iguanas, are as green as the leaves they feed upon, and the slender
whip-snakes are rendered almost invisible as they glide among the
foliage by a similar colouration. How difficult it is sometimes to catch
sight of the little green tree-frogs sitting on the leaves of a small
plant enclosed in a glass case in the Zoological Gardens; yet how much
better concealed must they be among the fresh green damp foliage of a
marshy forest. There is a North-American frog found on lichen-covered
rocks and walls, which is so coloured as exactly to resemble them, and
as long as it remains quiet would certainly escape detection. Some of
the geckos which cling motionless on the trunks of trees in the tropics,
are of such curiously marbled colours as to match exactly with the bark
they rest upon.
In every part of the tropics there are tree-snakes that twist among
boughs and shrubs, or lie coiled up on the dense masses of foliage.
These are of many distinct groups, and comprise both venomous and
harmless genera; but almost all of them are of a beautiful green
colour, sometimes more or less adorned with white or dusky bands and
spots. There can be little doubt that this colour is doubly useful to
them, since it will tend to conceal them from their enemies, and will
lead their prey to approach them unconscious of danger. Dr. Gunther
informs me that there is only one genus of true arboreal snakes (Dipsas)
whose colours are rarely green, but are of various shades of black,
brown, and olive, and these are all nocturnal reptiles, and there can be
little doubt conceal themselves during the day in holes, so that the
green protective tint would be useless to them, and they accordingly
retain the more usual reptilian hues.
Fishes present similar instances. Many flat fish, as for example the
flounder and the skate, are exactly the colour of the gravel or sand on
which they habitually rest. Among the marine flower gardens of an
Eastern coral reef the fishes present every variety of gorgeous colour,
while the river fish even of the tropics rarely if ever have gay or
conspicuous markings. A very curious case of this kind of adaptation
occurs in the sea-horses (Hippocampus) of Australia, some of which bear
long foliaceous appendages resembling seaweed, and are of a brilliant
red colour; and they are known to live among seaweed of the same hue, so
that when at rest they must be quite invisible. There are now in the
aquarium of the Zoological Society some slender green pipe-fish which
fasten themselves to any object at the bottom by their prehensile
tails, and float about with the current, looking exactly like some
simple cylindrical algae.
It is, however, in the insect world that this principle of the
adaptation of animals to their environment is most fully and strikingly
developed. In order to understand how general this is, it is necessary
to enter somewhat into details, as we shall thereby be better able to
appreciate the significance of the still more remarkable phenomena we
shall presently have to discuss. It seems to be in proportion to their
sluggish motions or the absence of other means of defence, that insects
possess the protective colouring. In the tropics there are thousands of
species of insects which rest during the day clinging to the bark of
dead or fallen trees; and the greater portion of these are delicately
mottled with gray and brown tints, which though symmetrically disposed
and infinitely varied, yet blend so completely with the usual colours of
the bark, that at two or three feet distance they are quite
undistinguishable. In some cases a species is known to frequent only one
species of tree. This is the case with the common South American
long-horned beetle (Onychocerus scorpio) which, Mr. Bates informed me,
is found only on a rough-barked tree, called Tapiriba, on the Amazon. It
is very abundant, but so exactly does it resemble the bark in colour and
rugosity, and so closely does it cling to the branches, that until it
moves it is absolutely invisible! An allied species (O. concentricus)
is found only at Para, on a distinct species of tree, the bark of which
it resembles with equal accuracy. Both these insects are abundant, and
we may fairly conclude that the protection they derive from this strange
concealment is at least one of the causes that enable the race to
flourish.
Many of the species of Cicindela, or tiger beetle, will illustrate this
mode of protection. Our common Cicindela campestris frequents grassy
banks, and is of a beautiful green colour, while C. maritima, which is
found only on sandy sea-shores, is of a pale bronzy yellow, so as to be
almost invisible. A great number of the species found by myself in the
Malay islands are similarly protected. The beautiful Cicindela gloriosa,
of a very deep velvety green colour, was only taken upon wet mossy
stones in the bed of a mountain stream, where it was with the greatest
difficulty detected. A large brown species (C. heros) was found chiefly
on dead leaves in forest paths; and one which was never seen except on
the wet mud of salt marshes was of a glossy olive so exactly the colour
of the mud as only to be distinguished when the sun shone, by its
shadow! Where the sandy beach was coralline and nearly white, I found a
very pale Cicindela; wherever it was volcanic and black, a dark species
of the same genus was sure to be met with.
