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+The Project Gutenberg EBook of The Dancing Mouse, by Robert M. Yerkes
+
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+Title: The Dancing Mouse
+       A Study in Animal Behavior
+
+Author: Robert M. Yerkes
+
+Release Date: August, 2005 [EBook #8729]
+[Yes, we are more than one year ahead of schedule]
+[This file was first posted on August 4, 2003]
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+Edition: 10
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+Language: English
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+Character set encoding: ISO-8859-1
+
+*** START OF THE PROJECT GUTENBERG EBOOK THE DANCING MOUSE ***
+
+
+
+
+Produced by Juliet Sutherland, Michael Oltz,
+Charles Franks and the Online Distributed Proofreading Team.
+
+
+
+
+[Illustration: DANCING MICE--SNIFFING AND EATING.]
+
+
+
+THE ANIMAL BEHAVIOR SERIES. VOLUME I
+
+
+THE DANCING MOUSE
+
+A Study in Animal Behavior
+
+BY
+
+ROBERT M. YERKES, Ph.D.
+INSTRUCTOR IN COMPARATIVE PSYCHOLOGY IN HARVARD
+UNIVERSITY
+
+The Cartwright Prize of the Alumni Association of the College of
+Physicians and Surgeons, Columbia University, was awarded, in 1907, for an
+Essay which comprised the first twelve chapters of this volume.
+
+
+1907
+
+
+
+IN LOVE AND GRATITUDE
+THIS BOOK IS DEDICATED TO
+MY MOTHER
+
+
+
+
+
+PREFACE
+
+
+This book is the direct result of what, at the time of its occurrence,
+seemed to be an unimportant incident in the course of my scientific work--
+the presentation of a pair of dancing mice to the Harvard Psychological
+Laboratory. My interest in the peculiarities of behavior which the
+creatures exhibited, as I watched them casually from day to day, soon
+became experiment-impelling, and almost before I realized it, I was in the
+midst of an investigation of their senses and intelligence.
+
+The longer I observed and experimented with them, the more numerous became
+the problems which the dancers presented to me for solution. From a study
+of the senses of hearing and sight I was led to investigate, in turn, the
+various forms of activity of which the mice are capable; the ways in which
+they learn to react adaptively to new or novel situations; the facility
+with which they acquire habits; the duration of habits; the roles of the
+various senses in the acquisition and performance of certain habitual
+acts; the efficiency of different methods of training; and the inheritance
+of racial and individually acquired forms of behavior.
+
+In the course of my experimental work I discovered, much to my surprise,
+that no accurate and detailed account of this curiously interesting animal
+existed in the English language, and that in no other language were all
+the facts concerning it available in a single book. This fact, in
+connection with my appreciation of the exceptional value of the dancer as
+a pet and as material for the scientific study of animal behavior, has led
+me to supplement the results of my own observation by presenting in this
+little book a brief and not too highly technical description of the
+general characteristics and history of the dancer.
+
+The purposes which I have had in mind as I planned and wrote the book are
+three: first, to present directly, clearly, and briefly the results of my
+investigation; second, to give as complete an account of the dancing mouse
+as a thorough study of the literature on the animal and long-continued
+observation on my own part should make possible; third, to provide a
+supplementary text-book on mammalian behavior and on methods of studying
+animal behavior for use in connection with courses in Comparative
+Psychology, Comparative Physiology, and Animal Behavior.
+
+It is my conviction that the scientific study of animal behavior and of
+animal mind can be furthered more just at present by intensive special
+investigations than by extensive general books. Methods of research in
+this field are few and surprisingly crude, for the majority of
+investigators have been more deeply interested in getting results than in
+perfecting methods. In writing this account of the dancing mouse I have
+attempted to lay as much stress upon the development of my methods of work
+as upon the results which the methods yielded. In fact, I have used the
+dancer as a means of exhibiting a variety of methods by which the behavior
+and intelligence of animals may be studied. As it happens the dancer is an
+ideal subject for the experimental study of many of the problems of animal
+behavior. It is small, easily cared for, readily tamed, harmless,
+incessantly active, and it lends itself satisfactorily to a large number
+of experimental situations. For laboratory courses in Comparative
+Psychology or Comparative Physiology it well might hold the place which
+the frog now holds in courses in Comparative Anatomy.
+
+Gratefully, and with this expression of my thanks, I acknowledge my
+indebtedness to Professor Hugo Münsterberg for placing at my command the
+resources of the Harvard Psychological Laboratory and for advice and
+encouragement throughout my investigation; to Professor Edwin B. Holt for
+valuable assistance in more ways than I can mention; to Professor Wallace
+C. Sabine for generous aid in connection with the experiments on hearing;
+to Professor Theobald Smith for the examination of pathological dancers;
+to Miss Mary C. Dickerson for the photographs of dancing mice which are
+reproduced in the frontispiece; to Mr. Frank Ashmore for additional
+photographs which I have been unable to use in this volume; to Mr. C. H.
+Toll for the drawings for Figures 14 and 20; to Doctors H. W. Rand and C.
+S. Berry for valuable suggestions on the basis of a critical reading of
+the proof sheets; and to my wife, Ada Watterson Yerkes, for constant aid
+throughout the experimental work and in the preparation of this volume.
+
+R. M. Y.
+
+CAMBRIDGE, MASSACHUSETTS,
+
+August, 1907.
+
+
+
+
+
+CONTENTS
+
+
+LIST OF ILLUSTRATIONS
+
+LITERATURE ON THE DANCING MOUSE
+
+
+CHAPTER I
+
+CHARACTERISTICS, ORIGIN, AND HISTORY
+
+Peculiarities of the dancing mouse--Markings and method of keeping record
+of individuals--The dancer in China and Japan (Kishi, Mitsukuri, Hatai)--
+Theories concerning the origin of the race: selectional breeding; the
+inheritance of an acquired character; mutation, inheritance, and
+selectional breeding; pathological changes; natural selection--Instances
+of the occurrence of dancers among other kinds of mice--Results of
+crossing dancer with other kinds of mice.
+
+
+CHAPTER II
+
+FEEDING, BREEDING, AND DEVELOPMENT OF THE YOUNG
+
+Methods of keeping and caring for dancers--Cages, nest-boxes, and
+materials for nest--Cleansing cages--Food supply and feeding--Importance
+of cleanliness, warmth, and pure food--Relations of males and females,
+fighting--The young, number in a litter--Care of young--Course of
+development--Comparison of young of dancer with young of common mouse--
+Diary account of the course of development of a typical litter of dancers.
+
+
+CHAPTER III
+
+BEHAVIOR: DANCE MOVEMENTS
+
+Dancing--Restlessness and excitability--Significance of restlessness--
+Forms of dance: whirling, circling, and figure-eights--Direction of
+whirling and circling: right whirlers, left whirlers, and mixed whirlers--
+Sex differences in dancing--Time and periodicity of dancing--Influence of
+light on activity--Necessity for prolonged observation of behavior.
+
+
+CHAPTER IV
+
+BEHAVIOR: EQUILIBRATION AND DIZZINESS
+
+Muscular coordination--Statements of Cyon and Zoth concerning behavior--
+Control of movements, orientation, equilibration, movement on inclined
+surfaces, climbing--The tracks of the dancer--Absence of visual
+dizziness--Comparison of the behavior of the dancer with that of the
+common mouse when they are rotated in a cyclostat--Behavior of blinded
+dancers (Cyon, Alexander and Kreidl, Kishi)--Cyon's two types of dancer--
+Phenomena of behavior for which structural bases are sought: dance
+movements; lack of response to sounds; deficiency in equilibrational
+ability; lack of visual and rotational dizziness.
+
+
+CHAPTER V
+
+STRUCTURAL PECULIARITIES AND BEHAVIOR
+
+The functions of the ear--Structure of the ear of the dancer as described
+by Rawitz, by Panse, by Baginsky, by Alexander and Kreidl, and by Kishi--
+Cyon's theory of the relation of the semicircular canals to space
+perception--Condition of the auditory organs--Condition of the
+equilibrational organs--Condition of the sound-transmitting organs--The
+bearing of the results of anatomical investigations upon the facts of
+behavior.
+
+
+CHAPTER VI
+
+THE SENSE OF HEARING
+
+Experiments on hearing in the dancer made by Rawitz, by Panse, by Cyon, by
+Alexander and Kreidl, by Zoth, and by Kishi--Hearing and the voice--
+Methods of testing sensitiveness to sounds--Results of tests with adults--
+Importance of indirect method of experimentation--Results of tests with
+young--The period of auditory sensitiveness--Individual differences.
+
+
+CHAPTER VII
+
+THE SENSE OF SIGHT: BRIGHTNESS VISION
+
+What is known concerning sight in the dancer--Brightness vision and color
+vision--Methods of testing brightness vision, the visual discrimination
+apparatus--Motives for discrimination and choice--Punishment versus reward
+as an incentive in animal experiments--Hunger as an incentive--An electric
+stimulus as an incentive--Conditions for brightness vision tests--
+White-black vision--Evidence of preference--Check experiments--Conclusion.
+
+
+CHAPTER VIII
+
+
+THE SENSE OF SIGHT: BRIGHTNESS VISION (_Continued_)
+
+The delicacy of brightness discrimination--Methods of testing the dancer's
+ability to detect slight differences in brightness--Results of tests with
+gray papers--Relation of intensity of visual stimuli to the threshold of
+discrimination--Weber's law apparatus and method of experimentation--
+Results of Weber's law tests--Practice effects, the training of vision--
+Description of the behavior of the dancer in the discrimination box
+experiments--Modes of choice: by affirmation; by negation; by comparison--
+Evidence of indiscriminable visual conditions.
