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

only 1000, the other of 100,000 individuals, and that the quota required

annually in the same district for the instruction of young insectivorous

birds is 500. By the larger species this loss will be hardly felt; to

the smaller it will mean the most dreadful persecution resulting in a

loss of half the total population. But, let the two species become

superficially alike, so that the birds see no difference between them.

The quota of 500 will now be taken from a combined population of 101,000

butterflies, and if proportionate numbers of each suffer, then the weak

species will only lose five individuals instead of 500 as it did before.

Now we know that the different species of Heliconidae are not equally

abundant, some being quite rare; so that the benefit to be derived in

these latter cases would be very important. A slight inferiority in

rapidity of flight or in powers of eluding attack might also be a cause

of danger to an inedible species of scanty numbers, and in this case too

the being merged in another much more abundant species, by similarity

of external appearance, would be an advantage.

distasteful butterflies till they have learned by bitter experience what

species to avoid? On this point Dr. Müller has fortunately been able to

obtain some direct evidence, by capturing several Acraeas and

Heliconidae which had evidently been seized by birds but had afterwards

escaped, as they had pieces torn out of the wing, sometimes

symmetrically out of both wings, showing that the insect had been seized

when at rest and with the two pairs of wings in contact. There is,

however, a general impression that this knowledge is hereditary, and

does not need to be acquired by young birds; in support of which view

Mr. Jenner Weir states that his birds always disregarded inedible

caterpillars. When, day by day, he threw into his aviary various larvae,

those which were edible were eaten immediately, those which were

inedible were no more noticed than if a pebble had been thrown before

the birds.
The cases, however, are not strictly comparable. The birds were not

young birds of the first year; and, what is more important, edible

larvae have a comparatively simple coloration, being always brown or

green and smooth. Uneatable larvae, on the other hand, comprise all that

are of conspicuous colours and are hairy or spiny. But with butterflies

there is no such simplicity of contrast. The eatable butterflies

comprise not only brown or white species, but hundreds of Nymphalidae,

Papilionidae, Lycaenidae, etc., which are gaily coloured and of an

immense variety of patterns. The colours and patterns of the inedible

kinds are also greatly varied, while they are often equally gay; and it

is quite impossible to suppose that any amount of instinct or inherited

habit (if such a thing exists) could enable young insectivorous birds to

distinguish all the species of one kind from all those of the other.

There is also some evidence to show that animals do learn by experience

what to eat and what to avoid. Mr. Poulton was assured by Rev. G.J.

Bursch that very young chickens peck at insects which they afterwards

avoid. Lizards, too, often seized larvae which they were unable to eat

and ultimately rejected.

coloration and pattern, yet, in proportion to the number of distinct

species in each district, the types of coloration are few and very well

marked, and thus it becomes easier for a bird or other animal to learn

that all belonging to such types are uneatable. This must be a decided

advantage to the family in question, because, not only do fewer

individuals of each species need to be sacrificed in order that their

enemies may learn the lesson of their inedibility, but they are more

easily recognised at a distance, and thus escape even pursuit. There is

thus a kind of mimicry between closely allied species as well as between

species of distinct genera, all tending to the same beneficial end. This

may be seen in the four or five distinct species of the genus Heliconius

which all have the same peculiar type of coloration--a yellow band

across the upper wings and radiating red stripes on the lower,--and are

all found in the same forests of the Lower Amazon; in the numerous very

similar species of Ithomia with transparent wings, found in every

locality of the same region; and in the very numerous species of Papilio

of the "Aeneas" group, all having a similar style of marking, the

resemblance being especially close in the females. The very uniform type

of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of

the same character.[107] In all these cases the similarity of the allied

species is so great, that, when they are on the wing at some distance

off, it is difficult to distinguish one species from another. But this

close external resemblance is not always a sign of very near affinity;

for minute examination detects differences in the form and scalloping of

the wings, in the markings on the body, and in those on the under

surface of the wings, which do not usually characterise the closest

allies. It is to be further noted, that the presence of groups of very

similar species of the same genus, in one locality, is not at all a

common phenomenon among unprotected groups. Usually the species of a

genus found in one locality are each well marked and belong to somewhat

distinct types, while the closely allied forms--those that require

minute examination to discriminate them as distinct species--are most

generally found in separate areas, and are what are termed

representative forms.
The extension we have now given to the theory of mimicry is important,

since it enables us to explain a much wider range of colour phenomena

than those which were first imputed to mimicry. It is in the richest

butterfly region in the world--the Amazon valley--that we find the most

abundant evidence of the three distinct sets of facts, all depending on

the same general principle. The form of mimicry first elucidated by Mr.

