Section C consists of much weaker and slower-flying insects, often

[_From the_ TRANSACTIONS _of the_ LINNEAN SOCIETY, vol. xxv.]

I. _On the Phenomena of Variation and Geographical Distribution as

illustrated by the_ Papilionidæ _of the Malayan Region. By_ ALFRED R.

WALLACE, _Esq._

(Plates I.–VIII.)

Read March 17, 1864.

When the naturalist studies the habits, the structure, or the affinities

of animals, it matters little to which group he especially devotes

himself; all alike offer him endless materials for observation and

research. But, for the purpose of investigating the phenomena of

geographical distribution and of local or general variation, the several

groups differ greatly in their value and importance. Some have too

limited a range, others are not sufficiently varied in specific forms,

while, what is of most importance, many groups have not received that

amount of attention over the whole region they inhabit, which could

furnish materials sufficiently approaching to completeness to enable us

to arrive at any accurate conclusions as to the phenomena they present

as a whole. It is in those groups which are and have long been

favourites with collectors that the student of distribution and

variation will find his materials the most satisfactory, from their

comparative completeness.

Preeminent among such groups are the diurnal Lepidoptera or Butterflies,

whose extreme beauty and endless diversity have led to their having been

assiduously collected in all parts of the world, and to the numerous

species and varieties having been figured in a series of magnificent

works, from those of Cramer, the contemporary of Linnæus, down to the

inimitable productions of our own Hewitson. But, besides their

abundance, their universal distribution, and the great attention that

has been paid to them, these insects have other qualities that

especially adapt them to elucidate the branches of inquiry already

alluded to. These are the immense development and peculiar structure of

the wings, which not only vary in form more than those of any other

insects, but offer on both surfaces an endless variety of pattern,

colouring, and texture. The scales with which they are more or less

completely covered imitate the rich hues and delicate surfaces of satin

or of velvet, glitter with metallic lustre, or glow with the changeable

tints of the opal. This delicately painted surface acts as a register of

the minutest differences of organization,—a shade of colour, an

additional streak or spot, a slight modification of outline continually

recurring with the greatest regularity and fixity, while the body and

all its other members exhibit no appreciable change. The wings of

Butterflies, as Mr. Bates has well put it[1], “serve as a tablet on

which Nature writes the story of the modifications of species;” they

enable us to perceive changes that would otherwise be uncertain and

difficult of observation, and exhibit to us on an enlarged scale the

effects of the climatal and other physical conditions which influence

more or less profoundly the organization of every living thing.

Footnote 1:

See ‘The Naturalist on the Amazons,’ 2nd edit. p. 412.

A proof that this greater sensibility to modifying causes is not

imaginary may, I think, be drawn from the consideration that while the

Lepidoptera as a whole are of all insects the least essentially varied

in form, structure, or habits, yet in the number of their specific forms

they are not much inferior to those orders which range over a much wider

field of nature, and exhibit more deeply seated structural

modifications. The Lepidoptera are all vegetable-feeders in their

larva-state, and suckers of juices or other liquids in their perfect

form. In their most widely separated groups they differ but little from

a common type, and offer comparatively unimportant modifications of

structure or of habits. The Coleoptera, the Diptera, or the Hymenoptera,

on the other hand, present far greater and more essential variations. In

either of these orders we have both vegetable- and animal-feeders,

aquatic, and terrestrial, and parasitic groups. Whole families are

devoted to special departments in the economy of nature. Seeds, fruits,

bones, carcases, excrement, bark, have each their special and dependent

insect tribes from among them; whereas the Lepidoptera are, with but few

exceptions, confined to the one function of devouring the foliage of

living vegetation. We might therefore anticipate that their population

would be only equal to those of the sections of the other orders that

have a similar uniform mode of existence; and the fact that their

numbers are at all comparable with those of entire orders, so much more

varied in organization and habits, is, I think, a proof that they are in

general highly susceptible of specific modification.

The Papilionidæ are a family of diurnal Lepidoptera which have hitherto,

by almost universal consent, held the first rank in the order; and

though this position has recently been denied them, I cannot altogether

acquiesce in the reasoning by which it has been proposed to degrade them

to a lower rank. In Mr. Bates’s most excellent paper on the

Heliconidæ[2], he claims for that family the highest position, chiefly

because of the imperfect structure of the fore legs, which is there

carried to an extreme degree of abortion, and thus removes them further

than any other family from the Hesperidæ and Heterocera, which all have

perfect legs. Now it is a question whether any amount of difference

which is exhibited merely in the imperfection or abortion of certain

organs, can establish in the group exhibiting it a claim to a high grade

of organization; still less can this be allowed when another group,

along with perfection of structure in the same organs, exhibits

modifications peculiar to it, together with the possession of an organ

which in the remainder of the order is altogether wanting. This is,

however, the position of the Papilionidæ. The perfect insects possess

two characters quite peculiar to them. Mr. Edward Doubleday, in his

‘Genera of Diurnal Lepidoptera,’ says, “The Papilionidæ may be known by

the apparently four-branched median nervule and the spur on the anterior

tibiæ, characters found in no other family.” The four-branched median

nervule is a character so constant, so peculiar, and so well marked, as

to enable a person to tell, at a glance at the wings only of a

butterfly, whether it does or does not belong to this family; and I am

not aware that any other group of Butterflies, at all comparable to this

in extent and modifications of form, possesses a character in its

neuration to which the same degree of certainty can be attached. The

spur on the anterior tibiæ is also found in some of the Hesperidæ, and

is therefore supposed to show a direct affinity between the two groups;

but I do not imagine it can counterbalance the differences in neuration

and in every other part of their organization. The most characteristic

feature of the Papilionidæ, however, and that on which I think

insufficient stress has been laid, is undoubtedly the peculiar structure

of the larvæ. These all possess an extraordinary organ situated on the

neck, the well-known =Y=-shaped tentacle, which is entirely concealed in

a state of repose, but which is capable of being suddenly thrown out by

the insect when alarmed. When we consider this singular apparatus, which

in some species is nearly half an inch long, the arrangement of muscles

for its protrusion and retraction, its perfect concealment during

repose, its blood-red colour, and the suddenness with which it can be

thrown out, we must, I think, be led to the conclusion that it serves as

a protection to the larva by startling and frightening away some enemy

when about to seize it, and is thus one of the causes which has led to

the wide extension and maintained the permanence of this now dominant

group. Those who believe that such peculiar structures can only have

arisen by very minute successive variations, each one advantageous to

its possessor, must see, in the possession of such an organ by one

group, and its complete absence in every other, a proof of a very

ancient origin and of very long-continued modification. And such a

positive structural addition to the organization of the family,

subserving an important function, seems to me alone sufficient to

warrant us in considering the Papilionidæ as the most highly developed

portion of the whole order, and thus in retaining it in the position

which the size, strength, beauty, and general structure of the perfect

insects have been generally thought to deserve.

Footnote 2:

Transactions of the Linnean Society, vol. xxiii. p. 495.

The Papilionidæ are pretty widely distributed over the earth, but are

especially abundant in the tropics, where they attain their maximum of

size and beauty and the greatest variety of form and colouring. South

America, North India, and the Malay Islands are the regions where these

fine insects occur in the greatest profusion, and where they actually

become a not unimportant feature in the scenery. In the Malay Islands in

particular the giant Ornithopteræ may be frequently seen about the

borders of the cultivated and forest districts, their large size,

stately flight, and gorgeous colouring rendering them even more

conspicuous than the generality of birds. In the shady suburbs of the

town of Malacca two large and handsome Papilios (_Memnon_ and

_Nephelus_) are not uncommon, flapping with irregular flight along the

roadway, or, in the early morning, expanding their wings to the

invigorating rays of the sun. In Amboyna and other towns of the

Moluccas, the magnificent _Deiphobus_ and _Severus_, and occasionally

even the azure-winged _Ulysses_, frequent similar situations, fluttering

about the orange-trees and flower-beds, or sometimes even straying into

the narrow bazaars or covered markets of the city. In Java the

golden-dusted _Arjuna_ may often be seen at damp places on the roadside

in the mountain districts, in company with _Sarpedon_, _Bathycles_, and

_Agamemnon_, and less frequently the beautiful swallow-tailed

_Antiphates_. In the more luxuriant parts of these islands one can

hardly take a morning’s walk in the neighbourhood of a town or village

without seeing three or four species of _Papilio_, and often twice that

number. No less than 120 species of the family are now known to inhabit

the Archipelago, and of these ninety-six were collected by myself.

Twenty-nine species are found in Borneo, being the largest number in any

one island, twenty-three species having been obtained by myself in the

vicinity of Sarawak; Java has twenty-seven species; Celebes and the

Peninsula of Malacca twenty-three each. Further east the numbers

decrease, Batchian producing seventeen, and New Guinea only thirteen,

though this number is certainly too small, owing to our present

imperfect knowledge of that great island.