+
+
+CHAPTER IX
+
+THE SENSE OF SIGHT: COLOR VISION
+
+Does the dancer see colors?--The food-box method of testing color vision--
+Waugh's food-box method--Results of tests--Tests by the use of colored
+papers in the visual discrimination box--Yellow-red vision--Blue-orange
+vision--Brightness vision _versus_ color vision--Brightness check
+tests--Green-blue vision--Violet-red vision--Conclusions.
+
+
+CHAPTER X
+
+THE SENSE OF SIGHT: COLOR VISION (_Continued_)
+
+The use of color filters--Testing color vision by the use of transmitted
+light--Green-blue vision--Green-red vision--Blue-red vision--Stimulating
+value of different portions of the spectrum--Does red appear darker to the
+dancer than to us?--Conclusions concerning color vision--Structure of the
+retina of the dancer and its significance.
+
+
+CHAPTER XI
+
+THE ROLE OF SIGHT IN THE DAILY LIFE OF THE DANCER.
+
+Sight and general behavior--Behavior of blinded dancers--Experimental
+tests of ability to perceive form--Visual guidance in mazes--Following
+labyrinth paths in the dark--The relative importance of visual, olfactory,
+and kinaesthetic stimuli--Conditions for the acquisition of a motor
+habit--Conditions for the execution of an habitual act.
+
+
+CHAPTER XII
+
+EDUCABILITY: METHODS OF LEARNING
+
+The modifiability of behavior--Educational value of experimental studies
+of modifiability--Methods: the problem method; the labyrinth method; the
+discrimination method--Relation of method to characteristics of animal--
+Simple test of the docility of the dancer--Lack of imitative tendency--
+Persistence of useless acts--Manner of profiting by experience--Individual
+differences in initiative.
+
+
+CHAPTER XIII
+
+HABIT FORMATION: THE LABYRINTH HABIT
+
+The labyrinth method--Problems--Preliminary tests--Comparison of the
+behavior of the dancer in a maze with that of the common mouse--Evolution
+of a labyrinth method--Records of time and records of errors--Simple and
+effective method of recording the path--Curves of habit formation--Regular
+and irregular labyrinths--Points for a standard labyrinth--Values and
+defects of the labyrinth method.
+
+
+CHAPTER XIV
+
+HABIT FORMATION: THE DISCRIMINATION METHOD
+
+Quantitative _versus_ qualitative results--Motives--Precautions--
+Preference--Results of systematic habit-forming experiments--Curves of
+habit formation--Meaning of irregularity in curve--Individual
+differences--Comparison of curves for discrimination habits with those for
+labyrinth habits--Averages--The index of modifiability as a measure of
+docility--Reliability of the index.
+
+
+CHAPTER XV
+
+THE EFFICIENCY OF TRAINING METHODS
+
+Importance of measuring the efficiency of educational methods--Rapidity of
+learning and permanency of modifications wrought by training--Results of a
+study of the efficiency of discrimination methods--Comparison by means of
+indices of modifiability--Number of tests per series versus number of
+series--Efficiency as measured by memory tests.
+
+
+CHAPTER XVI
+
+THE DURATION OF HABITS: MEMORY AND RE-LEARNING
+
+Measures of the permanency of modifications in behavior--The duration of
+brightness and color discrimination habits--The relation of learning to
+re-learning--Can a habit which has been lost completely be re-acquired
+with greater facility than it was originally acquired?--Relation of
+special training to general efficiency--Does the training in one form of
+labyrinth aid the dancer in acquiring other labyrinth habits?
+
+
+CHAPTER XVII
+
+INDIVIDUAL, AGE, AND SEX DIFFERENCES IN BEHAVIOR
+
+Individual peculiarities in sensitiveness, docility, and initiative--The
+relation of docility to age--The individual result and the average--How
+averages conceal facts--Sex differences in docility and initiative--
+Individual differences of motor capacity which seem to indicate
+varieties--Is the dancer pathological?
+
+
+CHAPTER XVIII
+
+THE INHERITANCE OF FORMS OF BEHAVIOR
+
+Characteristics of the race--Inheritance of the tendency to whirl in a
+particular way--Tests of the inheritance of individually acquired forms of
+behavior.
+
+
+INDEX
+
+
+
+
+
+
+ILLUSTRATIONS
+
+Dancing Mice--sniffing and eating    _Frontispiece_
+
+FIGURE
+
+1. Color patterns of dancers. Record blanks
+
+2. Double cage, with nest-boxes and water dishes
+
+3. Double cages in frame
+
+4. Photographs of dancers climbing (After Zoth)
+
+5. Tracks of common mouse (After Alexander and Kreidl)
+
+6. Tracks of dancer (After Alexander and Kreidl)
+
+7. The inner ear of the rabbit (Retzius)
+
+8. The membranous labyrinth of the ear of the dancer (After Rawitz)
+
+9. Same
+
+10. Same
+
+it. Model of the ear of the dancer (After Baginsky)
+
+12. Ear of the dancer (After Kishi)
+
+13. Ear of the dancer (After Kishi)
+
+14. Discrimination box
+
+15. Ground plan of discrimination box
+
+16. Nendel's gray papers
+
+17. Weber's law apparatus
+
+18. Food-box apparatus
+
+19. Waugh's food-box apparatus
+
+20. Color discrimination apparatus
+
+21. Ground plan of color discrimination apparatus
+
+22. Cards for form discrimination
+
+23. Labyrinth B
+
+24. Labyrinth B on electric wires
+
+25. Labyrinth A
+
+26. Curves of habit formation for labyrinth B
+
+27. Plan of labyrinth C, and path records
+
+28. Labyrinth D
+
+29. Curve of learning for white-black discrimination, twenty individuals
+
+30. Curve of learning for white-black discrimination, thirty individuals
+
+31. Curve of habit formation for labyrinth D
+
+32. Curves of learning and re-learning
+
+33. Plasticity curves
+
+
+
+
+
+LITERATURE ON THE DANCING MOUSE
+
+1. ALEXANDER, G. UND KREIDL, A. "Zur Physiologie des Labyrinths der
+Tanzmaus." _Archiv für die gesammte Physiologie_, Bd. 82: 541-552.
+1900.
+
+2. ALEXANDER, G. UND KREIDL, A. "Anatomisch-physiologische Studien über
+das Ohrlabyrinth der Tanzmaus." II Mittheilung. _Archiv für die gesammte
+Physiologie_. Bd. 88: 509-563. 1902.
+
+3. ALEXANDER, G. UND KREIDL, A. "Anatomisch-physiologische Studien über
+das Ohrlabyrinth der Tanzmaus." III Mittheilung. _Archiv für die
+gesammte Physiologie_, Bd. 88: 564-574. 1902.
+
+4. BAGINSKY, B. "Zur Frage über die Zahl der Bogengänge bei japanischen
+Tanzmäusen." _Centralblatt für Physiologie_, Bd. 16: 2-4. 1902.
+
+5. BATESON, W. "The present state of knowledge of colour-heredity in mice
+and rats." _Proceedings of the Zoölogical Society of London_, Vol. 2:
+71-99. 1903.
+
+6. BREHM, A. E. "Tierleben." Dritte Auflage. Saugetiere, Bd. 2: 513-514.
+1890.
+
+7. BREHM, A. E. "Life of Animals." Translated from the third German
+edition of the "Tierleben" by G. R. Schmidtlein. Mammalia, p. 338.
+Marquis, Chicago. 1895.
+
+8. CYON, E. DE. "Le sens de l'espace chez les souris dansantes
+japonaises." _Cinquantenaire de la Société de Biologie_ (Volume
+jubilaire). p. 544-546. Paris. 1899.
+
+9. CYON, E. VON. "Ohrlabyrinth, Raumsinn und Orientirung." _Archiv für
+die gesammte Physiologie_, Bd. 79: 211-302. 1900.
+
+10. CYON, E. DE. "Presentation de souris dansantes japonaises." _Comptes
+rendus du XIII Congrès International de Paris, Section de
+physiologie_, p. 160-161. 1900.
+
+11. CYON, E. VON. "Beiträge zur Physiologie des Raumsinns." I Theil. "Neue
+Beobachtungen an den japanischen Tanzmäusen." _Archiv für die gesammte
+Physiologie_, Bd. 89: 427-453. 1902.
+
+12. CYON, E. DE. "Le sens de l'espace." Richet's "Dictionnaire de
+physiologie," T. 5: 570-571. 1901.
+
+13. DARBISHIRE, A. D. Note on the results of crossing Japanese waltzing
+mice with European albino races. _Biometrica_, Vol. 2: 101-104. 1902.
+
+14. DARBISHIRE, A. D. Second report on the result of crossing Japanese
+waltzing mice with European albino races. _Biometrica_, Vol.2:
+165-173. 1903.
+
+15. DARBISHIRE, A. D. Third report on hybrids between waltzing mice and
+albino races. _Biometrica_, Vol. 2: 282-285. 1903.
+
+16. DARBISHIRE, A. D. On the result of crossing Japanese waltzing with
+albino mice. _Biometrica_, Vol 3: 1-51. 1904.
+
+17. GUAITA, G. v. "Versuche mit Kreuzungen von verschiedenen Rassen der
+Hausmaus." _Berichte der naturforschenden Gesellschaft zu Freiburg i.