Bates is characterised by the presence in each locality of certain

butterflies, or other insects, themselves edible and belonging to edible

groups, which derived protection from having acquired a deceptive

resemblance to some of the inedible butterflies in the same localities,

which latter were believed to be wholly free from the attacks of

insectivorous birds. Then came the extension of the principle, by Dr. F.

Müller, to the case of species of distinct genera of the inedible

butterflies resembling each other quite as closely as in the former

cases, and like them always found in the same localities. They derive

mutual benefit from becoming, in appearance, one species, from which a

certain toll is taken annually to teach the young insectivorous birds

that they are uneatable. Even when the two or more species are

approximately equal in numbers, they each derive a considerable benefit

from thus combining their forces; but when one of the species is scarce

or verging on extinction, the benefit becomes exceedingly great, being,

in fact, exactly apportioned to the need of the species.

allied species of the same genera of inedible butterflies into sets,

each having a distinct type of coloration, and each consisting of a

number of species which can hardly be distinguished on the wing. This

must be useful exactly in the same way as in the last case, since it

divides the inevitable toll to insectivorous birds and other animals

among a number of species. It also explains the fact of the great

similarity of many species of inedible insects in the same locality--a

similarity which does not obtain to anything like the same extent among

the edible species. The explanation of the various phenomena of

resemblance and mimicry, presented by the distasteful butterflies, may

now be considered tolerably complete.

_Mimicry in other Orders of Insects._
A very brief sketch of these phenomena will be given, chiefly to show

that the same principle prevails throughout nature, and that, wherever a

rather extensive group is protected, either by distastefulness or

offensive weapons, there are usually some species of edible and

inoffensive groups that gain protection by imitating them. It has been

already stated that the Telephoridae, Lampyridae, and other families of

soft-winged beetles, are distasteful; and as they abound in all parts of

the world, and especially in the tropics, it is not surprising that

insects of many other groups should imitate them. This is especially the

case with the longicorn beetles, which are much persecuted by

insectivorous birds; and everywhere in tropical regions some of these

are to be found so completely disguised as to be mistaken for species of

the protected groups. Numbers of these imitations have been already

recorded by Mr. Bates and myself, but I will here refer to a few others.

beetles of Central America[108] there are numbers of beautifully

coloured figures of the new species; and on looking over them we are

struck by the curious resemblance of some of the Longicorns to species

of the Malacoderm group. In some cases we discover perfect mimics, and

on turning to the descriptions we always find these pairs to come from

the same locality. Thus the Otheostethus melanurus, one of the

Prionidae, imitates the malacoderm, Lucidota discolor, in form, peculiar

coloration, and size, and both are found at Chontales in Nicaragua, the

species mimicked having, however, as is usual, a wider range. The

curious and very rare little longicorn, Tethlimmena aliena, quite unlike

its nearest allies in the same country, is an exact copy on a somewhat

smaller scale of a malacoderm, Lygistopterus amabilis, both found at

Chontales. The pretty longicorn, Callia albicornis, closely resembles

two species of malacoderms (Silis chalybeipennis and Colyphus

signaticollis), all being small beetles with red head and thorax and

bright blue elytra, and all three have been found at Panama. Many other

species of Callia also resemble other malacoderms; and the longicorn

genus Lycidola has been named from its resemblance to various species of

the Lycidae, one of the species here figured (Lycidola belti) being a

good mimic of Calopteron corrugatum and of several other allied species,

all being of about the same size and found at Chontales. In these cases,

and in most others, the longicorn beetles have lost the general form and

aspect of their allies to take on the appearance of a distinct tribe.

Some other groups of beetles, as the Elateridae and Eucnemidae, also

deceptively mimic malacoderms.