In estimating these numbers I have had the usual difficulty to

encounter, of determining what to consider species and what varieties.

The Malayan region, consisting of a large number of islands of generally

great antiquity, possesses, compared to its actual area, a great number

of distinct forms, often indeed distinguished by very slight characters,

but in most cases so constant in large series of specimens, and so

easily separable from each other, that I know not on what principle we

can refuse to give them the name and rank of species. One of the best

and most orthodox definitions is that of Pritchard, the great

ethnologist, who says, that “_separate origin and distinctness of race,

evinced by a constant transmission of some characteristic peculiarity of

organization_,” constitutes a species. Now leaving out the question of

“origin,” which we cannot determine, and taking only the proof of

separate origin, “_the constant transmission of some characteristic

peculiarity of organization_,” we have a definition which will compel us

to neglect altogether the _amount_ of difference between any two forms,

and to consider only whether the differences that present themselves are

_permanent_. The rule, therefore, I have endeavoured to adopt is, that

when the difference between two forms inhabiting separate areas seems

quite constant, when it can be defined in words, and when it is not

confined to a single peculiarity only, I have considered such forms to

be species. When, however, the individuals of each locality vary among

themselves, so as to cause the distinctions between the two forms to

become inconsiderable and indefinite, or where the differences, though

constant, are confined to one particular only, such as size, tint, or a

single point of difference in marking or in outline, I class one of the

forms as a variety of the other.

I find as a general rule that the constancy of species is in an inverse

ratio to their range. Those which are confined to one or two islands are

generally very constant. When they extend to many islands, considerable

variability appears; and when they have an extensive range over a large

part of the Archipelago, the amount of unstable variation is very large.

These facts are explicable on Mr. Darwin’s principles. When a species

exists over a wide area, it must have had, and probably still possesses,

great powers of dispersion. Under the different conditions of existence

in various portions of its area, different variations from the type

would be selected, and, were they completely isolated, would soon become

distinctly modified forms; but this process is checked by the dispersive

powers of the whole species, which leads to the more or less frequent

intermixture of the incipient varieties, which thus become irregular and

unstable. Where, however, a species has a limited range, it indicates

less active powers of dispersion, and the process of modification under

changed conditions is less interfered with. The species will therefore

exist under one or more permanent forms according as portions of it have

been isolated at a more or less remote period.

What is commonly called variation consists of several distinct phenomena

which have been too often confounded. I shall proceed to consider these

under the heads of—1st, simple variability; 2nd, polymorphism; 3rd,

local forms; 4th, coexisting varieties; 5th, races or subspecies; and

6th, true species.

1. _Simple variability._—Under this head I include all those cases in

which the specific form is to some extent unstable. Throughout the whole

range of the species, and even in the progeny of individuals, there

occur continual and uncertain differences of form, analogous to that

variability which is so characteristic of domestic breeds. It is

impossible usefully to define any of these forms, because there are

indefinite gradations to each other form. Species which possess these

characteristics have always a wide range, and are more frequently the

inhabitants of continents than of islands, though such cases are always

exceptional, it being far more common for specific forms to be fixed

within very narrow limits of variation. The only good example of this

kind of variability which occurs among the Malayan Papilionidæ is in

_Papilio Severus_, a species inhabiting all the islands of the Moluccas

and New Guinea, and exhibiting in each of them a greater amount of

individual difference than often serves to distinguish well-marked

species. Almost equally remarkable are the variations exhibited in most

of the species of _Ornithoptera_, which I have found in some cases to

extend even to the form of the wing and the arrangement of the nervures.

Closely allied, however, to these variable species are others which,

though differing slightly from them, are constant and confined to

limited areas. After satisfying oneself, by the examination of numerous

specimens captured in their native countries, that the one set of

individuals are variable and the others are not, it becomes evident that

by classing all alike as varieties of one species we shall be obscuring

an important fact in nature, and that the only way to exhibit that fact

in its true light is to treat the invariable local form as a distinct

species, even though it does not offer better distinguishing characters

than do the extreme forms of the variable species. Cases of this kind

are the _Ornithoptera Priamus_, which is confined to the islands of

Ceram and Amboyna, and is very constant in both sexes, while the allied

species inhabiting New Guinea and the Papuan Islands is exceedingly

variable; and in the island of Celebes is a species closely allied to

the variable _P. Severus_, but which, being exceedingly constant, I have

described as a distinct species under the name of _Papilio Pertinax_.

2. _Polymorphism or dimorphism._—By this term I understand the

coexistence in the same locality of two or more distinct forms, not

connected by intermediate gradations, and all of which are occasionally

produced from common parents. These distinct forms generally occur in

the female sex only, and the intercrossing of two of these forms does

not generate an intermediate race, but reproduces the same forms in

varying proportions. I believe it will be found that a considerable

number of what have been classed as _varieties_ are really cases of

_polymorphism_. Albinoism and melanism are of this character, as well as

most of those cases in which well-marked varieties occur in company with

the parent species, but without any intermediate forms. Under these

circumstances, if the two forms breed separately, and are never

reproduced from a common parent, they must be considered as distinct

species, contact without intermixture being a good test of specific

difference. On the other hand, intercrossing without producing an

intermediate race is a test of dimorphism. I consider, therefore, that

under any circumstances the term ‘variety’ is wrongly applied to such

cases.

The Malayan Papilionidæ exhibit some very curious instances of

polymorphism, some of which have been recorded as varieties, others as

distinct species; and they all occur in the female sex. _Papilio

Memnon_, L., is one of the most striking, as it exhibits the mixture of

simple variability, local and polymorphic forms, all hitherto classed

under the common title of varieties. The polymorphism is strikingly

exhibited by the females, one set of which resemble the males in form,

with a variable paler colouring; the others have a large spatulate tail

to the hinder wings and a distinct style of colouring, which causes them

closely to resemble _P. Coon_, a species of which the sexes are alike

and inhabiting the same countries, but with which they have no direct

affinity. The tailless females exhibit simple variability, scarcely two

being found exactly alike even in the same locality. The males of the

island of Borneo exhibit constant differences of the under surface, and

may therefore be distinguished as a local form, while the continental

specimens, as a whole, offer such large and constant differences from

those of the islands that I am inclined to separate them as a distinct

species—_P. Androgeus_, Cr. We have here, therefore, distinct species,

local forms, polymorphism, and simple variability, which seem to me to

be distinct phenomena, but which have been hitherto all classed together

as varieties. I may mention that the fact of these distinct forms being

one species is doubly proved. The males, the tailed and tailless

females, have all been bred from a single group of the larvæ, by Messrs.

Payen and Bocarmé, in Java, and I myself captured in Sumatra a male _P.

Memnon_, L., and a tailed female _P. Achates_, Cr., “in copulâ.”

_Papilio Pammon_, L., offers a somewhat similar case. The female was

described by Linnæus as _P. Polytes_, and was considered to be a

distinct species till Westermann bred the two from the same larvæ (see

Boisduval, ‘Species Générales des Lépidoptères,’ p. 272). They were

therefore classed as sexes of one species by Mr. Edward Doubleday, in

his ‘Genera of Diurnal Lepidoptera,’ in 1846. Later, female specimens

were received from India closely resembling the male insect, and this

was held to overthrow the authority of M. Westermann’s observation, and

to reestablish _P. Polytes_ as a distinct species; and as such it

accordingly appears in the British Museum List of Papilionidæ in 1856,

and in the Catalogue of the East India Museum in 1857. This discrepancy

is explained by the fact of _P. Pammon_ having two females, one closely

resembling the male, while the other is totally different from it. A

long familiarity with this insect (which, replaced by local forms or by

closely allied species, occurs in every island of the Archipelago) has

convinced me of the correctness of this statement; for in every place

where a male allied to _P. Pammon_ is found, a female resembling _P.

Polytes_ also occurs, and sometimes, though less frequently than on the

continent, another female closely resembling the male; while not only

has no male specimen of _P. Polytes_ yet been found, but the female

(_Polytes_) has never yet been found in localities to which the male

(_Pammon_) does not extend. In this case, as in the last, distinct

species, local forms, and dimorphic specimens have been confounded under

the common appellation of varieties.

But, besides the true _P. Polytes_, there are several allied forms of

females to be considered, namely, _P. Theseus_, Cr., _P. Melanides_, De

Haan, _P. Elyros_, G. R. G., and _P. Romulus_, L. The dark female

figured by Cramer as _P. Theseus_ seems to be the common and perhaps the

only form in Sumatra, whereas in Java, Borneo, and Timor, along with

males quite identical with those of Sumatra, occur females of the

_Polytes_ form, although a single specimen of the true _P. Theseus_,

Cr., taken at Lombock would seem to show that the two forms do occur

together. In the allied species found in the Philippine ♀Islands (_P.

