Chapter 3

CHAPTER VI.Spots and Stripes.

BEARING in mind the great tendency to repetition and symmetry of marking we have shown to exist, it becomes an interesting question to work out the origin of the peculiar spots, stripes, loops and patches which are so prevalent in nature. The exquisite eye-spots of the argus pheasant, the peacock, and many butterflies and moths have long excited admiration and scientific curiosity, and have been the subject of investigation by Darwin,[11]the Rev. H. H. Higgins,[12]Weismann,[13]and others, Darwin having paid especial attention to the subject.

His careful analysis of the ocelli or eye-spots in the Argus pheasant and peacock have led him to conclude that they are peculiar modifications of the bars of colour as shown by his drawings. Our own opinion, founded upon a long series of observations, is that this is not the whole case, but that, in the first place, bars are the result of the coalescence of spots. It is not pretended that a bar of colour is the result of the running together of a series of perfect ocelli like those in the so-called tail of the peacock, but merely that spots of colour are the normal primitive commencement of colouring, and that these spots may be developed on the one hand into ocelli or eye-spots, and on the other into bars or even into great blotches of a uniform tint, covering large surfaces.

Let us first take the cases of abnormal marking as shown in disease. An ordinary rash, as in measles, begins as a set of minute red spots, and the same is the case with small pox, the pustules of which sometimes run together, and becoming confluent form bars,which again enlarging meet and produce a blotch or area abnormally marked. It was these well-known facts that induced us to re-examine this question. Colouration and discolouration arise from the presence or absence of pigment in cells, and thus having, as it were independent sources, we should expect colour first to appear in spots. We have already stated, and shall more fully show in the sequel, how colouration follows structure, and would here merely remark that it seems as if any peculiarity of structure, or intensified function modifying structure, has a direct tendency to influence colour. Thus in the disease known as frontal herpes, as pointed out to us by Mr. Bland Sutton, of the Middlesex Hospital, the affection is characterized by an eruption on the skin corresponding exactly to the distribution of the ophthalmic division of the fifth cranial nerve, mapping out all its little branches, even to the one which goes to the tip of the nose. Mr. Hutchinson, F.R.S., the President of the Pathological Society, who first described this disease, has favoured us with another striking illustration of the regional distribution of the colour effects of herpes. In this case decolouration has taken place. The patient was a Hindoo, and upon his brown skin the pigment has been destroyed in the arm along the course of the ulnar nerve, with its branches along both sides of one finger and the half of another. In the leg the sciatic and saphenous nerves are partly mapped out, giving to the patient the appearance of an anatomical diagram.[14]

In these cases we have three very important facts determined. First the broad fact that decolouration and colouration in some cases certainly follow structure; second, that the effect begins as spots; thirdly, that the spots eventually coalesce into bands and blotches.

In birds and insects we have the best means of studying these phenomena, and we will now proceed to illustrate the case more fully. The facts seem to justify us in considering that starting with a spot we may obtain, according to the development, either an ocellus, a stripe or bar, or a blotch, and that between, these may have any number of intermediate varieties.

Among the butterflies we have numerous examples of the development from spots, as illustrated in plates. A good example is seen in our common English Brimstone (Gonepteryx rhamni)Fig. 2, Plate III.In this insect the male (figured) is of a uniform sulphur yellow, with a rich orange spot in the cell of each wing; the female is much paler in colour, and spotted similarly. In an allied continental species (G. Cleopatra)Fig. 1, Plate III., the female is like that ofrhamnionly larger; but the male, instead of having an orange spot in the fore-wing, has nearly the whole of the wing suffused with orange, only the margins, and the lower wings showing the sulphur ground-tint like that ofrhamni. Intermediate forms between these two species are known. In a case like this we can hardly resist the conclusion that the discoidal spot has spread over the fore-wing and become a blotch, and in some English varieties ofrhamniwe actually find the spot drawn out into a streak.

Plate III.Plate IIIBUTTERFLIES.

Plate III.

BUTTERFLIES.

