SOME CRITICISMS
The facts related in the last two chapters are sufficient to make it clear
that these remarkable resemblances between species belonging as a rule to
widely different groups constitute a real phenomenon, and as such demand an
explanation. One explanation, that in terms of the theory of mimicry, has
already been outlined, and we may now turn to consider it in more detail.
Some years ago Wallace[24], combating the suggestion that these instances
of resemblance might be mere coincidences, laid down five conditions which
he stated were applicable to all such cases, and rendered utterly
inadequate any explanation other than in terms of natural selection. These
five conditions are of historical interest and may also serve as a peg for
sundry criticisms in connection with the mimicry theory. They are as
follows:
(1) That the imitative species occur in the same area and occupy the very
same station as the imitated.
(2) That the imitators are always the more defenceless. {51}
(3) That the imitators are always less numerous in individuals.
(4) That the imitators differ from the bulk of their allies.
(5) That the imitation, however minute, is _external_ and _visible_ only,
never extending to internal characters or to such as do not affect the
external appearance.
In offering certain criticisms of the mimicry explanation it will be
convenient to do so in connection with these five conditions which Wallace
regarded as constant for all cases of mimetic resemblance.
(1) _That the imitative species occur in the same area and occupy the very
same station as the imitated._
This on the whole is generally true. It is well shewn in some of the most
striking cases such as those of the Old-World Papilios that mimic Danaines,
or of the Dismorphias and their Ithomiine models. In many of these cases
the range of neither model nor mimic is a very wide one, yet the mimic is
found strictly inside the area inhabited by the model. _Papilio agestor_,
for instance, is only found where _Caduga tytia_ occurs, nor is _P. mendax_
known outside the area frequented by _Euploea rhadamanthus_. Even more
striking in this respect are some of the Ithomiine-Dismorphia resemblances
in the New World. The Ithomiine models are as a rule very local though very
abundant. Two hundred miles away the predominant Ithomiine often bears
quite a distinct pattern, and when this is the case the mimicking
_Dismorphia_ is generally changed in the same sense. {52} But though mimic
and model may be found together in the same locality, they do not always
occupy the same station in the sense that they fly together. According to
Seitz[25] the Dismorphias themselves do not fly with the Ithomiines which
they mimic. The occurrence of butterflies is largely conditioned by the
occurrence of the plants on which the larva feeds, and this is especially
true of the female, which, as has already been noticed, is more commonly
mimetic than the male. The female of _Papilio polytes_, for instance, is
found flying where are to be found the wild citronaceous plants on which
its larva feeds. On the other hand, its so-called models, _Papilio hector_
and _P. aristolochiae_, are generally in the proximity of the Aristolochias
on which their larvae feed. The two plants are not always found together,
so that one frequently comes across areas where _P. polytes_ is very
abundant while the models are scarce or absent.
Though in the great majority of cases the imitator and the imitated occur
in the same locality, this is not always so. The female of the Fritillary
_Argynnis hyperbius_ (Pl. IV, fig. 3), for instance, is exceedingly
difficult to distinguish from _Danais plexippus_ when flying, although when
at rest the difference between the two is sufficiently obvious. Both
insects are plentiful in Ceylon but inhabit different stations. The Danaid
is a low-country insect, while the Fritillary is not found until several
thousand feet up. The two species affect entirely different stations and
hardly {53} come into contact with each other. Where one is plentiful the
other is not found. It has been suggested that migratory birds may have
come into play in such cases. The bird learns in the low country that _D.
plexippus_ is unpleasant, and when it pays a visit to the hills it takes
this experience with it and avoids those females of the Fritillary which
recall the unpleasant Danaine.
