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

free from attack by insectivorous creatures; they possess a peculiar

form and mode of flight, and do not seek concealment; while their

colours--although very varied, ranging from deep blue-black, with white,

yellow, or vivid red bands and spots, to the most delicate

semitransparent wings adorned with pale brown or yellow markings--are

yet always very distinctive, and unlike those of all the other families

of butterflies in the same country. It is, therefore, clear that if any

other butterflies in the same region, which are eatable and suffer great

persecution from insectivorous animals, should come to resemble any of

these uneatable species so closely as to be mistaken for them by their

enemies, they will obtain thereby immunity from persecution. This is the

obvious and sufficient reason why the imitation is useful, and therefore

why it occurs in nature. We have now to explain how it has probably been

brought about, and also why a still larger number of persecuted groups

have not availed themselves of this simple means of protection.
From the great abundance of the Heliconidae[99] all over tropical

America, the vast number of their genera and species, and their marked

distinctions from all other butterflies, it follows that they constitute

a group of high antiquity, which in the course of ages has become more

and more specialised, and owing to its peculiar advantages has now

become a dominant and aggressive race. But when they first arose from

some ancestral species or group which, owing to the food of the larvae

or some other cause, possessed disagreeable juices that caused them to

be disliked by the usual enemies of their kind, they were in all

probability not very different either in form or coloration from many

other butterflies. They would at that time be subject to repeated

attacks by insect-eaters, and, even if finally rejected, would often

receive a fatal injury. Hence arose the necessity for some

distinguishing mark, by which the devourers of butterflies in general

might learn that these particular butterflies were uneatable; and every

variation leading to such distinction, whether by form, colour, or mode

of flight, was preserved and accumulated by natural selection, till the

ancestral Heliconoids became well distinguished from eatable

butterflies, and thenceforth comparatively free from persecution. Then

they had a good time of it. They acquired lazy habits, and flew about

slowly. They increased abundantly and spread all over the country, their

larvae feeding on many plants and acquiring different habits; while the

butterflies themselves varied greatly, and colour being useful rather

than injurious to them, gradually diverged into the many coloured and

beautifully varied forms we now behold.
But, during the early stages of this process, some of the Pieridae,

inhabiting the same district, happened to be sufficiently like some of

the Heliconidae to be occasionally mistaken for them. These, of course,

survived while their companions were devoured. Those among their

descendants that were still more like Heliconidae again survived, and at

length the imitation would become tolerably perfect. Thereafter, as the

protected group diverged into distinct species of many different

colours, the imitative group would occasionally be able to follow it

with similar variations,--a process that is going on now, for Mr. Bates

informs us that in each fresh district he visited he found closely

allied representative species or varieties of Heliconidae, and along

with them species of Leptalis (Pieridae), which had varied in the same

way so as still to be exact imitations. But this process of imitation

would be subject to check by the increasing acuteness of birds and other

animals which, whenever the eatable Leptalis became numerous, would

surely find them out, and would then probably attack both these and

their friends the Heliconidae in order to devour the former and reject

the latter. The Pieridae would, however, usually be less numerous,

because their larvae are often protectively coloured and therefore

edible, while the larvae of the Heliconidae are adorned with warning

colours, spines, or tubercles, and are uneatable. It seems probable that

the larvae and pupae of the Heliconidae were the first to acquire the

protective distastefulness, both because in this stage they are more

defenceless and more liable to fatal injury, and also because we now

find many instances in which the larvae are distasteful while the

perfect insects are eatable, but I believe none in which the reverse is

the case. The larvae of the Pieridae are now beginning to acquire

offensive juices, but have not yet obtained the corresponding

conspicuous colours; while the perfect insects remain eatable, except

perhaps in some Eastern groups, the under sides of whose wings are

brilliantly coloured although this is the part which is exposed when at

rest.
It is clear that if a large majority of the larvae of Lepidoptera, as

well as the perfect insects, acquired these distasteful properties, so

as seriously to diminish the food supply of insectivorous and nestling

birds, these latter would be forced by necessity to acquire

corresponding tastes, and to eat with pleasure what some of them now eat

only under pressure of hunger; and variation and natural selection would

soon bring about this change.

being produced by the accumulation of fortuitous variations, but if the

reader will call to mind the large amount of variability that has been

shown to exist in all organisms, the exceptional power of rapid increase

possessed by insects, and the tremendous struggle for existence always

going on, the difficulty will vanish, especially when we remember that

nature has the same fundamental groundwork to act upon in the two

groups, general similarity of forms, wings of similar texture and

outline, and probably some original similarity of colour and marking.