The family ofPieridæ, or whites, again afford us admirable examples of the development of spots. The prevailing colours are white, black and yellow: greenappearsto occur in the Orange-tips (Anthocaris), but it is only the optical effect of a mixture of yellow and grey or black scales. The species are very variable, as a rule, and hence of importance to us; and there are many intermediate species on the continent and elsewhere which render the group a most interesting study.

The wood white (Leucophasia sinapis)Fig. 1, Plate IV., is a pure white species with an almost square dusky tip to the fore-wings of the male. In the female this tip is very indistinct or wanting,Fig. 4, Plate IV.In the varietyDiniensis,Fig. 2, Plate IV., this square tip appears as a round spot.

The Orange-tips, of which we have only one species in Britain (Anthocaris cardamines) belongs to a closely allied genus, as does also the continental genus Zegris. The male Orange-tip (A. cardamines) is white with a dark grey or black tip, and a black discoidal spot. A patch of brilliant orange extends from the dark tip to just beyond the discoidal spots. In the female this is wanting, but the dark tip and spot are larger than in the male.

Let us first study the dark tip. InL. sinapiswe have seen that it extends right to the margin of the wing in the male, but in the female is reduced to a dusky spot away from the margin. InA. cardaminesthe margin is not coloured quite up to the edge, but a row of tiny white spots, like a fringe of seed pearls, occupies the inter-spaces of the veins. On the underside these white spots are prolonged into short bars, seePlate IV. In the continental speciesA. belemiawe see the dark tip to be in a very elementary condition, being little more than an irregular band formed of unitedspots, there being as much white as black in the tip,Fig. 5, Plate IV.InA. belia,Fig. 6, Plate IV., the black tip is more developed, and in the varietysimploniastill more so,Fig. 7, Plate IV.We here see pretty clearly that this dark tip has been developed by the confluence of irregular spots.

Turning now to the discoidal spot we shall observe a similar development. Thus in:—

We here find two distinct types of variation. InA. beliawe have a tendency to form an ocellus, and inA. euphenothe spot of the female is expanded into a band in the male.

The orange flush again offers us a similar case; and with regard to this colour we may remark that it seems to be itself a development from the white ground-colour of the family in the direction of the red end of the spectrum. Thus in the Black-veined white (Aporia cratægi) we have both the upper and under surfaces of the typical cream-white, for there is no pure white in the family. In the true whites the under surface of the hind-wings is lemon-yellow, in the female ofA. euphenothe ground of the upper surface is faint lemon-yellow, and in the male this colour is well-developed. The rich orange, confined to a spot inG. rhamnibecomes a flush inG. Cleopatra, and a vivid tip inA. cardamines. These changes are all developments from the cream white, and may be imitated accurately by adding more and more red to the primitive yellow, as the artist actually did in drawing the plate.

InA. cardaminesthe orange flush has overflowed the discoidal spot, as it were, in the male, and is absent in the female. But inA. euphenowe have an intermediate state, for as the figures show, in the female,Fig. 8, the orange tip only extends half-way to the discoidal spot, and in the male it reaches it. Moreover it is to be noticed that the flow of colour, to continue the simile, is unchecked by the spot incardamines, but where the spot has expanded to a bar ineuphenoit has dammed the colour up and ponded it between bar and tip. An exactly intermediate case between these two species is seen inA. euphemoides,Fig. 10, Plate IV., in which the spot is elongated, and dribbles off into an irregular band, into which the orange has trickled, as water trickles through imperfect fascines. This series of illustrations might be repeated in almost any group of butterflies, but sufficient has been said to show how spots can spread into patches, either by the spreading of one or by the coalescence of several.

Plate IV.Plate IVSPOTS and STRIPES.

Plate IV.

SPOTS and STRIPES.