There are in the East small beetles of the family Buprestidae which
generally rest on the midrib of a leaf, and the naturalist often
hesitates before picking them off, so closely do they resemble pieces of
bird's dung. Kirby and Spence mention the small beetle Onthophilus
sulcatus as being like the seed of an umbelliferous plant; and another
small weevil, which is much persecuted by predatory beetles of the genus
Harpalus, is of the exact colour of loamy soil, and was found to be
particularly abundant in loam pits. Mr. Bates mentions a small beetle
(Chlamys pilula) which was undistinguishable by the eye from the dung of
caterpillars, while some of the Cassidae, from their hemispherical forms
and pearly gold colour, resemble glittering dew-drops upon the leaves.
A number of our small brown and speckled weevils at the approach of any
object roll off the leaf they are sitting on, at the same time drawing
in their legs and antennae, which fit so perfectly into cavities for
their reception that the insect becomes a mere oval brownish lump, which
it is hopeless to look for among the similarly coloured little stones
and earth pellets among which it lies motionless.
The distribution of colour in butterflies and moths respectively is very
instructive from this point of view. The former have all their brilliant
colouring on the upper surface of all four wings, while the under
surface is almost always soberly coloured, and often very dark and
obscure. The moths on the contrary have generally their chief colour on
the hind wings only, the upper wings being of dull, sombre, and often
imitative tints, and these generally conceal the hind wings when the
insects are in repose. This arrangement of the colours is therefore
eminently protective, because the butterfly always rests with his wings
raised so as to conceal the dangerous brilliancy of his upper surface.
It is probable that if we watched their habits sufficiently we should
find the under surface of the wings of butterflies very frequently
imitative and protective. Mr. T. W. Wood has pointed out that the little
orange-tip butterfly often rests in the evening on the green and white
flower heads of an umbelliferous plant, and that when observed in this
position the beautiful green and white mottling of the under surface
completely assimilates with the flower heads and renders the creature
very difficult to be seen. It is probable that the rich dark colouring
of the under side of our peacock, tortoiseshell, and red-admiral
butterflies answers a similar purpose.
Two curious South American butterflies that always settle on the trunks
of trees (Gynecia dirce and Callizona acesta) have the under surface
curiously striped and mottled, and when viewed obliquely must closely
assimilate with the appearance of the furrowed bark of many kinds of
trees. But the most wonderful and undoubted case of protective
resemblance in a butterfly which I have ever seen, is that of the common
Indian Kallima inachis, and its Malayan ally, Kallima paralekta. The
upper surface of these insects is very striking and showy, as they are
of a large size, and are adorned with a broad band of rich orange on a
deep bluish ground. The under side is very variable in colour, so that
out of fifty specimens no two can be found exactly alike, but every one
of them will be of some shade of ash or brown or ochre, such as are
found among dead, dry, or decaying leaves. The apex of the upper wings
is produced into an acute point, a very common form in the leaves of
tropical shrubs and trees, and the lower wings are also produced into a
short narrow tail. Between these two points runs a dark curved line
exactly representing the midrib of a leaf, and from this radiate on each
side a few oblique lines, which serve to indicate the lateral veins of a
leaf. These marks are more clearly seen on the outer portion of the base
of the wings, and on the inner side towards the middle and apex, and it
is very curious to observe how the usual marginal and transverse striae
of the group are here modified and strengthened so as to become adapted
for an imitation of the venation of a leaf. We come now to a still more
extraordinary part of the imitation, for we find representations of
leaves in every stage of decay, variously blotched and mildewed and
pierced with holes, and in many cases irregularly covered with powdery
black dots gathered into patches and spots, so closely resembling the
various kinds of minute fungi that grow on dead leaves that it is
impossible to avoid thinking at first sight that the butterflies
themselves have been attacked by real fungi.
But this resemblance, close as it is, would be of little use if the
habits of the insect did not accord with it. If the butterfly sat upon
leaves or upon flowers, or opened its wings so as to expose the upper
surface, or exposed and moved its head and antennae as many other
butterflies do, its disguise would be of little avail. We might be sure,
however, from the analogy of many other cases, that the habits of the
insect are such as still further to aid its deceptive garb; but we are
not obliged to make any such supposition, since I myself had the good
fortune to observe scores of Kallima paralekta, in Sumatra, and to
capture many of them, and can vouch for the accuracy of the following
details. These butterflies frequent dry forests and fly very swiftly.
They were never seen to settle on a flower or a green leaf, but were
many times lost sight of in a bush or tree of dead leaves. On such
occasions they were generally searched for in vain, for while gazing
intently at the very spot where one had disappeared, it would often
suddenly dart out, and again vanish twenty or fifty yards further on. On
one or two occasions the insect was detected reposing, and it could then
be seen how completely it assimilates itself to the surrounding leaves.
It sits on a nearly upright twig, the wings fitting closely back to
back, concealing the antennae and head, which are drawn up between their
bases. The little tails of the hind wing touch the branch, and form a
perfect stalk to the leaf, which is supported in its place by the claws
of the middle pair of feet, which are slender and inconspicuous. The
irregular outline of the wings gives exactly the perspective effect of a
shrivelled leaf. We thus have size, colour, form, markings, and habits,
all combining together to produce a disguise which may be said to be
absolutely perfect; and the protection which it affords is sufficiently
indicated by the abundance of the individuals that possess it.