+B_., Bd. 10: 317-332. 1898.
+
+18. GUAITA, G. v. "Zweite Mitteilung uber Versuche mit Kreuzungen von
+verschiedenen Hausmausrassen." _Berichte der naturforschenden
+Gesellschaft zu Freiburg i. B_., Bd. 11: 131-138. 1900.
+
+19. HAACKE, W. "Ueber Wesen, Ursachen und Vererbung von Albinismus und
+Scheckung und über deren Bedeutung für vererbungstheoretische und
+entwicklungsmechanische Fragen." _Biologisches Centralblatt_, Bd.
+15: 44-78. 1895.
+
+19a. HUNTER, M. S. "A Pair of Waltzing Mice." _The Century Magazine_,
+Vol. 73: 889-893. April, 1907.
+
+20. KAMMERER, P. "Tanzende Waldmaus und radschlagende Hausmaus."
+_Zoölogische Garten_, Bd. 41: 389-390. 1900.
+
+21. KISHI, K. "Das Gehörorgan der sogenannten Tanzmaus." _Zeitschrift
+für wissenschaftliche Zoölogie_, Bd. 71: 457-485. 1902.
+
+22. LANDOIS, H. "Chinesische Tanzmäuse." _Jahresbericht des Westfähschen
+Provinzial-Vereins_, Munster, 1893-1894: 62-64.
+
+22a. LOSE, J. "Waltzing Mice." _Country Life in America_, September,
+1904. p. 447.
+
+23. PANSE, R. Zu Herrn Bernhard Rawitz' Arbeit: "Das Gehörorgan der
+japanischen Tanzmäuse." _Archiv für Anatomie und Physiologie_,
+Physiologische Abtheilung, 1901: 139-140.
+
+24. PANSE, R. "Das Gleichgewichts- und Gehörorgan der japanischen
+Tanzmäuse." _Münchener medicinische Wochenschrift_, Jahrgang 48, Bd.
+I: 498-499. 1901.
+
+25. RAWITZ, B. "Das Gehörorgan der japanischen Tanzmäuse." _Archiv für
+Anatomie und Physiologie_, Physiologische Abtheilung, 1899: 236-243.
+
+26. RAWITZ, B. "Neue Beobachtungen über das Gehörorgan japanischer
+Tanzmäuse." _Archiv für Anatomie und Physiologie_, Physiologische
+Abtheilung, 1901, Supplement: 171-176.
+
+27. RAWITZ, B. "Zur Frage über die Zahl der Bogengänge bei japanischen
+Tanzmäusen." _Centralblatt für Physiologie_, Bd. 15: 649-651. 1902.
+
+28. SAINT-LOUP, R. "Sur le mouvement de manège chez les souris."
+_Bulletin de la Société Zoölogique de France_, T. 18: 85-88. 1893.
+
+29. SCHLUMBERGER, C. "A propos d'un netzukè japonais." _Memoires de la
+Société Zoölogique de France_, T. 7: 63-64. 1894.
+
+30. WELDON, W. F. R. Mr. Bateson's revisions of Mendel's theory of
+heredity. _Biometrica_, Vol. 2: 286-298. 1903.
+
+31. ZOTH, O. "Ein Beitrag zu den Beobachtungen und Versuchen an
+japanischen Tanzmäusen." _Archiv für die gesammte Physiologie_, Bd.
+86: 147-176. 1901.
+
+32. ANONYMOUS. "Fancy Mice: Their Varieties, Management, and Breeding."
+Fourth edition. London: L. Upcott Gill. No date.
+
+
+
+
+
+
+
+
+CHAPTER I
+
+
+CHARACTERISTICS, ORIGIN, AND HISTORY
+
+The variety of mouse which is known as the Japanese dancing or waltzing
+mouse has been of special interest to biologists and to lovers of pets
+because of its curious movements. Haacke in Brehm's "Life of Animals" (7
+p. 337)[1] writes as follows concerning certain mice which were brought to
+Europe from China and Japan: "From time to time a Hamburg dealer in
+animals sends me two breeds of common mice, which he calls Chinese
+climbing mice (Chinesische Klettermäuse) and Japanese dancing mice
+(Japanische Tanzmäuse). It is true that the first are distinguished only
+by their different colors, for their climbing accomplishments are not
+greater than those of other mice. The color, however, is subject to many
+variations. Besides individuals of uniform gray, light yellow, and white
+color, I have had specimens mottled with gray and white, and blue and
+white. Tricolored mice seem to be very rare. It is a known fact that we
+also have white, black, and yellow mice and occasionally pied ones, and
+the Chinese have profited by these variations of the common mouse also, to
+satisfy their fancy in breeding animals. The Japanese, however, who are no
+less enthusiastic on this point, know how to transform the common mouse
+into a really admirable animal. The Japanese dancing mice, which perfectly
+justify their appellation, also occur in all the described colors. But
+what distinguishes them most is their innate habit of running around,
+describing greater or smaller circles or more frequently whirling around
+on the same spot with incredible rapidity. Sometimes two or, more rarely,
+three mice join in such a dance, which usually begins at dusk and is at
+intervals resumed during the night, but it is usually executed by a single
+individual."
+
+[Footnote 1: The reference numbers, of which 7 is an example, refer to the
+numbers in the bibliographic list which precedes this chapter.]
+
+As a rule the dancing mouse is considerably smaller than the common mouse,
+and observers agree that there are also certain characteristic
+peculiarities in the shape of the head. One of the earliest accounts of
+the animal which I have found, that of Landois (22 p. 62), states,
+however, that the peculiarities of external form are not remarkable.
+Landois further remarks, with reason, that the name dancing mouse is ill
+chosen, since the human dance movement is rather a rhythmic hopping motion
+than regular movement in a circle. As he suggests, they might more
+appropriately be called "circus course mice" (22 p. 63).
+
+Since 1903 I have had under observation constantly from two to one hundred
+dancing mice. The original pair was presented to the Harvard Psychological
+Laboratory by Doctor A.G. Cleghorn of Cambridge. I have obtained
+specimens, all strikingly alike in markings, size, and general behavior,
+from animal dealers in Washington, Philadelphia, and Boston. Almost all of
+the dancers which I have had, and they now number about four hundred, were
+white with patches, streaks, or spots of black. The black markings
+occurred most frequently on the neck, ears, face, thighs, hind legs, about
+the root of the tail, and occasionally on the tail itself. In only one
+instance were the ears white, and that in the case of one of the offspring
+of a male which was distinguished from most of his fellows by the
+possession of one white ear. I have had a few individuals whose markings
+were white and gray instead of white and black.
+
+The method by which I was able to keep an accurate record of each of my
+dancers for purposes of identification and reference is illustrated in
+Figure 1. As this method has proved very convenient and satisfactory, I
+may briefly describe it. With a rubber stamp[1] a rough outline of a
+mouse, like that of Figure 1 A, was made in my record book. On this
+outline I then indicated the black markings of the individual to be
+described. Beside this drawing of the animal I recorded its number,
+sex,[2] date of birth, parentage, and history. B, C, and D of Figure 1
+represent typical color patterns. D indicates the markings of an
+individual whose ears were almost entirely white. The pattern varies so
+much from individual to individual that I have had no trouble whatever in
+identifying my mice by means of such records as these.
+
+[Footnote 1: For the use of the plate from which this stamp was made, I am
+indebted to Professor W.E. Castle, who in turn makes acknowledgment to
+Doctor G.M. Allen for the original drawing.]
+
+[Footnote 2: I have found it convenient to use the even numbers for the
+males and the odd numbers for the females. Throughout this book this usage
+is followed. Wherever the sex of an individual is not specially given, the
+reader therefore may infer that it is a male if the number is even; a
+female if the number is odd.]
+
+All of my dancers had black eyes and were smaller as well as weaker than
+the albino mouse and the gray house mouse. The weakness indicated by their
+inability to hold up their own weight or to cling to an object curiously
+enough does not manifest itself in their dancing; in this they are
+indefatigable. Frequently they run in circles or whirl about with
+astonishing rapidity for several minutes at a time. Zoth (31 p. 173), who
+measured the strength of the dancer in comparison with that of the common
+mouse, found that it can hold up only about 2.8 times its own weight,
+whereas the common white mouse can hold up 4.4 times its weight. No other
+accurate measurements of the strength, endurance, or hardiness of the
+dancer are available. They are usually supposed to be weak and delicate,
+but my own observations cause me to regard them as exceptionally strong in
+certain respects and weak in others.
+
+[Illustration: FIGURE I.--Typical markings of dancers. A, blank outline of
+mouse for record. B, markings of No. 2 [symbol for male], born September
+7, 1905, of unknown parents, died March 30,1907. C, markings of No 43
+[symbol for female], born November 10, 1906, of 212 and 211.  D,
+markings of No. 151 [symbol for female], born February 28, 1906, of 1000
+and 5, died February 26, 1907.]
+
+What the Japanese have to say about the dancing mouse is of special
+importance because Japan is rather commonly supposed to be its home. For
+this reason, as well as because of the peculiar interest of the facts
+mentioned, I quote at length from Doctor Kishi (21 p. 457). "The dancing
+mouse has received in Europe this name which it does not bear in its own
+home, because of the fact that the circular movements which it makes are
+similar to the European (human) dance. Sometimes it is also called the
+Japanese or Chinese mouse; originally, however, China must have been its
+home, since in Japan it is mostly called '_Nankin nesumi_,' the mouse from
+Nankin. When this animal came from China to Japan I shall inquire at a
+later opportunity. There were originally in Japan two different species of
+mouse, the gray and the white; therefore in order to distinguish our
+dancing mouse from these it was necessary to use the name of its native
+city.
