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Title: Mimicry in Butterflies
Author: Punnett, Reginald Crundall, 1875-
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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Transcriber's note: The conventional male and female symbols are rendered
as [M] and [F] in this edition. In the discussion of _Papilio polytes_
the male symbol [M] must be distinguished carefully from the unbracketted
M denoting the male-like variety of females.

       *       *       *       *       *


  CAMBRIDGE UNIVERSITY PRESS
  C. F. CLAY, MANAGER
  LONDON: FETTER LANE, E.C.
  EDINBURGH: 100 PRINCES STREET

[Illustration]

  NEW YORK: G. P. PUTNAM'S SONS
  BOMBAY, CALCUTTA AND MADRAS: MACMILLAN AND CO.
  TOKYO: THE MARUZEN-KABUSHIKI-KAISHA



_All rights reserved_



MIMICRY IN BUTTERFLIES



BY

REGINALD CRUNDALL PUNNETT, F.R.S.

  Fellow of Gonville and Caius College.
  Arthur Balfour Professor of Genetics in the University of Cambridge



  Cambridge:
  at the University Press
  1915

       *       *       *       *       *


PREFACE

This little book has been written in the hope that it may appeal to several
classes of readers.

Not infrequently I have been asked by friends of different callings in life
to recommend them some book on mimicry which shall be reasonably short,
well illustrated without being very costly, and not too hard to understand.
I have always been obliged to tell them that I know of nothing in our
language answering to this description, and it is largely as an attempt to
remedy this deficiency that the present little volume has been written.

I hope also that it will be found of interest to those who live in or visit
tropical lands, and are attracted by the beauty of the butterfly life
around them. There are few such countries without some of these cases of
close resemblance between butterflies belonging to different families and
groups, and it is to those who have the opportunity to be among them that
we must look for fuller light upon one of the most fascinating of all
nature's problems. If this little book serves to smooth the path of some
who would become acquainted with that problem, and desire to use their
opportunities of observation, the work that has gone to its making will
have been well repaid.

To those who cultivate biological thought from the more philosophical point
of view, I venture to hope that what I have written may not be without
appeal. At such a time as the present, big with impending changes in the
social fabric, few things are more vital than a clear conception of the
scope and workings of natural selection. Little enough is our certain
knowledge of these things, and small though the butterfly's contribution
may be I trust that it will not pass altogether unregarded.

In conclusion I wish to offer my sincere thanks to those who have helped me
in different ways. More especially are they due to my friends Dr Karl
Jordan for the loan of some valuable specimens, and to Mr T. H. Riches for
his kindly criticism on reading over the proof-sheets.

R. C. P.

  _February, 1915_

CONTENTS

  CHAP.                                                 PAGE
     I. INTRODUCTORY                                       1
    II. MIMICRY--BATESIAN AND MÜLLERIAN                    8
   III. OLD-WORLD MIMICS                                  18
    IV. NEW-WORLD MIMICS                                  37
     V. SOME CRITICISMS                                   50
    VI. "MIMICRY RINGS"                                   61
   VII. THE CASE OF _Papilio polytes_                     75
  VIII. THE CASE OF _Papilio polytes_ (_cont._)           93
    IX. THE ENEMIES OF BUTTERFLIES                       104
     X. MIMICRY AND VARIATION                            125
    XI. CONCLUSION                                       139
        APPENDIX I                                       154
        APPENDIX II                                      157
        PLATES I-XVI AND DESCRIPTIONS                    160 ff
          I-V.  ORIENTAL MOTHS AND BUTTERFLIES.
         VI-IX. AFRICAN BUTTERFLIES.
        X-XIII. SOUTH AMERICAN BUTTERFLIES.
          XIV.  SCALES OF LEPIDOPTERA.
          XV.   CENTRAL AND SOUTH AMERICAN BUTTERFLIES.
          XVI.  NORTH AMERICAN BUTTERFLIES.
        INDEX                                            183

    "The process by which a mimetic analogy is brought about in nature is a
    problem which involves that of the origin of all species and all
    adaptations."--H. W. BATES, 1861.

    "With mimesis, above all, it is wise, when the law says that a thing is
    black, first to inquire whether it does not happen to be white."--HENRI
    FABRE.

       *       *       *       *       *


{1}

CHAPTER I

INTRODUCTORY

It is now more than fifty years since Darwin gave the theory of natural
selection to the world, and the conception of a gradual evolution has long
ago become part of the currency of thought. Evolution for Darwin was
brought about by more than one factor. He believed in the inherited effects
of the use and disuse of parts, and he also regarded sexual selection as
operating at any rate among the higher animals. Yet he looked upon the
natural selection of small favourable variations as the principal factor in
evolutionary change. Since Darwin's time the trend has been to magnify
natural selection at the expense of the other two factors. The doctrine of
the inherited effects of use and disuse, vigorously challenged by Weismann,
failed to make good its case, and it is to-day discredited by the great
majority of biologists. Nor perhaps does the hypothesis of sexual selection
command the support it originally had. At best it only attempted to explain
those features, more especially among the higher animals, in which the
sexes differ from one another in pattern, ornament, and the like. With the
lapse of time there has come about a tendency to {2} find in natural
selection alone a complete explanation of the process of evolution, and to
regard it as the sole factor by which all evolutionary change is brought
about. Evolution on this view is a gradual process depending upon the slow
accumulation by natural selection of small variations, which are more or
less inherited, till at last a well-marked change of type is brought about.
Could we have before us all the stages through which a given form has
passed as natural selection transforms it into another, they would
constitute a continuous series such that even refined scrutiny might fail
to distinguish between any two consecutive terms. If the slight variations
are not of service they will get no favour from natural selection and so
can lead to nothing. But if of use in the struggle for existence natural
selection preserves them and subsequent variations in the same direction
until at length man recognises the accumulation as a new form. Moreover
when the perfect thing is once elaborated natural selection will keep it
perfect by discouraging any tendency to vary from perfection.

Upon this view, of which the most distinguished protagonist was Weismann,
natural selection is the sole arbiter of animal and plant form. Through it
and it alone the world has come to be what it is. To it must be ascribed
all righteousness, for it alone is the maker. Such in its extreme form is
the modern development of Darwin's great contribution to philosophy.

But is it true? Will natural selection really serve to explain all? Must
all the various characters of {3} plants and animals be supposed to owe
their existence to the gradual operation of this factor working upon small
variations?

Of recent years there has arisen a school of biologists to whom the terms
mutationist and Mendelian are frequently applied. Influenced by the
writings of Bateson and de Vries, and by the experimental results that have
flowed from Mendel's discovery in heredity, they have come to regard the
process of evolution as a discontinuous one. The new character that
differentiates one variety from another arises suddenly as a sport or
mutation, not by the gradual accretion of a vast number of intermediate
forms. The white flowered plant has arisen suddenly from the blue, or the
dwarf plant from the tall, and intermediates between them need never have
existed. The ultimate fate of the new form that has arisen through causes
yet unknown may depend upon natural selection. If better endowed than the
parent form in the struggle for existence it may through natural selection
come to supplant it. If worse endowed natural selection will probably see
to its elimination. But if, as may quite possibly happen, it is neither
better nor worse adapted than the form from which it sprang, then there
would seem to be no reason for natural selection having anything to do with
the relation of the new form to its parent.

Between the older and the newer or mutationist point of view an outstanding
difference is the rôle ascribed to natural selection. On the one view it
{4} builds up the new variety bit by bit, on the other the appearance of
the new variety is entirely independent of it. From this there follows a
radical difference with regard to the meaning of all the varied characters
of plants and animals. Those who uphold the all-powerfulness of natural
selection are bound to regard every character exhibited by an animal or
plant as of service to it in the struggle for existence. Else it could not
have arisen through the operation of natural selection. In other words
every character in plant or animal must be adaptive. On the mutationist
view this of course does not follow. If the new character which arises
independently of natural selection is neither of service nor disservice to
its possessors in the struggle for existence, there seems no reason why it
should not persist in spite of natural selection. In attempting to decide
between the two conflicting views the study of adaptation is of the first
importance.

It was perhaps in connection with adaptation that Darwin obtained the most
striking evidence in support of his theory, and it is clear from his
writings that it was in this field he laboured with most delight. The
marvellous ways in which creatures may be adapted in structure and habit
for the life they lead had not escaped the attention of the older
naturalists. John Ray wrote a book[1] upon the subject in which he pointed
out that all things in the Universe, from the fixed stars to the structure
of a bird, or the tongue of {5} a chameleon, or the means whereby some
seeds are wind distributed, are "argumentative of Providence and Design"
and must owe their existence to "the Direction of a Superior Cause." Nor
have there been wanting other authors who have been equally struck by the
wonders of adaptation. But their studies generally led to the same
conclusion, an exhortation to praise the infinite Wisdom of Him Who in the
days of Creation had taken thought for all these things.

The advent of natural selection threw a new light upon adaptation and the
appearance of design in the world. In such books as those on _The
Fertilization of Orchids_ and _The Forms of Flowers_ Darwin sought to shew
that many curious and elaborate structures which had long puzzled the
botanist were of service to the plant, and might therefore have arisen
through the agency of natural selection. Especially was this the case in
orchids where Darwin was able to bring forward striking evidence in favour
of regarding many a bizarre form of flower as specially adapted for
securing the benefits of cross-fertilization through the visits of insects.
In these and other books Darwin opened up a new and fascinating field of
investigation, and thenceforward the subject of adaptation claimed the
attention of many naturalists. For the most part it has been an
observational rather than an experimental study. The naturalist is struck
by certain peculiarities in the form or colour or habits of a species. His
problem is to account for their presence, and as nearly all students of
adaptation have been close {6} followers of Darwin, this generally means an
interpretation in terms of natural selection. Granted this factor it
remains to shew that the character in question confers some advantage upon
the individuals that possess it. For unless it has a utilitarian value of
some sort it clearly cannot have arisen through the operation of natural
selection. However when it comes to the point direct proof of this sort is
generally difficult to obtain. Consequently the work of most students of
adaptation consists in a description of the character or characters studied
together with such details of its life-history as may seem to bear upon the
point, and a suggestion as to how the particular character studied _may_ be
of value to its possessors in the struggle for existence. In this way a
great body of most curious and interesting facts has been placed on record,
and many ingenious suggestions have been made as to the possible use of
this or that character. But the majority of workers have taken natural
selection for granted and then interested themselves in shewing how the
characters studied by them might be of use. Probably there is no structure
or habit for which it is impossible to devise some use[2], and the pursuit
has doubtless provided many of its devotees with a pleasurable and often
fascinating exercise of the imagination. So it has come about that the
facts {7} instead of being used as a test of the credibility of natural
selection, serve merely to emphasise the pæan of praise with which such
exercises usually conclude. The whole matter is too often approached in
much the same spirit as that in which John Ray approached it two centuries
ago, except that the Omnipotency of the Deity is replaced by the
Omnipotency of Natural Selection. The vital point, which is whether Natural
Selection _does_ offer a satisfactory explanation of the living world, is
too frequently lost sight of. Whether we are bound or not to interpret all
the phenomena of life in terms of natural selection touches the basis of
modern philosophy. It is for the biologist to attempt to find an answer,
and there are few more profitable lines of attack than a critical
examination of the facts of adaptation. Though "mimicry" is but a small
corner in this vast field of inquiry it is a peculiarly favourable one
owing to the great interest which it has excited for many years and the
consequently considerable store of facts that has been accumulated. If then
we would attempt to settle this most weighty point in philosophy there is
probably nothing to which we can appeal with more confidence than to the
butterfly.

       *       *       *       *       *


{8}

CHAPTER II

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.

       *       *       *       *       * {18}


CHAPTER III

OLD-WORLD MIMICS

The earlier naturalists who studied butterflies made use of colour and
pattern very largely in arranging and classifying their specimens. Insects
shewing the same features in these respects were generally placed together
without further question, especially if they were known to come from the
same locality. In looking through old collections of butterflies from the
tropics it is not infrequent to find that the collector was deceived by a
mimetic likeness into placing model and mimic together. During the last
century, however, more attention was paid to the anatomy of butterflies,
with the result that their classification was placed upon a basis of
structure. As in all work of the sort certain features are selected, partly
owing to their constancy and partly for their convenience, the insects
being arranged according as to whether they present these features or not.
Everybody knows that the butterflies as a group are separated from the
moths on the ground that their antennae are club shaped at the end, while
those of the moth are generally filamentary and taper to a fine point. {19}

[Illustration: Figs. 1-8. Terminal portion of front legs of butterflies
belonging to different families. (After Eltringham.)]

  1. _Hypolimnas misippus_,  [F]  (Nymphalidae).
  2.      "          "       [M]  (     "     ).
  3. _Abisara savitri_,      [F]  (Erycinidae).
  4.     "       "           [M]  (     "    ).
  5. _Lycaena icarus_,       [F]  (Lycaenidae).
  6. _Cupido zoë_,           [M]  (     "    ).
  7. _Ganoris rapae_,        [M]  (Pieridae).
  8. _Papilio echerioides_,  [F]  (Papilionidae).

{20} The butterflies themselves may be subdivided into five main groups or
families[10] according to the structure of the first of their three pairs
of legs. In the Papilionidae or "swallow-tails," the first pair of legs is
well developed in both sexes (Fig. 8). In the Pieridae or "whites," the
front legs are also similar in both sexes, but the claws are bifid and a
median process, the empodium, is found between them (Fig. 7). In the
remaining three families the front legs differ in the two sexes. The
females of the Lycaenidae or "blues" have well-developed front legs in
which the tarsus is terminated by definite claws (Fig. 5), whereas in the
males the terminal part of the leg, or tarsus, is unjointed and furnished
with but a single small claw (Fig. 6). This reduction of the front legs has
gone somewhat further in the Erycinidae (Figs. 3 and 4), a family
consisting for the most part of rather small butterflies and specially
characteristic of South America. In the great family of the Nymphalidae the
reduction of the front legs is well marked in both sexes. Not only are they
much smaller than in the other groups, but claws are lacking in the female
as well as in the male (Figs. 1 and 2).

Though the structure of the fore limbs is the character specially chosen
for separating these different families from one another, it is of course
understood that they differ from one another in various other distinctive
features. The chrysalis of the Nymphalidae for example hangs head downwards
suspended by the {21} tail, whereas in the Pieridae and Papilionidae
metamorphosis takes place with the chrysalis attached by the tail but
supported also by a fine girdle of silk round the middle so that the head
is uppermost. The larvae also afford characters by which some of the
families may be distinguished--those of the Papilionidae for example having
a process on the back which can be extruded or retracted.

Owing to the great size of the family of the Nymphalidae, in which the
number of species approaches 5000, it is convenient to deal with the eight
sub-groups into which it has been divided. The characters serving to mark
off the sub-groups from one another are various. Sometimes it is the
minuter structure of the tarsus, at others the form of the caterpillar or
the chrysalis, at others the arrangement of the nervures that form the
skeleton of the wing. Into these systematic details, however, we need not
enter more fully here[11]. What is important from the standpoint of mimicry
is that these divisions, made solely on anatomical structure, correspond
closely with the separation of models from mimics. Of the eight
sub-families into which the Nymphalidae are divided four, viz. the
Danainae, Acraeinae, Heliconinae, and Ithomiinae, provide models and some,
but far fewer, mimics; two, the Satyrinae and Nymphalinae, provide many
mimics and but few models, while two groups, the Morphinae and Brassolinae,
practically do not enter into the mimicry story. {22}

Simple mimicry, explicable, at any rate in theory, on the lines laid down
by Bates, is a phenomenon of not infrequent occurrence in tropical
countries, though rare in more temperate lands. In each of the three great
divisions of the tropical world we find certain groups of butterflies
serving as models, and being mimicked by butterflies belonging as a rule to
quite different groups. Speaking generally the models of any given region
are confined to a few groups, while the mimics are drawn from a greater
number. In Asia the principal models belong to the Danaines, the
Euploeines, and to a group of swallow-tails which from the fact that their
larvae feed on the poisonous _Aristolochia_ plant are generally
distinguished as the "Poison-eaters," or _Pharmacophagus_ group. Of these
the Danaines and Euploeines are closely related and have much in common.
They are usually butterflies of medium size, of rather flimsy build and
with a somewhat slow and flaunting flight. In spite, however, of their
slight build they are toughly made and very tenacious of life. Most
butterflies are easily killed by simply nipping the thorax. There is a
slight crack and the fly never recovers. But the collector who treats a
Danaid in a way that would easily kill most butterflies is as likely as not
many hours after to find it still alive in his collecting box or in the
paper to which it may have been transferred when caught. They give one the
impression of being tougher and more "rubbery" in consistence than the
majority of Lepidoptera. Moreover, the juices of their bodies seem {23} to
be more oily and less easily dried up. In general colour scheme they vary a
great deal. Some, such as _Danais chrysippus_ (Pl. IV, fig. 1), are
conspicuous with their bright fulvous-brown ground colour and the sharp
white markings on the black tips of their fore wings. Others again such as
_Danais septentrionis_ (Pl. I, fig. 3), with a dark network of lines on a
pale greenish ground, are not nearly so conspicuous. Of the Euploeines some
have a beautiful deep blue metallic lustre (cf. Pl. II, fig. 4), though
many are of a plain sombre brown relieved only by an inconspicuous border
of lighter markings (cf. Pl. I, fig. 10).

Both Danaines and Euploeines serve as models for a great variety of species
belonging to different groups. _Danais septentrionis_ (Pl. I, fig. 3) is a
very abundant species in India and Ceylon, and in the same region there are
several other very similar species. Flying with them in Northern India are
two species of _Papilio_, _P. macareus_ and _P. xenocles_ (Pl. I, fig. 4),
which resemble these Danaids fairly closely. In Southern India and Ceylon
one of the two forms of _Papilio clytia_ (Pl. I, fig. 7) is also regarded
as a mimic of these Danaids. In the same part of the world there is a
Pierine of the genus _Pareronia_, whose female is very like these Danaines
on the upper surface (Pl. I, fig. 1). The male of this Pierine is quite
distinct from the female (Pl. I, fig. 2).

The common _Danais chrysippus_ (Pl. IV, fig. 1), found in this region, has
been described as probably the most abundant butterfly in the world, and
serves {24} as a model for several species belonging to different groups.
It and its mimics will, however, be described in more detail later on.
Mention must also be made of the striking case of the Danaid, _Caduga
tytia_ and its Papilionine mimic _P. agestor_ from Sikkim (Pl. II, figs. 2
and 3). In both species the fore wings are pale blue broken by black; while
the hind wings are pale with a deep outer border of rusty red. Not only in
colour but also in shape the swallow-tail bears a remarkable resemblance to
the Danaid. _C. tytia_ is also mimicked by a rare Nymphaline _Neptis
imitans_, which exhibits the same striking colour scheme so very different
from that of most of its allies.

No less remarkable are some of the cases in which the Euploeines serve as
models. _E. rhadamanthus_, for example, is mimicked by the scarce _Papilio
mendax_, and a glance at Figs. 8 and 9 on Plate II shews how well this
butterfly deserves its name. _Euploea rhadamanthus_ also serves as a model
for one of the several forms of female of the Nymphaline species _Euripus
halitherses_. In some Euploeines the sexes are different in appearance--a
somewhat unusual thing among butterflies serving as models in cases of
mimetic resemblance. Such a difference is found in _Euploea mulciber_, the
male being predominantly brown with a beautiful deep blue suffusion, while
the female is a rather lighter insect with less of the blue suffusion and
with hind wings streaked with lighter markings (Pl. II, figs. 4 and 5). It
is interesting to find that _Elymnias malelas_, a Satyrid which mimics this
species, {25} shews a similar difference in the two sexes (Pl. II, figs. 6
and 7).

It is remarkable that similar sexual difference is also shewn by the rare
_Papilio paradoxus_, the two sexes here again mimicking respectively the
two sexes of _Euploea mulciber_.

Many of the Euploeines, more especially those from Southern India and
Ceylon, lack the blue suffusion, and are sombre brown insects somewhat
relieved by lighter markings along the hinder border of the hind wings.
_Euploea core_ (Pl. I, fig. 10), a very common insect, is typical of this
group. A similar coloration is found in one of the forms of _Papilio
clytia_ (Pl. I, fig. 8) from the same region as well as in the female of
the Nymphaline species _Hypolimnas bolina_ (Pl. I, fig. 6). The male of
this last species (Pl. I, fig. 5) is quite unlike its female, but is not
unlike the male of the allied species, _H. misippus_, which it resembles in
the very dark wings each with a white patch in the centre, the junction of
light and dark being in each case marked by a beautiful purple-blue
suffusion. There is also a species of _Elymnias_ (_E. singhala_) in this
part of the world which in general colour scheme is not widely dissimilar
from these brown Euploeas (Pl. I, fig. 9).

The third main group of models characteristic of this region belongs to the
Papilionidae. It was pointed out by Haase some 20 years ago that this great
family falls into three definite sections, separable on anatomical grounds
(see Appendix II). One of these sections he termed the _Pharmacophagus_ or
"poison-eating" {26} group owing to the fact that the larvae feed on the
poisonous climbing plants of the genus _Aristolochia_. It is from this
group that all Papilios which serve as models are drawn. No mimics of other
unpalatable groups such as Danaines are to be found among the Oriental
Poison-eaters. In the other two sections of the genus mimics are not
infrequent (cf. Appendix II), though probably none of them serve as models.
To the Pharmacophagus group belong the most gorgeous insects of
Indo-Malaya--the magnificent Ornithoptera, largest and most splendid of
butterflies. It is not a large proportion of the members of the group which
serve as models, and these on the whole are among the smaller and less
conspicuous forms. In all cases the mimic, when a butterfly, belongs to the
_Papilio_ section of the three sections into which Haase divided the family
(cf. Appendix II). _Papilio aristolochiae_ (Pl. V, fig. 5), for example, is
mimicked by a female form of _Papilio polytes_, and the geographical
varieties of this widely spread model are generally closely paralleled by
those of the equally wide spread mimic. For both forms range from Western
India across to Eastern China. Another poison-eater, _P. coon_, provides a
model for one of the females of the common _P. memnon_. It is curious that
in those species of the poison-eaters which serve as models the sexes are
practically identical in pattern, and are mimicked by certain females only
of the other two Papilio groups, whereas in the Ornithoptera, which also
belong to the poison-eaters, the difference between the sexes is
exceedingly striking. {27}

Though the Pharmacophagus Papilios are mimicked only by other Papilios
among butterflies they may serve occasionally as models for certain of the
larger day-flying moths. _Papilio polyxenus_, for example, is mimicked not
only by the unprotected _P. bootes_ but also by the moth _Epicopeia
polydora_ (Pl. III, figs. 5 and 6). Like the butterfly the _Epicopeia_,
which is comparatively rare, has the white patch and the outer border of
red marginal spots on the hind wing. Though it is apparently unable to
provide itself with an orthodox tail it nevertheless makes a creditable
attempt at one. There are several other cases of mimetic resemblance
between day-flying moths and Pharmacophagus swallow-tails--the latter in
each case serving as the model. Rarely it may happen that the rôle of
butterfly and moth is reversed, and the butterfly becomes the mimic. A very
remarkable instance of this is found in New Guinea where the rare _Papilio
laglaizei_ mimics the common day-flying moth _Alcidis agathyrsus_. Viewed
from above the resemblance is sufficiently striking (Pl. III, figs. 1 and
2), but the most wonderful feature concerns the underneath. The ventral
half of the moth's abdomen is coloured brilliant orange. When the wings are
folded back they cover and hide from sight only the dorsal part of the
abdomen, so that in this position the orange neutral surface is
conspicuous. When, however, the wings of the butterfly are folded they
conceal the whole of the abdomen. But the butterfly has developed on each
hind wing itself a bright orange patch in such a position that when the
{28} wings are folded back the orange patch lies over the sides of the
abdomen. In this way is simulated the brilliant abdomen of the moth by a
butterfly, in which, as in its relations, this part is of a dark and sombre
hue.

A few models are also provided in the Oriental region by the genus
_Delias_, which belongs to the Pierines. A common form, _Delias eucharis_,
is white above but the under surface of the hind wings is conspicuous with
yellow and scarlet (Pl. II, fig. 1). It has been suggested that this
species serves as a model for another and closely allied Pierine,
_Prioneris sita_, a species distinctly scarcer than the _Delias_. There is
some evidence that the latter is distasteful (cf. p. 115), but nothing is
known of the _Prioneris_ in this respect. Other species of _Delias_ are
said to function as models for certain day-flying moths belonging to the
family Chalcosiidae, which may bear a close resemblance to them. In certain
cases it may happen that the moth is more abundant than the Pierine that it
resembles[12].

Tropical Africa is probably more wealthy in mimetic analogies than
Indo-Malaya, and the African cases have recently been gathered together by
Eltringham in a large and beautifully illustrated memoir[13]. The principal
models of the region are furnished by the Danainae and the allied group of
the Acraeinae. Of the Danaines one well-known model, _Danais chrysippus_,
{29} is common to Africa and to Indo-Malaya. Common also to the two regions
are the mimics, _Argynnis hyperbius_ and _Hypolimnas misippus_ (cf. Pl. IV,
figs. 3 and 7). The case of the last named is peculiarly interesting
because it presents well-marked varieties which can be paralleled by
similar ones in _D. chrysippus_. In addition to the typical form with the
dark tipped fore wing relieved by a white bar there is in each species a
form uniformly brown, lacking both the dark tip and the white bar of the
fore wing. There is also another form in the two species in which the hind
wing is almost white instead of the usual brown shade. In both species,
moreover, the white hind wing may be associated either with the uniformly
brown fore wing or with the typical form. There is also another common
African butterfly, _Acraea encedon_, in which these different patterns are
closely paralleled (cf. Pl. IX). Several other species of butterflies and a
few diurnal moths bear a more or less close resemblance to _D. chrysippus_.

Danaine butterflies with the dark interlacing fines on a pale greenish-blue
ground, so characteristic of the Oriental region, are represented in Africa
by the species _Danais petiverana_ (Pl. VI, fig. 1) ranging across the
continent from Sierra Leone to British East Africa. A common Papilio, _P.
leonidas_ (Pl. VI, fig. 2) has a similar extensive range, and has been
regarded as a mimic of the Danaine. In S. Africa _P. leonidas_ is
represented by the variety _brasidas_ in which the white spots are reduced
and the blue-green ground is lacking. _Brasidas_ bears a strong resemblance
to the tropical {30} Danaine _Amauris hyalites_ (Pl. VI, fig. 3) of which
it has been regarded as a mimic. It must however be added that it is only
over a small part of their respective ranges, viz. in Angola, that the two
species are to be met with together.