Yet there is evidently considerable difficulty in the process, or with

these great resources at her command nature would have produced more of

these mimicking forms than she has done. One reason of this deficiency

probably is, that the imitators, being always fewer in number, have not

been able to keep pace with the variations of the much more numerous

imitated form; another reason may be the ever-increasing acuteness of

the enemies, which have again and again detected the imposture and

exterminated the feeble race before it has had time to become further

modified. The result of this growing acuteness of enemies has been, that

those mimics that now survive exhibit, as Mr. Bates well remarks, "a

palpably intentional likeness that is perfectly staggering," and also

"that those features of the portrait are most attended to by nature

which produce the most effective deception when the insects are seen in

nature." No one, in fact, can understand the perfection of the imitation

who has not seen these species in their native wilds. So complete is it

in general effect that in almost every box of butterflies, brought from

tropical America by amateurs, are to be found some species of the

mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae,

placed together under the impression that they are the same species. Yet

more extraordinary, it sometimes deceives the very insects themselves.

Mr. Trimen states that the male Danais chrysippus is sometimes deceived

by the female Diadema bolina which mimics that species. Dr. Fritz

Müller, writing from Brazil to Professor Meldola, says, "One of the most

interesting of our mimicking butterflies is Leptalis melite. The female

alone of this species imitates one of our common white Pieridae, which

she copies so well that even her own male is often deceived; for I have

repeatedly seen the male pursuing the mimicked species, till, after

closely approaching and becoming aware of his error, he suddenly

returned."[100] This is evidently not a case of true mimicry, since the

species imitated is not protected; but it may be that the less abundant

Leptalis is able to mingle with the female Pieridae and thus obtain

partial immunity from attack. Mr. Kirby of the insect department of the

British Museum informs me that there are several species of South

American Pieridae which the female Leptalis melite very nearly

resembles. The case, however, is interesting as showing that the

butterflies are themselves deceived by a resemblance which is not so

great as that of some mimicking species.

_Other Examples of Mimicry among Lepidoptera._
In tropical Asia, and eastward to the Pacific Islands, the Danaidae take

the place of the Heliconidae of America, in their abundance, their

conspicuousness, their slow flight, and their being the subjects of

mimicry. They exist under three principal forms or genera. The genus

Euploea is the most abundant both in species and individuals, and

consists of fine broad-winged butterflies of a glossy or metallic

blue-black colour, adorned with pure white, or rich blue, or dusky

markings situated round the margins of the wings. Danais has generally

more lengthened wings, of a semitransparent greenish or a rich brown

colour, with radial or marginal pale spots; while the fine Hestias are

of enormous size, of a papery or semitransparent white colour, with

dusky or black spots and markings. Each of these groups is mimicked by

various species of the genus Papilio, usually with such accuracy that it

is impossible to distinguish them on the wing.[101] Several species of

Diadema, a genus of butterflies allied to our Vanessas, also mimic

species of Danais, but in this case the females only are affected, a

subject which will be discussed in another chapter.
Another protected group in the Eastern tropics is that of the beautiful

day-flying moths forming the family Agaristidae. These are usually

adorned with the most brilliant colours or conspicuous markings, they

fly slowly in forests among the butterflies and other diurnal insects,

and their great abundance sufficiently indicates their possession of

some distastefulness which saves them from attack. Under these

conditions we may expect to find other moths which are not so protected

imitating them, and this is the case. One of the common and wide-ranging

species (Opthalmis lincea), found in the islands from Amboyna to New

Ireland, is mimicked in a wonderful manner by one of the Liparidae (the

family to which our common "tussock" and "vapourer" moths belong). This

is a new species collected at Amboyna during the voyage of the

_Challenger_, and has been named Artaxa simulans. Both insects are

black, with the apex of the fore wings ochre coloured, and the outer

half of the hind wings bright orange. The accompanying woodcuts (for the

use of which I am indebted to Mr. John Murray of the _Challenger_

Office) well exhibit their striking resemblance to each other.
[Illustration: FIG. 24.--Opthalmis lincea (Agaristidae). Artaxa simulans

(Liparidae).]

Diadema mimicking Danaidae or Acraeidae with the most curious accuracy.