+
+"In Japan, as in Europe, the animal lives as a house animal in small
+cages, but the interest which is taken in it there is shown in quite
+another way than in Europe, where the whirling movements, to which the
+name dancing mouse is due, are of chief interest. For this reason in
+Europe it is given as much room as possible in its cage that it may dance
+conveniently. In Japan also the circular movements have been known for a
+long time, but this has had no influence upon our interest in the animal,
+for the human fashion of dancing with us is quite different from that in
+Europe. What has lent interest to the creature for us are its prettiness,
+its cleverness in tricks, and its activity. It is liked, therefore, as an
+amusement for children. For this purpose it is kept in a small cage,
+usually fifteen centimeters square, sometimes in a somewhat broader wooden
+box one of whose walls is of wire netting. In this box are built usually a
+tower, a tunnel, a bridge, and a wheel. The wheel is rather broad, being
+made in the form of a drum and pierced with holes on one side through
+which the animal can slip in and out. Running around on the inside, the
+mouse moves the wheel often for hours at a time, especially in the
+evening. Moreover, there are found in the box other arrangements of
+different kinds which may be set in motion by the turning of the wheel. No
+space remains in the box in which the animal may move about freely, and
+therefore one does not easily or often have an opportunity to observe that
+the animal makes circular movements, whether voluntarily or involuntarily.
+This is the reason that in its home this interesting little animal has
+never been studied by any one in this respect."
+
+It is odd indeed that the remarkable capacity of the dancer for the
+execution of quick, graceful, dextrous, bizarre, and oft-repeated
+movements has not been utilized in America as it has in Japan. The mice
+are inexhaustible sources of amusement as well as invaluable material for
+studies in animal behavior and intelligence.
+
+Concerning the origin and history of this curious variety of mouse little
+is definitely known. I have found no mention of the animal in scientific
+literature previous to 1890. The fact that it is called the Chinese
+dancing mouse, the Japanese dancing mouse, and the Japanese waltzing mouse
+is indicative of the existing uncertainty concerning the origin of the
+race.
+
+Thinking that Japanese literature might furnish more information bearing
+on the question of racial history than was available from European
+sources, I wrote to Professor Mitsukuri of the University of Tokyo, asking
+him whether any reliable records of the dancer existed in Japan. He
+replied as follows: "I have tried to find what is known in Japan about the
+history of the Japanese waltzing mice, but I am sorry to say that the
+results are wholly negative. I cannot find any account of the origin of
+this freak, either authentic or fictitious, and, strange as it may seem to
+you, no study of the mice in a modern sense has been made, so you may
+consider the literature on the mouse in the Japanese language as
+absolutely _nil_." In explanation of this somewhat surprising ignorance of
+the origin of the race in what is commonly supposed to be its native land,
+Professor Mitsukuri adds: "The breeders of the mice have mostly been
+ignorant men to whom writing is anything but easy."
+
+In response to similar inquiries, I received the following letter,
+confirmatory of Professor Mitsukuri's statements, from Doctor S. Hatai of
+Wistar Institute, Philadelphia: "If I remember rightly the so-called
+Japanese dancing mouse is usually called by us _Nankin-nedzumi_. _Nankin_
+means anything which has been imported from China, and _nedzumi_ means
+rat-like animal, or in this case mouse, or Chinese mouse. I referred to
+one of the standard Japanese dictionaries and found the following
+statement: 'The _Nankin-nedzumi_ is one of the varieties of _Mus
+spiciosus_ (_Hatszuka-nedzumi_), and is variously colored. It was imported
+from China. These mice are kept in cages for the amusement of children,
+who watch their play.' _Mus spiciosus_, if I remember correctly, is very
+much like _Mus musculus_ in color, size, and several other
+characteristics, if not the same altogether."
+
+In Swinhoe's list of the mammals of China, which appeared in the
+_Proceedings of the Zoological Society of London_ for 1870, _Mus musculus
+L_. is mentioned as occurring in houses in South China and in Formosa. It
+is further stated that black and white varieties which are brought from
+the Straits are often kept by the Chinese (p. 637).
+
+The statements of Kishi, Mitsukuri, and Hatai which have been quoted,
+taken in connection with the opinions expressed by various European
+scientists who have studied the dancer, make it seem highly probable that
+the race appeared first in China, and was thence introduced into Japan,
+from which country it has been brought to Europe and America. Accepting
+for the present this conclusion with reference to the place of origin of
+the dancer, we may now inquire, how and when did this curious freak, as
+Professor Mitsukuri has called it, come into existence? Concerning these
+matters there is wide divergence of opinion.
+
+Haacke (6 p. 514), as quoted in Brehm's "Tierleben," says that an animal
+dealer with whom he discussed the question of the possible origin of the
+dancer maintained that it came from Peru, where it nests in the full
+cotton capsules, arranging the cotton fibers in the form of a nest by
+running about among them in small circles. Hence the name cotton mouse is
+sometimes applied to it. Haacke himself believes, however, that the race
+originated either in China or Japan as the result of systematic
+selectional breeding. Of this he has no certainty, for he states that he
+failed to find any literature on the "beautiful mice of China and Japan."
+Whether Haacke's description of the dancing mouse was published elsewhere
+previous to its appearance in Brehm's "Tierleben" I am unable to state; I
+have found nothing written on the subject by him before 1890. Zoth (31 p.
+176) also thinks that the race was developed by systematic breeding, or in
+other words, that it is a product of the skill of the Asiatic animal
+breeders.
+
+Another account of the origin of the race is that accepted by Kishi (21 p.
+481) and some other Japanese biologists. It is their belief that the forms
+of movement acquired by the individual as the result of confinement in
+narrow cages are inherited. Thus centuries of subjection to the conditions
+which Kishi has described (p. 6) finally resulted in a race of mice which
+breed true to the dance movement. It is only fair to add, although Kishi
+does not emphasize the fact, that in all probability those individuals in
+which the dancing tendency was most pronounced would naturally be selected
+by the breeders who kept these animals as pets, and thus it would come
+about that selectional breeding would supplement the inheritance of an
+acquired character. Few indeed will be willing to accept this explanation
+of the origin of the dancer so long as the inheritance of acquired
+characters remains, as at present, unproved.
+
+Still another mode of origin of the mice is suggested by the following
+facts. In 1893 Saint Loup (28 p. 85) advanced the opinion that dancing
+individuals appear from time to time among races of common mice. The
+peculiarity of movement may be due, he thinks, to an accidental nervous
+defect which possibly might be transmissible to the offspring of the
+exceptional individual. Saint Loup for several months had under
+observation a litter of common mice whose quick, jerky, nervous movements
+of the head, continuous activity, and rapid whirling closely resembled the
+characteristic movements of the true dancers of China. He states that
+these mice ran around in circles of from 1 to 20 cm. in diameter. They
+turned in either direction, but more frequently to the left, that is,
+anticlockwise. At intervals they ran in figure-eights ([Symbol: figure
+eight]) as do the true dancers. According to Saint Loup these exceptional
+individuals were healthy, active, tame, and not markedly different in
+general intelligence from the ordinary mouse. One of these mice produced a
+litter of seven young, in which, however, none of the peculiarities of
+behavior of the parents appeared.
+
+In view of this proof of the occurrence of dancing individuals among
+common mice, Saint Loup believes that the race of dancers has resulted
+from the inheritance and accentuation of an "accidental" deviation from
+the usual mode of behavior. It is scarcely necessary to say that this
+opinion would be of far greater weight had he observed, instead of
+postulating, the inheritance of the peculiarities of movement which he has
+described. It might be objected, to the first of his so-called facts, that
+the litter resulted from the mating of mice which possessed dancer blood.
+Until the occurrence of dancers among varieties of mice which are known to
+be unmixed with true dancers is established, and further, until the
+inheritance of this peculiar deviation from the normal is proved, Saint
+Loup's account of the origin of the dancing mouse race must be regarded as
+an hypothesis.
+
+The occurrence of dancing individuals among common mice has been recorded
+by several other observers. Kammerer (20 p. 389) reports that he found a
+litter of young wood mice (_Mus sylvaticus L_.) which behaved much as do
+the spotted dancers of China. He also observed, among a lot of true
+dancers, a gray individual which, instead of spinning around after the
+manner of the race, turned somersaults at frequent intervals. It is
+Kammerer's opinion, as a result of these observations, that the black and
+white dancers of China and Japan have been produced by selectional
+breeding on the basis of this occasional tendency to move in circles.
+Among albino mice Rawitz (25 p. 238) has found individuals which whirled
+about rapidly in small circles. He states, however, that they lacked the
+restlessness of the Chinese dancers. Some shrews (_Sorex vulgaris L_.)
+which exhibited whirling movements and in certain other respects resembled
+the dancing mouse were studied for a time by Professor Häcker of Freiburg
+in Baden, according to a report by von Guaita (17 p. 317, footnote).
+Doctor G. M. Allen of Cambridge has reported to me that he noticed among a
+large number of mice kept by him for the investigation of problems of
+heredity[1] individuals which ran in circles; and Miss Abbie Lathrop of
+Granby, Massachusetts, who has raised thousands of mice for the market,
+has written me of the appearance of an individual, in a race which she
+feels confident possessed no dancer blood, which whirled and ran about in
+small circles much as do the true dancers.
