ornament2
CHAPTER III.Introductory Sketch.
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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 ofBougainvillea, 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 seento 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 isregional, 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.
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 exposedto 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 asVertebrata, or backboned animals, because the vertebral-column or backbone is always present. The other section is called theInvertebrata, 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 theCœlenterata; 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,[20]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.
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 principle of Repetition.
The termEmphasishas 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, shouldemphasize, point out, or make clear to the eye, the use orfunction 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 partsin order toemphasize 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 arecontinued 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 evolutionof 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.
CHAPTER IV.Colour, its Nature and Recognition.
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THIS chapter will be devoted to a slight sketch of the nature of light and colour, and to proofs that niceties of colour are distinguished by animals.
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 226ten-millionthsof an inch from crest to crest. The tiny billows roll outwards and onwards from theirsource 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 theyfall 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 ofcolour, 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 certaindirection 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, anda priorireasoning 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.
Yet another point in this connection. If this idea be correct, it follows that a uniformly coloured flower or animal must be of extreme rarity, since it necessitates not merely the entire suppression of the tendency to emphasize important regions in colour, but also the adjustment of all the varying parts of the organism to one uniform molecular condition, which enables it to absorb all but a certain closely related series of light waves no matter how varied the functions of the parts. Now, such "self-coloured" species, as florists would call them, are not only rare, but, as all horticulturists know, are extremely difficult to produce. When a pansy grower, for instance, sets to work to produce a self-coloured flower—say a white pansy without a dark eye—his difficulties seem insurmountable. And, in truth, this result has never been quite obtained; for he has to fight against every natural tendency of the plant to mark out its corolla-tube in colour, and when this is overcome, to still restrain it, so as to keep it within those limits which alone allow it to reflect the proper waves of light.
The production of black and white, then, being the acme of colour production, we should expect to find these tints largely used for very special purposes. Such is actually the case. The sense organs are frequently picked out with black, as witness the noses of dogs, the tips of their ears, the insertion of their vibrissæ, or whiskers, and so on; and white is the most usual warning or distinctive colour, as we see in the white stripes of the badger and skunk, the white spots of deer, and the white tail of the rabbit.
Plate I.KALLIMA INACHUS.
Plate I.
KALLIMA INACHUS.
Colour, then, as expressed in definite tints and patterns, is no accident; for although, as Wallace has well said, "colour is the normal character," yet we think that this colour would, if unrestrained and undirected, be indefinite, and could not produce definite tints, nor the more complicated phenomenon of patterns, in which definite hues are not merely confined to definite tracts, but so frequently contrasted in the most exquisite manner. As we write, the beautiful Red Admiral (V. atalanta) is sporting in the garden; and who can view its glossy black velvet coat, barred with vividest crimson, and picked out with purest snow white, and doubt for an instant that its robe is not merely the product of law, but the supreme effort of an important law? Mark the habits of this lovely insect. See how proudly it displays its rich decorations; sitting with expanded wings on the branch of a tree, gently vibrating them as it basks in the bright sunshine; and you know, once and for all, that the object of that colour is display. But softly—we have moved too rudely, and it is alarmed. The wings close, and where is its beauty now? Hidden by the sombre specklings of its under wings. See, it has pitched upon a slender twig, and notice how instinctively (shall we say?) it arranges itself in the line of the branch: if it sat athwart it would be prominent, but as it sits there motionless it is not only almost invisible,but it knows it; for you can pick it up in your hands, as we have done scores of times. It is not enough, if we would know nature, to study it in cabinets. There is too much of this dry-bone work in existence. The object of nature islife; and only in living beings can we learn how and why they fulfil their ends.
Here, in this common British butterfly, we have the whole problem set before us—vivid colour, the result of intense and long continued effort; grand display, the object of that colour; dusky, indefinite colour, for concealment; and the "instinctive" pose, to make that protective colour profitable. The insectknowsall this in some way. How it knows we must now endeavour to find out.