The Rev. Joseph Greene has called attention to the striking harmony
between the colours of those British moths which are on the wing in
autumn and winter, and the prevailing tints of nature at those seasons.
In autumn various shades of yellow and brown prevail, and he shows that
out of fifty-two species that fly at this season, no less than forty-two
are of corresponding colours. Orgyia antiqua, O. gonostigma, the genera
Xanthia, Glaea, and Ennomos are examples. In winter, gray and silvery
tints prevail, and the genus Chematobia and several species of Hybernia
which fly during this season are of corresponding hues. No doubt if the
habits of moths in a state of nature were more closely observed, we
should find many cases of special protective resemblance. A few such
have already been noticed. Agriopis aprilina, Acronycta psi, and many
other moths which rest during the day on the north side of the trunks of
trees can with difficulty be distinguished from the grey and green
lichens that cover them. The lappet moth (Gastropacha querci) closely
resembles both in shape and colour a brown dry leaf; and the well-known
buff-tip moth, when at rest is like the broken end of a lichen-covered
branch. There are some of the small moths which exactly resemble the
dung of birds dropped on leaves, and on this point Mr. A. Sidgwick, in a
paper read before the Rugby School Natural History Society, gives the
following original observation:--"I myself have more than once mistaken
Cilix compressa, a little white and grey moth, for a piece of bird's
dung dropped upon a leaf, and _vice versa_ the dung for the moth.
Bryophila Glandifera and Perla are the very image of the mortar walls on
which they rest; and only this summer, in Switzerland, I amused myself
for some time in watching a moth, probably Larentia tripunctaria,
fluttering about quite close to me, and then alighting on a wall of the
stone of the district which it so exactly matched as to be quite
invisible a couple of yards off." There are probably hosts of these
resemblances which have not been observed, owing to the difficulty of
finding many of the species in their stations of natural repose.
Caterpillars are also similarly protected. Many exactly resemble in tint
the leaves they feed upon; others are like little brown twigs, and many
are so strangely marked or humped, that when motionless they can hardly
be taken to be living creatures at all. Mr. Andrew Murray has remarked
how closely the larva of the peacock moth (Saturnia pavonia-minor)
harmonizes in its ground colour with that of the young buds of heather
on which it feeds, and that the pink spots with which it is decorated
correspond with the flowers and flower-buds of the same plant.
The whole order of Orthoptera, grasshoppers, locusts, crickets, &c., are
protected by their colours harmonizing with that of the vegetation or
the soil on which they live, and in no other group have we such striking
examples of special resemblance. Most of the tropical Mantidae and
Locustidae are of the exact tint of the leaves on which they habitually
repose, and many of them in addition have the veinings of their wings
modified so as exactly to imitate that of a leaf. This is carried to the
furthest possible extent in the wonderful genus, Phyllium, the "walking
leaf," in which not only are the wings perfect imitations of leaves in
every detail, but the thorax and legs are flat, dilated, and leaf-like;
so that when tho living insect is resting among the foliage on which it
feeds, the closest observation is often unable to distinguish between
the animal and the vegetable.
The whole family of the Phasmidae, or spectres, to which this insect
belongs, is more or less imitative, and a great number of the species
are called "walking-stick insects," from their singular resemblance to
twigs and branches. Some of these are a foot long and as thick as one's
finger, and their whole colouring, form, rugosity, and the arrangement
of the head, legs, and antennae, are such as to render them absolutely
identical in appearance with dead sticks. They hang loosely about shrubs
in the forest, and have the extraordinary habit of stretching out their
legs unsymmetrically, so as to render the deception more complete. One
of these creatures obtained by myself in Borneo (Ceroxylus laceratus)
was covered over with foliaceous excrescences of a clear olive green
colour, so as exactly to resemble a stick grown over by a creeping moss
or jungermannia. The Dyak who brought it me assured me it was grown over
with moss although alive, and it was only after a most minute
examination that I could convince myself it was not so.
We need not adduce any more examples to show how important are the
details of form and of colouring in animals, and that their very
existence may often depend upon their being by these means concealed
from their enemies. This kind of protection is found apparently in every
class and order, for it has been noticed wherever we can obtain
sufficient knowledge of the details of an animal's life-history. It
varies in degree, from the mere absence of conspicuous colour or a
general harmony with the prevailing tints of nature, up to such a minute
and detailed resemblance to inorganic or vegetable structures as to
realize the talisman of the fairy tale, and to give its possessor the
power of rendering itself invisible.