Mr. Trimen, who studied this subject in South Africa, has recorded eight

species or varieties of Diadema, and eight of Papilio, which each mimic

some species of Danais; while eight species or varieties of Panopaea

(another genus of Nymphalidae), three of Melanitis (Eurytelidae), and

two of Papilio, resemble with equal accuracy some species of

Acraea.[102] He has also independently observed the main facts on which

the explanation of the phenomenon rests,--the unpleasant odour of the

Danais and Acraea, extending to their larvae and pupae; their great

abundance, slow flight, and disregard of concealment; and he states that

while lizards, mantidae, and dragonflies all hunt butterflies, and the

rejected wings are to be found abundantly at some of their

feeding-places, those of the two genera Danais and Acraea were never

among them.
The two groups of the great genus Papilio (the true swallow-tailed

butterflies) which have been already referred to as having the special

characteristics of uneatable insects, have also their imitators in other

groups; and thus, the belief in their inedibility--derived mainly from

their style of warning coloration and their peculiar habits--is

confirmed. In South America, several species of the "Aeneas" group of

these butterflies are mimicked by Pieridae and by day-flying moths of

the genera Castnia and Pericopis. In the East, Papilio hector, P.

diphilus, and P. liris, all belonging to the inedible group, are

mimicked by the females of other species of Papilio belonging to very

distinct groups; while in Northern India and China, many fine day-flying

moths (Epicopeia) have acquired the strange forms and peculiar colours

of some of the large inedible Papilios of the same regions.
In North America, the large and handsome Danais archippus, with rich

reddish-brown wings, is very common; and it is closely imitated by

Limenitis misippus, a butterfly allied to our "white admiral," but which

has acquired a colour quite distinct from that of the great bulk of its

allies. In the same country there is a still more interesting case. The

beautiful dark bronzy green butterfly, Papilio philenor, is inedible

both in larva and perfect insect, and it is mimicked by the equally dark

Limenitis ursula. There is also in the Southern and Western States a

dark female form of the yellow Papilio turnus, which in all probability

obtains protection from its general resemblance to P. philenor. Mr. W.H.

Edwards has found, by extensive experiment, that both the dark and

yellow females produce their own kinds, with very few exceptions; and he

thinks that the dark form has the advantage in the more open regions and

in the prairies, where insectivorous birds abound. But in open country

the dark form would be quite as conspicuous as the yellow form, if not

more so, so that the resemblance to an inedible species would be there

more needed.[103]
The only probable case of mimicry in this country is that of the moth,

Diaphora mendica, whose female only is white, while the larva is of

protective colours, and therefore almost certainly edible. A much more

abundant moth, of about the same size and appearing about the same time,

is Spilosoma menthrasti, also white, but in this case both it and its

larva have been proved to be inedible. The white colour of the female

Diaphora, although it must be very conspicuous at night, may, therefore,

have been acquired in order to resemble the uneatable Spilosoma, and

thus gain some protection.[104]

_Mimicry among Protected (Uneatable) Genera._

colours and of mimicry that occur in the animal kingdom, it will be well

to notice a curious phenomenon which long puzzled entomologists, but

which has at length received a satisfactory explanation.

comparatively scarce and much persecuted species obtained protection by

its close external resemblance to a much more abundant uneatable species

inhabiting its own district; and this rule undoubtedly prevails among

the great majority of mimicking species all over the world. But Mr.

Bates also found a number of pairs of species of different genera of

Heliconidae, which resembled each other quite as closely as did the

other mimicking species he has described; and since all these insects

appear to be equally protected by their inedibility, and to be equally

free from persecution, it was not easy to see why this curious

resemblance existed, or how it had been brought about. That it is not

due to close affinity is shown by the fact that the resemblance occurs

most frequently between the two distinct sub-families into which (as Mr.

Bates first pointed out) the Heliconidae are naturally divided on

account of very important structural differences. One of these

sub-families (the true Heliconinae) consists of two genera only,

Heliconius and Eueides, the other (the Danaoid Heliconinae) of no less

than sixteen genera; and, in the instances of mimicry we are now

discussing, one of the pairs or triplets that resemble each other is

usually a species of the large and handsome genus Heliconius, the others

being species of the genera Mechanitis, Melinaea, or Tithorea, though

several species of other Danaoid genera also imitate each other. The

following lists will give some idea of the number of these curious

imitative forms, and of their presence in every part of the Neotropical

area. The bracketed species are those that resemble each other so

closely that the difference is not perceptible when they are on the

wing.
In the Lower Amazon region are found--

{ Heliconius sylvana.