In attacking this problem we must ask ourselves, What are thepurposes that colouration, and, especially, decoration, can alone subserve? We can only conceive it of use in three ways: first, as protection from its enemies; second, as concealment from its prey; third, as distinctive for its fellows. To the third class may be added a sub-class—attractiveness to the opposite sex.
The first necessity would seem to be distinctness of species; for, unless each species were separately marked, it would be difficult for the sexes to discriminate mates of their own kind, in many instances; and this is, doubtless, the reason why speciesaredifferently coloured.
But protective resemblance, as inKallima,[7]the Leaf-butterfly, and mimicry, as inD. niaviusandP. merope,[8]sometimes so hide the specific characters that this process seems antagonistic to the prime reason for colouration, by rendering species less distinct. Now, doubtless, protective colouring could not have been so wonderfully developedif the organ of sight were the only means of recognition. But it is not. Animals possess other organs of recognition, of which, as everyone knows, smell is one of the most potent. A dog may have forgotten a face after years of absence, but, once his cold nose has touched your hand, the pleased whine and tail-wagging of recognition, tells of awakened memories. Even with ourselves, dulled as our senses are, the odour of the first spring violet calls up the past; as words and scenes can never do. What country-bred child forgets the strange smell of the city he first visits? and how vividly the scene is recalled in after years by a repetition of that odour!
But insects, and, it may be, many other creatures, possess sense organs whose nature we know not. The functions of the antennæ and of various organs in the wings, are unknown; and none can explain the charm by which the female Kentish Glory, or Oak Egger moths lure their mates. You may collect assiduously, using every seduction in sugars and lanterns, only to find how rare are these insects; but if fortune grant you a virgin female, and you cage her up, though no eye can pierce her prison walls, and though she be silent as the oracles, she will, in some mysterious way, attract lovers; not singly, but by the dozen; not one now and another in an hour, but in eager flocks. Many butterflies possess peculiar scent-pouches on their wings, and one of these, aDanais, is mimicked by several species. It is the possession of these additional powers of recognition that leaves colouration free to run to the extreme of protective vagary, when the species is hard pressed in the struggle for life.
Plate II.MIMICRY.
Plate II.
MIMICRY.
Nevertheless, though animals have other means of recognition, the distinctive markings are, without doubt, the prime means of knowledge. Who, that has seen a peacock spread his glorious plumes like a radiant glory, can doubt its fascination? Who, that has wandered in America, and watched a male humming-bird pirouetting and descending in graceful spirals, its whole body throbbing with ecstasy of love and jealousy, can doubt? Who can even read of the Australian bower-bird, lowliest and first of virtuosi, decorating his love-bower with shells and flowers, and shining stones, running in and out with evident delight, and re-arranging his treasures, as a collector does his gems, and not be certain that here, at least, we have the keenest appreciation, not only of colour, but of beauty—a far higher sense?
It has been said that butterflies must be nearly blind, because they seldom fly directly over a wall, but feel their way up with airy touches. Yet every fact of nature contradicts the supposition. Why have plants their tinted flowers, but to entice the insects there? Why are night-blooming flowers white, or pale yellows and pinks, but to render them conspicuous? Why are so many flowers striped in the direction of the nectary, but to point the painted way to the honey-treasures below? The whole scheme of evolution, the whole of the new revelation of the meanings of nature, becomes a dead letter if insects cannot appreciate the hues of flowers. The bee confines himself as much as possible to one species of flower at a time, and this, too, shows that it must be able to distinguish them with ease. We may, then, take it as proven that the power of discriminating colours is possessed by the lower animals.
CHAPTER V.The Colour Sense.
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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 fromthe 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 mighta prioriexpect that animals would show sensibility to light as distinguished from darkness, before they had the power of discriminating between different kinds of light.