+
+[Footnote 1: Allen, G.M. "The Heredity of Coat Color in Mice." Proc. Amer.
+Academy, Vol. 40, 59-163, 1904.]
+
+Although it is possible that some of these cases of the unexpected
+appearance of individuals with certain of the dancer's peculiarities of
+behavior may have been due to the presence of dancer blood in the parents,
+it is not at all probable that this is true of all of them. We may,
+therefore, accept the statement that dancing individuals now and then
+appear in various races of mice. They are usually spoken of as freaks,
+and, because of their inability to thrive under the conditions of life of
+the race in which they happen to appear, they soon perish.
+
+Another and a strikingly different notion of the origin of the race of
+dancers from those already mentioned is that of Cyon (11 p. 443) who
+argues that it is not a natural variety of mouse, as one might at first
+suppose it to be, but instead a pathological variation. The pathological
+nature of the animals is indicated, he points out, by the exceptionally
+high degree of variability of certain portions of the body. According to
+this view the dancing is due to certain pathological structural conditions
+which are inherited. Cyon's belief raises the interesting question, are
+the mice normal or abnormal, healthy or pathological? That the question
+cannot be answered with certainty off-hand will be apparent after we have
+considered the facts of structure and function which this volume presents.
+
+Everything organic sooner or later is accounted for, in some one's mind,
+by the action of natural selection. The dancing mouse is no exception, for
+Landois (22 p. 62) thinks that it is the product of natural selection and
+heredity, favored, possibly, by selectional breeding in China. He further
+maintains that the Chinese dancer is a variety of _Mus musculus L._ in
+which certain peculiarities of behavior appear because of bilateral
+defects in the brain. This author is not alone in his belief that the
+brain of the dancer is defective, but so far as I have been able to
+discover he is the only scientist who has had the temerity to appeal to
+natural selection as an explanation of the origin of the race.
+
+Milne-Edwards, as quoted by Schlumberger (29 p. 63), is of the opinion
+that the Chinese dancer is not a natural wild mouse race, but instead the
+product of rigid artificial selection. And in connection with this
+statement Schlumberger describes a discovery of his own which seems to
+have some bearing upon the problem of origin. In an old Japanese wood
+carving which came into his possession he found a group of dancing mice.
+The artist had represented in minute detail the characteristics of the
+members of the group, which consisted of the parents and eight young. The
+father and mother as well as four of the little mice are represented as
+white spotted with black. Of the four remaining young mice, two are
+entirely black and two entirely white. The two pure white individuals have
+pink eyes, as has also the mother. The eyes of all the others are black.
+From these facts Schlumberger infers that the dancer has resulted from the
+crossing of a race of black mice with a race of albinos; the two original
+types appear among the offspring in the carving.
+
+Experimental studies of the inheritance of the tendency to dance are of
+interest in their bearing upon the question of origin. Such studies have
+been made by Haacke (19), von Guaita (17, 18), and Darbishire (13, 14, 15,
+16), and the important results of their investigations have been well
+summarized by Bateson (5).
+
+By crossing dancing mice with common white mice both Haacke and von Guaita
+obtained gray or black mice which are very similar to the wild house mouse
+in general appearance and behavior. The characteristic movements of the
+dancers do not appear. As the result of a long series of breeding
+experiments, Darbishire (16 pp. 26, 27) says: "When the race of waltzing
+mice is crossed with albino mice which do not waltz, the waltzing habit
+disappears in the resulting young, so that waltzing is completely
+recessive in Mendel's sense; the eye-color of the hybrids is always dark;
+the coat-color is variable, generally a mixture of wild-gray and white,
+the character of the coat being distinctly correlated with characters
+transmitted both by the albino and by the colored parent." When hybrids
+produced by the cross described by Darbishire are paired, they produce
+dancers in the proportion of about one to five.
+
+Bateson (5 p. 93, footnote), in discussing the results obtained by Haacke,
+von Guaita, and Darbishire, writes: "As regards the waltzing character,
+von Guaita's experiments agree with Darbishire's in showing that it was
+always recessive to the normal. No individual in F1 [thus the first hybrid
+generation is designated] or in families produced by crossing F1 with the
+pure normal, waltzed. In Darbishire's experiments F1 x F1 [first hybrids
+mated] gave 8 waltzers in 37 offspring, indicating 1 in 4 as the probable
+average. From von Guaita's matings in the form DR x DR the totals of
+families were 117 normal and 21 waltzers.... There is therefore a large
+excess of normals over the expected 3 to 1. This is possibly due to the
+delicacy of the waltzers, which are certainly much more difficult to rear
+than normals are. The small number in von Guaita's litters makes it very
+likely that many were lost before such a character as this could be
+determined."
+
+Bateson does not hazard a guess at the origin of the dancer, but merely
+remarks (5 p. 86) that the exact physiological basis of the dancing
+character is uncertain and the origin of this curious variation in
+behavior still more obscure. "Mouse fanciers have assured me," he
+continues, "that something like it may appear in strains inbred from the
+normal type, though I cannot find an indubitable case. Such an occurrence
+may be nothing but the appearance of a rare recessive form. Certainly it
+is not a necessary consequence of inbreeding, witness von Guaita's long
+series of inbred albinos." (von Guaita (17 p. 319) inbred for twenty-eight
+generations.)
+
+From the foregoing survey of the available sources of information
+concerning the origin and history of the race of dancing mice the
+following important facts appear. There are four theories of the origin of
+the race: (1) origin by selectional breeding (Haacke, Zoth, Milne-
+Edwards); (2) origin through the inheritance of an acquired character
+(Kishi); (3) origin by mutation, inheritance, and selectional breeding
+(Saint Loup, Kammerer, Cyon); (4) origin by natural selection, and
+inheritance, favored by selectional breeding (Landois). Everything
+indicates that the race originated in China. It is fairly certain that
+individuals with a tendency to move in circles appear at rare intervals in
+races of common mice. It seems highly probable, in view of these facts,
+that the Chinese took advantage of a deviation from the usual form of
+behavior to develop by means of careful and patient selectional breeding a
+race of mice which is remarkable for its dancing. Even if it should be
+proved that the mutation as it appears among common mice is not inherited,
+the view that slight deviations were taken advantage of by the breeders
+would still be tenable. The dancing tendency is such in nature as to unfit
+an individual for the usual conditions of mouse existence, hence, in all
+probability human care alone could have produced and preserved the race of
+dancers.
+
+In answer to the question, how and when did the race of dancers originate,
+it may be said that historical research indicates that a structural
+variation or mutation which occasionally appears in _Mus musculus_, and
+causes those peculiarities of movement which are known as dancing, has
+been preserved and accentuated through selectional breeding by the Chinese
+and Japanese, until finally a distinct race of mice which breeds true to
+the dance character has been established. The age of the race is not
+definitely known, but it is supposed to have existed for several
+centuries.
+
+
+
+
+CHAPTER II
+
+FEEDING, BREEDING, AND DEVELOPMENT OF THE YOUNG
+
+In this chapter I shall report, for the benefit of those who may wish to
+know how to take care of dancing mice, my experience in keeping and
+breeding the animals, and my observations concerning the development of
+the young. It is commonly stated that the dancer is extremely delicate,
+subject to diseases to an unusual degree and difficult to breed. I have
+not found this to be true. At first I failed to get them to breed, but
+this was due, as I discovered later, to the lack of proper food. For three
+years my mice have bred frequently and reared almost all of their young.
+During one year, after I had learned how to care for the animals, when the
+maximum number under observation at any time was fifty and the total
+number for the year about one hundred, I lost two by disease and one by an
+accident. I very much doubt whether I could have done better with any
+species of mouse. There can be no doubt, however, that the dancer is
+delicate and demands more careful attention than do most mice. In March,
+1907, I lost almost all of my dancers from what appeared to be an
+intestinal trouble, but with this exception I have had remarkably good
+luck in breeding and rearing them.
+
+My dancers usually were kept in the type of cage of which Figure 2 is a
+photograph.[1] Four of these double cages, 70 cm. long, 45 cm. wide, and
+10 cm. deep in front, were supported by a frame as is shown in Figure 3.
+The fact that the covers of these cages cannot be left open is of
+practical importance. A similar type of cage, which I have used to some
+extent, consists of a wooden box 30 by 30 cm. by 15 cm. deep, without any
+bottom, and with a hinged cover made in part of 1 cm. mesh wire netting.
+Such a cage may be placed upon a piece of tin or board, or simply on a
+newspaper spread out on a table. The advantage of the loose bottom is that
+the box may be lifted off at any time, and the bottom thoroughly cleansed.
+I have had this type of cage constructed in blocks of four so that a
+single bottom and cover sufficed for the block. If the mice are being kept
+for show or for the observation of their movements, at least one side of
+the cages should be of wire netting, and, as Kishi suggests, such objects
+as a wheel, a tower, a tunnel, a bridge, and a turntable, if placed in the
+cage, will give the animals excellent opportunity to exhibit their
+capacity for varied forms of activity.
+
+[Footnote 1: This cage was devised by Professors W.E. Castle and E.L.
+Mark, and has been used in the Zoological Laboratories of Harvard
+University for several years.]
+
+[Illustration: FIGURE 2.--Double cage, with nest boxes and water dishes.]
