Experiment Station Library
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Hugh B. Cott pinx.
Bombinator igneus Atelopus stelzneri Phrynomerus bifasciatus
Hyperolius marmoratus Salamandra maculosa
Dendrobates tinctorius Dendrobates tinctorius Dendrobates tinctorius
HUGH B. COTT
M.A., D.SC., F.R.P.S., F.Z.S.
Lecturer in Zoology and Strickland Curator
WITH AN INTRODUCTION BY
JULIAN S. HUXLEY
METHUEN & CO LTD
36 ESSEX STREET STRAND LONDON WC2
First published 28 March 1940
Reprinted with minor corrections 1957
CATALOGUE NO 3005/u
PRINTED IN GREAT BRITAIN
BY BRADFORD AND DICKENS DRAYTON HOUSE LONDON WC I
AND BOUND BY JAMES BURN AND CO LTD ESHER
The day has passed delightfully. Delight itself, however, is a weak term to express the feelings of a naturalist, who, for the first time, has wandered by himself in a Brazilian forest. . To a person fond of natural history, such a day brings with it a deeper pleasure than he can hope to experience again.' DARWIN
Happy the man whose lot it is to know
The secrets of the earth. He hastens not
To work his fellows' hurt by unjust deeds,
But with rapt admiration contemplates
Immortal Nature's ageless harmony,
And how and when her order came to be.'
AMONG a certain section of experimental biologists, any time in these last thirty years, it has been fashionable and indeed almost a matter of professional conscience to display a radical scepticism on the subject of adaptations, especially colour adaptations, and most particularly mimetic adaptations. Upholders of the theories of protective and warning coloration and of mimicry have often been attacked as armchair theorists (whereas they have in fact almost invariably been first and foremost field naturalists), insufficiently acquainted with modern work in genetics, which for some unexplained reason is held to do away with adaptive interpretations. I
Thus, for instance, to cite but one recent author, A. F. Shull in his book on Evolution (1936) writes (P. 212):
if the doctrine [of natural selection] can emerge minus its sexual election, its warning colors, its mimicry and its signal colors, the reaction over the end of the century will have been a distinct advantage. These special forms of the selection idea . seem destined to be dropped, or at least relegated to very minor places in Evolution discussion.
Elsewhere (p. 175) he states that the theories of aggressive and alluring resemblance must probably be set down as products of fancy belonging to uncritical times
Dr. Cott, in this important book, has turned the tables with a vengeance on objectors of this type. He has shown that it is they who are the armchair critics, or, one might say, the laboratory-bench critics. Had they taken the trouble to acquaint themselves with even a fraction of the relevant facts to be found in nature, they could never have ventured to enunciate such sweeping criticisms : their objections are a measure of their ignorance. Further, by applying optical and psychological principles, he has pushed the analysis of visual alkesthetic characters to a new level, and shown that many of them constitute adaptations of a quite unsuspected degree of refinement. Far from genetics in any way throwing doubt on their adaptive interpretation, the facts of cryptic, warning and mimetic coloration pose searching questions to the geneticist, and demand a recasting of many current views on the efficacy and mechanisms of selection.
This analysis is the most original part of Dr. Cott's book. With the aid of his own remarkable drawings and photographs, he demonstrates how naturein this case via the operation of natural selection-employs the most elaborate vii
ADAPTIVE COLORATION IN ANIMALS
optical-psychological devices to enhance conspicuousness where conspicuousness is advantageous, to reduce it where obliteration. is the biological aim.
Cases in which an unusual resting position is adopted provide beautiful evidence of his contentions. Counter-shading is reversed in fish which swim upside-down and caterpillars which rest inverted. Bark-haunting moths have their markings running parallel with the vertical cracks of the bark, whether they rest with body vertical or horizontal, with wings folded or expanded.
The correlation of behaviour and markings, indeed, as Poulton long ago pointed out, is one of the strongest supports for theories of adaptive coloration. Even if it were possible for a case-hardened sceptic to dismiss as accidental such isolated facts as the co-existence, in various stingless insects, of a resemblance to bees or wasps on the one hand and on the other of motions simulating stinging, and even of the protrusion of an imitation sting, the enormous array of less spectacular but equally significant correlations provided by Dr. Cott is overwhelming.
Dr. Cott is a true follower of Darwin in driving his conclusions home by sheer weight of examples. Faced with his long lists of demonstrative cases, the reader is tempted to wonder why adaptive theories of coloration have been singled out for attack by anti-selectionists. As Dr. Cott observes, physiologists and geneticists do not deny that wings exhibit characters which make them suitable as mechanisms for flying, or that legs, beaks and teeth are adaptively correlated with mode of life.
A proper analysis, such as that undertaken in. this book, demonstrates conclusively that optical-psychological principles promoting increase or decrease of conspicuousness in correlation with mode of life are just as effectively employed in animal coloration as are aerodynamic principles in the construction of flying animals. There is no more justification for denying the functional (and therefore adaptive) significance of the one than of the other.
Let it not be supposed, however, that the experimental approach has been neglected. Here again the anti-Darvinian critics of the theories of adaptive coloration seem not to have taken the trouble to acquaint themselves with the facts. The experimental data conclusively demonstrate that cryptic, warning and mimetic forms do in point of fact enjoy a degree of immunity from predators which constitutes a considerable selective advantage in the struggle for existence. Some of the recent work is especially convincing.
Another novel and interesting feature of this book is the constant crossreference to human affairs. Dr. Cott shows how the same optical principles automatically utilized by natural selection in animal coloration have over and over again been employed deliberately and of set purpose by men to achieve the same effects. It is good to know that he has been called on to apply the principles he has studied to such good effect in animals to the practice of camouflage in war.
Adaptive Coloration in Animals is perhaps the most satisfactory book yet written on adaptation. It brings together great masses of data, from the field, the museum and the laboratory, analyses them in the light of established physical
and psychological principles, and deals satisfactorily with the methodological problem of establishing proof of their adaptive nature. It is a worthy successor to Sir Edward Poulton's The Colours of Animals, published nearly half a century ago. The one was a pioneer study, the other is in many respects the last word on the subject. It is very gratifying to see that one of the great traditions of British biology-the tradition of scientific natural history, in which the comparative treatment of patiently accumulated data is made to yield generalizations of first-class importance-is being so worthily upheld.
JULIAN S. HUXLEY
THERE are many ways of regarding living creatures. They appear very differently to different men. What we see in an animal depends both upon our outlook and experience. To the morphologist, it presents problems of structure and descent ; to the systematist, it raises questions of relationship and classification ; the embryologist interests himself in its development; the psychologist, in its behaviour; to the stock-farmer, it has market value ; to the artist, beauty ; to the naturalist, a place to live and a part to play in the world of life. Even within the confines of a specialized study, like that of animal coloration, the same wide differences in approach and outlook exist. Here the biochemist is concerned with chemical problems of pigmentation; the physicist, with the optics of structural colours ; the geneticist attempts to analyse colour-characters in terms of heredity ; the systematist traces in them an indication of relationship ; the physiologist studies their functions in the body ; the naturalist, their functions in the field.
In the interrelationships between animals of the same, or of different species
-as between predator and prey, between rival males, or the opposite sexes, between parent and offspring, or between members of the group-characters which exert their influence from a distance, by sound, by sight, or scent, take a prominent place. To such characters Julian Huxley has applied the useful and comprehensive term Alkoesthetic. In this volume I have intentionally confined myself to a special study of the various manifestations and functions of coloration in the interspecific relations of animals-that is to say, in the relations between predator and prey. Thus it will be apparent at the outset that auditory and olfactory characters have not been dealt with, except in so far as their mention may be relevant to my purpose; and that, on the other hand, I have excluded as beyond the scope of the present volume that large and important class of coloration-phenomena whose function has to do with intraspecific relationscoloration, in other words, used for threat by rival males, for courtship by members of the opposite sex, and for recognition, by members of the family, or flock.
One of the fundamental facts affecting living creatures is the interspecific warfare known to biology as the struggle for existence. Animals, like men, are beset by many and great dangers. The problem of self-preservation in the field is very real, very urgent, and often difficult enough to solve ; but it is one with which all forms of animal life are faced. Broadly speaking, individual survival depends upon the satisfaction of two vital needs-security and sustenance. These are the two primary claims of life. In a world peopled with potential enemies and pregnant with hunger and the possibility of starvation, if an animal is to Xi
Xii ADAPTIVE COLORATION IN ANIMALS
survive, it must eat, and avoid being eaten. It is the old question of the relation between the aggressor and the victim of aggression, between hunter and hunted.
The very urgency of this central biological problem of self-preservation is reflected in the variety and specialization of nature's adaptive experiments in offence and defence, as it is also in the parallel inventions and contrivancies of man. Indeed, the primeval struggle of the jungle, and the refinements of civilized warfare, have here very much the same story to tell. In both realms we see the results of an armament race and an invention race, which has led to a state of preparedness for offence and defence as complex as it is interesting. In both, methods mainly similar are employed: we have the evolution of speed, on land, in the air, and under water, by pursuer and pursued; the employment of stealth and surprise, of deception and ambush; the display of warning signals and of alluring baits ; the elaboration of smoke-screens, traps, nets, parachutes, of electrocution and booby-traps ; the adoption of fossorial and nocturnal habits ; the development of poison, and of deadly apparatus in the form of fangs or stings or arrows for its injection into the bodies of enemies or prey ; the protection afforded by armour-plating, spines and barbed wire; the use of chemical warfare, which is practised, for instance, by certain insects ; and of poison gas, by creatures like the skunk.
Among all these evolutionary achievements, perhaps none are more important, more widely used, and more highly developed, than those characters which serve to elude, to attract, or to deceive the eye, and so to facilitate escape from enemies or the pursuit of prey. Indeed, it is no exaggeration to say that the modification of outward appearance by visual characters, directed towards a seeing public, and serving either to facilitate recognition or to frustrate it, has been one of the main results attained in the evolution of the higher animals ; and such characters comprise some of the most outstanding examples of adaptation in the whole field of biology.
In most spheres of modern warfare man has now, though in some cases only very recently, advanced far ahead of the so-called brute creation in his equipment for protection and aggression-in regard, for instance, to the development of armour and mobility, to the use of projectiles, and of devices such as the balloonbarrage and submarine-net (which in principle are gigantic spiders' webs), of smoke-screens (which are employed with effect by cuttle-fishes), and of delicate instruments like range-finders and sound- detectors. Yet, while there is the closest analogy between the needs for concealment and deception in nature and in war, in this sphere the coloration of animals has attained a degree of perfection
far beyond the comparatively crude attempts at camouflage with which we are too easily satisfied-attempts which often neglect to make use of the very principles revealed in the coloration of innumerable snakes, caterpillars, birds, fishes and other organisms.
Because of recent developments in aerial warfare, and the ever-increasing part played by aircraft in military and naval operations of all kinds-for reconnaissance, photography, and bombing-camouflage has assumed to-day a new
and vital function, whose significance can scarcely be overstated. But we have lagged far behind nature, and have much leeway to make up before we can approach the efficiency attained by different forms of wild life in the field. We should do well, therefore, to follow advice from the Book of Job: 'But ask now the beasts, and they shall teach thee ; and the fowls of the air, and they shall tell thee . and the fishes of the sea shall declare unto thee.'
A satisfactory system of classification, while primarily a matter of convenience, is nevertheless one of great importance to the serious study of any phenomena. Many of us had an early introduction to classification when, as children, we learned to play the game 'Animal, Vegetable and Mineral'. Nowthephenomena, of adaptive coloration likewise fall into three main classes, according to the visible results produced. I have accordingly divided the present work into parts devoted, respectively, to questions of concealment, of advertisement, and of disguise. The biological function of these elusive, attractive, or deceptive devices varies widely according to circumstance, Through reduced visibility, they may facilitate the capture of food, or escape from the aggressor. Through increased conspicuousness, they may serve as a warning, or as bluff, to potential enemies, or as an allurement to prey. Through deceptive or mimetic effects, they may mislead the observer as to an animal's whereabouts, attitude, or identity.
It would be great presumption for any one to attempt a book on functional aspects of animal coloration without acknowledging a deep debt of gratitude to 'Sir Edward Poulton, D.Sc., F.R.S., whose classical work on The Colours of Animals appeared just half a century ago. Whatever the subsequent developments of our knowledge in this field of natural history, Poulton's analysis and classification of the phenomena must remain the essential structure upon which others may build. The great array of facts which have since been accumulated are largely due to his untiring labours, and in the preparation of the present work I have time and again had occasion to refer to his writings. Reference has been made in the text to the original publications from which I have drawn examples and evidence.
Although such a study as this is somewhat specialized, it is nevertheless one bearing directly upon that central problem of biology-the mechanism of evolution. In this book I have been less concerned with the origin of adaptive coloration, than with its various manifestations and uses. Nevertheless, those who are m6re interested in questions of origin, than in effects and applications, will find assembled in these pages a body of facts and evidence from which they may draw their own conclusions. The theory of natural selection is to-day receiving support from a number of sources. One of these sources is the scientific study of natural history, and in particular of adaptive coloration. Indeed, in the light of modern knowledge, the claim may well be repeated and emphasized, which was made in 1898 by Poulton, when he wrote that 'the explanation of these deeply interesting facts, which form so fascinating and important a department of natural history in the tropics, is one of the most notable triumphs ever won by the great Theory of Natural Selection '.
ADAPTIVE COLORATION IN ANIMALS
In a book which deals, as this book deals, with optical principles and outward appearances, illustrations must take an important place, and I have spared no pains in the preparation of a series of photographs and drawings which will, I trust, materially add to any usefulness the work may possess. I have heard the view expressed that photographs do not provide reliable evidence as to the effectiveness of concealing coloration, since, it is argued, animals are more difficult to detect in the photographs than they would be in the field. Now this opinion does contain an element of truth when applied to some photographs-bad photographs. Every one has seen reproductions intended to illustrate some cryptic insect or bird, in which the creature remains unfound, not because of its cryptic coloration, but because the picture is technically deficient-lacking in clarity of outline, or in range of tones, or in scale, or because it is badly printed on vile paper. On the other hand, with illustrations of good quality-and in these days of ultra-rapid sensitive materials and ultra-precise cameras there is really no excuse for indifferent photography-the reverse is certainly true. In the sphere of aerial reconnaissance, it is the camera, and not the observer, which yields the exact data so necessary for the detection of camouflage. The photographic record is more accurate and discriminating than the retinal record, and camouflage is more readily spotted from the former than from the latter. Moreover, the limited field over which the eye must explore greatly facilitates the detection in a print of a cryptic insect which in nature may well escape observation since not even its approximate whereabouts is known. I hope, therefore, that the photographs reproduced in this volume will be of real value as a scientific record of the wonderful power of cryptic and deceptive coloration in nature. They have been obtained over a period of some fifteen years, from many parts of the world, and, unless otherwise stated, they represent wild animals, free and in their natural surroundings.
All the figures in the text I have drawn specially for the book. Figs. 8, 24(1)) 28(2)) 48) 50) 54) 69(2), 73) 75, 82 and 83(2, 3 and 4) are copies in line of existing half-tone illustrations whose source is named in the text. Fig- 56 is based partly upon photographs of the Crested Rat in display, kindly given me by Mr. David Seth Smith, of the Zoological Society of London. Fig. 69(1), representing the cryptic attitude of Zabrochilus australis, is taken from a photograph of the living insect by Mr. A. J. Nicholson, who generously supplied me both with a print and with a specimen of this remarkable grasshopper. In Fig- 74 the cryptic attitude of the Poor-me-one is taken from a photograph by A. Muir, published in the Ibis, 1925. The remaining figures are original.
Sir John Graham Kerr, F.R.S., M.P., has paid me the compliment of showing a lively interest in this work since it began to take shape, nearly five years ago, in the Department of Zoology, Glasgow University ; and his never-failing encouragement, guidance, and friendship, backed by his own exceptional store of knowledge and experience both as a field naturalist and as an authority on questions of camouflage, have contributed much towards the confident spirit in which I have been able to approach the work. In addition to the untiring support he has given me, he has generously found time to read through and correct the
entire proofs. To Dr. Julian S. Huxley, D.Sc., F.R.S., I am also under very real obligations. He, too, has read the whole book in proof, and has spared no pains in giving helpful criticism. I appreciate my good fortune in having had the benefit of advice and numerous valuable suggestions from one whose own important contributions to knowledge in the field of adaptive coloration is so widely recognized.
It is not possible to mention here the names of all those who, in one way or another, have generously given advice or assistance. They know, and I appreciate, in what ways they have helped. To various officials of the British Museum (Natural History) I am indebted for facilities which they have so courteously granted, and in particular I wish to thank Mr. H. W. Parker, M.A., and Mr. J. R. Norman, of the Department of Zoology, and Mr. W. E. China, of the Department of Entomology, for giving me access to specimens in their charge, and for helpful suggestions and many kindnesses. At Oxford, Professor G. D. Hale Carpenter, M.D., has kindly allowed me to examine and photograph specimens in the Hope Department. I am also especially indebted to my friend and former travelling companion, Mr. L. C. Bushby, of the Zoological Society of London, for the opportunities he has afforded for the study of living insects in the Society's Gardens.
I owe a debt of gratitude to the Colston Research Society, Bristol University, for a grant towards expenses incurred during a zoological expedition to the Canary islands in 1931 ; and to the Carnegie Trustees for the Universities of Scotland, for financial assistance received in 1937-38 during my tenure of a Carnegie Teaching Fellowship in the University of Glasgow.
To my publishers I should like to express my indebtedness for the courteous consideration they have shown me, and for the care which they have bestowed on the book in all stages of its production.
Finally, I must record a deep sense of gratitude to my wife, who has typed part of the manuscript, checked the final proofs, helped me to compile the index, and who in innumerable other ways has lightened my work.
This book was written in a period of unsettled peace, in which the nations of Europe were preparing for, or against, war. Now the war has come, and with it an intensification of industrial energy and of that preoccupation with machines which has long been an accompaniment of modern life. In these days that lie ahead, when too exclusive an interest in mechanical and scientific contrivancies must tend to encourage the development of what Lord Dawson once called the 'gadget-mind', which is restless, unreflective, and unemotional, the study of natural history provides a welcome antidote-not indeed as a way of escape from reality, but rather as a means of seeing, as from a mountain-top, and in clearer and wider perspective, that struggle for existence which is the lot of men not less than of animals.
In war, as in peace, to young and old alike, animals may be, and should be,
XVI ADAPTIVE COLORATION IN ANIMALS
a fount of joy and inspiration. It was Thomas 'a Kempis who said: If indeed thy heart were right, then would every creature be to thee a mirror of life, and a book of holy doctrine.' All men, provided they are not too ignorant, too proud, or too sophisticated, are bound to take a delight in animal life ; and fortunate are those who have learned to see, in the wild things of nature, something to be loved, something to be wondered at, something to be reverenced, for they will have found the key to a never-failing source of recreation and refreshment.
Unmindful of the affairs of men, the pageant of nature marches on: we can recognize and give names to most of the actors; but of their make-up, their parts, and their inner lives we still know little enough. In whatever direction we look, there appear problems, old and new, awaiting investigation. Much, then, remains to be done. For the research worker with patience, resolution, and imagination, the future is full of promise. Experimental embryologist and biochemist, physiologist and psychologist, systematist and ecologist, each views the players from a different angle ; each has a special light to shed upon them; each may help to banish the dark shadows and dusty corners into which we cannot yet see clearly. I hope that with the present volume I have been successful in sending a ray or two across this old stage of nature-a stage crossed by so fine and varied a cast that those privileged for a few fleeting years to see the show may well look on in wonder, and in gratitude.