Alphenor_, Cr., _P. Ledebouria_, Eschsch., ♀ _P. Elyros_, G. R. G.)

forms corresponding to these extremes occur along with a number of

intermediate varieties, as shown by a fine series in the British Museum.

We have here an indication of how dimorphism may be produced; for let

the extreme Philippine forms be better suited to their conditions of

existence than the intermediate connecting links, and the latter will

gradually die out, leaving two distinct forms of the same insect, each

adapted to some special conditions. As these conditions are sure to vary

in different districts, it will often happen, as in Sumatra and Java,

that the one form will predominate in the one island, the other in the

adjacent one. In the island of Borneo there seems to be a third form;

for _P. Melanides_, De Haan, evidently belongs to this group, and has

all the chief characteristics of _P. Theseus_, with a modified

coloration of the hind wings. I now come to an insect which, if I am

correct, offers one of the most interesting cases of variation yet

adduced. _Papilio Romulus_, L., a butterfly found over a large part of

India and Ceylon, and not uncommon in collections, has always been

considered a true and independent species, and no suspicions have been

expressed regarding it. But a male of this form does not, I believe,

exist. I have examined the fine series in the British Museum, in the

East India Company’s Museum, in the Hope Museum at Oxford, in Mr.

Hewitson’s and several other private collections, and can find nothing

but females; and for this common butterfly no male partner can be found

except the equally common _P. Pammon_, a species already provided with

two wives, and yet to whom we shall be forced, I believe, to assign a

third. On carefully examining _P. Romulus_, I find that in all essential

characters,—the form and texture of the wings, the length of the

antennæ, the spotting of the head and thorax, and even the peculiar

tints and shades with which it is ornamented,—it corresponds exactly

with the other females of the _Pammon_ group; and though, from the

peculiar marking of the fore wings, it has at first sight a very

different aspect, yet a closer examination shows that every one of its

markings could be produced by slight and almost imperceptible

modifications of the various allied forms. I fully believe, therefore,

that I shall be correct in placing _P. Romulus_ as a third Indian form

of the female _P. Pammon_, corresponding to _P. Melanides_, the third

form of the Malayan _P. Theseus_. I may mention here that the females of

this group have a superficial resemblance to the _Polydorus_ group, as

shown by _P. Theseus_ having been considered to be the female of _P.

Antiphus_, and by _P. Romulus_ being arranged next to _P. Hector_. There

is no close affinity between these two groups of _Papilio_, and I am

disposed to believe that we have here a case of mimicry, brought about

by the same causes which Mr. Bates has so well explained in his account

of Heliconidæ, and which thus led to the singular exuberance of

polymorphic forms in this and allied groups of the genus _Papilio._ I

shall have to devote a section of my paper to the consideration of this

subject.

The third example of polymorphism I have to bring forward is _Papilio

Ormenus_, Guér., which is closely allied to the well-known _P.

Erechtheus_, Don., of Australia. The most common form of the female also

resembles that of _P. Erechtheus_; but a totally different-looking

insect was found by myself in the Aru Islands, and figured by Mr.

Hewitson under the name of _P. Onesimus_, which subsequent observation

has convinced me is a second form of the female of _P. Ormenus_.

Comparison of this with Boisduval’s description of _P. Amanga_, a

specimen of which from New Guinea is in the Paris Museum, shows the

latter to be a closely similar form; and two other specimens were

obtained by myself, one in the island of Goram and the other in Waigiou,

all evidently local modifications of the same form. In each of these

localities males and ordinary females of _P. Ormenus_ were also found.

So far there is no evidence that these light-coloured insects are not

females of a distinct species, the males of which have not been

discovered. But two facts have convinced me this is not the case. At

Dorey, in New Guinea, where males and ordinary females closely allied to

_P. Ormenus_ occur (but which seem to me worthy of being separated as a

distinct species), I found one of these light-coloured females closely

followed in her flight by three males, exactly in the same manner as

occurs (and, I believe, occurs only) with the sexes of the same species.

After watching them a considerable time, I captured the whole of them,

and became satisfied that I had discovered the true relations of this

anomalous form. The next year I had corroborative proof of the

correctness of this opinion by the discovery in the island of Batchian

of a new species allied to _P. Ormenus_, all the females of which,

either seen or captured by me, were of one form, and much more closely

resembling the abnormal light-coloured females of _P. Ormenus_ and _P.

Pandion_ than the ordinary specimens of that sex. Every naturalist will,

I think, agree that this is strongly confirmative of the supposition

that both forms of female are of one species; and when we consider,

further, that in four separate islands, in each of which I resided for

several months, the two forms of female were obtained and only one form

of male ever seen, and that about the same time M. Montrouzier in

Woodlark Island, at the other extremity of New Guinea (where he resided

several years, and must have obtained all the large Lepidoptera of the

island), obtained females closely resembling mine, which, in despair at

finding no appropriate partners for them, he mates with a widely

different species,—it becomes, I think, sufficiently evident that this

is another case of polymorphism of the same nature as those already

pointed out in _P. Pammon_ and _P. Memnon_. This species, however, is

not only _dimorphic_, but _trimorphic_; for, in the island of Waigiou, I

obtained a third female quite distinct from either of the others, and in

some degree intermediate between the ordinary female and the male. The

specimen is particularly interesting to those who believe, with Mr.

Darwin, that extreme difference of the sexes has been gradually produced

by what he terms sexual selection, since it may be supposed to exhibit

one of the intermediate steps in that process which has been

accidentally preserved in company with its more favoured rivals, though

its extreme rarity (only one specimen having been seen to many hundreds

of the other form) would indicate that it may soon become extinct.

The only other case of polymorphism in the genus _Papilio_, at all equal

in interest to those I have now brought forward, occurs in America; and

we have, fortunately, accurate information about it. _Papilio Turnus_,

L., is common over almost the whole of temperate North America; and the

female resembles the male very closely. A totally different-looking

insect both in form and colour, _Papilio Glaucus_, L., inhabits the same

region; and though, down to the time when Boisduval published his

‘Species Général,’ no connexion was supposed to exist between the two

species, it is now well ascertained that _P. Glaucus_ is a second female

form of _P. Turnus_. In the ‘Proceedings of the Entomological Society of

Philadelphia,’ Jan. 1863, Mr. Walsh gives a very interesting account of

the distribution of this species. He tells us that in the New England

States and in New York all the females are yellow, while in Illinois and

further south all are black; in the intermediate region both black and

yellow females occur in varying proportions. Lat. 37° is approximately

the southern limit of the yellow form, and 42° the northern limit of the

black form; and, to render the proof complete, both black and yellow

insects have been bred from a single batch of eggs. He further states

that, out of thousands of specimens, he has never seen or heard of

intermediate varieties between these forms. In this interesting example

we see the effects of latitude in determining the proportions in which

the individuals of each form should exist. The conditions are _here_

favourable to the one form, _there_ to the other; but we are by no means

to suppose that these conditions consist in climate alone. It is highly

probable that the existence of enemies, and of competing forms of life,

may be the main determining influences; and it is much to be wished that

such a competent observer as Mr. Walsh would endeavour to ascertain what

are the adverse causes which are most efficient in keeping down the

numbers of each of these contrasted forms.

Dimorphism of this kind in the animal kingdom does not seem to have any

direct relations to the reproductive powers, as Mr. Darwin has shown to

be the case in plants, nor does it appear to be very general. One other

case only is known to me in another family of my eastern Lepidoptera,

the _Pierulæ_; and but few occur in the Lepidoptera of other countries.

The spring and autumn broods of some European species differ very

remarkably; and this must be considered as a phenomenon of an analogous

though not of an identical nature[3]. _Araschnia prorsa_, of Central

Europe, is a striking example of this alternate or seasonal dimorphism.

Mr. Pascoe has pointed out two forms of the male sex in some species of

Coleoptera belonging to the family Anthribidæ, in seven species of the

two genera _Xenocerus_ and _Mecocerus_ (Proc. Ent. Soc. Lond., 1862, p.

71); and no less than six European Water-beetles, of the genus

_Dytiscus_, have females of two forms, the most common having the elytra

deeply sulcate, the rarer smooth as in the males. The three, and

sometimes four or more, forms under which many Hymenopterous insects

(especially Ants) occur must be considered as a related phenomenon,

though here each form is specialized to a distinct function in the

economy of the species. Among the higher animals, albinoism and melanism

may, as I have already stated, be considered as analogous facts; and I

met with one case of a bird, a species of Lory (_Eos fuscata_, Blyth),

clearly existing under two forms, since I obtained both sexes of each

from a single flock.

Footnote 3:

Among our nocturnal Lepidoptera, I am informed, many analogous cases

occur; and as the whole history of many of these has been investigated

by breeding successive generations from the egg, it is to be hoped

that some of our British Lepidopterists will give us a connected

account of all the abnormal phenomena which they present.

