COLOUR, ITS NATURE AND RECOGNITION.
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 226
_ten-millionths_ of an inch from crest to crest. The tiny billows roll
outwards and onwards from their source 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 they
fall 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 of colour,
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 certain direction 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, and _a priori_ reasoning
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.
[Illustration: Plate I.
KALLIMA INACHUS.]
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.
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 is _life_; 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 insect _knows_ all this in some way.
How it knows we must now endeavour to find out.
In attacking this problem we must ask ourselves, What are the purposes
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 species _are_ differently
coloured.
But protective resemblance, as in _Kallima_,[7] the Leaf-butterfly, and
mimicry, as in _D. niavius_ and _P. 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
developed _if 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, a
_Danais_, 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.
[Illustration: 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.
[Illustration]
[7] Pl. I., Figs 1-3.
[8] Pl. II., Figs. 1-3.