HUGH B. COTT
UNIVERSITY MUSEUM OF ZOOLOGY,
PART I: CONCEALMENT
THE METHODS BY WHICH CONCEALMENT IS
ATTAINED IN NATURE
GENERAL COLOUR RESEMBLANCE PAGE I GENERAL RESEMBLANCE IN DIFFERENT ENVIRONMENTS 5
11 CONVERGENCE IN COLOUR RESEMBLANCE: COMMON CRYPTIC COLORATION 6 III ADAPTIVE RADIATION IN COLOUR RESEMBLANCE I
IV COLOUR RESEMBLANCE IN DIFFERENT LOCALITIES: LOCAL RACES 13
V COLOUR RESEMBLANCE IN DIFFERENT SEASONS: SEASONAL DIMORPHISM 17 VI COLOUR RESEMBLANCE DUE TO CHOICE OF APPROPRIATE BACKGROUNDS 18
VARIABLE COLOUR RESEMBLANCE
I CHANGES OF COLOUR CORRELATED WITH THE LIFE HISTORY 20
II CHANGES OF COLOUR CORRELATED WITH THE SEASONS Z2
III SLOW ADJUSTABLE COLOUR RESEMBLANCE: MORPHOLOGICAL COLOUR
IV RAPID ADJUSTABLE COLOUR RESEMBLANCE: PHYSIOLOGICAL COLOUR
3. OBLITERATIVE SHADING
I THE PRINCIPLE OF CONCEALMENT BY COUNTERSHADING 35
II COUNTERSHADING PRODUCED BY BLENDED PATTERNS 38
III THE FUNCTION OF OBLITERATIVE SHADING IN ANIMALS 40
IV THE RELATION BETWEEN COUNTERSHADING AND THE CONDITIONS OF LIFE 4z
4. DISRUPTIVE COLORATION
I THE FUNCTION OF DISRUPTIVE COLORATION 48
11 THE COLOURS OF DISRUPTIVE PATTERNS: DIFFERENTIAL BLENDING 49
III THE TONES OF DISRUPTIVE PATTERNS: MAXIMUM DISRUPTIVE CONTRAST 51 IV THE RELATION BETWEEN ADJACENT ELEMENTS IN THE PATTERN 55
V CONSTRUCTIVE SHADING AND PICTORIAL RELIEF 62
xviii AI)APTIVE COLORATION IN ANIMALS
~.COINCIDENT DISRUPTIVE COLORATION PAGE
I CONCEALMENT OF THE APPENDAGES 68
11 CONCEALMENT OF THE EYE 82
III THE BEARING OF COINCIDENT DISRUPTIVE COLORATION ON THE THEORY OF CONCEALING COLORATION 91
IV CONCEALMENT OF THE CONTOUR 93
V BACKGROUND PICTURING: THE RELATION BETWEEN PATTERN, ATTITUDE, AND ENVIRONMENT 98
6. CONCEALMENT OF THE SHADOW
I SHADOW CAMOUFLAGE IN ANIMALS OF COMPRESSED FORM 104
11 SHADOW CAMOUFLAGE IN ANIMALS OF DEPRESSED FORM io6
III IMITATION SHADOWS SUGGESTED BY DISRUPTIVE PATTERNS 112
THE FUNCTION OF CONCEALING COLORATION IN NA TURE
7. CONCEALMENT IN DEFENCE, MAINLY AS ILLUSTRATED BY BIRDS
I CONCEALMENT CONSIDERED IN REFERENCE TO OTHER MEANS OF PROTECTION 117
II CONCEALMENT IN RELATION TO NOCTURNAL HfABITS 119
III CONCEALMENT IN RELATION TO NESTING HABITS 122
1V THE GENERAL RELATION BETWEEN CRYPTIC APPEARANCE AND CRYPTIC BEHAVIOUR 131
8. CONCEALMENT IN OFFENCE I SURPRISE AS A FACTOR IN THE ATTACK 140
II THE REDUCTION OF VISIBLE MOVEMENT 141
III ADAPTIVE SILENCE 144
IV MASKING THE SCENT: THE APPROACH UP-WIND 146
9. OBJECTIONS AND EVIDENCE BEARING ON THE
THEORY OF CONCEALING COLORATION
I THAT CRYPTIC RESEMBLANCES ARE INCIDENTAL EFFECTS RATHER THAN
ADAPTIVE MODIFICATIONS 147
II THAT CRYPTIC COLORATION IS THE RESULT OF PHYSICAL OR CHEMICAL
III THAT ANIMALS WHICH LACK CONCEALING COLORATION APPEAR TO FARE AS WELL IN NATURE AS THOSE WHICH POSSESS IT 153
IV THAT ANIMALS DO NOT RESTRICT THEMSELVES TO THE BACKGROUNDS WHICH THEY RESEMBLE 155
V THAT SUPPOSED CRYPTIC RESEMBLANCES ARE DEVELOPED BEYOND THE POINT OF USEFULNESS 156
VI THAT BIRDS AND OTHER KEEN-SIGHTED ENEMIES ARE NOT DECEIVED EVEN BY THE MOST PERFECT CRYPTIC RESEMBLANCES 159
VII THAT CONCEALMENT DEPENDS UPON STILLNESS RATHER THAN UPON
VIII THAT CONCEALING COLORATION CANNOT BE ADAPTIVE, SINCE MANY
ANIMALS LACK COLOUR VISION 163
IX THAT CRYPTIC COLORATION CANNOT BE EFFECTIVE AGAINST ANIMALS WHICH HUNT BY SCENT, OR AGAINST NOCTURNAL PREDATORS i66
X THAT PROTECTEDI ANIMALS ARE SUBJECT TO ATTACK BY PREDATORY ENEMIES 167
XI THAT THE THEORIES OF ADAPTIVE COLORATION ARE BASED UPON ANTHROPOMORPHIC CONCEPTIONS 171
io. THE EFFECTIVENESS OF CONCEALING COLORATION
I EVIDENCE FROM EXPERIMENTS WITH ANIMALS IN CAPTIVITY 174
II EVIDENCE FROM EXPERIMENTS WITH WILD PREDATORS 178
III DIRECT EVIDENCE FROM OBSERVATIONS ON THE BEHAVIOUR OF PREDATORS IN THE FIELD 185
PART II: ADVERTISEMENT
THE METHODS BY WHICH CONSPICUOUSNESS IS ATTAINED IN NATURE
i. THE APPEARANCE AND BEHAVIOUR OF APOSEMATIC ANIMALS
I THE RELATION BETWEEN VISUAL ADVERTISEMENT AND VISUAL PERCEPTION 191
II THE RELATION BETWEEN THE APPEARANCE OF ADVERTISEMENTS AND THEIR USES 192
III THE COLORATION OF APOSEMATIC ANIMALS 193
IV APOSEMATIC COLOURS IN RELATION TO ENVIRONMENT AND HABITS 196
V FREE EXPOSURE OF APOSEMATIC ANIMALS 198
VI SLUGGISH BEHAVIOUR OF APOSEMATIC ANIMALS 199
VII GREGARIOUS HABITS OF APOSEMATIC ANIMALS 200
VIII. DIURNAL AND SEASONAL ACTIVITY OF APOSEMATIC ANIMALS 203
II III IV
ADAPTIVE COLORATION IN ANIMALS
.2. WARNING DISPLAYS DISPLAYS DEPENDING UPON AN INCREASE IN SIZE APPARENT INCREASE IN BULK BY ADAPTIVE ORIENTATION SUDDEN EXHIBITIONS OF CONSPICUOUS COLOUR ADVERTISEMENT BY MOVEMENT WARNING SOUNDS
3. ADVENTITIOUS WARNING COLORATION
INTIMATE PARTNERSHIPS BETWEEN CRUSTACEA AND APOSEMATIC ANIMALS PERMANENT ASSOCIATIONS BETWEEN SPIDER CRABS AND SEA ANEMONES PERMANENT ASSOCIATIONS BETWEEN DAMSEL-FISHES AND SEA ANEMONES NESTING ASSOCIATIONS BETWEEN BIRDS AND ACULEATE HYMENOPTERA
234 236 237 238
WARNING COLORATION IN REFERENCE TO PREY
4. THE NATURE AND FUNCTION OF WARNING COLORATION,
AS ILLUSTRATED BY THE MAMMALIA
PROTECTIVE ADAPTATIONS IN PORCUPINES PROTECTIVE ADAPTATIONS IN MUSTELINE CARNIVORA THE HABITS AND ATTRIBUTES OF APOSEMATIC MAMMALS PROTECTIVE ADAPTATIONS IN HEDGEHOGS AND SHREWS
5. THE PROTECTIVE ATTRIBUTES OF APOSEMATIC
ANIMALS- IN GENERAL
POISON IN DEFENCE DEFENSIVE SECRETIONS NAUSEOUS TASTE PROTECTIVE INTEGUMENT TENACITY OF LIFE
6. THE RELATION BETWEEN WARNING COLOURS AND DISTASTEFUL ATTRIBUTES METHODS OF INVESTIGATION WARNING COLORATION IN THE AMPHIBIA WARNING COLORATION IN OTHER GROUPS OF ANIMALS
7. THE EFFECTIVENESS OF PROTECTIVE ATTRIBUTES ASSOCIATED WITH WARNING COLOURS
241 242 244 246
253 255 256 259 259
261 264 269
FROM EXPERIMENTS WITH CAPTIVE ANIMALS FROM EXPERIMENTS WITH WILD ANIMALS FROM THE EXAMINATION OF STOMACH CONTENTS
224 225 228
II III IV
II III IV
II III IV
271 272 273
WARNING COLORATION IN REFERENCE TO PREDA TOR Y ENEMIES
8. EXPERIMENTAL EVIDENCE THAT VERTEBRATE ENEMIES LEARN BY EXPERIENCE
I AVOIDANCE OF APOSEMATIC PREY IS NOT INSTINCTIVE, BUT ACQUIRED 275 II EVIDENCE OF LEARNING BY EXPERIENCE IN MAMMALIA 276
III EXPERIMENTAL TASTING AND HABIT FORMATION IN BIRDS 277
IV FORMATION OF THE AVOIDING HABIT B Y LIZARDS 278
V INTELLIGENCE AND DISCRIMINATION IN THE ANURA 279
VI FEEDING REACTIONS, HABIT FORMATION, AND MEMORY IN THE TOAD 280
9. EVIDENCE OF SELECTIVE FEEDING BY VERTEBRATE
ENEMIES IN A STATE OF NATURE
I FOOD PREFERENCES EXHIBITED BY MAMMALS 290
II DISCRIMINATION AND SELECTIVE FEEDING BY BIRDS 292
III DISCRIMINATION BY LIZARDS 294
IV DISCRIMINATION BY SNAKES 295
V FOOD PREFERENCES AND DISCRIMINATION IN ANURA 297
VI FOOD PREFERENCES AND DISCRIMINATION IN FISHES 303
VII THE EFFECTIVENESS OF DISPLAYS IN PREVENTING ATTACK 304
PART III: DISGUISE
SPECIAL PROTECTIVE AND AGGRESSIVE RESEMBLANCE
SPECIAL RESEMBLANCE TO PARTICULAR OBJECTS
I LEAF-RESEMBLANCE IN VERTEBRATES 311
11 DIFFERENT METHODS BY WHICH THE APPEARANCE OF THINNESS IS PRODUCED *317 III SPECIAL RESEMBLANCE TO BARK 322
IV SPECIAL RESEMBLANCE TO LICHEN 324
V SPECIAL RESEMBLANCE TO LIANAS 328
VI SPECIAL RESEMBLANCE TO EXCREMENT 330
VII SPECIAL CRYPTIC APPEARANCES EFFECTED BY WIDELY DISSIMILAR MEANS 333 VIII DISSIMILAR CRYPTIC APPEARANCES FOUND WITHIN PARTICULAR GROUPS
OF ANIMALS 336
IX SPECIAL RESEMBLANCES IN THE SEA 337
xxii ADAPTIVE COLORATION IN ANIMALS
2. ADAPTIVE BEHAVIOUR IN RELATION TO SPECIAL
CRYPTIC RESEMBLANCE PAGE
I SPECIAL PROTECTIVE RESEMBLANCE EFFECTED BY GREGARIOUS HABITS 343 II SPECIAL RESEMBLANCES ENHANCED BY ADAPTIVE MOVEMENTS 345
III SPECIAL PROTECTIVE RESEMBLANCES IN RELATION TO THE TIME OF REST 347 IV SPECIAL RESEMBLANCES IN RELATION TO THE PLACE OF REST 348
V SPECIAL RESEMBLANCES IN RELATION TO THE ATTITUDE OF REST 350
VI SPECIAL PROTECTIVE RESEMBLANCE EFFECTED BY SPECIALLY PREPARED SURROUNDINGS 355
3. ADVENTITIOUS CONCEALING COLORATION
I CONCEALMENT AFFORDED BY MASKS OF ADVENTITIOUS MATERIAL 359
11 THE TRANSITION FROM ADVENTITIOUS CONCEALMENT TO ADVENTITIOUS ADVERTISEMENT 361
III THE TRANSITION FROM ADVENTITIOUS CONCEALMENT TO ADVENTITIOUS
IV THE TRANSITION FROM ADVENTITIOUS CONCEALMENT TO BURROWING
CONSPICUOUS LOCALIZED CHARACTERS
4. DEFLECTIVE MARKS
I CHARACTERS WHICH DEFLECT THE ATTACK OF ENEMIES FROM THE MORE TO THE LESS VITAL PARTS OF THE BODY 367
11 CHARACTERS WHICH DEFLECT THE ATTACK OF ENEMIES FROM THE MORE TO THE LESS VULNERABLE MEMBERS OF THE SPECIES 370
III CHARACTERS WHICH MISDIRECT THE ATTACK OF ENEMIES BY MISREPRESENTING THE POSTURE OF THEIR PREY 372
IV CHARACTERS WHICH MISDIRECT THE ATTACK OF ENEMIES BY MISREPRESENTING THE WHEREABOUTS OF THEIR PREY 374
5. DIRECTIVE MARKS
I CHARACTERS WHICH DIVERT THE ATTENTION OF PREY FROM THE MOST
DANGEROUS PART OF THEIR ENEMY 382
Il CHARACTERS WHICH ALLURE PREY TO THE MOST DANGEROUS PART OF THEIR ENEMY 383
III CHARACTERS WHICH ATTRACT THE ATTENTION OF ENEMIES TO THE MOST
I DANGEROUS ATTRIBUTE OF THEIR PREY 386
IV CHARACTERS WHICH ATTRACT THE ATTENTION OF ENEMIES TO AN APPARENTLY DANGEROUS ATTRIBUTE OF THEIR PREY t 387
ALLURING AND MIMETIC RESEMBLANCES
6. ALLURING COLORATION
I ADVENTITIOUS ALLUREMENT 391
II SPECIAL ALLURING COLORATION 392
7. MIMICRY: THE ATTRIBUTES OF MIMICS
I THE RELATION BETWEEN CRYPTIC AND MIMETIC RESEMBLANCE 396
II THE RELATION BETWEEN BATESIAN AND MULLERIAN MIMICRY 398
III THE GEOGRAPHICAL RELATIONS BETWEEN MODEL AND MIMIC 399
IV THE TOPOGRAPHICAL RELATIONS BETWEEN MODEL AND MIMIC 399
V MIMICS DEPART WIDELY FROM THEIR CONGENERS IN APPEARANCE 400
VI MIMICS DEPART WIDELY FROM THEIR CONGENERS IN BEHAVIOUR 400
VII THE RESEMBLANCES BETWEEN MODEL AND MIMIC ARE NOT DUE TO
SIMILARITIES OF LIFE-HISTORY- 402
VIII MIMETIC RESEMBLANCES ARE INDEPENDENT OF AFFINITY 403
IX MIMETIC RESEMBLANCES ARE INDEPENDENT OF ANATOMY 404
X SIMILAR APPEARANCES MAY BE PRODUCED BY THE MOST WIDELY
DIFFERENT METHODS 405
XI MIMETIC RESEMBLANCES ONLY AFFECT VISIBLE CHARACTERS 407
XII MIMETIC RESEMBLANCES INVOLVE MANY INDEPENDENT MODIFICATIONS 407 XIII MODIFICATION OF CONTOUR IN MIMICS OF HYMENOPTERA 408
XIV MIMETIC MODIFICATIONS OF THE ANTENNAE 411
XV THE EFFECTIVENESS OF MIMICRY 413
8. BREEDING PARASITISM AND MIMICRY IN CUCKOOS
I CUCULUS CANORUS" THE RELATIONSHIP BETWEEN PARASITE AND
FOSTERER a 417
II THE FACTORS UPON WHICH EGG-MIMICRY DEPENDS 418
III THE DEGREE OF MIMETIC RESEMBLANCE ACHIEVED 420
IV THE RELATIONS BETWEEN CUCKOO-MIMICRY AND INSECT-MIMICRY 422
ADAPTIVE COLORATION IN ANIMALS
I ADAPTIVE COLORATION AND OPTICAL PRINCIPLES 427
II ADAPTIVE COLORATION AND VISUAL PERCEPTION 428
III PATTERN AND ANATOMY 430
IV PATTERN AND AFFINITY 431
V COLORATION AND ADAPTATION 431
VI ADAPTIVE COLORATION AND MODE OF LIFE 433
VII ADAPTIVE COLORATION AND SPECIAL BEHAVIOUR 433
VIII ADAPTIVE COLORATION AND PROTECTIVE ATTRIBUTES 434
IX ADAPTIVE COLORATION AND SURVIVAL VALUE 435
X ADAPTIVE COLORATION AND APPLIED COLORATION 436
INDEX OF SCIENTIFIC NAMES 467
INDEX OF SUBJECTS AND AUTHORS' NAMES
WARNING COLORATION IN AMPHIBIA, in colour Frontispiece
At end of Book
I POLYCHRUS MARMOR4TUS IN NATURAL SURROUNDINGS ABBESS LIZARD (CORYTHOPHANES CRISTATUS)
2 HORNED VIPER (CERASTES CERASTES)
POGGE (AGONUS CATAPHRACTUS)
3 LEPTOPHYES PUNCTATISSIMA: PANCHROMATIC PHOTOGRAPH
THE SAME: INFRA-RED PHOTOGRAPH
4 EIDER DUCK: PANCHROMATIC PHOTOGRAPH
THE SAME: INFRA-RED PHOTOGRAPH
5 LARVA OF EYED HAWK-MOTH: PANCHROMATIC PHOTOGRAPH
THE SAME: INFRA-RED PHOTOGRAPH
6 HYLA C(ERULEA: PANCHROMATIC PHOTOGRAPH
THE SAME: INFRA-RED PHOTOGRAPH
7 WHITE COCK AGAINST WHITE SHEET, SHOWING RELIEF
BUSH BUCK: AN EXAMPLE OF OBLITERATIVE SHADING
8 LARVA OF EYED HAWK-MOTH: SHOWING EFFECT OF COUNTERSHADING
THE SAME: INVERTED
9 RANA ADSPERSA
IO GARDEN CARPET MOTH (XANTHORHOE FLUCTUATA) ON WOOD I I OAK BEAUTY MOTH (PACHYS STRATARIA) ON BARK OF OAK 12 GARDEN CARPET MOTH (XANTHORHOE FLUCTUATA) ON ELM BARK 13 YOUNG RINGED PLOVER ON SHINGLE 14 SCOPS OWL
15 NIGHTJAR BROODING OVER EGGS 16 SNIPE AT NEST
WOODCOCK AT NEST 17 EIDER DUCK AT NEST
GOLDEN PLOVER AT NEST
18 NEST AND EGGS OF RINGED PLOVER
NEST AND EGGS OF LAPWING
ADAPTIVE COLORATION IN ANIMALS
19 YOUNG WOODCOCK IN NATURAL SURROUNDINGS 20 YOUNG ARCTIC TERN IN NATURAL SURROUNDINGS 21 KASSINA SENEGALENSIS
22 SHINISAURUS CROCODILURUS
VIPERA SUPERCILIARIS 23 EDALORHINA BUCKLEYI
BUFO VALLICEPS BUFO TYPHONIUS
24 COMMON FROG (RANA TEMPORARIA)
SOUTH AMERICAN BOA (CONSTRICTOR CONSTRICTOR) 25 GABOON VIPER (BITIS GABONICA)
GEMSBOK (ORYX GAZELLA)
z6 LARVA OF PALE TUSSOCK MOTH (DASYCHIRA PUDIBUNDA)
BLOOD-VEIN MOTH (TIMANDRA AMATA)
27 MARBLED TREE-FROG (HYPEROLIUS MARMORATUS) IN NATURAL SURROUNDINGS 28 GREY TREE-FROG (CHIROMANTIS XERAMPELINA) ON BARK
COMMON FROG (RANA TEMPORARIA) IN GRASS 29 GECKO (TARENTOLA DELALANDII) ON ROCK
MARBLED BEAUTY MOTH (BRYOPHILA PERLA) ON WALL
30 HAWK-MOTH (XANTHOPAN M. MORGANI) ON CASUARINA TREE 31 PAINTED LADY (PYRAMEIS CARDUI)
GRAYLING (SATYRUS SEMELE) ON GROUND
32 SCARCE TISSUE MOTH (EUCOSMIA CERTATA) IN NATURAL SURROUNDINGS 33 WILLOW BEAUTY MOTH (BOARMIA GEMMARIA) IN NATURAL SURROUNDINGS 34 TROPICAL RAIN FOREST. PARA 35 BUFO TYPHONIUS ON FOREST FLOOR 36 LEAF-INSECT (PHYLLIUM CRURIFOLIUM)
MANTIS (SPHODROMANTIS VIRIDIS)
37 BUFF-TIP MOTH (PHALERA BUCEPHALA)
BUFO SUPERCILIARIS 38 CYCLOPTERA SP.
C YCLOPTERA EXCELLENS
CHITONISCUS FEEDJEANUS 39 DRACONIA RUSINA
40 STAGMATOPTERA SP.
CH(ERADODIS RHOMBOIDEA 41 HYLA LANGSDORFII
42 BARK GECKO (UROPLATES FIMBRIATUS) 43 PHALANGIUM OPILIO ON BARK OF SCOTS PINE
LITHINUS NIGROCRISTATUS 44 ACRIDA TURRITA ON GRASS
EREMOCHARIS INSIGNIS AMONG STONES 45 CILIX GLAUCATA
NEMEOBIUS LUCINA EPINEPHELE IANIRA HAETERA DIAPHANA 46 EDALORHINA PEREZI
47 HAETERA DIAPHANA : EXTREMITY OF HIND-WING
48 OCELLUS OF CALIGO EURYLACHUS
ILLUSTRATIONS IN THE TEXT
I DIAGRAMS ILLUSTRATING THAYER'S PRINCIPLE OF OBLITERATIVE SHADING 37 2 GRADED TONES PRODUCED BY PATTERNS 39
3 LARVA OF EYED HAWK-MOTH (SMERINTHUS OCELLATUS) SHOWING OBLITERATIVE FUNCTION OF COUNTERSHADING 44
4 THE PRINCIPLE OF OBLITERATIVE SHADING AS APPLIED TO GUNS 45
5 YOUNG WOODCOCK 47
6 DIAGRAMS ILLUSTRATING THE PRINCIPLE OF DIFFERENTIAL BLENDING 50
7 DIAGRAMS ILLUSTRATING THE PRINCIPLE OF MAXIMUM DISRUPTIVE
8 NASSAU GROUPER (EPINEPHELUS STRIATUS) 54
9 STAGES IN THE CONSTRUCTION OF A DISRUPTIVE PATTERN 56
10 DIAGRAMS SHOWING THE EFFECT OF ADJACENT CONTRASTED TONES 56
II DIAGRAMS OF THE DISRUPTIVE PATTERNS OF VARIOUS SNAKES 58
12 CARDIOGLOSSA GRACILIS 59
13 EQUES LANCEOLATUS 59
14 NEST OF RINGED PLOVER, WITH EGGS AND NEWLY-HATCHED YOUNG 60
15 LARVA OF PUSS MOTH (CERURA VINULA) 61
16 DIAGRAMS ILLUSTRATING THE CORRESPONDENCE BETWEEN PICTORIAL RELIEF
ON A FLAT SURFACE, AND LIGHT AND SHADE ON A MODELLED SURFACE 63 17 DIAGRAM SHOWING DISRUPTIVE COLORATION IN TERMS OF PICTORIAL RELIEF,
BASED UPON THE UNDERSIDE PATTERN OF THE MEADOW BROWN BUTTERFLY (EPINEPHELE IANIRA) 65
18 COMPARATIVE DIAGRAMS SHOWING THE EFFECTIVENESS OF DISRUPTIVE CONTRAST AND PICTORIAL RELIEF, BASED UPON THE PATTERN OF THE
COPPERHEAD SNAKE (AGKISTRODON MOKASEN) 66-67
19 MEGALIXALUS FORNASINII, ILLUSTRATING THE PRINCIPLE OF COINCIDENT
DISRUPTIVE COLORATION 69
20 HYLA LEUCOPHYLLATA 70
21 HIND-LIMBS OF THE COMMON FROG (RANA TEMPORARIA) 71
22 DASCYLLUS ARUANUS 73
23 HENIOCHUS MACROLEPIDOTUS 74
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ILLUSTRATIONS IN THE TEXT xxxi
46 DIAGRAMS ILLUSTRATING THE USE OF THE TAIL-FLANGE FOR SHADOW
47 DIAGRAMS ILLUSTRATING THE USE OF AN INTERRUPTED FRINGE FOR SHADOW
ELIMINATION I IO
48 SHADE- AND SHADOW-PATTERNS IN SOUTH AMERICAN SPHINGID LARVAE :
I ENYO J. JAPIX, 5TH INSTAR; 2 EPISTOR GORGON, FINAL INSTAR;
3 EPISTOR CAVIFER 1 12
49 GRAPH SHOWING SELECTIVE ELIMINATION OF MANTIS RELIGIOSA BY PREDATORS 18o
50 THE SPHINGID LARVA PSEUDOSPHINX TETRIO, AS AN EXAMPLE OF WARNING
51 ANTHIA SEXGUTTATA 205
52 TETRODON FLUVIATILIS, SHOWING THE FISH INFLATED AND DEFLATED 209 53 WARNING DISPLAY BROUGHT ABOUT BY INFLATION COMBINED WITH ADAPTIVE
ORIENTATION IN THE GIANT TOAD (BUFO MARINUS) 212
54 LARVA OF LEUCORHAMPHA ORNATUS: I CRYPTIC RESTING ATTITUDE;
2 TERRIFYING ATTITUDE 215
55 WARNING DISPLAY OF THE FRILLED LIZARD (CHLAMYDOSAURUS KINGII) 218 56 WARNING DISPLAY OF THE CRESTED RAT (LOPHIOMYS IBEANUS) 219
57 DIAGRAM SHOWING RATE OF LEARNING IN THE COMMON TOAD 284
58 GRAPH SHOWING RATE OF LEARNING IN THE COMMON TOAD 284
59 DIAGRAM SHOWING RESULT OF MEMORY EXPERIMENTS WITH THE COMMON
60 GRAPH SHOWING RESULT OF MEMORY EXPERIMENTS WITH THE COMMON
6I GRAPHS SHOWING FOOD-PREFERENCES OF RANA TEMPORARIA AND BUFO B.
