INTRODUCTORY SKETCH.
Natural science has shown us how the existing colouration of an animal
or plant can be laid hold of and modified in almost infinite ways under
the influence of natural or artificial evolution.
It shows us, for example, how the early pink leaf-buds have been
modified into attractive flowers to ensure fertilisation; and it has
tracked this action through many of its details. It has explained the
rich hue of the bracts of _Bougainvillea_, in which the flowers
themselves are inconspicuous, and the coloured flower-stems in other
plants, as efforts to attract notice of the flower-frequenting insects.
It has explained how a blaze of colour is attained in some plants, as in
roses and lilies by large single flowers; how the same effect is
produced by a number of small flowers brought to the same plane by
gradually increasing flower-stalks, as in the elderberry, or by still
smaller flowers clustered into a head, as in daisies and sunflowers.
It teaches us again how fruits have become highly coloured to lure
fruit-eating birds and mammals, and how many flowers are striped as
guides to the honey-bearing nectary.
Entering more into detail, we are enabled to see how the weird
walking-stick and leaf-insects have attained their remarkable protective
resemblances, and how the East Indian leaf-butterflies are enabled to
deceive alike the birds that would fain devour them, and the naturalist
who would study them. Even the still more remarkable cases of protective
mimicry, in which one animal so closely mimics another as to derive all
the benefits that accrue to its protector, are made clear.
All these and many other points have been deeply investigated, and are
now the common property of naturalists.
But up to the present no one has attempted systematically to find out
the principles or laws which govern the distribution of colouration;
laws which underlie natural selection, and by which alone it can work.
Natural selection can show, for instance, how the lion has become almost
uniform in colour, while the leopard is spotted, and the tiger striped.
The lion living on the plains in open country is thus rendered less
conspicuous to his prey, the leopard delighting in forest glades is
hardly distinguishable among the changing lights and shadows that
flicker through the leaves, and the tiger lurking amid the jungle
simulates the banded shades of the cane-brake in his striped mantle.
Beyond this, science has not yet gone; and it is our object to carry the
study of natural colouration still further: to show that the lion's
simple coat, the leopard's spots, and the tiger's stripes, are but
modifications of a deeper principle.
Let us, as an easy and familiar example, study carefully the colouration
of a common tabby cat. First, we notice, it is darker on the back than
beneath, and this is an almost universal law. It would, indeed, be quite
universal among mammals but for some curious exceptions among monkeys
and a few other creatures of arboreal habits, which delight in hanging
from the branches in such a way as to expose their ventral surface to
the light. These apparent exceptions thus lead us to the first general
law, namely, that colouration is invariably most intense upon that
surface upon which the light falls.
As in most cases the back of the animal is the most exposed, that is the
seat of intensest colour. But whenever any modification of position
exists, as for instance in the side-swimming fishes like the sole, the
upper side is dark and the lower light.
The next point to notice in the cat is that from the neck, along the
back to the tail, is a dark stripe. This stripe is generally continued,
but slighter in character across the top of the skull; but it will be
seen clearly that at the neck the pattern changes, and the skull-pattern
is quite distinct from that on the body.
From the central, or what we may call the back-bone stripe, bands pass
at a strong but varying angle, which we may call rib-stripes.
Now examine the body carefully, and the pattern will be seen to change
at the shoulders and thighs, and also at each limb-joint. In fact, if
the cat be attentively remarked, it will clearly be seen that the
colouration or pattern is _regional_, and dependent upon the structure
of the cat.
Now a cat is a vertebrate or backboned animal, possessing four limbs,
and if we had to describe its parts roughly, we should specify the head,
trunk, limbs and tail. Each of these regions has its own pattern or
decoration. The head is marked by a central line, on each side of which
are other irregular lines, or more frequently convoluted or twisted
spots. The trunk has its central axial backbone stripe and its lateral
rib-lines. The tail is ringed; the limbs have each particular stripes
and patches. Moreover, the limb-marks are largest at the shoulder and
hip-girdles, and decrease downwards, being smallest, or even wanting, on
the feet; and the changes take place at the joints.