Many longicorn beetles in the tropics exactly mimic wasps, bees, or

ants. In Borneo a large black wasp, whose wings have a broad white patch

near the apex (Mygnimia aviculus), is closely imitated by a heteromerous

beetle (Coloborhombus fasciatipennis), which, contrary to the general

habit of beetles, keeps its wings expanded in order to show the white

patch on their apex, the wing-coverts being reduced to small oval

scales, as shown in the figure. This is a most remarkable instance of

mimicry, because the beetle has had to acquire so many characters which

are unknown among its allies (except in another species from Java)--the

expanded wings, the white band on them, and the oval scale-like

elytra.[109] Another remarkable case has been noted by Mr. Neville

Goodman, in Egypt, where a common hornet (Vespa orientalis) is exactly

imitated in colour, size, shape, attitude when at rest, and mode of

flight, by a beetle of the genus Laphria.[110]
The tiger-beetles (Cicindelidae) are also the subjects of mimicry by

more harmless insects. In the Malay Islands I found a heteromerous

beetle which exactly resembled a Therates, both being found running on

the trunks of trees. A longicorn (Collyrodes Lacordairei) mimics

Collyris, another genus of the same family; while in the Philippine

Islands there is a cricket (Condylodeira tricondyloides), which so

closely resembles a tiger-beetle of the genus Tricondyla that the

experienced entomologist, Professor Westwood, at first placed it in his

cabinet among those beetles.
[Illustration: FIG. 26.--Mygnimia aviculus (Wasp). Coloborhombus

fasciatipennis (Beetle).]

a. Doliops sp. (Longicorn)

mimics Pachyrhynchus orbifae, (b) (a hard curculio).

c. Doliops curculionoides mimics (d) Pachyrhynchus sp.

e. Scepastus pachyrhynchoides (a grasshopper),

mimics (f) Apocyrtus sp. (a hard curculio).

g. Doliops sp. mimics (h) Pachyrhynchus sp.

i. Phoraspis (grasshopper) mimics (k) a Coccinella.

colours of the insects themselves renders the mimicry much more complete

in nature than it appears in the above figures.]
One of the characters by which some beetles are protected is excessive

hardness of the elytra and integuments. Several genera of weevils

(Curculionidae) are thus saved from attack, and these are often mimicked

by species of softer and more eatable groups. In South America, the

genus Heilipus is one of these hard groups, and both Mr. Bates and M.

Roelofs, a Belgian entomologist, have noticed that species of other

genera exactly mimic them. So, in the Philippines, there is a group of

Curculionidae, forming the genus Pachyrhynchus, in which all the species

are adorned with the most brilliant metallic colours, banded and spotted

in a curious manner, and are very smooth and hard. Other genera of

Curculionidae (Desmidophorus, Alcides), which are usually very

differently coloured, have species in the Philippines which mimic the

Pachyrhynchi; and there are also several longicorn beetles (Aprophata,

Doliops, Acronia, and Agnia), which also mimic them. Besides these,

there are some longicorns and cetonias which reproduce the same colours

and markings; and there is even a cricket (Scepastus pachyrhynchoides),

which has taken on the form and peculiar coloration of these beetles in

order to escape from enemies, which then avoid them as uneatable.[111]

The figures on the opposite page exhibit several other examples of these

mimicking insects.

must now pass on to consider a few of the very remarkable, but much

rarer instances, that are found among the higher animals.

_Mimicry among the Vertebrata._

harmless snakes which mimic poisonous species. The genus Elaps, in

tropical America, consists of poisonous snakes which do not belong to

the viper family (in which are included the rattlesnakes and most of

those which are poisonous), and which do not possess the broad

triangular head which characterises the latter. They have a peculiar

style of coloration, consisting of alternate rings of red and black, or

red, black, and yellow, of different widths and grouped in various ways

in the different species; and it is a style of coloration which does not

occur in any other group of snakes in the world. But in the same regions

are found three genera of harmless snakes, belonging to other families,

some few species of which mimic the poisonous Elaps, often so exactly

that it is with difficulty one can be distinguished from the other. Thus

Elaps fulvius in Guatemala is imitated by the harmless Pliocerus

equalis; Elaps corallinus in Mexico is mimicked by the harmless

Homalocranium semicinctum; and Elaps lemniscatus in Brazil is copied by

Oxyrhopus trigeminus; while in other parts of South America similar

cases of mimicry occur, sometimes two harmless species imitating the

same poisonous snake.
A few other instances of mimicry in this group have been recorded. There

is in South Africa an egg-eating snake (Dasypeltis scaber), which has

neither fangs nor teeth, yet it is very like the Berg adder (Clothos

atropos), and when alarmed renders itself still more like by flattening

out its head and darting forward with a hiss as if to strike a foe.[112]

Dr. A.B. Meyer has also discovered that, while some species of the genus

Callophis (belonging to the same family as the American Elaps) have

large poison fangs, other species of the same genus have none; and that

one of the latter (C. gracilis) resembles a poisonous species (C.

