facts than that the females first assumed the protective colouring,
and that the males followed later, and more slowly. Whether this is
due to inheritance on the female side, and thus ensues as a mechanical
necessity, in virtue of laws of inheritance still unknown to us, or
whether it arose because there was a certain advantage in protection
to the males--though not such a marked one--and that these, therefore,
followed independently along the same path of evolution as the females,
has yet to be investigated. Personally, I incline to the latter view,
because there are protected mimetic species, in which the female
mimics one immune model, and the male another, quite different from
the female's. A case in point is that of an Indian butterfly, _Euripus
haliterses_, and also _Hypolimnas scopas_, in the latter of which
the male resembles the male of _Euplœa pyrgion_, and the female is
like the somewhat different female of the same protected species. The
Indian _Papilio paradoxus_, too, seems to show the independence of the
processes of mimetic adaptation, for the male is like the blue male
of the immune _Euplœa binotata_ (Pl. III, Fig. 25), while the female
resembles the radially-striped female of _Euplœa midamus_ (Pl. III,
Fig. 27), and this double adaptation is repeated in another of the
persecuted butterflies, _Elymnias leucocyma_ (Pl. III, Fig. 26, 28).
Many objections have been made to the interpretation of mimicry by
selection. It has been asserted that butterflies are exposed to injury
from birds only to an inconsiderable extent, not sufficient to account
for such an intense and persistent process of selection, because they
are not very welcome morsels, on account of the large and uneatable
wings and the relatively small body. Doubt has also been raised as to
the immunity of the models, which has not been proved in many of the
species in regard to which it is assumed. Finally, it is maintained
that the advantage which resemblance to an immune model brings is
not proved, but is purely hypothetical; and that it is probable that
the birds do not distinguish the colours and markings of the flying
butterflies at all, but are at the most only deceived by resemblances
in their manner of flight.
The last objection contains a certain amount of truth, inasmuch as
the manner of flight always plays a part in the mimicry of a strange
species. We shall see later how much the instincts of a species
contribute to the deception in all cases of protective colouring.
It is, therefore, not improbable that, in many cases, the imitation
of the flight of an immune species, and a gradually increasing
familiarity with the habitats of the same immune species, preceded the
modification of the colour. Indeed, the slow flight of immune species
(Heliconiidæ) has been unanimously emphasized by observers, as a factor
in facilitating the recognition of the butterflies by the sharp-sighted
birds.
That it was not only in earlier ages of the world's history that
butterflies were much persecuted, as some have supposed, but that they
are so still, seems to me indisputable in view of the observations
of the last quarter of a century. Even in this country, where both
butterflies and insect-eating birds are being more and more crowded out
through cultivation, a considerable number of butterflies in flight
fall victims to the birds. Kennel gives observations on this point in
regard to the white-throat; Caspari for the swallows. The latter let
about a hundred little tortoiseshell butterflies (_Vanessa antiopa_)
fly from his window, 'but not ten of them reached the neighbouring
wood,' all the rest being eaten by swallows, 'which congregated in
numbers in front of his window.' Kathariner observed, in the highlands
of Asia Minor, a flock of bee-eaters (_Merops_) which caught in flight
and swallowed a great many individuals of a very beautiful diurnal
butterfly (_Thais cerisyi_).
Finally, Pastor Slevogt has collected much evidence to show that
our indigenous butterflies have a great deal to suffer in the way
of persecution from birds. And in regard to tropical countries, the
chase of butterflies by insectivorous birds has long been known. Thus
Pöppig says that in the primitive forests one can easily recognize
the place which has been selected by one of the Jacamars (Galbulidæ)
as its favourite resting-place, for the wings of the largest and most
beautiful butterflies, whose bodies alone are eaten, lie on the ground
in a circle for a distance of several paces. We owe direct observations
on the hunting of insects by birds of the primitive forest especially
to Dr. Hahnel, who found many opportunities for observation in the
course of his enthusiastic collecting journeys in Central and South
America. He writes: 'No other family of butterflies suffered so much
from birds as the Pieridæ (Whites), and these freebooters often snapped
away the prettiest and freshest specimens from quite close to me. Every
time I was amazed anew at the unfailing security of their flight, and I
gladly paid for the spectacle by the loss of a few specimens.' Of the
pursuit of one of the large _Caligo_ species, whose leaf-like under
surface, marked with eye-spots, I have already described, (Fig. 6, p.