All this seems to have some general relation to the internal structure
of the animal. Such we believe to be the case; and this brings us to the
second great law of colouration, namely, that it is dependent upon the
anatomy of the animal. We may enunciate these two laws as follows:--
I. THE LAW OF EXPOSURE. Colouration is primarily dependent upon the
direct action of light, being always most intense upon that surface
upon which the light falls most directly.
II. THE LAW OF STRUCTURE. Colouration, especially where
diversified, follows the chief lines of structure, and changes at
points, such as the joints, where function changes.
It is the enunciation and illustration of these two laws that form the
subject of the present treatise.
In the sequel we shall treat, in more or less detail, of each point as
it arises; but in order to render the argument clearer, this chapter is
devoted to a general sketch of my views.
Of the first great law but little need be said here, as it is almost
self-evident, and has never been disputed. It is true not only of the
upper and under-sides of animals, but also of the covered and uncovered
parts or organs.
For example, birds possess four kinds of feathers, of which one only,
the contour feathers, occur upon the surface and are exposed to the
light. It is in these alone that we find the tints and patterns that
render birds so strikingly beautiful, the underlying feathers being
invariably of a sober grey. Still further, many of the contour feathers
overlap, and the parts so overlapped, being removed from the light are
grey also, although the exposed part may be resplendent with the most
vivid metallic hues. A similar illustration can be found in most
butterflies and moths. The upper wing slightly overlaps the lower along
the lower margin, and although the entire surface of the upper wing is
covered with coloured scales, and the underwing apparently so as well,
it will be found that the thin unexposed margin is of an uniform grey,
and quite devoid of any pattern.
The law of structure, on the other hand, is an entirely new idea, and
demands more detailed explanation. Speaking in the broadest sense, and
confining ourselves to the animal kingdom, animals fall naturally into
two great sections, or sub-kingdoms, marked by the possession or absence
of an internal bony skeleton. Those which possess this structure are
known as _Vertebrata_, or backboned animals, because the
vertebral-column or backbone is always present. The other section is
called the _Invertebrata_, or backboneless animals.
Now, if we take the Vertebrata, we shall find that the system of
colouration, however modified, exhibits an unmistakably strong tendency
to assume a vertebral or axial character. Common observation confirms
this; and the dark stripes down the backs of horses, asses, cattle,
goats, etc., are familiar illustrations. The only great exception to
this law is in the case of birds, but here, again, the exception is more
apparent than real, as will be abundantly shown in the sequel. This
axial stripe is seen equally well in fishes and reptiles.
For our present purpose we may again divide the vertebrates into limbed
and limbless. Wherever we find limbless animals, such as snakes, the
dorsal stripe is prominent, and has a strong tendency to break up into
vertebra-like markings. In the limbed animals, on the other hand, we
find the limbs strongly marked by pattern, and thus, in the higher forms
the system of colouration becomes axial and appendicular.
As a striking test of the universality of this law we may take the
cephalopoda, as illustrated in the cuttle-fishes. These creatures are
generally considered to stand at the head of the Mollusca, and are
placed, in systems of classification, nearest to the Vertebrata;
indeed, they have even been considered to be the lowest type of
Vertebrates. This is owing to the possession of a hard axial organ,
occupying much the position of the backbone, and is the well-known
cuttle-bone. Now, these animals are peculiar amongst their class, from
possessing, very frequently, an axial stripe. We thus see clearly that
the dorsal stripe is directly related to the internal axial skeleton.
Turning now to the invertebrata, we are at once struck with the entire
absence of the peculiar vertebrate plan of decoration; and find
ourselves face to face with several distinct plans.
From a colouration point of view, we might readily divide the animal
kingdom into two classes, marked by the presence or absence of distinct
organs. The first of these includes all the animals except the
Protozoa--the lowest members of the animal kingdom--which are simply
masses of jelly-like protoplasm, without any distinct organs.