62 GRAPHS SHOWING FOOD-PREFERENCES OF HYPEROLIUS BA YONI AND MEGALIXALUS FORNASINII 300
63 MONOCIRRHUS POLYACANTHUS 32
64 PLATAX VESPERTILIO 314
65 RHAMPHOLEON BOULENGERI 3 17
66 SPECIAL RESEMBLANCE TO LEAVES IN A VARIETY OF ANIMALS: I PLATAX
VESPERTILIO; 2 TIMANDRA AMATA; 3 KALLIMA PARALEKTA ; 4 SYSTELLA RAFFLESII; 5 CYCLOPTERA SP.; 6 MONOCIRRHUS POLYACANTHUS; 7 RHAMPHOLEON BOULENGERI; 8 SMERINTHUS OCELLATUS, LARVA; 9 POLYCHRUS MARMORATUS ; 10 BUFO TYPHONIUS; I I MINIODES ORNATA; I 2 PHYLLIUM CRURIFOLIUM;
13 CHCERADODIS RHOMBOIDEA 320
67 HEMIDACTYLUS RICHARDSONI 325
xxxii ADAPTIVE COLORATION IN ANIMALS
68 SPECIAL RESEMBLANCE TO BROKEN TWIGS: I DUOMITUS LEUCONOTUS;
PHALERA BUCEPHALA 334
69 SPECIAL RESEMBLANCE TO GROWING TWIGS, SHOWING CRYPTIC RESTING
ATTITUDE: I, OF THE GRASSHOPPER ZABROCHILUS AUSTRALIS; AND
2, OF THE STICK-INSECT PARASOSIBIA PARVA 335
70 ANTENNARIUS MARMORATUS 340
7l PTEROPHRYNE TUMIDA 340
72 SEA DRAGON (PHYLLOPTERYX EQUES) 341
73 SLENDER TRIGGER-FISH (MONACANTHUS SCRIPTUS), IN CRYPTIC ATTITUDE
AMONG EEL-GRASS (THALASSIA) 351
74 POOR-ME-ONE (NYCTIBIUS GRISEUS) ON NEST, SHOWING SPECIAL PROTECTIVE
RESEMBLANCE AND CRYPTIC ATTITUDE 353
75 I COLOBOPSIS PARADOXUS VAR. JANITOR, FRONT AND LATERAL VIEWS OF
THE JANITOR'S HEAD; 2 ENTRANCE TO NEST OF C. CULMICOLA IN
BAMBOO, WHEN OPEN, AND WHEN CLOSED BY THE HEAD OF A JANITOR 364 76 CHA~TODON CAPISTRATUS 373
77 ANTENNARIUS NOTOPHTHALMUS 373
78 POMACANTHUS IMPERATOR 374
79 EXAMPLES OF FLASH COLORATION IN A VARIETY OF ANIMALS: I HEMISCIERA
(HOMOPTERA); 2 CATOCALA (LEPIDOPTERA); 3 ORNITHACRIS (ORTHOPTERA) ; 4 LACCOTREPHES (HETEROPTERA); 5 FULGORA
(HOMOPTERA); 6 PHYLLOMEDUSA (ANURA); 7 DRACO (LACERTILIA) 375 80 FLASH COLOURS AND WARNING COLOURS IN STICK-INSECTS: I ARUANOIDEA
GRUBANERI ; 2 PODACANTHUS TYPHON; 3 TROPIDODERUS RHOMBUS; 4 GRAEFFA COCCOPHAGA; 5 CNIPSUS RHACHIS 377
81 WARNING DISPLAY OF PSEUDOCREOBOTRA WAHLBERGI 388
82 ORNITHOSCATOIDES DECIPIENS: SPIDER AND WEB RESEMBLING A BIRD'S
83 MIMICRY OF ANTS AND WASPS BY STOUT-BODIED INSECTS, ACHIEVED BY
CAMOUFLAGE OF THE WAIST-LINE: I NABIS LATIVENTRIS; 2 OBEREA 410
BREVICOLLIS; 3 AND 4 MYRMECOPHANA FALLAX
84 TROPICAL RAIN FOREST. PARA. 437
A' D. Al R 7 I
It is the glory of God to conceal a thing : but the honour of kings is to
search out a matter.
PROVERBS XXV, 2
SHORTLY before sunset on a clear evening in summer, when foliage is fresh and full, the leaves of an oak are seen from certain directions to be aflame with orange light: the familiar tree becomes transmuted into a veritable burning bush-though only the elect have eyes to see it so. This reality may readily be proved by holding a flame-coloured nasturtium petal between the tree and the eye and comparing the tints thus brought together. But most of us make a mental reservation, based upon long years of experience, and we reject the evidence of the senses. Leaves, we know, are green in summer, not golden; and unless we are specially observant or specially trained, we refuse to accept the facts of colour as they are-we refuse to see that the distant firs are pale grey, or that the snow on the hills is blue where it lies in shade and warmly tinted where bathed in sunlight, that indeed it may be almost any tint except the white which we think it is. Experience has taken much of the colour and variety out of life, and we go on our way almost unconscious of the real appearance of the visual signs we have learned from earliest childhood to construe.
So accustomed are we to reject what the eye sees in nature, so dull and dead have we become as a result of visual experience, that to appreciate the wonder and wealth of colour around us we must be shown our surroundings in some novel or unusual manner-in a picture, for instance, or as they appear when we stand on our heads, or when seen inverted in the focusing screen of a camera. Indeed, so largely does experience enter into and modify our perception of objects, that many people are quite unable to accept what the eye gives them, but only what they have learned to expect it is giving them : they see only what they know. They have lost that power which artists by patient striving have recovered, and which Ruskin calls the innocence of the eye '. These remarks are true not merely of colour phenomena. All the sensations which enable us to appreciate depth and distance, modelling and texture of the objects around us come to have their special significance only as a result of individual experience. Thus we interpret the effects of light and shade in terms of sculpturing and relief ; we project perspective into three-dimensional space we translate separate
patches of colour into objects with separate surfaces.
Now what I wish to emphasize here is that these processes in the psycho2
logical plane cause us to overlook the fact that in the physical plane all optical effects whatsoever are fundamentally due to differences of colour and brightness, and of light and shade. They are solely due, in other words, to radiations differing in frequency and intensity. All the varied signs which enable us to perceive objects are capable of analysis into these simple terms. Once this important matter is grasped, we are in a position to appreciate not only the visual phenomena upon which recognition depends, but to understand the optical methods by which recognition can be delayed or altogether prevented. For these must depend ultimately upon modifications of the visual signs which reach the observer's eye.
To put the matter in another way: vision rests upon a threefold basis lying in the realms of psychology, physiology and physics. There are the physical facts which happen in the world outside the eye ; the physiological reactions which occur within it ; and the psychological processes which take place behind it. Now the visual effects making for concealment, or camouflage, while of course producing their results through the operation of the eye and the attributes of the mind, are fundamentally those of the physical properties of light-they are nothing more than effects of colour variously shaded.
Before considering the methods by which different animals are rendered so wonderfully inconspicuous and difficult to detect in the field, it will therefore be helpful first to enquire what are the optical principles upon which recognition by sight depends. How is it that we are able to isolate and recognize any single object from the surroundings which form its immediate background ? What do the various visual clues amount to-clues which the eye receives from the outer world and which we have learned to interpret and project in the language of solidity and space ?
All solid objects which we see in the world about us are presented to the eye merely as patches of colour occupying a particular area in the visual field. These patches may differ widely from one another in various respects-notably in their saturation or dilution, in their darkness or paleness, in their size and texture and form. Actually, we see nothing but flat stains of colour-variously shaded and variously shaped.
When we recognize anything by sight-a leaf, for instance-the means by which the eye is enabled to distinguish it are fourfold, and it is essential for the proper understanding of our subject that these should be clearly appreciated. Firstly, the object appears in the field of vision as a continuous area of colour, differing more or less markedly in hue and purity and depth from its immediate surroundings, against which it is therefore seen to stand out in contrast. The leaf may appear, for instance, as a dark green shape against the blue sky, or as a brown patch lying on the lawn.
Secondly, owing to the effect of unequal illumination on an uneven surface, the leaf is not seen simply as a wash of flat colour-even when in actual fact it may be uniform in colour. For it is thrown into relief by the effect of light and shade, which- enables the eye to detect surface curvature, model-
ling, texture and such details or irregularities as the midrib, lateral veins, and so on.
Thirdly, although natural objects are not, or are but rarely, bounded by lines-in the way that an outline drawing is-nevertheless the surface of every visible body is framed by a contour or outline which divides the area where it is from the area where it is not in the visual field. And this contour frequently has a characteristic or familiar shape, enabling the form of a familiar object to be recognized : it may tell us, for instance, that the leaf we are looking at is oak or elm, or holly, or some other kind.
Fourthly, under certain conditions of illumination a shadow will be thrown by the object upon its background. Shadows are caused when opaque bodies intercept light upon some surface other than their own, and are not to be confused with shade, which is found on parts of the body that are turned away from the light. By framing the outline of the object, as well as by virtue of their own shape and conspicuousness, shadows tend to facilitate recognition.
The point which cannot be too strongly emphasized here is that visible form can only be distinguished when it is exhibited by differences of colour or tone, or of light and shade.: with the reduction of such differences an animal or any other object becomes more and more difficult to recognize.: in their absence it becomes unrecognizable.
It follows from these theoretical considerations, that four fundamental steps towards effective camouflage must lie in the direction (1) of colour resemblance-i.e. the agreement in colour between an- object and the background against which it is seen ; (2) of obliterative shading-i.e. counter lightening and darkening which abolishes the appearance of roundness or relief due to light and shade ; (3) of disruptive coloration-i.e. a superimposed pattern of contrasted colours and tones serving to blur the outline and to break up the real surface form, which is replaced by an apparent but unreal configuration ; and (4.) by shadow elimination-i.e. the effacement of cast shadows by modifications of form or adaptive orientation.
Now it is a very remarkable fact, and one of much significance, that these theoretical principles of colour resemblance, obliterative shading, disruptive coloration and shadow elimination, together with various additional devices and instincts, are those actually found to operate in nature, whereby different animals
-fishes and wild-fowl, toads and tree-frogs, dabs and crabs, cats and caterpillars and innumerable others-are rendered so extraordinarily difficult to recognize when seen in their natural surroundings.
THE METHODS BY WHICH CONCEALMENT
IS ATTAINED IN NATURE
x. GENERAL COLOUR RESEMBLANCE
There are also divers other kinds of worms, which for colour and shape alter even as the ground out of which they are got.
I. GENERAL RESEMBLANCE IN DIFFERENT ENVIRONMENTS
THE general resemblance borne by various animals to the different surroundings in which they live is a theme more or less familiar to every one. The Ptarmigan nesting among the lichen-covered rocks of the mountain summit; the Golden Plover on the neighbouring moorland; the Woodcock among the bracken and fallen oak leaves; the Ringed Plover on its pebble beach; the Stone-Curlew in its native breckland ; the Bittern standing motionless among the tawny reeds; the Parrakeet screaming from the luxuriant foliage of a mango tree; the Saharan Nightjar crouching invisible in a barren waste; the Frogmouth perched on some rotten tree-stump--each is afforded concealment by the hues and tones demanded of its particular environment.
It is, of course, easy to point to exceptions: but the fact remains that innumerable animals, inhabiting all kinds of surroundings, tend to wear on their bodies a cryptic dress. The traveller who visits one of the arid regions of the earth, such as the Kalahari, Sahara, or the deserts of North-western India or Southern California, will look there in vain for the brilliant greens which beautify many tree-dwellers all the world over-parrots and woodpeckers, tree-frogs and tree-snakes, chameleons and iguanas, grasshoppers and mantids : he will look almost in vain for the blue tints of tunny and mackerel and flying fish which are typical of many inhabitants of the ocean's surface waters: nor will he find there the immaculate white dresses worn by members of the snowland fauna.
Instead he will observe that those creatures hardy enough to eke out an existence in such inhospitable places are, with few exceptions, clad in colours borrowed from the desert itself-ochre, buff, brown and sandy-grey, broken perhaps with patterns of dark-brown, black and white. Moreover, these colours prevail in distantly related groups of animals: they are seen alike in the fur of mammals, such as Jackal and Fennec, Gerbil and Jerboa; they are reproduced in the feathers of birds-the Desert and Crested Larks, the Egyptian Nightjar, Houbara, Cream-coloured Courser, Sand-Grouse and Quail: they are repeated
on the scales of desert lizards-geckos, skinks, monitors, horned toads ', and many others; and of snakes like the Horned Viper: and they occur once more in the chitinous covering of many desert insects.
It is a significant fact, to which I have referred on another page, that the only other major environment where somewhat similar colours predominate is one the very opposite in most respects from the desert, namely, the mud of estuaries and the sandy and gravelly bottom of inshore seas. And here again the fauna, though so entirely different from that we have been considering, wears the same sober hues of brown and orange, ochre and grey, such as are seen in flatfish like the Flounder and Plaice, and in many crustaceans and cephalopods. Here, too, we find that the colours and patterns worn are not indiscriminately brown or buff or yellow, for there is generally a marked colour-harmony between the animal and the particular bottom on which it rests.
Facts such as these, regarding general cryptic resemblance in the major environments of the world, are too well known to require detailed description here. Certain controversial aspects of the subject will be deferred until a later page. The broad facts are clear enough. In every well-illuminated region having a dominant type of colour, the tendency is apparent for a considerable percentage of the fauna to reproduce more or less closely its prevailing colours and tones. For instance, green-which occurs so widely and so vividly in the colour scheme of foliage haunting birds, lizards, snakes, frogs, beetles, bugs, grasshoppers, mantids, moths, cater pillars and other forms from evergreen forests-is found to predominate in other regions only where green is also a constant feature of the environment, namely, on land among grass and ground herbage generally, and in the sea among sea grass and green alg~e. There is no environment in the world where white is predominantly worn by the fauna except in the frozen North, where white is also the dominant colour of the landscape.
Not least remarkable in this connexion is the evolution of transparency in the surface waters of the sea, a feature shared in the larval or adult state by such widely different pelagic organisms as ccdlenterates, Gastropod molluscs, Polychoete worms, crustaceans, tunicates and fishes-members of which in this particular differ completely from their opaque and pigmented nearest relatives, or adult stages, dwelling on the sea bottom and seashore.
II. CONVERGENCE IN COLOUR RESEMBLANCE : COMMON CRYPTIC COLORATION
The phenomena of general cryptic resemblance involve many striking demonstrations of the. principle of convergence-which implies a superficial similarity (in this case of colour) that is independent of affinity but correlated with similar conditions of life. As with all types of adaptive convergence, the superficial resemblance between different animals seen in Common Cryptic Coloration (for which Poulton has introduced the term Syncryptic) is quite incidental, and in this respect utterly different both in function and origin from mimetic resemblances where the superficial similarity between different animals appears to have been developed as an end in itself.
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 7
(i) Green Coloration in Tropical Tree-Snakes-We may illustrate the point by reference to the green coloration of tropical tree-snakes, which has arisen on a number of different occasions, in several remotely related families, and in every tropical region of both Old and New Worlds. The following table indicates the systematic position of various genera containing vividly green arboreal species-of which a single example from each genus is cited. The list could, of course, be considerably extended, and is merely intended to show the widespread occurrence of a colour which every one with experience of tropical nature knows to be highly effective as a cryptic dress among foliage.
Family. Sub-family. Genus. Species. Distribution.
Boidae Boine Boa B. canina South America
Pythonine Chondropython C. viridis Papua
Colubride Colubrine Leimadophis L. viridis South America
Leptophis L. mexicanus Mexico
Chlorophis C. neglectus Tropical Africa
Philothamnus P. nitidus Tropical Africa
Gastropyxis G. smaragdina Africa
Elaphe E. prasina China
Opheodrys O. aestivus North America
Boigine Passerita P. prasina South-East Asia
Oxybelis O. fulgidus Central America
Philodryas P. viridissimus Peru
Elapide Dendraspis D. viridis West Africa
Viperide Viperine Atheris A. squamigera Tropical Africa
Crotalinae Trimeresurus T. albolabris Asia
Bothrops B. bicolor Central America
The above list embodies several striking cases of convergence-of form as well as colour: for example, as between the Bush Vipers (Atheris) of tropical Africa and the Palm Vipers (Bothrops) of tropical America-the former being true vipers (Viperinx) and the latter pit vipers (Crotaline).
Parallel phenomena are of world-wide occurrence among other members of the forest fauna, as seen in the green uniforms evolved independently in numerous genera and families of lizards, frogs, and birds, not to mention Coleoptera, Lepidoptera, Hemiptera and Orthoptera.
(2) Green Coloration due to a Variety of Causes-A further series of facts which lends indirect support to the adaptive interpretation of green colour
concerns the wide variety of methods, both physical and chemical, by which the appearance of green is itself produced in different groups of animals.
Many years ago Poulton proved that the green coloration of various caterpillars was due to a modified- chlorophyll derived from their food, and present in their blood (499). It is remarkable, as he points out, that this colouring matter, unlike any other known solution of chlorophyll, is stable under the prolonged action of light, and that spectroscopically the green blood shows a closer resemblance to the unaltered chlorophyll in the leaf, than the latter does to chlorophyll in alcoholic solution-a fact which seems the more striking when account is taken of chemical processes involved in the passage of the pigment through the wall of the alimentary tract.
It is interesting to note that in cases like Smerinthus ocellatus the green larval pigment passes by way of the pupa into the eggs, and so to the young larvae, which are thereby rendered green on hatching before they have had the opportunity of acquiring fresh chlorophyll from the food plant (525).
Sometimes, as in caterpillars of the Angle Shades Moth (Phlogophora meticulosa), the green coloration is caused by the food in the alimentary tract showing through the transparent body wall (496). In other cases green pigments are synthesized by the insect, being developed independently of the nature of the food even in phytophagous species, and betraying no close relation to chlorophyll. This is shown by their production in stick insects, locusts and many caterpillars when reared upon food containing no pigment (170, 207, 412). And their independent nature has been confirmed spectroscopically. Thus Faure (170) finds that in green individuals of the solitary phase of the African Migratory Locust (Locusta migratoria subsp. migratoroides) the pigment involved differs from chlorophyll in showing a single absorption band with its centre at 6,7oo A, and no trace of red fluorescence. In contrast to what has been said above, we have to note that various leaf-mining dipterous larvae, which have no need of cryptic resemblance, do not assume a green coloration though feeding upon chlorophyllbearing tissue (281).
The independent nature of the green pigment in certain crustaceans has been demonstrated by Gamble (198), who showed that the development of green coloration in specimens of Hippolyte varians, when reared against a background of green weed, was uninfluenced by diet; for prawns fed with colourless substances, with red ovary, and with fine brown weed, became green under these conditions.
Green in mature insects is frequently either structural in nature, or produced by the combined effect of pigment and structure. Thus Onslow (449) has shown that the brilliant green of the butterfly Ornithoptera poseidon is due to the modification of a yellow pigment by a structural blue; and that the green colour of beetles such as Heterorrhina elegans and H. africana is due to a similar cause.
Coloration in the Amphibia depends upon the arrangement of three kinds of pigment cells or chromatophores-namely, melanophores, lipophores and
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 9
guanophores, whose pigments are black or brown, yellow or red, and white respectively. Now it is interesting to find that by means of, and in spite of so limited a palette, many foliage-haunting tree-frogs, such as Hyla arborea and Leptopelisjohnstoni, are able to appear dressed in brightest leaf-green tints. This effect is due partly to the selective absorption of rays of greater wave-length by the deep-seated background of melanophores, and partly to the action of the guanophores, which reflect back the waves of shorter length. This scattered light would appear blue, for the same reason that the sky is blue, were it not for the filtering action of the overlying colour screen formed by the layer of lipophores, which transmit only the green light (445).
In the case of snakes, it appears that nothing has yet been published on the nature of green coloration. However, Mr. H. W. Parker, of the British Museum (Natural History), informs me that green snakes react in alcohol in two different ways. Either they remain green; or else they become blue. In the former case, the alcohol is liable to be discoloured green; in the latter, it is discoloured yellow. He points out that presumably this different behaviour is due to different methods of producing green colour-the first by a green pigment slightly soluble in alcohol; the second by a combination of structural blue which is unaffected by alcohol, and a soluble yellow pigment.
Similar conditions appear to obtain among lizards. Many species belong to the second category, and in certain cases post-mortem colour changes have led to mistaken ideas of the true coloration in life. A striking instance of this came to my notice 'in Brazil. A rare and very handsome lizard of the most brilliant green hue which I took on Maraj6 Island was subsequently identified with a species known to science by the inappropriate name of Urocentron azureum, whose colour had repeatedly been described in systematic literature as un beau bleu '-doubtless owing to its appearance in spirit (07).
With birds, especially among arboreal species, green is a common colour. In the Musophagide, or Plantain-eaters, this is due to a green pigment containing iron. In all other birds, green is a structural colour-being produced by a combination of yellow or grey or brown pigment associated with a special colourless superstructure (439). Its nature is well seen in species like the Bluefronted Amazon Parrot, whose green feathers turn a dull brown when the bird is thoroughly drenched after a bath.
Finally, we may mention here the well-known, not to say highly curious case of the South American Sloths Bradypus tridactylus and Cholcepus didactylus, whose coarse, shaggy hair is given a greenish-grey tint by the presence of a symbiotic alga of the Protococcus group.
(3) Green, Coloration and Infra-red Radiation-A very interesting question bearing upon the nature and quality of light reflected from green animals is opened up by the comparatively new technique of infra-red photography. It is well known that chlorophyll reflects light in the infra-red region of the spectrum, and that as a consequence grass or foliage appear snowy-white in the infra-red photograph. But different green animals, which to the eye appear
similar in tone and colour both to one another and to their surroundings, differ greatly in their absorption of infra-red light. Consequently when twin photographs of these are taken respectively on panchromatic and on infra-red plates, the results in certain cases are most illuminating-those forms which absorb infra-red rays becoming in the latter case strongly differentiated from their environment, and appearing as dark objects standing out conspicuously from their light surroundings.
This is well seen in the photographs of the Tettigoniid grasshopper (Leptophyes punctatissima (Plate 3), which stands out vividly as a dark patch against the light infra-red rendering of the rhubarb leaf on which it is resting. A similar effect is obtained with the Eider Duck (Plate 4), which in the infra-red photograph becomes as conspicuous as it is in normal circumstances inconspicuous.
There is an aspect of this subject of practical importance. During the Great War the development of aerial photography and reconnaissance rendered the concealment of ammunition dumps, battery positions, concentrations of troops, and other objectives a matter of vital importance. To this end various devices were used, such as the erection of overhead cover of a suitable colour and texture, and disposed so as to eliminate shadows, approaches, spoil, blast-marks and so forth. Because such screens are effective against direct observation and ordinary photography, by no means does it follow that they will be hidden in the infra-red photograph. Comparison of aerial photographs taken simultaneously on panchromatic and infra-red plates will reveal much that before the advent of this new technique would have been adequately camouflaged, and a new difficulty has thus been added to the difficult problem of concealment by deception.