70), he says: 'With incredible skill this fairly large insect avoided
every blow of the bill of the bird which followed it in close chase,
and saved itself by flying from one shrub to another, till at last it
was lost to sight in the thickest tangle of branches, and the exhausted
bird gave up further attempts at pursuit.'
But, in addition to the birds, the butterflies of the primitive forest
have to dread the persecution of other insects, especially of the large
predaceous dragon-flies, which throw themselves upon them in the midst
of their flight. Hahnel often saw a specimen of the large, beautiful,
blue _Morpho cisseis_, which was fluttering peacefully about the crown
of a tree, suddenly shoot head downwards, 'like an ox with horns
lowered, and then reascended apparently with difficulty, after it had
torn itself free from its sudden assailant, whose jaws left distinct
short scars.'
In addition to birds and predatory insects the butterflies are
persecuted by the whole army of lizards. In order to entice the
butterflies, Hahnel laid bait in the wood, 'sugar-cane, little sweet
bananas, and such like.' Various kinds of butterfly settled on it,
'Satyrides, Ageroniæ, _Adelpha_ and other Nymphalidæ.' He saw that they
'were persistently stalked and attacked by greedy lizards, which, in
spite of their plump figure and uncouth gait, showed themselves able
to spring suddenly out and snatch their prey with great adroitness. It
is, however, very wonderful to see the agility such a persecuted insect
displays in evading the repeated attacks of these marauders.' Thus on
one occasion an _Adelpha_ was driven off a dozen times from the exposed
bait by a lizard, which pounced upon it, but it always settled down
for a short time on a leaf, and soon returned to its repast, whereupon
the enemy 'instantaneously rushed upon it in a fury, until at last he
was obliged to give in,' abandoning the attempt to catch a creature so
adept in retreat.
Many butterflies assemble at midday on sandbanks in the middle
of the river, in order to drink, and there, too, the lizards are
always lurking about. Hahnel gives a pretty and undoubtedly accurate
description of the protective value of the long tail borne by many
of the sail-like Papilios at the end of the posterior wing; they
'quite obviously' afford protection against the lizards, 'which, after
snapping, often find themselves obliged to be content with the tail
alone, while the rest of the animal flies away practically uninjured.'
Not only is the great persecution of the butterflies a fact, the
immunity of the known species, which are models for mimicry, is
also certain. For numerous species, at any rate, this has now been
established. First of all--as has already been said--this is true of
the Heliconiidæ, in regard to which Wallace long ago showed that, if
the thorax be pressed, they exude a yellowish juice of unpleasant
smell. This is probably the blood of the insect, but that does not
hinder the repulsive odour of the living butterfly being perceptible at
a distance of 'several paces,' as Seitz observed in _Heliconius besei_.
Repeated experiments have been made, which have shown that such
butterflies are rejected not only by the insectivorous birds of the
primitive forest, but also by tame turkeys, pheasants and partridges,
usually so greedy. Hahnel has recently repeated these experiments in
Brazil with hens, and he obtained the same result. The hens, 'which
otherwise devoured all butterflies eagerly,' rejected all Ithomidæ,
Heliconiidæ, the white Papilios, as also some of the gaily coloured
Heliconiid-like moths which fly by day, such as _Esthema bicolor_ and
_Pericopis lycorea_. Obviously, the gay or conspicuous colour of these
Lepidoptera acts as a warning signal of their unpalatability, and
protects them from attempts on the part of the birds to investigate
their flavour. Hence we find that the under surface of these insects
is coloured like the upper. Even the numbers of these species which
fly about indicates that they must be little decimated, and, in point
of fact, we never find the wings of Heliconiidæ lying on the ground in
the forests of South America, while those of the Nymphalidæ and other
butterflies are by no means uncommonly seen as the remains of birds'
meals.
There is just as little room for doubt, as in the case of the
Heliconiidæ and their allies, that the Danaidæ, Acræidæ, and the
Euplœidæ in the tropical regions of the Old World enjoy a certain
immunity on account of their repulsive odour and taste. Here, too,
observation and experiment have shown that birds, lizards, and
predaceous insects leave the butterflies of these families unmolested.
