external form depends almost entirely on the proportions and arrangement
of that skeleton, which again is strictly adapted to the functions
necessary for the well-being of the animal. The form cannot therefore be
rapidly modified by variation, and the thin and flexible integument will
not admit of the development of such strange protuberances as occur
continually in insects. The number of species of each group in the same
country is also comparatively small, and thus the chances of that first
accidental resemblance which is necessary for natural selection to work
upon are much diminished. We can hardly see the possibility of a mimicry
by which the elk could escape from the wolf, or the buffalo from the
tiger. There is, however, in one group of Vertebrata such a general
similarity of form, that a very slight modification, if accompanied by
identity of colour, would produce the necessary amount of resemblance;
and at the same time there exist a number of species which it would be
advantageous for others to resemble, since they are armed with the most
fatal weapons of offence. We accordingly find that reptiles furnish us
with a very remarkable and instructive case of true mimicry.
_Mimicry among Snakes._
There are in tropical America a number of venomous snakes of the genus
Elaps, which are ornamented with brilliant colours disposed in a
peculiar manner. The ground colour is generally bright red, on which are
black bands of various widths and sometimes divided into two or three by
yellow rings. Now, in the same country are found several genera of
harmless snakes, having no affinity whatever with the above, but
coloured exactly the same. For example, the poisonous Elaps fulvius
often occurs in Guatemala with simple black bands on a coral-red ground;
and in the same country is found the harmless snake Pliocerus equalis,
coloured and banded in identically the same manner. A variety of Elaps
corallinus has the black bands narrowly bordered with yellow on the same
red ground colour, and a harmless snake, Homalocranium semicinctum, has
exactly the same markings, and both are found in Mexico. The deadly
Elaps lemniscatus has the black bands very broad, and each of them
divided into three by narrow yellow rings; and this again is exactly
copied by a harmless snake, Pliocerus elapoides, which is found along
with its model in Mexico.
But, more remarkable still, there is in South America a third group of
snakes, the genus Oxyrhopus, doubtfully venomous, and having no
immediate affinity with either of the preceding, which has also the same
curious distribution of colours, namely, variously disposed rings of
red, yellow, and black; and there are some cases in which species of all
three of these groups similarly marked inhabit the same district. For
example, Elaps mipartitus has single black rings very close together. It
inhabits the west side of the Andes, and in the same districts occur
Pliocerus euryzonus and Oxyrhopus petolarius, which exactly copy its
pattern. In Brazil Elaps lemniscatus is copied by Oxyrhopus trigeminus,
both having black rings disposed in threes. In Elaps hemiprichii the
ground colour appears to be black, with alternations of two narrow
yellow bands and a broader red one; and of this pattern again we have an
exact double in Oxyrhopus formosus, both being found in many localities
of tropical South America.
What adds much to the extraordinary character of these resemblances is
the fact, that nowhere in the world but in America are there any snakes
at all which have this style of colouring. Dr. Gunther, of the British
Museum, who has kindly furnished some of the details here referred to,
assures me that this is the case; and that red, black, and yellow rings
occur together on no other snakes in the world but on Elaps and the
species which so closely resemble it. In all these cases, the size and
form as well as the colouration, are so much alike, that none but a
naturalist would distinguish the harmless from the poisonous species.
Many of the small tree-frogs are no doubt also mimickers. When seen in
their natural attitudes, I have been often unable to distinguish them
from beetles or other insects sitting upon leaves, but regret to say I
neglected to observe what species or groups they most resembled, and the
subject does not yet seem to have attracted the attention of naturalists
abroad.
_Mimicry among Birds._
In the class of birds there are a number of cases that make some
approach to mimicry, such as the resemblance of the cuckoos, a weak and
defenceless group of birds, to hawks and Gallinaceae. There is, however,
one example which goes much further than this, and seems to be of
exactly the same nature as the many cases of insect mimicry which have
been already given. In Australia and the Moluccas there is a genus of
honeysuckers called Tropidorhynchus, good sized birds, very strong and
active, having powerful grasping claws and long, curved, sharp beaks.