The fact of the two sexes of one species differing very considerably is

so common, that it attracted but little attention till Mr. Darwin showed

how it could in many cases be explained by what he termed sexual

selection. For instance, in most polygamous animals the males fight for

the possession of the females, and the victors, always becoming the

progenitors of the succeeding generation, impress upon their male

offspring their own superior size, strength, or unusually developed

offensive weapons. It is thus that we can account for the spurs and the

superior strength and size of the males in Gallinaceous birds, and also

for the large canine tusks in the males of fruit-eating Apes. So the

superior beauty of plumage and special adornments of the males of so

many birds can be explained by supposing (what there are many facts to

prove) that the females prefer the most beautiful and perfect-plumaged

males, and that thus slight accidental variations of form and colour

have been accumulated till they have produced the wonderful train of the

Peacock and the gorgeous plumage of the Bird of Paradise. Both these

causes have no doubt acted partially in insects, so many species

possessing horns and powerful jaws in the male sex only, and still more

frequently the males alone rejoicing in rich colours or sparkling

lustre. But there is here another cause which has led to sexual

differences, viz. a special adaptation of the sexes to diverse habits or

modes of life. This is well seen in female Butterflies (which are

generally weaker and of slower flight), often having colours better

adapted to concealment; and in certain South American species (_Papilio

torquatus_) the females, which inhabit the forests, resemble the _Æneas_

group, which abound in similar localities, while the males, which

frequent the sunny open riverbanks, have a totally different coloration.

In these cases, therefore, natural selection seems to have acted

independently of sexual selection; and all such cases may be considered

as examples of the simplest dimorphism, since the offspring never offer

intermediate varieties between the parent forms.

The distinctive character therefore of dimorphism is this, that the

union of these distinct forms does not produce intermediate varieties,

but reproduces them unchanged. In simple varieties, on the other hand,

as well as when distinct local forms or distinct species are crossed,

the offspring never resembles either parent exactly, but is more or less

intermediate between them. Dimorphism is thus seen to be a specialized

result of variation, by which new physiological phenomena have been

developed; the two should therefore, whenever possible, be kept

separate[4].

Footnote 4:

The phenomena of _dimorphism_ and _polymorphism_ may be well

illustrated by supposing that a blue-eyed, flaxen-haired Saxon man had

two wives, one a black-haired, red-skinned Indian squaw, the other a

woolly-headed, sooty-skinned negress—and that instead of the children

being mulattoes of brown or dusky tints, mingling the separate

characteristics of their parents in varying degrees, all the boys

should be pure Saxon boys like their father, while the girls should

altogether resemble their mothers. This would be thought a

sufficiently wonderful fact; yet the phenomena here brought forward as

existing in the insect-world are still more extraordinary; for each

mother is capable not only of producing male offspring like the

father, and female like herself, but also of producing other females

exactly like her fellow-wife, and altogether differing from herself.

If an island could be stocked with a colony of human beings having

similar physiological idiosyncrasies with _Papilio Pammon_ or _Papilio

Ormenus_, we should see white men living with yellow, red, and black

women, and their offspring always reproducing the same types; so that

at the end of many generations the men would remain pure white, and

the women of the same well-marked races as at the commencement.

3. _Local form, or variety._—This is the first step in the transition

from variety to species. It occurs in species of wide range, when groups

of individuals have become partially isolated in several points of its

area of distribution, in each of which a characteristic form has become

segregated more or less completely. Such forms are very common in all

parts of the world, and have often been classed as varieties or species

alternately. I restrict the term to those cases where the difference of

the forms is very slight, or where the segregation is more or less

imperfect. The best example in the present group is _Papilio Agamemnon_,

L., a species which ranges over the greater part of tropical Asia, the

whole of the Malay archipelago, and a portion of the Australian and

Pacific regions. The modifications are principally of size and form,

and, though slight, are tolerably constant in each locality. The steps,

however, are so numerous and gradual that it would be impossible to

define many of them, though the extreme forms are sufficiently distinct.

_Papilio Sarpedon_, L., presents somewhat similar but less numerous

variations.

4. _Coexisting variety._—This is a somewhat doubtful case. It is when a

slight but permanent and hereditary modification of form exists in

company with the parent or typical form, without presenting those

intermediate gradations which would constitute it a case of simple

variability. It is evidently only by direct evidence of the two forms

breeding separately that this can be distinguished from _dimorphism_.

The difficulty occurs in _Papilio Jason_, Esp., and _P. Evemon_, Bd.,

which inhabit the same localities, and are almost exactly alike in form,

size, and coloration, except that the latter always wants a very

conspicuous red spot on the under surface, which is found not only in

_P. Jason_, but in all the allied species. It is only by breeding the

two insects that it can be determined whether this is a case of a

coexisting variety or of _dimorphism_. In the former case, however, the

difference being constant and so very conspicuous and easily defined, I

see not how we could escape considering it as a distinct species. A true

case of coexisting forms would, I consider, be produced, if a slight

variety had become fixed as a local form, and afterwards been brought

into contact with the parent species with little or no intermixture of

the two; and such instances do very probably occur.

5. _Race, or subspecies._—These are local forms completely fixed and

isolated; and there is no possible test but individual opinion to

determine which of them shall be considered as species and which

varieties. If stability of form and “_the constant transmission of some

characteristic peculiarity of organization_” is the test of a species

(and I can find no other test that is more certain than individual

opinion), then every one of these fixed races, confined as they almost

always are to distinct and limited areas, must be regarded as a species;

and as such I have in most cases treated them. The various modifications

of _Papilio Ulysses_, _P. Peranthus_, _P. Codrus_, _P. Eurypilus_, _P.

Helenus_, &c., are excellent examples; for while some present great and

well-marked, others offer slight and inconspicuous differences, yet in

all cases these differences seem equally fixed and permanent. If,

therefore, we call some of these forms species, and others varieties, we

introduce a purely arbitrary distinction, and shall never be able to

decide where to draw the line. The races of _Papilio Ulysses_, L., for

example, vary in amount of modification from the scarcely differing New

Guinea form to those of Woodlark Island and New Caledonia, but all seem

equally constant; and as most of these had already been named and

described as species, I have added the New Guinea form under the name of

_P. Penelope_. We thus get a little group of Ulyssine Papilios, the

whole comprised within a very limited area, each one confined to a

separate portion of that area, and, though differing in various amounts,

each apparently constant. Few naturalists will doubt that all these may

and probably have been derived from a common stock; and therefore it

seems desirable that there should be a unity in our method of treating

them: either call them all _varieties_ or all _species_. Varieties,

however, continually get overlooked; in lists of species they are often

altogether unrecorded; and thus we are in danger of neglecting the

interesting phenomena of variation and distribution which they present.

I think it advisable, therefore, to name all such forms; and those who

will not accept them as species may consider them as subspecies or

races.

6. _Species._—Species are merely those strongly marked races or local

forms which, when in contact, do not intermix, and when inhabiting

distinct areas are generally believed to have had a separate origin, and

to be incapable of producing a fertile hybrid offspring. But as the test

of hybridity cannot be applied in one case in ten thousand, and even if

it could be applied, would prove nothing, since it is founded on an

assumption of the very question to be decided—and as the test of

separate origin is in every case inapplicable—and as, further, the test

of non-intermixture is useless, except in those rare cases where the

most closely allied species are found inhabiting the same area, it will

be evident that we have no means whatever of distinguishing so-called

“true species” from the several modes of variation here pointed out, and

into which they so often pass by an insensible gradation. It is quite

true that, in the great majority of cases, what we term “species” are so

well marked and definite that there is no difference of opinion about

them; but as the test of a true theory is, that it accounts for, or at

the very least is not inconsistent with, the whole of the phenomena and

apparent anomalies of the problem to be solved, it is reasonable to ask

that those who deny the origin of species by variation and selection

should grapple with the facts in detail, and show how the doctrine of

the distinct origin and permanence of species will explain and harmonize

them. It has been recently asserted by a high authority that the

difficulty of limiting species is in proportion to our ignorance, and

that just as groups or countries are more accurately known and studied

in greater detail the limits of species become settled[5]. This

statement has, like many other general assertions, its portion of both

truth and error. There is no doubt that many uncertain species, founded

on few or isolated specimens, have had their true nature determined by

the study of a good series of examples: they have been thereby

established as species or as varieties; and the number of times this has

occurred is doubtless very great. But there are other and equally

trustworthy cases in which, not single species, but whole groups have,

by the study of a vast accumulation of materials, been proved to have no

definite specific limits. A few of these must be adduced. In Dr.

Carpenter’s ‘Introduction to the Study of the Foraminifera,’ he states

that “_there is not a single specimen of plant or animal of which the

range of variation has been studied by the collocation and comparison of

so large a number of specimens as have passed under the review of

Messrs. Williamson, Parker, Rupert Jones, and myself, in our studies of

the types of this group_;” and the result of this extended comparison of

specimens is stated to be, “_The range of variation is so great among

the Foraminifera as to include not merely those differential characters

which have been usually accounted_ SPECIFIC, _but also those upon which

the greater part of the_ GENERA _of this group have been founded, and

even in some instances those of its_ ORDERS” (Foraminifera, Preface, x).

