The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

Macmillan's _Magazine_, and more fully in my volume on _Tropical

Nature_. Subsequently Mrs. Barber gave a few examples under the head of

"Indicative or Banner Colours," but she applied it to the distinctive

colours of the males of birds, which I explain on another principle,

though this may assist.]
[Footnote 87: Quoted by Darwin in _Descent of Man_, p. 317.]
[Footnote 88: In the _American Naturalist_ of March 1888, Mr. J.E. Todd

has an article on "Directive Coloration in Animals," in which he

recognises many of the cases here referred to, and suggests a few

others, though I think he includes many forms of coloration--as

"paleness of belly and inner side of legs"--which do not belong to this

class.]
[Footnote 89: For numerous examples of this protective colouring of

marine animals see Moseley's _Voyage of the Challenger_, and Dr. E.S.

Morse in _Proc. of Bost. Soc. of Nat. Hist._, vol. xiv. 1871.]

_Proc. Bost. Soc. of Nat. Hist._, 1874, p. 284; and _Mammals and Winter

Birds of Florida_, pp. 233-241.]

CHAPTER IX

The skunk as an example of warning coloration--Warning colours

among insects--Butterflies--Caterpillars--Mimicry--How mimicry

has been produced--Heliconidae--Perfection of the

imitation--Other cases of mimicry among Lepidoptera--Mimicry

among protected groups--Its explanation--Extension of the

principle--Mimicry in other orders of insects--Mimicry among the

vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among

birds--Objections to the theory of mimicry--Concluding remarks

on warning colours and mimicry.

We have now to deal with a class of colours which are the very opposite

of those we have hitherto considered, since, instead of serving to

conceal the animals that possess them or as recognition marks to their

associates, they are developed for the express purpose of rendering the

species conspicuous. The reason of this is that the animals in question

are either the possessors of some deadly weapons, as stings or poison

fangs, or they are uneatable, and are thus so disagreeable to the usual

enemies of their kind that they are never attacked when their peculiar

powers or properties are known. It is, therefore, important that they

should not be mistaken for defenceless or eatable species of the same

class or order, since in that case they might suffer injury, or even

death, before their enemies discovered the danger or the uselessness of

the attack. They require some signal or danger-flag which shall serve as

a warning to would-be enemies not to attack them, and they have usually

obtained this in the form of conspicuous or brilliant coloration, very

distinct from the protective tints of the defenceless animals allied to

them.
_The Skunk as illustrating Warning Coloration._

Central Pacific Railway, I strolled out one evening after dinner, and on

the road, not fifty yards from the house, I saw a pretty little white

and black animal with a bushy tail coming towards me. As it came on at a

slow pace and without any fear, although it evidently saw me, I thought

at first that it must be some tame creature, when it suddenly occurred

to me that it was a skunk. It came on till within five or six yards of

me, then quietly climbed over a dwarf wall and disappeared under a small

outhouse, in search of chickens, as the landlord afterwards told me.

This animal possesses, as is well known, a most offensive secretion,

which it has the power of ejecting over its enemies, and which

effectually protects it from attack. The odour of this substance is so

penetrating that it taints, and renders useless, everything it touches,

or in its vicinity. Provisions near it become uneatable, and clothes

saturated with it will retain the smell for several weeks, even though

they are repeatedly washed and dried. A drop of the liquid in the eyes

will cause blindness, and Indians are said not unfrequently to lose

their sight from this cause. Owing to this remarkable power of offence

the skunk is rarely attacked by other animals, and its black and white

fur, and the bushy white tail carried erect when disturbed, form the

danger-signals by which it is easily distinguished in the twilight or

moonlight from unprotected animals. Its consciousness that it needs only

to be seen to be avoided gives it that slowness of motion and

fearlessness of aspect which are, as we shall see, characteristic of

most creatures so protected.

_Warning Colours among Insects._

abundant. We all know how well marked and conspicuous are the colours

and forms of the stinging wasps and bees, no one of which in any part of

the world is known to be protectively coloured like the majority of

defenceless insects. Most of the great tribe of Malacoderms among

beetles are distasteful to insect-eating animals. Our red and black

Telephoridae, commonly called "soldiers and sailors," were found, by Mr.