+
+The floors of the cages were covered with a thin layer of sawdust for the
+sake of cleanliness, and in one corner of each cage a nest box of some
+sort was placed. During the warm months I found it convenient and
+satisfactory to use berry boxes, such as appear in Figure 2, with a small
+entrance hole cut in one side; and during the cold months cigar boxes,
+with an entrance hole not more than 5 cm. in diameter at one end. In the
+nest box a quantity of tissue paper, torn into fragments, furnished
+material for a nest in which the adults could make themselves comfortable
+or the female care for her young. Cotton should never be used in the nest
+boxes, for the mice are likely to get it wound about their legs with
+serious results. Apparently they are quite unable to free themselves from
+such an incumbrance, and their spinning motion soon winds the threads so
+tightly that the circulation of the blood is stopped.
+
+[Illustration: FIGURE 3.--Double cages in frame.]
+
+The cages and nest boxes were emptied and thoroughly cleaned once a week
+with an emulsion made by heating together one part of kerosene and one
+part of water containing a little soap. This served to destroy whatever
+odor the cages had acquired and to prevent vermin from infesting the
+nests. In hot weather far greater cleanliness is necessary for the welfare
+of the mice than in cold weather. The animals attend faithfully to their
+own toilets, and usually keep themselves scrupulously clean.
+
+For water and food dishes I have used heavy watch glasses[1] 5 cm. in
+diameter and 1/2 cm. deep. They are convenient because they are durable,
+easily cleaned, and not large enough for the young mice to drown in when
+they happen to spin into one which contains water. It is said that mice do
+not need water, but as the dancers seem very fond of a little, I have made
+it a rule to wash the watch glasses thoroughly and fill them with pure
+fresh water daily. The food, when moist, may be placed in the cages in the
+same kind of watch glass.
+
+[Footnote 1: Minot watch glasses.]
+
+There is no need of feeding the animals oftener than once a day, and as
+they eat mostly in the evening and during the night, it is desirable that
+the food should be placed in the cage late in the afternoon. For almost a
+year I kept a pair of dancers on "force"[1] and water. They seemed
+perfectly healthy and were active during the whole time, but they produced
+no young. If the animals are kept as pets, and breeding is not desired, a
+diet of "force," "egg-o-see,"[1] and crackers, with some bird-seed every
+few days, is likely to prove satisfactory. As with other animals, a
+variety of food is beneficial, but it appears to be quite unnecessary. Too
+much rich food should not be given, and the mice should be permitted to
+dictate their own diet by revealing their preferences. They eat
+surprisingly little for the amount of their activity. I have had excellent
+success in breeding the mice by feeding them a mixture of dry bread-
+crumbs, "force," and sweet, clean oats slightly moistened with milk. The
+food should never be made soppy. A little milk added thus to the food
+every other day greatly increases fertility. About once a week a small
+quantity of some green food, lettuce for example, should be given. It is
+well, I have found, to vary the diet by replacing the bread and "force" at
+intervals with crackers and seeds. Usually I give the food dry every other
+day, except in the case of mice which are nursing litters. One person to
+whom I suggested that lettuce was good for the dancers lost four,
+apparently because of too much of what the mice seemed to consider a good
+thing. This suggests that it should be used sparingly.
+
+[Footnote 1: A cereal food.]
+
+Success in keeping and breeding dancing mice depends upon three things:
+cleanliness, warmth, and food supply. The temperature should be fairly
+constant, between 60° and 70° Fahr. They cannot stand exposure to cold or
+lack of food. If one obtains good healthy, fertile individuals, keeps them
+in perfectly clean cages with soft nesting materials, maintains a
+temperature of not far above or below 65°, and regularly supplies them
+with pure water and food which they like, there is not likely to be
+trouble either in keeping or breeding these delicate little creatures.
+Several persons who have reported to me difficulty in rearing the young or
+in keeping the adults for long periods have been unable to maintain a
+sufficiently high or constant temperature, or have given them food which
+caused intestinal trouble.
+
+The males are likely to fight if kept together, and they may even kill one
+another. A male may be kept with one or more females, or several females
+may be kept together, for the females rarely, in my experience, fight, and
+the males seldom harm the females. Unless the male is removed from the
+cage in which the female is kept before the young are born, he is likely
+to kill the newborn animals. When a female is seen to be building a nest
+in preparation for a litter, it is best to place her in a cage by herself
+so that she may not be disturbed.
+
+The sex of individuals may be determined easily in most cases, at the age
+of 10 to 12 days, by the appearance of teats in the case of females.
+
+The period of gestation is from 18 to 21 days. The maximum number born by
+my dancers in any single litter was 9, the minimum number 3. In 25 litters
+of which I have accurate records, 135 individuals were born, an average of
+5.4. The average number of males per litter was precisely the same, 2.7,
+as the number of females.
+
+On the birth of a litter it is well to see that the female has made a nest
+from which the young are not likely to escape, for at times, if the nest
+is carelessly made, they get out of it or under some of the pieces of
+paper which are used in its construction, and perish. Several times I have
+observed nests so poorly built that almost all of the young perished
+because they got too far away to find their way back to the mother. It is
+surprising that the female should not take more pains to keep her young
+safe by picking them up in her mouth, as does the common mouse, and
+carrying them to a place where they can obtain warmth and nourishment.
+This I have never seen a dancing mouse do. For the first day or two after
+the birth of a litter the female usually remains in the nest box almost
+constantly and eats little. About the second day she begins to eat
+ravenously, and for the next three or four weeks she consumes at least
+twice as much food as ordinarily. Alexander and Kreidl (3 p. 567) state
+that the female does not dance during the first two weeks after the birth
+of a litter, but my experience contradicts their statement. There is a
+decreased amount of activity during this period, and usually the whirling
+movement appears but rarely; but in some cases I have seen vigorous and
+long-continued dancing within a few hours after the birth of a litter.
+There is a wide range of variability in this matter, and the only safe
+statement, in the light of my observations, is that the mother dances less
+than usual for a few days after a litter is born to her.
+
+The development of the young, as I have observed it in the cases of twenty
+litters, for ten of which (Table I) systematic daily records were kept,
+may be sketched as follows. At birth the mice have a rosy pink skin which
+is devoid of hair and perfectly smooth; they are blind, deaf, and
+irresponsive to stimulation of the vibrissae on the nose. During the first
+week of post-natal life the members of a litter remain closely huddled
+together in the nest, and no dance movements are exhibited. The mother
+stays with them most of the time. On the fourth or fifth day colorless
+hairs are visible, and by the end of the week the body is covered with a
+coat which rapidly assumes the characteristic black and white markings of
+the race. For the first few days the hind legs are too weak to support the
+body weight, and whatever movements appear are the result of the use of
+the fore legs. As soon as the young mice are able to stand, circling
+movements are exhibited, and by the end of the second week they are
+pronounced. Somewhere about the tenth day the appearance of the teats in
+the case of the females serves to distinguish the sexes plainly. Between
+the tenth and fifteenth days excitability, as indicated by restless jerky
+movements in the presence of a disturbing condition, increases markedly;
+the auditory meatus opens, and, in the case of some individuals, there are
+signs of hearing. On or after the fifteenth day the eyes open and the
+efforts to escape from the nest box rapidly become more vigorous. About
+this time the mother resumes her dancing with customary vigor, and the
+young, when they have opportunity, begin to eat of the food which is given
+to her. They now dance essentially as do the adults. From the end of the
+third week growth continues without noteworthy external changes until
+sexual maturity is attained, between the fourth and the sixth week. For
+several weeks after they are sexually mature the mice continue to increase
+in size.
+
+
+
+TABLE I
+
+DEVELOPMENT OF THE YOUNG
+
+
+         NUMBER                  JERKY              REACT
+           IN   HAIR    TEATS    MOVE-     EARS      TO       EYES
+PARENTS LITTER VISIBLE VISIBLE   MENTS     OPEN     SOUND     OPEN
+         M F                     APPEAR
+
+152+151  5 0  4th day    --     13th day  14th day  14th day 16th day
+152+151  1 3  4th day  9th day  10th day  12th day  13th day 15th day
+410+415  4 1  5th day 11th day  14th day  15th day  15th day 17th day
+410+415  2 4  5th day 10th day  13th day  14th day  14th day 16th day
+420+425  0 2  4th day 10th day  12th day  14th day  14th day 16th day
+210+215  4 1    --      --      17th day  13th day  17th day 15th day
+210+215  3 3  5th day 11th day  11th day  14th day    No     16th day
+212+211  1 3  4th day 10th day  15th day  14th day    No     15th day
+220+225  2 4  4th day 10th day  16th day  14th day    No     15th day
+220+225  3 3  4th day 10th day  17th day  13th day    No     15th day
+
+
+A course of development very similar to that just described was observed
+by Alexander and Kreidl (3 p. 565) in three litters of dancing mice which
+contained 3, 5, and 7 individuals respectively. These authors, in
+comparing the development of the dancer with that of the common mouse, say
+that at birth the young in both cases are about 24 mm. in length. The
+young common mouse grows much more rapidly than the dancer, and by the
+ninth day its length is about 43 mm. as compared with 31 mm. in the case
+of the dancer. According to Zoth (31 p. 148) the adult dancer has a body
+length of from 7 to 7.5 cm., a length from tip of nose to tip of tail of
+from 12 to 13 cm., and a weight of about 18 grams. The movement of the
+dancer from the first tends to take the form of circles toward the middle
+of the nest; that of the common mouse has no definite tendency as to
+direction. When the common mouse does move in circles, it goes first in
+one direction, then in the other, and not for any considerable period in
+one direction as does the true dancer. Neither the young dancer nor the
+common mouse is able to equilibrate itself well for the first few days
+after birth, but the latter can follow a narrow path with far greater
+accuracy and steadiness than the former. The uncertain and irregular
+movements of the common mouse are due to muscular weakness and to
+blindness, but the bizarre movements of the young dancer seem to demand
+some additional facts as an explanation.
