MIMICRY--BATESIAN AND MÜLLERIAN
Mimicry is a special branch of the study of adaptation. The term has
sometimes been used loosely to include cases where an animal, most
frequently an insect, bears a strong and often most remarkable resemblance
to some feature of its inanimate surroundings. Many butterflies with wings
closed are wonderfully like dead leaves; certain spiders when at rest on a
leaf look exactly like bird-droppings; "looper" caterpillars simulate small
twigs; the names of the "stick-" and "leaf-" insects are in themselves an
indication of their appearance. Such cases as these, in which the creature
exhibits a resemblance to some part of its natural surroundings, should be
classified as cases of "protective resemblance" in contradistinction to
mimicry proper. Striking examples of protective resemblance are abundant,
and though we possess little critical knowledge of the acuity of perception
in birds and other insect feeders it is plausible to regard the
resemblances as being of definite advantage in the struggle for existence.
However, it is with mimicry and not with protective coloration in general
that we are here directly {9} concerned, and the nature of the phenomenon
may perhaps best be made clear by a brief account of the facts which led to
the statement of the theory.
In the middle of last century the distinguished naturalist, H. W. Bates,
was engaged in making collections in parts of the Amazon region. He paid
much attention to butterflies, in which group he discovered a remarkably
interesting phenomenon[3]. Among the species which he took were a large
number belonging to the group Ithomiinae, small butterflies of peculiar
appearance with long slender bodies and narrow wings bearing in most cases
a conspicuous pattern (cf. Pl. X, fig. 7). When Bates came to examine his
catch more closely he discovered that among the many Ithomiines were a few
specimens very like them in general shape, colour, and markings, but
differing in certain anatomical features by which the Pierinae, or
"whites," are separated from other groups. Most Pierines are very different
from Ithomiines. It is the group to which our common cabbage butterfly
belongs and the ground colour is generally white. The shape of the body and
also of the wings is in general quite distinct from what it is in the
Ithomiines. Nevertheless in these particular districts certain of the
species of Pierines had departed widely from what is usually regarded as
their ancestral pattern (Pl. X, fig. 1) and had come to resemble very
closely the far more abundant Ithomiines among whom they habitually flew
(cf. Pl. X, figs. 2 and 3). To {10} use Bates' term they "mimicked" the
Ithomiines, and he set to work to devise an explanation of how this could
have come about. The _Origin of Species_ had just appeared and it was
natural that Bates should seek to interpret this peculiar phenomenon on the
lines there laid down. How was it that these Pierines had come to depart so
widely from the general form of the great bulk of their relations, and to
mimic so closely in appearance species belonging to an entirely different
group, while at the same time conserving the more deeply seated anatomical
features of their own family? If the change was to be regarded as having
come about through the agency of natural selection it must clearly be of
advantage to the mimicking forms; otherwise natural selection could not
come into operation. What advantage then have the Ithomiines over the
majority of butterflies in those parts? They are small insects, rather
flimsy in build, with comparatively weak powers of flight, and yet so
conspicuously coloured that they can hardly be mistaken for anything else.
In spite of all this they are little subject to the attacks of enemies such
as birds, and Bates attributed this to the fact that the juices of their
bodies are unpalatable. According to him their striking and conspicuous
pattern is of the nature of a warning coloration, advertising their
disagreeable properties to possible enemies. A bird which had once
attempted to eat one would find it little to its taste. It would
thenceforward associate the conspicuous pattern with a disagreeable flavour
{11} and in future leave such butterflies severely alone. The more
conspicuous the pattern the more readily would it be noticed by the enemy,
and so it would be of advantage to the Ithomiine to possess as striking a
pattern as possible. Those butterflies shewing a tendency to a more
conspicuous pattern would be more immune to the attacks of birds and so
would have a better chance of leaving progeny than those with a less
conspicuous pattern. In this way variations in the direction of greater
conspicuousness would be accumulated gradually by natural selection, and so
would be built up in the Ithomiine the striking warning coloration by which
it advertises its disagreeable properties. Such is the first step in the
making of a mimicry case--the building up through natural selection of a
conspicuous pattern in an unpalatable species by means of which it is
enabled to advertise its disagreeable properties effectively and thereby
secure immunity from the attacks of enemies which are able to appreciate
the advertisement. Such patterns and colours are said to be of a "warning"
nature. The existence of an unpalatable model in considerable numbers is
the first step in the production of a mimetic resemblance through the
agency of natural selection.
We come back now to our Pierine which must be assumed to shew the general
characters and coloration of the family of whites to which they belong (cf.
Pl. X, fig. 1). Theoretically they are not specially protected by nauseous
properties from enemies and hence their conspicuous white coloration
renders {12} them especially liable to attack. If, however, they could
exchange their normal dress for one resembling that of the Ithomiines it is
clear that they would have a chance of being mistaken for the latter and
consequently of being left alone. Moreover, in certain cases these Pierines
_have_ managed to discard their normal dress and assume that of the
Ithomiines. On theoretical grounds this must clearly be of advantage to
them, and being so might conceivably have arisen through the operation of
natural selection. This indeed is what is supposed to have taken place on
the theory of mimicry. Those Pierines which exhibited a variation of colour
in the direction of the Ithomiine "model" excited distrust in the minds of
would-be devourers, who had learned from experience to associate that
particular type of coloration with a disagreeable taste. Such Pierines
would therefore have a rather better chance of surviving and of leaving
offspring. Some of the offspring would exhibit the variation in a more
marked degree and these again would in consequence have a yet better chance
of surviving. Natural selection would encourage those varying in the
direction of the Ithomiine model at the expense of the rest and by its
continuous operation there would gradually be built up those beautiful
cases of resemblance which have excited the admiration of naturalists.