_Theory of Protective Colouring._
We will now endeavour to show how these wonderful resemblances have most
probably been brought about. Returning to the higher animals, let us
consider the remarkable fact of the rarity of white colouring in the
mammalia or birds of the temperate or tropical zones in a state of
nature. There is not a single white land-bird or quadruped in Europe,
except the few arctic or alpine species, to which white is a protective
colour. Yet in many of these creatures there seems to be no inherent
tendency to avoid white, for directly they are domesticated white
varieties arise, and appear to thrive as well as others. We have white
mice and rats, white cats, horses, dogs, and cattle, white poultry,
pigeons, turkeys, and ducks, and white rabbits. Some of these animals
have been domesticated for a long period, others only for a few
centuries; but in almost every case in which an animal has been
thoroughly domesticated, parti-coloured and white varieties are produced
and become permanent.
It is also well known that animals in a state of nature produce white
varieties occasionally. Blackbirds, starlings, and crows are
occasionally seen white, as well as elephants, deer, tigers, hares,
moles, and many other animals; but in no case is a permanent white race
produced. Now there are no statistics to show that the normal-coloured
parents produce white offspring oftener under domestication than in a
state of nature, and we have no right to make such an assumption if the
facts can be accounted for without it. But if the colours of animals do
really, in the various instances already adduced, serve for their
concealment and preservation, then white or any other conspicuous colour
must be hurtful, and must in most cases shorten an animal's life. A
white rabbit would be more surely the prey of hawk or buzzard, and the
white mole, or field mouse, could not long escape from the vigilant owl.
So, also, any deviation from those tints best adapted to conceal a
carnivorous animal would render the pursuit of its prey much more
difficult, would place it at a disadvantage among its fellows, and in a
time of scarcity would probably cause it to starve to death. On the
other hand, if an animal spreads from a temperate into an arctic
district, the conditions are changed. During a large portion of the
year, and just when the struggle for existence is most severe, white is
the prevailing tint of nature, and dark colours will be the most
conspicuous. The white varieties will now have an advantage; they will
escape from their enemies or will secure food, while their brown
companions will be devoured or will starve; and as "like produces like"
is the established rule in nature, the white race will become
permanently established, and dark varieties, when they occasionally
appear, will soon die out from their want of adaptation to their
environment. In each case the fittest will survive, and a race will be
eventually produced adapted to the conditions in which it lives.
We have here an illustration of the simple and effectual means by which
animals are brought into harmony with the rest of nature. That slight
amount of variability in every species, which we often look upon as
something accidental or abnormal, or so insignificant as to be hardly
worthy of notice, is yet the foundation of all those wonderful and
harmonious resemblances which play such an important part in the economy
of nature. Variation is generally very small in amount, but it is all
that is required, because the change in the external conditions to which
an animal is subject is generally very slow and intermittent. When these
changes have taken place too rapidly, the result has often been the
extinction of species; but the general rule is, that climatal and
geological changes go on slowly, and the slight but continual
variations in the colour, form, and structure of all animals, has
furnished individuals adapted to these changes, and who have become the
progenitors of modified races. Rapid multiplication, incessant slight
variation, and survival of the fittest--these are the laws which ever
keep the organic world in harmony with the inorganic, and with itself.
These are the laws which we believe have produced all the cases of
protective resemblance already adduced, as well as those still more
curious examples we have yet to bring before our readers.
It must always be borne in mind that the more wonderful examples, in
which there is not only a general but a special resemblance--as in the
walking leaf, the mossy phasma, and the leaf-winged butterfly--represent
those few instances in which the process of modification has been going
on during an immense series of generations. They all occur in the
tropics, where the conditions of existence are the most favourable, and
where climatic changes have for long periods been hardly perceptible. In
most of them favourable variations both of colour, form, structure, and
instinct or habit, must have occurred to produce the perfect adaptation
we now behold. All these are known to vary, and favourable variations
when not accompanied by others that were unfavourable, would certainly
survive. At one time a little step might be made in this direction, at
another time in that--a change of conditions might sometimes render
useless that which it had taken ages to produce--great and sudden
physical modifications might often produce the extinction of a race
just as it was approaching perfection, and a hundred checks of which we
can know nothing may have retarded the progress towards perfect
adaptation; so that we can hardly wonder at there being so few cases in
which a completely successful result has been attained as shown by the
abundance and wide diffusion of the creatures so protected.
_Objection that Colour, as being dangerous, should not exist in Nature._
It is as well here to reply to an objection that will no doubt occur to
many readers--that if protection is so useful to all animals, and so
easily brought about by variation and survival of the fittest, there
ought to be no conspicuously-coloured creatures; and they will perhaps
ask how we account for the brilliant birds, and painted snakes, and
gorgeous insects, that occur abundantly all over the world. It will be
advisable to answer this question rather fully, in order that we may be
prepared to understand the phenomena of "mimicry," which it is the
special object of this paper to illustrate and explain.