Yet this same group had been divided by D’Orbigny and other authors into

a number of clearly defined _families_, _genera_, and _species_, which

these careful and conscientious researches have shown to have been

almost all founded on incomplete knowledge.

Footnote 5:

See Dr. J. E. Gray “On the Species of Lemuroids,” Proc. Zool. Soc.

1863, p. 134.

Professor DeCandolle has recently given the results of an extensive

review of the species of _Cupuliferæ_. He finds that the best-known

species of oaks are those which produce most varieties and subvarieties,

that they are often surrounded by provisional species; and, with the

fullest materials at his command, two-thirds of the species he considers

more or less doubtful. His general conclusion is, that “_in botany the

lowest series of groups_, SUBVARIETIES, VARIETIES, _and_ RACES _are very

badly limited; these can be grouped into_ SPECIES _a little less vaguely

limited, which again can be formed into sufficiently precise_ GENERA.”

This general conclusion is entirely objected to by the writer of the

article in the ‘Natural History Review,’ who, however, does not deny its

applicability to the particular order under discussion, while this very

difference of opinion is another proof that difficulties in the

determination of species do not, any more than in the higher groups,

vanish with increasing materials and more accurate research.

Another striking example of the same kind is seen in the genera _Rubus_

and _Rosa_, adduced by Mr. Darwin himself; for though the amplest

materials exist for a knowledge of these groups, and the most careful

research has been bestowed upon them, yet the various species have not

thereby been accurately limited and defined so as to satisfy the

majority of botanists.

Dr. Hooker seems to have found the same thing in his study of the Arctic

flora. For though he has had much of the accumulated materials of his

predecessors to work upon, he continually expresses himself as unable to

do more than group the numerous and apparently fluctuating forms into

more or less imperfectly defined species[6].

Footnote 6:

In his paper on the “Distribution of Arctic Plants,” Trans. Linn. Soc.

xxiii. p. 310, Dr. Hooker says:—

“The most able and experienced descriptive botanists vary in their

estimate of the value of the ‘specific term’ to a much greater

extent than is generally supposed.”

“I think I may safely affirm that the ‘specific term’ has three

different standard values, all current in descriptive botany, but

each more or less confined to one class of observers.”

“This is no question of what is right or wrong as to the real value

of the specific term; I believe each is right according to the

standard he assumes as the specific.”

Lastly, I will adduce Mr. Bates’s researches on the Amazons. During

eleven years he accumulated vast materials, and carefully studied the

variation and distribution of insects. Yet he has shown that many

species of Lepidoptera, which before offered no special difficulties,

are in reality most intricately combined in a tangled web of affinities,

leading by such gradual steps from the slightest and least stable

variations to fixed races and well-marked species, that it is very often

impossible to draw those sharp dividing-lines which it is supposed that

a careful study and full materials will always enable us to do.

These few examples show, I think, that in every department of nature

there occur instances of the instability of specific form, which the

increase of materials aggravates rather than diminishes. And it must be

remembered that the naturalist is rarely likely to err on the side of

imputing greater indefiniteness to species than really exists. There is

a completeness and satisfaction to the mind in defining and limiting and

naming a species, which leads us all to do so whenever we

conscientiously can, and which we know has led many collectors to reject

vague intermediate forms as destroying the symmetry of their cabinets.

We must therefore consider these cases of excessive variation and

instability as being thoroughly well established; and to the objection

that, after all, these cases are but few compared with those in which

species can be limited and defined, and are therefore merely exceptions

to a general rule, I reply that a true law embraces all apparent

exceptions, and that to the great laws of nature there are no real

exceptions—that what appear to be such are equally results of law, and

are often (perhaps indeed always) those very results which are most

important as revealing the true nature and action of the law. It is for

such reasons that naturalists now look upon the study of _varieties_ as

more important than that of well-fixed species. It is in the former that

we see nature still at work, in the very act of producing those

wonderful modifications of form, that endless variety of colour, and

that complicated harmony of relations, which gratify every sense and

give occupation to every faculty of the true lover of nature.

_Variation as specially influenced by Locality._

The phenomena of variation as influenced by locality have not hitherto

received much attention. Botanists, it is true, are acquainted with the

influences of climate, altitude, and other physical conditions in

modifying the forms and external characteristics of plants; but I am not

aware that any peculiar influence has been traced to locality,

independent of climate. Almost the only case I can find recorded is

mentioned in that repertory of natural-history facts, ‘The Origin of

Species,’ viz. that herbaceous groups have a tendency to become arboreal

in islands. In the animal world, I cannot find that any facts have been

pointed out as showing the special influence of locality in giving a

peculiar _facies_ to the several disconnected species that inhabit it.

What I have to adduce on this matter will therefore, I hope, possess

some interest and novelty.

On examining the closely allied species, local forms, and varieties

distributed over the Indian and Malayan regions, I find that larger or

smaller districts, or even single islands, give a special character to

the majority of their Papilionidæ. For instance: 1. The species of the

Indian region (Sumatra, Java, and Borneo) are almost invariably smaller

than the allied species inhabiting Celebes and the Moluccas; 2. The

species of New Guinea and Australia are also, though in a less degree,

smaller than the nearest species or varieties of the Moluccas; 3. In the

Moluccas themselves the species of Amboyna are the largest; 4. The

species of Celebes equal or even surpass in size those of Amboyna; 5.

The species and varieties of Celebes possess a striking character in the

form of the anterior wings, different from that of the allied species

and varieties of all the surrounding islands; 6. Tailed species in India

or the Indian region become tailless as they spread eastward through the

archipelago.

Having preserved the finest and largest specimens of Butterflies in my

own collection, and having always taken for comparison the largest

specimens of the same sex, I believe that the tables I now give are

sufficiently exact. The differences of expanse of wings are in most

cases very great, and are much more conspicuous in the specimens

themselves than on paper. It will be seen that no less than fourteen

Papilionidæ inhabiting Celebes and the Moluccas are from one-third to

one-half greater in extent of wing than the allied species representing

them in Java, Sumatra, and Borneo. Six species inhabiting Amboyna are

larger than the closely allied forms of the northern Moluccas and New

Guinea by about one-sixth. These include almost every case in which

closely allied species can be compared.

PAPILIONIDÆ.

Species of the Moluccas and Celebes│Closely allied species of Java and

(large). │ the Indian region (small).

Expanse. │ Expanse.

inches. │ inches.

Ornithoptera Helena 7·6│O. Pompeus 5·8

(Amboyna) │

„ „ │O. Amphrisius 6·0

───────────────────────────────────┼───────────────────────────────────

Papilio Macedon 5·8│P. Peranthus 3·8

(Celebes) │

P. Philippus (Moluccas) 4·8│ „ „

───────────────────────────────────┼───────────────────────────────────

P. Blumei (Celebes) 5·4│P. Brama 4·0

P. Alphenor (Celebes) 4·8│P. Theseus 3·6

P. Gigon (Celebes) 5·4│P. Demolion 4·0

P. Deucalion (Celebes) 4·6│P. Macareus 3·7

P. Agamemnon, var. 4·4│P. Agamemnon, var. 3·8

(Celebes) │

───────────────────────────────────┼───────────────────────────────────

P. Eurypilus (Moluccas) 4·0│P. Jason 3·4

P. Telephus (Celebes) 4·3│ „ „

───────────────────────────────────┼───────────────────────────────────

P. Ægisthus (Moluccas) 4·4│P. Rama 3·2

P. Miletus (Celebes) 4·4│P. Sarpedon 3·8

P. Androcles (Celebes) 4·8│P. Antiphates 3·7

P. Polyphontes 4·6│P. Diphilus 3·9

(Celebes) │

Leptocircus Curtius 2·0│L. Meges 1·8

(Celebes) │

│

Species inhabiting Amboyna (large).│ Allied species of New Guinea and

│ the North Moluccas (smaller).

Papilio Ulysses 6·1│P. Penelope 5·2

„ „ │P. Telegonus 4·0

───────────────────────────────────┼───────────────────────────────────

P. Polydorus 4·9│P. Leodamas 4·0

P. Deiphobus 6·8│P. Deiphontes 5·8

───────────────────────────────────┼───────────────────────────────────

P. Gambrisius 6·4│P. Ormenus 5·6

„ „ │P. Tydeus 6·0

───────────────────────────────────┼───────────────────────────────────

P. Codrus 5·1│P. Codrus, var. 4·3

│ _papuensis_

Ornithoptera Priamus, ♂ 8·0│Orn. Poseidon, ♂ 7·0

The differences of form are equally clear.