We will now take an illustration of the formation of stripes or bars from spots, and in doing so must call attention to the rarity of true stripes in butterflies. By a true stripe I mean one that has even edges, that is, whose sides are uninfluenced by structure. In all our British species such asP. machaon,M. artemis,M. athalia,V. atalanta,L. sibilla,A. iris, and some of the Browns, Frittilaries and Hair-streaks, which can alone be said to be striped, the bands are clearly nothing more than spots which have spread up to the costæ, and still retain traces of their origin either in the different hue of the costæ which intersect them, or in curved edges corresponding with the interspaces of the costæ. This in itself is sufficient to indicate their origin. But in many foreign species true bands are found, though they are by no means common. Illustrations are given inPlate IV., of two Swallow-tails,Papilio machaon,Fig. 11, andP. podalirius,Fig. 12, in which the development of a stripe can readily be seen.

Inmachaonthe dark band inside the marginal semi-lunar spots of the fore-wings retain traces of their spot-origin in the speckled character of the costal interspaces, and in the curved outlines of those parts. Inpodaliriusthe semi-lunar spots have coalesced into a stripe, only showing its spot-origin in the black markings of the intersecting costæ; and the black band has become a true stripe, with plain edges. Had only such forms as this been preserved, the origin of the spots would have been lost to view.

It may, however, be said, though I think not with justice, that we ought not to take two species, however closely allied, to illustrate such a point. But very good examples can be found in the same species. A common German butterfly,Araschnia Levana, has two distinct varieties,Levanabeing the winter, andprorsathe summer form; and between these an intermediate form,porima, can be bred from the summer form by keeping the pupæ cold. Dr. Weismann, who has largely experimented on this insect, has given accurate illustrations of the varieties.Plate V.is taken from specimens in our possession. In the males of bothLevana,Fig. 4, andprorsa,Fig. 1, the hind-wing has a distinct row of spots, and a less distinct one inside it, and in the females of both these are represented by dark stripes. Inporimawe get every intermediate form of spots and stripes, both in the male and female, and as these were hatched from the same batch of eggs, or, are brothers and sisters, it is quite impossible to doubt that here, at least, we have an actual proof of the change of spots into stripes.

Fig. 1. Part of secondary feather of Argus Pheasant.a.a.Elongated spots, incipient ocelli.b.Interspaces.c.c.Axial line.d.d.Double spots, incipient ocelli.e.Minute dottings.f.f.Shaft.k.k.Line of feathering.

Fig. 1. Part of secondary feather of Argus Pheasant.

Fig. 2. Part of secondary wing feather of Argus Pheasant.a.Oval. Axis at right angles.b.Round.c.c.Shaft.d.Imperfect ocellus.e.Expansion of stripe.f.Interspace.g.Stalk.h.Edge of feather.k.Line of feathering.

Fig. 2. Part of secondary wing feather of Argus Pheasant.

Plate V.Plate VSEASONAL VARIETIES.

Plate V.

SEASONAL VARIETIES.

The change of spots more or less irregular into eye-spots, or ocelli, is equally clear; and Darwin's drawing of the wings ofCyllo leda[16]illustrates the point well. "In some specimens," he remarks, "large spaces on the upper surfaces of the wings are coloured black, and include irregular white marks; and from this state a complete gradation can be traced into a tolerably perfect ocellus,and this results from the contraction of the irregular blotches of colour. In another series of specimens a gradation can be followed from excessively minute white dots, surrounded by a scarcely visible black line, into perfectly symmetrical and larger ocelli." In the words we have put in italics Darwin seems to admit these ocelli to be formed from blotches; and we think those of the Argus pheasant can be equally shown to arise from spots.

Darwin's beautiful drawings show, almost as well as if made for the purpose, that the bars are developed from spots.[17]InFig. 1is shown part of a secondary wing feather, in which the linesk. k.mark the direction of the axis, along which the spots are arranged, perfectly on the right, less so on the left. The lengthening out of the spots towards the shaft is well seen on the right, and the coalescence into lines on the left. InFig. 2we have part of another feather from the same bird, showing on the left elongated spots, with a dark shading round them, and on the right double spots, like twin stars, with one atmosphere around them. Increase the elongation of these latter, and you have the former, and both are nascent ocelli. We here, then, have a regular gradation between spots, bands, and ocelli, just as we can see in insects.