{ Melinaea egina.
{ Heliconius numata.

{ Melinaea mneme.

{ Tithorea harmonia.
{ Methona psidii.

{ Thyridia ino.

{ Melinaea mnasias.

In Central America are found--

{ Heliconius zuleika.

Nicaragua { Melinaea hezia.

{ Mechanitis sp.

{ Tithorea penthias.

{ Melinaea imitata.

In the Upper Amazon region--

{ Heliconius pardalinus.

{ Melinaea pardalis.

{ Melinaea lucifer.

In New Grenada--

{ Heliconius ismenius.

{ Melinaea messatis.
{ Heliconius messene.

{ Melinaea mesenina.

{ (?) Mechanitis sp.
{ Heliconius hecalesia.

{ Tithorea hecalesina.

{ Tithorea bonplandi.

In Eastern Peru and Bolivia--

{ Heliconius aristona.

{ Melinaea cydippe.

{ (?) Mechanitis mothone.

In Pernambuco--

{ Heliconius ethra.

{ Mechanitis nesaea.

In Rio Janeiro--

{ Helieonius eucrate.

{ Mechanitis lysimnia.

In South Brazil--

{ Thyridia megisto.

{ Ituna ilione.
{ Acraea thalia.

{ Eueides pavana.

Besides these, a number of species of Ithomia and Napeogenes, and of

Napeogenes and Mechanitis, resemble each other with equal accuracy, so

that they are liable to be mistaken for each other when on the wing; and

no doubt many other equally remarkable cases are yet unnoticed.

Thyridia megisto, female.]

species, from Dr. Fritz Müller's original paper in _Kosmos_, will serve

to show the considerable amount of difference, in the important

character of the neuration of the wings, between these butterflies,

which really belong to very distinct and not at all closely allied

genera. Other important characters are--(1) The existence of a small

basal cell in the hind wings of Ituna which is wanting in Thyridia; (2)

the division of the cell between the veins 1_b_ and 2 of the hind wings

in the former genus, while it is undivided in the latter; and (3) the

existence in Thyridia of scent-producing tufts of hair on the upper edge

of the hind wing, while in Ituna these are wanting; but in place of them

are extensible processes at the end of the abdomen, also emitting a

powerful scent. These differences characterise two marked subdivisions

of the Danaoid Heliconinae, each containing several distinct genera; and

these subdivisions are further distinguished by very different forms of

larvae, that to which Ituna belongs having from two to four long

threadlike tentacles on the back, while in that containing Thyridia

these are always absent. The former usually feed on Asclepiadeae, the

latter on Solanaceae or Scrophulariaceae.
The two species figured, though belonging to such distinct and even

remote genera, have acquired almost identical tints and markings so as

to be deceptively alike. The surface of the wings is, in both,

transparent yellowish, with black transverse bands and white marginal

spots, while both have similar black-and white-marked bodies and long

yellow antennae. Dr. Müller states that they both show a preference for

the same flowers growing on the edges of the forest paths.[105]
We will now proceed to give the explanation of these curious

similarities, which have remained a complete puzzle for twenty years.

Mr. Bates, when first describing them, suggested that they might be due

to some form of parallel variation dependent on climatic influences; and

I myself adduced other cases of coincident local modifications of

colour, which did not appear to be explicable by any form of

mimicry.[106] But we neither of us hit upon the simple explanation given

by Dr. Fritz Müller in 1879.

insect-eating birds only learn by experience to distinguish the edible

from the inedible butterflies, and in doing so necessarily sacrifice a

certain number of the latter. The quantity of insectivorous birds in

tropical America is enormous; and the number of young birds which every

year have to learn wisdom by experience, as regards the species of

butterflies to be caught or to be avoided, is so great that the

sacrifice of life of the inedible species must be considerable, and, to

a comparatively weak or scarce species, of vital importance. The number

thus sacrificed will be fixed by the quantity of young birds, and by the

number of experiences requisite to cause them to avoid the inedible

species for the future, and not at all by the numbers of individuals of

which each species consists. Hence, if two species are so much alike as

to be mistaken for one another, the fixed number annually sacrificed by

inexperienced birds will be divided between them, and both will benefit.

But if the two species are very unequal in numbers, the benefit will be

comparatively slight for the more abundant species, but very great for

the rare one. To the latter it may make all the difference between

safety and destruction.
To give a rough numerical example. Let us suppose that in a given

limited district there are two species of Heliconidae, one consisting of