_Papilio Pammon_ everywhere on the continent is tailed in both sexes. In

Java, Sumatra, and Borneo, the closely allied _P. Theseus_ has a very

short tail, or tooth only, in the male, while in the females the tail is

retained. Further east, in Celebes and the South Moluccas, the hardly

separable _P. Alphenor_ has quite lost the tail in the male, while the

female retains it, but in a narrower and less spatulate form. A little

further, in Gilolo, _P. Nicanor_ has completely lost the tail in both

sexes.

_Papilio Agamemnon_ exhibits a somewhat similar series of changes. In

India it is always tailed; in the greater part of the archipelago it has

a very short tail; while far east, in New Guinea and the adjacent

islands, the tail has almost entirely disappeared.

In the _Polydorus_-group two species, _P. Antiphus_ and _P. Diphilus_,

inhabiting India and the Indian region, are tailed, while the two which

take their place in the Moluccas, New Guinea, and Australia, _P.

Polydorus_ and _P. Leodamas_, are destitute of tail, the species

furthest east having lost this ornament the most completely.

Western species, tailed. Eastern species (closely allied),

less tailed.

Papilio Pammon (India) tailed. P. Thesus (islands) very short

tail.

P. Agamemnon, var. tailed. P. Agamemnon, var. not tailed.

(India) (islands)

P. Antiphus (India, tailed. P. Polydorus (Moluccas) not tailed.

Java)

P. Diphilus (India, tailed. P. Leodamas (New not tailed.

Java) Guinea)

The most conspicuous instance of local modification of form, however, is

exhibited in the island of Celebes, which in this respect, as in some

others, stands alone and isolated in the whole archipelago. Almost every

species of _Papilio_ inhabiting Celebes has the wings of a peculiar

shape, which distinguishes them at a glance from the allied species of

every other island. This peculiarity consists, first, in the upper wings

being generally more elongate and falcate; and secondly, in the costa or

anterior margin being much more curved, and in most instances exhibiting

near the base an abrupt bend or elbow, which in some species is very

conspicuous. This peculiarity is visible, not only when the Celebesian

species are compared with their small-sized allies of Java and Borneo,

but also, and in an almost equal degree, when the large forms of Amboyna

and the Moluccas are the objects of comparison, showing that this is

quite a distinct phenomenon from the difference of size which has just

been pointed out.

In the following Table I have arranged the chief Papilios of Celebes in

the order in which they exhibit this characteristic form most

prominently. (See Plate VIII.)

Papilios of Celebes, having the Closely allied Papilios of the

wings falcate or with abruptly surrounding islands, with less

curved costa. falcate wings and slightly curved

costa.

1. P. Gigon, n. s. P. Demolion (Java).

2. P. Telephus, n. s. P. Jason (Sumatra).

3. P. Miletus, n. s. P. Sarpedon (Moluccas, Java).

4. P. Agamemnon, var. P. Agamemnon, var. (Borneo).

5. P. Macedon, n. s. P. Peranthus (Java).

6. P. Ascalaphus. P. Deiphontes, n. s. (Gilolo).

7. P. Hecuba, n. s. P. Helenus (Java).

8. P. Blumei. P. Brama (Sumatra).

9. P. Androcles. P. Antiphates (Borneo).

10. P. Rhesus. P. Aristæus (Moluccas).

11. P. Theseus, var., ♂. P. Thesus, ♂ (Java).

12. P. Codrus, var. P. Codrus (Moluccas).

13. P. Encelades. P. Leucothoë (Malacca).

It thus appears that every species of _Papilio_ exhibits this peculiar

form in a greater or less degree, except one, _P. Polyphontes_, Bd.,

allied to _P. Diphilus_ of India and _P. Polydorus_ of the Moluccas.

This fact I shall recur to again, as I think it helps us to understand

something of the causes that may have brought about the phenomenon we

are considering. Neither do the genera _Ornithoptera_ and _Leptocircus_

exhibit any traces of this peculiar form. In several other families of

Butterflies this characteristic form reappears in a few species. In the

Pieridæ the following species exhibit it distinctly:—

1. Eronia tritæa │compared with Eronia Valeria (Java).

2. Iphias Glaucippe, var.│ „ „ Iphias Glaucippe (Java).

3. Pieris Zebuda │ „ „ Pieris Descombesi (India).

4. P. Zarinda │ „ „ P. Nero (Malacca).

5. P., n. s. │ „ „ P. Hyparete (Java).

──────────────────────────┬────────────────────────────────────────────

6. P. Hombronii │ have the same form, but are isolated

│ species.

7. P. Ithome │ „

──────────────────────────┼────────────────────────────────────────────

8. P. Eperia, _Bd._ │compared with P. Coronis (Java).

9. P. Polisma │ „ „ P., n. s. (Malacca).

10. Terias, n. s. │ „ „ P. Tilaha (Java).

The other species of _Terias_, one or two _Pieris_, and the genus

_Callidryas_ do not exhibit any perceptible change of form.

In the other families there are but few similar examples. The following

are all that I can find in my collection:—

Cethosia Æole compared with Cethosia Biblis (Java).

Junonia, n. s. „ „ Junonia Polynice (Borneo).

Limenitis Limire „ „ Limenitis Procris (Java).

Cynthia Arsinoë, var. „ „ Cynthia Arsinoë (Java, Sum.,

Born.).

All these belong to the family of the Nymphalidæ. Many other genera of

this family, as _Diadema_, _Adolias_, _Charaxes_, and _Cyrestis_, as

well as the entire families of the Danaidæ, Satyridæ, Lycænidæ, and

Hesperidæ, present no examples of this peculiar form of the upper wing

in the Celebesian species.

The facts now brought forward seem to me of the highest interest. We see

that almost all the species in two important families of the Lepidoptera

(Papilionidæ and Pieridæ) acquire, in a single island, a characteristic

modification of form distinguishing them from the allied species and

varieties of all the surrounding islands. In other equally extensive

families no such change occurs, except in one or two isolated species.

However we may account for these phenomena, or whether we may be quite

unable to account for them, they furnish, in my opinion, a strong

corroborative testimony in favour of the doctrine of the origin of

species by successive small variations; for we have here slight

varieties, local races, and undoubted species, all modified in exactly

the same manner, indicating plainly a common cause producing identical

results. On the generally received theory of the original distinctness

and permanence of species, we are met by this difficulty: one portion of

these curiously modified forms are admitted to have been produced by

variation and some natural action of local conditions; whilst the other

portion, differing from the former only in degree, and connected with

them by insensible gradations, are said to have possessed this

peculiarity of form at their first creation, or to have derived it from

unknown causes of a totally distinct nature. Is not the _à priori_

evidence in favour of the assumption of an identity of the causes that

have produced such similar results? and have we not a right to call upon

our opponents for some proofs of their own doctrine, and for an

explanation of its difficulties, instead of their assuming that they are

right, and laying upon us the burthen of disproof?

Let us now see if the facts in question do not themselves furnish some

clue to their own explanation. Mr. Bates has shown that certain groups

of butterflies have a defence against insectivorous animals,

independent of swiftness of motion. These are generally very abundant,

slow, and weak fliers, and are more or less the objects of mimicry by

other groups, which thus gain an advantage in a freedom from

persecution similar to that enjoyed by those they resemble. Now the

only Papilios which have not in Celebes acquired the peculiar form of

wing belong to a group which is imitated both by other species of

_Papilio_ and by Moths of the genus _Epicopeia_, West. This group is

of weak and slow flight; and we may therefore fairly conclude that it

possesses some means of defence (probably in a peculiar odour or

taste) which saves it from attack. Now the arched costa and falcate

form of wing is generally supposed to give increased powers of flight,

or, as seems to me more probable, greater facility in making sudden

turnings, and thus baffling a pursuer. But the members of the

_Polydorus_-group (to which belongs the only unchanged Celebesian

_Papilio_), being already guarded against attack, have no need of this

increased power of wing; and “natural selection” would therefore have

no tendency to produce it. The whole family of Danaidæ are in the same

position: they are slow and weak fliers; yet they abound in species

and individuals, and are the objects of mimicry. The Satyridæ have

also probably a means of protection—perhaps their keeping always near

the ground and their generally obscure colours; while the Lycænidæ and

Hesperidæ may find security in their small size and rapid motions. In

the extensive family of the Nymphalidæ, however, we find that several

of the larger species, of comparatively feeble structure, have their

wings modified (_Cethosia_, _Limenitis_, _Junonia_, _Cynthia_), while

the large-bodied powerful species, which have all an excessively rapid

flight, have exactly the same form of wing in Celebes as in the other

islands. On the whole, therefore, we may say that all the butterflies

of rather large size, conspicuous colours, and not very swift flight

have been affected in the manner described, while the smaller-sized

and obscure groups, as well as those which are the objects of mimicry,

and also those of exceedingly swift flight, have remained unaffected.

It would thus appear as if there must be (or once have been) in the

island of Celebes, some peculiar enemy to these larger-sized butterflies

which does not exist, or is less abundant, in the surrounding islands.