They assemble together in groups and small flocks, and they have a very
loud bawling note, which can be heard at a great distance, and serves to
collect a number together in time of danger. They are very plentiful
and very pugnacious, frequently driving away crows, and even hawks,
which perch on a tree where a few of them are assembled. They are all of
rather dull and obscure colours. Now in the same countries there is a
group of orioles, forming the genus Mimeta, much weaker birds, which
have lost the gay colouring of their allies the golden orioles, being
usually olive-green or brown; and in several cases these most curiously
resemble the Tropidorhynchus of the same island. For example, in the
island of Bouru is found the Tropidorhynchus bouruensis, of a dull
earthy colour, and the Mimeta bouruensis, which resembles it in the
following particulars:--The upper and under surfaces of the two birds
are exactly of the same tints of dark and light brown; the
Tropidorhynchus has a large bare black patch round the eyes; this is
copied in the Mimeta by a patch of black feathers. The top of the head
of the Tropidorhynchus has a scaly appearance from the narrow
scale-formed feathers, which are imitated by the broader feathers of the
Mimeta having a dusky line down each. The Tropidorhynchus has a pale
ruff formed of curious recurved feathers on the nape (which has given
the whole genus the name of Friar birds); this is represented in the
Mimeta by a pale band in the same position. Lastly, the bill of the
Tropidorhynchus is raised into a protuberant keel at the base, and the
Mimeta has the same character, although it is not a common one in the
genus. The result is, that on a superficial examination the birds are
identical, although they have important structural differences, and
cannot be placed near each other in any natural arrangement. As a proof
that the resemblance is really deceptive, it may be mentioned that the
Mimeta is figured and described as a honeysucker in the costly "Voyage
de l'Astrolabe," under the name of Philedon bouruensis!
Passing to the island of Ceram, we find allied species of both genera.
The Tropidorhynchus subcornutus is of an earthy brown colour washed with
yellow ochre, with bare orbits, dusky cheeks, and the usual pale
recurved nape-ruff. The Mimeta forsteni is absolutely identical in the
tints of every part of the body, the details of which are imitated in
the same manner as in the Bouru birds already described. In two other
islands there is an approximation towards mimicry, although it is not so
perfect as in the two preceding cases. In Timor the Tropidorhynchus
timoriensis is of the usual earthy brown above, with the nape-ruff very
prominent, the cheeks black, the throat nearly white, and the whole
under surface pale whitish brown. These various tints are all well
reproduced in Mimeta virescens, the chief want of exact imitation being
that the throat and breast of the Tropidorhynchus has a very scaly
appearance, being covered with rigid pointed feathers which are not
imitated in the Mimeta, although there are signs of faint dusky spots
which may easily furnish the groundwork of a more exact imitation by the
continued survival of favourable variations in the same direction. There
is also a large knob at the base of the bill of the Tropidorhynchus
which is not at all imitated by the Mimeta. In the island of Morty
(north of Gilolo) there exists the Tropidorhynchus fuscicapillus, of a
dark sooty brown colour, especially on the head, while the under parts
are rather lighter, and the characteristic ruff of the nape is wanting.
Now it is curious that in the adjacent island of Gilolo should be found
the Mimeta phaeochromus, the upper surface of which is of exactly the
same dark sooty tint as the Tropidorhynchus, and is the only known
species that is of such a dark colour. The under side is not quite light
enough, but it is a good approximation. This Mimeta is a rare bird, and
may very probably exist in Morty, though not yet found there; or, on the
other hand, recent changes in physical geography may have led to the
restriction of the Tropidorhynchus to that island, where it is very
common.
Here, then, we have two cases of perfect mimicry and two others of good
approximation, occurring between species of the same two genera of
birds; and in three of these cases the pairs that resemble each other
are found together in the same island, and to which they are peculiar.
In all these cases the Tropidorhynchus is rather larger than the Mimeta,
but the difference is not beyond the limits of variation in species, and
the two genera are somewhat alike in form and proportion. There are, no
doubt, some special enemies by which many small birds are attacked, but
which are afraid of the Tropidorhynchus (probably some of the hawks),
and thus it becomes advantageous for the weak Mimeta to resemble the
strong, pugnacious, noisy, and very abundant Tropidorhynchus.