This appears to be the case, for Engelmann has shown[9]thatmany of the lowest forms of life, which are almost mere specks of protoplasm, are influenced by light, some seeking and others shunning it. He found, too, that in the case ofEuglena viridisit would seek the light only if it "were allowed to fall upon the anterior part of the body. Here there is a pigment spot; but careful experiment showed that this was not the point most sensitive to light, a colourless and transparent area of protoplasm lying in front of it being found to be so." Commenting upon this Romanes observes, "it is doubtful whether this pigment spot is or is not to be regarded as an exceedingly primitive organ of special sense." Haeckel has also made observations upon those lowest forms of life, which, being simply protoplasm without the slightest trace of organization, not even possessing a nucleus, form his divisionProtista, occupying the no-man's-land between the animal and vegetable kingdoms. He finds that "already among the microscopic Protista there are some that love light, and some that love darkness rather than light. Many seem also to have smell and taste, for they select their food with great care.... Here, also, we are met by the weighty fact that sense-function is possible without sense organs, without nerves. In place of these, sensitiveness is resident in that wondrous, structureless, albuminous substance, which, under the name of protoplasm, or organic formative material, is known as the general and essential basis of all the phenomena of life."[10]
Now, whether Romanes be correct in doubting whether the pigment-spot in Euglena is a sense organ or not, matters little to our present enquiry, but it certainly does seem that the spot,with its accompanying clear space, looks like such an organ. And when we are further told that after careful experiment it is found thatEuglena viridisprefers blue to all the colours of the spectrum, the fundamental fact seems to be established that even as low down as this the different parts of the spectrum affect differently the body of creatures very nearly at the bottom of the animal scale. This implies a certain selection of colour, and, equally, an abstention from other colours.
It is not part of our scheme, however, to follow out in detail the development of the organs of special sense, and the reader must be referred to the various works of Mr. Romanes, who has worked long and successfully at this and kindred problems. Suffice it to say that in this and other cases he has been led to adopt the theory ofinherited memory, though not, as we believe, in the fulness with which it must ultimately be acquired.
This, however, seems certain, that the development, not only of the sense organs, but of organs in general—that is, the setting aside of certain portions for the performance of special duties, and the modifications of those parts in relation to their special duties, is closely related to the activity of the organism. Thus, we find in those animals, like some of the Cœlenterata, which pass some portion of their existence as free-swimming beings, and the remainder in a stationary or sessile condition, that the former state is the most highly organized. This is shown to a very remarkable degree in the Sea Squirts (Ascidians), a class of animals that are generally grouped with the lower Mollusca, but which Prof. Ray Lankester puts at the base of the Vertebrata.
These animals are either solitary or social, fixed or free; but even when free, have little or no power of locomotion, simply floating in the sea. Their embryos are, however, free-swimming, and some of the most interesting beings in nature. Some are marvellously like young tadpoles, and possess some of the distinctive peculiarities of the Vertebrata. Thus, the body is divided into a head and body, or tail, as in tadpoles. The head contains a large nerve centre, corresponding with the brain, which is produced backwards into a chord, corresponding to the spinal chord. In the head, sense organs are clearly distinguishable; there is a well-marked eye, an equally clear ear, and a less clearly marked olfactory organ. Besides this, the spinal-cord is supported below by a rod-like structure, called the notochord. In the vertebrate embryo this structure always precedes the development of the true vertebral column, and in the lowest forms is persistent through life.
We have thus, in the ascidian larva, a form which, if permanent, would most certainly entitle it to a place in the vertebrate sub-kingdom. It is now an active free-swimming creature, but as maturity approaches it becomes fixed, or floating, and all this pre-figurement of a high destiny is annulled. The tail, with its nervous cord and notochord atrophies, and in the fixed forms, not only do the sense organs pass away, but the entire nervous system is reduced to a single ganglion, and the creature becomes little more than an animated stomach. It is, as Ray Lankester has pointed out, a case of degeneration. In the floating forms, which still possess acertain power of locomotion, this process is not carried to such extremes, and the eye is left.
Now, cases of this kind are important as illustrating the direct connection between an active life and advancement; and they also add indirectly to the view Wallace takes of colouration, namely, that the most brilliant colour is generally applied to the most highly modified parts, and is brightest in the seasons of greatest activity.