In some larvæ, those of theSphingidæespecially, ocelli occur, and these may be actually watched as they grow from dots to perfect eye-spots, with the maturity of the larva.

Even in some mammals the change from spots to stripes can beseen. Thus, the young tiger is spotted, and so is the young lion; but, whereas in the former case the spots change into the well-known stripes (which are really loops), in the latter they die away. The horse, as Darwin long ago showed, was probably descended from a striped animal, as shown by the bars on a foal's leg. But before this the animal must have been spotted; and the dappled horses are an example of this; and, moreover, almost every horse shows a tendency to spottiness, especially on the haunches. In the museum at Leiden a fine series of the Java pig (Sus vittatus) is preserved. Very young animals are banded, but have spots over the shoulders and thighs; these run into stripes as the animal grows older; then the stripes expand, and, at last meeting, the mature animal is a uniform dark brown. Enough has now, I trust, been said upon this point to show that from spots have been developed the other markings with which we are familiar in the animal kingdom.

The vegetable kingdom illustrates this fact almost as well. Thus, the beautiful leaves of the Crotons are at first green, with few or no coloured spots; the spots then grow more in number, coalesce, form irregular bands, further develop, and finally cover the whole, or almost the whole, of the leaf with a glow of rich colour. Some of the pretty spring-flowering orchid callitriche have sulphur-yellow petals, with dark rich sepia spots; these often develop to such an extent as to overspread nearly all the original yellow. Many other examples might be given.

Hitherto we have started with a spot, and traced its development. But a spot is itself a developed thing, inasmuch as it is an aggregation of similarly coloured cells. How they come about may, perhaps, be partly seen by the following considerations. Definite colour-pattern has a definite function—that of being seen. We may, therefore, infer that the more definite colour is of newer origin than the less definite. Hence, when we find the two sexes differently coloured, we may generally assume that the more homely tinted form is the more ancient. For example, some butterflies, like the gorgeous Purple Emperor (Apatura iris), have very sombre mates; and it seems fair to assume that the emperor's robes have been donned since his consort's dress was originally fashioned.

That the object of brilliant colour is display is shown partly by the fact that in those parts of the wings of butterflies which overlap the brilliant colour is missing, and partly by the generally brighterhues of day-flying butterflies and moths than of the night-flying species. Now, the sombre hues of nocturnal moths are not so much protective (like the sober tints of female butterflies and birds), because night and darkness is their great defender, as the necessary result of the darkness: bright colours are not produced, because they could not be seen and appreciated. In these cases it is very noticeable how frequently the colour is irregularly dotted about—irrorated or peppered over the wings, as it were. This irregular distribution of the pigment cells, if it were quite free from any arrangement, might be looked upon as primitive colouring, undifferentiated either into distinct colour or distinct pattern. If we suppose a few of the pigment cells here and there to become coloured, we should have irregular brilliant dottings, just as we actually see in many butterflies, along the costa. The grouping together of these colour dots would give rise to a spot, from which point all is clear.

That some such grouping or gathering together, allied to segregation, does take place, a study of spots, and especially of eye-spots, renders probable. What the nature of the process is we do not know, nor is it easy to imagine. But let us suppose a surface uniformly tinted brown. Then, if we gather some of the colouring matter into a dark spot we shall naturally leave a lighter area around it, just as we see in all our Browns and Ringlets. In this way we can see how a ring-spot can be formed. To make it a true eye-spot, with a light centre, we must also suppose a pushing away of the colour from that centre. A study of ocelli naturally suggests such a process, which is analogous to the banding of agates, and all concentric nodules. Darwin, struck with this, seems to adopt it as a fact, for he says, "Appearances strongly favour the belief that, on the one hand, a dark spot is often formed by the colouring matter being drawn towards a central point from a surrounding zone, which is thus rendered lighter. And, on the other hand, a white spot is often formed by the colour being driven away from a central point, so that it accumulates in a surrounding darker zone."[18]The analogy between ocelli and concretions may be a real one. At any rate beautiful ocelli of all sizes can be seen forming in many iron-stained sand-stones. The growth of ocelli may thus be a mechanical process adapted by the creature for decorative purposes, but the artistic colouring of many eye-spots implies greater effort.