The butterflies belonging to the genus _Amauris_ are among the most
abundant and characteristic Danaine models of Africa. Some of the black and
white species such as _A. niavius_ (Pl. VIII, fig. 6) are conspicuous
insects in a cabinet. Others again, such as _A. echeria_ (Pl. VIII, fig.
7), are relatively sombre-looking forms. Among the best known mimics of the
genus is a species of _Hypolimnas_[14]--_H. dubius_. This interesting form
is polymorphic and mimics different species of _Amauris_. The variety
_wahlbergi_, for example, is very like _A. niavius_, while _mima_ strongly
resembles _A. echeria_ (Pl. VIII, figs. 8 and 9). It was at one time
supposed that these two varieties of _Hypolimnas dubius_ were different
species and the matter was only definitely settled when the two forms were
bred from the eggs of the same female. Other mimics of _Amauris_ are found
among the Papilios and the Nymphaline genus _Pseudacraea_.

But among all the mimics of Danaines in Africa and elsewhere _Papilio
dardanus_ is pre-eminent, and has been described by more than one writer as
the most important case of mimicry in existence. Not only does it shew
remarkable resemblances to various {31} Danaids, but it presents features
of such peculiar interest that it must be considered in more detail.
_Papilio dardanus_ in its various sub-races is spread over nearly all the
African continent south of the Sahara. Over all this area the male, save
for relatively small differences, remains unchanged--a lemon-yellow insect,
tailed, and with black markings on fore and hind wings (Pl. VIII, fig. 1).
The female, however, exhibits an extraordinary range of variation. In South
Africa she appears in three guises, (1) the _cenea_ form resembling
_Amauris echeria_, (2) the _hippocoon_ form like _Amauris niavius_, and (3)
the _trophonius_ form which is a close mimic of the common _Danais
chrysippus_[15]. Except that _cenea_ does not occur on the West Coast these
three forms of female are found over almost all the great continental range
of _dardanus_ and its geographical races. Northwards in the latitude of
Victoria Nyanza occurs a distinct form of female, _planemoides_, which
bears a remarkable resemblance to the common and distasteful _Planema
poggei_, and is found only where the latter is abundant. All of these four
forms are close mimics of a common Danaine or Acraeine model. Other forms
of female, however, are known, of which two, _dionysus_ and _trimeni_, are
sufficiently distinct and constant to have acquired special names.
_Dionysus_ may be said to unite the fore wing of the _hippocoon_ form with
the hind wing of the _trophonius_ form, except that the colour of the last
part is yellow instead of {32} bright brown. It is a western form and is
unlike any model. _Trimeni_ also is unlike any model but is of peculiar
interest in that it is much more like the male with its pale creamy-yellow
colour and the lesser development of black scales than occurs in most of
the forms of female. At the same time the general arrangement of the darker
markings is on the whole similar to that in the _hippocoon_ and in the
_trophonius_ form. _Trimeni_ is found on the Kikuyu Escarpment, near Mt
Kenia, along with the four mimicking forms.

Continental Africa, south of the equator, has produced no female similar to
the male. But in Abyssinia is found another state of things. Here, so far
as is known, occur three forms, all tailed, of which one is similar in
general colour and pattern to the male, while the other two, _niavioides_
and _ruspina_[16], resemble respectively a tailed _hippocoon_ and a tailed
_trophonius_. Lastly we have to record that _Papilio dardanus_ is also
found as the geographical race _humbloti_ on Comoro Island, and as
_meriones_ on Madagascar. In both forms the females are tailed, and
resemble the males.

From this long series of facts it is concluded that the male of _P.
dardanus_ represents the original form of both sexes. On the islands of
Comoro and Madagascar this state of things still survives. But it is
supposed that on the African continent existed enemies which persecuted the
species more than on the islands {33} and encouraged the development of
mimetic forms in the female. The original female still lingers in Abyssinia
though it is now accompanied by the two mimetic forms _niavioides_ and
_ruspina_. Over the rest of the area occupied by _dardanus_ the females are
always tailless and, with the exception of _trimeni_ and _dionysus_,
wonderfully close mimics. _Trimeni_, the intermediate form, provides the
clue to the way in which the mimetic females have been derived from the
male, viz. by the prolongation across the fore wing of the dark costal bar
already found in the females of the Madagascar and Abyssinian races, by the
deepening of the dark edging to the wings, and by the loss of the tail.
Through the gradual accumulation of small variations _trimeni_ came from
the male-like female, and by further gradual accumulation of small
favourable variations the mimetic forms came from _trimeni_. South of the
equator the male-like form and the intermediate _trimeni_ have disappeared
owing to the stringency of selection being greater. Moreover the likeness
of mimic to model is closer than in the north, a further proof of the
greater stringency of natural selection in these parts. Such in brief is
the explanation in terms of mimicry of the remarkable and complex case of
_dardanus_.

Although the Euploeinae are not represented on the African continent, it is
the headquarters of another distasteful family of butterflies--the
Acraeinae--which is but sparingly represented in the Oriental region[17].
{34} Of smaller size than the Danaines they are characterised, like this
group, by their tenacity of life and by the presumably distasteful
character of their body juices. They are said also to possess an offensive
odour apparently exuded through the thorax. The majority of the members of
the group fall into the two genera _Acraea_ and _Planema_. Species of
Acraea are on the whole characterised by their general bright red-brown
colour and by the conspicuous black spots on both fore and hind wings. A
typical Acraeine pattern is that of _Acraea egina_ (Pl. VI, fig. 7) which
is mimicked remarkably closely by the Nymphaline _Pseudacraea boisduvali_
and by the Swallow-tail _Papilio ridleyanus_ (Pl. VI, figs. 5 and 6).

In the genus _Planema_ the spots are as a rule fewer and clustered near the
body, while on both fore and hind wings there is a tendency to develop
clear wide band-like areas of orange or white (cf. Pl. VII).

Like the Acraeas the Planemas are principally mimicked by species of
_Pseudacraea_ and of _Papilio_. Some of the cases of resemblance between
_Planema_ and _Pseudacraea_ are among the most striking known. _Planema
macarista_ is one of those comparatively rare instances in which a model
shews a marked difference in the pattern of the two sexes. The clear area
on the fore wing of the male is deep orange, whereas in the female it is
somewhat different in shape, and, like the area on the hind wing, is white
(cf. Pl. VII, figs. 1 and 2). {35} _Pseudacraea eurytus hobleyi_ (Pl. VII,
figs. 6 and 7) shews a similar difference in the sexes, the male and female
of this species mimicking respectively the male and female of _Planema
macarista_. The case is made even more remarkable by the fact that both of
the sexual forms of _Planema macarista_ are mimicked by the Satyrine
_Elymnias phegea_ (Pl. VII, fig. 9), though in this species either the
black and white, or the black, white, and orange form may occur in either
sex. Among the best Papilionine mimics of the Planemas is _Papilio cynorta_
whose female is extraordinarily like the common _Planema epaea_ (Pl. VII,
figs. 5 and 10). The resemblance of the _planemoides_ female of _P.
dardanus_ to _P. poggei_ has already been noticed.

A striking feature of the African continent is the frequency with which
mimetic forms are found among the Lycaenidae. As a rule the "blues" rarely
exhibit mimetic analogies, but in Africa there are several species,
especially those of the genus _Mimacraea_, which closely resemble
Acraeines. Others again bear a marked resemblance to certain small
Pierines, _Citronophila similis_ from S. Nigeria for example being
extraordinarily like the common _Terias brigitta_, a small bright yellow
Pierine with black-edged wings.

A remarkable feature of the African continent is the absence of the
Pharmacophagus Swallow-tails. Of such Papilios as exhibit mimicry, and as
compared with the total number of the group present the proportion is
large, the majority resemble one or other {36} of the characteristic
Danaines, while a few such as _P. ridleyanus_ and _P. cynorta_ resemble
either an Acraeoid or a Planemoid model.

As in the Oriental region the African Pierines do not offer many instances
of mimetic analogies. The genus _Mylothris_, in which certain species are
characterised by orange patches at the bases of the undersurfaces of the
fore wings, is regarded by some authors as providing models for allied
genera such as _Belenois_ and _Phrissura_. But as neither models nor mimics
offer a marked divergence in appearance from the ordinary Pierine facies it
is doubtful whether much stress can be laid on these cases.

Africa also offers a few striking instances of mimicry in which day-flying
moths play a part. The conspicuous Geometer _Aletis helcita_ is an abundant
form, and with its strong red colour and black wing margins broken by white
it is a striking object in the preserved state. Among the forms which bear
a close resemblance to it are the Nymphaline _Euphaedra ruspina_, and the
Lycaenid _Telipna sanguinea_[18].

       *       *       *       *       * {37}


CHAPTER IV

NEW-WORLD MIMICS

Of all the continents South America affords the greatest wealth of
butterfly life, and it is in the tropical part of this region that many of
the most beautiful and striking cases of mimicry are to be found. Viewed as
a whole the butterfly population presents several features which serve to
mark it off from that of the other two great tropical areas. In the first
place the proportion of gaily coloured forms is higher. Bright red, yellow
or fulvous brown contrasted with some deep shade approaching black form the
dominant notes. Sombre coloured species are relatively scarcer than in the
Oriental and African regions. In the second place when looking over
collections from this part of the world one cannot help being struck by the
frequency with which similar colour combinations occur over and over again
in different as well as in the same groups. Now it is a simple scheme of
black with an oblique scarlet band upon the fore wings--now an arrangement
with alternating stripes of bright brown and black relieved with patches of
clear yellow--now again a scheme of pure transparency and black. {38} Gay
and pleasing as are the designs turned out the palette is a small one and
invention is circumscribed. Under such conditions it might well be supposed
that instances of close resemblance between different species would be
numerous, and this in effect is what we find.

As in Asia with its Euploeines and Danaines, and in Africa with its
Danaines and Acraeines, so in S. America are the fashions set by two
dominant groups of models. These are the Heliconinae and the Ithomiinae,
both peculiar to this region and both characterised, like the Old-world
Danaids, by slow flight and great tenacity of life. Both live on poisonous
plants--the Heliconines on Passifloras and the Ithomiines on Solanaceae. In
both groups, but more especially in the Ithomiinae, the species are
numerous, and the number of individuals in a species often beyond
computation. From the point of view of mimicry these two groups have so
much in common that they may conveniently be considered together.

It was from among the Ithomiines, as already pointed out, that the models
came for the Pierine mimics of the genus _Dismorphia_ upon which Bates
founded the theory of mimicry. Though the Pierine mimics are the most
striking the Heliconines and Ithomiines are mimicked by members of other
groups. A few Papilios (Pl. X, fig. 8), certain Nymphalines such as
_Protogonius_ (Pl. X, fig. 9), _Eresia_, _Phyciodes_ and _Colaenis_ (Pl.
XI, fig. 4), together with various day-flying moths, more particularly of
the genera {39} _Castnia_ and _Pericopis_, are among the well-known mimics
of this group of models. The models themselves are very variable in
appearance. In one locality the predominant pattern is black with a warm
red-brown diagonal bar occupying rather more than a third of the fore wing
(Pl. XV, fig. 5), in another it consists of parallel bands of black and
fulvous brown with clear yellow patches at the tips of the fore wings (cf.
Pl. X, fig. 7), while in yet another locality it is different again.
Different localities often have their own peculiar pattern and this affects
the various mimics as well as the Ithomiine and Heliconine models.

These groups of different species, some belonging to palatable and some to
unpalatable groups, all exhibiting a close resemblance in colour and
pattern, are far more strikingly developed in S. America than in either
Asia or Africa, and it is not uncommon for eight or ten species to enter
into such an association. A group of this sort which possesses unusual
interest is the so-called "Transparency Group" from certain parts of the
Amazon region. It was originally described by Bates with seven species
belonging to six different genera. To-day it is said that no less than 28
species of this peculiar facies are known, though some are excessively
rare. The majority are Ithomiines, but two species of the Danaine genus
_Ituna_, the Pierine _Dismorphia orise_ (Pl. XII, fig. 2), the Swallow-tail
_Papilio hahneli_, and several species of diurnal moths belonging to
different families (cf. Pl. XII, fig. 4) also enter into the combination.
{40} In connection with it there is a feature of peculiar interest in that
the transparent effect is not always produced in the same way. In the
Ithomiines such as _Thyridia_, where there are normally two kinds of
scales, the wider ones for the most part lose their pigment, become much
reduced in size and take on the shape of a stumpy V (Pl. XIV, fig. 3). Also
they stand out for the most part more or less at right angles to the
wing[19], and the neck by which they are joined to the wing membrane is
very short. The longer and narrow form of scales also tend to lose their
pigment and become reduced to fine hairs. In _Dismorphia_ the scales, which
are of one sort, are also reduced in size though apparently not in number.
Like the wider scales of the _Thyridia_ they tend sometimes to project at
right angles to the wing membrane, though not to the same extent as in the
Ithomiine: possibly because the neck of the scale is not so short. As in
_Thyridia_ these reduced scales lose their pigment except in the transition
region round the borders of the transparent patches. In _Ituna_ there is a
difference. The scales are not reduced to the same extent in point of size.
Their necks are longer as in normal scales and they lie flat on the wing
membrane. The majority of the scales, as in the preceding cases, lose their
pigment, but mixed up with them is a certain proportion, about one-quarter,
{41} in which the pigment is retained. In _Castnia_ and in _Anthomysa_ the
scales on the transparent parts which are without pigment are also somewhat
reduced in size, being stumpier than the normal ones. At the same time they
tend to stand out at right angles to the wing membrane[20]. The neck here
again is shorter in the transparent than in the pigmented scales. A good
deal of stress has been laid upon this case by some supporters of the
theory of mimicry, since it is supposed to shew that a similar effect can
be brought about in a variety of ways; consequently the existence of this
assembly of similar transparent forms belonging to various families cannot
be put down as due to the effect of similar conditions, but must be
regarded as having arisen in each instance in a different manner through
the independent action of natural selection[21]. It is doubtful, however,
whether such a conclusion necessarily follows from the facts. In all of the
cases the process would appear to be similar: loss of pigment, reduction in
the size of the scales, and eventually a tendency for the scales to stand
at right angles to the wing--this last part of the process apparently
depending upon the reduction of the neck of the scale. It has been said
that greater transparency is brought about by the scales standing out at
right angles in this way, but as the scales {42} themselves are already
transparent there would appear to be no reason why this should be so. Of
course the process has not proceeded in all of the forms to the same
extent. There is least change in _Ituna_ where the scales are not much
reduced in size and where a fair proportion are still pigmented. There is
probably most in an Ithomiine such as _Thyridia_, where the scales are not
only small and entirely without pigment, but also are for the most part
neckless so that they stand out at right angles to the wing. Having regard
to the fact that several widely separate genera with different types of
scaling formed the starting points, the final results do not seem to
preclude the supposition that the transparency has arisen through a similar
process in all of them.

It is somewhat remarkable that no Satyrine exhibits mimicry in S. America,
in spite of the fact that transparency of the wings, as in so many of the
butterflies of this region, is quite common in the group. On the other hand
the relatively large number of more or less mimetic Pierines is a striking
feature of S. America. For the most part they belong to the genera
_Dismorphia_ and _Perrhybris_, and resemble the yellow, black, and brown
Heliconines and Ithomiines, though some of the former genus are mimics of
the small transparent Ithomiines. Some of the species of _Pereute_ with
their dark ground colour and the bright red bar across the fore wing (Pl.
XI, fig. 6) resemble _Heliconius melpomene_, as also does _Papilio
euterpinus_. But some of the most interesting Pierine {43} mimics are
several forms belonging to the genus _Archonias_ (Pl. XI, fig. 10) which
exhibit the simple and striking arrangement of black, red and white so
characteristic of the Swallow-tail Poison-eaters of S. America. They form
one of the rare instances of a Pharmacophagus Papilio being mimicked by a
butterfly which does not belong to the Swallow-tail group.

As everywhere in the tropics the Papilios of S. America supply a goodly
proportion of the mimicry cases. A few, such as _P. zagreus_ (Pl. X, fig.
8), enter into the black-brown and yellow Ithomiine-Heliconine combination;
_P. euterpinus_ resembles _Heliconius melpomene_ (Pl. XI, fig. 5); _P.
pausanias_ is like _Heliconius sulphurea_ (Pl. XI, figs. 1 and 2). But this
practically exhausts the list of Papilios which mimic Heliconines and
Ithomiines. The great majority of mimicking Swallow-tails in S. America
find their models among the Poison-eaters of their own family, offering in
this respect a contrast to those of Asia where the majority of models are
among the Danaines and Euploeines, and of Africa where they are exclusively
Acraeines or Danaines.

The Poison-eaters of S. America fall into two well-marked groups which we
may call the red-spotted and the dark green groups respectively. The red
spotted group form a remarkably compact and uniform assemblage. The general
ground colour is a deep black-brown (Pl. XI, figs. 8 and 9), the hind wings
are almost invariably marked with red near the centre or towards the outer
margin, and the fore wing may {44} or may not bear a patch which is
generally whitish in the female, though often of a brilliant blue or green
in the male. This simple colour scheme with variations runs throughout
about three-quarters (some 40 species) of the Poison-eaters. The same
general colour scheme is also found in about two dozen species of the
unprotected Swallow-tails. As the total number of the unprotected species
is placed by Seitz at less than 100 this means that fully one-quarter of
them fall into the general colour scheme adopted by the majority of the
Poison-eaters. In many cases the resemblance between mimic and model is so
close as to have deceived the most expert entomologists before the
structural differences between the groups had been appreciated (cf.
Appendix II). The matter is further complicated by the fact that
polymorphism is not uncommon, especially among the females of the mimetic
forms. _Papilio lysithous_ for instance has no less than six distinct forms
of female, which differ chiefly in the extent and arrangement of the white
markings on the wings, one form lacking them entirely. Several of these
forms may occur together in a given locality, and may resemble as many
distinct species of Poison-eaters. Thus the three forms _lysithous_, with
white on both wings, _rurik_, with white on the fore wing only, and
_pomponius_ without any white, all fly together in Rio Grande do Sul and
respectively mimic the three distinct Pharmacophagus species _nephalion_,
_chamissonia_, and _perrhebus_ (Pl. XIII). It is worthy of note that mimics
are provided by both unprotected {45} groups of Swallow-tails in S.
America, whereas in Asia the Cosmodesmus division never provides mimics for
Pharmacophagus models (cf. Appendix II).

In the second and smaller group of the Pharmacophagus Swallow-tails the
general colour scheme is a more or less dark metallic blue-green with a
tendency towards the obliteration of light markings. Some idea of their
appearance may be got from the figure of the Central and N. American _P.
philenor_ on Pl. XVI, fig. 1. Though one or two unprotected Papilios in S.
America fall more or less into this colour scheme, the group, from the
point of view of mimicry, is not nearly so important as the red-spotted
one.

Nevertheless the blue-green Pharmacophagus group as represented by _P.
philenor_ is supposed to play a considerable part in mimicry in N. America.
_P. philenor_ is found throughout the greater part of the Eastern United
States, straggling up as far as the Canadian border. On the west it is also
found reaching up to North California. Over considerable parts of its range
are three other Swallow-tails, belonging to the unprotected Papilios, which
are regarded by Professor Poulton and others as mimics of _philenor_[22].
One of these, _P. troilus_, is dark brown with a dusting of blue scales
over the hind wing (Pl. XVI, fig. 2). The sexes here are more or less
alike. _Troilus_ stretches up into North-west Canada some way beyond the
limits reached by its model. _P. glaucus_ is a black and yellow
Swallow-tail with two forms of female. {46} One of these resembles the male
while the other is darker and is said to mimic _philenor_. It is known as
the _turnus_ form and is found more commonly in the southern part of the
range of the species, _i.e._ in the country where _philenor_ is more
plentiful. The third species, _P. asterius_, has a more southerly
distribution. Its female is darker and nearer to _philenor_ than the male.
It must, however, be admitted that none of the three species bears a very
close resemblance to _philenor_. It is suggested that this is because _P.
philenor_ is a tropical form which has only recently invaded N. America.
The crossing of _philenor_ has, as it were, induced the three mimicking
_Papilios_ to turn dark, but the model has not been long enough in contact
with them for the likeness to become a close one. The explanation, however,
hardly accounts for the fact that the best mimic of the three, _P.
troilus_, in which both sexes are dark, is found far north of _philenor_.
Either the dark colour was established without the influence of the
Pharmacophagus model, or else the species rapidly extended its range
northwards after having been modified under the influence of _philenor_ in
the south. But in that case the critic may ask why it does not revert to
the original pattern now that it has got beyond the model's sphere of
influence. On the whole it seems at present quite doubtful whether any
relation of a mimetic nature exists between _P. philenor_ and these three
species of _Papilio_.

_P. philenor_ is also regarded as serving as a model {47} for two
Nymphaline butterflies in the United States. One of these is the large
Fritillary _Argynnis diana_ of which the dark female has a markedly blue
tint on the upper surface (Pl. XVI, fig. 3). The other is a _Limenitis_[23]
related to our own White Admiral. This form, _L. astyanax_ (Pl. XVI, fig.
5), is a dark form with a bluish iridescence on the upper surface. It is
found, like _P. philenor_, over the greater part of the Eastern States,
while to the north, near the Canadian boundary, its place is taken by _L.
arthemis_ with prominent white bar across both wings (Pl. XVI, fig. 4).
There is reason for believing that where the two overlap there is
occasional inbreeding, and that the hybrid is the form known as
_proserpina_, resembling _astyanax_ more than _arthemis_. It must be
admitted that in general appearance _L. astyanax_ and _Argynnis diana_ are
more like _Papilio troilus_ than _P. philenor_. In explanation it has been
suggested that all the mimics are on the way to resembling _P. philenor_,
and consequently we should expect them at certain stages to shew more
resemblance to one another than to the form they have all as it were set
out to mimic. On this view they will all arrive at a close resemblance to
_philenor_ in time. Another explanation is that favoured by Professor
Poulton on which it is assumed that we are here dealing with a case of
Müllerian Mimicry, all of the species in question being distasteful with
the exception perhaps of _A. diana_. Thus _troilus_ and _astyanax_ though
distasteful are less so than {48} _philenor_. Hence it is of advantage to
them to have even a chance of being mistaken for the more obnoxious
_philenor_, and so the one has come from the black and yellow Swallow-tail
pattern and the other from the white-banded _arthemis_ form to what they
are, _i.e._ more alike to one another than to _philenor_. They now form a
Müllerian combination for mutual protection along with the dark females of
_glaucus_ and _asterius_. But they are themselves still moderately
distasteful so that it is to the advantage of the female of _Argynnis
diana_ to mimic them. Whether they are all on the way to resembling
_philenor_ more closely, or whether they have sufficiently vindicated their
inedible properties and are now stationary, it is for the future to reveal
to posterity. Lastly we have the view that these different species have
attained their present coloration entirely independently of one another,
and that we are not here concerned with mimicry at all. Since the sole
evidence available at present is that based on general appearance and
geographical distribution, the view taken of this case must rest largely
upon personal inclination.

Though the cases just quoted are only very problematically mimetic, N.
America has yet several examples of resemblance between distantly related
forms as close as any that occur in the tropics. In this region are found
two species of the genus _Danais_--_D. archippus_ occurring all over the
United States and reaching up northwards into Canada, _D. berenice_ found
in the South-eastern States, _e.g._ in Florida, where it is said to be more
abundant than _archippus_. {49} _D. archippus_ (Pl. XVI, fig. 8) is very
similar to the oriental _D. plexippus_ (Pl. IV, fig. 2), from which perhaps
its most notable difference lies in the extent and arrangement of the white
spots near the tip of the fore wing. _D. berenice_ is not unlike
_archippus_ in its general colour scheme but is smaller and darker (Pl.
XVI, fig. 9).

We have already had occasion to mention the common Nymphaline, _Limenitis
arthemis_ (Pl. XVI, fig. 4) which is found in Canada and the Northeastern
States. Widely spread over N. America is a close ally of this species, _L.
archippus_, which, though so similar in structure and habits, is very
different in external appearance. As appears from Pl. XVI, fig. 6, _L.
archippus_ is remarkably like the Danaid which bears the same specific
name. In the Southern States _L. archippus_ is replaced by a form slightly
different in details of pattern and distinctly darker, _L. floridensis_ (=
_eros_) (Pl. XVI, fig. 7). In Florida occurs also the darker N. American
Danaid, _D. berenice_, to which the colour of _L. floridensis_ approximates
more than to _D. archippus_, and it is of interest that although the last
named is also found in this locality it is said to be much less abundant
than _D. berenice_. Nevertheless it appears to be true that the range of
_L. floridensis_ is much more extensive than that of its model; in other
words, that there are considerable regions where _L. floridensis_ and _D.
archippus_ coexist, and from which _L. archippus_ and _D. berenice_ are
wanting.

       *       *       *       *       *


{50}

CHAPTER V

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.

       *       *       *       *       *


{61}

CHAPTER VI

"MIMICRY RINGS"

Having reviewed briefly some of the most striking phenomena of what has
been termed mimicry, we may now inquire whether there are good grounds for
supposing that these resemblances have been brought about through the
operation of natural selection or whether they are due to some other cause.
If we propose to offer an explanation in terms of natural selection we are
thereby committed to the view that these resemblances are of the nature of
adaptation. For unless we grant this we cannot suppose that natural
selection has had anything to do either with their origin or with their
survival. Granting then for the present the adaptational nature of these
mimetic resemblances, we may attempt to deduce from them what we can as to
the mode of operation of natural selection. In doing so we shall bear in
mind what may be called the two extreme views: viz. (_a_) that the
resemblance has been brought about through the gradual accumulation of very
numerous small variations in the right direction through the operation of
natural selection, and (_b_) that the mimetic form came into being as a
sudden sport or {62} mutation, and that natural selection is responsible
merely for its survival and the elimination of the less favoured form from
which it sprang.