Now, on our view, that colouration follows structure, we ought to find
an absence of decoration in this structureless group. This is what we
actually do find. The lowest Protozoa are entirely without any system of
colouring; being merely of uniform tint, generally of brown colour. As
if to place this fact beyond doubt, we find in the higher members a
tendency to organization in a pulsating vesicle, which constantly
retains the same position, and may, hence, be deemed an incipient organ.
Now, this vesicle is invariably tinged with a different hue from the
rest of the being. We seem, indeed, here to be brought into contact with
the first trace of colouration, and we find it to arise with the
commencement of organization, and to be actually applied to the
incipient organ itself.
Ascending still higher in the scale, we come to distinctly organized
animals, known as the _Coelenterata_; of which familiar examples are
found in the jelly-fishes and sea anemonies. These animals are
characterized by the possession of distinct organs, are transparent, or
translucent, and the organs are arranged radially.
No one can have failed to notice on our coasts, as the filmy
jelly-fishes float by, that the looped canals of the disc are delicately
tinted with violet; and closer examination will show the radiating
muscular bands as pellucid white lines; and the sense organs fringing
the umbrella are vividly black--the first trace of opaque colouration in
the animal kingdom.
These animals were of yore united with the star-fishes and sea-urchins,
to form the sub-kingdom Radiata, because of their radiate structure.
Now, in all these creatures we find the system of colouration to be
radiate also.
Passing to the old sub-kingdom Articulata, which includes the worms,
crabs, lobsters, insects, etc., we come to animals whose structure is
segmental; that is to say, the body is made up of a number of distinct
segments. Among these we find the law holds, rigidly that the
colouration is segmental also, as may be beautifully seen in lobsters
and caterpillars.
Lastly, we have the Molluscs, which fall for our purpose into two
classes, the naked and the shelled. The naked molluscs are often most
exquisitely coloured, and the feathery gills that adorn many are
suffused with some of the most brilliant colours in nature. The shelled
molluscs differ from all other animals, in that the shell is a
secretion, almost as distinct from the animals as a house is from its
occupant. This shell is built up bit by bit along its margin by means of
a peculiar organ known as the mantle--its structure is marginate--its
decoration is marginate also.
We have thus rapidly traversed the animal kingdom, and find that in all
cases the system of decoration follows the structural peculiarity of the
being decorated. Thus in the:--
Structureless protozoa there is no varying colouration.
Radiate animals--the system is radiate.
Segmented " " segmental.
Marginate " " marginal.
Vertebrate " " axial.
We must now expound this great structural law in detail, and we shall
find that all the particular ornamentations in their various
modifications can be shown to arise from certain principles, namely--
1. The principle of Emphasis,
2. The " Repetition.
The term _Emphasis_ has been selected to express the marking out or
distinguishing of important functional or structural regions by
ornament, either as form or colour. It is with colour alone that we have
to deal.
Architects are familiar with the term emphasis, as applied to the
ornamentation of buildings. This ornamentation, they say, should
_emphasize_, point out, or make clear to the eye, the use or function
of the part emphasized. They recognise the fact that to give sublimity
and grace to a building, the ornamentation must be related to the
character of the building as a whole, and to its parts in particular.
Thus in a tower whose object or function is to suggest height, the
principal lines of decoration must be perpendicular, while in the body
of a building such as a church, the chief lines must be horizontal, to
express the opposite sentiment. So, too, with individual parts. A banded
column, such as we see in Early English Gothic, looks weak and incapable
of supporting the superincumbent weight. It suggests the idea that the
shaft is bound up to strengthen it. On the other hand, the vertical
flutings of a Greek column, at once impress us with their function of
bearing vertical pressure and their power to sustain it.
This principle is carried into colour in most of our useful arts. The
wheelwright instinctively lines out the rim and spokes and does not
cross them, feeling that the effect would be to suggest weakness.
Moreover, in all our handicraft work, the points and tips are emphasized
with colour.
This principle seems to hold good throughout nature. It is not suggested
that the colouration is applied to important parts _in order to_
emphasize them, but rather that being important parts, they have become
naturally the seats of most vivid colour. How this comes about we cannot
here discuss, but shall refer to it further on.