Wallace was the next after Bates to interest himself in mimicry and, from
his study of the butterflies of the Oriental region[4], shewed that in this
part of {13} the world too there existed these remarkable resemblances
between species belonging to different families. Perhaps the most important
part of Wallace's contribution was the demonstration that in some species
not only was it the female alone that "mimicked" but that there might be
several different forms of female mimicking different models, and in some
cases all unlike the male of their own species. One of the species studied
by Wallace, _Papilio polytes_, is shewn on Plate V. We shall have occasion
to refer to this case later on, and it is sufficient here to call attention
to the three different forms of female, of which one is like the male while
the other two resemble two other species of _Papilio_, _P. hector_ and _P.
aristolochiae_, which occur in the same localities. Instances where the
female alone of some unprotected species mimics a model with obnoxious
properties are common in all tropical countries. It has been suggested that
this state of things has come about owing to the greater need of protection
on the part of the female. Hampered by the disposal of the next generation
the less protected female would be at a greater disadvantage as compared
with the mimic than would the corresponding male whose obligations to
posterity are more rapidly discharged. The view of course makes the
assumption that the female transmits her peculiar properties to her
daughters but not to her sons.
A few years later Trimen[5] did for Africa what Bates had done for America
and Wallace for {14} Indo-Malaya. It was in this paper that he elucidated
that most remarkable of all cases of mimicry--_Papilio dardanus_ with his
harem of different consorts, all tailless, all unlike himself, and often
wonderfully similar to unpalatable forms found in the same localities (cf.
p. 30).
We may now turn to one of the most ingenious developments of the theory of
mimicry. Not long after Bates' original memoir appeared attention was
directed to a group of cases which could not be explained on the simple
hypothesis there put forward. Many striking cases of resemblance had been
adduced in which both species obviously belonged to the presumably
unpalatable groups. Instances of the sort had been recorded by Bates
himself and are perhaps most plentiful in South America between species
belonging respectively to the Ithomiinae and Heliconinae. On the theory of
mimicry all the members of both of these groups must be regarded as
specially protected owing to their conspicuous coloration and distasteful
properties. What advantage then can an Ithomiine be supposed to gain by
mimicking a Heliconine, or _vice versâ_? Why should a species exchange its
own bright and conspicuous warning pattern for one which is neither
brighter nor more conspicuous? To Fritz Müller, the well-known
correspondent of Darwin, belongs the credit of having suggested a way out
of the difficulty. Müller's explanation turns upon the education of birds.
Every year there hatch into the world fresh generations of young birds, and
each {15} generation has to learn afresh from experience what is pleasant
to eat and what is not. They will try all things and hold fast to that
which is good. They will learn to associate the gay colours of the
Heliconine and the Ithomiine with an evil taste[6] and they will
thenceforward avoid butterflies which advertise themselves by means of
these particular colour combinations. But in a locality where there are
many models, each with a different pattern and colour complex, each will
have to be tested separately before the unpalatableness of each is
realised. If for example a thousand young birds started their education on
a population of butterflies in which there were five disagreeable species,
each with a distinct warning pattern, it is clear that one thousand of each
would devote their lives to the education of these birds, or five thousand
butterflies in all[7]. But if these five species, instead of shewing five
distinct warning patterns, all displayed the same one it is evident that
the education of the birds would be accomplished at the price of but one
thousand butterfly existences instead of five. Even if one of the five
species were far more abundant than the others it would yet be to its
advantage that the other four should exhibit the same warning pattern. Even
though the losses were distributed _pro rata_ the more abundant species
would profit to some extent. For {16} the less abundant species the gain
would of course be relatively greater. Theoretically therefore, all of the
five species would profit if in place of five distinct warning patterns
they exhibited but a single one in common. And since it is profitable to
all concerned what more natural than that it should be brought about by
natural selection?
Müller's views are now widely accepted by students of mimicry as an
explanation of these curious cases where two or more evidently distasteful
species closely resemble one another. Indeed the tendency in recent years
has been to see Müllerian mimicry everywhere, and many of the instances
which were long regarded as simple Batesian cases have now been relegated
to this category. The hypothesis is, of course, based upon what appears to
man to be the natural behaviour of young birds under certain conditions. No
one knows whether young birds actually do behave in the way that they are
supposed to. In the absence of any such body of facts the Müllerian
hypothesis cannot rank as more than a plausible suggestion, and, as will
appear later, it is open to severe criticism on general grounds.
Perhaps the next contribution to the subject of mimicry which must rank of
the first importance was that of Erich Haase[8], to whose book students of
these matters must always be under a heavy obligation. It was the first and
still remains the chief work of general scope. Since Haase's day great
numbers of {17} fresh instances of mimetic resemblance have been recorded
from all the great tropical areas of the world, and the list is being added
to continually. Most active in this direction is the Oxford School under
Professor Poulton to whose untiring efforts are largely due the substantial
increases in our knowledge of African butterflies contributed by various
workers in the field during the past few years. Whatever the interpretation
put upon them, there can be no question as to the value of the facts
brought together, more especially those referring to the nature of the
families raised in captivity from various mimetic forms. With the
considerable additions from Africa[9] during the past few years several
hundreds of cases of mimicry must now have been recorded. Some of the best
known and most striking from among these will be described briefly in the
next two chapters.
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