There is a serious difficulty in the way of accepting the former of these
two views. If our two species, model and would-be mimic are, to begin with,
markedly different in pattern, how can we suppose that a slight variation
in the direction of the model on the part of the latter would be of any
value to it? Take for example a well-known South American case--the
resemblance between the yellow, black, and brown Ithomiine, _Mechanitis
saturata_ (Pl. X, fig. 7) and the Pierine, _Dismorphia praxinoe_ (Pl. X,
fig. 3). The latter belongs to the family of the "whites," and
entomologists consider that in all probability its ancestral garb was white
with a little black like the closely allied species _D. cretacea_ (Pl. X,
fig. 1). Can we suppose that in such a case a small development of brown
and black on the wings would be sufficient to recall the Ithomiine and so
be of service to the _Dismorphia_ which possessed it? Such a relatively
slight approach to the Ithomiine colouring is shewn by the males of certain
South American "whites" belonging to the genus _Perrhybris_ (Pl. X, figs. 4
and 5). But the colour is confined to the under-surface and the butterflies
possessing it could hardly be confused with a _Mechanitis_ more than their
white relations which entirely lack such a patch of colour. If birds
regarded white butterflies as edible it is difficult to suppose that they
would be checked in their attacks {63} by a trifling patch of colour while
the main ground of the insect was still white. But unless they avoided
those with the small colour patch there would be an end of natural
selection in so far as the patch was concerned, and it would have no
opportunity of developing further through the operation of that factor.
This is the difficulty of the initial variation which has been clearly
recognised by most of the best known supporters of the theory of mimicry.
Bates himself offered no suggestion as to the way in which such a form as a
Pierid could be conceived of as beginning to resemble an Ithomiine[30].
Wallace supposed that the Ithomiines were to start with not so distinct
from many of the edible forms as they are to-day, and that some of the
Pierines inhabiting the same district happened to be sufficiently like some
of the unpalatable forms to be mistaken for them occasionally[31].

The difficulty of the initial variation had also occurred to Darwin, and he
discusses it in an interesting passage which is so important that we may
quote it here in full:

    It should be observed that the process of imitation probably never
    commenced between forms widely dissimilar in colour. But starting with
    species already somewhat like each other, the closest resemblance, if
    beneficial, could readily be gained by the above means; and if the
    imitated form was subsequently and gradually {64} modified through any
    agency, the imitating form would be led along the same track, and thus
    be altered to almost any extent, so that it might ultimately assume an
    appearance or colouring wholly unlike that of the other members of the
    family to which it belonged. There is, however, some difficulty on this
    head, for it is necessary to suppose in some cases that ancient members
    belonging to several distinct groups, before they had diverged to their
    present extent, accidentally resembled a member of another and
    protected group in a sufficient degree to afford some slight
    protection; this having given the basis for the subsequent acquisition
    of the most perfect resemblance[32].

Both Darwin and Wallace recognised clearly this difficulty of the initial
variations, and both suggested a means of getting over it on similar lines.
Both supposed that in general colour and pattern the groups to which model
and mimic belonged were far more alike originally than they are to-day.
They were in fact so much alike that comparatively small variations in a
favourable direction on the part of the mimic would lead to its being
confused with the unpalatable model. Then as the model became more and more
conspicuously coloured, as it developed a more and more striking pattern
warning would-be enemies of its unpleasant taste, the mimic gradually kept
pace with it through the operation of natural selection, in the shape of
the discriminating enemy, eliminating those most unlike the model. The
mimic travelled closely in the wake of the model, coaxed as it were by
natural selection, till at last it was far removed in general appearance
from the great majority of its near relations. {65}

In this way was offered a comparatively simple method of getting over the
difficulty of applying the principle of natural selection to the initial
variations in a mimetic approach on the part of one species to another. But
it did not escape Darwin's penetration that such an argument would not
always be easy of application--that there might be cases where a given
model was mimicked by members of several groups of widely differing
ancestral pattern, and that in these cases it would be difficult to
conceive of members of each of the several groups shewing simultaneous
variations which would render them liable to be mistaken for the protected
model. The difficulty may perhaps be best illustrated if we consider a
definite case.

It is a feature of mimetic resemblances among butterflies that a given
species in a given locality may serve as a model for several other species
belonging to unrelated groups. Generally such mimics belong to presumably
palatable species, but other presumably unpalatable species may also
exhibit a similar coloration and pattern. In this way is formed a combine
to which the term "mimicry ring" has sometimes been applied. An excellent
example of such a mimicry ring is afforded by certain species of
butterflies in Ceylon, and is illustrated on Plate IV. It is made up in the
first place of two species belonging to the presumably distasteful Danaine
group, viz. _Danais chrysippus_ and _D. plexippus_. The latter is a rather
darker insect but presents an unmistakable general likeness to _D.
chrysippus_. Those who believe in {66} Müllerian mimicry would regard it as
an excellent example of that phenomenon. For those who believe only in
Batesian mimicry _D. plexippus_, being the scarcer insect, must be regarded
as the mimic and _D. chrysippus_ as the model. In both of these species the
sexes are similar, whereas in the other three members of the "ring" the
female alone exhibits the resemblance. One of these three species is the
common Nymphaline, _Hypolimnas misippus_, of which the female bears an
extraordinary likeness to _D. chrysippus_ when set and pinned out on cork
in the ordinary way. The male, however (Pl. IV, fig. 8), is an insect of
totally different appearance. The upper surfaces of the wings are velvety
black with a large white patch bordered with purple in the middle of
each[33]. The "ring" is completed by the females of _Elymnias undularis_
and _Argynnis hyperbius_. The former of these belongs to the group of
Satyrine butterflies and the female is usually regarded as a mimic of _D.
plexippus_, which it is not unlike in so far as the upper surface of the
wings is concerned. Here again the male is an insect of totally dissimilar
appearance. Except for a border of lighter brown along the outer edges of
the hind wings the upper surface is of a uniform deep purple-brown {67} all
over (Pl. IV, fig. 6). In _Argynnis hyperbius_ the appearance is in general
that of the Fritillary group to which it belongs. But in the female the
outer portion of the fore wings exhibits much black pigment and is crossed
by a broad white band similar to that found in the same position on the
wing of _D. plexippus_ (Pl. IV, fig. 2).

Of the five species constituting this little "mimicry ring" in Ceylon two,
on the current theory of mimicry, are to be regarded as definitely
unpalatable, one (_H. misippus_) as doubtfully so, while the Satyrine and
the Fritillary are evidently examples of simple or Batesian mimicry.

Now such examples as this of simultaneous mimicry in several species are of
peculiar interest for us when we come to inquire more closely into the
process by which the resemblances can be supposed to have been brought
about. Take for example the case of _E. undularis_. The male is evidently
an unprotected insect in so far as mimicry is concerned, while the female
exhibits the general pattern and coloration characteristic of the warningly
coloured and presumably distasteful species _D. plexippus_ or _D.
chrysippus_. If we are to suppose this to have been brought about by the
operation of natural selection it is clear that we must regard the colour
and pattern of the male as the original colour and pattern of both sexes.
For natural selection cannot be supposed to have operated in causing the
male to pass from a protected to an unprotected condition, or even in
causing him {68} to change one unprotected condition for another. Probably
all adherents of the mimicry theory would be agreed in regarding the male
of _Elymnias undularis_ as shewing the ancestral coloration of the species,
and in looking upon the female as having been modified to her own advantage
in the direction of _D. plexippus_. The question that we have to try to
decide is how this has come about--whether by the accumulation of slight
variations, or whether by a sudden change or mutation in the pattern and
colour of the female by which she came to resemble closely the Danaine. It
is clear that if _D. plexippus_ were what it is to-day before the mimetic
approach on the part of _E. undularis_ began, small variations in the
latter would have been of no service to it. The difference between the two
species would have been far too great for individuals exhibiting slight
variation in the direction of _D. plexippus_ to stand any chance of being
confused with this species. And unless such confusion were possible natural
selection could not work. There is, however, an immediate way out of the
difficulty. We may suppose that the coloration of the male of the mimic,
_E. undularis_, is not only the ancestral colour of its own species but
also of the model. _D. plexippus_ on this supposition was very like _E.
undularis_, of which both sexes were then similar to what the male is
to-day. The pattern is, however, an inconspicuous one, and it can be
imagined that it might be to the advantage of _D. plexippus_ to don a
brighter garb for the advertisement of its unpleasant qualities. {69}
Variations in the direction of a more conspicuous pattern would for that
reason tend to be preserved by natural selection, until eventually was
evolved through its means the well-marked pattern so characteristic of the
model to-day. If in the meantime variations in the same direction occurred
among the females of _E. undularis_ these would tend to be preserved
through their resemblance to the developing warning pattern of the
distasteful Danaine model. The development of model and mimic would proceed
_pari passu_, but if the sexes of the mimic differ, as in this case, we
must suppose the starting-point to have been the condition exhibited by the
male of the mimicking species.

But _Argynnis hyperbius_ is also a species in which the female mimics _D.
plexippus_; and by using the same argument as that just detailed for
_Elymnias undularis_ we can shew that the Danaine model, _D. plexippus_,
must also have been like the male of _Argynnis hyperbius_. And if the
resemblance of _A. hyperbius_ was developed subsequently to that of _E.
undularis_, then both _D. plexippus_ and _E. undularis_ must at one time
have been like the male of _A. hyperbius_, a proposition to which few
entomologists are likely to assent. Further, since the female of _H.
misippus_ also comes into the _plexippus-chrysippus_ combine we must
suppose that these species must at some time or another have passed through
a pattern stage like that of the _misippus_ male.

It is scarcely necessary to pursue this argument {70} further, for even the
most devoted adherents of the theory of mimicry as brought about by the
operation of natural selection on small variations are hardly likely to
subscribe to the phylogenetic consequences which it must entail in cases
where a model is mimicked by the females of several species whose males are
widely dissimilar in appearance.

Even if we suppose the two Danaines to have been originally like the male
of one of the three mimics, we must still suppose that the females of the
other two originated as "sports," sufficiently near to Danaines to be
confused with them. But if such sports can be produced suddenly by some
mutational process not at present understood, why should not these sports
be the females of the three mimicking species as we see them at present?
Why need we suppose that there were intermediate stages between the
mimicking female and the original hypothetical female which was like the
male? If a sport occurred which was sufficiently similar to an unpalatable
species to be confused with it, it is theoretically demonstrable that,
although relatively scarce to start with, it would rapidly increase at the
expense of the unprotected male-like female until the latter was
eliminated. We shall, however, return in a later chapter (p. 96) to the
argument by which this view can be supported.

So far we have discussed what we called the two extreme views as to the way
in which a mimetic resemblance may be supposed to have originated. Of the
two that which assumes the resemblance to have {71} been brought about by a
succession of slight variations must also assume that model and mimic were
closely alike to start with, and this certainly cannot be true in many
cases. On the other hand, there is so far no reason against the idea of
supposing the resemblance to have originated suddenly except what to most
minds will probably appear its inherent improbability.

There are writers on these questions of mimicry who adopt a view more or
less intermediate between those just discussed. They regard the resemblance
as having arisen in the first place as a sport of some magnitude on the
part of the mimic, rendering it sufficiently like the model to cause some
confusion between the two. A rough-hewn resemblance is first brought about
by a process of mutation. Natural selection is in this way given something
to work on, and forthwith proceeds to polish up the resemblance until it
becomes exceedingly close. Natural selection does not originate the
likeness, but, as soon as a rough one has made its appearance, it comes
into operation and works it up through intermediate stages into the
finished portrait. It still plays some part in the formation of a mimetic
resemblance though its rôle is now restricted to the putting on of the
finishing touches. Those who take this view hold also that the continued
action of natural selection is necessary in order to keep the likeness up
to the mark. They suppose that if selection ceases the likeness gradually
deteriorates owing to the coming into operation of a mysterious {72}
process called regression. This idea involves certain conceptions as to the
nature of variation which we shall discuss later.

Though it is difficult to regard Batesian mimicry as produced by the
accumulation of small variations through natural selection, it is perhaps
rather more plausible to suppose that such a process may happen in
connection with the numerous instances of Müllerian mimicry. For since the
end result is theoretically to the advantage of both species instead of but
one, it is possible to argue that the process would be simplified by their
meeting one another halfway, as Müller[34] himself originally suggested.
Variations on the part of each in the direction of the other would be
favourably selected, the mimicry being reciprocal.

Difficulties, however, begin to arise when we inquire into the way in which
this unification of pattern may be conceived of as having come about. By no
one have these difficulties been more forcibly presented than by
Marshall[35] in an able paper published a few years ago, and perhaps the
best way of appreciating them is to take a hypothetical case used by him as
an illustration.

Let us suppose that in the same area live two equally distasteful species A
and _B_, each with a conspicuous though distinct warning pattern, and each
sacrificing 1000 individuals yearly to the education of young {73} birds.
Further let it be supposed that A is a common species of which there are
100,000 individuals in the given area, while _B_ is much rarer, and is
represented by 5000. The toll exacted by young birds falls relatively more
lightly upon A than upon _B_, for A loses only 1%, whereas _B_'s loss is
20%. Clearly if some members of _B_ varied so that they could be mistaken
for A it would be greatly to their advantage, since they would pass from a
population in which the destruction by young birds was 20% to one in which
it would now be rather less than 1%. Moreover, as the proportion of _B_
resembling A gradually increased owing to this advantage, the losses
suffered by those exhibiting the original _B_ pattern would be relatively
heavier and heavier until the form was ultimately eliminated. In other
words, it is theoretically conceivable that of two distasteful species with
different patterns the rarer could be brought to resemble the more
abundant.

We may consider now what would happen in the converse case in which the
more numerous species exhibited a variation owing to which it was confused
with the rarer. Suppose that of the 100,000 individuals of A 10,000 shewed
a variation which led to their being mistaken for _B_, so that there are
90,000 of the A pattern and 15,000 of the _B_ pattern of which 10,000
belong to species A. A will now lose 1000 out of the 90,000 having the A
pattern, and 2/3 × 1000 out of the 10,000 of species A which exhibit the
_B_ pattern. The toll of the birds will be 1/90 of those keeping the
original A pattern, and 2/30 of those of species A which have {74} assumed
the _B_ pattern. The mortality among the mimetic members of A is six times
as great as among those which retain the type form. It is clear therefore
that a variation of A which can be mistaken for _B_ is at a great
disadvantage as compared with the type form[36], and consequently it must
be supposed that the Müllerian factor, as the destruction due to
experimental tasting by young birds is termed, cannot bring about a
resemblance on the part of a more numerous to a less numerous species.
Further, as Marshall goes on to shew, there can be no approach of one
species to the other when the numbers are approximately equal. A condition
essential for the establishing of a mimetic resemblance on Müllerian lines,
no less than on Batesian, is that the less numerous species should take on
the pattern of the more numerous. Consequently the argument brought forward
in the earlier part of this chapter against the establishing of such a
likeness by a long series of slight variations is equally valid for
Müllerian mimicry[37].

       *       *       *       *       * {75}


CHAPTER VII

THE CASE OF _PAPILIO POLYTES_

Many instances of mimicry are known to-day, but comparatively few of them
have been studied in any detail. Yet a single carefully analysed case is
worth dozens which are merely superficially recorded. In trying to arrive
at some conception of the way in which the resemblance has come about we
want to know the nature and extent of the likeness in the living as well as
in the dead; the relative abundance of model and mimic; what are likely
enemies and whether they could be supposed to select in the way required,
whether the model is distasteful to them; whether intermediate forms occur
among the mimics; how the various forms behave when bred together, etc.,
etc. Probably the form that from these many points of view has, up to the
present, been studied with most care is that of the Swallow-tail, _Papilio
polytes_. It is a common butterfly throughout the greater part of India and
Ceylon, and closely allied forms, probably to be reckoned in the same
species, reach eastwards through China as far as Hongkong. _P. polytes_ is
one of those species which exhibit polymorphism in the female sex. Three
distinct forms of female are known, of which one is like the male, while
the other two are very different. Indeed {76} for many years they were
regarded as distinct species, and given definite specific names. To Wallace
belongs the credit of shewing that these three forms of female are all to
be regarded as wives of the same type of male[38]. He shewed that there
were no males corresponding to two of the females; also that the same one
male form was always to be found wherever any of the females occurred. As
the result of breeding experiments in more recent years Wallace's
conclusions have been shewn to be perfectly sound.

The male of _polytes_ (Pl. V, fig. 1) is a handsome blackish insect with a
wing expanse of about four inches. With the exception of some
yellowish-white spots along their outer margin the fore wings are entirely
dark. Similar spots occur along the margin of the hind wing also, while
across the middle runs a series of six yellowish-white patches producing
the appearance of a broad light band. The thorax and abdomen are full
black, though the black of the head is relieved by a few lighter yellowish
scales. The under surface is much like the upper, the chief difference
being a series of small and slightly reddish lunules running outside the
light band near the margin of the hind wing (Pl. V, fig. 1 _a_). In some
specimens these markings are almost absent. One form of female is almost
exactly like the male (Pl. V, fig. 2), the one slight difference being that
the lunules on the under surface of the hind wing are generally a trifle
larger. For brevity she may be called the M form. The second form of female
{77} differs in many respects from the male and the M female. Instead of
being quite dark, the fore wings are marked by darker ribbed lines on a
lighter ground[39] (Pl. V, fig. 3). The hind wings shew several marked
differences from those of the male. Of the series of six patches forming
the cross band the outermost has nearly disappeared, and the innermost has
become smaller and reddish. The middle four, on the other hand, have become
deeper, reaching up towards the insertion of the wing, and are pure white.
A series of red lunules occurs on the upper surface outside the white band,
and the yellowish-white marginal markings tend to become red. These
differences are equally well marked on the under surface (Pl. V, fig. 3
_a_). The colour of the body, however, remains as in the male. From the
resemblance shewn by this form to another species of Swallow-tail, _Papilio
aristolochiae_ (Pl. V, fig. 5), we shall speak of it as the A form.

The third form of female is again very distinct from the other two. The
fore wings are dark but are broken by an irregular white band running
across the middle (Pl. V, fig. 4), and there is also an irregular white
patch nearer the tips of the wing. The hind wings, on the other hand, are
characterised by having only red markings. The yellowish-white band of the
male is much reduced and is entirely red, while the red lunules are much
larger than in the A form. The under surface (Pl. V, fig. 4 _a_)
corresponds closely with the {78} upper. The body remains black as in all
the other forms. This type of female bears a resemblance to _Papilio
hector_ (Pl. V, fig. 6), and for that reason we shall speak of it as the H
form. It should be added that these three forms of female are quite
indistinguishable in the larval and chrysalis stages.

It was Wallace who first offered an explanation of this interesting case in
terms of mimicry. According to this interpretation _P. polytes_ is a
palatable form. The larva, which feeds on citronaceous plants, and the
chrysalis are both inconspicuous in their natural surroundings. They may be
regarded as protectively coloured, and consequently edible and liable to
persecution. The original coloration is that of the male and the M female.
From this the other two forms of female have diverged in the direction of
greater instead of less conspicuousness, although the presumed edibility of
the insect might have led us to think that a less conspicuous coloration
would have been more to its advantage. But these two females resemble the
two species _Papilio aristolochiae_ and _Papilio hector_, which, though
placed in the same genus as _P. polytes_, belong to a very different
section of it[40]. The larvae of these two species are conspicuously
coloured black and red with spiny tubercles. They feed upon the poisonous
_Aristolochia_ plants. For these reasons and also from the fact that the
butterflies themselves are both conspicuous and plentiful it is inferred
that they are unpalatable. In short, they are the models upon {79} which
the two _polytes_ females that are unlike the male have been built up by
natural selection.

The suggestion of mimicry in this case is supported by the fact that there
is a general correspondence between the areas of distribution of model and
mimic. _P. hector_ is not found outside India and Ceylon, and the H female
of _P. polytes_ is also confined to this area. _P. aristolochiae_, on the
other hand, has a much wider range, almost as wide indeed as that of _P.
polytes_ itself. Generally speaking the A female accompanies _P.
aristolochiae_ wherever the latter species is found. Beyond the range of
_P. aristolochiae_, in northern China, the M female alone is said to occur.
On the other hand, as the matter comes to be more closely studied
exceptions are beginning to turn up. The H female, for instance, is found
on the lower slopes of the Himalayas, far north of the range of _P.
hector_, and there are indications that a careful study of the distribution
in China and Japan may prove of importance.

Moreover, the investigation of a smaller area may also bring to light
points of difficulty. In Ceylon, for example, _P. polytes_ is common up to
several thousand feet, while _P. hector_ is rare at half the height to
which _polytes_ ascends. Nevertheless the H form of female is relatively
just as abundant up-country where _hector_ is rarely found as it is low
down where _hector_ is plentiful[41]. On the other hand, _P. aristolochiae_
may be exceedingly abundant at altitudes where _hector_ is scarce. Yet the
A form of _polytes_ is no more relatively abundant {80} here than elsewhere
on the island. All over Ceylon, in fact, the relative proportions of the
three forms of female appear to be the same, quite irrespective of the
abundance or scarcity of either of the models. As, however, we shall have
to return to this point later, we may leave it for the moment to consider
other features of this case of _P. polytes_.

In collections of insects from India or Ceylon it is not unusual to find
specimens of the A form of female of _polytes_ placed with _P.
aristolochiae_, and the H form with _P. hector_. When the insects are old
and faded and pinned out on cork the mistake is a very natural one. But
after all the enemies of _polytes_ do not hunt it in corked cabinets, and
any estimation of resemblance to be of use to us must be based upon the
living insects. Are the resemblances of the mimics to the models when alive
so close that they might be expected to deceive such enemies[42] as prey
upon them and have no difficulty in distinguishing the male form of
_polytes_ from _P. aristolochiae_ or _P. hector_?

To answer for a bird is a hazardous undertaking. We know so little of the
bird's perceptive faculties whether of taste or sight. But on general
grounds, from the specialization of their visual apparatus, it is probable
that the sense of sight is keen, though whether the colour sense is the
same as our own is doubtful[43]. On the other hand, the olfactory apparatus
{81} is relatively poorly developed in birds, and from this we can only
argue that the senses of smell and taste are not especially acute. Really
we can do little more than to describe how these mimetic resemblances
appear to our own senses, and to infer that they do not appear very
different to the bird. If there is any difference in keenness of perception
we shall probably not be far wrong in presuming that the advantage rests
with the bird. After all if there is any truth in the theory of mimicry the
bird has to depend largely upon its keenness of sight in making its living,
at any rate if that living is to be a palatable one. If natural selection
can bring about these close resemblances among butterflies it must
certainly be supposed to be capable of bringing the bird's powers of vision
to a high pitch of excellence.

Returning now to the case of _P. polytes_, there is not the least doubt
that to the ordinary man accustomed to use his eyes the A form of female is
easily distinguishable from _P. aristolochiae_, as also is the H form from
_P. hector_. The two models have a feature in common in which they both
differ from their respective mimics. In both of them the body and head are
largely of a brilliant scarlet, whereas neither of the mimics has a touch
of red on the body. In the living insect when the body is swelled by its
natural juices the effect is very striking[44]. It gives at once a
"dangerous" look {82} to the insect when settled, even at a distance of
several yards, and this although one may be perfectly familiar with its
harmless nature. The mimics on the other hand with their sombre-coloured
bodies never look otherwise than the inoffensive creatures that they are.
The "dangerous" look due to the brilliant scarlet of the body and head of
_hector_ and _aristolochiae_ is reinforced by the quality of the red on the
markings of the wings. In both models it is a strong clamorous red
suggestive of a powerful aniline dye, whereas such red as occurs in the
mimics is a softer and totally distinct colour. The difference in quality
is even more marked on the under than on the upper surface (Pl. V, figs. 3
_a_--6 _a_), and the net result is that when settled, with wings either
expanded or closed, there is no possibility of an ordinarily observant man
mistaking mimic for model in either case, even at a distance of several
yards.

It may, however, be argued that it is not when at rest but during flight
that the mimetic resemblance protects the mimic from attack. Actually this
can hardly be true, for the mode of flight constitutes one of the most
striking differences between model and mimic. _P. hector_ and _P.
aristolochiae_ fly much in the same way. They give one the impression of
flying mainly with their fore wings, which vibrate rapidly, so that the
course of the insect, though not swift, is on the whole sustained and even.
The flight of all the different forms of _polytes_ is similar and quite
distinct from that of the models. It is a strong but rather {83} heavy and
lumbering up-and-down flight. One gets the impression that all the wing
surface is being used instead of principally the fore wings as appears in
_P. hector_ and _P. aristolochiae_. The difference is difficult to put into
words, but owing to these peculiarities of flight the eye has no difficulty
in distinguishing between model and mimic even at a distance of 40 to 50
yards. Moreover, colour need not enter into the matter at all. It is even
easier to distinguish model from mimic when flying against a bright
background, as for instance when the insect is between the observer and a
sunlit sky, than it is to do so by reflected light. I have myself spent
many days in doing little else but chasing _polytes_ at Trincomalee where
it was flying in company with _P. hector_, but I was never once lured into
chasing the model in mistake for the mimic. My experience was that whether
at rest or flying the species are perfectly distinct, and I find it
difficult to imagine that a bird whose living depended in part upon its
ability to discriminate between the different forms would be likely to be
misled. Certainly it would not be if its powers of discrimination were
equal to those of an ordinary civilised man. If the bird were unable to
distinguish between say the A form of female and _P. aristolochiae_ I think
that it would be still less likely to distinguish between the same A form
and the male or the M form of female. For my experience was that at a
little distance one could easily confuse the A form of _polytes_ with the
male. Except when one was quite close the red on the A form was apt to be
lost, the {84} white markings on the hind wing were readily confused with
those of the male, and one had to depend entirely on the lighter fore wing.
Unless the bird were keener sighted than the man the A form would be more
likely to be taken in mistake for its unprotected relative than avoided for
its resemblance to the presumably unpalatable model. On the other hand, if
the bird were sufficiently keen sighted never to confuse the A female with
the male form its sight would be too keen to be imposed upon by such
resemblance as exists between the A female and _P. aristolochiae_.

