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}