Now, when we return to the problem in its bearing upon animals, it is found that this difficulty is one which can be, and has been, solved in nature. In the first place, as might be expected, certain green caterpillars like Smerinthus ocellatus, whose green coloration is due to a modified chlorophyll, do not thus become differentiated in the infra-red plate from the leaves among which they are resting, but retain their cryptic appearance (Plate 5)However, something much more remarkable is seen in the case of certain tree-frogs, such as Hyla coerulea (Plate 6). Here the green coloration is purely subjective, being due-as already mentioned-to a combination of pigment and structure. Yet in the infra-red photograph the skin is rendered as though its coloration were due to chlorophyll (with which, of course, it has no physical or chemical relation whatsoever). Thus in the infra-red picture, or to the infrared-sensitive eye, it retains the harmony of tone upon which its inconspicuousness depends, appearing in waxy white pallor amidst the snow-white foliage.
In view of the possibility that certain animals may have a visual range which extends to, or lies within, the deep-red and infra-red region of the spectrumas claimed by Vanderplank (639) in the case of the Tawny Owl '-the whole
1 In a recent letter to Nature (1939, 143, 983), L. Harrison Matthews and Bryan H. C. Matthews describe experiments which lend no support to the view that owls can see a
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE ii
subject becomes one of deep interest to the naturalist, as it is of necessity to the camouflage expert; and it obviously opens up a profitable field for future investigation.
III. ADAPTIVE RADIATION IN COLOUR RESEMBLANCE
The same principle may be studied from the complementary point of view, namely, that of the adaptive range of coloration exhibited by particular groups of animals whose habits and relationships are such as to place a premium on concealment, whether for offensive or defensive purposes. The fact is that with such species the divergent colour differences exhibited within different families and orders is commonly correlated with the diversity of surroundings occupied by the several species and genera. Such a relation is found, for instance, in groups so diverse in habits and affinity as spiders, lizards, and cats.
(i) General Resemblances among Spiders-Various members of the first group well illustrate the point. Thus species that live on bark, like Marpessa muscosa, are commonly brown; others, like Salticus scenicus, which haunt stone walls and granite rocks, wear a disruptive dress of black and white; others, such as Trochosa picta, which dwell on sandy ground, are sandy in hue, and vary in different localities according to the shade of sand prevalent in their particular neighbourhood ; others again, like Sitticus pubescens, which commonly frequent walls and fences, are greyish in colour; lichen-haunting species, such as the North American Epeira prompta, often bear a close resemblance to lichen; many grass-frequenting species, like the South American Tetragnatha extensa, are dressed in green; while certain flower-loving forms, like Misumena vatia, are whitish, pink, yellow or pale green according to the colour of their immediate surroundings.
(2) General Resemblances among Lizards-Lizards show the same general relation between habits, haunts and habiliment in a striking manner. In the forest, as already mentioned, many species living among twigs and foliage are predominantly green; and this coloration bears no particular relation to the animals' systematic position. It occurs, for instance, in a number of Iguanid genera such as Anolis, Iguana and Polychrus (Plate i); in many Agamidae, such as Cophotis ceylanica, a species found on moss-covered tree-trunks in Ceylon; in a number of Geckos such as Phelsuma; and typically among Chameleons like the little South African C. pumilus and the East African C. dilepis. Other lizards, living on boughs and trunks, are variegated with browns and greys and resemble bark and lichen, as, for instance, in the East African Agama atricollis, in Corythophanes cristatus (Plate i) from Southern Burma, and in Ptychozoon kuhli from the Malay Peninsula. Other species such as the Brazilian Ameiva surinamensis, living on the light-flecked ground, are dark-hued with light spots. In the
small mammal in darkness by its own infra-red radiation. These authors detected no response in the Tawny Owl's eye to radiation from a black body at temperatures between 400 C. and 4000 C. ; and they found that transmission through the eye practically ceases for long infra-red wave-lengths.
savannah, spotted liveries tend to give place to a striped dress harmonizing with the linear configuration of grasses, as seen in the African Skink Mabuia quinqueteniata.
In the desert, stripes are discarded for drab dustcoats, which with their darker markings so well blend with the weathered rock and sand of a sun-baked wilderness. Such species seem almost invariably to be coloured like their surroundings, irrespective of the family to which they belong. As examples the following may be cited-Phrynocephalus maculatus (Agamida) from Persia; Phrynosoma modestum (Iguanide) from Mexico ; Palmatogecko rangei (Gekkonidge) from South-West Africa; and Varanus griseus (Varanidae) from Arabia and North-West India.
We may also note in passing a point that will be taken up again later in connexion with Mammals, Birds and Insects, that different races and species frequently show the closest possible resemblance to the colour of the particular ground on which they occur. This is well seen in lizards of the genus Phrynosoma, the well-known horned lizard of the United States. Bryant (65) states that this lizard resembles the colour of the substratum so closely that it is practically invisible except when in motion. The large P. douglassii ornatissimum resembles the vari-coloured rocks of the Painted Desert of Arizona; P. d. douglassii the uni-coloured soil of Oregon; P. platyrhinos, at Ash Meadows in the Amargosa Desert, occurs as a very white form living on white alkali soil; others from the black lava belts are almost black in colour ; and P. blainvillii frontale-the only known horned lizard which inhabits the forest belt near Pacific Grove, Monterey County, simulates the colour of the pine-needle carpet.
(3) General Resemblances among Cats-Analogous colour schemes have been evolved in different Mammalian orders. Thus among members of the Felida, colour and pattern are generally related to habit and environment. The Snow Leopard (Felis uncia), whose long fur is white in ground colour, is confined to the almost treeless highlands of Central Asia. The vertical tawnyorange and black stripes of the Tiger (F. tigris) assimilate with the tall parallel grass stems and reeds of the swamps and grassy plains where it lives. The more arboreal cats, both large and small, are boldly marked with disruptive spotted or streaked patterns : this is especially true of the South American Jaguar (F. onfa), whose tawny-yellow and reddish coat, broken with black spots, constitutes an effective anticryptic disguise for forest foliage. A modification of this dress is worn by the handsome Ocelot (F. pardalis), exclusively a forest animal, also from South America; and by the Clouded Leopard (F. nebulosa), a thoroughly arboreal forest species from South Eastern Asia. The Fishing Cat (F. viverrina), of the same region, whose general ground colour of grey is broken by elongated dark spots and stripes, lives in lake-side and river-side thickets where its coloration is doubtless highly cryptic. The coat of the British Wild Cat (F. silvestris) is similarly adapted to close country. In less degree we find the relation exhibited by the Leopard (F. pardus) which frequently inhabits wooded districts and is partly arboreal. On the other hand, species living in open barren
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 13
country tend to be uniformly coloured, or but lightly marked: this is, of course, the case with the Lion (F. leo), primarily an inhabitant of sandy plains and rocky places with low scrub; and with such smaller species as the Caracal (F. caracal), a uniformly coloured fawn or rufous species from India and Arabia, Pallas's Cat (F. manul), a greyish or buff-coloured form from the Mongolian desert, and the Pampas Cat (F. pajeros), a yellowish-grey species with oblique straw-coloured bands on the flanks, found in the grassy regions of Argentina.
IV. COLOUR RESEMBLANCE IN DIFFERENT LOCALITIES: LOCAL RACES
A refinement of the principle we have been considering is found in the close colour adaptation presented by different animals to the particular locality which they frequent. We shall here refer to a few of the more striking instances provided by mammals, birds, and insects.
(i) Local Races among Mammals-The existence of pale races of wild animals upon isolated beaches, sandy islands, and patches of desert, has been recorded by many writers. In 1898 Jameson (285) published an account of a pale-coloured race of the House Mouse (Mus musculus) found inhabiting sandhills on a small island called the North Bull, in Dublin Bay. It appears that this island first arose between 1775 and 18oo, and the race had evidently evolved during the intervening hundred years or so. Jameson observed that in autumn and winter the sandhills were frequented by Short-eared Owls, and that hawks from the mainland were regular visitors and could be seen any day in pursuit of prey on the North Bull. He concluded that the protectively coloured race owes its origin to the selective elimination by these birds (both of which hunt by sight) of the more conspicuous dark mice, combined with the effect of isolation in preventing immigration of the darker type from the mainland.
Many similar observations have been recorded in North America. W. H. Osgood, in his valuable Revision of the Mice of the American Genus Peromyscus (45o), described a pale sub-race of the Deer-mouse (Peromyscus maniculatus rubidus) inhabiting a practically isolated sandy peninsula near Samoa, Humboldt Bay, on the northern Californian coast. The occurrence of this pallid form and the related darker Deer-mice from the redwoods of the mainland adjoining has since been analysed by Sumner (595), who discusses the r6le of isolation in the formation of the narrowly-localized pallid race.
Conversely, as is now well known, there is a prevailing tendency for different mammals living on black lava beds to become deeply pigmented. Dark forms of mice and other rodents, differing in their dusky coloration from their nearest relatives of the lighter-hued adjacent desert, have been described from blackish lava beds by Merriam (408), Goldman (2IO), Osgood (450), Sumner and Swarth (6o2), Benson (46), and others.
For example, Merriam (4o8) mentions three mammals captured in the lava fields of Arizona-a Squirrel (Citellus spilosoma obsidianus), a Mouse (Onychomys leucogaster fuliginosus), and a Pocket-mouse (Perognathus flavus fuliginosus)-which differed strikingly from their relatives of the neighbouring desert.
Broadly speaking, it would seem that we must look either to atmospheric or to optical factors for an explanation of the facts. The correlation between dark pigment and high humidity, and between pale coloration and arid conditions, has long been recognized, not only among different mammals and birds but also in various reptiles and insects. Distributional studies have led some authorities to support the first hypothesis-namely, that the pallid hues of desert animals stand in some direct relation to the low humidity of their surroundings.
Sumner's earlier studies of rodents on the North Californian Coast (595) and in the Mojave Desert (596) led him to doubt the validity of protective coloration as an explanation of the phenomena-as inapplicable to animals which are almost wholly nocturnal in their habits; which (in the case of Gophers) spend most of their time beneath the ground; and which show depigmentation in parts of the body (soles of the feet and fur of the belly) not exposed to view.
Moreover, he describes the occurrence (596), on an isolated and dark-coloured lava field in the Mojave Desert, of Peromyscus crinitus stephensi, whose pale coat colour showed no difference as compared with a control series of Deer-mice from pale sand in another part of the same desert. On these and other grounds he inclined to the view that the observed colour differences are probably correlated with climatic factors, and especially with differences of atmospheric humidity.
Dice's (141) investigations in the distribution of rodents in the Tularosa Basin, New Mexico-an area peculiarly well suited to biogeographical studiesappear to support the adaptive interpretation. He describes certain colour forms, occurring in close proximity and under almost identical climatic conditions, which closely harmonize with two extreme types of background-namely, black lava and white sand. The prevailing colours of the mammals living on the desert plains and slopes are the pale buff and tawny hues of the soil. But on an extensive lava flow in this region the White-throated Wood-rat and the Rock Pocket-mouse, characteristic of the adjacent mountain slopes, are replaced by black races (Neotoma albigula melas and Perognathus intermedius ater) : these black races appear to be restricted to the lava formation. On the neighbouring White Sands region there occurs a small nearly white pocket-mouse (Perognathus gypsi), whose colour closely approaches that of the gypsum dunes where it lives: this species was not taken elsewhere than on the White Sands. Dice concludes that the presence of these peculiar species and subspecies cannot be correlated with climatic differences, since the desert and lava stations lie but a few miles apart in the same valley, and at practically the same elevation. He believes that the coloration is in some way correlated with the colour of the surroundings.
In 1931 Benson (47) carried out a more detailed survey of the mammalian fauna of the same region; and in his paper on 'Concealing Coloration among some Desert Rodents of the Southwestern United States' discusses the possible effect of background upon the coloration of mammals inhabiting the white sanddune and black lava flow areas. His observations support and extend those of Dice, and confirm the view that the observed colour differences are not due to climatic factors. The author concludes that: The hypothesis of the operation
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 15
of natural selection toward producing protective coloration, together with the factor of isolation, appears to be the best explanation of the development of the races peculiar to the dunes and to the lava .
It now seems clear that in certain cases at any rate depigmentation results from a life on a pallid background ; and it is very evident that an appeal to the physical effects of light or atmosphere involve great difficulties. More recent researches by Sumner led him to modify his earlier conclusions. In an important paper dealing with the subspecies of Peromyscus (598) he brings forward evidence which points towards the operation of visual rather than physical factors, and he admits that earlier arguments for the direct responsibility of atmospheric agents in bringing about pallid hues have lost some of their cogency.
Thus, on an isolated reef of pure white quartz sand off North-west Florida he found Peromyscus polionotus polionotus of the mainland represented by a distinct and extraordinarily pallid race, P. p. leucocephalus. In this race most of the hair was white, and the pigment of the skin was greatly reduced. Yet the environment is one of high rainfall and high atmospheric humidity. Here, then, the relation between humidity and pigmentation is reversed, while that between the coloration of the Mice and that of their background provides a striking example of approximation. Sumner expresses the opinion that, in the absence of direct evidence, protective coloration, achieved through differential survival of the paler variants, seems to be the most plausible hypothesis to meet this case.
Reviewing the phenomena, Sumner appears to favour a compromise verdict' between the alternative interpretations. It is not improbable ', he says, that both of these explanations must be reckoned with. Pronounced differences of atmospheric humidity may be capable of bringing about genetic changes in pigment formation, independently of selection, while selection, on the basis of concealing coloration, may enhance these effects in the case of many desert animals.'
Dice and Blossom (41a) discuss a number of instances of local, non-climatic colour-adaptation in their comprehensive work on mammalian ecology of southwestern North America. For instance, a most complete series of colour-races is presented by the Rock Pocket-mouse (Perognathus intermedius), the Cactus Mouse (Peromyscus eremicus), the White-throated Wood-rat (Neotoma albigula) and the Cactus Wood-rat (N. lepida). All occur in parallel series of pale, intermediate, and dark-coloured races- on pale, intermediate and dark-coloured rocks; and the authors point out that this correspondence cannot be the result of coincidence, nor can it be due to differences of climate or vegetation.
The facts, then, point unmistakably towards adaptation, and it is difficult to avoid the conclusion that the coloration of these mammals is to be explained in terms of concealment from predators.
(2) Local Races among Birds-Many analogous facts are known in other groups of animals. I have referred elsewhere to the similarity exhibited by lizards of the genus Phrynosoma to different surroundings. Among birds, a classical instance is afforded by Desert Larks of the genus Ammomanes, whose
local races are wonderfully adapted to the colour of the ground on which they nest (94). On the desert between Ammam and Baghdad Meinertzhagen found an almost black race (A. deserti anna) on a narrow belt of black iron-pan rock. On the neighbouring sandy plain this was replaced by a pale race (A. d. coxi). Both exactly matched the colour of the soil on which they lived, and doubtless nested. Major Cheesman points out that: It cannot be contended that these two forms are due to humidity, as the climate is exactly the same for both.' This author found another subspecies (A. d. azizi), the palest of the genus, on white chalky sandstone hills at Hufuf, where he states that the harmony of colour between the bird and the pinky-white rubble was marvellous.
What is in some respects one of the most singular cases yet recorded of colour adaptation to a particular background has been described by Stuart Baker
(i3), and relates to the eggs of the Yellow-wattled Lapwing (Lobipluvia malabarica). Breeding throughout the greater part of India, this bird nests on the bare ground, the eggs usually being placed in a slight depression absolutely in the open: but their earthy coloration and dark markings render them very inconspicuous and difficult to find. Search for nests in 1915 by Mr. J. Stuart on a narrow sandy strip along the Malabar coast, where the soil consists of brick-red laterite with scattered nodules of black ironstone, led to the discovery that throughout this particular area the Yellow-wattled Lapwing lays a curious erythritic form of egg-from pale to deep buff-red in ground colour, with blackishbrown or reddish-brown specks and blotches. These eggs are 'exactly like the ground upon which they are deposited', and when in situ are practically invisible. In subsequent years a large series of eggs was collected both from the red laterite and from the dark-soiled surrounding country. In the latter region the eggs were of the normal earthy type: on the red laterite they were almost consistently of the erythritic form. On the rare occasions on which eggs were found on soil contrasting with their colour, they stood up so conspicuously that it was obvious they must have speedily attracted the attention of vermin passing anywhere close by.'
(3) Local Races among Insects-Among insects also many cases are known of colour races which closely harmonize with the prevailing hues of the particular region which they inhabit. A notable example is provided by the Geometrid moth Camptogramma bilineata-a widely spread and common British species. On a small rock islet called Dursey Island, off the Kerry coast, Kane (299) discovered a melanic variety (var. isolata), which had in that locality wholly superseded the yellow type. Melanism was advanced, both fore and hind wings being a 'uniform sooty black', which harmonized with the local dark slate formation. Kane attributed its existence to the effect of isolation and to the selective agency of bats, Rock Pipits, Wheatears and smaller gulls which haunted the rock in active pursuit of insects. It will be remembered that these moths habitually rest on the rock-face with outstretched wings, and he believed that the intense struggle for existence on a small exposed area out at sea had led to the thinning out of the paler individuals, and to the survival of the better con-
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 17
cealed dark form. Although Kane produces no direct evidence to support this explanation, its probability is supported by evidence of the selective value of concealing coloration and of discrimination by insectivorous enemies which will be considered on a later page. What we may notice here are the further significant facts that dark aberrations of C. bilineata were also found on the dark cliffs of the adjoining mainland, and also on the dark peat bogs of Connemara; but that on the pale grey limestone tracts of Galway Bay and in the neighbouring Aran Islands, the same species, varying procryptically in the opposite direction, had assumed a light patternless form.
Of special interest is the well-known phenomenon of industrial melanism in Lepidoptera, recently reviewed by E. B. Ford (I8ia). Many different species of moths, belonging to several very distinct groups, have become black in industrial areas in England and on the Continent. The change has been brought about during the last eighty years. It is still actively proceeding. And, as Ford remarks, 'It represents one of the few evolutionary modifications ever actually witnessed in nature, and is certainly the most considerable of them.'
The earliest instance was provided by the Pepper and Salt Moth (Biston betularia), whose melanic (and almost entirely black) variety carbonaria was apparently first noticed in 185o at Manchester. It has now completely replaced the typical form in English industrial areas, whence it has also spread into neighbouring country districts.
Good examples are seen in the Geometrid moths Boarmia extersaria and B. punctinalis. The first occurs in southern England, and here its black variety cornelseni is rare; but in Germany this has replaced the normal form where the species occurs in manufacturing districts, though not elsewhere in unpolluted country. Similarly, the black variety humperti of B. punctinalis has established itself where the species occurs in German industrial areas.
The distribution of such melanic varieties, coinciding as it often does with industrialism, is very significant. So is the fact that the melanic forms have not, or have but rarely, become established, even though they are present, in rural districts like the South of England-and this although it has been demonstrated that in a number of cases the black variety is actually hardier than the normal form.
This is apparently to be accounted for, in Ford's view, by the counterbalancing disadvantage of conspicuousness-for black coloration would be a handicap in unpolluted areas, but not so in the smoke-blackened countryside of many manufacturing districts. Consequently in the latter the physiological advantages inherent in melanism, perhaps aided by the ecological advantage of inconspicuousness, have apparently led to its replacement of the normal form.
V. COLOUR RESEMBLANCE IN DIFFERENT SEASONS: SEASONAL DIMORPHISM
Various insects having more than one annual brood exhibit seasonal dimorphism, the colours of successive broods being correlated with the changing conditions produced by seasonal phenomena. In the following brief reference
to the subject, I have drawn my information mainly from Poulton's Essays on Evolution. Among butterflies, well-marked differences often occur between wet- and dry-season broods, involving modifications both of form and colour. Typically, the latter may be characterized by an elongated and bent apex to the fore wing, and by the much-produced angle of the hind wing, which gives an outline more in keeping with the curled and warped leaves of the dry period.
Such modifications, which are well seen in the Indian Kallima (43), have been developed independently in a number of groups-Satyrinx, Nymphalinx and Pierinax. They also occur in various northern species, such as Selenia bilunaria. In Africa, where-as compared with India-the two seasons are more distinctly contrasted, the differences between seasonal phases of butterflies, especially as regards colour, tend to be more marked. Of these two phases, that of the dry season is usually the one better adapted for concealment. Moreover it sometimes happens, as with Precis sesamus, P. antilope and P. actia, that the two phases differ from one another very markedly-the dry forms having the under wing surfaces highly cryptic in appearance, while those of the wet form are highly conspicuous.
Poulton has correlated this striking difference with the conditions of life in the two seasons. During the wet season, butterflies are plentiful ; they are much on the wing ; and they are less subject to long periods of rest. The dry season, on the other hand, is one of comparative scarcity for insectivorous foes. Insects are then much reduced in numbers and are in greater demand. They are also less upon the wing; more inclined to a state of hibernation; and obliged to face the risks of longer periods of enforced repose. Also, the succession of broods is almost at a standstill, so that individuals must face the hazards of life for longer periods. Hence the dry season is a time of far greater stress. These considerations led Poulton to conclude that while conspicuousness may be advantageous to relatively distasteful species in the wet season, when more palatable food is plentiful, concealment is a necessity during the time of drought,
-when food is scarce and the struggle for existence especially severe.
VI. COLOUR RESEMBLANCE DUE TO CHOICE OF APPROPRIATE BACKGROUNDS
A number of cases are known in which colour harmony is dependent upon a definite escape instinct, which leads animals-including such diverse types as grasshoppers and butterflies, fishes and molluscs-to seek shelter in appropriately coloured surroundings. I shall refer to this habit on a later page, in connexion with special cryptic resemblances, and will confine myself here to a single instance observed by Doflein in Martinique (152).
This relates to three species of lizards, belonging to the genus Anolis, which were found occupying one part of the island. Though coloured very differently
-being respectively brownish, green and light grey marbled with darker spots
-they lived in the same surroundings and were in the habit of hunting together for insects over the same ground. When disturbed, however, Doflein noticed that they would dart off for a short distance and then disappear from view. After
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATrURE 19
a little time he recognized that this was due to a peculiar sorting of the lizards, each species seeking refuge in surroundings of appropriate colour; the green individuals running to the green vegetation, the brown to dry withered bushes, and the grey species to cover among the grey tree-trunks, whose sun-flecked bark corresponded closely with their own marbled coloration. Having gained the protection of these surroundings, the three species would remain completely motionless, as though in the knowledge that they were perfectly safe.
We may also include here cases in which colour harmony appears to be correlated with special nesting habits. There is evidence that certain groundnesting birds select a special type of site whose coloration is in closer harmony with that of their eggs than are the general surroundings. Thus the eggs come to be better concealed than would be possible in a site chosen indiscriminately.
The Four-banded Sand-Grouse (Pterocles quadricinctus quadricinctus) lays pinkish eggs which would seem to make them conspicuous, since they are laid on bare ground. However, Mr. A. L. Butler (cited Bannerman 18) observed in the Sudan 'the interesting fact that all the nests he found were amongst the ,scattered fallen leaves of the Camel's Foot Tree (Baukinia), which dry exactly the same clay-pink as the eggs, and amongst which the eggs are very difficult to detect '.
It appears that Lapwings may obtain similar protection for their eggs when nesting on pasture, where they show a tendency to deposit them among the scattered droppings of farm animals. In May 1938 I found in a large grass field near Strathaven, Lanark, a number of Lapwings nesting, and the immediate association of the nests with the scattered brown droppings of horses requires some comment. In several cases the nests and eggs were situated right in the centre of the scattered excrement, and in this situation they were undoubtedly less conspicuous than when surrounded by clean grass. The distribution in this case was as follows :
5 nests containing eggs: 4 in horse dung; i in grass.