Increased powers of flight, or rapidity of turning, was advantageous in

baffling this enemy; and the peculiar form of wing necessary to give

this would be readily acquired by the action of “natural selection” on

the slight variations of form that are continually occurring. Such an

enemy one would naturally suppose to be an insectivorous bird; but it is

a remarkable fact that most of the genera of Fly-catchers of Borneo and

Java on the one side (_Muscipeta_, _Philentoma_), and of the Moluccas on

the other (_Monarcha_, _Rhipidura_), are almost entirely absent from

Celebes. Their place seems to be supplied by the Caterpillar-catchers

(_Graucalus_, _Campephaga_), of which six or seven species are known

from Celebes and are very numerous in individuals. We have no positive

evidence that these birds pursue butterflies on the wing, but it is

highly probable that they do so when other food is scarce[7]. However

this may be, the fauna of Celebes is undoubtedly highly peculiar in

every department of which we have any knowledge; and though we may not

be able to trace it satisfactorily, there can, I think, be little doubt

that the singular modification in the wings of so many of the

butterflies of that island is an effect of that complicated action and

reaction of all living things upon each other in the struggle for

existence, which continually tends to readjust disturbed relations, and

to bring every species into harmony with the varying conditions of the

surrounding universe.

Footnote 7:

Mr. Bates has suggested that the larger Dragon-flies (_Æshna_, &c.)

prey upon butterflies; but I did not notice that they were more

abundant in Celebes than elsewhere.

But even the conjectural explanation now given fails us in the other

cases of local modification. Why the species of the western islands

should be smaller than those further east,—why those of Amboyna should

exceed in size those of Gilolo and New Guinea—why the tailed species of

India should begin to lose that appendage in the islands, and retain no

trace of it on the borders of the Pacific, are questions which we cannot

at present attempt to answer. That they depend, however, on some general

principle is certain, because analogous facts have been observed in

other parts of the world. Mr. Bates informs me that, in three distinct

groups, Papilios which on the Upper Amazon and in most other parts of

South America have spotless upper wings obtain pale or white spots at

Para and on the Lower Amazon; and also that the _Æneas_-group of

Papilios never have tails in the equatorial regions and the Amazons

valley, but gradually acquire tails in many cases as they range towards

the northern or southern tropic. Even in Europe we have somewhat similar

facts; for the species and varieties of butterflies peculiar to the

island of Sardinia are generally smaller and more deeply coloured than

those of the mainland, and _Papilio Hospiton_ has lost the tail, which

is a prominent feature of the closely allied _P. Machaon_.

Facts of a similar nature to those now brought forward would no doubt be

found to occur in other groups of insects, were local faunas carefully

studied in relation to those of the surrounding countries; and they seem

to indicate that climate and other physical causes have, in some cases,

a very powerful effect in modifying specific form, and thus directly aid

in producing the endless variety of nature.

I may state that I can adduce facts perfectly analogous to these from

other families of Lepidoptera, especially the Danaidæ; but as the

greater part of the species are still undescribed, I can only now assert

that similar phenomena do occur there.

_Mimicry._

I need scarcely say that I entirely agree with Mr. Bates’s explanation

of the causes which have led to one group of insects mimicking another

(Trans. Linn. Soc. vol. xxiii. p. 495). I have, therefore, only now to

adduce such illustrations of this curious phenomenon as are furnished by

the Eastern Papilionidæ, and to show their hearing upon the phenomena of

variation already mentioned. As in America, so in the Old World, species

of Danaidæ are the objects which the other families most often imitate.

But, besides these, some genera of Morphidæ and one section of the genus

_Papilio_ are also less frequently copied. Many species of _Papilio_

mimic other species of these three groups so closely that they are

undistinguishable when on the wing; and in every case the pairs which

resemble each other inhabit the same locality.

The following list exhibits the most important and best-marked cases of

mimicry which occur among the Papilionidæ of the Malayan region and

India:—

Mimickers[8]. Species mimicked. │Common habitat.

DANAIDÆ.

1. Papilio paradoxa, Euplœa Midamus, _Cr._, ♂│Sumatra, &c.

_Zink._, ♂ │

—— ——, ♀ —— ——, ♀ │ „

──────────────────────────────────────────────────────┼────────────────

2. —— ——, _West._ E. Rhadamanthus │Sumatra, &c.

3. P. Caunus, E., sp. │Borneo.

4. P. Thule, _Wall._ Danais sobrina, _Bd._ │New Guinea.

5. P. Macareus, _Godt._ D. Aglaia, _Cr._ │Malacca, Java.

6. P. Agestor, _G. R. G._ D. Tytia, _G. R. G._ │Northern India.

7. P. idæoides, _Hewits._ Hestia Leuconoë, │Philippines.

_Erichs._ │

8. P. Delessertii, _Guér._ Hestia, sp. │Penang.

│

MORPHIDÆ.

9. P. Pandion, _Wall._, ♀ Drusilla bioculata, │New Guinea.

_Guér._ │

│

PAPILIO (POLYDORUS- and COON-groups).

10. P. Pammon, _L._ (Romulus, Papilio Hector, _L._ │India.

_L._), ♀ │

11. P. Theseus, _Cr._, var., P. Antiphus, _Fab._ │Sumatra, Borneo.

♀ │

12. P. Theseus, _Cr._, var., P. Diphilus, _Esp._ │Sumatra, Java.

♀ │

13. P. Memnon, var. P. Coon, _Fab._ │Sumatra.

_Achates_, ♀ │

14. P. Androgeus, var. P. Doubledayi, _Wall._ │Northern India.

_Achates_, ♀ │

15. P. Œnomaus, _God._, ♀ P. Liris, _God._ │Timor.

Footnote 8:

The terms “_mimicry_” and “_mimickers_” have been objected to on the

ground that they imply _voluntary action_ on the part of the insects.

This appears to me of little importance compared with the advantages

of convenience, flexibility, and expressiveness which they undoubtedly

possess, especially as the whole theory propounded by the originator

of the term in this sense excludes all idea of voluntary action. The

only approximately synonymous words, not implying will, are

_resemblance_, _similarity_, and _likeness_; and it is evident that

none of these can be applied intelligibly under the variety of forms

required, and to which Mr. Bates’s expression so readily lends itself

in the terms _mimic_, _mimickers_, _mimicry_, _mimicked_. Add to this

the inconvenience of changing a term which, from the interest and wide

discussion of the subject, must be already very generally understood,

and I think it will be admitted that nothing would be gained by

altering it, even if a better word were pointed out, which has not yet

been done.

We have therefore fifteen species or marked varieties of _Papilio_ which

so closely resemble species of other groups in their respective

localities, that it is not possible to impute the resemblance to

accident. The first two in the list (_Papilio paradoxa_ and _P. Caunus_)

are so exactly like _Euplœa Midamus_ and _E. Rhadamanthus_ on the wing,

that, although they fly very slowly, I was quite unable to distinguish

them. The first is a very interesting case, because the male and female

differ considerably, and each mimics the corresponding sex of the

_Euplœa_. A new species of _Papilio_ which I discovered in New Guinea

resembles _Danais sobrina_, Bd., from the same country, just as _Papilio

Macareus_ resembles _Danais Aglaia_ in Malacca, and (according to Dr.

Horsfield’s figure) still more closely in Java. The Indian _Papilio

Agestor_ closely imitates _Danais Tytia_, which has quite a different

style of colouring from the preceding; and the extraordinary _Papilio

idæoides_ from the Philippine Islands must, when on the wing, perfectly

resemble the _Hestia Leuconoë_ of the same region, as also does the _P.

Delessertii_, Guér., imitate an undescribed species of _Hestia_ from

Penang. Now in every one of these cases the Papilios are very scarce,

while the Danaidæ which they resemble are exceedingly abundant—most of

them swarming so as to be a positive nuisance to the collecting

entomologist by continually hovering before him when he is in search of

newer and more varied captures. Every garden, every roadside, the

suburbs of every village are full of them, indicating very clearly that

their life is an easy one, and that they are free from persecution by

the foes which keep down the population of less favoured races. This

superabundant population has been shown by Mr. Bates to be a general

characteristic of all American groups and species which are objects of

mimicry; and it is interesting to find his observations confirmed by

examples on the other side of the globe.

The remarkable genus _Drusilla_, a group of pale-coloured butterflies,

more or less adorned with ocellate spots, is also the object of mimicry

by three distinct genera (_Melanitis_, _Hyantis_, and _Papilio_). These

insects, like the _Danaidæ_, are abundant in individuals, have a very

weak and slow flight, and do not seek concealment, or appear to have any

means of protection from insectivorous creatures. It is natural to

conclude, therefore, that they have some hidden property which saves

them from attack; and it is easy to see that when any other insects, by

what we call accidental variation, come more or less remotely to

resemble them, the latter will share to some extent in their immunity.