Jenner Weir, to be refused by small birds. These and the allied

Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by

Mr. Belt's tame monkey and by his fowls, though most other insects were

greedily eaten by them. The Coccinellidae or lady-birds are another

uneatable group, and their conspicuous and singularly spotted bodies

serve to distinguish them at a glance from all other beetles.
These uneatable insects are probably more numerous than is supposed,

although we already know immense numbers that are so protected. The most

remarkable are the three families of butterflies--Heliconidae, Danaidae,

and Acraeidae--comprising more than a thousand species, and

characteristic respectively of the three great tropical regions--South

America, Southern Asia, and Africa. All these butterflies have

peculiarities which serve to distinguish them from every other group in

their respective regions. They all have ample but rather weak wings, and

fly slowly; they are always very abundant; and they all have conspicuous

colours or markings, so distinct from those of other families that, in

conjunction with their peculiar outline and mode of flight, they can

usually be recognised at a glance. Other distinctive features are, that

their colours are always nearly the same on the under surface of their

wings as on the upper; they never try to conceal themselves, but rest on

the upper surfaces of leaves or flowers; and, lastly, they all have

juices which exhale a powerful scent, so that when one kills them by

pinching the body, the liquid that exudes stains the fingers yellow, and

leaves an odour that can only be removed by repeated washings.

very offensive to us, is so to most insect-eating creatures. Mr. Bates

observed that, when set out to dry, specimens of Heliconidae were less

subject to the attacks of vermin; while both he and I noticed that they

were not attacked by insect-eating birds or dragonflies, and that their

wings were not found in the forest paths among the numerous wings of

other butterflies whose bodies had been devoured. Mr. Belt once observed

a pair of birds capturing insects for their young; and although the

Heliconidae swarmed in the vicinity, and from their slow flight could

have been easily caught, not one was ever pursued, although other

butterflies did not escape. His tame monkey also, which would greedily

munch up other butterflies, would never eat the Heliconidae. It would

sometimes smell them, but always rolled them up in its hand and then

dropped them.

the Eastern Danaidae. The Hon. Mr. Justice Newton, who assiduously

collected and took notes upon the Lepidoptera of Bombay, informed Mr.

Butler of the British Museum that the large and swift-flying butterfly

Charaxes psaphon, was continually persecuted by the bulbul, so that he

rarely caught a specimen of this species which had not a piece snipped

out of the hind wings. He offered one to a bulbul which he had in a

cage, and it was greedily devoured, whilst it was only by repeated

persecution that he succeeded in inducing the bird to touch a

Danais.[92]

the great genus Papilio--the true swallow-tailed butterflies--which have

all the characteristics of uneatable insects. They have a special

coloration, usually red and black (at least in the females), they fly

slowly, they are very abundant, and they possess a peculiar odour

somewhat like that of the Heliconidae. One of these groups is common in

tropical America, another in tropical Asia, and it is curious that,

although not very closely allied, they have each the same red and black

colours, and are very distinct from all the other butterflies of their

respective countries. There is reason to believe also that many of the

brilliantly coloured and weak-flying diurnal moths, like the fine

tropical Agaristidae and burnet-moths, are similarly protected, and that

their conspicuous colours serve as a warning of inedibility. The common

burnet-moth (Anthrocera filipendula) and the equally conspicuous

ragwort-moth (Euchelia jacobeae) have been proved to be distasteful to

insect-eating creatures.

is, however, furnished by caterpillars, because in this case the facts

have been carefully ascertained experimentally by competent observers.

In the year 1866, when Mr. Darwin was collecting evidence as to the

supposed effect of sexual selection in bringing about the brilliant

coloration of the higher animals, he was struck by the fact that many

caterpillars have brilliant and conspicuous colours, in the production

of which sexual selection could have no place. We have numbers of such

caterpillars in this country, and they are characterised not only by

their gay colours but by not concealing themselves. Such are the mullein

and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of

the buff-tip, and many others. Some of these caterpillars are

wonderfully conspicuous, as in the case of that noticed by Mr. Bates in

South America, which was four inches long, banded across with black and

yellow, and with bright red head, legs, and tail. Hence it caught the

eye of any one who passed by, even at the distance of many yards.

coloration; and, having been recently interested in the question of the

warning coloration of butterflies, I suggested that this was probably a

similar case,--that these conspicuous caterpillars were distasteful to

birds and other insect-eating creatures, and that their bright

non-protective colours and habit of exposing themselves to view, enabled

their enemies to distinguish them at a glance from the edible kinds and

thus learn not to touch them; for it must be remembered that the bodies

of caterpillars while growing are so delicate, that a wound from a

bird's beak would be perhaps as fatal as if they were devoured.[93] At

this time not a single experiment or observation had been made on the

subject, but after I had brought the matter before the Entomological

Society, two gentlemen, who kept birds and other tame animals, undertook

to make experiments with a variety of caterpillars.