There is, however, one set of colour lines in birds and insects that do not seem to arise from spots in the ordinary way. These are the coloured feather-shafts of birds, and the coloured nerves or veins in a butterfly's wing, In these the colour has a tendency to flow all along the structure in lines.

Conclusion.The results arrived at in this chapter may be thus summarised:—

Spots, ocelli, stripes, loops, and patches may be, and nearly always are, developed from more or less irregular spots.This is shown both by the study of normal colouring, or by abnormal colouring, or decolouring in disease.Even the celebrated case of the Argus Pheasant shows that the bands from which the ocelli are developed arose from spots.

Spots, ocelli, stripes, loops, and patches may be, and nearly always are, developed from more or less irregular spots.

This is shown both by the study of normal colouring, or by abnormal colouring, or decolouring in disease.

Even the celebrated case of the Argus Pheasant shows that the bands from which the ocelli are developed arose from spots.

CHAPTER VII.Colouration in the Invertebrata.

IF the principle of the dependence of colour-pattern upon structure, enunciated in the preceding pages be sound, we ought to find certain great schemes of colouration corresponding to the great structural subdivisions of the animal kingdom. This is just what we do find; and before tracing the details, it will be as well to group the great colour-schemes together, so that a general view of the question can be obtained at a glance.

The animal kingdom falls naturally into two divisions, but the dividing line can be drawn in two ways. If we take the most simple classification, we have:—

1.Protozoa, animals with no special organs.2.Organozoa, animals possessing organs.

1.Protozoa, animals with no special organs.

2.Organozoa, animals possessing organs.

Practically this classification is not used, but we shall see that from our point of view it is a useful one. In the most general scheme the divisions are:—

1.Invertebrata, animals without backbones.2.Vertebrata, animals with backbones.

1.Invertebrata, animals without backbones.

2.Vertebrata, animals with backbones.

The invertebrata are divided into sub-kingdoms, of which the protozoa form one. These protozoa possess, as it were, only negative properties. In their simplest form they are mere masses of protoplasm, even lacking an investing membrane or coat, and never, even in the highest forms, possessing distinct organs. It is this simplicity which at once separates them entirely from all other animals.

The other sub-kingdoms are:—

Cœlenterata, of which the jelly-fishes are a type; animals possessing an alimentary canal, fully communicating with the general cavity of the body, but without distinct circulatory or nervous systems.

Annuloida, of which the star-fishes are a type; animals having the alimentary canal shut off from the body-cavity, and possessing a nervous system, and in some a true circulatory system.

Annulosa, of which worms, lobsters, and insects are types; animals composed of definite segments, arranged serially, always possessing true circulatory and nervous systems.

Mollusca, of which oysters and whelks are types; animals which are soft-bodied, often bearing a shell, always possessing a distinct nervous system and mostly with a distinct heart.

In old systems of classification, theCœlenterataandAnnuloidawere united into one sub-kingdom, theRadiata, in consequence of their radiate or star-like structures.

As colouration, according to the views here set forth, depends upon structure, we may classify the Invertebrata thus:—

The mollusca are said to be marginate in structure because, in those possessing shells—the mollusca proper—the shell is formed by successive additions to the margin or edge of the shell, by means of the margin of the mantle, or shell-secreting organ.

Now we shall proceed to show that the schemes of colouration follow out these structure-plans, and thus give additional force to the truth of the classification, as well as showing that, viewed on a broad scale, the present theory is a true one.

We can, in fact, throw the whole scheme into a table, as follows:—

SYSTEMS OF COLOURATION.

Protozoa.The protozoa are generally very minute, and always composed of structureless protoplasm. Their peculiarities are rather negative than positive, there being neither body segments, muscular, circulatory, nor nervous systems. Even the denser exterior portion (ectosarc) possessed by some of them seems to be rather a temporary coagulation of the protoplasm than a real differentiation of that material.