But they have a higher meaning also, for they may point us to the prime cause of the divergence of the animal and vegetable kingdoms. In thinking over this matter, one of us ventured to suggest that probably the reason why animals dominate the world, and not plants, is, that plants are, as a rule, stationary, and animals lead an active existence. We can look back to the period prior to the divergence of living protoplasm into the two kingdoms. Two courses only were open to it, either to stay at home, and take what came in its way, or to travel, and seek what was required. The stay-at-homes became plants, and the gad-abouts animals. In a letter it was thus put; "It is a truly strange fact that a free-swimming, sense-organ-bearing animal should degenerate into a fixed feeding and breeding machine. It seems to me that the power of locomotion is asine qua nonfor active development of type, as it necessarily sharpens the wits by bringing fresh experiences and unlooked-for adventures to the creature. I almost think, and this, I believe may be a great fundamental fact, that the only reason why animals rule the world instead of plants is that plants elected to stay at home, and animals did not. They had equal chances. Both start as active elements; the one camps down, and the other looks about him."
Talking over this question with Mr. Butler, he astonished the writer by quoting from his work, "Alps and Sanctuaries" (p. 196), the following passage:—
"The question of whether it is better to abide quiet, and take advantage of opportunities that come, or to go farther afield in search of them, is one of the oldest which living beings have to deal with. It was on this that the first great schism or heresy arose in what was heretofore the catholic faith of protoplasm. The schism still lasts, and has resulted in two great sects—animals and plants. The opinion that it is better to go in search of prey is formulated in animals; the other—that it is better, on the whole, to stay at home,and profit by what comes—in plants. Some intermediate forms still record to us the long struggle during which the schism was not yet complete.
"If I may be pardoned for pursuing this digression further, I would say that it is the plants, and not we, who are the heretics. There can be no question about this; we are perfectly justified, therefore, in devouring them. Ours is the original and orthodox belief, for protoplasm is much more animal than vegetable. It is much more true to say that plants have descended from animals than animals from plants. Nevertheless, like many other heretics, plants have thriven very fairly well. There are a great many of them, and, as regards beauty, if not wit—of a limited kind, indeed, but still wit—it is hard to say that the animal kingdom has the advantage. The views of plants are sadly narrow; all dissenters are narrow-minded; but within their own bounds they know the details of their business sufficiently well—as well as though they kept the most nicely-balanced system of accounts to show them their position. They are eaten, it is true; to eat them is our intolerant and bigoted way of trying to convert them: eating is only a violent mode of proselytizing, or converting; and we do convert them—to good animal substance of our own way of thinking. If we have had no trouble we say they have 'agreed' with us; if we have been unable to make them see things from our point of view, we say they 'disagree' with us, and avoid being on more than distant terms with them for the future. If we have helped ourselves to too much, we say we have got more than we can 'manage.' And an animal is no sooner dead than a plant will convert it back again. It is obvious, however, that no schism could have been so long successful without having a good deal to say for itself.
"Neither party has been quite consistent. Whoever is or can be? Every extreme—every opinion carried to its logical end will prove to be an absurdity. Plants throw out roots and boughs and leaves: this is a kind of locomotion; and as Dr. Erasmus Darwin long since pointed out, they do sometimes approach nearly to what is called travelling; a man of consistent character will never look at a bough, a root, or a tendril, without regarding it as a melancholy and unprincipled compromise. On the other hand, many animals are sessile; and some singularly successful genera, as spiders, are in the main liers-in-wait."
This exquisitively written passage the writer was quite unaware of having read, though he possessed and had perused the work quoted, nor can he understand how such an admirable exposition could have escaped notice. Had he read it: had he assimilated it so thoroughly as to be unconscious of its existence; is this a case of rapid growth of automatism? He cannot say.
To return to the main point, it would seem that specialization is directly proportionate to activity, and when we compare the infinitely diverse organization of the animal with the comparative simplicity of the vegetable world, this conclusion seems to be inevitable.