These, however, are not the only criticisms of the theory of mimicry which
the study of this species forces upon us. _Papilio polytes_ is one of the
few mimetic species that has been bred, and in no other case of
polymorphism is the relation between the different forms so clearly
understood. For this result we are indebted mainly to the careful
experiments of Mr J. C. F. Fryer, who recently devoted the best part of two
years to breeding the different forms of this butterfly in Ceylon[45].
Fryer came to the conclusion that an explanation of this curious case is
possible on ordinary Mendelian lines. At first sight the breeding results
appear complicated, for any one of the three forms of female can behave in
several different ways. For the sake of simplicity we may for the moment
class together the A and H females as the mimetic females, the non-mimetic
being represented by the M or male-like females. {85} The different kinds
of families which each of the three females can produce may be tabulated as
follows:--

  ([alpha]) The M form may give either:--
    (1)  M only.
    (2)  M and mimetics in about equal numbers.
    (3)  Mimetics only.

  ([beta]) The A form may give either:--
    (1)  M and mimetics in about equal numbers.
    (2)  M and mimetics in the ratio of about 1:3.
    (3)  Mimetics only.

  ([gamma]) The H form may give either:--
    (1)  M and mimetics in about equal numbers.
    (2)  M and mimetics in the ratio of about 1:3.
    (3)  Mimetics only.

The males are in all cases alike to look at but it must nevertheless be
supposed that they differ in their transmitting powers. In fact the
evidence all points to there being three different kinds of male
corresponding to the three different kinds of female. But they cannot shew
any difference outwardly because there is always present in the male a
factor which inhibits the production of the mimetic pattern even though the
factor for that pattern be present.

Returning now to the records of the females it will be noticed that
although the M form may breed true the mimetics never give the M form
alone. Where they give the M form among their progeny they produce mimetics
and non-mimetics either in the ratio 1:1 or of 3:1. This at once suggests
that the {86} non-mimetic is recessive to the mimetic forms--that the
mimetics contain a factor which does not occur in the non-mimetics. If this
factor, which may be called X, be added to the constitution of a
non-mimetic female it turns it into a mimetic. If X be added to a male such
an individual, though incapable of itself exhibiting the mimetic pattern
owing to the inhibitory factor always present in that sex, becomes capable
of transmitting the mimetic factor to its offspring. Expressed in the usual
Mendelian way the formulae for these different butterflies are as
follows:--

  M[F]       =      iixx  |  Iixx = [M] (1)
  Mimetic  } = {    iiXX  |  IiXX = [M] (2)
    [F][F] }   { or iiXx  |  IiXx = [M] (3)

where X stands for the mimetic factor and I for the factor which inhibits
the action of X. All males are heterozygous for I, but during the
segregation of characters at some stage in the formation of the families
only the male-producing sperms come to contain the factor I. It is lacking
in all the female-producing sperms formed by the male.

[M] (1) does not contain the factor for the mimetic condition and gives
only daughters of the M form when mated with an M[F]. [M] (2) on the
other hand is homozygous for the factor X, and consequently all of his germ
cells contain it. This is the male that gives nothing but mimetic daughters
with whatever form of female he is bred. [M] (3) is heterozygous for X;
that is to say, one half of his germ cells contain it, the other half not.
With the M[F] he must give equal numbers {87} of offspring with and
without X, _i.e._ half of his daughters will be mimetic and the other half
non-mimetic. With a heterozygous mimetic female (iiXx), which is also
producing germ cells with and without X in equal numbers, he may be
expected to give the usual result, viz. dominants and recessives in the
ratio 3:1; or in other words mimetic and non-mimetic females in the ratio
3:1.

One of Fryer's experiments may be given here in illustration of the nature
of the evidence upon which the above hypothesis depends.

          H[F](wild)                 H[F](wild)
              |                          |
     +--------+----------+            +--+------+----------+
     |        |          |            |         |          |
  18[M][M]  10M[F][F]  7H[F][F]    26[M][M]  7M[F][F]  26H[F][F]
              |                       |
              +---------+      +------+
                        |      |
                       M[F] × [M]
                            |
                      +---------+
                      |         |
                   7[M][M]  12H[F][F]
                      |         |
                +-----+         |
                |      \        |
                |     +---------+--------------+
                |     |  \                     |
              [M] × H[F]  ------------- [M] × H[F]
                  |                         |
     +---------+--+-----+         +---------+----------+
     |         |        |         |         |          |
  14[M][M]  6H[F][F]  1M[F]    8[M][M]  10H[F][F]  2M[F][F]

Families were reared from the two wild H females of whom nothing was known
either as to ancestry or husband. The first family contained 10 M and 7 H
females. Hence the original wild mother was probably iiXx and had mated
with a male of the constitution {88} Iixx. The family from the second wild
H female contained 26 H and 7 M females; _i.e._ the ratio in which these
two forms appeared was not far from 3:1. Hence the wild female was probably
iiXx and her husband IiXx. If this were so some of the 26 [M][M] should
receive the X factor from both parents and consequently be IiXX in
constitution. This was almost certainly so in the case of the single male
in this brood tested by mating with an M female from the other brood. All
of his 12 daughters were of the H form, as should have been the case had
his constitution been IiXX. Supposing this to be so, all his offspring, of
both sexes, must be heterozygous for X. Consequently any pair mated
together should give both H and M females in the ratio of three of the
former to one of the latter. In Mr Fryer's experiment two males and two
females chosen at random were mated together. In the one case six H and one
M female were produced, in the other ten H and two M females. As was
expected both classes of female appeared, and the looked-for ratio of three
H to one M was, in view of the smallness of the numbers, not departed from
widely in either instance.

In the experiments selected as an illustration, the mimetic females happen
to be all of the H form. In other experiments, however, both the H form and
the A form occurred. As the result of his experiments Mr Fryer came to the
conclusion that here again the difference is one of a single hereditary
factor. All mimetic females contain the X factor, but the H {89} females
contain in addition a factor which we may call Y. The function of the Y
factor is to carry the change made by the X factor a step further, and to
turn the A form of female into the H form. Y is a modifier of X, but unless
X is present Y can produce no effect. All the different individuals which
are to be found among _P. polytes_ in Ceylon may be represented as
follows:--

  [M][M]  |  M[F][F]  |  A[F][F]  |  H[F][F]  |
          |           |           |           |
  IixxYY  |  iixxYY   |    --     |    --     |
  IixxYy  |  iixxYy   |    --     |    --     |
  Iixxyy  |  iixxyy   |    --     |    --     |
  IiXxYY  |    --     |    --     |  iiXxYY   |
  IiXxYy  |    --     |    --     |  iiXxYy   |
  IiXxyy  |    --     |  iiXxyy   |    --     |
  IiXXYY  |    --     |    --     |  iiXXYY   |
  IiXXYy  |    --     |    --     |  iiXXYy   |
  IiXXyy  |    --     |  iiXXyy   |    --     |

In this way is offered a simple explanation in terms of three Mendelian
factors which serves at once to explain the various results of the breeding
experiments, and the fact that intermediates between the different forms of
female are not found.

The only other experiments comparable with these on _P. polytes_ are some
made by Jacobsen on _Papilio memnon_ in Java[46]. Here again there are
three forms of female, one of which, _laomedon_, is something like the
male, while the other two, _agenor_ and _achates_, are quite distinct. Of
these three _achates_, unlike the male and the other two females, is
tailed, and resembles {90} the species _Papilio coon_ which belongs to the
same presumably distasteful group as _P. aristolochiae_. These experiments
of Jacobsen's are not so complete as the series on _P. polytes_, but
Professor de Meijere and Mr Fryer have both pointed out that they are
capable of being interpreted on the same simple lines.

Another instance of experimental breeding involving polymorphism and
mimicry in the female sex is that of the African _Papilio dardanus_, but
the case is here complicated by the greater number of female forms (cf. pp.
30-33). The data, too, are far more scanty than in the other two cases, but
so far as they go there is nothing to preclude an explanation being
eventually arrived at on similar lines[47].

And now we may consider briefly the bearing of these experiments on the
theory of mimicry. Throughout the work no individuals intermediate between
the three well-marked forms of _polytes_ were met with. There is no
difference in appearance between the heterozygous and the homozygous
mimetic insects, whether they belong to the A or to the H form. The factor
X, whether inherited from both parents, or from one only, produces its full
effect, and the same is also true of the action of the factor Y. Now the
most generally accepted hypothesis as to the formation of these mimetic
resemblances supposes that they have been brought about through the gradual
operation of natural selection accumulating slight variations. {91}
Professor Poulton, for example, a prominent exponent of this school,
considers that the A form of female was first evolved gradually from the M
form, and later on the H form came by degrees from the A form. If this be
true we ought, by mingling the M germ plasm with the H germ plasm and by
subsequently breeding from the insects produced, to get back our series of
hypothetical intermediates, or at any rate some of them. We ought as it
were to reverse the process by which the evolution of the different forms
has taken place. But as is shewn by the experiment of Mr Fryer, which was
quoted above, nothing of the sort happens.

From experiments with cultivated plants such as primulas and sweet peas, we
have learnt that this discontinuous form of inheritance which occurs in _P.
polytes_ is the regular thing. Moreover, we have plenty of historical
evidence that the new character which behaves in this way is one that has
arisen suddenly without the formation of intermediate steps. The dwarf
"Cupid" form of sweet pea, for instance, behaves in heredity towards the
normal form as though the difference between them were a difference of a
single factor. It is quite certain that the "Cupid" arose as a sudden sport
from the normal without the intervention of anything in the way of
intermediates. And there is every reason to suppose that the same is true
for plenty of other characters involving colour and pattern as well as
structure, both in the sweet pea, the primula, and other species. Since the
forms of _polytes_ female behave in breeding like the various {92} forms of
sweet pea and primula there is every reason to suppose that they _arose_ in
the same way, that is to say, as sudden sports or mutations and not by the
gradual accumulation of slight differences.

But if we take this view, which is certainly most consonant with the
evidence before us, we must assign to natural selection a different rôle
from that which is generally ascribed to it. We cannot suppose that natural
selection has played any part in the _formation_ of a mimetic likeness. The
likeness turned up suddenly as a sport quite independently of natural
selection. But although natural selection may have had nothing to do with
its production, it may nevertheless have come into play in connection with
the _conservation_ of the new form. If the new form possesses some
advantage over the pre-existing one from which it sprang, is it not
conceivable that natural selection will come into operation to render it
the predominant form? To this question we shall try to find an answer in
the next chapter.

       *       *       *       *       *


{93}

CHAPTER VIII

THE CASE OF _PAPILIO POLYTES_ (_cont._)

It was suggested in the last chapter that if a new variation arose as a
sport--as a sudden hereditary variation--and if that variation were,
through resemblance to a different and unpalatable species, to be more
immune to the attacks of enemies than the normal form, it was conceivable
that the newer mimetic sport would become established, and in time perhaps
come to be the only form of the species. We may suppose, for example, that
the A female of _P. polytes_ arose suddenly, and that owing to its likeness
to the presumably distasteful _P. aristolochiae_ it became rapidly more
numerous until in some localities it is the commonest or even the only
form. However, before discussing the establishing of a mimetic form in this
manner we must first deal with certain general results which may be
expected to follow on a process of selection applied to members of a
population presenting variations which are inherited on ordinary Mendelian
lines.

Let us suppose that we are dealing with the inheritance of a character
which depends upon the presence of the genetic factor X; and let us also
suppose that the heterozygous form () is indistinguishable {94} from the
homozygous form () in appearance. In other words the character dependent
upon X exhibits complete dominance. With regard to X then all the members
of our population must belong to one or other of three classes. They may be
homozygous (XX) for X, having received it from both parents, or they may be
heterozygous (Xx) because they have received it from only one parent, or
they may be devoid of X, _i.e._ pure recessives (xx). An interesting
question arises as to the conditions under which a population containing
these three kinds of individuals remains stable. By stability is meant that
with the three kinds mating freely among themselves and being all equally
fertile, there is no tendency for the relative proportions of the three
classes to be disturbed from generation to generation. The question was
looked into some years ago by G. H. Hardy, who shewed that if the mixed
population consist of p XX individuals, 2q Xx individuals and r xx
individuals, the population will be in stable equilibrium with regard to
the relative proportions of these three classes so long as the equation pr
= q^2 is satisfied[48].

Now let us suppose that in place of equality of conditions selection is
exercised in favour of those individuals which exhibit the dominant
character. It has been shewn by Mr Norton that even if the selection
exercised were slight the result in the end would be that the recessive
form would entirely disappear. The total time required for bringing this
about would {95} depend upon two things, (1) the proportion of dominants
existing in the population before the process of selection began, and (2)
the intensity of the selection process itself. Suppose, for example, that
we started with a population consisting of pure dominants, heterozygotes,
and recessives in the ratio 1:4:4. Since these figures satisfy the equation
pr = q^2, such a population mating at random within itself is in a state of
stable equilibrium. Now let us suppose that the dominant form (including of
course the heterozygotes) is endowed with a selection advantage over the
recessives of 10%, or in other words that the relative proportion of the
recessives who survive to breed is only 90% of the proportion of dominants
that survive[49]. It is clear that the proportion of dominants must
gradually increase and that of the recessives diminish.

At what rate will this change in the population take place? Mr Norton has
worked this out (see App. I) and has shewn that at the end of 12
generations the proportions of pure dominants, heterozygotes, and
recessives will be 1:2:1. The population will have reached another position
of equilibrium, but the proportion of recessives from being four-ninths of
the {96} total is now reduced to one-quarter. After 18 more generations the
proportions 4:4:1 are reached, the recessives being only one-ninth of the
total; after 40 further generations of the process they become reduced to
one-fortieth. In other words a selective advantage of 10% operating against
the recessives will reduce their numbers in 70 generations from nearly
one-half of the population to less than one-fortieth.

With a less stringent selective rate the number of generations elapsing
before this result is brought about will be larger. If, for example, the
selective rate is diminished from 10% to 1% the number of generations
necessary for bringing about the same change is nearly 700 instead of
70--roughly ten times as great. Even so, and one can hardly speak of a 1%
selective rate as a stringent one, it is remarkable in how brief a space of
time a form which is discriminated against, even lightly, is bound to
disappear. Evolution, in so far as it consists of the supplanting of one
form by another, may be a very much more rapid process than has hitherto
been suspected, for natural selection, if appreciable, must be held to
operate with extraordinary swiftness where it is given established
variations with which to work.

We may now consider the bearing of these theoretical deductions upon the
case of _Papilio polytes_ in Ceylon. Here is a case of a population living
and breeding together under the same conditions, a population in which
there are three classes depending upon the presence or absence of two
factors, X and Y, {97} exhibiting ordinary Mendelian inheritance. For the
present we may consider one of these factors, X, which involves the
proportion of mimetic to non-mimetic forms. It is generally agreed among
observers who have studied this species that of the three forms of female
the M form is distinctly the most common, while of the other two the H form
is rather more numerous than the A form. The two dominant mimetic forms
taken together, however, are rather more numerous than the recessive M
form. The most recent observer who studied this question, Mr Fryer,
captured 155 specimens in the wild state as larvae. When reared 66 turned
out to be males, while of the females there were 49 of the two mimetic
forms and 40 of the M form, the ratio of dominants to recessives being
closely 5:4[50]. Now as has already been pointed out the ratio 5:4 of
dominants and recessives is characteristic of a population exhibiting
simple Mendelian inheritance when in a state of stable equilibrium. The
natural deduction from Mr Fryer's figures is that with regard to the factor
that differentiates the mimetic forms from the non-mimetic, the _polytes_
population is, for the moment at any rate, in a position of stable
equilibrium. This may mean one of two things. Either the population is
definitely in a state of equilibrium which has lasted for a period of time
in the past {98} and may be expected to endure for a further period in the
future, or else the population is in a condition of gradual change as
regards the numerical proportion of mimetics and non-mimetics, progressing
towards the elimination of the one or the other, the present state of
equilibrium being merely transitory and accidental. In this connection a
few scraps of historical evidence are of interest. Of the various forms of
_P. polytes_ the A form of female was the first to be described in 1758,
and not long after (1776) the H form was registered as a species under the
name of _Papilio Eques Trojanus romulus_. Later on the female resembling
the male found its way into the literature as _Papilio pammon_. From the
fact that the mimetic forms were known before the non-mimetic, it is
unlikely that they can have been scarce a century and a half ago. As _P.
polytes_ certainly produces at least four broods a year in Ceylon this
period of time represents something like 600 generations in the life of the
species, and we have already seen that even if the mimetic forms have but a
1% advantage over the non-mimetic the proportion of the latter would
decrease from nearly equality down to but 1 in 40 in about 700 generations.
Actually for _P. polytes_ the decrease would not be so marked because the
male is non-mimetic. Owing to this peculiar feature the rapidity of change
in the proportion of the different forms is reduced to about one-half of
what it would be if the males were also mimetic. Nevertheless the change
from nearly equality to about one non-mimetic in 40 would have taken place
{99} during the time _P. polytes_ has been known if a 2% selection
advantage had operated during that period in favour of the mimetic. If
there has been any appreciable selection going on during that time mimetics
must have been far rarer when the species was first discovered, but the
fact that both the mimetic forms made their way into collections before the
non-mimetic tells distinctly against this supposition. Nor is there any
reason to suppose that the non-mimetic form has been dwindling in numbers
relatively to the mimetics during the last half century. Moore[51] in 1880
records an earlier observation of Wade's that "These three butterflies are
very common, especially those of the first form; the second being perhaps
least so." The first form alluded to is the M form, and the second is the A
form, so that at the time Wade wrote the relative proportions of these
three forms must have been very much what they are to-day. Even during half
a century and with such a relatively weak selection rate as 2% in favour of
the mimetics, the proportion of non-mimetics should drop from about 4:5
down to about 1:5. Therefore we must either infer that in respect of
mimetic resemblances natural selection does not exist for _P. polytes_ in
Ceylon, or else we must suppose its force to be so slight that in half a
century certainly, and perhaps in a century and a half, it can produce no
effect appreciable to the necessarily rough method of estimation employed.
{100}

It may, however, be argued that even an exceedingly low selection rate is
able to bring about the elimination of one or other type provided that it
acts for a sufficiently long time. This is perfectly true. A selective rate
of .001% would reduce the proportion of recessives to dominants from 4:5
down to 1:40 in the course of about 1,400,000 generations where the mimetic
resemblance is already established. Such a form of selection entails the
death of but one additional non-mimetic in 100,000 in each generation. If,
however, the mimetic resemblance is not fully established and the mimic
bears only what supporters of the mimicry theory term a "rough" resemblance
to the model, it is clear that it will have far less chance of being
mistaken for the model. Its advantage as compared with the non-mimetic form
will be very much less. Even supposing that the slight variations concerned
are inherited, an intensity of selection which would produce a certain
change in 1,400,000 generations where a mimetic resemblance is already
established must be supposed to take an enormously greater time where an
approach to a model has to take place from a "rough" resemblance.

From the data as to the relative proportions of the polymorphic females of
_P. polytes_ during the past and at present, and from the behaviour of
their different forms in breeding, the following conclusions only can be
drawn. Either natural selection, from the point of view of mimicry, is
non-existent for this species in Ceylon, or else it is so slight as to be
unable in half a {101} century to produce an appreciable diminution in the
proportion of non-mimetic females. For even if the mimetic resemblance
brings about but the survival of one additional protected form in 100 as
compared with the unprotected, this means a marked diminution in the
proportion of M females in 50 years--a diminution such as there are no
grounds for supposing to have taken place.

It has been argued that in populations exhibiting Mendelian heredity even a
relatively low selection rate must bring about a rapid change in the
constitution of a mixed population. Have we any grounds for supposing that
populations of this sort can undergo such rapid changes? In cases where
mimetic resemblances are involved we have no examples of the sort. But some
interesting evidence as to the rate at which a population may change is to
be gathered from the study of melanism in certain moths. It is well known
that in some parts of England the common peppered moth, _Amphidasys
betularia_ has been almost entirely supplanted by the darker melanic form
_doubledayaria_. It first made its appearance near Manchester in 1850, and
from that centre has been gradually spreading over northern England, the
Midlands, and the south-eastern counties. At Huddersfield, for instance,
fifty years ago only the type form _betularia_ existed; to-day there is
nothing but _doubledayaria_. In Lancashire and Cheshire the type is now
rare. On the continent, too, there is the same story to be told. The
melanic form first appeared in Rhenish {102} Prussia in 1888; to-day it is
much more abundant than the older type. There, too, it is spreading
eastwards and southwards to Thuringia, to Saxony, to Silesia. What
advantage this new dark form has over the older one we do not know[52].
Some advantage, however, it must have, otherwise it could hardly supplant
_betularia_ in the way that it is doing. From our present standpoint two
things are of interest in the case of the peppered moth--the rapidity with
which the change in the nature of the population has taken place, and the
fact that the two forms exhibit Mendelian heredity, _doubledayaria_ being
dominant and _betularia_ recessive[53]. Moreover, mixed broods have been
reared from wild females of both sorts, and so far as is known the two
forms breed freely together where they co-exist. This case of the peppered
moth shews how swiftly a change may come over a species[54]. It is not at
all improbable that the establishing of a new variety at the expense of an
older one in a relatively short space of time is continually going on,
especially in tropical lands where {103} the conditions appear to be more
favourable to exuberance of variation and where generations succeed one
another in more rapid succession. At present, however, we are without data.
A form reported by an old collector as common is now rare; a variety once
regarded as a great prize is now easily to be found. Such to-day is the
sort of information available. For the solution of our problem it is, of
course, useless. The development of Mendelian studies has given us a
method, rough perhaps but the best yet found, of testing for the presence,
and of measuring the intensity, of natural selection. Much could be learned
if some common form were chosen for investigation in which, as in _P.
polytes_, there are both mimetic and non-mimetic forms. Large numbers
should be caught at stated intervals, large enough to give trustworthy data
as to the proportions of the different forms, mimetic or non-mimetic, that
occurred in the population. Such a census of a polymorphic species, if done
thoroughly, and done over a series of years at regular intervals, might be
expected to give us the necessary data for deciding whether the relative
proportion of the different forms was changing--whether there were definite
grounds for supposing natural selection to be at work, and if so what was
the rate at which it brought the change about.

       *       *       *       *       *


{104}

CHAPTER IX

THE ENEMIES OF BUTTERFLIES

The theory of mimicry demands that butterflies should have enemies, and
further that those enemies should exercise a certain discrimination in
their attacks. They must be sufficiently observant to notice the difference
between the mimetic and the non-mimetic form; they must be sufficiently
unobservant to confuse the mimetic form with the unpalatable model. And, of
course, they must have enough sense of taste to dislike the unpalatable and
to appreciate the palatable varieties. What these enemies are and whether
they can be supposed to play the part required of them we may now go on to
consider.

Butterflies are destroyed in the imago state principally by three groups of
enemies--predaceous insects, lizards, and birds. It is known that monkeys
also devour butterflies to some extent, but such damage as they inflict is
almost certainly small in comparison with that brought about by the three
groups already mentioned. In view of the very different nature of these
groups it will be convenient to consider them separately. {105}

I. _Predaceous Insects._ Butterflies are known to be preyed upon by other
insects of different orders, and a considerable number of observations have
recently been gathered together from various sources and put on record by
Professor Poulton[55]. These observations shew that butterflies may be
devoured by mantids, dragon-flies, and blood-sucking flies of the families
Empiidae and Asilidae. For mantids the records are scanty, but they have
been observed to kill presumably distasteful forms as often as those which
are considered palatable. An interesting set of experiments was made by G.
A. K. Marshall on captive mantids in Africa[56]. Of the eleven individuals
representing several species with which he experimented, some ate every
butterfly offered, including the distasteful Danaines and Acraeines.
Others, however, shewed some distaste of the Acraeines and would not devour
them so freely as butterflies of other species. There are no grounds,
however, for supposing that the mantids had any appreciation of the warning
coloration of the Acraeines. Whether completely eaten or not the Acraeines
were apparently sufficiently damaged to prevent their taking any further
part in the propagation of their species. Warning coloration is not of much
service to its possessor who has to be tasted and partially eaten before
being eventually rejected. Even if some mantids shew distaste of certain
unpalatable butterflies, that distaste is probably seldom {106} exercised
with a gentleness sufficient to ensure that the butterfly reaps the reward
of its disagreeable nature. And unless, of course, the butterfly is allowed
to do so the enemy can play no part in the production or maintenance of a
mimetic resemblance.

What is true for mantids is probably also true for the other groups of
predaceous insects. Dragon-flies and wasps have been recorded as attacking
the distasteful as well as butterflies of unprotected groups. Among the
most serious enemies of butterflies must probably be reckoned the
blood-sucking Asilids. These powerful and ferocious flies seize butterflies
on the wing with their strong claws and plunge their proboscis into the
thorax. Apparently they inject some swift poison, for the butterfly is
instantly paralysed, nor is there any sign of struggle. The Asilid flies
off with its victim, sucking the juices as it goes. There can be no doubt
in the mind of any one who has watched these creatures hawking butterflies
that their natural gifts are such as to enable them to exercise
discrimination in their food. Most insect life is at their mercy but they
appear to exercise no choice, seizing and devouring the first flying thing
that comes within easy reach. Certainly as regards butterflies palatability
or the reverse makes no difference, and they are known to feed
indiscriminately both upon the evil-flavoured and upon the good. Taking it
all together the evidence is such that we cannot suppose predaceous insects
to pay any attention to warning colours, and, therefore, we cannot regard
them as playing any part in connection with mimetic resemblance. {107}

II. _Lizards._ In those parts of the world where lizards of larger size are
abundant there is plenty of evidence that certain species are very
destructive to butterfly life. As might be expected this is especially true
of forms which are either arboreal or semi-arboreal in habit. Among the
reptiles of Ceylon, for example, are several species of the genus
_Calotes_, of which two, _C. ophiomachus_ and _C. versicolor_, are
particularly abundant. In appearance and habits they are not unlike
chameleons though far more active in their movements. Like chameleons, too,
they are able to change colour, and the fact that they can assume a
brilliant scarlet hue about the head and neck has probably led to their
popular name of "blood-suckers." It is not impossible that the assumption
of this scarlet coloration may serve as a lure to bring insects within
range. These lizards have often been observed to seize and devour
butterflies. Moreover, it is a common thing to find butterflies with a
large semi-circular patch bitten out of the hind wings, and there is little
doubt but that such injuries have been inflicted by lizards. There is,
however, no evidence to suggest that they exercise any discrimination in
their choice of the butterflies which they attack. This is borne out by
their behaviour towards various species offered to them, both when at
liberty and when caged. In an ingenious series of experiments Col. Manders
brought various butterflies within reach of a _Calotes_ by the help of a
fishing-rod and a long line of fine silk, by this means simulating natural
conditions as far as possible. {108} He found that the lizards ate the
so-called distasteful forms such as _Danais chrysippus_, _Euploea core_,
_Acraea violae_, and _Papilio hector_, as readily as the presumably more
palatable forms[57]. In captivity, too, they will take any butterfly as
readily as another. Experiments by Finn[58] and by the writer[59] proved
that they ate Danaids, Euploeas, and _Papilio aristolochiae_ without any
hesitation so long as the insects were alive and moving. When, too, a
mixture of different species, some with and some without warning
coloration, was given to them all were eaten, nor was there any
discrimination evidenced in the order in which they were taken. The lizard
simply took the first that came within reach and went on until the whole
lot was devoured, wings and all.