Here, then, we have to deal with the simplest forms of life, and if colouration depends upon structure, these structureless transparent creatures should lack all colour-pattern, and such is really the case. Possessing no organs, they have no colouration, and are generally either colourless or a faint uniform brown colour, and through their colourless bodies the food particles show, often giving a fictitious appearance of colouring.

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'smagnificent volume on Freshwater Rhizopods.

Cœlenterata.These animals fall into two groups, theHydrozoa, of which the hydra and jelly-fishes are types, and theActinozoa, of which the sea-anemonies and corals are types. Most of the cœlenterataare 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.

TheCorynida, 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, orgonophores, 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-purpleVelella, 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 theActinozoa, 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. Thewealth 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 colouringmustbe external in an opaque animal; itmaybe 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 protectivecolour 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 commonActinea 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 ofsimilar blue runs round the base of the body.

Turning now to the corals, which are simply like colonies of single anemonies with a stony skeleton, we have quite a different arrangement of hues. No sight is more fascinating than that of a living-coral reef, as seen through the clear waters of a lagoon. The tropical gardens ashore cannot excel these sea-gardens in brilliancy or variety of colour. Reds, yellows, purples, browns of every shade, almost bewilder the eye with their profusion; and here again we find structural decoration carried out to perfection. The growing points of white branching corals (Madrepores) are frequently tipped with vivid purple, and the tiny polyps themselves are glowing gem-stars. In the white brain-corals, the polyps are vivid red, green, yellow, purple and so on; but in almost every case vividly contrasting with the surrounding parts, the colour changing as the function changes.

TheAlcyonariæ, which include the sea-fans, sea-pens, and the red coral of commerce, practically bring us to the end of theCœlenterata, and afford us fresh proof of the dependence of colour upon structure and function. The well-known organ-pipe coral (Tubipora musica) is of a deep crimson colour, and the polyps themselves are of the most vivid emerald green, a contrast that cannot be excelled. Almost equally beautiful is the commercial coral (Corallium rubrum) whose vivid red has given a name to a certain tint. In this coral the polyps are of a milk-white colour.

It must be remembered that in these cases the colour seems actually to be intentional, so as to form a real and not merely an accidental contrast between the stony polypidom and the polyp, for the connecting tissue (cœnosarc) is itself as colourless as it is structureless.

Gathering together the facts detailed in this chapter we find:—

1. That the Protozoa are practically colourless and structureless.2. That in those species which possess a rudimentary organ (contractilevesicle) a slight decoration is applied to that organ.3. That in the Cœlenterata the colouration is directly dependent upon the structure.4. That in transparent animals the colouration is applied directly to the organ whether it be internal as in the canals or ovaries, or external, as in the eye-specks.5. That in opaque animals, as in the sea-anemonies, the colouring is entirely external.6. That it is very variable in hue, but not in pattern.7. That the most highly differentiated parts (tentacles, eye-specks), are the most strongly coloured.8. That in the corals an emphatic difference occurs between the colour of the polypidom (or "coral") and the polyp.

1. That the Protozoa are practically colourless and structureless.

2. That in those species which possess a rudimentary organ (contractilevesicle) a slight decoration is applied to that organ.

3. That in the Cœlenterata the colouration is directly dependent upon the structure.

4. That in transparent animals the colouration is applied directly to the organ whether it be internal as in the canals or ovaries, or external, as in the eye-specks.

5. That in opaque animals, as in the sea-anemonies, the colouring is entirely external.

6. That it is very variable in hue, but not in pattern.

7. That the most highly differentiated parts (tentacles, eye-specks), are the most strongly coloured.

8. That in the corals an emphatic difference occurs between the colour of the polypidom (or "coral") and the polyp.

CHAPTER VIII.Details of Protozoa.

THE Protozoa are divided into three orders.

I.—Gregarinidæ.II.—Rhizopoda.III.—Infusoria.

I. TheGregarinidæconsist of minute protozoa, parasitic in the interior of insects, &c., and like other internal parasites are colourless, as we should expect.