I need only mention the observation of Trimen that, under an acacia
much visited by butterflies, on which Mantides--the so-called
praying-insects--caught and devoured large numbers, the wings of an
_Acræa_ or a _Danais_ were never found. These unpalatable butterflies
also possess a motley or at least striking dress, recognizable from
afar, and alike on both surfaces; and they also have a slow flight,
by which they are readily recognized. They, too, usually assemble
in large swarms, and both sexes are alike, or resemble each other
closely in colouring, or at least they are both equally conspicuous.
But even these cases do not complete the list of butterflies which are
protected by their unpalatability; among the otherwise much-persecuted
and therefore palatable Pieridæ (Whites) there is an Asiatic genus,
_Delias_, which in all probability belongs to the immune butterflies,
as their gaily coloured under surface indicates, and among the
nocturnal Lepidoptera of different countries and families there are
isolated generations which are very gaily and conspicuously coloured,
and which are rejected by birds, their unpleasant odour being
perceptible at a distance of several feet (Chalcosiidæ and Eusemiidæ).
The latter no longer fly under cover of night, like their relatives,
but have assumed diurnal habits.
It is to be supposed that the repulsiveness of such 'unpalatable'
butterflies is associated with the food-plant on which the caterpillar
lives. Acrid, nauseous, astringent, and actually poisonous substances
are produced in many plants, and we shall see later that this is to
their own advantage; these substances pass into the insect, and they
do so probably in part unaltered, in part certainly altered, but still
they are protective, perhaps even in an increased degree. This is
borne out by the fact that many caterpillars of immune butterflies
live on more or less poisonous plants: the Acræidæ and Heliconiidæ on
Passiflores, which contain nauseous substances; the Danaidæ on the
poisonous Asclepiadæ, which are rich in milky juice or latex; the
Euplœæ on the poisonous species of _Ficus_, the Neotropinæ on the
Solanaceæ, and so on. But there are many genera, rich in species,
and distributed over the whole earth, the caterpillars of which live
on plants of very various families and characters, and of these the
majority of species are palatable, though a few are repulsive in
taste and odour, and therefore immune. This is the case in the genus
_Papilio_. As far back as the sixties Wallace discovered that there
were immune species of _Papilio_, and that these were mimicked
by other species. Later it was shown that these immune species
live chiefly on poisonous plants (in the wide sense), on various
Aristolochiæ; and Haase has recently grouped these together as
poison-eaters (Aristolochia-butterflies or Pharmacophagæ). They are
distinguished by a conspicuous red on the body. In some of them, as in
_Papilio philoxenus_, a repulsive odour as of decomposing urine has
been detected in the living animal.
We see, then, that the much-persecuted and easily injured butterflies
make use of a poisonous substance (in the widest sense), prepared in
the plant for its own protection, and, wherever their own metabolism
makes it possible, they use it to protect themselves. We need not
wonder, therefore, that so many butterflies are immune, nor that among
the numerous palatable species a small proportion have endeavoured to
become like the protected species, as far as natural selection was able
to bring such a resemblance about.
There is hardly any adaptation phenomenon so widely distributed and
diverse in its manifestations, which has been at the same time so much
observed and followed out into all its details, as Mimicry; and it must
surely be regarded as a justification of the validity of interpreting
it in terms of Natural Selection that all the observed phenomena tally
so beautifully with the deductions from the theory. I at least know of
no facts which contradict the theory, but of many which might have been
predicted from it.
For instance, it might have been predicted from the theory alone that
an immune species would often have several mimics, as, in point of
fact, is frequently the case, and it would be easy to give numerous
examples of this. Thus the two Danaids of South and Central Africa,
_Amauris echeria_ and _Amauris niavius_, are mimicked, not only
by the two female forms of _Papilio merope_, as we have already
described in detail, but the latter is also mimicked by Nymphalid,
which requires protection, _Diadema anthedon_, and the former by two
diurnal butterflies of different families, _Diadema nuina_ and _Papilio
echerioides_.
Similarly, the black-and-red coloured _Heliconius melpomene_ in Brazil
is mimicked both by the female of a White (_Archonias teuthamis_),
and by a _Papilio_, which has received the name of _P. euterpinus_ on
account of this resemblance. Thus, too, the immune _Methona psidii_,
Cr. of Brazil, with its half-transparent wings marked with black bands,
has five mimics, belonging to five different genera, and one of these
is not a true diurnal butterfly at all, but one of the day-flying
species of the genus _Castnia_, whose systematic position is doubtful.