Some experiments by Miss Pritchett on the American lizard _Sceleporus
floridanus_ point to the same conclusion[60]. She found that it took
without hesitation any butterfly offered to it including the presumably
distasteful models _Danais archippus_ and _Papilio philenor_ (cf. pp. 45
and 49). On the other hand, another species of lizard with which Miss
Pritchett experimented, _Gerrhonotus infernalis_, refused all the
butterflies offered to it, though it fed freely on Orthopterous insects as
well as on spiders and scorpions.

It seems clear from these various observations and {109} experiments that
certain lizards devour butterflies freely, but that they do not exercise
any discrimination in the species which they attack. All are caught and
devoured indiscriminately, so that in spite of the fact that such lizards
are among the most serious enemies of butterflies we cannot suppose them to
play any part in establishing a mimetic resemblance.

III. _Birds._ The relations which exist between butterflies and their bird
enemies have for many years been the subject of keen discussion. It is
generally recognised that if mimetic resemblances become established
through the agency of discriminating enemies those enemies must be birds.
Hence those interested in the question of mimicry have for some years past
turned their attention to birds more than to the other enemies of
butterflies. That many birds systematically feed on butterflies is a fact
that does not admit of doubt. It is true that, as Mr Marshall points out in
the valuable paper in which he has summarised the evidence[61],
observations of birds eating butterflies are relatively scanty. Though, as
he points out, this is equally true for other groups of insects besides
butterflies, bird attacks on butterflies, owing to the conspicuous nature
of the victim, are much more likely to attract attention than attacks on
other groups. We are still without much information as to the extent to
which birds destroy butterflies and as to whether they shew any decided
preference for certain species over others. A careful examination of the
contents of the {110} stomachs of large numbers of insectivorous birds in a
tropical area would go some way towards deciding the matter, but at present
such information is lacking. We have to rely upon the existing observations
of birds attacking butterflies in the wild state, and upon certain feeding
experiments made with captive birds.

Observations on birds attacking butterflies where mimetic forms occur have
been made almost entirely in certain parts of Africa, in India, and in
Ceylon. For Africa, Marshall has collected some forty-six observations of
which almost half are concerned with Pierines. The remainder include four
instances of attacks on species of _Acraea_, a genus which on the mimicry
theory must be regarded as among the most unpalatable of butterflies.

The records from the Indo-Malayan region (principally India and Ceylon) are
somewhat more numerous and here again more than one-third of them refer to
Pierines. Among the others are records of the distasteful forms _Euploea
core_, _E. rafflesii_, _Acraea violae_, and _Papilio hector_ being taken
and devoured.

There is one interesting record which seems to suggest that Swinhoe's
Bee-Eater (_Melittophagus swinhoei_) may exercise that discrimination in
the butterflies it attacks which is demanded on the mimicry theory.
Lt.-Col. Bingham on one occasion in Burma noticed this species hawking
butterflies. He records that they took _Papilio erithonius_, _P. sarpedon_,
_Charaxes athamas_, _Cyrestis thyodamas_, and _Terias hecabe_, and probably
also species of the genera _Prioneris_, _Hebomoia_ (Pierines), {111}
_Junonia_ and _Precis_ (Vanessids). And he goes on to say: "I also
particularly noticed that the birds never went for a _Danais_ or _Euploea_,
or for _Papilio macareus_ and _P. xenocles_, which are mimics of Danais,
though two or three species of _Danais_, four or five of _Euploea_, and the
two above-mentioned mimicking _Papilios_ simply swarmed along the whole
road[62]."

Marshall also quotes a case of attack by a green bee-eater on a _Danais_ in
which the butterfly was caught and subsequently rejected, after which it
flew away. Little stress, however, can be laid upon this case in view of
the more recent data brought together by Col. Manders and Mr Fryer.
Discussing the attacks of birds on butterflies in Southern India and
Ceylon, Col. Manders gives the following quotation[63] from a letter of Mr
T. N. Hearsy, Indian Forest Service:

"Coimbatore, 6. 6. 10.... I have frequently seen the common green bee-eater
(_Merops viridis_) and the king-crow (_Buchanga atra_) take butterflies on
the wing, the butterflies being _Catopsilia pyranthe_, _C. florella_,
_Terias hecabe_ and _Papilio demoleus_. The bee-eater I have also seen
taking _Danais chrysippus_ and _Danais septentrionis_, and I remember to
have been struck with their taste for those latter...."

Col. Manders also brings forward evidence for these Danaids and Euploeas
being eaten by Drongos and by the paradise flycatcher. Still more recently
an interesting contribution to the matter has been made by {112} Mr J. C.
F. Fryer[64]. The Ashy Wood-swallow (_Artamus fuscus_) had been recorded on
two occasions as having attacked _Euploea core_. Mr Fryer was fortunate in
coming across this bird in the gardens at Peradeniya, near Kandy, at a time
when _Euploea core_ and _Danais septentrionis_ were particularly abundant,
and he watched a number of them systematically hawking these presumably
unpalatable species. As he observes, "in Ceylon a resemblance to the genera
_Danais_ and _Euploea_ is doubtfully of value; in fact in the neighbourhood
of Wood-swallows it is a distinct danger." Fryer also noted that the
mimetic forms of _P. polytes_ were taken as well as the non-mimetic.

For tropical Central and South America, that other great region where
mimetic forms are numerous, there are unfortunately hardly any records of
butterflies attacked by birds. Bates stated that the Pierines were much
persecuted by birds, and his statement is confirmed by Hahnel, but exact
observations for this region are remarkably scanty. Belt observed a pair of
birds bring butterflies and dragon-flies to their young, and noticed that
they brought no Heliconii to the nest although these swarmed in the
neighbourhood[65]. On the other hand, Mr W. Schaus[66], from an experience
of many years spent in the forests of Central America, considers that the
butterflies of this region are hardly, if ever, attacked by birds. {113}

For North America Marshall records over 80 cases of birds attacking
butterflies. Among them is an interesting record of a bird seizing and
rejecting a specimen of _Anosia plexippus_ (= _Danais archippus_), one of
the few Danaines found in this region.

It must be admitted that the data at present available with regard to the
attacks of birds upon butterflies under natural conditions are too meagre
to allow of our coming to definite conclusions on the points at issue. It
is safe to say that a number of species of birds have been known to attack
butterflies--that a few out of the number feed upon butterflies
systematically--that some of the most persistent bird enemies devour the
presumably protected forms as freely as the unprotected--but that in a few
instances there is some reason for supposing that the bird discriminates.
Beyond this it is unsafe to go at present.

In attempting to come to a decision as to the part played by birds in the
destruction of butterflies an evident desideratum is a knowledge of the
contents of the stomachs of freshly killed birds. Unfortunately few
systematic observations of this nature exist. G. L. Bates[67], when
collecting in the Southern Cameroons, noted the stomach contents of a
considerable number of birds. The remains of beetles were recognised in 213
cases: Orthoptera in 177: ants in 57 (mostly in stomachs of birds of the
genus _Dendromus_): other Hymenoptera in 8: coccids in 32: bugs in 19:
white ants in 31: slugs and snails in 24: spiders in 85 {114} (mostly in
Sunbirds): millipedes in 20; but in no single instance were the remains of
butterflies found. More recently Bates' account has been criticized by
Swynnerton[68] who comments on the difficulty of identifying butterfly
remains as compared with those of beetles and grasshoppers. He states that
the pellets ejected by captive birds after a meal of butterflies contain
only fine debris which is very difficult to identify. Further, he found
that of twenty small bird excreta collected in the forest no less than
eighteen contained scales and small wing fragments of Lepidoptera.

Some attention has been paid to the relation between birds and butterflies
in the United States, and under the auspices of the Department of
Agriculture a large number of birds' stomachs have been investigated.
Careful examination of some 40,000 stomachs of birds shot in their natural
habitats resulted in the discovery of butterfly remains in but four. It
cannot, therefore, be supposed that birds play much part in connection with
such mimetic resemblances as are found in North America (cf. pp. 45-49).
Nevertheless, it is known that on occasion large numbers of butterflies may
be destroyed by birds. An interesting case is described by Bryant[69] of an
outbreak in North California of _Eugonia californica_, a close relative of
the tortoiseshell. The butterfly was so abundant as to be a plague, and
five species of birds took advantage of its great abundance to prey largely
upon it. From {115} his examination of the stomachs Bryant came to the
conclusion that some 30% of the food of these five species was composed of
this butterfly. The stomachs of many other species were examined without
ever encountering butterfly remains. Nor did field observations support the
view that any species, other than the five specially noted, ever attacked
these butterflies. The case is of interest in the present discussion as
evidence that the identification of butterfly remains in the stomachs of
birds is by no means so difficult as some observers suggest.

Besides this evidence derived from observations upon birds in the wild
state some data have been accumulated from the experimental feeding of
birds in captivity. Of such experiments the most extensive are those of
Finn[70] in South India. He experimented with a number of species of
insectivorous birds belonging to different groups. Of these he found that
some, among which may be mentioned the King-crow, Starling, and
Liothrix[71], objected to Danaines, _Papilio aristolochiae_ and _Delias
eucharis_, a presumably distasteful Pierine with bright red markings on the
under surface of the hind wings (Pl. II, fig. 1). In some cases the bird
refused these forms altogether, while in others they were eaten in the
absence of more palatable forms. The different species of birds often
differed in {116} their behaviour towards these three "nauseous" forms. The
Hornbill, for example, refused the Danaines and _P. aristolochiae_
absolutely, but ate _Delias eucharis_. Some species again, notably the
Bulbuls (_Molpastes_) and Mynahs, shewed little or no discrimination, but
devoured the "protected" as readily as the "unprotected" forms. Finn also
states that "_Papilio polytes_ was not very generally popular with birds,
but much preferred to its model, _P. aristolochiae_."

In many of Finn's experiments both model and mimic were given to the birds
simultaneously so that they had a choice, and he says that "in several
cases I saw the birds apparently deceived by mimicking butterflies. The
Common Babbler was deceived by _Nepheronia hippia_[72] and Liothrix by
_Hypolimnas misippus_. The latter bird saw through the disguise of the
mimetic _Papilio polites_, which, however, was sufficient to deceive the
Bhimraj and King-crow. I doubt if any bird was impressed by the mimetic
appearance of the female _Elymnias undularis_" (cf. Pl. IV, fig. 5). Finn
concluded from his experiments that on the whole they tended to support the
theory of Bates and Wallace, though he admits that the unpalatable forms
were commonly taken without the stimulus of actual hunger and generally
without signs of dislike. Certainly it is as well to be cautious in drawing
conclusions from experiments with captive birds. The King-crow, for
instance, according to Finn shewed a marked dislike for Danaines in
captivity; yet Manders records this {117} species as feeding upon Danaines
under natural conditions (cf. p. 111).

A few further experiments with the birds of this region were carried out by
Manders[73] in Ceylon. The results are perhaps to be preferred to Finn's,
as the birds were at liberty. Manders found that the Brown Shrike (_Lanius
cristatus_) would take butterflies which were pinned to a paling. In this
way it made off with the mimetic females of _Hypolimnas bolina_ and _H.
misippus_, as well as with _Danais chrysippus_ and _Acraea violae_ which
were successively offered to it. Evidently this species had no repugnance
to unpalatable forms. Manders also found that a young Mynah allowed
complete liberty in a large garden would eat such forms as _Acraea violae_
and _Papilio hector_. As the result of his experience Manders considers
that the unpalatability of butterflies exhibiting warning coloration has
been assumed on insufficient data, and he is further inclined to doubt
whether future investigations will reveal any marked preference in those
birds which are mainly instrumental in the destruction of butterflies.

A few experiments on feeding birds with South African butterflies are
recorded by Marshall. A young Kestrel (_Cerchneis naumanni_) was fed from
time to time with various species of butterflies. In most cases the
butterflies offered were eaten even when they were species of _Acraea_. On
the other hand _Danais chrysippus_ was generally rejected after being
partly {118} devoured. When first offered this unpalatable species was
taken readily and it was only after it had been tasted that the bird
rejected it. When offered on several subsequent occasions it was partly
eaten each time, and the behaviour of the Kestrel did not suggest that it
associated a disagreeable flavour even with this conspicuous pattern.
Another young Kestrel (_Cerchneis rupicoloides_) was also used for
experiment. At first it would not take butterflies and at no time did it
shew any fondness for them. Indeed it is doubtful from the way in which
they seem to have shaped at the insects whether either of these Kestrels
had had any experience of butterflies before the experiments began.

A Ground Hornbill with which Marshall also experimented ate various
species, including _Acraea_, but, after crushing it, refused the only
_Danais chrysippus_ offered. It is hardly likely that this large omnivorous
bird operates as a selecting agent in cases of mimicry.

In an interesting paper published recently McAtee[74] discusses the value
of feeding experiments with animals in captivity as a means of indicating
their preference for different articles of diet. After reviewing the
various evidence brought forward he concludes that the food accepted or
rejected by captive animals is very little guide to its preferences under
natural conditions. He points out that a bird in captivity not infrequently
rejects what is known to form a main staple of its diet in nature, and that
conversely it may eagerly accept something which, in the wild state, it
{119} would have no opportunity of obtaining. Great caution must,
therefore, be exercised in the interpretation of feeding experiments made
with birds in captivity.

It appears to be generally assumed that colour perception in birds is
similar to what it is among human beings, but some experiments made by
Hess[75] render it very doubtful whether this is really the case. In one of
these experiments a row of cooked white grains of rice was illuminated by
the whole series of spectral colours from violet to deep red. Hens which
had been previously kept in the dark so that their eyes were adapted to
light of low intensity were then allowed to feed on the spectral rice. The
grains illuminated by green, yellow, and red were quickly taken, but the
very dark red, the violet, and the blue were left, presumably because the
birds were unable to perceive them. Again, when the birds were given a
patch of rice grains of which half was feebly illuminated by red light and
the other half more strongly by blue light, they took the red but left the
blue. Previous experiment had shewn that with ordinary white light the
birds always started on the best illuminated grains. It seems reasonable to
conclude, therefore, that in the red-blue experiment the feebly illuminated
red grains were more visible than the far more strongly lighted blue ones.
It might be objected that the birds had a prejudice against blue, but, as
Hess points out, this is almost certainly not the case because they took
grains {120} which were very strongly illuminated with blue. Results of a
similar nature were also obtained from pigeons, and from a kestrel which
was fed with pieces of meat lighted with different colours.

On the whole these experiments of Hess convey a strong suggestion that the
colour perceptions of birds may be quite different from our own, more
especially where blue is concerned. Great caution is needed in discussing
instances of mimicry in their relation to the bird, for we have no right to
assume that the bird sees things as we do. On the other hand, it is a
matter of much interest to find that in general blue plays relatively
little part in cases of mimetic resemblance among butterflies; some
combination of a dark tint with either red, white, brown, or yellow being
far more common.

It will probably be admitted by most people that the evidence, taken all
together, is hardly sufficient for ascribing to birds that part in the
establishing of a mimetic likeness which is required on the theory of
mimicry. That birds destroy butterflies in considerable numbers is
certainly true, but it is no less true that some of the most destructive
birds appear to exercise no choice in the species of butterfly attacked.
They simply take what comes first and is easiest to catch. It is probably
for this reason that the Wood-swallow feeds chiefly on Euploeines and
Danaines (cf. p. 112). It is probably for this reason also that such a
large proportion of the records of attacks on butterflies under natural
conditions refer to the Pierines; for owing to {121} their light colour it
is probable that the "Whites" are more conspicuous and offer a better mark
for a bird in pursuit than darker coloured species.

_Mammals._ Apart from man it is clear that only such mammals as are of
arboreal habits are likely to cause destruction among butterflies in the
imago state. Apparently there are no records of any arboreal mammal, except
monkeys, capturing butterflies in the wild state, nor is there much
evidence available from feeding experiments. But such evidence as exists is
of considerable interest. As the result of feeding butterflies of different
sorts to an Indian Tree-shrew (_Tupaia ferruginea_) Finn[76] found that it
shewed a strong dislike to Danaids and to _Papilio aristolochiae_ though it
took readily _Papilio demoleus_, _Neptis kamarupa_, and _Catopsilia_ (a
Pierine). It is fairly certain that if the Tree-shrew is an enemy of
butterflies in the wild state it is a discriminating one.

The other mammals with which experiments have been made are the common
baboon, a monkey (_Cercopithecus pygerythrus_), and a mongoose (_Herpestes
galera_)--all by Marshall[77] in South Africa. The mongoose experiments
were few and inconclusive, nor is this a matter of much moment as it is
unlikely that this mammal is a serious enemy of butterflies.

The monkey ate various forms of _Precis_ (a Vanessid), after which it was
given _Acraea halali_. This distasteful form was "accepted without
suspicion, but when {122} the monkey put it into his mouth, he at once took
it out again and looked at it with the utmost surprise for some seconds,
and then threw it away. He would have nothing to do with an _Acraea
caldarena_ which I then offered him[78]."

The experiments with the baboons were more extensive. Two species of
_Acraea_, _halali_ and _axina_, were recognised when first offered and
refused untasted. _Danais chrysippus_, on the other hand, was tasted on
being offered for the first time, and then rejected. This species was twice
offered subsequently and tasted each time before being rejected. When
offered the fourth time it was rejected at sight. The baboon evidently
learned to associate an unpleasant taste with the _chrysippus_ pattern. At
this stage it would have been interesting to have offered it some
well-known mimic of _chrysippus_, such as the female of _Hypolimnas
misippus_ or the _trophonius_ form of _Papilio dardanus_, but this
experiment was unfortunately not made. Marshall did, however, offer it at
the same time a specimen each of _Byblia ilithyia_ (a Vanessid) and of
_Acraea axina_ to which it bears a general resemblance. The baboon took the
former but neglected the latter altogether. The general resemblance between
the two species was not sufficiently close to deceive it.

These experiments with mammals, though few in number, are of unusual
interest. Should they be substantiated by further work it is not impossible
{123} that, as a factor in the establishing of a mimetic likeness, a
stronger case may be made out for the monkey than the bird. The monkey
apparently eats butterflies readily[79]: owing probably to a keener sense
of smell it shews far less hesitation as to its likes and dislikes: its
intelligence is such that one can easily imagine it exercising the
necessary powers of discrimination; in short it is the ideal enemy for
which advocates of the mimicry theory have been searching--if only it could
fly. As things are its butterfly captures must be made when the insect is
at rest, probably near sunrise and sunset, and this leads to a difficulty.
Most butterflies rest with their wings closed. In many of the well-known
cases of mimicry the pattern on the under surface of the mimic's wings
which would meet the monkey's eye is quite different from that of its
model. It is difficult in such cases to imagine the monkey operating as a
factor in establishing a resemblance between the upper surfaces of the
wings of the two unrelated species. On the other hand, some butterflies,
{124} _e.g._ _Papilio polytes_, rest with wings outspread, and there are
rare cases, such as that of _P. laglaizei_ (p. 27), where the most striking
point about the resemblance is only to be appreciated when the insects are
at rest with their wings closed. In such cases it is conceivable that the
monkey may play a part in the elimination of the non-mimetic elements of a
palatable species which at the same time possessed a mimetic form closely
resembling another species disagreeable to the monkey's taste. As has been
pointed out earlier (p. 96) even a slight persecution directed with
adequate discrimination will in time bring about a marked result where the
mimetic likeness is already in existence. It is not impossible therefore
that the establishing of such a likeness may often be due more to the
discrimination of the monkey than to the mobility of the bird.

       *       *       *       *       *


{125}

CHAPTER X

MIMICRY AND VARIATION

It is clear from the last few chapters that the theory of mimicry in
butterflies with its interpretation of the building up of these likenesses
by means of natural selection in the form of predaceous birds and other
foes is open to destructive criticism from several points of view. The
evidence from mimicry rings makes it almost certain that in some cases the
resemblance must be founded on an initial variation of such magnitude that
the mimic could straightway be confused with the model. Till the mimic can
be mistaken for the model natural selection plays no part. The evidence
from breeding suggests strongly that in certain cases (_e.g._ _Papilio
polytes_) the likeness arose in the form in which we know it to-day. In
such cases there is no reason for supposing that natural selection has had
anything to do with the formation of the finished mimic. Considerations of
this nature may be said to have destroyed the view, current until quite
recently, that in the formation of a mimetic resemblance the exclusive
agent was natural selection. During the past few years it has come to be
admitted by the staunchest upholders of the theory of mimicry that natural
{126} selection would not come into play until the would-be mimic was
sufficiently like the model to be confused with it under natural
conditions[80]. The part now often attributed to natural selection is to
put a polish on the resemblance and to keep it up to the mark by weeding
out those which do not reach the required standard. It is supposed that if
natural selection ceases to operate the mimetic resemblance is gradually
lost owing to the appearance of variations which are no longer weeded out.
An interesting case has recently been brought forward by Carpenter[81] and
explained on these lines: The Nymphaline _Pseudacraea eurytus_ is a
polymorphic species found in Central Africa. In Uganda it occurs in several
distinct forms which were originally supposed to be distinct species. Three
of these forms bear a marked resemblance to three species of the Acraeine
genus _Planema_.

               Mimic                                     Model
       _Pseudacraea eurytus_                           _Planema_
  Form  _hobleyi_[82] (Pl. VII, figs. 6, 7)   _macarista_ (Pl. VII, fig. 2)
        _terra_ (Pl. VII, fig. 8)             _tellus_ (Pl. VII, fig. 3)
        _obscura_                             _paragea_ (Pl. VII, fig. 4)

These different species occur round Victoria Nyanza and also on some of the
islands in the lake. Some {127} interesting points are brought out by a
comparison of the occurrence and variation of the species on the mainland
with what is found on Bugalla Island in the Sesse Archipelago. On the
mainland the Pseudacraeas are abundant but the Planemas even more so,
outnumbering the former by about 5:2[83]. Moreover, it is rare to find
individuals more or less intermediate between the three forms, though they
are known to occur. On Bugalla Island, however, a different state of things
is found. The Pseudacraeas are very abundant, whereas the Planemas, owing
doubtless to the scarcity of their food plant, are relatively rare, and are
very greatly outnumbered by the Pseudacraeas. At the same time the
proportion of transitional forms among the Pseudacraeas is definitely
higher than on the mainland. These facts are interpreted by Carpenter as
follows:--

On the mainland where the models are abundant there is a vigorous action on
the part of natural selection. The mimetic forms have a strong advantage
and the non-mimetics have been gradually weeded out. But on the island,
where the Pseudacraeas outnumber the models, the advantage obtained through
mimicry is not so great. The so-called transitional forms are little, if at
all, worse off than those closely resembling the scarce models, and
consequently have as good a chance of surviving as any of the typical
mimetic forms. On {128} the mainland, however, the enemies of _Pseudacraea_
are well acquainted with the _Planema_ models which are here common, and
discriminate against individuals which are not close mimics of the
Planemas. The result is that on the mainland transitional forms are scarcer
than on the island. Natural selection maintains a high standard for the
mimetic likeness on the mainland owing to the abundance of the model; but
when the model is scarce the likeness ceases to be kept up to the mark
strictly, and tends to become lost owing to the appearance of fresh
variations which are no longer weeded out.

Here it should be stated that the various Pseudacraeas form a population in
which the different forms mate freely with one another. In the few breeding
experiments that Dr Carpenter was able to make he found that _obscura_
could produce _terra_, and that _tirikensis_ was able to give _obscura_,
the male in each case being, of course, unknown. Far too little work has as
yet been done on the genetics of these various forms, and it would be rash
to make assumptions as to the nature of the intermediates until the method
of experimental breeding has been more extensively employed in analysing
their constitution. Possibly it is not without significance that the
abundance or scarcity of the _obscura_ form runs parallel with the
abundance or scarcity of the intermediates. It suggests that the
intermediates are heterozygous in some factor for which the typical
_obscura_ is homozygous, and the fact that the intermediates are more
numerous than {129} _obscura_ is what is to be looked for in a population
mating at random. This case of the polymorphic _Pseudacraea eurytus_ is one
of the greatest interest, but it would be hazardous to draw any
far-reaching deductions from such facts as are known at present. When the
genetics of the various typical forms and of the intermediates has been
worked out it will be disappointing if it does not throw clear and
important light on these problems of mimetic resemblance.