II. TheRhizopodamay, for our purpose, be divided into the naked forms likeAmœba, and those which possess a skeleton, such as the Radiolaria, the Foraminifera and the Spongia.

Of these the naked forms are colourless, or uniformly tinted, excepting the flush already described as emphasizing the contractile vesicle.

TheForaminiferaare the earliest animals that possess a skeleton or shell, and though generally very small, this shell is often complex, and of extreme beauty, though their bodies retain the general simplicity of the protozoa, indeed, they are said to possess no contractile vesicle. Still the complexity of their shells places them on a higher level than the naked rhizopoda.

In these animals we find the first definite colour, not as a pattern, but as simple tinting of the protoplasm. The general hue is yellowish-brown (as inAmœba), but deep red is not uncommon. The deepest colour is found in the oldest central chambers, becoming fainter towards the periphery, where it is often almost unrecognisable.[19]

TheRadiolariaare minute organisms with still more complex skeletons, and are considered by Haeckel[20]to be more highly organized than the preceding order. They consist of a central portion containing masses of minute cells, and an external portion containing yellow cells. Here we have the first differentiation of parts in the external coating and internal capsule, and side by side with this differentiation we find colour more pronounced, and even taking regional tints in certain forms.

We may notice the following genera as exhibiting fine colour:—

Red.Eucecryphalus, Arachnocorys, Eucrytidium, Dictyoceras.Yellow.Carpocanium, Dictyophimus, Amphilonche.Purple.Eucrytidium, Acanthostratus.Blue.Cyrtidosphæra, Cœlodendrum.Green.Cladococeus, Amphilonche.Brown.Acanthometra, Amphilonche.

Examples of these may be seen in the plates of Haeckel's fine work, and as an illustration of regional decoration we citeAcanthostratus purpuraceus, in which the central capsule is seen to run from red to orange, and the external parts to be colourless, with red markings in looped chains.

Spongocycliaalso exhibits this regional distinction of colour very clearly, the central capsule being red and the external portion yellow.

TheSpongida, or sponges, are, broadly speaking, assemblages or colonies of amœba-like individuals, united into a common society. Individually the component animals are low, very low, in type, but their union into colonies, and the necessity for a uniform or common government has given rise to peculiarities that in a certain sense raise them even above the complex radiolaria. Some, it is true, are naked, and do not possess the skeleton that supports the colony, which skeleton forms what we usually call the sponge; but even amongst these naked sponges the necessity for communal purposes over and above the mere wants of the individual, raises them a step higher in the animal series. A multitude of individuals united by a common membrane, living in the open sea, it must have happened that some in more immediate contact with the food-producing waters, would have thriven at the expense of those in the interior who could only obtain the nutriment that had passed unheeded by the peripheral animals. But just as in higher communities we have aninflowing system of water and an out-flowing system of effete sewerage quite uncontrolled, and, alas, generally quite unheeded by the individuals whose wants are so supplied; so in the sponges we have a system of inflowing food-bearing water and an out-flowing sewage, or exhausted-water system. This is brought about by a peculiar system of cilia-lined cells which, as it were, by their motion suck the water in, bringing with it the food, and an efferent system by which the exhausted liquid escapes. These cilia-lined cells are the first true organs that are to be found in the animal kingdom, and according to the views we hold, they ought to be emphasized with colour, even though their internal position renders the colouration less likely. This we find actually to be the case, and these flagellated cells, as they are called, are often the seat of vividest colour.

The animal matter, or sarcode, or protoplasm of sponges falls into three layers, just as we find the primitive embryo of the highest animals; and just as the middle membrane of a mammalian ovum develops into bone, muscle and nerve, so the middle membrane (mesosarc) of the sponges develops the hard skeleton, and in this most important part we find the colour cells prevail. Sollas, one of our best English authorities upon sponges, writes, "The colours of sponges, which are very various, are usually due to the presence of pigment granules, interbedded either in theendosarc of the flagellated cells, or in the mesodermic cells, usually of the skin only, but sometimes of the whole body."[21]

We can, then, appeal most confidently to the protozoa as illustrating the morphological character of colouration.

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