birds in his aviary, and he found that none of them would eat the

following smooth-skinned conspicuous caterpillars--Abraxas

grossulariata, Diloba caeruleocephala, Anthrocera filipendula, and

Cucullia verbasci. He also found that they would not touch any hairy or

spiny larvae, and he was satisfied that it was not the hairs or the

spines, but the unpleasant taste that caused them to be rejected,

because in one case a young smooth larva of a hairy species, and in

another case the pupa of a spiny larva, were equally rejected. On the

other hand, all green or brown caterpillars as well as those that

resemble twigs were greedily devoured.[94]
Mr. A.G. Butler also made experiments with some green lizards (Lacerta

viridis), which greedily ate all kinds of food, including flies of many

kinds, spiders, bees, butterflies, and green caterpillars; but they

would not touch the caterpillar of the gooseberry-moth (Abraxas

grossulariata), or the imago of the burnet-moth (Anthrocera

filipendula). The same thing happened with frogs. When the gooseberry

caterpillars were first given to them, "they sprang forward and licked

them eagerly into their mouths; no sooner, however, had they done so,

than they seemed to become aware of the mistake that they had made, and

sat with gaping mouths, rolling their tongues about, until they had got

quit of the nauseous morsels, which seemed perfectly uninjured, and

walked off as briskly as ever." Spiders seemed equally to dislike them.

This and another conspicuous caterpillar (Halia wavaria) were rejected

by two species--the geometrical garden spider (Epeira diadema) and a

hunting spider.[95]
Some further experiments with lizards were made by Professor Weismann,

quite confirming the previous observations; and in 1886 Mr. E.B. Poulton

of Oxford undertook a considerable series of experiments, with many

other species of larvae and fresh kinds of lizards and frogs. Mr.

Poulton then reviewed the whole subject, incorporating all recorded

facts, as well as some additional observations made by Mr. Jenner Weir

in 1886. More than a hundred species of larvae or of perfect insects of

various orders have now been made the subject of experiment, and the

results completely confirm my original suggestion. In almost every case

the protectively coloured larvae have been greedily eaten by all kinds

of insectivorous animals, while, in the immense majority of cases, the

conspicuous, hairy, or brightly coloured larvae have been rejected by

some or all of them. In some instances the inedibility of the larvae

extends to the perfect insect, but not in others. In the former cases

the perfect insect is usually adorned with conspicuous colours, as the

burnet and ragwort moths; but in the case of the buff-tip, the moth

resembles a broken piece of rotten stick, yet it is partly inedible,

being refused by lizards. It is, however, very doubtful whether these

are its chief enemies, and its protective form and colour may be needed

against insectivorous birds or mammals.

has found so many of the eggs and larvae destroyed by hymenopterous and

dipterous parasites that he thinks at least nine-tenths, perhaps a

greater proportion, never reach maturity. Yet he has never found any

evidence that such parasites attack either the egg or the larva of the

inedible Danais archippus, so that in this case the insect is

distasteful to its most dangerous foes in all the stages of its

existence, a fact which serves to explain its great abundance and its

extension over almost the whole world.[96]
One case has been found of a protectively coloured larva,--one,

moreover, which in all its habits shows that it trusts to concealment to

escape its enemies--which was yet always rejected by lizards after they

had seized it, evidently under the impression that from its colour it

would be eatable. This is the caterpillar of the very common moth Mania

typica; and Mr. Poulton thinks that, in this case, the unpleasant taste

is an incidental result of some physiological processes in the organism,

and is itself a merely useless character. It is evident that the insect

would not conceal itself so carefully as it does if it had not some

enemies, and these are probably birds or small mammals, as its

food-plants are said to be dock and willow-herb, not suggestive of

places frequented by lizards; and it has been found by experiment that

lizards and birds have not always the same likes and dislikes. The case

is interesting, because it shows that nauseous fluids sometimes occur

sporadically, and may thus be intensified by natural selection when

required for the purpose of protection. Another exceptional case is

that of the very conspicuous caterpillar of the spurge hawk-moth

(Deilephila euphorbiae), which was at once eaten by a lizard, although,

as it exposes itself on its food-plant in the daytime and is very

abundant in some localities, it must almost certainly be disliked by

birds or by some animals who would otherwise devour it. If disturbed

while feeding it is said to turn round with fury and eject a quantity of

green liquid, of an acid and disagreeable smell similar to that of the

spurge milk, only worse.[97]