intestinalis) so closely, that only an exact comparison will discover

the difference of colour and marking. A similar kind of resemblance is

said to exist between another harmless snake, Megaerophis flaviceps, and

the poisonous Callophis bivirgatus; and in both these cases the harmless

snake is less abundant than the poisonous one, as occurs in all examples

of true mimicry.[113]
In the genus Elaps, above referred to, the very peculiar style of colour

and marking is evidently a "warning colour" for the purpose of

indicating to snake-eating birds and mammals that these species are

poisonous; and this throws light on the long-disputed question of the

use of the rattle of the rattlesnake. This reptile is really both

sluggish and timid, and is very easily captured by those who know its

habits. If gently tapped on the head with a stick, it will coil itself

up and lie still, only raising its tail and rattling. It may then be

easily caught. This shows that the rattle is a warning to its enemies

that it is dangerous to proceed to extremities; and the creature has

probably acquired this structure and habit because it frequents open or

rocky districts where protective colour is needful to save it from being

pounced upon by buzzards or other snake-eaters. Quite parallel in

function is the expanded hood of the Indian cobra, a poisonous snake

which belongs also to the Elapidae. This is, no doubt, a warning to its

foes, not an attempt to terrify its prey; and the hood has been

acquired, as in the case of the rattlesnake, because, protective

coloration being on the whole useful, some mark was required to

distinguish it from other protectively coloured, but harmless, snakes.

Both these species feed on active creatures capable of escaping if their

enemy were visible at a moderate distance.

_Mimicry among Birds._

them of the phenomena of warning colours or of mimicry; and the extreme

development of their instincts and reasoning powers, as well as their

activity and their power of flight, usually afford them other means of

evading their enemies. Yet there are a few imperfect, and one or two

very perfect cases of true mimicry to be found among them. The less

perfect examples are those presented by several species of cuckoos, an

exceedingly weak and defenceless group of birds. Our own cuckoo is, in

colour and markings, very like a sparrow-hawk. In the East, several of

the small black cuckoos closely resemble the aggressive drongo-shrikes

of the same country, and the small metallic cuckoos are like glossy

starlings; while a large ground-cuckoo of Borneo (Carpococcyx radiatus)

resembles one of the fine pheasants (Euplocamus) of the same country,

both in form and in its rich metallic colours.

orioles in the Malay Archipelago and a genus of large honey-suckers--the

Tropidorhynchi or "Friar-birds." These latter are powerful and noisy

birds which go in small flocks. They have long, curved, and sharp beaks,

and powerful grasping claws; and they are quite able to defend

themselves, often driving away crows and hawks which venture to approach

them too nearly. The orioles, on the other hand, are weak and timid

birds, and trust chiefly to concealment and to their retiring habits to

escape persecution. In each of the great islands of the Austro-Malayan

region there is a distinct species of Tropidorhynchus, and there is

always along with it an oriole that exactly mimics it. All the

Tropidorhynchi have a patch of bare black skin round the eyes, and a

ruff of curious pale recurved feathers on the nape, whence their name of

Friar-birds, the ruff being supposed to resemble the cowl of a friar.

These peculiarities are imitated in the orioles by patches of feathers

of corresponding colours; while the different tints of the two species

in each island are exactly the same. Thus in Bouru both are earthy

brown; in Ceram they are both washed with yellow ochre; in Timor the

under surface is pale and the throat nearly white, and Mr. H.O. Forbes

has recently discovered another pair in the island of Timor Laut. The

close resemblance of these several pairs of birds, of widely different

families, is quite comparable with that of many of the insects already

described. It is so close that the preserved specimens have even

deceived naturalists; for, in the great French work, _Voyage de

l'Astrolabe_, the oriole of Bouru is actually described and figured as a

honey-sucker; and Mr. Forbes tells us that, when his birds were

submitted to Dr. Sclater for description, the oriole and the

honey-sucker were, previous to close examination, considered to be the

same species.

_Objections to the Theory of Mimicry._

need a large and copiously illustrated volume; and no more interesting

subject could be taken up by a naturalist who has access to our great

collections and can devote the necessary time to search out the many

examples of mimicry that lie hidden in our museums. The brief sketch of

the subject that has been here given will, however, serve to indicate

its nature, and to show the weakness of the objections that were at

first made to it. It was urged that the action of "like conditions,"

with "accidental resemblances" and "reversion to ancestral types," would

account for the facts. If, however, we consider the actual phenomena as