12 empty nests or scrapes: 6 in horse dung; 6 in grass.
The area of this field actually covered by droppings was possibly about i per cent., and was sufficiently small to exclude altogether the explanation of these results as due to chance ; and the facts seem to point to definite selection of a particular site.
-The above facts acquire additional significance when they are considered in relation to the known selection by cuckoos of particular foster parents, whose eggs are similar in colour to their own. This subject will be considered under the heading of Mimicry. I merely wish here to draw attention to the common end achieved by these nesting instincts, so widely different in other respects, but both of which lead to colour-resemblance between the eggs and their immediate surroundings-a resemblance which in the one case is protective, in the other mimetic.
2. VARIABLE COLOUR RESEMBLANCE
As a vesture shalt thou change them, and they shall be changed.
PSALM CII, 26
We now come to an important class of cases, in which cryptic resemblance depends upon adaptive colour changes in the individual. The nature of these adjustments in appearance differs widely according to circumstances. They may be permanent, or temporary; specific and unvarying, or adjustable and capable of repetition; correlated with the life cycle, or with the cycle of the seasons; built up gradually, or assumed in an instant.
I. CHANGES OF COLOUR CORRELATED WITH THE LIFE HISTORY
Adaptive changes associated with the life history are presented by many insects and other animals which, in the succeeding stages of their lives, bear a strong resemblance to the successive stations which they occupy. Among Lepidoptera, for instance, examples are familiar to every one. Many Geometridae, like the Oak Beauty (Pachys strataria), bear in the larval form a wonderful similarity, exact to the smallest detail, to twigs of the food plant on which they rest; as pupae, occurring in and wearing the colour of the soil; while the habits and coloration of the adult are admirably adapted for concealment on bark.
Many Sphingidae, like the Pine Hawk Moth (Hyloicus pinastri), are dressed in the similitude of the needles, soil, and bark with which they are intimately associated in the larval, pupal and adult states. In certain Brazilian butterflies of the genus Ageronia, where the pupa are suspended among leaves in full view of enemies, this stage' is elaborately modified both in form and colour, so that it bears an even closer resemblance to a rolled leaf than the adult does to the grey bark of a tree.
Poulton has pointed out that fully fed larvae of the Privet Hawk Moth (Sphinx ligustri) change from green to brown and are thus rendered inconspicuous on brown earth prior to burying for pupation. At a corresponding period larvae of the August Thorn Moth (Eunomos angularia) undergo exactly the reverse change from brown to green, preparatory to pupating in a cocoon of elm leaves (496).
Frequently particular phases of early life are correlated with special temporary habits and attitudes. An interesting example has been described by Hale Carpenter from Uganda. When very young the larvae of the Bombycid 20
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 21
moth Triloqua obliquissima are chalky white in colour, and since they feed crowded together on the surface of a leaf, they simulate a bird's dropping. But with growth their appearance and habits alter. They become 'rich brown, like the petioles, with greenish and purplish mottlings and numerous excrescences like scales at the bases of the petioles : and now they no longer crowd together, but feed on the edge of the leaves and rest individually on the twigs and petioles
Several instances of a similar nature are given by Poulton in The Colours of Animals. Thus, various stick-like Geometrid caterpillars do not sit upon the branches, but upon the leaves of their food-plant. In such a situation a stick-like posture would be the reverse of deceptive. Certain of these caterpillars, such as Ephyra omicronaria, are at first green in colour. Others are brown, but adopt special attitudes, being twisted into an irregular spiral Ephyra pendularia, or into an irregular zigzag, as in Selenia bilunaria, so that instead of resembling twigs, they simulate torn or withered fragments of a leaf, or the excrement of birds or snails (496).
Under the present heading we must also include those far-reaching changes seen in many animals like the Eel (Anguilla anguilla) and other fishes-as, indeed, in innumerable molluscs and crustaceans-which as pelagic larvx are transparent and colourless, and as littoral adults opaque and pigmented like their immediate surroundings of weed, rock or sand.
Change in an opposite direction occurs in species like the Flying-fish Cypsilurus furcatus, whose young are coloured buff, with the 'wings' marbled in greys, yellows and orange-brown, when as juveniles they shelter among drifting seaweeds; but later, as adults, they assume the typical pelagic uniform of bluegrey and silver.
An interesting example of the same principle is presented by the Opisthobranch Mollusc Aplysia punctata, whose colour, varying with age from rosered to olive green, is correlated with changes in the algal background during its migration from deep to shallow waters (see page 349).
Among mammals and birds the first liveries acquired by the young-whether this happens before or after birth-often differ widely from the full dress of their parents. But it must not be assumed that such differences are necessarily adaptive. Lion cubs have spotted coats, and their tails are ringed. Puma cubs have the face marked with black stripes and spots, while the legs and under parts are spotted. Young Lynxes are liberally spotted with black. Since the kittens of all these animals, as well as of smaller carnivores such as Genets and Civets, Ichneumons and Mongooses, are born in sheltered dens or holes, carefully hidden or guarded by the mother, the spotted pattern can hardly be explained as protective.
The fact that many self-coloured adults have spotted young, while self-coloured young never acquire spots as adults, points to the view that these markings of young carnivores represent a primitive pattern, to be explained on ancestral rather than ecological grounds. What is significant from our point of view is
that while the juvenile spotted livery is retained and intensified in Leopards and Jaguars, Servals and Ocelots, in whose wooded surroundings it is turned to useful account, it has been modified as a pattern of stripes in the Tiger, and obliterated in the self-coloured cats of the open country-Lion, Puma and Lynx.
The colour differences between young and adult pigs are hardly to be regarded as adaptive. It is true that the striped young of River Hogs and Wart Hogs haunt grassy surroundings where a striped pattern might tend towards concealment, but as Chalmers Mitchell points out: . the parents guard their young with so great devotion and with so powerful weapons that the little pigs have no need of concealment' (413).
Differences between the young and adult pattern are common among deer. The majority of species have spotted young, and frequently, as in the Red Deer, Roe Deer, Wapiti and Chinese Water Deer, the spotted coat of the fawn is shed in a few months and the spots never reappear. Such white flecks as are worn by fawns which haunt the neighbourhood of trees and lie at rest beneath their shade cannot fail to make them less visible.
With birds, differences of colour and pattern correlated with growth are very striking. The coloration of nestlings is referred to on a later page. There can be little doubt that the disruptive down-patterns worn by newly hatched Rheas, Emus and Cassowaries, by Ducks of many species, by Partridges, Quails and Pheasants, by Gulls and Terns, by Grebes, and by wading birds generally, represent schemes of colour whose function is concealment. And it is to be noted that such cryptic costumes are typical of nidifugous young which run and fend for themselves almost as soon as they leave the egg ; while helpless nidicolous young living in sheltered nests are naked and weak at birth and do not subsequently acquire disruptive downy coats. It is interesting to find that this distinction is maintained even between related species. Thus the Sand-Grouse, although related to the pigeons, nest on the ground like game birds, and have precocious young born in an advanced, active condition, and clad in a highly cryptic dress of cream or buff, strongly marked with rich brown or black.
II. CHANGES OF COLOUR CORRELATED WITH THE SEASONS
We have seen in an earlier section that the coloration of successive broods of insects may be correlated with seasonal changes in the surroundings. With various birds and mammals a somewhat similar end is achieved in an entirely different way, involving seasonal changes, not in successive generations of individuals, but in the succeeding dresses worn by one and the same individual.
(i) Seasonal Change in Snowlands-The coloration and colour-changes of northern mammals and birds present many puzzling features. The fact that a number of arctic and subarctic species, such as the Ptarmigan, Willow Grouse, Mountain Hare, Prairie Hare, Arctic Fox, Stoat and others undergo a change to white at the approach of winter is known to every one. But this assumption of a cryptic winter dress is by no means universal in the snowlands. In the fir-st place, certain forms such as the Polar Bear, Snowy Owl, Greenland
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 23
Falcon and American Polar Hare are white all the year round. On the other hand, others, like Moose, Musk Ox, Glutton, Reindeer, Rabbit and Raven, do not become white during any part of the year even in the coldest parts of their range.
Among those which are subject to seasonal changes, closely allied species may differ considerably in this respect. For instance, the Hudson's Bay Lemming (Cuniculus torquatus) assumes a white winter coat, while the Scandinavian Lemming (Myodes lemmus) does not (3) In a number of animals the degree of seasonal change varies with latitude and climate. For instance, in the extreme north of its range, the Arctic Fox (Alopex lagopus) nearly always discards its bluish-brown summer dress for a pure white winter coat; but in Iceland, where the winter is less severe, the change to white is exceptional. The same is true of the Stoat (Mustela erminea), which in winter regularly turns white in the north of Scotland, but less completely and less frequently in England. Similarly the Weasel (M. nivalis) always assumes white in northern Europe, but rarely does so in Great Britain.
The Mountain Hare (Lepus timidus) of northern Europe, and its local races in Scotland and Ireland, illustrate the same principle. In Scandinavia it habitually changes into white ; in the Scottish Highlands it generally does so ; but in Ireland and the south of Sweden the grey coat is retained throughout the year. In North America the Prairie Hare (L. campestris) and Varying Hare (L. americanus) turn white in winter; the Wood Hare (L. sylvaticus), which ranges farther south, retains its summer coloration; while the northern Polar Hare (L. arcticus) remains white all the year round.
There is evidence, in certain cases at any rate, that low temperature is the factor which causes the whitening of the hair (496). On the other hand, there are many facts which this explanation will not meet. In the first place, with certain forms like the Stoat, the change may occur in early autumn, before the onset of cold weather: -while, as Dr. Julian Huxley informs me, whitening takes place regularly in Arctic Foxes living in the Zoological Society's Gardens. Such facts do not support the view that the change is due to the direct influence of the climate. Moreover, the fact that some arctic and subarctic animals-both mammals and birds-change in winter while others do not, renders improbable the sufficiency of any such explanation.
On the other hand, the exceptional cases, in which the dark summer coloration is retained throughout the winter, lend indirect support to an adaptive, rather than a climatic, explanation of the phenomena. They are 'the exceptions which prove the rule '-since the outstanding examples, which include the Raven, Sable, Glutton, Pine Marten, Musk Ox, Moose and Reindeer, are animals having no special need for concealment (see p. 154) Moreover, it is significant that those species, like the Polar Bear, Arctic Fox, Snowy Owl, Greenland Falcon, Stoat, Weasel, Willow Grouse, Ptarmigan, and Mountain Hare, which as potential predators or prospective prey may be assumed to need cryptic coloration for purposes of offence and defence, are those which in winter, whether by change or constancy, wear a white uniform.
(z) Seasonal Change in Deciduous Forest-We have now to mention briefly a type of colour change in certain respects the very opposite of that considered above. Various inhabitants of forest and savannah in tropical and subtropical countries have their bodies decorated with white spots-such as are worn by the Axis Deer (Axis axis) of India, the Bush Buck (Tragelaphus scriptus sylvaticus) of Africa, and the Spotted Cavy (Cuniculus paca) of South America. In surroundings where shafts from the sun shine through between the leaves and sca4:ter everywhere flecks of sunlight which are bright in contrast to the adjacent shade, such markings distributed over an animal's back and flanks form an important and effective cryptic element.
On the other hand, in colder latitudes, where the leaves fall in autumn, a spotted livery would be the reverse of protective during the winter months. Now it is interesting to find that certain species, such as the European Fallow Deer (Dama dama), and the Japanese Deer (Sika nippon), which inhabit deciduous forests, undergo a seasonal colour change. During the summer they are decorated with white spots; but in winter these are lost, and they assume a uniform greyish or brown coloration. In the more tropical climate of Formosa, however, where the forests are evergreen, the latter species is represented by a race whose white spots show a tendency to persist during winter.
When we compare the present seasonal changes with those found among Arctic animals, it will be noted that a similar tendency-namely, to develop white hairs-is correlated with precisely opposite climatic conditions. In the snowlands it is a phenomenon of winter and in the forests of summer occurrence. But in both regions, these opposite methods bring about the same result, namely, cryptic resemblance to a changing environment-resemblance, that is to say, in the one case to fields of snow, and in the other to flecks of sunlight.
III. SLOW ADJUSTABLE COLOUR RESEMBLANCE : MORPHOLOGICAL COLOURCHANGE
Many circumstances in the lives of animals place a premium upon the ability to change colour. With more or less sedentary species, different individuals may find themselves facing life in incongruous surroundings. Active animals, on the other hand, are constantly coming into contact with different backgrounds. In neither case can a fixed type of coloration be altogether satisfactory. Varying conditions, whether for different individuals at one time or for one individual at different times, demand variable camouflage; and it will surprise no one who is at all familiar with the wonderful variety and versatility of nature's adaptive arrangements to discover that this most difficult problem-of matching an animal to a variable or changing background-has been successfully overcome, and this not once, but over and over again by various members of the animal kingdom.
We are concerned in the present section with a type of colour adjustment especially applicable to relatively inactive insects and other creatures which undergo development against a specific background, and which become adjusted
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 25
to the predominant tones of their surroundings as development proceeds. Such modifications, known as Morphological Colour Changes, tend to be both slow and permanent in effect, in contrast to Physiological Colour Changes which, as we shall see in the following section, may be both rapid and reversible.
(i) Adjustable Resemblances among Lepidopterous Larva and PupaIn a classical series of experiments conducted many years ago, Poulton proved that certain Lepidopterous larve and pupae possess the power of acquiring the coloration of their immediate surroundings (490). He showed with Vanessa urtica and V. io, and also Pieris brassicee and P. rapee, that the pupal adjustment was due to extreme sensibility of the larvae to reflected light when in their final resting position, preparatory to pupation.
A further interesting discovery was that certain larve are capable of adaptive adjustment not merely to colour, but also to pattern, in their surroundings. Poulton (502) found that caterpillars of the Scalloped Hazel Moth, Odontopera bidentata, which rest by day on twigs and branches, are highly sensitive to various shades of brown and grey commonly associated with the bark of their food plant, but that when the larvae were exposed to the influence of lichen-covered branches, their brown bark-like bodies assumed green lichen-like markings. Similar results were obtained with young larvae of the Lappet Moth, Gastropacha quercifolia.
(2) Adjustable Resemblances among Orthoptera-The resemblance of certain grasshoppers to surroundings of different hues in different localities is well known, and there is strong evidence that such insects react to radiation from their environment, and possess marked powers of individual colour adaptation (514, 207). In I90I Poulton observed that the grasshopper Stauroderus bicolor always matched the reddish-brown earth in Heligoland, but that on a neighbouring sandbank called the Dine grasshoppers of the same species were sand-coloured. Not a single dark example was seen on the Dine, nor one pale specimen on the dark soil. There is little doubt ', he writes, that these results, observed without a single exception in such large numbers, in two localities so near to each other, point to the conclusion that the species possesses the power of adjusting its colours to those of its normal surroundings, although these may differ widely from one another.'
More recent researches by Faure (I7o) and Hertz and Imms (248) have fully confirmed this prediction, and it is now known that the immature forms of various Acridiide possess powers of slow colour adjustment within wide limits, and that this morphological colour change may lead to remarkable colour resemblances between the insect and its immediate environment. I have referred briefly elsewhere to the very wide range of cryptic colour exhibited by grasshoppers in the Canary Islands. A striking instance has been described by Mrs. H. H. Brindley from the Russian steppes (232). Referring to Acrida turritaa sluggish grass-feeding grasshopper--she says: 'In June, while the grass is still fresh and green, the Tryxalids are green, with a silvery bloom shading into purple on the antenna and margins of the elytra. At this time they are indis-
tinguishable among the blades and purple panicles of the grass. In August, when the herbage is dried and yellow, the grasshoppers still feed in the same spots, but now they are brown and scorched-looking, in the perfect similitude of bits of straw.'
(3) Adjustable Resemblance of Insects to Areas blackened by Grass Fires-More remarkable, however, is the case discussed by Poulton (514) of African insects which bear a resemblance to the blackened areas caused by grass fires. A variety of species belonging to several families taken by Bacot, Carpenter and Swynnerton on areas recently blackened by fire, and including a cockroach, a mantis, a cricket, many species of Acridiid grasshoppers, a Pentatomid bug, and a Noctuid larva, all showed more or less of the blackened or charred appearance of their surroundings. For instance, the larve, apparently those of Spodoptera abyssinia-taken on an island in Lake Victoria by Carpenter
-were coloured in longitudinal stripes of coal-black and bright grass-green, a disruptive uniform whose tints harmonized perfectly with those of the fireblackened stems and new green shoots where they were feeding. The bug Macrina juvenca, found under similar conditions by Carpenter in Uganda, was the colour of a partially burnt chip of wood.
Direct evidence that these resemblances are due to individual adjustment, and not to seasonal forms appearing at the time when the grass is burnt, is lacking. But the former explanation is supported by observations by Sj6stedt (575) and is rendered likely by the knowledge that different Orthoptera and Lepidoptera do possess the power of colour-change in conformity with their surroundings.
(4) Adjustable Resemblance of Spiders to Flowers-Many other
animals, including various Crustacea and Arachnida, exhibit slow reactions of a similar kind. The Crab-spider Misumena vatia has a considerable capacity for colour-adaptation to different backgrounds. These spiders have adopted the interesting, and doubtless profitable, habit of lurking among the petals of flowers, where they lie in ambush for insect prey. They exhibit a fair range of coloration, and it is well known that individuals tend to resemble closely the tint of the particular flower inhabited. For instance, Packard (452) found in Maine that in early summer, during June and July, when the greater mass of flowers such as White Fleabane and Ox-eye are white, specimens of M. vatia were white also, no yellow individuals being detected. Subsequently, during late July and early August, when the Golden Rod begins to flower, a few yellow specimens made their appearance. But by the middle of the month and through September almost without exception the spiders were pale or deep yellow in colour.
Parallel phenomena were observed by Banks in Virginia, where the white spiders occurring in spring on the white Wake-robin (Trillium) afterwards became yellow on the flowers of the Yellow Dog-tooth Violet. In England specimens taken on Heather are usually pink in colour. Kerville (3o8) and Gabritschevsky (191) have shown experimentally that these resemblances are
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 27
due to individual colour adjustment, and that the reaction, moreover, is a reversible one. Thus when Kerville transferred a specimen with a yellow abdomen from the yellow corolla of Ranunculus acris to the greenish-white Viola tricolor, it gradually assumed the colour of its new surroundings; while a white-andgreen individual became clear yellow when removed from the white flower of Calystegia sepium to the yellow Chrysanthemum coronarium. Gabritschevsky found that the immature spiderlings were incapable of assuming white or yellow coloration when raised on papers of these colours ; but that after the last moult they became sensitive to reflected yellow or white light-white individuals changing to yellow on a yellow background in from ten to twenty days, while they reassumed their white coloration on a white background after five or six days.
IV. RAPID ADJUSTABLE COLOUR RESEMBLANCE: PHYSIOLOGICAL COLOURCHANGE
The type of colour-change now to be considered differs from the last in that the effects produced are both rapid and capable of repetition, being brought about by the rearrangement of pigment granules within specialized cells-the chromatophores. The power of rapid, and sometimes almost instantaneous, adjustment to the colour of the background is one especially suited to active animals which are constantly passing and repassing from one kind of background to another. Its value lies in enabling such animals to depend upon concealment while retaining freedom of movement; and, at its best, it represents undoubtedly the most wonderful automatic cryptic device in existence.
We have to notice in the first place that the mechanism is essentially an attribute of the 'higher' animals. Molluscs, arthropods, and vertebrates represent the three highest lines of evolutionary specialization in the animal kingdom, and it is in these three phyla that chromatic change has been established. Chromatophores are developed in Cephalopoda-the final goal of molluscan evolution; in Crustacea, and exceptionally among insects; and in the three lowest classes of vertebrates-fishes, amphibians, and reptiles. Among mammals and birds chromatophoral activity is of course rendered impracticable, since the skin is usually hidden beneath hair or feathers.
The physiological mechanism responsible for the changes is a complex one, involving reflex activities induced through the sense of sight, control by hormones in the blood, or in some cases being due to the direct action of light on the skin. Parker (455) points out that all the animals concerned have welldeveloped sense-organs, especially eyes, and that the eye seems to play an important part in initiating the colour response, which is, however, essentially a reflex action rather than a higher nervous response. In general, colour-changes which affect the animal as a whole are due to the action of hormones, while pattern formation, which involves the independent activity of different parts of the body, is under nervous control.
(i) Semi-permanent Adjustments among Fishes-Before discussing this type of colour-change, I must refer to another and more permanent type
of adjustment commonly possessed by fishes, which is brought about by a decrease or increase in the number of chromatophores, rather than by the contraction or expansion of their pigment. The latter arrangement might be compared in ourselves to a sudden flushing or pallor, rapid in onset and temporary in duration, due to emotional disturbances such as anger or fear: the former to a gradual darkening or paling of the complexion, progressive in onset and more permanent in duration, due to sunburn or sickness. It is interesting to note that in their effect on an observer these two types of adjustment in fishes are precisely parallel to the two methods by which an artist can deepen the tone of a stippled ink drawing-that is to say, by increasing the number of dots in a particular area, or by increasing the size of existing dots in the same area.
Adjustments of chromatophore distribution take time to mature, and though reversible, are semi-permanent. Consequently they are suited to, and appear most frequently in, species passing different periods of their lives in different environments. For instance, the production of semi-permanent spots is seen in the Plaice (Pleuronectes platessa), whose colour patterns have been studied by Hewer. The young, which normally inhabit inshore sandy grounds, are highly marked and possess numerous white spots which agree well with the surroundings. On the other hand, adults taken from a muddy bottom are mainly greyish-brown, usually without white spots, though these become numerous in specimens kept on whitish pebbly bottoms (255).
Such adjustments of coloration coinciding with the tone, or colour, or pattern of the fishes' environment, are familiar to every naturalist. In certain cases the results produced are very striking. Jordan states that on the coast of California there is a band-shaped Blenny (Apodichthys flavidus), which appears in three colour schemes-blood-red, grass-green, and olive-yellow-according to its surroundings. Here the red coloration also is essentially protective, for the region inhabited by such individuals is the zone of red alge. The Demon Stinger (Inimicus japonicus), of Japanese waters, is blackish in colour when taken among lava rocks, but when found among red algx it wears bright red, broken with a few irregular markings of yellowish-brown (293).
Changes of a similar nature are recorded of certain American Pipe-fishes, which may vary from dark to light green when transferred from tufts of eelgrass to pale weeds; or from brown to brick-red when taken respectively from mud or from red seaweeds (446). Many other Teleost fishes including Wrasse (Crenilabrus), Trout (Salmo), Stickleback (Gasterosteus), Bullheads (Cottus), and Minnows (Phoxinus) are likewise subject to a considerable range of variation which is in general correlated with that of their natural surroundings.
Colour-changes in Elasmobranch fishes have received less attention, but it is now known that various species, such as the Dogfish Mustelus canis (457), and the Skate Raja erinacea, possess a considerable range of variation, and that these changes tend to conform to changes in the environment. Parker carried out experiments with R. erinacea which showed the cryptic nature of the adjustment (456). Two individuals, indistinguishable as regards colour, were placed
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 29
respectively in white-walled and black-walled sea tanks: after eighteen hours the 'white' fishwas light brown, and the 'dark' one dark brown. They were then transferred, and after nine hours the light skate had become dark. With the other individual the reverse change was slower, but twenty-one hours after transference to the white tank it was itself pinkish-white in colour.