[Illustration: FIG. 18. Upper surfaces of _A_, _Acræa egina_, from the
Gold Coast, immune. _B_, _Papilio ridleyanus_, from Gaboon, not immune.
_C_, _Pseudacræa boisduvalii_, from the Gold Coast, not immune.]
The West African immune Acræid, _Acræa gea_ (Pl. II, Fig. 21), is
deceptively mimicked, both as to the narrow, long shape of the wing
and its blackish-brown and white mottled markings, by a Nymphalid,
_Pseudacræa hirce_, by the female of a Papilio (_P. cynorta_) whose
mate is quite different, and by the female of a Satyrid (_Elymnias
phegea_) (Pl. II, Fig. 20). In the _Papilio_ the resemblance extends to
the peculiar pitch-black shining spot on the under side of the base of
the posterior wing, and all three are like the model on both surfaces,
and therefore in flight as well as in the resting attitude.
On the same West African coast occurs the strange greyish-black _Acræa
egina_, with brick-red spots and bands, and coal-black dots (Fig. 18,
_A_). This immune species is deceptively mimicked in its native country
by two other butterflies--a Nymphalid, _Pseudacræa boisduvalii_ (Fig.
18, _C_), and by a female _Papilio_ (_P. ridleyanus_) (Fig. 18, _B_),
by the latter not so exactly as by the former, but quite sufficiently
to be confused with its model in flight.
It would have been less easy to predict with certainty from the theory
that, conversely, the different species of a genus which stood in need
of protection would be able to mimic quite different immune models, for
who would have ventured to prophesy how far the capacity of a species
for variation might go, and how many different kinds of coloration it
was able to assume? But the facts teach us that there is a wide range
of possibility in this respect.
Most interesting in this respect is, perhaps, the Asiatic-African
genus _Elymnias_, a Satyrid whose numerous (over thirty) species
all seem to be in need of protection, for many of them mimic immune
butterflies, while the rest are inconspicuous and are provided with
protective colouring on the under surface. On Plates II and III some
of the former are depicted beside their models. The single African
species (_Elymnias phegea_) (Pl. II, Fig. 20) mimics, as has been
already mentioned, the prevalent _Acræa gea_ (Pl. II, Fig. 21). Many
of the Asiatic Elymniidæ are mimics of the immune Euplœæ, especially
the dark-brown species with steel-blue shimmer, such as _E. patna_ in
India, _E. beza_ in Borneo, and _E. penanga_ in Borneo. In Amboina
there flies an _E. vitellia_, the female of which mimics accurately the
plain, light-brown, inconspicuous _Euplœa climena_ which occurs there.
The male of _Elymnias leucocyma_ (Pl. III, Fig. 26) resembles the brown
and blue shimmering _Euplœa binotata_ (Pl. III, Fig. 25), while the
female mimics the dusky, radially-striped female of _Euplœa midamus_
(Pl. III, Figs. 27 and 28): the male of _Elymnias cassiphone_ resembles
the blackish-brown and deep-blue iridescent _Euplœa claudia_, while the
female is like the female of _Euplœa midamus_. A number of species of
_Elymnias_ copy Danaids: thus both sexes of _E. lais_ are like _Danais
vulgaris_ (Pl. III, Figs. 29 and 30), and _E. ceryx_ and _E. timandra_
are like another similar Danaid, _D. tytia_. The female only of _E.
undularis_ of Ceylon mimics the brown-yellow _D. genutia_ (Pl. II, Fig.
22) in general appearance, though not minutely, while the male (Pl.
II, Fig. 24) seems to attempt an imitation of the blue EuplϾ. A rare
form, not often represented in collections, _Elymnias künstleri_, bears
a striking resemblance to the Danaid, _Ideopsis daos_ Boisd., with its
white wings spotted with black, while three species mimic the probably
immune Pierid genus _Delias_, especially on the under surface, which
is decorated with yellow and red. Perhaps the one which has diverged
farthest from the original type is _Elymnias agondas_ Boisd. (Pl. II,
Fig. 32) of the Papua region and the island of Waigeu, for it bears
two large blue eye-spots on the posterior wings, and thus, especially
in the case of the almost white female, closely resembles _Tenaris
bioculatus_ (Pl. III, Fig. 31). There are thus seven or eight types
of marking and colouring differing from one another, and belonging to
six different genera and a much greater number of species, which are
mimicked by this one genus _Elymnias_.