Migratory birds have also been appealed to in another case where the
resembling species are even further removed from one another than in the
last case. _Hypolimnas misippus_ is common and widely spread over Africa
and Indo-Malaya, and the male (Pl. IV, fig. 8) bears a simple and
conspicuous pattern--a large white spot bordered with purple on each of the
very dark fore and hind wings. The same pattern occurs in the males of two
other Nymphalines allied to _H. misippus_, viz. _Athyma punctata_ and
_Limenitis albomaculata_. The two species, however, have a distribution
quite distinct from that of _H. misippus_, being found in China. It has
nevertheless been suggested by Professor Poulton[26] that the case may yet
be one of mimicry. According to his explanation, _H. misippus_ is
unpalatable, the well-known association of its female with _Danais
chrysippus_ being an instance of Müllerian mimicry. Migratory birds did the
rest. Having had experience of _H. misippus_ in the south, on their arrival
in China they spared such specimens of _Athyma punctata_ and _Limenitis_
{54} _albomaculata_ as approached most nearly to _H. misippus_ in pattern,
and so brought about the resemblance. The explanation is ingenious, but a
simpler view will probably commend itself to most. Other cases are known in
which two butterflies bear a close resemblance in pattern and yet are
widely separated geographically. Several species of the S. American
Vanessid genus _Adelpha_ are in colour scheme like the African _Planema
poggei_ which serves as a model for more than one species. The little S.
American _Phyciodes leucodesma_ would almost certainly be regarded either
as a model for or a mimic of the African _Neptis nemetes_, did the two
occur together. Nevertheless examples of close resemblance between
butterflies which live in different parts of the world are relatively rare
and serve to emphasise the fact that the great bulk of these resemblance
cases are found associated in pairs or in little groups.
(2) _That the imitators are always the more defenceless._
In the case of butterflies "defence" as a rule denotes a disagreeable
flavour rendering its possessor distasteful to birds and perhaps to other
would-be devourers. Feeding experiments with birds (cf. Chapter IX) suggest
that certain groups of butterflies, notably the Danaines, Acraeines,
Heliconines, Ithomiines and Pharmacophagus Papilios--groups from which
models are generally drawn--are characterised by a disagreeable taste,
while as a rule this is not true for the mimics. This distasteful quality
is frequently accompanied by a more or less conspicuous type of coloration,
{55} though this is by no means always so. Many Euploeas are sombre
inconspicuous forms, and it is only some of the Ithomiines that sport the
gay colours with which that group is generally associated. The members of
the distasteful groups usually present certain other peculiarities. Their
flight is slower, they are less wary, their bodies are far tougher, and
they are more tenacious of life. The slow flight is regarded as an
adaptation for exhibiting the warning coloration to the best advantage, but
from the point of view of utility it is plausible to suggest that the
insect would be better off if in addition to its warning coloration it
possessed also the power of swift flight[27]. It is possible that the
peculiar slowness of flight of these unpalatable groups is necessitated by
the peculiar tough but elastic integument which may present an
insufficiently firm and resistant skeletal basis for sharp powerful
muscular contraction, and so render swift flight impossible. It is stated
that the flight of the mimics is like that of the model, and in some cases
this is undoubtedly true. But in a great many cases it certainly does not
hold good. _Papilio clytia_ (Pl. I, figs. 7 and 8) is a strong swift flyer
very unlike the Danaine and Euploeine which it is supposed to mimic. The
flight of the female of _Hypolimnas misippus_ (Pl. IV, fig. 7) is quite
distinct from that of _Danais chrysippus_, while the mimetic {56} forms of
_P. polytes_ fly like the non-mimetic one, a mode of flight so different
from that of the two models that there is no difficulty in distinguishing
them many yards away. Swift flight must be reckoned as one of the chief
modes of defence in a butterfly, and on this score the mimic is often
better off than the model. And of course it must not be forgotten that
where the mode of flight is distinct the protective value of the
resemblance must be very much discounted.
(3) _That the imitators are always less numerous in individuals._
In the majority of cases this is certainly true. Probably all the Old-World
Papilios that mimic Danaines are scarcer, and frequently very much scarcer,
than their models. This is very evident from a study of the more
comprehensive priced catalogues of Lepidoptera. The mimic is generally a
more expensive insect than the model, and not infrequently it costs as many
pounds as the model does shillings. But the rule is not universal. _Papilio
polytes_ is often much more common than either of its models. The
remarkable Pierines, _Archonias tereas_ and _A. critias_ (Pl. XI, fig. 10)
as a rule far outnumber the Pharmacophagus Swallow-tail which they mimic.
Or again the Chalcosid moth _Callamesia pieridoides_[28] is a more abundant
insect than the Bornean Pierine _Delias cathara_ which it closely
resembles.