As the result of modern experimental breeding work it is recognised that an
intermediate form between two definite varieties may be so because it is
heterozygous for a factor for which one variety is homozygous and which is
lacking in the other--because it has received from only one parent what the
two typical varieties receive from both parents or from neither. Its germ
cells, however, are such as are produced by the two typical forms, and the
intermediate cannot be regarded as a stage in the evolution of one variety
from the other. In these cases of mimicry the existence of intermediate
forms does not entail the deduction that they have played a part in the
evolution of one pattern from another under the influence of a given model.
It is quite possible that the new mimetic pattern appeared suddenly as a
sport and that the intermediates arose when the new form bred with that
which was already in existence. But before we are acquainted with the
genetic relationships between the various forms, both types and
intermediates, speculation as to their origin must remain comparatively
worthless. {130}

In this connection a few words on another source of variation may not be
out of place. The patterns of butterflies are often very sensitive to
changes in the conditions to which they are exposed during later larval and
pupal life. Many moths and butterflies in temperate climates are double
brooded. The eggs laid by the late summer brood hatch out, hibernate in the
larval or pupal state, and emerge in the following spring. This spring
brood produces the summer brood during the same year. In these cases it
often happens that the two broods differ in appearance from one another, a
phenomenon to which the term "Seasonal Dimorphism" has been applied. A
well-marked instance is that of the little European Vanessid, _Araschnia
levana_. The so-called _levana_ form which emerges in the spring is a small
black and orange-brown butterfly (Pl. VI, fig. 10). From the eggs laid by
this brood is produced another brood which emerges later on in the summer,
and is, from its very different appearance, distinguished as the _prorsa_
form (Pl. VI, fig. 9). It is very much darker than the spring form and is
characterised by white bands across the wings. The eggs laid by the
_prorsa_ form give rise to the _levana_ form which emerges in the following
spring. It has been shewn by various workers, and more especially by the
extensive experiments of Merrifield[84], that the appearance of the
_levana_ or the _prorsa_ form from any batch of eggs, whether laid by
_prorsa_ or _levana_, is dependent upon the conditions of temperature under
{131} which the later larval and early pupal stages are passed. By cooling
appropriately at the right stage _levana_ can be made to produce _levana_
instead of the _prorsa_ which it normally produces under summer conditions.
So also by appropriate warming _prorsa_ will give rise to _prorsa_.
Moreover, if the conditions are properly adjusted an intermediate form
_porima_ can be produced, a form which occurs occasionally under natural
conditions. The pattern is, in short, a function of the temperature to
which certain earlier sensitive stages in this species are submitted. What
is true of _A. levana_ is true also of a number of other species. In some
cases temperature is the factor that induces the variation. In other
countries where the year is marked by wet and dry seasons instead of warm
and cold ones moisture is the agent that brings about the change. In some
of the South African butterflies of the genus _Precis_ the seasonal change
may be even more conspicuous than in _A. levana_. In _Precis octavia_, for
example, the ground colour of the wet season form is predominantly red,
while in the dry season form of the same species the pattern is different,
blue being the predominating colour (cf. Pl. VI, figs. 11 and 12). Such
examples as these are sufficient to shew how sensitive many butterflies are
to changes in the conditions of later larval and earlier pupal life. The
variations brought about in this way are as a rule smaller than in the
examples chosen, but in no case are they known to be inherited, and in no
case consequently could variation of this nature play any part in {132}
evolutionary change. Before any given variation can be claimed as a
possible stage in the development of a mimetic likeness satisfactory
evidence must be forthcoming that it is not of this nature, but that it is
transmissible and independent of climatic and other conditions.

Many species of butterflies, especially such as are found over a wide
range, exhibit minor varieties which are characteristic of given
localities. These minor varieties may be quite small. In _Danais
chrysippus_, for example, African and Asiatic specimens can generally be
distinguished. On examples from India a small spot is seen just below the
bar on the fore wing and on the inner side of it. Eastwards towards China
this spot tends to become larger and confluent with the white bar, giving
rise to an L-shaped marking; westwards in Africa the spot tends to
disappear altogether. The existence of such local races has been used as an
argument for the hereditary transmission of very small variations--in the
present instance the size of a small white spot[85]. For if it can be
supposed that small differences of this nature are always transmitted, it
becomes less difficult to imagine that a mimetic resemblance has been
brought about by a long series of very small steps. But before this can be
admitted it is necessary to shew by experiment that the size of this spot
is independent of environmental conditions, both climatic and other. Apart
from temperature and moisture it is not improbable that the formation of
{133} pigment in the wings may depend in some degree upon the nature of the
food. The larvae of _D. chrysippus_ feed upon various Asclepiads, and it is
at any rate conceivable that the pigment formation, and consequently the
details of pattern, may be in slight measure affected by the plant species
upon which they have fed. The species of food plants are more likely to be
different at the extremities of the range of a widely distributed form like
_D. chrysippus_, and if they are really a factor in the pattern it is at
the extremities that we should expect to find the most distinct forms[86].
Actually we do find this in _D. chrysippus_, though it does not, of course,
follow that the cause suggested is the true one, or, if true, the only one.
Of the nature of local races too little at present is known to enable us to
lay down any generalization. We must first learn by experiment how far they
remain constant when transported from their own environment and bred in the
environment under which another distinct local race is living. The
behaviour of the transported race under the altered conditions would help
us in deciding whether any variation by which it is characterised had a
definite hereditary basis or was merely a fluctuation dependent upon
something in the conditions under which it had grown up. The decision as to
whether it is hereditary or not must depend upon the {134} test of
breeding, through which alone we can hope to arrive at a satisfactory
verdict upon any given case.

The particular geographical variation which has just been considered
happens to be a small one. But it may happen that a geographical variety is
much more distinct. Indeed it is not impossible that butterflies which are
at present ranked as distinct species may prove eventually to be different
forms of the same species. Especially is this likely to be true of many
forms in South America, of which Bates long ago remarked "that the
suspicion of many of the species being nothing more than local
modifications of other forms has proved to be well founded." Since Bates'
day more material has been forthcoming[87] and it has been shewn that
certain colour schemes are characteristic of distinct geographical regions
in South America where they may occur in species belonging to very
different genera and families. In Central America, for example, the pattern
common to many species is determined by horizontal and oblique black bands
on a bright fulvous brown ground, with two broken yellow bars towards the
tip of the fore wing. The general type is well shewn by _Mechanitis
saturata_ and the female of _Dismorphia praxinoe_ (Pl. X, figs. 7 and 3).
Belonging to this pattern group are a number of different species belonging
to various families, including several Heliconines and Ithomiines, Pierids
such as _Dismorphia_ and _Perrhybris_, Nymphalines of the genera _Eresia_
and {135} _Protogonius_, and other forms. In Eastern Brazil the predominant
pattern is one characterised by a yellow band across the hind wing and a
white or yellow apical fore wing marking (cf. Pl. XV, figs. 3 and 8). Here
also, with the exception of the _Perrhybris_, all the various genera which
figured in the last group are again represented. It is true that the
members of this second group are regarded as belonging to different species
from those of the first group, but as species here are made by the
systematist chiefly, if not entirely, on the colour pattern this fact may
not mean much. Passing now to Ega on the Upper Amazons the general ground
colour is a deep chestnut purple and the apical area of the fore wings
presents a much mottled appearance (cf. Pl. XV, figs. 4 and 9). In this
group again we find represented the different genera found in the other
groups, the only notable absentees being _Eresia_ and _Perrhybris_. Lastly
in Ecuador, Peru, and Bolivia the general pattern scheme consists of
orange-tawny markings on a black ground (cf. Pl. XV, figs. 5 and 10). This
group differs somewhat in composition from the preceding in that it
contains no Pierid and no Danaid. On the other hand its numbers have been
strengthened by the accession of a _Papilio_, an _Acraea_, and two species
of the Satyrid genus _Pedaliodes_. Certain writers have seen in the theory
of mimicry the only explanation of these peculiar geographical pattern
groups. The fashion is in each case set by the most abundant form,
generally an Ithomiine of the genus _Melinaea_. The rest are mimics of this
dominant species, either in the {136} Batesian or Müllerian sense. Batesian
mimics are such genera as _Dismorphia_ and _Protogonius_, to which there
are no reasons for attributing disagreeable properties. Of the nature of
Müllerian mimics on the other hand are the various Heliconines and
Ithomiines which enter into the combination. In each case the whole
assemblage is a great "mimicry ring," of which the pattern is dictated by
the Ithomiine that predominates in point of numbers. It is, however, very
doubtful whether this can be accepted as a satisfactory explanation. The
four groups which we have considered are all characterised by a peculiar
and distinctive coloration, and in each case the pattern must on the theory
of mimicry be regarded as a highly efficient warning pattern. One or other
of these patterns must doubtless be looked upon as the most primitive. If
so the question at once arises as to why a distasteful genus should change
from one efficient warning pattern to another quite distinct one. If the
newer pattern affords better protection we should expect it to have spread
and eventually to have ousted the older one. That it has not done so must
probably be attributed to the old pattern being as efficient as the new
one. But if this is so we are left without grounds for assuming the change
to have been brought about by natural selection through the agency of
enemies to whom warning colours appeal. For natural selection can only
bring about a change that is beneficial to the species. Hence we must
suppose the change on the part of the dominant model to have been
independent of natural selection by {137} enemies, and due to some
condition or set of conditions of which we are ignorant. It is not
inconceivable that the new colour scheme was associated with some
physiological peculiarity which was advantageous to the species in its
altered surroundings. If so natural selection may have favoured the new
variety, not because of its colour scheme, but owing to the underlying
physiological differences of which the pattern is but an outward sign. And
if this could happen in one species there seems to be no reason why it
should not happen in others. The weak point of the explanation on the
mimicry hypothesis is that it offers no explanation of the change in the
so-called dominant Ithomiine pattern as we pass from one region to another.
Whatever the cause of this change may be there would appear to be nothing
against it having also operated to produce similar changes in other
unrelated species, in which case the mimicry hypothesis becomes
superfluous. It is not unlikely that the establishing of these new forms
was due to natural selection. If they were associated with physiological
peculiarities better adapted for their environment it is reasonable to
suppose that natural selection would favour their persistence as opposed to
the older type until the latter was eliminated. But such action on the part
of natural selection is quite distinct from that postulated on the mimicry
hypothesis. On the one view the colour itself is selected because it is of
direct advantage to its possessor; on the other view the colour pattern is
associated with a certain physiological constitution which places the {138}
butterflies possessing it at an advantage as compared with the rest[88].

It is, nevertheless, possible that mimicry may have played some part in
connection with establishing the new colour pattern in some of these South
American species. For if the new pattern had become established in the
predominant distasteful species, and if some of the members of a palatable
form (_e.g._ _Protogonius_) were to shew a variation similar to that
already established in the distasteful species, and if further there be
granted the existence of appropriate enemies, then it would be almost
certain that the newer form in palatable species would eventually replace
the older form. In such a case the part played by natural selection would
be the preservation of a chance sport which happened to look like an
unpalatable form. There is no reason for regarding the change as
necessarily brought about by the gradual accumulation of a long series of
very small variations through the operation of natural selection.

       *       *       *       *       * {139}


CHAPTER XI

CONCLUSION

From the facts recorded in the preceding chapters it is clear that there
are difficulties in the way of accepting the mimicry theory as an
explanation of the remarkable resemblances which are often found between
butterflies belonging to distinct groups. Of these difficulties two stand
out beyond the rest, viz., the difficulty of finding the agent that shall
exercise the appropriate powers of discrimination, and the difficulty of
fitting in the theoretical process involving the incessant accumulation of
minute variations with what is at present known of the facts of heredity.

With regard to the former of these two difficulties we have seen that the
supporters of the theory regard birds as the main selective agent. At the
outset we are met with the fact that relatively few birds have been
observed to prey habitually on butterflies, while some at any rate of those
that do so shew no discrimination between what should be theoretically
pleasant to eat and what should not be pleasant. Even if birds are the
postulated enemies it must be further shewn that they exercise the
postulated discrimination. It is required of them that they should do two
things. {140} In the first place they must confuse an incipient or "rough"
mimic with a model sufficiently often to give it an advantage over those
which have not varied in the direction of the model. In other words, they
must be easily taken in. Secondly, they are expected to bring about those
marvellously close resemblances that sometimes occur by confusing the exact
mimicking pattern with the model, while at the same time eliminating those
which vary ever so little from it. In other words, they must be endowed
with most remarkably acute powers of discrimination. Clearly one cannot ask
the same enemy to play both parts. If, therefore, birds help to bring about
the resemblance we must suppose that it is done by different species--that
there are some which do the rough work, others which do the smoothing, and
others again which put on the final polish and keep it up to the mark. This
is, of course, a possibility, but before it can be accepted as a
probability some evidence must be forthcoming in its favour.

But even if the difficulty of the appropriate enemy be passed over, and it
be granted that a mimetic resemblance can be built up through a number of
small separate steps, which have become separately established through the
agency of separate species of birds with various but distinct
discriminating powers, we are left face to face with an even more serious
physiological difficulty. For why is it that when the end form which is
supposed to have resulted from this process is crossed back with the
original form all {141} the intermediate steps do not reappear? Why is it
that when the altered germplasm is mingled with the original germplasm the
various postulated stages between them are not reformed? For in various
cases where we know the course of evolution this does occur. The pale pink
sweet-pea has come from the wild purple by a series of definite steps, and
when it is crossed back with the wild form the resulting plants give the
series of stages that have occurred in the evolution of the pink. So also
when the orange rabbit is crossed with the wild grey form and the offspring
are inbred there are reproduced the black, the tortoiseshell, and the
chocolate, forms which are stages in the evolution of the orange from the
wild grey. If then, to take an example, the "aristolochiae" form of
_Papilio polytes_ has been derived from the male-like form by a series of
steps, why do we not get these steps reproduced after the germplasms of the
two forms have been mingled? From the standpoint of modern genetic work the
inference is that these postulated intermediate steps have never
existed--that the one form of _polytes_ female came directly from the
other, and was not built up gradually through a series of stages by the
selective agency of birds or any other discriminating enemy.

These two objections, viz. the difficulty of finding the appropriate enemy,
and the non-appearance of intermediates when the extreme forms are crossed,
may, perhaps, be said to constitute the main objections to the current
theory of mimicry. Others such as {142} the relative scarcity of mimicry in
the male sex and the existence of cases of polymorphism among females of a
species which cannot possibly be explained on mimetic lines have already
been mentioned. But while the main objections remain it is hardly necessary
to insist upon these others. Looked at critically in the light of what we
now know about heredity and variation the mimicry hypothesis is an
unsatisfactory explanation of the way in which these remarkable
resemblances between different species of butterflies have been brought
about. Sometimes this is admitted by those who nevertheless embrace the
theory with a mild aloofness. For they argue that even though it does not
explain all the facts no other theory explains so many. Others have sought
an explanation in what has sometimes been termed the hypothesis of external
causes, regarding these resemblances as brought about by similar conditions
of soil and climate, and so forth. It is not inconceivable that certain
types of colour and pattern may be the expression of deep-seated
physiological differences, which place their possessors at an advantage as
compared with the rest of the species. Were this so it is but reasonable to
suppose that they would become established through the agency of natural
selection. But it is difficult, if not impossible, to regard this as a
satisfactory solution, if for no other reason than that it offers no
explanation of polymorphism. For example, each of the three forms of
_polytes_ female holds its own and all must, therefore, be regarded as
equally well adapted to the circumstances under which {143} they live. They
are so distinct in colour that it is difficult on this hypothesis to
suppose that they are all on the same footing in respect to their
environment. Yet if one is better off than the others, how is it that these
still exist?

Those who have examined long series of these cases of resemblance among
butterflies find it hard to believe that there is not some connection
between them apart from climatic influence. One feels that they are too
numerous and too striking to be all explained away as mere coincidences
engendered by like conditions. Nor is it improbable that natural selection
in the form of the discriminating enemy may have played a part in
connection with them, though a different one from that advocated on the
current theory of mimicry. If we assume that sudden and readily appreciable
variations of the nature of "sports" turn up from time to time, and if
these variations happen to resemble a form protected by distastefulness so
closely that the two can be confused by an enemy which has learned to avoid
the latter, then there would appear to be good grounds for the mimicking
sport becoming established as the type form of the species. For it has
already been seen that a rare sport is not swamped by intercrossing with
the normal form, but that on the contrary if it possess even a slight
advantage, it must rapidly displace the form from which it sprang (cf.
Chap. VIII). On this view natural selection in the form of the
discriminating enemy will have played its part, but now with a difference.
Instead of building up a {144} mimetic likeness bit by bit it will merely
have conserved and rendered numerically preponderant a likeness which had
turned up quite independently. The function of natural selection in respect
of a mimetic likeness lies not in its formation but in its conservation. It
does not bring about the likeness, neither does it accentuate it: it brings
about the survival of those forms which happen to shew the likeness. Why
variations on the part of one species should bear a strong resemblance to
other, and often distantly related, species is another question altogether.

Even a superficial survey of the facts makes it evident that cases of
mimicry tend to run in series--that a closely related series of mimics,
though often of very different pattern and colour, tends to resemble a
closely related series of models. In Asia we have the Cosmodesmus Papilios
mimicking a series of Danaines, while the true Papilios (cf. Appendix II)
tend to resemble a series of the less conspicuous members of the
Pharmacophagus group. In the same region the various species of _Elymnias_
form a series resembling a series of Danaines. In Africa there stands out
the Cosmodesmus group again mimicking a Danaine series, and in part also an
Acraeine series. Overlapping the Acraeines again are various forms of the
Nymphaline genus _Pseudacraea_. It is also of interest that in _Danais
chrysippus_ and _Acraea encedon_ the Danaine and Acraeine series overlap
(cf. Pl. IX). Similar phenomena occur also in South America, where closely
parallel series of colour patterns are exhibited by several {145}
Ithomiines, by _Heliconius_, _Lycorea_, _Dismorphia_, and other genera (cf.
p. 39). On the other hand such mimetic resemblances as are shewn by the
South American Swallow-tails of the Papilio and Cosmodesmus groups are
almost all with the Pharmacophagus group, and almost all of the red-black
kind (cf. p. 43).

On the whole it may be stated that the majority of cases of mimicry fall
into one or other of such series as the above. If we select a case of
mimicry at random we shall generally find that there are at least several
close allies of the mimic resembling several close allies of the model.
Isolated cases such as the resemblance between _Pareronia_ and _Danais_ (p.
23), between _Archonias_ and a Pharmacophagus Papilio (p. 43), or the
extraordinary instance of _Papilio laglaizei_ and _Alcidis agathyrsus_,
must be regarded as exceptional.

We have before us then a number of groups of butterflies each with a series
of different colour patterns. In each group a portion of the series
overlaps a portion of the series belonging to another more or less
distantly related group. In the light of recent discoveries connected with
heredity and variation the natural interpretation to such a set of
phenomena would be somewhat as follows: Each group of Lepidoptera, such as
those just discussed, contains, spread out among its various members, a
number of hereditary factors for the determination of colour pattern.
Within the group differences of pattern depend upon the presence or absence
of this or that factor, the variety of pattern being the result of the many
possible permutations and {146} combinations of these colour factors.
Within the limits of each group is found a definite number of these
factors--more in one group, less in another. But some factors may be common
to two or more groups, in which case some of the permutations of the
factors would be similar in the groups and would result in identical or
nearly identical pattern. To take a simple example in illustration, let us
suppose that a given group, ([alpha]), contains the eight factors A-H.
Since any species in the group may exhibit any combination of one or more
of these factors it follows that a considerable number of different forms
are possible. Now suppose that another group, ([beta]), distinguished from
([alpha]) by definite structural features, also contains eight factors
within the group, and that these factors are F-M, F, G, and H being common
to both ([alpha]) and ([beta]). Any combination therefore in ([alpha])
lacking the factors A-E will be paralleled by any combination in ([beta])
lacking the factors I-M. For in both cases we should be dealing only with
the factors F, G, and H, which are common to each group. So again a third
group might have some factors in common with ([alpha]) and some with
([beta]), and so on for other groups. In this way certain of the series of
colour patterns found in ([beta]) would overlap certain of those in
([alpha]), while others of the groups ([beta]) and ([alpha]) might overlap
those found in different groups again. The striking resemblances not
infrequently found between species belonging to quite distinct groups would
on this view depend upon the hereditary factors for pattern and colour
being limited {147} in number, so that the same assortment might not
infrequently be brought together even though the group whose members
exhibited the resemblance might, owing to structural differences, be placed
in different families.

We know from recent experimental work that something of the sort is to be
found in the coat colours of different rodents. Agouti, black, chocolate,
blue-agouti, blue, and fawn form a series of colours common to the rabbit,
the mouse, and the guinea-pig. These colours are related to each other in
the same way in these different beasts. In the rat, on the other hand,
there occur of this range of colours only the agouti and the black. Each of
these species again has certain colour patterns which are peculiar to
itself, such as the "English" type in the rabbit, the tricolor pattern in
the guinea-pig, or the "hooded" variety in the rat. The total range of
colour and pattern is somewhat different for each species, but a few are
common to them all. Moreover, there are others which are common to the
mouse and the rabbit but are not found in the guinea-pig, and others again
which may occur in the rabbit and the guinea-pig but have not been met with
in the other two. In certain features the rabbit might be said to "mimic"
the mouse, and in other features the guinea-pig. It is not, of course,
suggested that the case of the butterflies is so simple as that of the
rodents, but so far as we can see at present there would seem to be no
reason why the explanation should not be sought along the same lines. {148}
On this view the various colour patterns found among butterflies depend
primarily upon definite hereditary factors of which the number is by no
means enormous. Many of these factors are common to several or many
different groups, and a similar aggregate of colour factors, whether in an
Ithomiine, a Pierid, or a Papilio, results in a similar colour scheme. The
likeness may be close without being exact because the total effect is
dependent in some degree on the size and relative frequency of the scales
and other structural features. In so far as pattern goes _Hypolimnas
dubius_ and _Amauris echeria_ (Pl. VIII, figs. 7 and 8) are exceedingly
close. But inspection at once reveals a difference in the quality of the
scaling, giving to the _Hypolimnas_, where the black and yellow meet, a
softness or even raggedness of outline, which is distinct from the sharper
and more clear-cut borders of the _Amauris_. It is not unreasonable to
suppose that these species carry identical factors for colour pattern, and
that the differences by which the eye distinguishes them are dependent upon
the minuter structural differences such as occur in the scaling. So the eye
would distinguish between a pattern printed in identical colours on a piece
of cretonne and a piece of glazed calico. Though pattern and colour were
the same the difference in material would yield a somewhat different
effect.

On the view suggested the occurrence of mimetic resemblances is the
expression of the fact that colour pattern is dependent upon definite
hereditary factors of which the total number is by no means very great.
{149} As many of the factors are common to various groups of butterflies it
is to be expected that certain of the colour patterns exhibited by one
group should be paralleled by certain of those found in another group. That
cases of resemblance should tend to run in parallel series in different
groups is also to be expected, for in some groups the number of factors in
common is likely to be greater than in other groups. In consonance with
this view is the fact that where polymorphism occurs among the females of a
mimicking species the models, though often widely different in appearance,
are, as a rule, closely related. Some of the Asiatic Papilios, for
instance, resemble Danaines, while others resemble Pharmacophagus Papilios.
But although the polymorphism exhibited by the females of a given species
may be very marked, we do not find one of them resembling a Danaine and
another a Pharmacophagus Swallow-tail. The models of a polymorphic mimic
are almost always closely related species[89].

In discussing the problems of mimicry more attention is naturally paid to
groups which exhibit the phenomenon than to those which either do not do
so, or else only do so to a very limited extent. Yet the latter may be of
considerable interest. Among the Pieridae of the Old World the phenomenon
of mimicry is very rare. _Pareronia_ and _Aporia agathon_ conform {150}
closely to the common Danaid type represented by _Danais vulgaris_ and
other species, but apart from these none of the many Pierids in Asia
resemble any of the recognised models. Africa is apparently destitute of
Pierids which mimic species belonging to other groups. Yet no group of
butterflies is more persecuted by birds. Of all the instances of bird
attacks collected together by Marshall[90] more than one-third are
instances of attacks upon this group alone. If birds are the agents by
which mimetic likenesses are built up through the cumulative selection of
small variations, how can the rarity or absence of mimetic Pierids in the
Old World be accounted for? For the species of Pierids, like the species of
other families, shew considerable variation, and if this process of
selection were really at work one would expect to find many more Pierid
mimics in these regions than actually occur. It is true that the white,
yellow, and red pigments found in Pierids differ from those of other
butterflies in being composed either of uric acid or of some substance
closely allied to that body[91]. These substances are generally found
between the two layers of chitin, of which the scale is composed, whereas
the black pigment is intimately associated with the chitin of the scale
itself. What is perhaps the principal factor in the formation of a mimetic
likeness is the distribution of the black pigment with reference to the
lighter pigments; and although the latter are chemically distinct {151} in
the Pierids as compared with other butterflies, there would seem to be no
reason why the same factors governing the distribution of black should not
be common to members of different groups. A distribution of black pigment
similar to that found in a model and its mimic may occur also in a
non-mimetic ally of the mimic. _Dismorphia astynome_, for example,
resembles the Ithomiine _Mechanitis lysimnia_ (Pl. XV, fig. 8) both in the
distribution of black as well as of yellow and bright brown pigments. A
similar distribution of the black pigment is also found in _Dismorphia
avonia_, but the yellow and bright brown of the other two species is here
replaced with white. By a slight though definite alteration in chemical
composition this white pigment could be changed into bright brown and
yellow with the result that _D. avonia_ would closely resemble _D.
astynome_ in its colour scheme and would in this way also become a mimic of
_Mechanitis lysimnia_. Another good instance is that of the females of
_Perrhybris demophile_ and _P. lorena_, the former being black and white,
whereas in the latter the white is replaced by yellow and bright brown,
giving the insect a typical Ithomiine appearance[92]. Here again a definite
small change in the composition of the pigment laid down in the scales
would result in the establishing of a mimetic likeness where there would
otherwise be not even a suggestion of it. It is in accordance with what we
know to-day of variation {152} that such a change should appear suddenly,
complete from the start. And if so there is no difficulty in supposing that
it might be of some advantage to its possessor through the resemblance to
an unpalatable form. Even were the advantage but a slight one it is clear
from previous discussion (p. 96) that the new variety would more or less
rapidly replace the form from which it had sprung. With the continued
operation of natural selection the new form would entirely supplant the
original one, but it is not impossible that in some cases the selecting
agent may be removed before this result has been achieved. In this event
the proportions of the new and the old form would fall into a condition of
equilibrium as in _P. polytes_ in Ceylon, until some other selective agent
arose to disturb the balance. On this view natural selection is a real
factor in connection with mimicry, but its function is to conserve and
render preponderant an already existing likeness, not to build up that
likeness through the accumulation of small variations, as is so generally
assumed. Recent researches in heredity and variation all point to this
restriction of the scope of natural selection. Hitherto an argument in
favour of the older view has been that derived from the study of
adaptation--of an apparent purpose, which, at first sight, appears to be
behind the manner in which animals fit into their surroundings. For many
the explanation of this apparent purpose has been found in the process of
natural selection operating gradually upon small variations, accumulating
some and rejecting {153} others, working as it were upon a plastic
organism, moulding it little by little to a more and more perfect
adaptation to its surroundings. On this view adaptation is easy to
understand. The simplicity of the explanation is in itself attractive. But
when the facts come to be examined critically it is evident that there are
grave, if not insuperable, difficulties in the way of its acceptance. To
outline some of these has been the object of the present essay. Though
suggestions have been made as to the lines along which an explanation may
eventually be sought it is not pretended that the evidence is yet strong
enough to justify more than suggestions. Few cases of mimicry have as yet
been studied in any detail, and until this has been done many of the points
at issue must remain undecided. Nevertheless, the facts, so far as we at
present know them, tell definitely against the views generally held as to
the part played by natural selection in the process of evolution.