An extraordinary dimorphic form of a female _Papilio_ has come to

resemble the Drusillas sufficiently to be taken for one of that group at

a little distance; and it is curious that I captured one of these

Papilios in the Aru Islands hovering along the ground, and settling on

it occasionally, just as it is the habit of the Drusillas to do. The

resemblance in this case is only general; but this form of _Papilio_

varies much, and there is therefore material for natural selection to

act upon so as ultimately to produce a copy as exact as in the other

cases.

The eastern Papilios allied to _Polydorus Coon_ and _P. Philoxenus_,

form a natural section of the genus resembling, in many respects, the

_Æneas_-group of South America, which they may be said to represent in

the East. Like them, they are forest insects, have a low and weak

flight, and in their favourite localities are rather abundant in

individuals; and like them, too, they are the objects of mimicry. We may

conclude, therefore, that they possess some hidden means of protection,

which makes it useful to other insects to be mistaken for them.

The Papilios which resemble them belong to a very distinct section of

the genus, in which the sexes differ greatly; and it is those females

only which differ most from the males, and which have already been

alluded to as exhibiting instances of dimorphism, which resemble species

of the other group.

The resemblance of _P. Romulus_ to _P. Hector_ is, in some specimens,

very considerable, and has led to the two species being placed to follow

each other in the British Museum Catalogues and by Mr. E. Doubleday. I

have shown, however, that _P. Romulus_ is probably a dimorphic form of

the female _P. Pammon_, and belongs to a distinct section of the

genus[9].

Footnote 9:

See Plate II. fig. 6.

The next pair, _P. Theseus_, Cr., and _P. Antiphus_, Fab., have been

united as one species both by De Haan and in the British Museum

Catalogues. The ordinary variety of _P. Theseus_ found in Java almost as

nearly resembles _P. Diphilus_, Esp., of the same country. The most

interesting case, however, is the extreme female form of _P. Memnon_

(_P. Achates_, Cr.)[10], which has acquired the general form and

markings of _P. Coon_, an insect which differs from the ordinary male

_P. Memnon_, as much as any two species differ which can be chosen in

this extensive and highly varied genus; and, as if to show that this

resemblance is not accidental, but is the result of law, when in India

we find a species closely allied to _P. Coon_, but with red instead of

yellow spots (_P. Doubledayi_, Wall.), the corresponding variety of _P.

Androgeus_ (_P. Achates_, Cram., 182, A, B,) has acquired exactly the

same peculiarity of having red spots instead of yellow. Lastly, in the

island of Timor, the female of _P. Œnomaus_ (a species allied to _P.

Memnon_) resembles so closely _P. Liris_ (one of the _Polydorus_-group),

that the two, which were often seen flying together, could only be

distinguished by a minute comparison after being captured.

Footnote 10:

See Plate I. fig. 4.

The last six cases of mimicry are especially instructive, because they

seem to indicate one of the processes by which dimorphic forms have been

produced. When, as in these cases, one sex differs much from the other,

and varies greatly itself, it may happen that occasionally individual

variations will occur having a distant resemblance to groups which are

the objects of mimicry, and which it is therefore advantageous to

resemble. Such a variety will have a better chance of preservation; the

individuals possessing it will be multiplied; and their accidental

likeness to the favoured group will be rendered permanent by hereditary

transmission, and, each successive variation which increases the

resemblance being preserved, and all variations departing from the

favoured type having less chance of preservation, there will in time

result those singular cases of two or more isolated and fixed forms

bound together by that intimate relationship which constitutes them the

sexes of a single species. The reason why the females are more subject

to this kind of modification than the males is, probably, that their

slower flight, when laden with eggs, and their exposure to attack while

in the act of depositing their eggs upon leaves, render it especially

advantageous for them to have some additional protection. This they at

once obtain by acquiring a resemblance to other species which, from

whatever cause, enjoy a comparative immunity from persecution.

This summary of the more interesting phenomena of variation presented by

the eastern Papilionidæ is, I think, sufficient to substantiate my

position, that the Lepidoptera are a group that offer especial

facilities for such inquiries; and it will also show that they have

undergone an amount of special adaptive modification rarely equalled

among the more highly organized animals. And, among the Lepidoptera, the

great and pre-eminently tropical families of Papilionidæ and Danaidæ

seem to be those in which complicated adaptations to the surrounding

organic and inorganic universe have been most completely developed,

offering in this respect a striking analogy to the equally

extraordinary, though totally different, adaptations which present

themselves in the _Orchideæ_, the only family of plants in which mimicry

of other organisms appears to play any important part, and the only one

in which striking cases of polymorphism occur; for such we must consider

to be the male, female, and hermaphrodite forms of _Catasetum

tridentatum_, which differ so greatly in form and structure that they

were long considered to belong to three distinct genera.

_Arrangement and Geographical Distribution of the Malayan_ Papilionidæ.

Although the species of Papilionidæ inhabiting the Malayan region are

very numerous, they all belong to three out of the nine genera into

which the family is divided. One of the remaining genera (_Eurycus_) is

restricted to Australia, and another (_Teinopalpus_) to the Himalayan

Mountains, while no less than four (_Parnassius_, _Doritis_, _Thais_,

and _Sericinus_) are confined to Southern Europe and to the

mountain-ranges of the Palæarctic region.

The genera _Ornithoptera_ and _Leptocircus_ are highly characteristic of

Malayan entomology, but are uniform in character and of small extent.

The genus _Papilio_, on the other hand, presents a great variety of

forms, and is so richly represented in the Malay islands, that more than

one-fourth of all the known species are found there. It becomes

necessary, therefore, to divide this genus into natural groups before we

can successfully study its geographical distribution.

Owing principally to Dr. Horsfield’s observations in Java, we are

acquainted with a considerable number of the larvæ of Papilios; and

these furnish good characters for the primary division of the genus into

natural groups. The manner in which the hinder wings are plaited or

folded back at the abdominal margin, the size of the anal valves, the

structure of the antennæ, and the form of the wings are also of much

service, as well as the character of the flight and the style of

coloration. Using these characters, I divide the Malayan Papilios into

four sections, and seventeen groups, as follows:—

Genus ORNITHOPTERA.

a. _Priamus_-group. Black and green.

b. _Pompeus_-group. Black and yellow.

c. _Brookeanus_-group.

Genus PAPILIO.

A. Larvæ short, thick, with numerous fleshy tubercles; purplish.

a. _Nox_-group. Abdominal fold in ♂ very large; anal valves small,

but swollen; antennæ moderate; wings entire, or tailed:

includes the Indian _Philoxenus_-group.

b. _Coon_-group. Abdominal fold in ♂ small; anal valves small, but

swollen; antennæ moderate; wings tailed.

c. _Polydorus_-group. Abdominal fold in ♂ small, or none; anal

valves small or obsolete, hairy; wings tailed or entire.

B. Larvæ with third segment swollen, transversely or obliquely

banded; pupa much bent. Imago with abdominal margin in ♂

plaited, but not reflexed; body weak; antennæ long; wings much

dilated, often tailed.

d. _Ulysses_-group.

e. _Peranthus_-group.

_Protenor_-group (Indian) is somewhat intermediate between

these, and is nearest to the _Nox_-group.

f. _Memnon_-group.

g. _Helenus_-group.

h. _Erectheus_-group.

i. _Pammon_-group.

k. _Demolion_-group.

C. Larvæ subcylindrical, variously coloured. Imago with abdominal

margin in ♂ plaited, but not reflexed; body weak; antennæ short,

with a thick curved club; wings entire.

l. _Erithonius_-group. Sexes alike, larva and pupa something like

those of _P. Demolion_.

m. _Paradoxa_-group. Sexes different.

n. _Dissimilis_-group. Sexes alike; larva bright-coloured; pupa

straight, cylindric.

D. Larvæ elongate, attenuate behind, and often bifid, with lateral

and oblique pale stripes, green. Imago with the abdominal margin

in ♂ reflexed, woolly or hairy within; anal valves small, hairy;

antennæ short, stout; body stout.

o. _Macareus_-group. Hind wings entire.

p. _Antiphates_-group. Hind wings much tailed (swallow-tails).

q. _Eurypylus_-group. Hind wings elongate or tailed.

Genus LEPTOCIRCUS.

making, in all, twenty distinct groups of Malayan Papilionidæ.

The first section of the genus _Papilio_ (A) comprises insects which,

though differing considerably in structure, have much general

resemblance. They all have a weak, low flight, frequent the most

luxuriant forest districts, seem to love the shade, and are the objects

of mimicry by other Papilios.

Section B consists of weak-bodied, large-winged insects, with an

irregular wavering flight, and which, when resting on foliage, often

expand the wings, which the species of the other sections rarely or

never do. They are the most conspicuous and striking of eastern

Butterflies.

Section C consists of much weaker and slower-flying insects, often

resembling in their flight, as well as in their colours, species of

Danaidæ.