. Now, this vesicle is invariably tinged with a different hue from the rest of the being. We seem, indeed, here to be brought into contact with the first trace of colouration, and we find it to arise with the commencement of organization, and to be actually applied to the incipient organ itself.

Ascending still higher in the scale, we come to distinctly organized animals, known as the _Coelenterata_; of which familiar examples are found in the jelly-fishes and sea anemonies. These animals are characterized by the possession of distinct organs, are transparent, or translucent, and the organs are arranged radially.

No one can have failed to notice on our coasts, as the filmy jelly-fishes float by, that the looped canals of the disc are delicately tinted with violet; and closer examination will show the radiating muscular bands as pellucid white lines; and the sense organs fringing the umbrella are vividly black--the first trace of opaque colouration in the animal kingdom.

These animals were of yore united with the star-fishes and sea-urchins, to form the sub-kingdom Radiata, because of their radiate structure. Now, in all these creatures we find the system of colouration to be radiate also.

Passing to the old sub-kingdom Articulata, which includes the worms, crabs, lobsters, insects, etc., we come to animals whose structure is segmental; that is to say, the body is made up of a number of distinct segments. Among these we find the law holds, rigidly that the colouration is segmental also, as may be beautifully seen in lobsters and caterpillars.

Lastly, we have the Molluscs, which fall for our purpose into two classes, the naked and the shelled. The naked molluscs are often most exquisitely coloured, and the feathery gills that adorn many are suffused with some of the most brilliant colours in nature. The shelled molluscs differ from all other animals, in that the shell is a secretion, almost as distinct from the animals as a house is from its occupant. This shell is built up bit by bit along its margin by means of a peculiar organ known as the mantle--its structure is marginate--its decoration is marginate also.

We have thus rapidly traversed the animal kingdom, and find that in all cases the system of decoration follows the structural peculiarity of the being decorated. Thus in the:--

Structureless protozoa there is no varying colouration. Radiate animals--the system is radiate. Segmented " " segmental. Marginate " " marginal. Vertebrate " " axial.

We must now expound this great structural law in detail, and we shall find that all the particular ornamentations in their various modifications can be shown to arise from certain principles, namely--

1. The principle of Emphasis, 2. The " Repetition.

The term _Emphasis_ has been selected to express the marking out or distinguishing of important functional or structural regions by ornament, either as form or colour. It is with colour alone that we have to deal.

Architects are familiar with the term emphasis, as applied to the ornamentation of buildings. This ornamentation, they say, should _emphasize_, point out, or make clear to the eye, the use or function of the part emphasized. They recognise the fact that to give sublimity and grace to a building, the ornamentation must be related to the character of the building as a whole, and to its parts in particular.

Thus in a tower whose object or function is to suggest height, the principal lines of decoration must be perpendicular, while in the body of a building such as a church, the chief lines must be horizontal, to express the opposite sentiment. So, too, with individual parts. A banded column, such as we see in Early English Gothic, looks weak and incapable of supporting the superincumbent weight. It suggests the idea that the shaft is bound up to strengthen it. On the other hand, the vertical flutings of a Greek column, at once impress us with their function of bearing vertical pressure and their power to sustain it.

This principle is carried into colour in most of our useful arts. The wheelwright instinctively lines out the rim and spokes and does not cross them, feeling that the effect would be to suggest weakness. Moreover, in all our handicraft work, the points and tips are emphasized with colour.

This principle seems to hold good throughout nature. It is not suggested that the colouration is applied to important parts _in order to_ emphasize them, but rather that being important parts, they have become naturally the seats of most vivid colour. How this comes about we cannot here discuss, but shall refer to it further on.

It is owing to this pervading natural principle, that we find the extreme points of quadrupeds so universally decorated. The tips of the nose, ears and tail, and the feet also proclaim the fact, and the decoration of the sense organs, even down to the dark spots around each hair of a cat's feelers, are additional proofs. Look, for instance, at a caterpillar with its breathing holes or spiracles along the sides, and see how these points are selected as the seats of specialized colour, eye-spots and stripes in every variety will be seen, all centred around these important air-holes.

This leads us to our second principle, that of repetition, which simply illustrates the tendency to repeat similar markings in like areas. Thus the spiracular marks are of the same character on each segment.

The principle of repetition, however, goes further than this, and tends to repeat the style of decoration upon allied parts. We see this strongly in many caterpillars in which spiracular markings are continued over the segments which lack spiracles; and it is probably owing to this tendency that the rib-like markings on so many mammals are continued beyond the ribs into the dorsal region.

Upon these two principles the whole of the colouration of nature seems to depend. But the plan is infinitely modified by natural selection, otherwise the result would have been so patent as to need no elucidation.

Natural selection acts by suppressing, or developing, structurally distributed colour. So far as our researches have gone, it seems most probable that the fundamental or primitive colouration is arranged in spots. These spots may expand into regular or irregular patches, or run into stripes, of which many cases will be given in the sequel. Now, natural selection may suppress certain spots, or lines, or expand them into wide, uniform masses, or it may suppress some and repeat others. On these simple principles the whole scheme of natural colouration can be explained; and to do this is the object of the following pages.

Into the origin of the colour sense it is not our province to enlarge; but, it will reasonably be asked, How are these colours of use to the creature decorated? The admiration of colour, the charm of landscape, is the newest of human developments. Are we, then, to attribute to the lower animals a discriminative power greater than most races of men possess, and, if so, on the theory of evolution, how comes it that man lost those very powers his remote ancestors possessed in so great perfection? To these questions we will venture to reply.

Firstly, then, it must be admitted that the higher animals do actually possess this power; and no one will ever doubt it if he watches a common hedge-sparrow hunting for caterpillars. To see this bird carefully seeking the green species in a garden, and deliberately avoiding the multitudes of highly coloured but nauseous larvæ on the currant bushes, arduously examining every leaf and twig for the protected brown and green larvæ which the keen eye of the naturalist detects only by close observation; hardly deigning to look at the speckled beauties that are feeding in decorated safety before his eyes, while his callow brood are clamouring for food--to see this is to be assured for ever that birds can, and do, discriminate colour perfectly. What is true of birds can be shown to be true of other and lower types; and this leads us to a very important conclusion--that colouration has been developed with the evolution of the sense of sight. We can look back in fancy to the far off ages, when no eye gazed upon the world, and we can imagine that then colour in ornamental devices must have been absent, and a dreary monotony of simple hues must have prevailed.

With the evolution of sight it might be of importance that even the sightless animals should be coloured; and in this way we can account for the decoration of coral polyps, and other animals that have no eyes; just as we find no difficulty in understanding the colouration of flowers.

Colour, in fact, so far as external nature is concerned, is all in all to the lower animals. By its means prey is discovered, or foes escaped. But in the case of man quite a different state of things exists. The lower animals can only be modified and adapted to their surroundings by the direct influence of nature. Man, on the other hand, can utilise the forces of nature to his ends. He does not need to steal close to his prey--he possesses missiles. His arm, in reality, is bounded, not by his finger tips, but by the distance to which he can send his bolts. He is not so directly dependent upon nature; and, as his mental powers increase, his dependence lessens, and in this way--the æsthetic principle not yet being awakened--we can understand how his colour sense, for want of practice, decayed, to be reawakened in these our times, with a vividness and power as unequalled as is his mastery over nature--the master of his ancestors.