It sometimes happens that in one and the same place there are two groups
of animals not very nearly related which are "doubles" of one another.
Investigation shows that the members of the one group, _always in the
majority_, are in some way specially protected, e.g. by being
unpalatable. They are the "mimicked." The members of the other group,
_always in the minority_, have not got the special protection possessed
by the others. They are the "mimickers," though the resemblance is not,
of course, associated with any conscious imitation. The theory is that
the mimickers live on the reputation of the mimicked. If the mimicked
are left alone by birds because they have a reputation for
unpalatability, or because they are able to sting, the mimickers
survive--although they are palatable and stingless. They succeed, not
through any virtue of their own, but because of their resemblance to the
mimicked, for whom they are mistaken. There are many cases of mimetic
resemblance so striking and so subtle that it seems impossible to doubt
that the thing works; there are other cases which are rather
far-fetched, and may be somewhat of the nature of coincidences. Thus
although Mr. Bates tells us that he repeatedly shot humming-bird moths
in mistake for humming-birds, we cannot think that this is a good
illustration of mimicry. What is needed for many cases is what is
forthcoming for some, namely, experimental evidence, e.g. that the
unpalatable mimicked butterflies are left in relative peace while
similar palatable butterflies are persecuted. It is also necessary to
show that the mimickers do actually consort with the mimicked. Some
beetles and moths are curiously wasplike, which may be a great
advantage; the common drone-fly is superficially like a small bee; some
harmless snakes are very like poisonous species; and Mr. Wallace
maintained that the powerful "friar-birds" of the Far East are mimicked
by the weak and timid orioles. When the model is unpalatable or
repulsive or dangerous, and the mimic the reverse, the mimicry is called
"Batesian" (after Mr. Bates), but there is another kind of mimicry
called Müllerian (after Fritz Müller) where the mimic is also
unpalatable. The theory in this case is that the mimicry serves as
mutual assurance, the members of the ring getting on better by
consistently presenting the same appearance, which has come to mean to
possible enemies a signal, _Noli me tangere_ ("Leave me alone"). There
is nothing out of the question in this theory, but it requires to be
taken in a critical spirit. It leads us to think of "warning colours,"
which are the very opposite of the disguises which we are now studying.
Some creatures like skunks, magpies, coral-snakes, cobras, brightly
coloured tree-frogs are obtrusive rather than elusive, and the theory
of Alfred Russel Wallace was that the flaunting conspicuousness serves
as a useful advertisement, impressing itself on the memories of
inexperienced enemies, who soon learn to leave creatures with "warning
colours" alone. In any case it is plain that an animal which is as safe
as a wasp or a coral-snake can afford to wear any suit of clothes it
likes.
[Illustration: DEAD-LEAF BUTTERFLY (_Kallima Inachis_) FROM INDIA
It is conspicuous on its upper surface, but when it settles down on a
twig and shows the underside of its wings it is practically invisible.
The colouring of the under surface of the wings is like that of the
withering leaf; there are spots like fungas spots; and the venation of
the wings suggests the mid-rib and veins of the leaf. A, showing upper
surface; B, showing under surface; C, a leaf.]
[Illustration: PROTECTIVE RESEMBLANCE BETWEEN A SMALL SPIDER (_to the
left_) AND AN ANT (_to the right_)
As ants are much dreaded, it is probably profitable to the spider to be
like an ant. It will be noted that the spider has four pairs of legs and
no feelers, whereas the ant has three pairs of legs and a pair of
feelers.]
[Illustration: _Photo: J. J. Ward, F.E.S._
THE WASP BEETLE, WHICH, WHEN MOVING AMONGST THE BRANCHES GIVES A
WASP-LIKE IMPRESSION]
[Illustration: HERMIT-CRAB WITH PARTNER SEA-ANEMONES
Hermit-crabs hide their soft tail in the shell of a whelk or some other
sea-snail. But some hermit-crabs place sea-anemones on the back of their
borrowed shell. The sea-anemones mask the hermit-crab and their
tentacles can sting. As for the sea-anemones, they are carried about by
the hermit-crab and they get crumbs from its table. This kind of
mutually beneficial external partnership is called commensalism, i.e.
eating at the same table.]
[Illustration: _Photo: G. P. Duffus._