       *       *       *       *       *


{154}

APPENDIX I

For the table on p. 155 I am indebted to the kindness of Mr H. T. J. Norton
of Trinity College, Cambridge. It affords an easy means of estimating the
change brought about through selection with regard to a given hereditary
factor in a population of mixed nature mating at random. It must be
supposed that the character depending upon the given factor shews complete
dominance, so that there is no visible distinction between the homozygous
and the heterozygous forms. The three sets of figures in the left-hand
column indicate different positions of equilibrium in a population
consisting of homozygous dominants, heterozygous dominants, and recessives.
The remaining columns indicate the number of generations in which a
population will pass from one position of equilibrium to another, under a
given intensity of selection. The intensity of selection is indicated by
the fractions 100/50, 100/75, etc. Thus 100/75 means that where the chances
of the favoured new variety of surviving to produce offspring are 100,
those of the older variety against which selection is operating are as 75;
there is a 25% selection rate in favour of the new form.

The working of the table may perhaps be best explained by a couple of
simple examples.

In a population in equilibrium consisting of homozygous dominants,
heterozygous dominants and recessives the last named class comprises 2.8%
of the total: assuming that a 10% selection rate now operates in its favour
as opposed to the two classes of dominants--in how many generations will
the recessive come to constitute one-quarter of the population? The answer
is to be looked for in column B (since the favoured variety is recessive)
under the fraction 100/90. The recessive passes from 2.8% to 11.1% of the
population in 36 generations, and from 11.1% to 25% in a further 16
generations--_i.e._ under a 10% selection rate in its favour the proportion
of the recessive rises from 2.8% to 25% in 52 generations.

{155}

  A: Where the new variety is dominant
  +-----------+-----------+----------+------------------------------------+
  |           |           |          |  Number of generations taken       |
  |Percentage |Percentage |Percentage|  to pass from one position         |
  |of total   |of total   |of total  |  to another as indicated           |
  |population |population |population|  in the percentages of different   |
  |formed by  |formed by  |formed by |  individuals in left-hand column.  |
  |old variety|the hybrids|the new   +--------+--------+--------+---------+
  |           |           |variety   | 100/50 | 100/75 | 100/90 |  100/99 |
  +-----------+-----------+----------+--------+--------+--------+---------+
  |   99.9    |     .09   |     .000 |        |        |        |         |
  |   98.0    |    1.96   |     .008 |     4  |    10  |     28 |     300 |
  |   90.7    |    9.0    |     .03  |     2  |     5  |     15 |     165 |
  |   69.0    |   27.7    |    2.8   |     2  |     4  |     14 |     153 |
  |   44.4    |   44.4    |   11.1   |     2  |     4  |     12 |     121 |
  |   25.     |   50.     |   25.    |     2  |     4  |     12 |     119 |
  |   11.1    |   44.4    |   44.4   |     4  |     8  |     18 |     171 |
  |    2.8    |   27.7    |   69.0   |    10  |    17  |     40 |     393 |
  |     .03   |    9.0    |   90.7   |    36  |    68  |    166 |    1632 |
  |     .008  |    1.96   |   98.0   |   170  |   333  |    827 |    8243 |
  |     .000  |     .09   |   99.9   |  3840  |  7653  | 19,111 | 191,002 |
  +-----------+-----------+----------+--------+--------+--------+---------+

  B: Where the new variety is recessive
  +-----------+-----------+----------+------------------------------------+
  |           |           |          |  Number of generations taken       |
  |Percentage |Percentage |Percentage|  to pass from one position         |
  |of total   |of total   |of total  |  to another as indicated           |
  |population |population |population|  in the percentages of different   |
  |formed by  |formed by  |formed by |  individuals in left-hand column.  |
  |old variety|the hybrids|the new   +--------+--------+--------+---------+
  |           |           |variety   | 100/50 | 100/75 | 100/90 |  100/99 |
  +-----------+-----------+----------+--------+--------+--------+---------+
  |   99.9    |     .09   |     .000 |        |        |        |         |
  |   98.0    |    1.96   |     .008 |  1920  |  5740  | 17,200 | 189,092 |
  |   90.7    |    9.0    |     .03  |    85  |   250  |    744 |   8,160 |
  |   69.0    |   27.7    |    2.8   |    18  |    51  |    149 |   1,615 |
  |   44.4    |   44.4    |   11.1   |     5  |    13  |     36 |     389 |
  |   25.     |   50.     |   25.    |     2  |     6  |     16 |     169 |
  |   11.1    |   44.4    |   44.4   |     2  |     4  |     11 |     118 |
  |    2.8    |   27.7    |   69.0   |     2  |     4  |     11 |     120 |
  |     .03   |    9.0    |   90.7   |     2  |     6  |     14 |     152 |
  |     .008  |    1.96   |   98.0   |     2  |     6  |     16 |     165 |
  |     .000  |     .09   |   99.9   |     4  |    10  |     28 |     299 |
  +-----------+-----------+----------+--------+--------+--------+---------+

{156}

If the favoured variety is dominant it must be borne in mind that it can be
either homozygous or heterozygous--that for these purposes it is
represented in the left-hand column by the hybrids as well as by the
homozygous dominant. In a population in equilibrium which contains about 2%
of a dominant form, the great bulk of these dominants will be heterozygous,
and the relative proportion of recessives, heterozygous, and homozygous
dominants is given in the second line of the left-hand column.

Let us suppose now that we want to know what will be the percentage of
dominants after 1000 generations if they form 2% of the population to start
with, and if, during this period, they have been favoured with a 1%
selection advantage. After 165 generations the proportion of recessives is
90.7, so that the proportion of dominants has risen to over 9%; after 153
further generations the percentage of dominants becomes 27.7 + 2.8 = 30.5;
after 739 generations it is 88.8%, and after 1122 generations it is 69.0 +
27.7 = 96.7. Hence the answer to our question will be between 89% and 97%,
but nearer to the latter figure than the former.

Mr Norton has informed me that the figures in the table are accurate to
within about 5%.

       *       *       *       *       *


{157}

APPENDIX II

The genus _Papilio_ is a large and heterogeneous collection. It was pointed
out by Haase[93] that it falls into three distinct sections, of which
one--the Pharmacophagus section--provides those members which serve as
models in mimicry; while in the other two sections are found mimics, either
of Pharmacophagus Swallow-tails, or of models belonging to other groups.
Though Haase's terms have not yet come into general use with systematists,
there is little doubt that the genus _Papilio_ as it now stands must
eventually be broken up on these lines. To say that one species of
_Papilio_ mimics another is therefore somewhat misleading; for the
differences between the Pharmacophagus group and the other two are such as
to constitute at any rate generic distinction in other groups. For
convenience of reference a table has been added in which the various
Papilios mentioned in the text have been assigned to their appropriate
sections, and referred to their respective models. {158}

  Pharmacophagus         Papilio                   Cosmodesmus

  (POISON-EATERS)        (FLUTED SWALLOW-TAILS)   (KITE SWALLOW-TAILS)

  Antennae without       Antennae without          Antennae scaled
    scales.                scales.                   on upper side.

  Outer ventral row      Outer ventral row         As in Papilio.
    of spines of tarsi     of spines of tarsi
    not separated          separated from
    from the dorsal        the dorsal spines
    spines by a spineless  by a spineless
    longitudinal           longitudinal depression.
    depression.

  Larva covered with     Larva either smooth       Larva with third
    short hairs--with      or with hard              thoracic segment
    fleshy tubercles       spiny tubercles.          enlarged
    but no                 Third and fourth          (known only in
    spines.                thoracic segments         a few species).
                           enlarged.

  Pupa with row of       Pupa wrinkled--generally  Pupa short with
    well-marked            with                      long four-sided
    humps on each          short dorsal              thoracic horn.
    side of abdomen.       horn. Humps
                           if present very
                           short.

  Larva feeds on         Larva does not feed       As in Papilio.
    _Aristolochia_.        on _Aristolochia_.

                         Abdominal margin          Abdominal margin
                           of hind wing              of hind wing
                           curved downwards          bent over in [M],
                           forming                   and with scent
                           a kind of groove.         organ in fold
                           No scent organ.           so formed.

{159}

LIST OF PAPILIONINE MIMICS

  ----------------+-----------------+---------------+----------------------
   PHARMACOPHAGUS |     PAPILIO     |  COSMODESMUS  |
      (MODEL)     |     (MIMIC)     |    (MIMIC)    |       (MODELS)
  ----------------+-----------------+---------------+----------------------
                                 ORIENTAL
                  |_agestor_        |               |_Caduga tytia_
                  |_clytia_         |               |_Danais septentrionis_
                  |   " var.        |               |
                  |     _dissimilis_|               |_Euploea core_
                  |_mendax_         |               |     " _rhadamanthus_
                  |_paradoxus_      |               |     " _mulciber_
  _hector_        |_polytes_ [F]    |               |
  _aristolochiae_ |   "      [F]    |               |
  _coon_          |_memnon_ [F]     |               |
  _polyxenus_     |_bootes_         |               |
                  |_laglaizei_      |               |_Alcidis agathyrsus_
                  |                 |_delesserti_   |_Ideopsis daos_
                  |                 |_macareus_     |_Danais septentrionis_
                  |                 |_xenocles_     |    "          "

                                 AFRICAN
                  |_dardanus_ [F]   |               |_Danais chrysippus_
                  |    "      [F]   |               |_Amauris niavius_
                  |    "      [F]   |               |    " _echeria_
                  |_echerioides_ [F]|               |    " _psyttalea_
                  |_cynorta_ [F]    |               |_Planema epaea_
                  |_rex_            |               |_Melinda formosa_
                  |                 |_ridleyanus_   |_Acraea egina_
                  |                 |_leonidas_     |_Danais petiverana_
                  |                 |_brasidas_     |_Amauris hyalites_

                                AMERICAN
  _hahneli_(mimic)|                 |               |_Methona confusa_
                  |_zagreus_        |               |} various Heliconinae
                  |_bachus_         |               |}   and Ithomiinae
                  |_euterpinus_     |               |_Heliconius melpomene_
                  |                 |_pausanias_    |     " _sulphurea_
  various species |_hippason_, etc. |_lysithous_    |
                  |                 |   etc.        |
  _philenor_      |_troilus_        |               |
                  |_turnus_ [F]     |               |
                  |_asterius_ [F]   |               |

       *       *       *       *       *


{160}

PLATE I

ORIENTAL BUTTERFLIES

  1.  _Pareronia ceylonica_               [F]  }  (Pieridae)
  2.       "        "                     [M]  }
  3.  _Danais septentrionis_                      (Danainae)
  4.  _Papilio xenocles_                          (Papilionidae)
  5.  _Hypolimnas bolina_                 [M]  }  (Nymphalinae)
  6.       "        "                     [F]  }
  7.  _Papilio clytia_ var. _dissimilis_       }  (Papilionidae)
  8.      "      "     var. _lankeswara_       }
  9.  _Elymnias singhala_                         (Satyrinae)
  10. _Euploea core_                              (Danainae)

[Illustration]

       *       *       *       *       *


PLATE II

ORIENTAL BUTTERFLIES

  1.  _Delias eucharis_               (Pieridae)
  2.  _Caduga tytia_                  (Danainae)
  3.  _Papilio agestor_               (Papilionidae)
  4.  _Euploea mulciber_      [M]  }  (Danainae)
  5.     "       "            [F]  }
  6.  _Elymnias malelas_      [M]  }  (Satyrinae)
  7.     "       "            [F]  }
  8.  _Euploea rhadamanthus_          (Danainae)
  9.  _Papilio mendax_                (Papilionidae)

[Illustration]

       *       *       *       *       *


PLATE III

ORIENTAL MOTHS AND BUTTERFLIES

 The three upper figures are those of moths, and the three lower ones are
those of butterflies.

  1. _Alcidis agathyrsus_ (New Guinea)
  2. _Papilio laglaizei_    "     "

 The moth is here supposed to serve as a model for the far rarer Papilio.

  3. _Cyclosia hestinioides_
  4. _Ideopsis daos_

 The butterfly is very common and must be regarded as the model, the rarer
moth as the mimic.

  5. _Epicopeia polydora_ (Assam)
  6. _Papilio bootes_        "

 Both of these species are to be regarded as mimics of the abundant
Pharmacophagus Papilio, _P. polyxenus_, which is very like _P. bootes_ in
appearance.

[Illustration]

       *       *       *       *       *


PLATE IV

ORIENTAL BUTTERFLIES

  1.  _Danais chrysippus_     [M]  }  (Danainae)
  2.     "   _plexippus_      [F]  }
  3.  _Argynnis hyperbius_    [F]  }  (Nymphalinae)
  4.      "        "          [M]  }
  5.  _Elymnias undularis_    [F]  }  (Satyrinae)
  6.      "        "          [M]  }
  7.  _Hypolimnas misippus_   [F]  }  (Nymphalinae)
  8.       "         "        [M]  }

The two Danaids together with the females of the other three species form a
"mimicry ring." For explanation see text, pp. 65-69.

[Illustration]

       *       *       *       *       *


PLATE V

ORIENTAL BUTTERFLIES

  1.  _Papilio polytes_  [M]
  2.     "       "       [F], var. _cyrus_   (M form)
  3.     "       "       [F], var. _polytes_ (A form)
  4.     "       "       [F], var. _romulus_ (H form)
  5.     "    _aristolochiae_
  6.     "    _hector_

The specimens figured on this plate were taken in Ceylon where they are all
plentiful.

Figures 1_a_--6_a_ represent the under surfaces of the hind wings belonging
to specimens 1--6.

[Illustration]

       *       *       *       *       *


PLATE VI

AFRICAN BUTTERFLIES

(except _A. levana_, Figs. 8--10, which is European)

   1. _Danais petiverana_                 (Danainae)
   2. _Papilio leonidas_                  (Papilionidae)
   3. _Amauris hyalites_                  (Danainae)
   4. _Papilio leonidas_ var. _brasidas_  (Papilionidae)
   5. _Pseudacraea boisduvali_            (Nymphalinae)
   6. _Papilio ridleyanus_                (Papilionidae)
   7. _Acraea egina_                      (Acraeinae)
   8. _Araschnia levana_ var. _porima_
   9.    "         "     var. _prorsa_
  10.    "         "
  11. _Precis octavia_ var. _sesamus_
  12.    "       "     var. _natalensis_

[Illustration]

       *       *       *       *       *


PLATE VII

TROPICAL AFRICAN BUTTERFLIES

   1. _Planema macarista_   [M]  (Acraeinae)
   2.     "        "        [F]      "
   3.     "   _tellus_               "
   4.     "   _paragea_              "
   5.     "   _epaea_                "
   6. _Pseudacraea hobleyi_ [M]  (Nymphalinae)
   7.       "         "     [F]       "
   8.       "     _terra_             "
   9. _Elymnias phegea_     [F]  (Satyrinae)
  10. _Papilio cynorta_     [F]  (Papilionidae)

(NOTE. _Pseudacraea hobleyi_ and _P. terra_ (Figs. 6--8) were at one time
regarded as separate species. More recently they have been shewn to be
forms of the polymorphic species, _Pseudacraea eurytus_.)

[Illustration]

       *       *       *       *       *


PLATE VIII

AFRICAN BUTTERFLIES

  1. _Papilio dardanus_ [M]
  2.     "       "      [F], var. _trophonius_
  3.     "       "      [F], var. _hippocoon_
  4.     "       "      [F], var. _cenea_
  5. _Danais chrysippus_ (Danainae)
  6. _Amauris niavius_       "
  7.     "   _echeria_       "
  8. _Hypolimnas dubius_ var. _mima_      (Nymphalinae)
  9.      "        "     var. _wahlbergi_       "

[Illustration]

       *       *       *       *       *


[Illustration]

  _Danais chrysippus_
  a. Typical form
  b. _Alcippus_ form
  c. _Dorippus_ form

  _Acraea encedon_
  d. Typical form
  e. _Alcippina_ form
  f. _Daira_ form

  _Hypolimnas misippus_ [F]
  g. Typical form
  h. _Alcippoides_ form
  i. _Inaria_ form

  (After Aurivillius)

       *       *       *       *       *


PLATE X

SOUTH AMERICAN BUTTERFLIES

  1.  _Dismorphia cretacea_     [M]                (Pieridae)
  2.       "     _praxinoe_     [M]                    "
  3.       "         "          [F]                    "
  4.  _Perrhybris malenka_      [M]                    "
  5.       "         "          [M] (under surface)    "
  6.       "         "          [F]                    "
  7.  _Mechanitis saturata_                        (Ithomiinae)
  8.  _Papilio zagreus_                            (Papilionidae)
  9.  _Protogonius tithoreides_                    (Nymphalinae)
  10. _Tithorea pseudonyma_                        (Ithomiinae)

(NOTE. The figure of the _Mechanitis_ (Fig. 7) is taken from a rather worn
specimen. The quality of the orange brown is better shewn by the specimen
illustrated in Fig. 7 on Plate XV.)

[Illustration]

       *       *       *       *       *


PLATE XI

SOUTH AMERICAN BUTTERFLIES

   1. _Heliconius sulphurea_   (Heliconinae)
   2. _Papilio pausanias_      (Papilionidae)
   3. _Heliconius telesiphe_   (Heliconinae)
   4. _Colaenis telesiphe_     (Nymphalinae)
   5. _Heliconius melpomene_   (Heliconinae)
   6. _Pereute charops_   [F]  (Pieridae)
   7.    "        "       [M]      "
   8. _Papilio osyris_    [M]  (Papilionidae)
   9.    "        "       [F]       "
  10. _Archonias critias_ [F]  (Pieridae)

[Illustration]

       *       *       *       *       *


[Illustration]

PLATE XII

SOUTH AMERICAN BUTTERFLIES

  _Methona confusa_  (Ithomiinae)
  _Dismorphia orise_ (Pierinae)
  _Ituna phenarete_  (Danainae)
  _Castnia sp._      (Heterocera)

       *       *       *       *       *


[Illustration]

PLATE XIII

  MODELS
  1. _Papilio nephalion_
  2.     "   _chamissonia_
  3.     "   _perrhebus_

  MIMICS
  4. _Papilio lysithous_ var. _lysithous_
  5.     "           "   var. _rurik_
  6.     "           "   var. _pomponius_

(For further details of this case see Jordan, _I^{er} Congr. Internat.
d'Entomologie_, Bruxelles, 1911, p. 396.)

       *       *       *       *       *


[Illustration]

PLATE XIV

  1. _Methona confusa_,   ×90 (Ithomiinae)
  2. _Dismorphia orise_, ×150 (Pierinae)
  3. _Thyridia themisto_, ×90 (Ithomiinae)
  4. _Ituna ilione_,      ×90 (Danainae)
  5. _Castnia sp._,       ×60 (Moth)

Microphotographs of the scales of various Lepidoptera in the S. American
"Transparency group." For explanation see text, pp. 39-42.

       *       *       *       *       *


PLATE XV

CENTRAL AND SOUTH AMERICAN BUTTERFLIES

Illustrating the closely parallel series of patterns occurring in the two
distinct groups Heliconinae and Ithomiinae.

  1.  _Heliconius mirus_
  2.        "    _telchinia_
  3.        "    _eucrate_
  4.        "    _pardalinus_
  5.        "    _splendens_
  6.  _Mechanitis elisa_
  7.        "    _saturata_
  8.        "    _lysimnia_
  9.        "    _egaensis_
  10.       "    _methona_

[Illustration]

       *       *       *       *       *


PLATE XVI

NORTH AMERICAN BUTTERFLIES

  1.  _Papilio philenor_                (Papilionidae)
  2.     "    _troilus_                       "
  3.  _Argynnis diana_ [F]              (Nymphalinae)
  4.  _Limenitis arthemis_                    "
  5.     "      _astyanax_                    "
  6.     "      _archippus_                   "
  7.     "      _floridensis_ (= _eros_)      "
  8.  _Danais archippus_                (Danainae)
  9.     "   _berenice_                     "

[Illustration]

       *       *       *       *       *


{183}

              INDEX

  _References to the plates are given in thicker type_

  _Acraea_, taken by kestrel, 118;
    _A. axina_, 122;
    _A. caldarena_, 122;
    _A. egina_, 34, VI. 7;
    _A. encedon_, patterns of different forms in relation to those of
        _Danais chrysippus_, 29, 144;
    typical form of, IX. D;
    _alcippina_ form of, IX. E;
    _daira_ form of, IX. F;
    _A. halali_, 122;
    _A. violae_, 33 note;
    eaten by lizards, 108;
    attacked by birds, 110, 117
  Acraeinae, as models for African butterflies, 33
  Adaptation and Natural Selection, 61
  _Adelpha_, 54
  African butterflies, mimicry among, 28-36
  _Alcidis agathyrsus_, 27, 145, III. 1
  _Aletis helcita_, 36
  _Amauris echeria_, 30, 148, VIII. 7;
    _A. hyalites_, 30, VI. 2;
    _A. niavius_, 30, VIII. 6
  _Amphidasys betularia_, rapidity of increase in melanic sport of, 101
  _Anosia plexippus_ (= _Danais archippus_), 113
  _Anthomysa_, 41
  _Aporia agathon_, 149
  _Araschnia levana_, seasonal dimorphism in, 130;
    typical form, VI. 10;
    _prorsa_ form, VI. 9;
    _porima_ form, VI. 8
  _Archonias_, 43, 56, 145;
    _A. critias_, XI. 10
  _Argynnis diana_, 47, XVI. 3;
    _A. hyperbius_, 29;
    as mimic of _Danais plexippus_, 52;
    in mimicry ring, 66, IV. 3, 4
  _Artamus fuscus_, 112
  Asilid flies, as enemies of butterflies, 106
  _Athyma punctata_, 53

  Bates, G. L., on contents of birds' stomachs, 113
  Bates, H. W., on mimicry, 9;
    on resemblances between unpalatable forms, 14;
    on initial variation in mimetic resemblance, 63;
    on S. American Pierines attacked by birds, 112
  Bateson, 3
  _Belenois_, 36
  Bingham, on birds eating butterflies, 110
  Birds, as enemies of butterflies, 109;
    stomach contents of, 113;
    feeding experiments with, 115;
    colour perception in, 119
  Bowater, on _Amphidasys betularia_, 102, 137 note
  Breeding experiments, with _Hypolimnas dubius_, 30;
    with _Papilio polytes_, 84;
    with _Papilio memnon_, 89;
    with _Papilio dardanus_, 90;
    with _Pseudacraea eurytus_, 128
  Bryant, on birds eating butterflies, 114
  _Buchanga atra_, 111
  _Byblia ilithyia_, 122
  {184}

  _Caduga tytia_, 24, 51, II. 2
  _Callamesia pieridoides_, 56
  _Calotes ophiomachus_, 107;
    _C. versicolor_, 107
  Carpenter, on intermediates in _Pseudacraea eurytus_, 126;
    on breeding experiments with _Pseudacraea eurytus_, 128
  _Castnia_, as mimic, 39, XII. 4;
    scales of, 41, XV. 5
  _Catopsilia_, 121;
    _C. florella_, 111;
    _C. pyranthe_, 111
  _Cerchneis rupicoloides_, 118;
    _C. naumanni_, 117
  _Cercopithecus pygerythrus_, 121
  _Charaxes athamas_, 110
  _Citronophila similis_, 35
  Classification of butterflies, 18-21
  _Colaenis telesiphe_, 38, XI. 4
  _Cyclosia hestinioides_, III. 3
  _Cymatophora or_, establishment of melanic sport in, 102 note
  _Cyrestis thyodamas_, 110

  Danainae, characteristics of, 22;
    as models for Oriental butterflies,23;
    as models for African butterflies, 28
  _Danais_, 111, 145;
    _D. archippus_, 48;
    eaten by lizard, 108;
    rejected by bird, 113, XVI. 8;
    _D.  berenice_, 48, XVI. 9;
    _D. chrysippus_, 23, 28;
    flight of, 55;
    in mimicry ring, 65;
    eaten by lizards, 108;
    eaten by Bee-eater, 111;
    eaten by Brown Shrike, 117;
    rejected by Kestrel, 118;
    rejected by baboon, 122;
    local variation in, 132;
    patterns overlapping with those of _Acraea encedon_, 144;
    _alcippus_ form, IX. B;
    _dorippus_ form, IX. C;
    typical form, IV. 1, VIII. 5;
    _D. plexippus_, as model for _Argynnis hyperbius_, 52;
    in mimicry ring, 65;
    eaten by Liothrix, 115 note, IV. 2;
    _D. petiverana_, 29, VI. 1;
    _D. septentrionis_, 23, 111, 112, I. 3;
    _D. vulgaris_, 150
  Darwin, on natural selection, 1;
    on adaptation, 5;
    on initial variation in mimetic resemblance, 63;
    on a difficulty of the mimicry theory, 65
  Defence in butterflies, 54
  _Delias cathara_, 56;
    _D. eucharis_, 28, 115, 116, II. 1
  de Meijere, on breeding _Papilio memnon_, 89
  de Vries, 3
  _Dismorphia_, as mimics of Ithomiinae, 38, 42;
    restricted range of many forms, 51;
    diversity of pattern in genus, 58;
    as Batesian mimics, 135;
    patterns parallel with those of Ithomiinae, 145;
    _D. astynome_, 151;
    _D. avonia_, 151;
    _D. cretacea_, 5, 8, 62, X. 1;
    _D. orise_, as mimic, 39, XII. 2;
    scales of, 40, XIV. 2;
    _D. praxinoe_, as mimic, 57, 62, X. 2, 3;
    as member of mimicry ring, 134
  Distasteful groups, characteristics of, 55