It is most interesting to note how these mimetic species give up, more
or less, the original sympathetic colouring of the under surface,
and use in establishing their mimicry the marking elements which
were originally directed towards concealment. According to the
beautiful observations of Erich Haase on this genus _Elymnias_, the
ground-colouring on the under surface must have been 'a grey, darkly
mottled protective one,' as still occurs, for instance, in several
mimetic species, such as _Elymnias lais_ (Pl. III, Fig. 30). This
leaf-colouring disappears more and more the more perfect the mimicry of
the model becomes, until, finally, the model is repeated on the under
surface also. Compare, for instance, Figs. 30 and 32. From this we
may conclude that a dress which makes Lepidoptera appear unpalatable
morsels is a more effective protection than resemblance to a leaf. That
might indeed be deduced even from the theory, for resemblance to a leaf
never protects _absolutely_, and does so, in any case, only during
rest, while apparent unpalatability repels assailants at all times.
Those unversed in butterfly lore usually ask, when these mimetic
relations are expounded to them, how we know that copies which are
so like their models really belong to a different genus, or even
family. There are certainly cases in which model and copy resemble
each other so closely that even a zoologist cannot tell one from the
other without close examination, as, for instance, in the case of
certain transparent-winged Heliconiidæ of Brazil (Ithomiides) and their
mimics belonging to the family of Whites. But even in such cases the
likeness only extends as far as is theoretically requisite, that is,
only to those characters that make the butterfly appear to the eye of
its pursuer like another species, known to it to be unpalatable. The
likeness does not extend to details, which can only be seen with a
magnifying-glass or a microscope, and above all, it does not extend
to the caterpillar, pupa, or egg. Thus, in the case cited, we may be
certain that the caterpillar of _Ithomia_ is quite different from
that of the mimicking White, since the former will be, in structure,
of the type of _Ithomia_ caterpillar, and the other of the usual type
of Whites. As yet, indeed, these two species are not known in their
caterpillar stages, but other cases are known. A species belonging to
the same genus as our indigenous 'kingfishers' (_Limenitis populi_), a
diurnal butterfly of North America, _Limenitis archippus_ (Pl. I, Fig.
9), strongly resembles the brown-yellow, immune _Danais erippus_ (Pl.
I, Fig. 8), while the caterpillars of both species are quite different,
that of _Danais erippus_ possessing the remarkable, soft and flexible
horn-like processes of the Danaid caterpillars (Pl. I, Fig. 10_a_),
while the caterpillar of _Limenitis archippus_ (Pl. I, Fig. 11_a_) is
at once recognizable by its blunt, club-shaped and spinose papillæ
as a _Limenitis_ caterpillar. The adaptation of the butterfly to its
protected model has thus exercised no influence upon the caterpillar.
Nor has it affected the pupa, which in both cases exhibits the very
different and quite characteristic form of the _Danais_ pupa and the
_Limenitis_ pupa respectively (Pl. I, Fig. 10_b_, and 11_b_).
But even in the butterfly itself nothing is altered, except what
increases the resemblance to the model. All else has remained
unchanged, above all, the venation of the wings. Since the painstaking
and valuable work of Herrich-Schäfer the venation has been made the
basis of the whole systematic arrangement of butterflies, and it
enables us, in point of fact, to distinguish with precision, not the
families alone, but often even the genera, for the course of the
veins in the different species of a single genus is the same, and
that is true for the mimetic species as well as for others. Thus the
Danaid-like _Limenitis_ has the usual _Limenitis_ venation, of the kind
seen in our own indigenous species of _Limenitis_, and the already
described _Elymnias_ species of the African and Indian forests and
grassy plains have always the venation characteristic of this genus,
whether they be protected only by sympathetic colouring or imitate
an immune _Euplœa_, a _Danais_, an _Acræa_, or a _Tenaris_. However
much the contour of the wing may vary, the venation is unaffected,
and we can distinguish model from copy by this means alone, so that,
even when there is the closest resemblance, no doubt is possible.
In its theoretical aspect this constancy of venation is obviously
important, for as nothing about the organism is incapable of variation,
the veining of the wings might have varied, as indeed it has varied
from genus to genus in the course of the phylogenetic history; but as
changes in venation could not be detected by the butterflies' enemies,
however sharp-sighted, there has been no reason in these cases for
variation in this respect.