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 of _rhamni_ only 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 of _rhamni_. 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 of _rhamni_ we actually find the spot drawn out into a streak.

The family of _Pieridæ_, or whites, again afford us admirable examples of the development of spots. The prevailing colours are white, black and yellow: green _appears_ to 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 variety _Diniensis_, 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. In _L. sinapis_ we 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. In _A. cardamines_ the 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, see Plate IV. In the continental species _A. belemia_ we see the dark tip to be in a very elementary condition, being little more than an irregular band formed of united spots, there being as much white as black in the tip, Fig. 5, Plate IV. In _A. belia_, Fig. 6, Plate IV., the black tip is more developed, and in the variety _simplonia_ still 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:--

_A. cardamines_, male, it is small and perfect. Do. female, " larger " _A. belemia_ " large " _A. belia_ " large with white centre. Do. _v. simplonia_ " small and perfect. [15]_A. eupheno_, female, " nearly perfect. Do. male, " a band.

We here find two distinct types of variation. In _A. belia_ we have a tendency to form an ocellus, and in _A. eupheno_ the 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 of _A. eupheno_ the 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 in _G. rhamni_ becomes a flush in _G. Cleopatra_, and a vivid tip in _A. 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.

In _A. cardamines_ the orange flush has overflowed the discoidal spot, as it were, in the male, and is absent in the female. But in _A. eupheno_ we 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 in _cardamines_, but where the spot has expanded to a bar in _eupheno_ it has dammed the colour up and ponded it between bar and tip. An exactly intermediate case between these two species is seen in _A. 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.

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 as _P. 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 in Plate IV., of two Swallow-tails, _Papilio machaon_, Fig. 11, and _P. podalirius_, Fig. 12, in which the development of a stripe can readily be seen.

In _machaon_ the 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. In _podalirius_ the 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, _Levana_ being the winter, and _prorsa_ the 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 both _Levana_, Fig. 4, and _prorsa_, 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. In _porima_ we 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.

The change of spots more or less irregular into eye-spots, or ocelli, is equally clear; and Darwin's drawing of the wings of _Cyllo 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] In Fig. 1 is shown part of a secondary wing feather, in which the lines _k. 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. In Fig. 2 we 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 the _Sphingidæ_ especially, 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 be seen. 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 brighter hues 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.

[11] Descent of Man, vol. ii., p. 132. [12] Quart. Journ. Sci., July 1868, p. 325. [13] Studies in the Theory of Descent. [14] See photographs in Hutchinson's Illustrations of Clinical Surgery. [15] See Plate IV. [16] Desc. Man, vol. ii, p. 133, fig. 52. [17] Compare his figs. 56 to 58 op. cit. [18] Desc. Man, vol. ii., p. 134.