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.
[Illustration: Plate III.
BUTTERFLIES.]
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.
[Illustration: Plate IV.
SPOTS AND STRIPES.]
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.
[Illustration: 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.]
[Illustration: 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.]
[Illustration: 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 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.
[Illustration]
[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.