My friend, Mr. Osbert Salvin, has given me another interesting case of
bird mimicry. In the neighbourhood of Rio Janeiro is found an
insect-eating hawk (Harpagus diodon), and in the same district a
bird-eating hawk (Accipiter pileatus) which closely resembles it. Both
are of the same ashy tint beneath, with the thighs and under
wing-coverts reddish brown, so that when on the wing and seen from below
they are undistinguishable. The curious point, however, is that the
Accipiter has a much wider range than the Harpagus, and in the regions
where the insect-eating species is not found it no longer resembles it,
the under wing-coverts varying to white; thus indicating that the
red-brown colour is kept true by its being useful to the Accipiter to be
mistaken for the insect-eating species, which birds have learnt not to
be afraid of.
_Mimicry among Mammals._
Among the Mammalia the only case which may be true mimicry is that of
the insectivorous genus Cladobates, found in the Malay countries,
several species of which very closely resemble squirrels. The size is
about the same, the long bushy tail is carried in the same way, and the
colours are very similar. In this case the use of the resemblance must
be to enable the Cladobates to approach the insects or small birds on
which it feeds, under the disguise of the harmless fruit-eating
squirrel.
_Objections to Mr. Bates' Theory of Mimicry._
Having now completed our survey of the most prominent and remarkable
cases of mimicry that have yet been noticed, we must say something of
the objections that have been made to the theory of their production
given by Mr. Bates, and which we have endeavoured to illustrate and
enforce in the preceding pages. Three counter explanations have been
proposed. Professor Westwood admits the fact of the mimicry and its
probable use to the insect, but maintains that each species was created
a mimic for the purpose of the protection thus afforded it. Mr. Andrew
Murray, in his paper on the "Disguises of Nature," inclines to the
opinion that similar conditions of food and of surrounding circumstances
have acted in some unknown way to produce the resemblances; and when the
subject was discussed before the Entomological Society of London, a
third objection was added--that heredity or the reversion to ancestral
types of form and colouration, might have produced many of the cases of
mimicry.
Against the special creation of mimicking species there are all the
objections and difficulties in the way of special creation in other
cases, with the addition of a few that are peculiar to it. The most
obvious is, that we have gradations of mimicry and of protective
resemblance--a fact which is strongly suggestive of a natural process
having been at work. Another very serious objection is, that as mimicry
has been shown to be useful only to those species and groups which are
rare and probably dying out, and would cease to have any effect should
the proportionate abundance of the two species be reversed, it follows
that on the special-creation theory the one species must have been
created plentiful, the other rare; and, notwithstanding the many causes
that continually tend to alter the proportions of species, these two
species must have always been specially maintained at their respective
proportions, or the very purpose for which they each received their
peculiar characteristics would have completely failed. A third
difficulty is, that although it is very easy to understand how mimicry
may be brought about by variation and the survival of the fittest, it
seems a very strange thing for a Creator to protect an animal by making
it imitate another, when the very assumption of a Creator implies his
power to create it so as to require no such circuitous protection. These
appear to be fatal objections to the application of the special-creation
theory to this particular case.
The other two supposed explanations, which may be shortly expressed as
the theories of "similar conditions" and of "heredity," agree in making
mimicry, where it exists, an adventitious circumstance not necessarily
connected with the well-being of the mimicking species. But several of
the most striking and most constant facts which have been adduced,
directly contradict both those hypotheses. The law that mimicry is
confined to a few groups only is one of these, for "similar conditions"
must act more or less on all groups in a limited region, and "heredity"
must influence all groups related to each other in an equal degree.
Again, the general fact that those species which mimic others are rare,
while those which are imitated are abundant, is in no way explained by
either of these theories, any more than is the frequent occurrence of
some palpable mode of protection in the imitated species. "Reversion to
an ancestral type" no way explains why the imitator and the imitated
always inhabit the very same district, whereas allied forms of every
degree of nearness and remoteness generally inhabit different countries,
and often different quarters of the globe; and neither it, nor "similar
conditions," will account for the likeness between species of distinct
groups being superficial only--a disguise, not a true resemblance; for
the imitation of bark, of leaves, of sticks, of dung; for the
resemblance between species in different orders, and even different
classes and sub-kingdoms; and finally, for the graduated series of the
phenomena, beginning with a general harmony and adaptation of tint in
autumn and winter moths and in arctic and desert animals, and ending
with those complete cases of detailed mimicry which not only deceive
predacious animals, but puzzle the most experienced insect collectors
and the most learned entomologists.
