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
species--population would be only equal to that of sections of the other
orders having 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.
_Question of the rank of the Papilionidae._
The Papilionidae 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 Heliconidae,
(published in the Transactions of the Linnaean Society, vol. xxiii., p.
495) 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 Hesperidae 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
Papilionidae. The perfect insects possess two characters quite peculiar
to them. Mr. Edward Doubleday, in his "Genera of Diurnal Lepidoptera,"
says, "The Papilionidae may be known by the apparently four-branched
median nervule and the spur on the anterior tibiae, 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 tibiae is also found
in some of the Hesperidae, 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 Papilionidae,
however, and that on which I think insufficient stress has been laid, is
undoubtedly the peculiar structure of the larvae. 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 Papilionidae 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.
In Mr. Trimen's paper on "Mimetic Analogies among African Butterflies,"
in the Transactions of the Linnaean Society, for 1868, he has argued
strongly in favour of Mr. Bates' views as to the higher position of the
Danaidae and the lower grade of the Papilionidae, and has adduced, among
other facts, the undoubted resemblance of the pupa of Parnassius, a
genus of Papilionidae, to that of some Hesperidae and moths. I admit,
therefore, that he has proved the Papilionidae to have retained several
characters of the nocturnal Lepidoptera which the Danaidae have lost, but
I deny that they are therefore to be considered lower in the scale of
organization. Other characters may be pointed out which indicate that
they are farther removed from the moths even than the Danaidae. The club
of the antennae is the most prominent and most constant feature by which
butterflies may be distinguished from moths, and of all butterflies the
Papilionidae have the most beautiful and most perfectly developed clubbed
antennae. Again, butterflies and moths are broadly characterised by their
diurnal and nocturnal habits respectively, and the Papilionidae, with
their close allies the Pieridae, are the most pre-eminently diurnal of
butterflies, most of them lovers of sunshine, and not presenting a
single crepuscular species. The great group of the Nymphalidae, on the
other hand (in which Mr. Bates includes the Danaidae and Heliconidae as
sub-families), contains an entire sub-family (Brassolidae) and a number
of genera, such as Thaumantis, Zeuxidia, Pavonia, &c., of crepuscular
habits, while a large proportion of the Satyridae and many of the
Danaidae are shade-loving butterflies. This question, of what is to be
considered the highest type of any group of organisms, is one of such
general interest to naturalists that it will be well to consider it a
little further, by a comparison of the Lepidoptera with some groups of
the higher animals.
Mr. Trimen's argument, that the lepidopterous type, like that of birds,
being pre-eminently aerial, "therefore a diminution of the ambulatory
organs, instead of being a sign of inferiority, may very possibly
indicate a higher, because a more thoroughly aerial form," is certainly
unsound, for it would imply that the most aerial of birds (the swift and
the frigate-birds, for example) are the highest in the scale of
bird-organization, and the more so on account of their feet being very
ill adapted for walking. But no ornithologist has ever so classed them,
and the claim to the highest rank among birds is only disputed between
three groups, all very far removed from these. They are--1st. The
Falcons, on account of their general perfection, their rapid flight,
their piercing vision, their perfect feet armed with retractile claws,
the beauty of their forms, and the ease and rapidity of their motions;
2nd. The Parrots, whose feet, though ill-fitted for walking, are perfect
as prehensile organs, and which possess large brains with great
intelligence, though but moderate powers of flight; and, 3rd. The
Thrushes or Crows, as typical of the perching birds, on account of the
well-balanced development of their whole structure, in which no organ
or function has attained an undue prominence.
Turning now to the Mammalia, it might be argued that as they are
pre-eminently the terrestrial type of vertebrates, to walk and run well
is essential to the typical perfection of the group; but this would give
the superiority to the horse, the deer, or the hunting leopard, instead
of to the Quadrumana. We seem here to have quite a case in point, for
one group of Quadrumana, the Lemurs, is undoubtedly nearer to the low
Insectivora and Marsupials than the Carnivora or the Ungulata, as shown
among other characters by the Opossums possessing a hand with perfect
opposable thumb, closely resembling that of some of the Lemurs; and by
the curious Galeopithecus, which is sometimes classed as a Lemur, and
sometimes with the Insectivora. Again, the implacental mammals,
including the Ornithodelphia and the Marsupials, are admitted to be
lower than the placental series. But one of the distinguishing
characters of the Marsupials is that the young are born blind and
exceedingly imperfect, and it might therefore be argued that those
orders in which the young are born most perfect are the highest, because
farthest from the low Marsupial type. This would make the Ruminants and
Ungulata higher than the Quadrumana or the Carnivora. But the Mammalia
offer a still more remarkable illustration of the fallacy of this mode
of reasoning, for if there is one character more than another which is
essential and distinctive of the class, it is that from which it derives
its name, the possession of mammary glands and the power of suckling
the young. What more reasonable, apparently, than to argue that the
group in which this important function is most developed, that in which
the young are most dependent upon it, and for the longest period, must
be the highest in the Mammalian scale of organization? Yet this group is
the Marsupial, in which the young commence suckling in a foetal
condition, and continue to do so till they are fully developed, and are
therefore for a long time absolutely dependent on this mode of
nourishment.
These examples, I think, demonstrate that we cannot settle the rank of a
group by a consideration of the degree in which certain characters
resemble or differ from those in what is admitted to be a lower group;
and they also show that the highest group of a class may be more closely
connected to one of the lowest, than some other groups which have
developed laterally and diverged farther from the parent type, but which
yet, owing to want of balance or too great specialization in their
structure, have never reached a high grade of organization. The
Quadrumana afford a very valuable illustration, because, owing to their
undoubted affinity with man, we feel certain that they are really higher
than any other order of Mammalia, while at the same time they are more
distinctly allied to the lowest groups than many others. The case of the
Papilionidae seems to me so exactly parallel to this, that, while I admit
all the proofs of affinity with the undoubtedly lower groups of
Hesperidae and moths, I yet maintain that, owing to the complete and
even development of every part of their organization, these insects best
represent the highest perfection to which the butterfly type has
attained, and deserve to be placed at its head in every system of
classification.
_Distribution of the Papilionidae._
The Papilionidae 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 Ornithopterae 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 roadways, 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 130 species of the family are
now known to inhabit the Archipelago, and of these ninety-six were
collected by myself. Thirty 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-eight
species; Celebes twenty-four, and the Peninsula of Malacca, twenty-six
species. Further east the numbers decrease; Batchian producing
seventeen, and New Guinea only fifteen, though this number is certainly
too small, owing to our present imperfect knowledge of that great
island.
_Definition of the word Species._
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.
_Laws and Modes of Variation._
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, co-existing 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 Papilionidae 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
co-existence 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 their offspring, instead of being hybrids, or
like the two parents, appear to reproduce all the distinct 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. If these
distinct forms breed independently, and are never reproduced from a
common parent, they must be considered as separate 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 Papilionidae 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
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 having the two sexes 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, to which the name P. Androgeus (Cramer) may be applied. 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 larvae, by Messrs. Payen and Bocarme, in Java, and I myself
captured, in Sumatra, a male P. Memnon, and a tailed female P. Achates,
under circumstances which led me to class them as the same species.
Papilio Pammon offers a somewhat similar case. The female was described
by Linnaeus as P. Polytes, and was considered to be a distinct species
till Westermann bred the two from the same larvae (see Boisduval,
"Species General des Lepidopteres," 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
re-establish P. Polytes as a distinct species; and as such it
accordingly appears in the British Museum List of Papilionidae 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 discovered, 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.