+
+A brief account of the development of the dancer given by Zoth (31 p. 149)
+adds nothing of importance to the description given by Alexander and
+Kreidl. As my own observations disagree with their accounts in certain
+respects, I shall now give, in the form of a diary, a description of the
+important changes observed from day to day in a normal litter. The litter
+which I have selected as typical of the course of development in the
+dancer grew rapidly under favorable conditions. I have observed many
+litters which passed through the various stages of development mentioned
+in this description anywhere from a day to a week later. This was usually
+due to some such obviously unfavorable condition as too little food or
+slight digestive or bowel troubles. According to the nature of the
+conditions of growth the eyes of the dancer open anywhere from the
+fourteenth to the twentieth day. This statement may serve to indicate the
+degree of variability as to the time at which a given stage of development
+is reached by different litters.
+
+On July 14, 1906, No. 151 (female) and No. 152 (male) were mated, and on
+August 3 a litter of six was born to them. The course of the development
+of this litter during the first three weeks was as follows:--
+
+_First day_ The skin is pink and hairless, several vibrissae are visible
+on the nose and lips, but there is no definite response when they are
+touched. The mice are both blind and deaf, but they are able to squeak
+vigorously. The mother was not seen to dance or eat during the day.
+
+_Second day_. There is a very noticeable increase in size. The vibrissae
+are longer, but touching them still fails to cause a reaction. No hairs
+are visible on the body. The mother danced rapidly for periods of a minute
+several times while the record was being made. She ate very little to-day.
+
+_Third day_. Scales began to appear on the skin to-day. The animals are
+rapidly increasing in strength; they can now crawl about the nest easily,
+but they are too weak to stand, and constantly roll over upon their sides
+or backs when they are placed on a smooth surface. Because of their
+inability to progress it is impossible to determine with certainty whether
+they have a tendency to move in circles. The mother was seen out of the
+nest dancing once to-day. She now eats ravenously.
+
+_Fourth day_. One of the six young mice was found under a corner of the
+nest this morning dead, and the others were scattered about the nest box.
+I gathered them together into a nest which I made out of bits of tissue
+paper, and the mother immediately began to suckle them. They are very
+sensitive to currents of air, but they do not respond to light or sound
+and seldom to contact with the vibrissae.
+
+_Fifth day_. When placed on a smooth surface, they tend to move in
+circles, frequently rolling over. When placed on their sides or backs,
+they immediately try to right themselves. They do not walk, for their legs
+are still too weak to support the weight of the body; instead they drag
+themselves about by the use of the fore legs. Fine colorless hairs are
+visible over the entire body surface. When the vibrissae are touched, the
+head is moved noticeably. The mother dances a great deal and eats about
+twice as much as she did before the birth of the litter.
+
+_Sixth day_. Certain regions of the skin, which were slightly darker than
+the remainder on the fourth and fifth days, are now almost black. It is
+evident that they are the regions in which the black hair is to appear.
+The movement in circles is much more definite today, although most of the
+individuals are still too weak to stand on their feet steadily for more
+than a few seconds at a time. Most of their time, when they are first
+taken from the nest, is spent in trying to maintain or regain an upright
+position. The hair is now easily visible, and the skin begins to have a
+white appearance as a result.
+
+_Seventh day_. Although they are strong enough to move about the nest
+readily, none of the young has attempted to leave the nest. They huddle
+together in the middle of it for warmth. The epidermal scales, which have
+increased in number since the third day, are dropping off rapidly. Contact
+with the vibrissae or with the surface of the body, frequently calls forth
+a motor reaction but neither light nor sound produces any visible change
+in behavior. The black and white regions of the skin are sufficiently
+definite now to enable one to distinguish the various individuals by their
+markings. The mother was seen to dance repeatedly today, and she ate all
+the food that was given to her.
+
+_Eighth day_. A fold is plainly visible where later the eyelids will
+separate. The black pigment in the skin has increased markedly.
+
+_Ninth day_. The eyelids are taking form rapidly, but they I have not
+separated. The body is covered with a thick coat of hair which is either
+pure white or black. Standing on the four legs is still a difficult task.
+
+_Tenth day_. To-day teats are plainly visible in the case of four of the
+five individuals of the litter. Up to this time I had thought, from
+structural indications, that there were three males and two females; it is
+now evident that there are four females and one male. The external ear,
+the pinna, is well formed, and has begun to stand out from the head, but
+no opening to the inner portion of the ear is present. The eyelids appear
+to be almost fully formed.
+
+_Eleventh day_. There are no very noticeable changes in appearance except
+in size, which continues to increase rapidly. They are able to regain
+their normal upright position almost immediately when they happen to roll
+over. The mother dances as usual.
+
+_Twelfth day_. It appears to-day as if the eyes were about to open. The
+ears are still closed, and there is no evidence of a sense of hearing.
+They squeaked considerably when in the nest, but not at all when I took
+them out to note their development. The mother stays outside of the nest
+box much of the time now, probably to prevent the young ones from sucking
+continuously.
+
+_Thirteenth day_. One of the little mice came out of the nest box while I
+was watching the litter this morning, and was able to find his way back
+directly despite the lack of sight. The mice are still dependent upon the
+mother for nourishment. I have not seen any of them attempt to eat the
+food which is given to the mother. They are extremely neat and clean. I
+watched one of them wash himself this morning. Each foot was carefully
+licked with the tongue. There seems to be special care taken to keep the
+toes perfectly clean.
+
+_Fourteenth day_. An opening into the ear is visible to-day. When tested
+with the Galton whistle, all five responded with quick, jerky movements of
+the head and legs. They evidently hear certain tones. During the past two
+days the ears have changed rapidly. In one of the females, which seems to
+be a little in advance of the others in development, certain peculiarities
+of behavior appeared to-day. She jumped and squeaked sharply when touched
+and sprang out of my hand when I attempted to take her up. This is in
+marked contrast with her behavior previously.
+
+_Fifteenth day_. The eyes are partly opened. All of the members of the
+litter came out of the nest box this morning and ran around the cage,
+dancing frequently and trying to eat with the mother. Three out of the
+five gave auditory reactions on first being stimulated; none of them
+responded to repetitions of the stimulus. All appeared to be less
+sensitive to sounds than yesterday. The quick, nervous, jerky movements
+are very noticeable.
+
+_Sixteenth day_. The eyes of all five are fully opened. They dance
+vigorously and are outside the nest much of the time.
+
+_Seventeenth day_. No reactions to sound could be detected to-day. The
+sense of sight gives evidence of being well developed. The nervous jumping
+movements persist.
+
+_Eighteenth day_. The young mice continue to suck, although they eat of
+the food which is given to the mother. They are now able to take care of
+themselves.
+
+_Nineteenth day_. There are no noteworthy changes except increase in size
+and strength.
+
+_Twentieth day_. No auditory reactions were obtained today, but other
+forms of stimulation brought about unmistakable responses.
+
+_Twenty-first day_. They are now about half grown and there is no other
+change of special interest to be recorded. Growth continues for several
+weeks. The statement made by Alexander and Kreidl to the effect that the
+dancer is almost full grown by the thirty-first day of life is false. At
+that age they may be sexually mature, but usually they are far from full
+grown.
+
+
+
+
+CHAPTER III
+
+BEHAVIOR: DANCE MOVEMENTS
+
+The peculiarities of behavior of the dancing mouse are responsible alike
+for the widespread interest which it has aroused, and for its name. In a
+little book on fancy varieties of mice, in which there is much valuable
+information concerning the care of the animals, one who styles himself "An
+old fancier" writes thus of the behavior of the dancer: "I believe most
+people have an idea that the waltzing is a stately dance executed on the
+hind feet; this is not so. The performer simply goes round and round on
+all fours, as fast as possible, the head pointing inwards. The giddy
+whirl, after continuing for about a dozen turns, is then reversed in
+direction, and each performance usually occupies from one to two minutes.
+Whether it is voluntary or not, is difficult to determine, but I am
+inclined to think the mouse can refrain if it wishes to do so, because I
+never see them drop any food they may be eating, and begin to waltz in the
+midst of their meal. The dance, if such it can be called, generally seizes
+the mouse when it first emerges from its darkened sleeping place, and this
+would lead one to suppose that the light conveys an impression of shock to
+the brain, through the eyes, which disturbs the diseased centers and
+starts the giddy gyrations. The mice can walk or run in a fairly straight
+line when they wish to do so." Some of the old fancier's statements are
+true, others are mere guesses. Those who have studied the mice carefully
+will doubtless agree that he has not adequately described the various
+forms of behavior of which they are capable. I have quoted his description
+as an illustration of the weakness which is characteristic of most popular
+accounts of animal behavior. It proves that it is not sufficient to watch
+and then describe. The fact is that he who adequately describes the
+behavior of any animal watches again and again under natural and
+experimental conditions, and by prolonged and patient observation makes
+himself so familiar with his subject that it comes to possess an
+individuality as distinctive as that of his human companions. To the
+casual observer the individuals of a strange race are almost
+indistinguishable. Similarly, the behavior of all the animals of a
+particular species seems the same to all except the observer who has
+devoted himself whole-heartedly to the study of the subject and who has
+thus become as familiar with their life of action as most of us are with
+that of our fellow-men; for him each individual has its own unmistakable
+characteristics.