_Mimicry by Female Insects only._
But there is yet another series of phenomena connected with this
subject, which considerably strengthens the view here adopted, while it
seems quite incompatible with either of the other hypotheses; namely,
the relation of protective colouring and mimicry to the sexual
differences of animals. It will be clear to every one that if two
animals, which as regards "external conditions" and "hereditary
descent," are exactly alike, yet differ remarkably in colouration, one
resembling a protected species and the other not, the resemblance that
exists in one only can hardly be imputed to the influence of external
conditions or as the effect of heredity. And if, further, it can be
proved that the one requires protection more than the other, and that in
several cases it is that one which mimics the protected species, while
the one that least requires protection never does so, it will afford
very strong corroborative evidence that there is a real connexion
between the necessity for protection and the phenomenon of mimicry. Now
the sexes of insects offer us a test of the nature here indicated, and
appear to furnish one of the most conclusive arguments in favour of the
theory that the phenomena termed "mimicry" are produced by natural
selection.
The comparative importance of the sexes varies much in different classes
of animals. In the higher vertebrates, where the number of young
produced at a birth is small and the same individuals breed many years
in succession, the preservation of both sexes is almost equally
important. In all the numerous cases in which the male protects the
female and her offspring, or helps to supply them with food, his
importance in the economy of nature is proportionately increased,
though it is never perhaps quite equal to that of the female. In
insects the case is very different; they pair but once in their lives,
and the prolonged existence of the male is in most cases quite
unnecessary for the continuance of the race. The female, however, must
continue to exist long enough to deposit her eggs in a place adapted for
the development and growth of the progeny. Hence there is a wide
difference in the need for protection in the two sexes; and we should,
therefore, expect to find that in some cases the special protection
given to the female was in the male less in amount or altogether
wanting. The facts entirely confirm this expectation. In the spectre
insects (Phasmidae) it is often the females alone that so strikingly
resemble leaves, while the males show only a rude approximation. The
male Diadema misippus is a very handsome and conspicuous butterfly,
without a sign of protective or imitative colouring, while the female is
entirely unlike her partner, and is one of the most wonderful cases of
mimicry on record, resembling most accurately the common Danais
chrysippus, in whose company it is often found. So in several species of
South American Pieris, the males are white and black, of a similar type
of colouring to our own "cabbage" butterflies, while the females are
rich yellow and buff, spotted and marked so as exactly to resemble
species of Heliconidae with which they associate in the forest. In the
Malay archipelago is found a Diadema which had always been considered a
male insect on account of its glossy metallic-blue tints, while its
companion of sober brown was looked upon as the female. I discovered,
however, that the reverse is the case, and that the rich and glossy
colours of the female are imitative and protective, since they cause her
exactly to resemble the common Euploea midamus of the same regions, a
species which has been already mentioned in this essay as mimicked by
another butterfly, Papilio paradoxa. I have since named this interesting
species Diadema anomala (see the Transactions of the Entomological
Society, 1869, p. 285). In this case, and in that of Diadema misippus,
there is no difference in the habits of the two sexes, which fly in
similar localities; so that the influence of "external conditions"
cannot be invoked here as it has been in the case of the South American
Pieris pyrrha and allies, where the white males frequent open sunny
places, while the Heliconia-like females haunt the shades of the forest.
We may impute to the same general cause (the greater need of protection
for the female, owing to her weaker flight, greater exposure to attack,
and supreme importance)--the fact of the colours of female insects being
so very generally duller and less conspicuous than those of the other
sex. And that it is chiefly due to this cause rather than to what Mr.
Darwin terms "sexual selection" appears to be shown by the otherwise
inexplicable fact, that in the groups which have a protection of any
kind independent of concealment, sexual differences of colour are either
quite wanting or slightly developed. The Heliconidae and Danaidae,
protected by a disagreeable flavour, have the females as bright and
conspicuous as the males, and very rarely differing at all from them.
The stinging Hymenoptera have the two sexes equally well coloured. The
Carabidae, the Coccinellidae, Chrysomelidae, and the Telephori have both
sexes equally conspicuous, and seldom differing in colours. The
brilliant Curculios, which are protected by their hardness, are
brilliant in both sexes. Lastly, the glittering Cetoniadae and
Buprestidae, which seem to be protected by their hard and polished coats,
their rapid motions, and peculiar habits, present few sexual differences
of colour, while sexual selection has often manifested itself by
structural differences, such as horns, spines, or other processes.
_Cause of the dull Colours of Female Birds._