lizards at first will be eaten if the lizards are very hungry, show that

there are differences in the amount of the distastefulness, and render

it probable that if other food were wanting many of these conspicuous

insects would be eaten. It is the abundance of the eatable kinds that

gives value to the inedibility of the smaller number; and this is

probably the reason why so many insects rely on protective colouring

rather than on the acquisition of any kind of defensive weapons. In the

long run the powers of attack and defence must balance each other. Hence

we see that even the powerful stings of bees and wasps only protect them

against some enemies, since a tribe of birds, the bee-eaters, have been

developed which feed upon them, and some frogs and lizards do so

occasionally.
The preceding outline will sufficiently explain the characteristics of

"warning coloration" and the end it serves in nature. There are many

other curious modifications of it, but these will be best appreciated

after we have discussed the remarkable phenomenon of "mimicry," which is

bound up with and altogether depends upon "warning colour," and is in

some cases the chief indication we have of the possession of some

offensive weapon to secure the safety of the species imitated.

_Mimicry._

one species so closely resembles another in external form and colouring

as to be mistaken for it, although the two may not be really allied and

often belong to distinct families or orders. One creature seems

disguised in order to be made like another; hence the terms "mimic" and

mimicry, which imply no voluntary action on the part of the imitator. It

has long been known that such resemblances do occur, as, for example,

the clear-winged moths of the families Sesiidae and Aegeriidae, many of

which resemble bees, wasps, ichneumons, or saw-flies, and have received

names expressive of the resemblance; and the parasitic flies (Volucella)

which closely resemble bees, on whose larvae the larvae of the flies

feed.
The great bulk of such cases remained, however, unnoticed, and the

subject was looked upon as one of the inexplicable curiosities of

nature, till Mr. Bates studied the phenomenon among the butterflies of

the Amazon, and, on his return home, gave the first rational explanation

of it.[98] The facts are, briefly, these. Everywhere in that fertile

region for the entomologist the brilliantly coloured Heliconidae abound,

with all the characteristics which I have already referred to when

describing them as illustrative of "warning coloration." But along with

them other butterflies were occasionally captured, which, though often

mistaken for them, on account of their close resemblance in form,

colour, and mode of flight, were found on examination to belong to a

very distinct family, the Pieridae. Mr. Bates notices fifteen distinct

species of Pieridae, belonging to the genera Leptalis and Euterpe, each

of which closely imitates some one species of Heliconidae, inhabiting

the same region and frequenting the same localities. It must be

remembered that the two families are altogether distinct in structure.

The larvae of the Heliconidae are tubercled or spined, the pupae

suspended head downwards, and the imago has imperfect forelegs in the

male; while the larvae of the Pieridae are smooth, the pupae are

suspended with a brace to keep the head erect, and the forefeet are

fully developed in both sexes. These differences are as large and as

important as those between pigs and sheep, or between swallows and

sparrows; while English entomologists will best understand the case by

supposing that a species of Pieris in this country was coloured and

shaped like a small tortoise-shell, while another species on the

Continent was equally like a Camberwell beauty--so like in both cases

as to be mistaken when on the wing, and the difference only to be

detected by close examination. As an example of the resemblance,

woodcuts are given of one pair in which the colours are simple, being

olive, yellow, and black, while the very distinct neuration of the wings

and form of the head and body can be easily seen.

(Pieridae).]

Erycinidae, three Castnias (a genus of day-flying moths), and fourteen

species of diurnal Bombycidae, all imitating some species of Heliconidae

which inhabited the same district; and it is to be especially noted that

none of these insects were so abundant as the Heliconidae they

resembled, generally they were far less common, so that Mr. Bates

estimated the proportion in some cases as not one to a thousand. Before

giving an account of the numerous remarkable cases of mimicry in other

parts of the world, and between various groups of insects and of higher

animals, it will be well to explain briefly the use and purport of the

phenomenon, and also the mode by which it has been brought about.

_How Mimicry has been Produced._

offensive odour and taste, which lead to their being almost entirely