It has sometimes been suggested in explanation {57} of the greater
abundance of the mimic that in such cases we are concerned with Müllerian
mimicry, that since both of the species concerned are distasteful there is
not, strictly speaking, either a mimic or a model, and consequently the
relative proportions have not the significance that they possess where the
mimicry is of the simple Batesian type. It is, however, very doubtful
whether such an explanation is of any value, for, as will appear later,
there are grave objections to accepting the current theory as to the way in
which a resemblance is established on Müllerian lines (cf. pp. 72-74).
(4) _That the imitators differ from the bulk of their allies._
What importance we attach to this condition must depend upon our
interpretation of the word "allies"--whether, for example, we use it for a
small group of closely connected species, for a genus, for a group of
genera, or in an even wider sense. Perhaps an example will serve to make
the difficulty more clear. As already noticed, the S. American genus
_Dismorphia_ belongs to the family of Pieridae or "whites." Also certain
species of _Dismorphia_ bear a close resemblance to certain species of
Ithomiines, a noteworthy example being _D. praxinoe_ and _Mechanitis
saturata_ (Pl. X, figs. 3 and 7), in which the pattern, colour, and shape
of the two species are all far removed from what is usually understood by a
"white." It must not be forgotten, however, that these matters are
generally discussed by European {58} naturalists who have grown up in a
region where the majority of the "whites" are more or less white. For this
reason the statement that _D. praxinoe_ differs from the bulk of its allies
is likely to meet with general acceptance, especially as some of the
species of the genus itself (e.g., _D. cretacea_, Pl. X, fig. 1) are
regular whites in appearance. But when we come to look at the genus
_Dismorphia_ as a whole the matter assumes another complexion. Seitz[29]
recognises 75 species of which about a dozen are predominantly white. The
rest present a wonderful diversity of colour and pattern. Black
predominates on the fore wings, and the insect is frequently marked with
gay patches of yellow, bright brown, scarlet, or blue. Forms which from
their colour are clearly not mimics present nevertheless the general
pattern and shape of other forms which bear a strong resemblance to some
Ithomiine. Sometimes a change of colour in certain patches from blue or
yellow to bright brown would make all the difference between a
non-imitative and an imitative species. Moreover, the non-imitative forms
frequently have the peculiar narrow wing, so unusual in a Pierine, which
enhances the resemblance of the mimicking species to the Ithomiine model,
and which to some extent occurs even in _D. cretacea_. Clearly we are not
justified in saying that _D. praxinoe_ differs from the bulk of its allies,
for inside the genus there are many non-imitative species which differ {59}
from it in some particulars and are alike it in others. There is a distinct
family resemblance among the bulk of the Dismorphias, including practically
all the mimetic forms, and on the whole the resemblances between the
imitative and the non-imitative forms are as noteworthy as the differences.
Though not exhibited in so striking a fashion, the same is to a large
extent true of a large proportion of the cases of mimicry. It is on the
whole unusual to find cases where a single species departs widely from the
pattern scheme of the other members of the genus and at the same time
resembles an unrelated species. Two of the best instances are perhaps those
of _Limenitis archippus_ (p. 49) and of the Pierid _Pareronia_ (p. 23). Of
the total number of mimicry instances a high proportion is supplied by
relatively few groups. In each region several main series of models and
mimics run as it were parallel to one another. In Asia, for example, we
have the Papilio-Danaine series where the colour-patterns of a series of
Danaines, all nearly related, are closely paralleled by those of a section
of the genus _Papilio_, and by those of the Satyrid genus _Elymnias_. In
Africa there is a similar Papilio-Danaine series though of less extent.
Africa has a group of models not found in Asia, and the Papilio-Danaine
series is as it were curtailed by the Papilio-Planema series with which to
some extent runs parallel the genus _Pseudacraea_. These phenomena of
parallel series have been mentioned here as shewing that mimicry tends to
run in certain groups and that in many cases at {60} any rate little
meaning can be attached to the statement that the imitators differ from the
bulk of their allies.
The fifth of Wallace's conditions is clear and needs no discussion.
It is evident that at any rate a large proportion of the instances of close
resemblance do not fulfil all of the conditions laid down by Wallace.
Nevertheless we should expect them to do so if the resemblance has been
brought about by the cumulative effect of natural selection on small
favourable variations. Clearly there is a _prima facie_ case for doubting
whether we must of necessity ascribe all resemblance of the kind to natural
selection, and in the next few chapters we shall discuss it in more detail
from several points of view.
* * * * *
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