An application of this semi-permanent type of adjustment is seen in the practice of 'conditioning' Minnows in a can with white sides, so that when subsequently used as bait they appear more conspicuous in the dark waters inhabited by Pike and Perch. As Poulton wrote in 189o: 'The fisherman knows that he stands an extra chance of catching his fish while the bait remains unadapted to its environment. This experience serves to prove in a practical way that the power of changing the colour is essentially protective' (496)-a conclusion strikingly confirmed in recent years by Sumner, whose experiments are described at some length on another page.
(2) Rapid Adjustment in Fishes-We have now to consider adjustments of a more specialized type. Many fishes-notably certain inhabitants of coral reefs-possess powers of rapid colour-change, which both in range and rapidity far exceed those of chameleons, whose classical achievements pale by comparison. In his detailed study of the colour-changes undergone by different reef fishes from Bermuda on exhibition in the New York Aquarium, Dr. Townsend records 'instantaneous changes' in twenty-eight tropical species. Some of these, like the Nassau Grouper (Epinephelus striatus), can assume six or even eight different liveries, which may be put on and off within a few moments. These colour reactions are frequently to be explained as an emotional response, correlated with excitement, anger, or fear, or with such activities as fighting or feeding. Allowing for this factor, however, there is no doubt that many adjustments are commonly induced by, and correlated with, changes in the background, and are adaptive in the sense that they make for concealment-as has been demonstrated by the work of Sumner (594), Mast (398), Hewer (255), and others.
The remarkable powers of adaptive adjustment in flat-fishes are too well known to merit detailed description here. The Flounder (Pleuronectes flesus) is generally greyish-olive in colour, more or less marbled with brown ; but it may be yellow, or almost black. Not only do the most responsive species possess a wide colour range from the palest sandy hues to the deepest brown tones, but they have the singular ability to respond to different degrees of coarseness in their surroundings-taking on a definite pattern which varies, within limits, with the texture of the bottom where they are lying. Sumner's work with the Turbotlike Rhomboidichthys podas at Naples, and the Sand-Dab Lophopsetta maculata and Flounders Paralichthys dentatus and Pseudopleuronectes americanus at Woods Hole, provides many striking examples of this phenomenon-the adjustment to the sand or mud or gravel upon which the fishes happen to be resting being often wellnigh perfect from a cryptic standpoint.
The cryptic nature of the colour-changes in--reef fishes has been proved by Longley, who was able to evoke different phases at will by leading individuals,
with the offer of food, from one characteristic environment to another (337, 339)., Rapid cryptic adjustments also follow vertical movements-a number of species readily exchanging their patterned bottom-colours for self-coloured wateruniforms when they leave the bottom to rise through deep water. 'No fish', says Longley (346), may more justly exemplify this class than the great surgeonfish Hepatus matoides, which, when near bottom, may be black except for its tail and particolored yellow pectoral fin, but which 25 feet higher in the water, away from the reef-face, appears in pure pale blue-grey of lowest visibility.' A similar and very interesting example is presented by a species of Thalassoma whose alternative colour-phases were originally attributed to two distinct species, T. nitidus and T. nitidissima-the former wearing,a slaty-blue colour scheme, the latter a predominantly yellow one. Longley showed that these phases are correlated with changes in the environment: the blue nitidus dress is worn chiefly by fishes swimming well above the bottom, and the yellow nitidissima dress by fishes near it. 'Repeatedly schools of 2 dozen or more fishes in the nitidus coloration passed over almost instantaneously to the nitidissima type when they dashed down to the bottom to feed upon a broken sea-urchin, and underwent the reverse change at once when they returned to their original position (347).
Another significant point brought out by Longley's pioneer submarine researches is that particular phases of pattern are frequently related to specific activities in a way which accords with the principles of optical illusion (344). For instance, different fishes which have alternate costumes of longitudinal stripes or uniform colour, and of transverse bars, wear the former when in motion (an arrangement which makes for concealment in that it tends to mask forward movement) and the latter when at rest (when bars better serve to break up the contour and surface form against a broken background). Moreover, precisely similar adjustments are found in certain squids, which wear stripes for swimming and bands for resting (345).
Even in cases where the colour-change appears to be primarily an emotional response, its result may tend towards concealment. For instance, in one of its several phases the Nassau Grouper wears a boldly banded and mottled pattern of black on a light ground-colour. As may be inferred from Fig. 8, the pattern, with its strong tonal contrasts, its obliterative ocular band, its coincident elements passing from body to fins, and its ruptive design which everywhere breaks across the contour of the body, is highly cryptic if viewed against a background of broken tones. Now Townsend tells us that it is this banded phase which 'is instantly assumed by all specimens when they are frightened and seek hiding-places among the rock-work' (629).
(3) Rapid Adjustment in Frogs-Many of the Anura are capable of more or less rapid colour adjustment, which in certain genera, such as Xenopus and Hyla, is remarkable for its considerable range from very dark to very light tones. This faculty appears to reach its highest and most perfect form in Hyla goughi, from Trinidad. In 1911 Boulengtr wrote that the rapid changes in colour which
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 31
this species undergoes are probably unparalleled in any other Amphibian (58). Individuals have at their disposal a range of tints varying from dark brown, through reddish-brown and various shades of yellow, to a very pale greyish white.
Yet more striking are the changes recorded for Phrynobatrachus plicatus by Ivan T. Sanderson in the Cameroons. One individual, whose head and back were green and brown when captured, became almost black above, except for a stripe along the back which turned terra-cotta, when the frog was taken into a tent; and Sanderson gives the followihg remarkable range of colours observed in different individuals of this species-' pure white, yellow, gold, orange, brick, various browns, maroon, purple, mauve, pink, sea-green, grass-green and dove-grey' (55 1).
Such changes in Amphibia are induced by a great variety of primary factors, including temperature, tactile and visual stimuli, direct action of light on the skin, respiratory disturbances, and so forth, the activation of the chromatophores being effected through the agency of hormones. But whatever their physiological cause, the fact remains that they do tend, in general, to produce colour-harmony with the surroundings. And Gadow (195), referring to the experimental physiologists' observations, says: 'All this sounds very well, but the observations and experiments are such as are usual in physiological laboratories, and the frogs, when observed in their native haunts, or even when kept under proper conditions, do not always behave as the physiologist thinks they should... in many cases the creature knows what it is about, and . the eye plays a very important part in the decision of what colour is to be produced. The sensory impression received through the skin of the belly is the same, no matter if the board be painted white, black, or green, and how does it then come to pass that the frog adjusts its colour to a nicety to the general hue or tone of its surroundings ?'
(4) Rapid Adjustment in Lizards-Among lizards, many species besides chameleons exhibit marked and rapid colour-adjustments, as shown, for instance, by the American Iguanid Anolis carolinensis, whose different phases range from dark-brown to pea-green; and by the Asiatic Agamid Calotes versicolor, whose habit of playing the harlequin causes it frequently to be misnamed the 'Chameleon' by Europeans. Certain geckos are also subject to a remarkable range of rapid adjustment. Malcolm Smith says that Gehyra mutilata can change colour with considerable rapidity (579), while according to Tytler Phelsuma andamanense possesses a colour-range from rich emerald green-which is the costume worn in the sun or strong light, to nearly black-which is that assumed in dark places or subdued light (634). It will be noted that all the above species are, like the chameleons themselves, typically arboreal, and as such (and in contrast to terrestrial forms) rely for safety mainly upon concealment, rather than upon alertness and speed, though colour-changes occur also in various ground lizards, such as Phrynosoma.
(5) Chromatic Response in Cephalopods-Very remarkable, both for its rapidity and range of action, is the power of colour-response possessed by
cephalopods-the squids, cuttle-fishes and devil-fishes. 'Under excitement a devil-fish may change from an ashen-gray to almost black and back again, and with effects that are described as cloud-like or shadow-like in their delicacy. The endless variety and change that such an animal may show can be compared only with sky and water. In the squids the same colour-play is observable except that it is ordinarily in tints of golden orange, red and brown' (455)The chromatic organs responsible for these extraordinary effects have been closely studied, in Loligo, by Bozler (59). Each is a complex structure, consisting of a highly specialized group of cells: a central element, the chromatophore proper, containing pigment; and a number of innervated muscle fibres radiating from it in the plane of the skin. Contraction of the radial fibres draws out the central cell into an expanded disc, whose contained pigment consequently occupies a much greater area than before. As they relax, the elasticity of the cell membrane causes the chromatophore to regain its original spherical form.
The cephalopod chromatophore is unique for its complexity not only from the structural, but from the functional standpoint. In his valuable review entitled Chromatophores Parker states: 'Its functional activities include many of the intricacies of muscle action. In its relation to the central nervous apparatus, it shows a remarkable degree of differentiation, for the whole chromatophoral system may be acted on locally in such a way as to produce the wave-like spread of colour-change so characteristic of these animals. Such an activity would be impossible without a high degree of central nervous specialization whereby a succession of superficial changes may be called forth. In this respect the cephalopod chromatophoral apparatus, peripheral and nervous together, must involve a complexity of communications and of controls such as are present in the modern electric sign over whose surface an ever-changing design may be made to pass' (455).
The peculiar nature of the cephalopod chromatophoral mechanism renders possible the greatest rapidity of colour-change. Solandt and Hill (58o) find that the whole transformation in a chromatophore of Sepia, from complete contraction to complete expansion, can take place in two-thirds of a second.
Evidence bearing on the adaptive significance of colour-change in Sepia officinalis has recently been brought forward by Holmes, whose observations point to the general conclusion that the assumed patterns have a protective function. He shows that this versatile cuttlefish has at its command an extensive wardrobe, that different dresses are worn under different conditions of background, and that usually the former are adjusted to the latter so as to produce a cryptic effect. For example, when resting against a white background, all the chromatophores contract, until the animal assumes an iridescent whiteness. On a well-illuminated sandy bottom, the exposed dorsal surface bears a mottled pattern of very light and slightly darker browns. In black surroundings the mottled pattern deepens in shade, and the animal becomes very dark in colour. On the other hand, against a dark background containing white objects, a strong disruptive pattern develops with the appearance of a bold white square, or of
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 33
two white transverse bands, on the back. This disruptive pattern is assumed only when light and dark objects are present simultaneously in the animal's visual field-the reaction in black and white surroundings being entirely distinct from that in surroundings which are either black or white (269).
(6) Chromatic Response in Crustacea-Various of the larger Crustacea
-amphipods, isopods, decapods-exhibit adaptive colour responses. Such changes may be said, in general, to make for harmony of colour between the animal and its environment-the chromatophores expanding against a dark background and contracting against a light one. Frequently there is adaptation not merely of shade, but of colour also. Crangon is stated by Koller (320) to assume an appropriate tint of yellow, orange or red against different surroundings. The chromatic changes in Hippolyte have been extensively studied by Keeble and Gamble (198, 199, 304), who found that when young or old prawns were exposed to the influence of white or of black backgrounds, the pigment became for the most part reduced to dots, or disappeared, in the first case; and in the second, maximal quantities of red and yellow pigments developed, giving the prawn a deep reddish tint. Under natural conditions, that is to say, against backgrounds of weed, H. varians exhibits close harmony with its surroundings
-becoming brown on brown weed, green on Ulva or Zostera, red on red algx. Gamble and Keeble found that young transparent specimens became respectively green or brown within forty-eight hours when placed on weed illuminated by direct sunlight (304). The production of sympathetic weed-colours in the shallower zones of the coast is significant, since the life of Hippolyte is intimately bound up with the presence of the algxe, among which it lives and feeds: and these authors remark that . of all things which characterize Hippolyte, its tenacious immobility on the weed of its choice is the most striking'.
The chromatophoral mechanism of Crustacea appears to be entirely under humoral control. Thus Koller (32o) has demonstrated that in shrimps belonging to the genera Crangon and Leander the eye-stalks produce a substance' contractin which on passing into the blood causes pigment in chromatophores of the region supplied to contract; the rostral 'black-organ' producing another substance expantin' having an opposite effect. However, the precise mechanism involved must be extremely complex, and as Parker remarks, we are still far from a complete understanding of the operation. In Crangon, for instance, the reactions are very varied, for the shrimp can well adapt itself to backgrounds as diverse in colour as white, red, orange, yellow and black.
(7) Physiological Colour-change in Carausius-Reversible, or physiological colour-changes are not unknown among insects, for the Phasmid Carausius becomes paler or darker in response to a variety of stimuli, including changes of temperature, osmotic changes in the blood, mechanical pressure, humidity, or visual sensations. The colour-responses of these stick insects have recently attracted the attention of several workers, notably Giersberg (207), Janda (286), Pantel and Sinety (454), and Priebatsch (528). The actual colour-change is effected by a clumping or spreading of pigment granules in the hypodermal A.C.A.-3
cells, which may therefore be regarded as unspecialized chromatophores. The parallel between Carausius and the more specialized condition in various vertebrates and invertebrates with true chromatophores extends to the method of control, in that optical stimuli acting through the insect's eye upon the visual centre of the brain apparently induce the secretion of a hormone into the blood which activates the pigmentary movements.
(8) Ecological Significance of Chromatic Response-We see in chromatophoral mechanisms what we find again and again in other classes of adaptive coloration phenomena: namely, a similar final result brought about by the most diverse means. Wide differences exist both in the number and arrangement of the various chromatic elements forming the physical basis for colour-change; and in the physiological method of control.
From a structural standpoint, the various types of chromatic element bear little resemblance. Those of vertebrates are relatively simple, consisting of single cells, each normally carrying a single kind of pigment. The crustacean element consists of a group of cells, or a syncytium, containing pigment of one or more kinds. In cephalopods the complex and unique structure of the chromatophore gives it the status of a simple organ.
From a physiological standpoint, equally striking differences are apparent in the methods of control. Cephalopod chromatic elements are mainly under nervous control, being apparently little influenced by hormones. In Crustacea, on the other hand, the chromatophores are without nervous connexions, and are apparently activated entirely by hormones. Melanophore activity in vertebrates may be either nervous or humoral-being predominantly of the former type in fishes, the latter in amphibians, and influenced by both agencies in reptiles (455).
But the significant fact must be noted that beneath all this diversity of structural equipment and physiological activity there is generally to be traced a common type of external stimulus and a common type of visible result. The primary sensation of background is received through the eyes, which form the essential approach to the chromatophore system. And the final effect is an approximation in colour and tone, and often in pattern, by the organism to its surroundings. It is, in other words, an adaptation to environment leading to reduced visibility.
3. OBLITERATIVE SHADING
Themselves but shadows, of a shadow world.
The second optical principle upon which concealing coloration depends is that of countershading. When an animal or any other solid object is observed out of doors in the open, it will be seen that its upper surface is more brightly illuminated than its under parts, owing to the direction of incident light from the sky. The effect of this top lighting is to lighten the tone of the upper parts, while the lower surfaces, which are in shade, appear to be darkened.
It is owing to the unequal reflection of light from its different surfaces that a solid object of uniform colour presents to the eye the well-known appearance of light and shade; and it is almost entirely by this optical appearance of relief
-due to light and shade-that we are able to recognize the object as a solid one.
This property of light alone enables us to distinguish at a glance between a disc and a sphere, or between the side of a cube and the side of a cylinder, even when the compared bodies appear to have identical outlines-a circle or square
-and in the complete absence of visual clues due to perspective, or stereoscopic effects.
I. THE PRINCIPLE OF CONCEALMENT BY COUNTERSHADING
Shade, then, is a serious matter, and must be taken into account in any serious attempt to reduce visibility-whether by animals in the interspecific struggle for existence, or by man in the international sphere of warfare.
Shade gives the appearance of projection, or of depth, and by means of light and shade alone a solid body can be distinguished even when it is placed before a background whose colour and texture exactly matches its own.
This is well illustrated in Plate 7 which shows a white cock standing in the open before a white background, illuminated by ordinary diffused daylight from the sky. Contrary to what might have been expected by any one lacking in artistic perception, the bird appears highly conspicuous, the back looking lighter, and the breast darker, than the background, although in actual fact, back, background, and breast are all pure white. The only parts of the bird's surface which match the shade of the background are those which, like the background itself, are approximately vertical and hence illuminated to the same degree.
Now, short of floodlighting the bird's belly, it would be quite impossible to obliterate the dark tone which betrays it-for we cannot make white whiter 3S
than white. But let us suppose for a moment that this bird were coloured instead a uniform brown tone, and were viewed against a background of exactly similar tone. It would then, of course, still look equally conspicuous; in fact it would appear precisely as in our photograph of the white bird, though in a lower key. But with brush and paint it would be an easy matter for an artist to lighten the undersurfaces and to darken the back with graded tints so as exactly to counteract the effect of shade and light. This treatment by countershading, if properly carried out, as in Thayer's famous models, would render the bird almost completely invisible from a short distance away. Now it is this very principle upon which the concealment of many animals in nature depends. The brush of nature has laid down in skin and scale, in fur and feather, darker pigments on the back, grading into lighter pigments on the belly; and in this way a further important step is taken towards the obliteration of form-a step which prepares the background for the application of the third great optical principle of disruptive coloration, to be considered later.
The theory of concealment by countershading will always be associated with the name of Abbott H. Thayer, who first fully grasped this important principle and applied it widely to problems of animal coloration. In his important discovery he was, however, anticipated in part by Poulton, who fifty years ago clearly showed how the appearance of roundness in the chrysalis of the Purple Emperor Butterfly (Apatura iris), was obliterated by the presence of white spots which-in a manner precisely analogous to the stippling of an artist-exactly neutralized the darker tones of its shaded surfaces, by which alone the impression of roundness is conveyed to the eye. 'By this beautiful and simple method,' he wrote, 'a pupa, which is 8.5 mm. from side to side in its thickest part, appears flat, and offers the most remarkable resemblance to a leaf which is a small fraction of i mm. in thickness' (495).
It will be noted that in countershading we have a system of colouring the exact opposite to that upon which an artist depends when painting a picture. A skilled artist is the most wonderful of all illusionists. By means of pigments he can create, upon a flat surface-in addition to impressions of objects in mere colour-the appearance of depth and distance, of light and atmosphere, of roundness and solidity. Now it is remarkable to find that in the countershading of nature the very opposite effect is produced by precisely opposite means. The artist, by the skilful use of light and shade, creates upon a flat surface the illusionary appearance of roundness : nature, on the other hand, by the precise use of countershading, produces upon a rounded surface the illusionary appearance of flatness.
By countershading surfaces normally directed towards the source of light and counterlightening those normally in shade, using properly graded tones, it is thus possible to eliminate the effects of relief, to destroy the appearance of depth and three-dimensional space, and to render an object, whether it be a Hare or a Herring, optically flat. When at the same time the animal happens to be seen against surroundings with which it agrees in hue, it will fade into a ghostly elusiveness and become completely invisible from a short distance-
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 37
its entire contour and surface blending into the background. In the words of A. H. Thayer, the effect of obliterative gradation of light and shade is 'to render the creature's actual surface unrecognizable as the surface of any object or objects of the immediate foreground, causing it to pass for an empty space through which the background is seen' (614).
The ghost-like effect of countershading is perhaps nowhere more perfectly displayed than by various fishes. The Gray Snapper (Lutianus griseus), which haunts waters over grey-white sand at Tortugas, is seen in a photograph published by Reighard (538) to be absolutely flattened by the application of this principle,
:~~.................. .....:...:...........:...................................:...... .... ....:......
S- ~ 2
S...... ... ......
.......................... . .. .
-.. . ..
... .. ....
. . .. .........
..... ........ ............................... . ......... ... . .
FIG. i.-Diagrams illustrating Thayer's principle of obliterative shading I The appearance of light and shade produced by a self-coloured body when illuminated from above ; 2 The appearance of a countershaded body uniformly illuminated; 3 The combined effect of top-lighting and countershading
and the fish thus provides a most striking example of general aggressive resemblance. Its prey is the Hard-head (Atherina laticeps), a plankton feeder which according to Reighard neither feeds nor shelters in the reefs, but swims over the sand, where it appears almost invisible, especially when seen from below. I quote the following passage from Reighard to indicate the effectiveness of its protective coloration. 'Photographs were obtained of this species in its natural environment. Although the photographs were excellent, the fish were so mconspicuous that the loss of contrast in the pictures, inevitable in the process of reproduction, made it inadvisable to make plates from them, as they would have shown practically nothing.'
The principle is nicely illustrated in the photograph on Plate 7 of a Bush
Buck (Tragelaphus scriptus). At a little distance the creature is seen to lose all appearance of solidity, an effect which is in this case enhanced by the white markings on the flanks. These resemble, or at any rate suggest, flecks of sunlight, and, as it were, carry the eye of the observer through the optically flattened surface of the body to the light-flecks that lie beyond.
The coat of the Hare has a similar grading of tone from dark above to light beneath. As she crouches motionless in the field with her long ears pressed down along her back, she is difficult enough to detect; but look at the difference when, having been killed, she lies on her side or back. Now the compensating effect of light and shade on dark and light surfaces is upset; the optical illusion is destroyed; and in death she becomes as conspicuous as in life she was difficult to see.
HI. COUNTERSHADING PRODUCED BY BLENDED PATTERNS
Countershading, as considered above, depends upon the employment of graded tones of colour. The view that certain patterns found on animals produce the same result when they have become blended with distance was put forward and discussed by J. C. Mottram in 1915 (4.24.). This device depends upon the fact that when a pattern composed of alternate black and white markingswhether of squares or lines or spots-is examined from successively increasing distances, a point will be reached from which the separate markings are no longer visible, but blend to form what appears to be a uniform grey half-tone. If in such a pattern the relative proportion of black to white be increased, or decreased, then the resulting tone of grey at blending distance will appear darker, or lighter. In this way it is possible to produce a, flat tone of any depth ranging from black to white.
Now it follows that if the proportion of black to white be varied over different parts of the pattern, the effect produced upon the eye at blending distance will be a variegated, or graded, rather than a uniform grey tone. This principle is, of course, one employed by every artist, but nowhere with such effect as in pen drawing. It also forms a basis for printed reproductions in half-tone, which consist entirely of alternate black and white spots whose relative size will give the required pictorial illusion at blending distance. Various methods employed by the artist or printer in producing a graded surface of grey, ranging from black to white, are shown in Fig. 2.
All of the ten designs here shown agree in this-that they present at blending distance a tone graded from darkest above to lightest beneath. But in each case the effect is produced by a different arrangement of white elements. The following changes will be seen to occur in these designs as one proceeds from the solid black area at the top to the solid white area at the bottom: (i) graded change in tone; (2) evenly spaced horizontal black bars becoming progressively narrower; (3) evenly spaced vertical black bars becoming progressively narrower;
(4.) vertical black stripes becoming less numerous ; (5) and (7) evenly spaced black marks becoming progressively smaller; (6) and (8) evenly spaced white
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 39
marks becoming progressively larger; (9) similar black marks becoming more widely scattered; (io) similar white marks becoming more densely crowded.