It is owing to this pervading natural principle, that we find the
extreme points of quadrupeds so universally decorated. The tips of the
nose, ears and tail, and the feet also proclaim the fact, and the
decoration of the sense organs, even down to the dark spots around each
hair of a cat's feelers, are additional proofs. Look, for instance, at a
caterpillar with its breathing holes or spiracles along the sides, and
see how these points are selected as the seats of specialized colour,
eye-spots and stripes in every variety will be seen, all centred around
these important air-holes.
This leads us to our second principle, that of repetition, which simply
illustrates the tendency to repeat similar markings in like areas. Thus
the spiracular marks are of the same character on each segment.
The principle of repetition, however, goes further than this, and tends
to repeat the style of decoration upon allied parts. We see this
strongly in many caterpillars in which spiracular markings are
continued over the segments which lack spiracles; and it is probably
owing to this tendency that the rib-like markings on so many mammals are
continued beyond the ribs into the dorsal region.
Upon these two principles the whole of the colouration of nature seems
to depend. But the plan is infinitely modified by natural selection,
otherwise the result would have been so patent as to need no
elucidation.
Natural selection acts by suppressing, or developing, structurally
distributed colour. So far as our researches have gone, it seems most
probable that the fundamental or primitive colouration is arranged in
spots. These spots may expand into regular or irregular patches, or run
into stripes, of which many cases will be given in the sequel. Now,
natural selection may suppress certain spots, or lines, or expand them
into wide, uniform masses, or it may suppress some and repeat others. On
these simple principles the whole scheme of natural colouration can be
explained; and to do this is the object of the following pages.
Into the origin of the colour sense it is not our province to enlarge;
but, it will reasonably be asked, How are these colours of use to the
creature decorated? The admiration of colour, the charm of landscape, is
the newest of human developments. Are we, then, to attribute to the
lower animals a discriminative power greater than most races of men
possess, and, if so, on the theory of evolution, how comes it that man
lost those very powers his remote ancestors possessed in so great
perfection? To these questions we will venture to reply.
Firstly, then, it must be admitted that the higher animals do actually
possess this power; and no one will ever doubt it if he watches a common
hedge-sparrow hunting for caterpillars. To see this bird carefully
seeking the green species in a garden, and deliberately avoiding the
multitudes of highly coloured but nauseous larvæ on the currant bushes,
arduously examining every leaf and twig for the protected brown and
green larvæ which the keen eye of the naturalist detects only by close
observation; hardly deigning to look at the speckled beauties that are
feeding in decorated safety before his eyes, while his callow brood are
clamouring for food--to see this is to be assured for ever that birds
can, and do, discriminate colour perfectly. What is true of birds can be
shown to be true of other and lower types; and this leads us to a very
important conclusion--that colouration has been developed with the
evolution of the sense of sight. We can look back in fancy to the far
off ages, when no eye gazed upon the world, and we can imagine that then
colour in ornamental devices must have been absent, and a dreary
monotony of simple hues must have prevailed.
With the evolution of sight it might be of importance that even the
sightless animals should be coloured; and in this way we can account for
the decoration of coral polyps, and other animals that have no eyes;
just as we find no difficulty in understanding the colouration of
flowers.
Colour, in fact, so far as external nature is concerned, is all in all
to the lower animals. By its means prey is discovered, or foes escaped.
But in the case of man quite a different state of things exists. The
lower animals can only be modified and adapted to their surroundings by
the direct influence of nature. Man, on the other hand, can utilise the
forces of nature to his ends. He does not need to steal close to his
prey--he possesses missiles. His arm, in reality, is bounded, not by his
finger tips, but by the distance to which he can send his bolts. He is
not so directly dependent upon nature; and, as his mental powers
increase, his dependence lessens, and in this way--the æsthetic
principle not yet being awakened--we can understand how his colour
sense, for want of practice, decayed, to be reawakened in these our
times, with a vividness and power as unequalled as is his mastery over
nature--the master of his ancestors.
[Illustration]