  Eltringham, 17 note, 32 note, 36 note
  _Elymnias_, patterns in genus compared with those of Danaidae, 59, 144;
    _E. malelas_, 24, II. 6, 7;
    _E. phegea_, 35, VII. 9;
    _E. singhala_, 25, I. 9;
    _E. undularis_, in mimicry ring, 66, 115 note, 116, IV. 5, 6
  _Epicopeia polydora_, 27, III. 5
  Equilibrium, conditions of in mixed population, 93
  _Eresia_, 134, 135
  _Eugonia californica_, 114
  _Euphaedra ruspina_, 36
  _Euploea core_, 25, 108, 110, 112, I. 10;
    _E. mulciber_, 24, 51, II. 4, 5;
    _E. rhadamanthus_, 24, 51, II. 8;
    _E. rafflesii_, 110
  Euploeinae, characteristics of, 22;
    as models for Oriental butterflies, 24;
    in relation to birds, 111, 112, 115 note
  {185}
  _Euripus halitherses_, 24

  Feeding experiments, with Mantids, 105;
    with lizards, 107;
    with birds, 115;
    with mammals, 121
  Finn, on feeding experiments with lizards, 108;
    on feeding experiments with Indian birds, 115;
    on feeding experiments with a Tree-shrew, 121
  Flight, different in model and mimic, 55;
    difference of in _Papilio polytes_ and its models, 82
  Fryer, on breeding _Papilio polytes_, 84;
    on relative abundance of females of_ Papilio polytes_ in Ceylon, 97;
    on birds eating "unpalatable" butterflies, 112

  _Gerrhonotus infernalis_, 108

  Haase, on mimicry, 16;
    on classification of Papilionidae, 25
  Hahnel, on S. American Pierines attacked by birds, 112
  Hardy, on conditions of equilibrium in a mixed population, 94
  Hearsy, on birds eating butterflies, 111
  _Hebomoia_, 110
  Heliconinae, as models for S. American butterflies, 38
  _Heliconius_, 145;
    _H. eucrate_, XV. 3;
    _H. melpomene_, as model, 42, 43, XI. 5;
    _H. mirus_, XV. 1;
    _H. pardalinus_, XV. 4;
    _H. splendens_, XV. 5;
    _H. sulphurea_, 43, XI. 1;
    _H. telchinia_, XV. 2;
    _H. telesiphe_, XI. 3
  _Herpestes galera_, 121
  Hess, on colour perception in birds, 119
  Hopkins, on pigment of Pierids, 150
  _Hypolimnas dubius_, polymorphism in, 30;
    as mimic of Danaines,30, VII. 8, 9;
    breeding experiments with, 30;
    var. _mima_ compared with model, 148;
    patterns of in relation to models, 149;
    _H. bolina_, 25, 117, I. 5, 6;
    _H. misippus_, 25, 29,
    as model, 53;
    flight of, 55;
    in mimicry ring, 66, 116;
    eaten by Brown Shrike, 117;
    _alcippoides_ form, IX. H;
    _inaria_ form, IX. I;
    typical form, IV. 7, 8, IX. G

  _Ideopsis daos_, III. 4
  Initial variation, difficulty of, 63
  Insect enemies of butterflies, 105
  Intermediates, between different forms of _Pseudacraea eurytus_, 128;
    in relation to mimicry,129, 140
  Ithomiinae, characteristics of, 10;
    as models for S. American butterflies, 38
  _Ituna_, 39;
    _I. ilione_, 40, XIV. 4;
    _I. phenarete_, XII. 3

  Jacobsen, experiments with _Papilio memnon_, 89
  Jordan, 40 note
  _Junonia_, 111

  _Lanius cristatus_, 117
  _Limenitis albomaculata_, 53;
    _L. archippus_, 49, 59, XVI. 6;
    _L. arthemis_, 47, 49, XVI. 4;
    _L. astyanax_, 47, XVI. 5;
    _L. floridensis (= eros)_, 49, XVI. 7;
    _L. proserpina_, 47
  Lizards, as enemies of butterflies, 107
  Local varieties, in connection with mimicry, 132
  Lycaenidae, as mimics in Africa, 35
  _Lycorea_, 145

  McAtee, on feeding experiments with birds, 118
  Mammals, as enemies of butterflies, 121
  Manders, on feeding experiments with lizards, 107;
    with birds, 117
  {186}
  Mantids, as enemies of butterflies, 105
  Marshall, on Müllerian mimicry, 72;
    on feeding experiments with Mantids, 105;
    on birds as enemies of butterflies, 107;
    on feeding experiments with S. African birds, 117;
    with monkeys, 121;
    on birds attacking Pierids, 150
  _Mechanitis egaensis_, XV. 9;
    _M. elisa_, XV. 6;
    _M. lysimnia_, 151, XV. 8;
    _M. methona_, XV. 10;
    _M. saturata_, as model for _Dismorphia praxinoe_, 57, 62;
    as member of mimicry ring, 134, XV. 7
  Melanic sports in moths, 101
  _Melinaea_, 135
  _Melinda formosa_, App. II
  _Melittophagus swinhoei_, 110
  _Merops viridis_, 111
  Merrifield, on seasonal dimorphism, 130
  _Methona confusa_, XII. 1, XIV. 1
  Migratory birds, suggested influence on mimicry of, 53
  _Mimacraea_, 35
  Mimetic resemblance, as induced through gradual slight changes, 64
  Mimic, occupying same station as model, 51;
    occupying station apart from model, 53;
    scarcer than model, 56;
    pattern of in relation to allies, 57
  Mimicry, Wallace's conditions of, 50;
    Batesian, 9;
    Müllerian, 14
  Mimicry rings, 65;
    in S. American butterflies, 134;
    and natural selection, 136
  Mimicry theory, difficulties of, 139
  Monkeys, as enemies of butterflies, 121
  Moths, mimicry in, 27, 36
  Moulton, on S. American mimicry rings, 134
  Müller, 14, 72
  Müllerian mimicry, 53, 57, 66;
    difficulties of, 72
  Mutation, see Sports
  _Mylothris_, 36

  Natural selection and mimicry, 10-12, 61, 92, 152
  Neal, on monkeys as enemies of butterflies, 123
  _Nepheronia (= Pareronia) hippia_, 116
  _Neptis imitans_, 24;
    _N. nemetes_, 54;
    _N. kamarupa_, 121
  North American butterflies, mimicry among, 45
  Norton, on rapidity of changes in mixed populations through natural
      selection, 94, App. I

  Oriental butterflies, mimicry among, 23
  Overlapping in patterns of different groups of butterflies, 144

  _Papilio aristolochiae_, as model for female of _P. polytes_, 13, 26, 52,
      77;
    range of, 79;
    likeness  to _P. polytes_, 80;
    characteristics of, 81;
    flight of, 82;
    eaten by lizards, 108;
    rejected by certain birds, 115, 116;
    disliked by Tree-shrew, 121, V. 5, 5A;
    _P. agestor_, 24, 51, II. 3;
    _P. asterius_, 46;
    _P. bachus_, App. II;
    _P. bootes_, 27, III. 6;
    _P. brasidas_, 29, VI. 4;
    _P. chamissonia_, 44, XIII. 2;
    _P. clytia_, 23, 25, 55, I. 7, 8;
    _P. coon_, 26, 89;
    _P. cynorta_, 35, 36, VII. 10;
    _P. dardanus_, investigated by Trimen, 14;
    mimicry in, 30;
    breeding experiments with, 90;
    polymorphic forms of in relation to models, 149 note;
    var. _humbloti_, 32;
    var. _meriones_, 32;
    [F] _cenea_, 31, VIII. 4;
    [F] _dionysus_, 31, 33;
    [F] _hippocoon_, 31, VIII. 3;
    [F] _niavioides_, 32, 33;
    [F] _planemoides_, 31;
    [F] _ruspina_, 33;
    [F] _trimeni_, 31, 32, 33;
  {187}
    [F] _trophonius_, 31, 122, VIII. 2;
    _P. delesserti_, App. II;
    _P. demoleus_, 111, 121;
    _P. echerioides_, App. II;
    _P. erithonius_, 110;
    _P. euterpinus_, 42, 43;
    _P. glaucus_, 45;
    var. _turnus_, 46;
    _P. hahneli_, 39;
    _P. hector_, model for female of _P. polytes_, 13, 52, 78;
    range of, 79;
    characteristics of, 81;
    flight of, 82;
    eaten by lizards, 108;
    eaten by birds, 110, 117, V. 6, 6A;
    _P. hippason_, App. II;
    _P. laglaizei_, 27, 124, III. 2;
    _P. leonidas_, 29, VI. 3;
    _P. lysithous_, polymorphism in, 44;
    [F] _lysithous_, XIII. 4;
    [F] _rurik_, XIII. 5;
    [F] _pomponius_, XIII. 6;
    _P. macareus_, 23, 111;
    _P. memnon_, 26, 89;
    _P. mendax_, 24, 51, II. 9;
    _P. nephalion_, 44, XIII. 1;
    _P. osyris_, XI. 8, 9;
    _P. paradoxus_, 25;
    _P. pausanias_, 43, XI. 2;
    _P. perrhebus_, 44, XIII. 3;
    _P. philenor_, as model, 45;
    taken by lizard, 108, XVI. 1;
    _P. polytes_, polymorphism in females of, 13, 75;
    mimic of Pharmacophagus Papilio, 26;
    habits of, 52, 124;
    often more abundant than models, 56;
    description of, 76-78;
    relative abundance of models in Ceylon, 79;
    breeding experiments with, 84;
    equilibrium among females of in Ceylon, 96;
    relative abundance of three forms of female of in Ceylon, 97;
    historical notes on abundance of forms of female in Ceylon, 98;
    origin of forms of female in, 125, 141;
    relation of polymorphic forms to models in, 149 note;
    preyed on by Wood-Swallow, 112;
    feeding experiments with, 116, V. 1-4, 1A-4A;
    _P. polyxenus_, 27;
    _P. rex_, App. II;
    _P. ridleyanus_, 34, 36, VI. 6;
    _P. sarpedon_, 110;
    _P. troilus_, 45, XVI. 2;
    _P. xenocles_, 23, 111, I. 4;
    _P. zagreus_, 43, X. 8
  Papilionidae, as mimics of Oriental models, 23-25;
    of African models, 29, 30, 35;
    of S. American models, 43;
    of N. American models, 45
  Parallel patterns, in different butterfly groups, 144
  _Pareronia_, 145, 149;
    _P. ceylanica_, 23, 59, 116 note, I. 1, 2
  Pattern and physiological properties, possible connection between, 137
  Patterns, overlapping series of in different groups of butterflies, 145
  _Pedaliodes_, 135
  _Pereute charops_, 42, XI. 6, 7
  _Pericopis_, 39
  _Perrhybris_, as mimics of Ithomiines, coloration of male in _P.
      malenka_, 62;
    as members of mimicry rings, 134, 135;
    _P. demophile_, 151;
    _P. lorena_, 151;
    _P. malenka_, X. 4, 5, 6
  Pharmacophagus Swallow-tails, characteristics of, 22, App. II;
    as models for Oriental butterflies, 25;
    absence of in Africa, 35;
    as models in S. America, 43;
    as models in N. America, 45
  _Phrissura_, 36
  _Phyciodes_, 38, 54
  Physiological properties, possible connection of with pattern, 137
  Pieridae, as models for Oriental butterflies, 28;
    mimicry in African, 36;
    mimicry in S. American, 43;
    frequency of bird attacks on, 150
  _Planema epaea_, 35, VII. 5;
    _P. macarista_, sexual difference in, 34, VII. 1, 2;
    mimicked by _Elymnias phegea_, 35;
    by _Pseudacraea eurytus_, 126;
    _P. poggei_, as model for _planemoides_ female of _Papilio dardanus_,
        31;
    _P. paragea_, 126, VII. 4;
    _P. tellus_, 126, VII. 3
  {188}
  Poison-eaters, see Pharmacophagus Swallow-tails
  Polymorphism, in females of mimicking species, 13;
    among females of _P. dardanus_, 30;
    among females of _P. polytes_, 75
  Population, conditions of equilibrium in mixed, 93
  Poulton, 17;
    on N. American mimetic butterflies, 45;
    on the "Transparency group," 41;
    on mimicry through agency of migratory birds, 53;
    on _Hypolimnas misippus_, 66 note;
    on the relation between mimetic forms of _P. polytes_, 90;
    on predaceous insects, 105;
    on relative proportion of different forms of _Pseudacraea eurytus_,
        127;
    on local variation in _D. chrysippus_, 132
  _Precis_, 111, 122, 131;
    _P. octavia_, seasonal dimorphism in, 131, VI. 11, 12
  _Prioneris_, 110;
    _P. sita_, 28
  Pritchett, feeding experiments with lizards, 108
  Protective resemblance, 8
  _Protogonius_, as mimics of Ithomiines, 38;
    as members of mimicry rings, 134, 135, 138;
    _P. tithoreides_, X. 9
  _Pseudacraea_, 59, 144;
    _P. boisduvali_, 34, VI. 5;
    _P. eurytus_, relative proportion of different forms in, 127;
    polymorphism of in relation to model, 149 note;
    var. _hobleyi_ as mimic of _Planema macarista_, 35, 127, VII. 6, 7;
    var. _terra_, as mimic of _Planema tellus_, 126, VII. 8;
    var. _obscura_ as mimic of _Planema paragea_, 126

  Ray, on adaptation, 4, 6
  Rodents, bearing on mimicry of recent genetic work with, 147

  Satyrinae, transparency in S. American, 42
  _Sceleporus floridanus_, 108
  Schaus, on birds as enemies of butterflies, 112
  Seasonal dimorphism, 130
  Seitz, 44, 52, 58
  Shelford, 56 note
  S. American butterflies, mimicry among, 38
  Sports, as foundation of mimetic resemblances, 70, 91, 143
  Sweet-peas, experiments on, 91
  Swynnerton, on contents of stomachs of birds, 114

  _Telipna sanguinea_, 36
  _Terias brigitta_, 35;
    _T. hecabe_, 110
  _Thyridia_, 40, XIV. 3
  _Tithorea pseudonyma_, X. 10
  "Transparency group," in S. America, 39
  Trimen, on mimicry in African butterflies, 13
  _Tupaia ferruginea_, 121

  Variation, difficulty of initial, 63

  Wade, on relative abundance of the three forms of _P. polytes_ in Ceylon,
      99
  Wallace, on mimicry in Oriental butterflies, 12;
    on the conditions of mimicry, 50;
    on the females of _P. polytes_, 76;
    on initial variation, 64
  Warning colours, 10, 11
  Weismann, 1, 2

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NOTES

[1] _The Wisdom of God manifested in the Works of the Creation_, London,
1691.

[2] Ray gives the case of an elephant "that was observed always when he
slept to keep his trunk so close to the ground, that nothing but Air could
get in between them," and explains it as an adaptation in habit to prevent
the mice from crawling into its lungs--"a strange sagacity and Providence
in this Animal, or else an admirable instinct."

[3] _Trans. Linn. Soc._ vol. 23, 1862.

[4] _Trans. Linn. Soc._ vol. 25, 1866.

[5] _Trans. Linn. Soc._ vol. 26, 1870.

[6] In attributing this quality to the butterflies in question I am merely
stating what is held by the supporters of the mimicry theory. I know of
scarcely any evidence either for or against the supposition.

[7] It is assumed that the intelligence of the birds is such that they can
learn a pattern after a single disagreeable experience of it.

[8] _Untersuchungen über die Mimikry_, 1893.

[9] The African mimetic butterflies have been recently monographed by
Eltringham in a large and beautifully illustrated work--_African Mimetic
Butterflies_, Oxford, 1910.

[10] Omitting the Hesperidae which hardly enter into questions of mimicry.

[11] The classification adopted is that used by Dr Sharp in the "Cambridge
Natural History," _Insects_, vol. 2, 1901.

[12] Cf. Shelford, _Proc. Zool. Soc._ 1902.

[13] _African Mimetic Butterflies_, Oxford, 1910.

[14] These African species of _Hypolimnas_ are frequently referred to the
genus _Euralia_.

[15] Corresponding to the _dorippus_ form of _D. chrysippus_ (cf. Pl. IX)
there is a rare form of _trophonius_ known as _dorippoides_.

[16] These two forms are figured on Plate 10 of Eltringham's _African
Mimetic Butterflies_.

[17] _Acraea violae_, the only representative of the group in S. India and
Ceylon, is nevertheless a very abundant insect. It cannot, however, be said
that it is definitely mimicked by any other species in this region.

[18] Coloured figures of these and of the other African species referred to
may be found in Eltringham's work on _African Mimetic Butterflies_.

[19] These descriptions are taken from preserved specimens which I owe for
the most part to the kindness of Dr Jordan. I have not had an opportunity
of examining fresh ones.

[20] This is more marked in _Castnia_ than in _Anthomysa_. It appears to be
a peculiarity of many members of the genus _Castnia_ that the scales do not
lie so tight as generally in moths. Owing to this, some of the large
whole-coloured species have a somewhat fluffy look.

[21] Cf. Poulton, _Essays on Evolution_, 1908, pp. 264-6.

[22] Cf. Poulton, _Darwin and the 'Origin,'_ 1909, pp. 177-186.

[23] The N. American members of this genus are often referred to as
_Basilarchia_.

[24] _Darwinism_, 1890 (1st Edition 1889), p. 264.

[25] _Macrolepidoptera of the World._ _Fauna Americana_, p. 98.

[26] _Essays on Evolution_, 1908, p. 381.

[27] These "unpalatable" butterflies are sometimes extensively preyed upon
by insectivorous birds, when they fall an easier prey owing to their
slowness (cf. p. 112).

[28] See Shelford, PROC. ZOOL. SOC. 1902, p. 260. A coloured figure of both
species is given in the paper.

[29] _Macrolepidoptera of the World._ _Fauna Americana_, pp. 98-104, Plates
28-30.

[30] "In what way our _Leptalis_ (= _Dismorphia_) originally acquired the
general form and colour of Ithomiae I must leave undiscovered." _Trans.
Linn. Soc._ vol. 23, 1862, p. 513.

[31] _Darwinism_, 1890, pp. 242-244.

[32] _Origin of Species_, 6th Edition, 1891, p. 354.

[33] _H. misippus_ was at one time regarded as a clear case of Batesian
mimicry. But in view of its plentifulness, of the fact that it may be
abundant outside the area inhabited by its model, and of the ease with
which it can establish itself in parts remote from its original habitat,
_e.g._ S. America, it has come to be regarded by certain supporters of the
mimicry theory as a Müllerian mimic. Cf. Poulton, _Essays on Evolution_,
1908, pp. 215-217.

[34] An English translation of Müller's paper is given by Meldola, _Proc.
Ent. Soc._, 1879, p. xx.

[35] _Trans. Ent. Soc. Lond._, 1908, p. 93.

[36] Provided of course that the type form remains in the majority. If the
variation occurred simultaneously in more than 50% of A the advantage would
naturally be with the variation.

[37] It is possible to imagine an exceptional case though most unlikely
that it would occur. Suppose for example that there were a number of
distasteful species, say 20, all of different patterns, and suppose that in
all of them a particular variation occurred simultaneously; then if the
total shewing that variation from among the 20 species were greater than
the number of any one of the species, all of the 20 species would come to
take on the form of the new variation. In this way it is imaginable that
the new pattern would gradually engulf all the old ones.

[38] _Trans. Linn. Soc._ vol. 24, 1866.

[39] These darker ribs are also present in the male and M female but are
obscured owing to the generally deeper colour.

[40] See Appendix II, p. 158.

[41] _Spolia Zeylanica_, 1910.

[42] We shall take it for the present that, from the point of view of
mimicry, birds are the main enemies of butterflies (cf. Chap. IX).

[43] See later, p. 119.

[44] The specimens figured on Pl. V were dried in papers when taken. The
body is consequently much compressed and the characteristic scarlet of _P.
hector_ and _P. aristolochiae_ is largely hidden.

[45] _Philosophical Transactions of the Royal Society_, vol. 204, 1913.

[46] _Tijdschr. voor Entomologie_, vol. 53, 1909. A more accessible account
is given by de Meijere, _Zeit. f. indukt. Abstamm. u. Vererbungslehre_,
vol. 3, 1910.

[47] For further information see Poulton, _Trans. Ent. Soc. Lond._ 1909,
and various notes in _Proc. Ent. Soc. Lond._ subsequent to this date.

[48] _Science_, July, 1908.

[49] If for example there were 5000 dominants and 4000 recessives, and if
only half of the population survives to mate, then we should be left with
2500 dominants and 2000 recessives as parents of the next generation. But
if there were also a 10% selective disadvantage working against the
recessives, their numbers would be further reduced from 2000 to 1800 and
the proportion of dominants to recessives would be changed from 5:4 to
25:18.

[50] As these larvae were for the most part found simply over a
considerable time it follows that they are the offspring of different
females and represent the relative proportions of the three forms in the
general population.

[51] _The Lepidoptera of Ceylon_, 1880.

[52] From the experience of breeders it would appear that the melanic form
is somewhat hardier, at any rate in captivity.

[53] Intermediates may also occur in some strains (cf. Bowater, _Journal of
Genetics_, vol. 3, no. 4, 1914).

[54] An interesting case of a similar nature has recently been published by
Hasebroek (_Die Umschau_, 1913, p. 1020). A melanic form of the moth,
_Cymatophora or_, suddenly appeared near Hamburg in 1904. This new form, to
which the name _albingensis_ was given, rapidly became the predominant one.
In 1911-1912 over 90% of the moths reared from caterpillars taken in the
open were of the _albingensis_ form; nor were any intermediates found
between it and the typical form. Some experiments were also made which shew
that the _albingensis_ form behaves as a dominant to the original type
form.

[55] _Trans. Ent. Soc. Lond. 1907._

[56] _Trans. Ent. Soc. Lond. 1902._

[57] _Proc. Zool. Soc._ 1911.

[58] _Journ. Roy. Asiat. Soc. Bengal_, vol. 65, 1897.

[59] _Spolia Zeylanica_, 1910.

[60] _Biological Bulletin_, vol. 5, 1903.

[61] _Trans. Ent. Soc. Lond._ 1909.

[62] _Trans. Ent. Soc. Lond._ 1902.

[63] _Trans. Ent. Soc. Lond._ 1911.

[64] _Proc. Zool. Soc. 1913._

[65] _A Naturalist in Nicaragua_, 1874, p. 316.

[66] _I^{er} Congr. Internat, d'Entomologie_, Bruxelles, 1911.

[67] _Ibis_, 1911.

[68] _Ibis_, 1912.

[69] _The Condor_, vol. 13, 1911, pp. 195-208.

[70] _Journ. Asiat. Soc. Bengal_, vol. 64, 1895, and vol. 66, 1897.

[71] Nevertheless a Liothrix is recorded as eating _Danais plexippus_ and a
_Euploea_ even though two male specimens of the palatable _Elymnias
undularis_ were in the cage.

[72] A form closely resembling _P. ceylonica_ figured on Pl. I, fig. 1.

[73] _Proc. Zool. Soc. Lond. 1911._

[74] _Proc. Acad. Nat. Sci. Philadelphia_, 1912.

[75] C. Hess, _Handbuch der vergleichenden Physiologie_ (herausgegeben von
H. Winterstein), Bd. 4, 1912, p. 563.

[76] _Journ. As. Soc. Bengal_, vol. 66^2, 1898.

[77] _Trans. Ent. Soc. Lond._ 1902.

[78] Marshall, _loc. cit._ p. 379.

[79] In this connection may be quoted a letter from Capt. N. V. Neal near
Lagos to Mr W. A. Lamborn which was recently published in the _Proceedings
of the Entomological Society_.

"You have asked me about monkeys eating butterflies. This is very common,
as every native will tell you. I have seen it myself. The monkey runs along
a path, sees some butterflies fluttering round some filth, goes very
quietly, and seizes one by the wings, puts the solid part (body) into his
mouth, then pulls the wings off. The poor butterfly goes down like any
oyster.... The dog-faced baboon and the large brown monkey with a very long
tail, which seems to be the most common species in this colony, are great
butterfly-eaters. The little spider-monkey also considers a butterfly a
treat, and prefers one to a spider."

[80] Cf. E. B. Poulton in _Bedrock_ for Oct. 1913, p. 301.

[81] _Trans. Ent. Soc. London, 1914._

[82] In the female _hobleyi_, with rare exceptions, the orange of the male
is replaced by white, and it has received the name _tirikensis_. The female
of _P. macarista_ also shews white in place of the orange of the male.

[83] Cf. Poulton, E. B., _I^{er} Congr. Internat. d'Entomol._, Bruxelles
1911. This proportion is founded on several hundreds caught at random.
Observers are agreed that _Pseudacraea_ is both a warier insect and a
stronger flyer than the various Planemas which it resembles.

[84] _I^{er} Congr. Internat. d'Entom._, Bruxelles 1911.

[85] Cf. Poulton, _Bedrock_, Oct. 1913, p. 300.

[86] The size of the white spot may shew much variation in specimens from
the same region. I have seen African specimens in which it is large, while
in the Ceylon specimen figured on Plate IV it is as small as in the typical
African specimen shewn on Plate VIII.

[87] See Moulton, J. C., _Trans. Ent. Soc. London_, 1909.

[88] In this connection it is of interest that a recent observer with
considerable breeding experience finds that the dark _doubledayaria_
variety of the Peppered Moth is more hardy than the typical form (cf. p.
101). The swift success of the dark variety led some to regard it as better
protected against bird enemies. It is, however, not unlikely that the
deeper pigmentation is associated with some physiological difference which
makes for greater hardiness. See Bowater, _Journal of Genetics_, vol. 3,
1914.

[89] As examples may be mentioned _P. polytes_, _Hypolimnas misippus_, _H.
dubius_, and _Pseudacraea hobleyi_. With the exception of the _planemoides_
form it is true also for _P. dardanus_, the most polymorphic of them all.

[90] _Trans. Ent. Soc. Lond. 1909._

[91] Cf. F. G. Hopkins, _Phil. Trans. Roy. Soc. 1895._

[92] Coloured representations of these two species will be found on Pl. 20
of Seitz, _Macrolepidoptera of the World, Fauna Americana_.

[93] _Untersuchungen über die Mimikry_, 1893.





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