Mottram has applied this principle to the study of patterns in animals, and points out that the appearance of obliterative shading can be, and is in certain cases, reproduced by means of patterns like some of the above, rather than by the more usual method of graded ground colour as understood by Thayer. As an example he cites Grant's Zebra (Equus burchelli granti), whose coloration consists of black stripes on a white ground which lacks countershading, being of the same tone throughout. Upon the back and other surfaces which receive full illumination, the black stripes are wide; but on the belly and under parts
/ ,./8','i ... "i /P 5
/// _A //
~FI..-Types.,. opt whhd a
FI. .-rypes of pattern which at blending distance produce graded tones similar to those seen in normal countershading
which are in shade they are narrow; while between the two extremes they are intermediate in width. This arrangement, which is essentially that seen in Fig. 2, No- 3, producesafter blending an appearance similar to that of countershaded patternless animals like the Ass. A similar device is seen in Hardwick's Civet (Hemigale hardwicki), which wears a series of broad, dark, saddle-shaped bands across the back, tapering to points on the flanks; and in the patterns of the Banded Duiker (Cephalophus doriae), and the Tasmanian Wolf (Thylacinus cyanocephalus). In other cases, as in the Striped Hyvena (Hyena hycena), there is a tendency for the stripes on the back to increase in number, rather in width, after the fashion of Fig. 2, No. 4.
Various other animals exhibit other arrangements which at blending distance produce similar results. For instance, the Cheetah (Cynaeurus jubatus) presents
little countershading ; but this deficiency is compensated for by the black spots, which are closer together on the back, and become more widely separated towards the under surfaces-a scheme based upon the arrangement seen in Fig. 2, No. 9. The Fishing Cat (Felis viverrina) illustrates the same principle. Here the dorsal stripes break up laterally into spots, which tend to become more widely separated on the flanks beneath. Similar graded effects are seen in the arrangement of spots in the Serval (Felis serval).
In the Leopard Cat (Felis bengalensis), and in the Pale Genet (Genetta tigrina) and related species, the same countershaded effect is produced at blending distance by dark markings which are larger on the back and become progressively smaller on the flanks, disappearing altogether on the under surfaces, as in Fig. 2, No. 7.
A similar result may be achieved by a great variety of other patterns. In the Guineafowl (Numida meleagris), for instance, the spots are light upon a dark background, instead of dark upon a light one; but here, in accordance with the same optical principle, they are small on the back, and become larger towards the ventral surface, as in Fig. 2, No. 8. One essential feature all these patterns have in common-namely, the decreasing area of dark, and increasing area of light colour as one passes from the dorsal to the ventral aspect. And by this means they each produce, when at sufficient distance for blending, the appearance of countershading.
III. THE FUNCTION OF OBLITERATIVE SHADING IN ANIMALS
Countershading is a fundamental principle of animal coloration, and is of wide occurrence in nature. It has been evolved alike in many unrelated groups of animals; by the hunter and the hunted; in the sea and on land. It tones the canvas on which are painted the Leopard's spots, the Tiger's stripes, and the patterns of smaller Carnivora such as Serval and Ocelot, Civet and Genet, Jackal and Hyamna. It is the dress almost universally worn by rodents, including the Vizcacha, Jerboas, Gerbils, Cavies, Agoutis, Hares, and many others. It is the essential uniform adopted by Conies, Asses, Antelopes, Deer, and other groups of ungulates. It is repeated extensively among the marsupials, as seen in the coloration of the Tasmanian Wolf, Opossums, Wallabies, and others. It forms a background to reveal the beautiful and subtle picture-patterns worn by Wheatears, Warblers, Pipits, Woodcock, Bustards, and innumerable other birds. It provides a basic livery for the great majority of snakes, lizards, and amphibians. Among insects it reaches a fine state of perfection in different caterpillars and grasshoppers. And the same is true of cephalopods like Sepia, where, as Holmes has recently shown, countershading may even be adjusted to suit different inclinations of the body.
It is, however, in rivers, and in the surface waters of the sea, that countershading reaches its maximum development and significance. Among active pelagic fishes belonging to many unrelated groups, and in different sea snakes (Hydrophide) and Cetaceans, this arrangement of pigment typically forms the foundation colour-scheme. In most oceanic forms, like the Blue Shark (Car-
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 41
charinus lamia), Mackerel Shark (Isurus oxyrhynchus), Yellow-finned Albacore (Neothunnus albacora), Tunny (Thunnus thynnus), Barracuda (Sphyrena barracuda), Tarpon (Megalops atlanticus), Flying-fish (Exoccetus volitans), Hake (Merluccius vulgaris), Herring (Clupea harengus), and Dolphin (Delphinus delphis), concealment is effected by countershading in uniform hues of blue, green or grey, more or less undecorated by a pattern. But others, like the Mackerel (Scomber scombrus), Spanish Mackerel (Scomberomorus maculatus), Sergeant Fish (Rachycentron canadum), Pilot-fish (Naucrates ductor), and certain Sea Snakes such as Enhydrina valakadien, have patterns of bars, blotches, stripes or spots superimposed upon a background of obliterative shading. In the variegated and brightly coloured surroundings of coral reefs obliterative shading likewise forms the essential background upon which patterns, themselves countershaded, are commonly superimposed. 'Countershading,' says Longley, 'appears almost universally upon reef fishes, and its absence, or relative deficiency, seems to be definitely correlated with some unusual habit or peculiar form' (340). Again, in inland waters the same phenomenon is of almost general occurrence, as in the Trout (Salmo trutta), Dace (Leuciscus leuciscus), Roach (L. rutilus), Perch (Perca fluviatilis), Bream (Abramis brama), Pike (Esox lucius), and many other fresh-water species.
Of singular interest in this connexion are certain transparent or semi-transparent fishes in which the principle of countershading is extended even to certain opaque internal organs. Thus Longley states that Coralliozetus cardone of the Tortugas reefs has the trunk musculature almost transparent; but the brown and yellow of the normal externally pigmented head extends backwards in a relatively elaborate pattern, with countershading such as normally appears upon external surfaces, but worn on the peritoneum (349).
What is the effect of such colour-schemes, and what is their value ? The answer to the first question is perfectly clear. That they confer upon their owners a degree of invisibility is a fact which cannot be reasonably disputed-a fact that is in no sense a scientific discovery, but one that must have been familiar to fishermen and sailors from time immemorial. What does not appear to have been recognized before Thayer made his important contributions to the subject are the principles upon which the inconspicuousness of these fishes depends. When viewed from the side such forms are rendered unsubstantial in appearance by countershading, as already explained. Under favourable circumstances they are made to melt perfectly into their watery surrounding background. But the dark back and silver belly so characteristic of surface-swimming ocean fishes, and of many fresh-water forms, also evidently reduce visibility in another way
-that is, when the creatures are viewed by enemies or prey from above, or from beneath. The back will be difficult to recognize, for it is effaced against the deep colour of the ocean which itself appears from above dark-grey, or leaden, or deep-blue; or it may be against the muddy or olive tones of the river bottom; and it is interesting to note that the dorsal colour appears frequently to be in close harmony with the dominant note in the environment-the blackish-brown
in the Herring (Clupea harengus) is practically invisible from above in boreal waters; the Barracuda (Sphyrena barracuda) from Atlantic Coasts of tropical America has its upper parts clad in dark green; the Flying-fish (Exoccetus volitans), the Pilot-fish (Naucrates ductor), and the young of Scombresox, wear the bright blue coloration almost peculiar to tropical surface-dwelling pelagic forms; while in the muddier, more opaque waters of rivers the metallic shades worn by marine forms are replaced by the familiar olivaceous and brownish tones displayed on the upper parts of numerous fresh-water fishes such as River Trout, Pike, and Perch. When, on the other hand, such forms are seen from beneath, the eye of an observer would recognize with equal difficulty the silvery sheen of the belly, whose iridescent surface displays the nearest possible approach to the bright surrounding background of sky, or surface film.
As regards the second question-the value in nature of obliterative coloration is a matter which still requires further observation and experimental proof. In cases such as those described above, the coloration appears to be adapted intimately to the conditions of life; and there has been a tendency for many writers to accept, on a priori grounds, the assumption that concealment is valuable to the fish, either as a means of escaping capture or of approaching prey; and this uncritical attitude has led others to doubt, or reject, any explanation of countershading in terms of adaptation. Further consideration of this important matter will be deferred until a later chapter, when the evidence will be discussed. Further (indirect) evidence is afforded by certain facts now to be considered.
IV. THE RELATION BETWEEN COUNTERSHADING AND THE CONDITIONS OF LIFE
So general is the application of Thayer's principle in the coloration of vertebrates, and especially of fishes, that exceptions are relatively few, and in these the lack of countershading can usually be ascribed to some peculiarity of form or of habit-for instance : where the body is already flattened structurally and therefore only requires incipient countershading; or where it is seen under conditions of lighting which do not normally cause strong relief ; or where, again, the normal attitude of the species is an abnormal one for the group. Under any of these circumstances strong countershading would be either redundant or harmful.
(i) Countershading in Relation to Form-Some modification is found in the distribution of ground-pigment in several unrelated genera of deep-bodied, laterally compressed forms, such as Zeus (Zeide), Chleatodipterus (Ilarchidw), Scatophagus (Scatophagids), Chetodon (Choetodontidw), Platax (Platacidx), Zanclus (Zanclidx), and Pterophyllum (Cichlide). These have their sides nearly vertical and presenting to an observer but slight convex curvature. With fish of this type strong countershading would defeat its own end, and it is significant that such forms are only slightly countershaded.
(2) Countershading in Relation to Environment-Countershading is found in various degrees of strength, and may be related to the conditions of illumination-and hence to an animal's appearance to an observer-in different surroundings. For instance, various fishes like the Glass-eyed Snapper (Pria-
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 43
canthus cruentatus), which dwell habitually in dim light, are only slightly countershaded (340). Again, the contrast between upper and under surfaces is slight in desert animals, which gain something from the light reflected upwards from the sand. The contrast is also slight for another reason in animals whose home is the interior of shady forests, where the light is diffused and dim and where consequently the differences between the illumination of upper and lower parts is less marked. In the deep waters of the sea, fishes and other animals living beyond the limit of light-penetration lack countershading altogether. On the other hand, contrast between the dorsal and ventral tones reaches its maximum in surface-swimming fishes, and in land animals which live on dark ground under an open sky, as in many deer, antelopes, and rodents of the open plains.
(3) Countershading in Relation to Habits-Among fishes which lack countershading, the following apparent exceptions to Thayer's principle prove on examination to be as significant as the examples which support it. The Remora or Shark Sucker (Echeneis naucrates) is in the habit of attaching itself by the sucker on top of its head to different parts of large fishes. It may consequently have any side uppermost, and, as pointed out by Longley, the lack of countershading is correlated with the fact that the fish maintains no constant position with reference to the source of light (340). The same author mentions the case of Fierasfer, which spends most of its time in retreat within the cloaca of large Sea Cucumbers (Holothuroidea) on the reef flats, and which, like many cave animals, not only lacks countershading, but has lost its external pigment. This state of affairs is reproduced in a colourless crab (Pinnotheres) occurring in the mantle cavity of the oyster. Finally, countershading, like colour-pattern, is typically absent from deep-sea forms living in the everlasting gloom of the abyss.
(4) Countershading in Relation to Attitude-More remarkable as exceptions, however, are different animals which have the countershading inverted, the back being light and the belly dark in tone. Such reversal of tones is found in the Nile Cat-fish (Synodontis batensoda), which has the singular habit of swimming with the belly upwards-' an attitude taken up by no other fish unless it be sick or dead' (446). In the pelagic snail Glaucus atlanticus inverted coloration is likewise correlated with inverted habits. The same arrangement occurs in different spiders, like members of the family Linyphiida, which suspend themselves in the web with the ventral surfaces uppermost. It is of great interest to notice that the same reversal of countershading is widespread among caterpillars, like those of Automeris io and Tropcea luna, which habitually feed or rest upside-down.
A beautiful example of this arrangement is furnished by the larva of the Eyed Hawk-moth (Smerinthus ocellatus). When resting among the leaves of sallow, the food plant, they do so with the back inclined in an inverted position, often hanging back with only the hinder pairs of claspers applied to the stem. In this position the underparts are fully illuminated from above, while the back is in shade. The caterpillars are beautifully countershaded, being pale applegreen on the back grading over the sides to dark greyish sage-green beneath.
This inverted countershading, correlated with the inverted attitude, effaces the larva so effectively that all appearance of solidity is destroyed. The illusion is completed by the superimposed design representing the effect of light and shade on lateral leaf-veins. So effective is the result, that in searching for such larve one finds the partly eaten leaves a far easier clue to its possible whereabouts than the presence of the larva itself, which may be as readily discovered with the hand as with the eye.
But such things have to be experienced in the field to be appreciated. In an attempt to record the optical conditions for the benefit of those who have
FIG. 3.-Larva of Eyed Hawk-moth. Left: Natural (e.g. 'up-side-down') resting attitude, showing the obliterative effect of inverted countershading. Right: Unnatural (e.g. right-wayup ') position, showing strong relief and solid appearance
not known these animals in the wild state, I have sketched (Fig. 3) the appearance of one resting on a stem of its food plant in two positions-that on the left, with the back downwards as in nature; that on the right with the back upwards, an attitude quite foreign to its instincts. The latter shows the insect in bold relief, which is exaggerated since the effect of countershading is added to that of illumination, and its solidity stands out in conspicuous contrast to the flat leaves. In the former, a wonderful appearance of flatness is produced. That this sketch does represent the phenomena in these respects is clearly proved by the two photographs on Plate 8 of a larva taken out of doors under an open sky, under conditions identical as regards exposure, stop, plate, and lighting; the only difference between them being that of orientation-the larva having been turned
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 45
over for the second exposure through about i8o degrees, so that it is illuminated from the opposite direction.
Many striking examples of countershading are described in Saiffert's detailed study of adaptive coloration in Lepidopterous larva and pupa (593). Thus, he has shown that sometimes, when the normal attitude of rest is perpendicular, as with the larva and pupa of the Purple Emperor (Apatura iris), the countershading is likewise in the long axis of the body: the former, which rests head uppermost, is most deeply shaded anteriorly ; but in the latter, which hangs head downwards, the countershading is also reversed.
Siiffert has further demonstrated that in certain cases, as with the caterpillar
FIG. 4.-The principle of obliterating shading as applied to guns: i The appearance due to top lighting; 2 and 3 The appearance due to obliterative paint; 4 and 5 The appearance due to a combination of top-lighting and obliterative paint. It will be noted that 2 and 4 show the incorrect method of applying paint; 3 and 5 the correct method
of the Clouded Yellow (Colias edusa), the appropriate attitude in relation to the direction of light is taken up as a response to the light itself. Thus, when subjected to light reflected upwards from a mirror, the insect takes its stand beneath, rather than above, the stem of its food plant. Experiments with Brimstone (Gonepteryx rhamni) larvae showed that the response is due to the perception of light by dermal sense organs in the general body-surface.
The same author also describes a type of countershading in which two regions of the body-separated by a pale contrasted element-are countershaded
independently. This optical device renders the body as two flat surfaces converging at an angle. It is seen, for instance, in the larva-and with especial effect in the pupa-of Colias edusa. We may note here that the same two-surface effect forms the basis of the remarkable leaf-like coloration of the toad Bufo superciliaris, described on page 3:21.In conclusion, some passing reference may be made here to those cases of inverted coloration which are not correlated with an up-side-down attitude. Under such circumstances the pigmentation will enhance, rather than efface, the animal's relief, thus tending to make it more, rather than less, conspicuous. It is significant that where this arrangement occurs, as, for instance, in the Skunks, Zorillas, and Badgers, there is often good reason to believe (as will be shown in a later chapter) that self-effacement is unnecessary, but that, on the contrary, self-advertisement is advantageous to the species.
(5) Applied Aspects of Countershading-Among the more perfect protective and aggressive adaptations of animals, few are without some special application to the art of war. And in this respect countershading is no exception. The conditions of light which affect the appearance of a caterpillar or a snake are the same as those which cause a gun or torpedo-tube to stand out conspicuously even against a background covered with exactly the same paint. Hence it is essential that such rounded objects be treated with paint properly graded in tone so as to counteract the effects of relief.
Photographs recently published in the press of camouflaged coastal-defence guns reveal a complete failure to apply the relevant system of coloration. On the contrary, the scheme favoured by the authorities is one which entirely misses the mark, as may be seen by reference to the accompanying diagram (Fig. 4).
During the early months of the Great War failure to apprehend the same principle led to a blunder which doubtless caused the loss of very many lives. I refer, of course, to the service cap with which our men went into action-a cap whose attributes as a reflector of light seemed especially designed (by the reversal of this principle) to present the sniper and machine-gunner with a conspicuous target.
The case of aircraft is in some respects analogous to that of surface-swimming fishes like the Mackerel. Such creatures, being dark blue on the back and silverywhite on the belly, -are relatively difficult to detect whether viewed against a background of water from above or of sky from beneath, Aircraft operating by day present an analogous case, and should be coloured on similar lines: the most brilliant white underneath-the colour which most nearly approaches the bright background of sky; and some darker tone above-calculated to harmonize with the ground when viewed from overhead. For special duty at night bombers should be treated with an absolutely matt black paint-the deepest black obtainable. A machine so painted would on a clear night become totally invisible from the ground, and would reflect downwards relatively few rays if caught in the beam of a searchlight.
Neither is there any creature that is not manifest in his sight : but all
things are naked and opened unto the eyes of him with whom we have to do.
HEBREWS IV. 13
We now come to what is perhaps the most interesting, and certainly the most important, set of principles relating to concealment-namely, the type of camouflage familiar to most people from its war-time uses under the quite inappropriate term dazzle', I and properly known as Disruptive Coloration.
FIG. 5.-Young Woodcock
1 The word dazzle ', which came into popular use during the War in connexion with camouflage painting, is an American term applied by Thayer in his work on Concealing Coloration to disruptive colouring in animals. It is based on the American slang I razzle-dazzle -, which has a meaning-expressive of active confusion-quite different from the correct English use for partial blinding by brilliant lights.
4. DISRUPTIVE COLORATION
It will be clear from what has already been said that under ideal conditions, colour-resemblance combined with countershading would suffice to render an animal absolutely invisible against a plain background, even at a short distance. But in nature, as in war, conditions are rarely ideal, and they never remain sofor they change. Most animals, like most machines, are active, and their movements bring them before a constantly varying background, which is itself rarely uniform in colour and pattern. Moreover, the light which falls upon them itself varies constantly in colour, intensity, and incidence.
Even allowing, therefore, for the most efficient colour-harmony and countershading, we still have to reckon with the fact that a uniformly coloured animal presents to the eye a continuous patch or area in the visual field, standing out more or less conspicuously against darker, or lighter, or differently coloured surrounding objects. It is this continuity of surface, bounded by a specific contour or outline, which chiefly enables us to recognize any object with whose shape we are familiar. Thus for effective concealment, it is essential that the tell-tale appearance of form should be destroyed. The difficulty of doing this is met, often with extraordinary success, by the application of optical principles involving the use of patterns.
I. THE FUNCTION OF DISRUPTIVE COLORATION
When a pickpocket intends to relieve you of your watch or wallet, he, or, his confederate, takes care to distract your attention from what he proposes to do by creating a diversion. He draws your eyes from what is really happening to what seems to be happening. Now the patterns worn by many animals such as Giraffes and Jaguars, Anacondas and Iguanas, Pipits and Plovers, and various Grass-frogs, Grasshoppers, Moths, and Mantids, operate in a somewhat analogous way. The function of a disruptive pattern is to prevent, or to delay as long as possible, the first recognition of an object by sight. Its success depends not only upon optical principles, but upon a psychological factor. When the surface of a fish, or of a factory, is covered with irregular patches of contrasted colours and tones, these patches tend to catch the eye of the observer and to draw his attention away from the shape which bears them. The patterns themselves may be conspicuous enough, but since they contradict the form on which they are superimposed, they concentrate attention upon themselves, and pass for part of the general environment, in the same way that the pickpocket's tactics of bluff pass for a commonplace incident.
Even the simplest disruptive patterns tend to hinder recognition and so to make for concealment. In its most elementary state, the principle is well shown in the coloration of certain tropical frogs and toads. For example, the East African Rana adspersa-a large frog whose general colour-scheme is a patchwork of subdued earthy-browns and olive-greens-wears on its back a conspicuous yellow stripe extending from the snout right along the middle of the back (Plate 9). So far from drawing attention to the animal, however, the effect of this stripe is the very reverse. In the first place, the yellow line-which, of
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 49
course, in itself bears no resemblance whatever to its wearer, but rather suggests to the casual glance a twig or blade of grass-stands out from the back and catches the observer's eye. Secondly, by providing a strong incident of colour, it serves to flatten by contrast the less-defined half-tones by which the real form of the frog is differentiated, from its surroundings. In the third place, it serves to bisect the form of the frog, so that the eye of an enemy is actually presented with a configuration of two half-frogs-which look very different from one whole one-and as likely as not the brain behind the eye will fail to join them together in recognition.
An analogous arrangement is found in countless animals of different kinds: cases which come to mind are the light dorsal stripes of different grasshoppers (Figs. 26, 27) ; and among birds the buff back-streaks of the Jack Snipe, the light head-stripes of the Common Snipe, those on the mantle of the Woodcock, and on the wings of the Stone-Curlew. An extremely interesting case which beautifully illustrates the principle in its simplest form is that of the flower-like mantis Hymenopus bicornis, observed in the Malay Peninsula by Annandale (7). Pl pink and white in colour, this remarkable insect has its body optically broken into two pieces by a bold transverse bar of deep sage-green on the thorax. When seen on an inflorescence of Melastoma polyanthum, Annandale states that the effect of the bar is to divide the insect into two parts which appear to have no connexion with one another on the inflorescence, a fact which seems significant when it is realized that the complete insect is ordinarily larger than the expanse of a single flower.
Whether or not the body is optically broken up by strongly accentuated patches of conspicuous local colour, such as those considered above, it is usual for the disruptive scheme to extend as an irregular patchwork of cryptic tones over the body-surface generally, or over such parts of it as are normally exposed to view. Beautiful examples of general disruptive coloration are afforded by the buff and black eggs of the Lapwing (Plate 18), and by the South American Ceratop hrys cornuta (Plate 9), with its wonderful camouflage scheme of gold and green and brown.
II. THE COLOURS OF DISRUPTIVE PATTERNS : DIFFERENTIAL BLENDING
The success in nature of the various devices, which may be lumped together under the general heading of disruptive coloration, depends upon a number of optical principles which we must now examine a little more closely. Provided an animal is seen against a broken background, it is probably true to say that any pattern of darker or lighter colours and tones will tend to hinder recognition by destroying to a greater or less degree its apparent form. But it must not be imagined that to effect concealment any pattern will do, or that any colours will do, or that any arrangement of tonal contrasts will do. In order to achieve effective results, the colours, tonal contrasts, and patterns employed must conform to definite optical principles. A little investigation shows that certain colours, certain degrees of contrast, and certain arrangements of design are more
effective than others for purposes of effacement ; and it is a fact of the greatest interest and significance that the particular disruptive devices which on theoretical grounds appear best for this purpose are those which occur on the bodies of innumerable animals which rely for safety, or for success in hunting, upon remaining incognito.
In the first place, the effect of a disruptive pattern is greatly strengthened when some of its components closely match the background, while others differ strongly from it. Under these conditions, by the contrast of some tones and the
FIG. 6.-Diagrams illustrating the principle of differential blending
blending of others, certain portions of the object fade out completely while others stand out emphatically. And it is to be noted that the shape of the latter-which alone can be distinguished-is such that their real identity cannot be determined.
The principle is illustrated by the simple diagrams shown in Fig. 6, where a series of forms-a fish, an egg, and a moth--are represented against different backgrounds. Seen as self-coloured objects without any pattern, as in the left-hand figure of each series, these forms are easily recognizable. If dressed in a disruptive pattern they become less distinguishable, even when seen, as in the second figure of each series, before a background against which each stands
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 51
out clearly. But when, as in the third and fourth, the background matches and absorbs one element of the colour-scheme, the difficulty is much increased, and the effect on the eye of an observer is such as greatly to delay, or even altogether to prevent, recognition. The photographs of the Garden Carpet Moth (Plate io) and Oak Beauty Moth (Plate i i) illustrate this principle of Differential Blending in operation.
We have to realize, consequently, that the actual range of some of the colours employed should in general be such as occur typically in an animal's environment. This theoretical point is sufficiently obvious. Many instances of its application in nature amongst all kinds of animals have been given in other parts of this book and are familiar ground to every field naturalist. We need therefore only mention here once again. the type of criticism which claims that cryptic animals are often seen against backgrounds against which their colour-scheme fails to harmonize; and that by searching it would be possible to find a background against which any animal would appear more or less concealed. Such critics entirely fail to grasp the vital fact that the backgrounds which different cryptic animals match are typically those where they normally rest, or in which they pass those periods of their lives when concealment is most necessaryfor instance, the diurnal resting-places of nocturnal moths, and the nesting-sites of ground-breeding birds. Plenty of evidence for this will be found on another page. No colour-scheme whatever could hide a Woodcock as it flies on migration over the sea, but at such times it relies not upon concealing coloration, but upon speed or the cover of darkness. On the other hand, during the weeks when it incubates and nurses its treasures in the woods, invisibility is vital, and the colours which it wears are precisely those best adapted for concealment in the surroundings in which it elects to carry out these parental duties.
III. THE TONES OF DISRUPTIVE PATTERNS : MAXIMUM DISRUPTIVE CONTRAST
Next we have to bear in mind the part played by tone, as opposed to colour, in patterns whose function is to break up continuity of surface. We have seen that the effect of a disruptive pattern is to break up what is really a continuous surface into what appears to be a number of discontinuous surfaces. These tend to be interpreted by the eye as separate objects-none of which suggests, but all of which contradict, the shape of the body on which they are superimposed.
Now this illusory appearance-this contradiction of the true form-is greatly intensified by the use of strongly contrasted tones. In general, very light markings on a dark object, and very dark markings on a light one, will be most, effective. The principle is similar to that which makes an open network curtain effective as a screen in preventing a casual passer-by from seeing into the interior of a room.
When any one glances at such a window, his gaze falls upon, and is arrested by, the curtain, whose excessive brightness in contrast to the relatively dark room interior prevents him from noticing what lies beyond it. Now it is not to be imagined that such a curtain intercepts the view, after the manner of an opaque screen. Details in the room beyond are openly exposed to view-although they
cannot be seen clearly. That the curtain acts in this way, merely dazzling and distracting the eye, and not, like an opaque screen, by actually intercepting vision, can easily be proved by dyeing it black, when it at once becomes almost useless as an optical barrier against an observer outside. But it will now tend to prevent those inside the room from seeing out. This reversal of effect is due to the same cause which renders a white curtain effective in the ordinary way, namely, to the contrast in tone between the curtain and the rest of the visual field behind it-in this case, as presented by a black screen silhouetted against the brightly illuminated view outside. From what has been said, it follows that while a white curtain is effective by day against observation from outside, though at the same time it allows those in the room to see out; at night, when the room is artificially illuminated and it is dark outside, a black curtain is what is needed for the same purpose.
I have referred to the present matter because it introduces on grounds familiar to every one. an important principle upon which the concealment of different animals depends, and which we may conveniently refer to as the principle of Maximum Disruptive Contrast. The greater the contrast in tone between adjacent elements in an animal's pattern, the greater will be its disruptive function. Broadly speaking (and provided the design used conforms to the nature of the environment), we may say that white marks on dark animals living in dark surroundings (such as forests) and black marks on light animals living in light surroundings (such as deserts) will be those most effective in breaking u the continuity of their surface, and in masking by contrast tell-tale half-tones of surface structures and modelling; and they are, in fact, the type of markings commonly found in these environments. To produce their greatest effect, these distracting marks need to.be used sparingly in relation to the total area of the body, or they may fail in their function-which is to prevent recognition by forcibly drawing the attention of an observer to themselves, so that the cryptic parts of the animal wearing them pass unnoticed.
The simplified diagrams in Fig- 7 illustrate the value and effectiveness of maximum disruptive contrasts better than any verbal description. In each series of diagrams the corresponding figures are shaded to exactly the same tone, which is one approximating to that of the background-this being black in the first, white in the second, and half-tone in the third set. The second figure of each horizontal series carries a superimposed pattern-white in the dark figure, black in the light figure, and black and white in the one of medium tone.
On looking at these drawings from a little distance, it will be seen that the conspicuous patches operate most efficiently in distracting attention from the form of the animals wearing them. By sheer force of their brightness, or blackness, or contrasts, they dominate the picture presented to the eye, apparently destroying form by levelling out the lesser contrasts between the animals themselves and their respective monochrome backgrounds.
The third figure of each series illustrates the effect of broken surroundings in further blending and confusing the picture. In nature, it is, of course, this
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 53
last which most nearly represents the impression presented to the eye of an observer. Any one who looks at the right-hand column before seeing the figures on the left will appreciate the value of these optical devices in delaying first recognition.
The principle is one with many applications to modern warfare. In the Great War it was utilized by the Germans when they introduced strongly marked incidents of white or black tone to conceal the fainter contrasts of tone made by the sloping sides of overhead camouflage -screens, or roofing, as seen from the air. The same principle has, of course, a special application in any attempt to
FIG. 7.-Diagrams illustrating the principle of maximum disruptive contrast, and showing the distractive effect upon the eye of patterns which contrast as violently as possible with the tone of their background
reduce the visibility of large objects of all kinds, such as ships, tanks, buildings, and aerodromes. The essential function of dazzle painting is to break up continuity of surface by means of violently contrasting tones, rather than by mere differences of tint. The latter are relatively unimportant : the former are all-important.
Various recent attempts to camouflage tanks, armoured cars, and the roofs of buildings with paint reveal an almost complete failure by those responsible to grasp the essential factor in the disguise of surface continuity and of contour. Such work must be carried out with courage and confidence, for at close range objects properly treated will appear glaringly conspicuous. But they are not painted for deception at close range, but at ranges at which big gun actions and
bombing raids are likely to be attempted. And at these distances differences of tint-mere blotches of brown and green and grey like those commonly used for Camouflaging' army vehicles-blend and thus nullify the effect and render the work practically valueless.
In nature vigorous disruptive contrasts are frequently seen at work, and their wonderful effectiveness in hindering recognition needs to be experienced in the field to be appreciated fully. Beautiful examples are seen in the white markings of the beetle Lithinus nigrocristatus (Plate 43); of newly hatched Little Grebes and Jaganas; of tree frogs such as Hyla leucophyllata (Fig. 20) from the Amazon and Megalixalus fornasinii (Fig. 19) from the Zambesi; in the disruptive phases of many reef-fishes like Epinephelus striatus (Fig. 8) ; and of bottom-living fishes-Lophius piscatorius, and cuttlefishes-Sepia officinalis: and on the other hand, in the black gorget of the Ringed Plover and the black cheek and breast markings of the Turnstone, in both cases associated with, and contrasted against, patches of
white. Both of
birds are splendidly
seen in their native
especially against a
pattern of pebbles,
the latter among the
tide wrack on rocky
FIG. 8.-Nassau Grouper (Epinephelus striatus). Heavily banded phase shores.. The extra(Drawn from a photograph by TOWNSEND) ordinary efficiency
of such strongly
contrasted markings is very well shown in Plate i9, which illustrates the appearance of newly hatched Woodcock chicks in their natural woodland surroundings.
A striking application of the curtain principle is found in the white silken bands with which various spiders decorate their cartwheel snares. In this case, however, the function of the conspicuous design (which might be compared to the patch of whitewash with which the builder marks a newly fitted window-pane) is not to conceal something beyond the web, but to protect something upon it, namely, the spider itself. These devices have been the subject of much careful and detailed field observation, by Major Hingston (265). The points to be noted here are: firstly, that the snare is almost invisible owing to the thinness and transparency of its lines ; and secondly, that the spider is therefore the only thing visible in an undecorated snare. Thus the white zigzag or spiral will be a source of distraction to any enemy hunting by sight. This is what Hingston calls a dispersing device '. 'The enemy, when it flutters in front of the web, is attracted and confused by this wavy spiral and drawn away from the spider's seat.' In order that it may serve this end, the distracting band should be as
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 55
conspicuous as possible, and it is significant that in the webs of different spiders, such as Cyclosa filiobliqua, it forms a most striking feature.
IV. THE RELATION BETWEEN ADJACENT ELEMENTS IN THE PATTERN
In the foregoing pages I have attempted to show that the success of a disruptive design depends upon the obliteration of form by means of superimposed patterns. Various objects included in the visual field are presented to the eye in such a way that they appear to be-more or less clearly segregated as separate patches or masses, differing from one another in colour, tone, shape, size, and texture. When therefore any single object like an animal bears upon its surface a superimposed pattern of contrasted colours and tones, these serve to mask its real form, which is replaced by an apparent but unreal series of separate forms. These forms tend to be interpreted as representing a number of different objects-none
of which suggests, by its shape or arrangement, the body which bears them.
This illusory appearance is an objective underlying all types of disruptive coloration. We have seen that the disruptive effect of a pattern depends, among other factors, upon the range of tones employed. Now a further important factor depends upon their mutual distribution upon the body. To make the point clear, it will be helpful to inquire first what are the visual characteristics of single objects of simple form-as opposed to complex forms seen in the aggregate. Look at a rounded cloud, illuminated from the side, and poised alone in azure space. -It has a light and a shadowed side. At one margin-that directed towards the sun-it is an intensely pure white, or ochreous, or warm rose in hue, according to circumstances. At the other, it is a soft neutral grey, or a dark, cold, slaty purple. At no point on the cloud can you make out abrupt changes of colour or tone, except at its edge. The passage from light to dark tones, and from warm to cold hues, is gradual, and delicately graded. Gentle gradations of tone, easy transitions of colour-these are characteristic of simple rounded surfaces, and by them the eye records and the brain perceives their form.
Compare with this a complex system of cumulus. Here sudden transitions of tint and shade are everywhere apparent at the billowing contours of each formation-where the mountain ranges of vapour are seen piled up one behind another, and where the margins of the nearer masses overlap or eclipse the surface of those beyond. Now what we have said of clouds applies to almost any objects seen in the aggregate, except that the more solid they are, the more irregular they are, and the more varied they are, the more contrasted, confused and complex will be the configuration or pattern presented to the eye. The passages of colour will be harsher, less delicate, and more abrupt; the shadows harder and deeper; the high lights brighter by contrast.
Such effects meet the observant eye everywhere in nature-the golden gleam of sunlight reflected from a curving grass-blade strikes out boldly from the surrounding gloom of shaded herbage and from the yet darker interstices of intense shadow: the fallen leaf, sodden and glistening on the forest floor, has its highest light just where the edge arches over the black tunnel of shade beneath
it : grey lichen tufts clothing the tree trunk with a delicate veil break the bark into a thousand irregular and varied masses of light, shade, and shadow. The same thing is seen in the broken surroundings of a desert, a rocky coast-line, or a shingle beach ; in the varied foliage patterns of leaves and lianas ; in the traceries of twigs or of corals-indeed in the varied patterns presented by different objects anywhere and everywhere.
What I have attempted to stress here is that a broken visual pattern made
FIG. .-Stages in the construction of a disruptive pattern, showing the effect of strong contrasts in tone between adjacent elements in the pattern
up of sudden transitions of colour, sharply contrasted passages of tone, and of irregular shapes of all kinds, are features typically associated in the mind-as a result of visual experience-with the appearance of objects in the aggregate, rather than of single objects of simple form. It therefore follows that the most effective disruptive patterns will be those which present these appearances to the observer's
FIG. io.-Showing the disruptive effect of adjacent contrasted tones (lower diagram) ; as compared with contrasts which are separated by half-tones (upper diagram)
eye. In particular, the maximum effect will be produced when the tones of greatest contrast-that is to say, those representing the highest lights and deepest shades
-occur adjacent to one another in the pattern ; and when the transition from one to the other is sharply defined, without intermediate gradations or half-tones-which
would at once tend to detract from the apparent discontinuity of surface.
These principles are clearly illustrated in Figs. 9 and io. The first shows the way in which a disruptive pattern of the- kind with which we are dealing
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 57
can be built up in progressive stages, each of which tends to be an improvement on its predecessor, and culminating in a very striking disruptive effect whose essential feature is the apposition of black patches contrasted with white ones. The second diagram illustrates the inferior effect produced when the brightest and blackest areas, instead of being adjacent, are separated by graded half-tones.
Now it may be suggested that these are theoretical considerations far removed from questions of animal coloration. Theoretical they are. Yet a little investigation shows that the underlying principles are made use of in the cryptic colour-schemes of innumerable animals. The fact is that in group after group we find disruptive patterns incorporating these very optical devices. Since, so far as I am aware, this is the first time attention has been drawn to the subject, it may be worth while to consider a few examples in greater detail.
Speaking generally, the coloration found among snakes is decidedly cryptic
-largely, no doubt, owing to the need for concealment from-their prey. While in some, such as the green Oxybelis fulgidus and the sandy Cerastes cerastes, this result is achieved solely by colour-harmony and countershading, in very many the deceptive appearance is perfected by the addition of a bold disruptive pattern. The value of such superimposed designs in breaking up the elongated serpentine form is obvious, and it is perhaps especially necessary for large-bodied snakes, whether constrictors like Python molurus or sluggish venomous species like Bitis gabonica. When we examine these patterns it is very interesting to notice that in snakes belonging to widely different families, and inhabiting totally dissimilar surroundings, the optical device under consideration is incorporated in the pattern. Although the designs used show almost infinite variety in the form, colour and arrangement of their component parts, over and over again we see that the lightest element in the pattern is opposed to, and off-sets by contrast, the deepest tones. The diagram (Fig. ii) shows the arrangement in a number of species. Here, for the sake of simplicity, the intermediate or half-tones have been represented by shading, while the deepest and lightest elements are shown respectively by black and white-but it must be remembered that the actual colours used are the cryptic tones of the environment, the 'white' parts, for instance, being actually light brown, buff, ochre, or pale green according to circumstances.
This principle of adjacent contrast is especially marked in head disguises. For instance, Constrictor constrictor (Plate 24) wears a light patch on the upper jaw immediately below the bold dark patch associated with the eye. Similar contrasts are presented by head markings in Python regius, Oxybelis acuminatus (Fig. 32), Bitis gabonica (Plate 25), Bothrops jararacussu, Crotalus confluentus, and many others. In all these the details of the head-mask differ widely, but all agree in the strong contrast effects of adjacent tones. Doubtless some day an analysis of these patterns in terms of embryology and physiology will be forthcoming: but what we are concerned with here is their ecological significance-namely, that the nature of the patterns is such as to employ the very principles by which concealment can be most effectively attained.
If we turn to other groups we find the same pictures repeated over and over again-in the scaly suits of lizards and fishes, in the skins of frogs and toads, in the feathered costumes of birds and the fur coats of mammals, as well as in innumerable insects and other invertebrates belonging to different orders. It would serve no useful purpose to describe these in detail here. Before leaving
FIG. i i.-Diagrams of the disruptive patterns of various snakes : i Python reghtius; 2 Python molurus ; 3 Constrictor constrictor ; 4 Ophibolus doliatus triangulus ; 5 Natrix fasciata ; 6 Bothrops atrox ; 7 Bothrops alternatus ; 8 Crotalus confluentus ; 9 Bothropsjararaca ; io Causus rhombeatus; i i Vipera russellii; iz Bitis arietans
the subject, however, I wish to draw attention to a few striking cases among various groups.
Many frogs, belonging to widely separated families, have the side of the head boldly painted with black or dusky blotches of irregular shape, which serve, as I shall show later, to camouflage the eye. Now it is a very common char-
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 59
acteristic for such species to have the upper or lower adjacent areas pale in hue, or white. In fact, the pattern reproduces, so far as its contrast effects are concerned, that occurring in snakes. This is beautifully seen in wild specimens of the Common Frog (Rana temporaria), where the value of this arrangement for concealment under natural conditions is well demonstrated in the photograph
FIG. I 2.-Cardioglossa gracilis
reproduced on Plate 24. Other examples are furnished by Rana oxyrhynchus, R. stenocephala, R. sphenocephala, Cardioglossa leucomystax, and C. gracilis (Figs. 12 and 31).
Many fishes show the same thing, dark ocular bands, be they longitudinal, diagonal or vertical, being bordered with areas of the lightest tone, which by
FIG. I13.-Eques lanceolatus
contrast enforce and exaggerate the deeply pigmented parts. And, of course, the arrangement is not confined to the head. It may occur on any part of the body, and frequently it also extends on to the fins and tail. Among examples which we have figured the reader is referred to the disguise of Eques lanceolatus (Fig. 13), Chatodon capistratus (Fig. 76), and Epinephelus striatus (Fig. 8).
Similar disruptive effects are seen in many lizards, such as Mabuia dorie
and Gymnodactylus albofasciatus-the former wearing a dark lateral stripe in a light setting; the latter a series of light transverse bars bordered with dark pigment.
Among birds, the Turnstone and the Ringed Plover provide striking examples of the same principle. As already mentioned, both of these species have on the head, throat, and neck a series of strongly contrasted black and white markings, whose function can best be appreciated by observing the actual appearance of these delightful species in nature-especially when brooding. Similar contrast effects are found in the Reeve, Cream-coloured Courser, Stone-Curlew, Bittern, Quail, and many other birds.
The principle is illustrated even better by the newly hatched young of the Ringed Plover. The coloration of these tiny down-clad chicks incorporates
t-', -' ..1-- .C ,.6
FIG. 14.-Nest of Ringed Plover, with eggs and newly-hatched young
in a wonderful way the three fundamental principles of colour-resemblance, countershading, and maximum disruptive contrast. As regards the last, it will be noticed (Plate 13) that the chick has its form optically broken into two pieces by a collar passing through the eyes and round the back of the head. This does not consist merely of a dark band breaking up the intermediate tone of the head and back. On the contrary, this narrow black band is accentuated by another band of white immediately behind it, and it is the combination of the two bands, one black and one white, and both in contrast to the general tone of the upper parts, which produces so deceptive an appearance. With some individuals this effect is repeated at the back of the body-a black mark on the flank dividing the greyish upper parts from the white down of the belly (Fig. 14).
METHODS BY WHICH CONCEALMENT IS ATTAINED IN NATURE 61
Various other nidifugous chicks, like those of the Golden Plover, OysterCatcher, Snipe, and Sandpiper, illustrate the same point. In every case the essential function of the pattern is to prevent recognition by breaking up visible form. In effect, these down-clad babes have rejected the injunction 'Rend your hearts and not your garments'. Instead, by rending their garments they have gone a long way towards saving their hearts from the claws and beaks of Peregrines, Lesser Black-backed Gulls, and other blood-thirsty robbers. How well they have succeeded will be best understood by those who have attempted to locate newly hatched young of birds like the Woodcock, Snipe or OysterCatcher. Indeed, if we except the insects, chicks such as these provide some of the most perfect and admirable examples of camouflage in the whole realm of nature.
Among mammals such complex patterns are less frequently found. However, it may be worth noticing that adjacent contrast effects of the type we are considering occur in a number of cases. For instance, the Brazilian Anteater
FIG. I5.-Larva of Puss Moth (Cerura vinula)
(Myrmecophaga jubata) has the side of its head and body decorated with a characteristic black patch which is bounded above and below by a band distinctly paler than the general tone of the body. On the other hand, the grey-haired Three-toed Sloth (Bradypus tridactylus) has its back broken with a bright orangecoloured patch, which is bordered with a zone of dense black. Here the relation between the dark and light masses has been reversed, but its effect is the same and undoubtedly tends to give concealment amidst the broken surroundings of the forest foliage. Two other mammals which appear to illustrate the same principle in a striking manner (though I cannot speak from any field acquaintance with them) are the Patagonian Cavy (Dolichotis patachonica) and the Vizcacha (Lagostomus trichodactylus) (Fig. 37).
Finally, it must be mentioned that the same phenomenon is of wide occurrence among invertebrate animals which depend upon concealment for safety. It is well seen in the Puss Moth larva (Fig. 15), whose dark saddle of brownishpurple is bordered by a narrow pale margin. This accentuates the disruptive dorsal patch, so that when the caterpillar is seen on its food plant, the misleading superimposed pattern is noticed in preference to the real form of the insect.
V. CONSTRUCTIVE SHADING AND PICTORIAL RELIEF
The principles and examples described above lead imperceptibly to a very subtle and singular type of optical illusion which is of great importance to our subject. The phenomena treated under the heading of Maximum Disruptive Contrast, though extremely effective in breaking up surface continuity, do so in a crude way. At best, flat patches of contrasted tones serve to throw the surface into a number of flat optical planes which to the eye strongly suggest discontinuity. But if, in addition to violent contrast between contiguous elements in the pattern, the individual patches of colour are themselves graded in tone, then at once we introduce a new factor-namely, the false appearance of relief. This illusion of surface modelling, in combination with the disruptive principle, is a brilliant achievement of organic evolution. But it is one to which little attention has been paid by biologists, since Darwin's detailed analysis of the pattern of the Argus Pheasant (136). In display the male exhibits a wonderful series of ocelli on the secondary wing feathers. Each ocellus consists of an intensely black circular ring, enclosing a space shaded so as exactly to resemble a ball illuminated from above-the high-light being represented by a patch of pure white, shading downwards through pale leaden tints into yellowish and brown tones which deepen gradually towards the lower part of the ball. A curious detail in this illusion is produced by the position of the white high-light ': in the inner feathers, which are held perpendicularly, the white marks are uppermost and distal in position ; but in the outer feathers, which are held almost horizontally, the white markings have shifted laterally, so that in spite of their different orientation the ocelli still appear as balls illuminated from above.
Dr. A. D. Imms has recently brought to my notice a paper by Schwanwitsch (560) on the so-called stereoeffect of cryptic colour-patterns. By stereoscopic methods, and by sculptured models, Schwanwitsch has reconstructed and interpreted the wing-patterns of various butterflies in terms of series of elevated and depressed areas, with here and there cast shadows. His methods undoubtedly open up a very promising line of investigation, but I do not follow him in his interpretation of different tones on a flat wing as representing different surfaces lying in the same plane but at different levels. Omitting the question of cast shadows, and considering only the illusion caused by light and shade, contrasted tones on a flat surface (Fig. 16, No. i) are the optical equivalent in relief of surfaces differently inclined towards the source of light (No. z) rather than of surfaces seen at various levels in one plane (No. 3) as shown in his figures. In other words, the optical effect is not one of stepped surfaces, but of sloping surfaces.
Nor does the matter end here. One of the most significant illusory effects produced by individual elements in the disruptive patterns of animals is an arrangement whereby curved, rather than angular relief, is suggested. This results from tones which are graded, as opposed to those just considered, which are uniform. A flat surface of graded tone (Fig. 16, No. 4) is the optical counterpart of a curved surface of uniform colour (Nos- 5 and 6).