+
+I shall now describe the behavior of the dancing mouse in the light of the
+results of the observation of scores of individuals for months at a time,
+and of a large number of experiments. From time to time I shall refer to
+points in the accounts of the subject previously given by Rawitz (25 p.
+236), Cyon (9 p. 214), Alexander and Kreidl (1 p. 542), Zoth (31 p. 147),
+and Kishi (21 p. 479).
+
+The most striking features of the ordinary behavior of the dancer are
+restlessness and movements in circles. The true dancer seldom runs in a
+straight line for more than a few centimeters, although, contrary to the
+statements of Rawitz and Cyon, it is able to do so on occasion for longer
+distances. Even before it is old enough to escape from the nest it begins
+to move in circles and to exhibit the quick, jerky head movements which
+are characteristic of the race. At the age of three weeks it is able to
+dance vigorously, and is incessantly active when not washing itself,
+eating, or sleeping. According to Zoth (31 p. 149) the sense of sight and
+especially the sense of smell of the dancer "seem to be keenly developed;
+one can seldom remain for some time near the cage without one or another
+of the animals growing lively, looking out of the nest, and beginning to
+sniff around in the air (_windet_). They also seem to have strongly
+developed cutaneous sensitiveness, and a considerable amount of curiosity,
+if one may call it such, in common with their cousin, the white mouse." I
+shall reserve what I have to say concerning the sense of sight for later
+chapters. As for the sense of smell and the cutaneous sensitiveness, Zoth
+is undoubtedly right in inferring from the behavior of the animal that it
+is sensitive to certain odors and to changes in temperature. One of the
+most noticeable and characteristic activities of the dancer is its
+sniffing. Frequently in the midst of its dancing it stops suddenly, raises
+its head so that the nose is pointed upward, as in the case of one of the
+mice of the frontispiece, and remains in that position for a second or
+two, as if sniffing the air.
+
+The restlessness, the varied and almost incessant movements, and the
+peculiar excitability of the dancer have repeatedly suggested to casual
+observers the question, why does it move about in that aimless, useless
+fashion? To this query Rawitz has replied that the lack of certain senses
+compels the animal to strive through varied movements to use to the
+greatest advantage those senses which it does possess. In Rawitz's opinion
+the lack of hearing and orientation is compensated for by the continuous
+use of sight and smell. The mouse runs about rapidly, moves its head from
+side to side, and sniffs the air, in order that it may see and smell as
+much as possible. In support of this interpretation of the restlessness of
+the dancer, Rawitz states that he once observed similar behavior in an
+albino dog which was deaf. This suggestion is not absurd, for it seems
+quite probable that the dancer has to depend for the guidance of its
+movements upon sense data which are relatively unimportant in the common
+mouse, and that by its varied and restless movements it does in part make
+up for its deficiency in sense equipment.
+
+The dancing, waltzing, or circus course movement, as it is variously
+known, varies in form from moment to moment. Now an individual moves its
+head rapidly from side to side, perhaps backing a little at the same time,
+now it spins around like a top with such speed that head and tail are
+almost indistinguishable, now it runs in circles of from 5 cm. to 30 cm.
+in diameter. If there are any objects in the cage about or through which
+it may run, they are sure to direct the expression of activity. A tunnel
+or a hole in a box calls forth endless repetitions of the act of passing
+through. When two individuals are in the same cage, they frequently dance
+together, sometimes moving in the same direction, sometimes in opposite
+directions. Often, as one spins rapidly about a vertical axis, the other
+runs around the first in small circles; or again, both may run in a small
+circle in the same direction, so that their bodies form a living ring,
+which, because of the rapidity of their movements, appears perfectly
+continuous. The three most clearly distinguishable forms of dance are (1)
+movement in circles with all the feet close together under the body, (2)
+movement in circles, which vary in diameter from 5 cm. to 30 cm., with the
+feet spread widely, and (3) movement now to the right, now to the left, in
+figure eights ([Symbol: figure eight]). For convenience of reference
+these types of dance may be called _whirling, circling_, and the _figure
+eight dance_. Zoth, in an excellent account of the behavior of the dancer
+(31 p. 156), describes "manège movements," "solo dances," and "centre
+dances." Of these the first is whirling, the second one form of circling,
+and the third the dancing of two individuals together in the manner
+described above.
+
+Both the whirling and the circling occur to the right (clockwise) and to
+the left (anticlockwise). As certain observers have stated that it is
+chiefly to the left and others that it is as frequently to the right, I
+have attempted to get definite information concerning the matter by
+observing a number of individuals systematically and at stated intervals.
+My study of this subject soon convinced me that a true conception of the
+facts cannot be got simply by noting the direction of turning from time to
+time. I therefore planned and carried out a series of experimental
+observations with twenty dancers, ten of each sex. One at a time these
+individuals were placed in a glass jar, 26 cm. in diameter, and the number
+of circle movements executed to the right and to the left during a period
+of five minutes was determined as accurately as possible. This was
+repeated at six hours of the day: 9 and 11 o'clock A.M., and 2, 4, 6, and
+8 o'clock P.M. In order that habituation to the conditions under which the
+counts of turning were made might hot influence the results for the group,
+with ten individuals the morning counts were made first, and with the
+others the afternoon counts. No attempt was made in the counting to keep a
+separate record of the whirling and circling, although had it been
+practicable this would have been desirable, for, as soon became evident to
+the observer, some individuals which whirl in only one direction, circle
+in both.
+
+In Table 2 the results of the counts for the males are recorded; in Table
+3 those for the females. Each number in the column headed "right" and
+"left" indicates the total number of circles executed by a certain dancer
+in a period of five minutes at the hour of the day named at the head of
+the column. I may point out briefly the curiously interesting and entirely
+unexpected new facts which this method of observation revealed to me.
+
+First, there are three kinds of dancers: those which whirl almost
+uniformly toward the right, those which whirl just as uniformly toward the
+left, and those which whirl about as frequently in one direction as in the
+other. To illustrate, No. 2 of Table 2 may be characterized as a "right
+whirler," for he turned to the right almost uniformly. In the case of the
+6 P.M. count, for example, he turned 285 times to the right, not once to
+the left. No. 152, on the contrary, should be characterized as a "left
+whirler," since he almost always turned to the left. From both of these
+individuals No. 210 is distinguished by the fact that he turned now to the
+left, now to the right. For him the name "mixed whirler" seems
+appropriate.
+
+Second, the amount of activity, as indicated by the number of times an
+individual turns in a circle within five minutes, increases regularly and
+rapidly from 9 A.M. to 8 P.M. According to the general averages which
+appear at the bottom of Table 2, the average number of circles executed by
+the males at 9 A.M. was 89.8 as compared with 207.1 at 8 P.M. In other
+words, the mice dance more in the evening than during the day.
+
+Third, as it appears in a comparison of the general averages of Tables 2
+and 3, the females dance more than the males, under the conditions of
+observation. At 9 A.M. the males circled 89.8 times, the females 151.0
+times; at 8 P.M. the males circled 207.1 times, the females, 279.0 times.
+
+Fourth, according to the averages for the six counts made with each
+individual, as they appear in Table 4, the males turn somewhat more
+frequently to the left than to the right (the difference, however, is not
+sufficient to be considered significant); whereas, the females turn much
+more frequently to the right than to the left. I do not wish to emphasize
+the importance of this difference, for it is not improbable that counts
+made with a larger number of animals, or even with another group of
+twenty, would yield different results.
+
+
+
+TABLE 2
+
+NUMBER or WHIRLS TO THE RIGHT AND TO THE LEFT DURING
+FIVE-MINUTE INTERVALS AS DETERMINED BY COUNTS MADE AT
+SIX DIFFERENT HOURS, FOR EACH OF TEN MALE DANCERS
+
+
+NUMBER        9 A.M           11 A.M.           2 P.M.
+  OF
+ANIMAL    RIGHT  LEFT      RIGHT  LEFT      RIGHT  LEFT
+
+  2         11     2         23     4        194     1
+ 30         20     1        134     1        109     2
+ 34          2    16          2    48          4    92
+ 36        194    21        180    11        143    65
+152          7    48          3   171          6    79
+156         63     8         53     9         27     6
+210          3     9          7    41        225    21
+220        168   105         39    43         47     5
+410          2    67         10    27          8   103
+420         15   142          5   214         16   238
+
+Averages  48.5  41.3       45.6  56.9       77.9  61.2
+
+Gen. Av.     89.8             102.5            139.1
+
+
+NUMBER        4 P.M            6 P.M.           8 P.M.
+  OF
+ANIMAL    RIGHT  LEFT      RIGHT  LEFT      RIGHT  LEFT
+
+  2         70     3        285     0        237    10
+ 30        154     0        107     6        134     5
+ 34          7   158          5   118          6   147
+ 36        173    14        170    11        325    19
+152          0    91         16   210          9   223
+156         85     2         72    26        139    26
+210        159    18         31    82         47   201
+220         45    38         78    17         69    33
+410          9   155          9   394         24    94
+420         18   243         16   291          3   320
+
+Averages  72.0  72.2       78.9 115.5       99.3 107.8
+
+Gen. Av.    144.2             194.4            207.1
+
+
+
+
+
+
+TABLE 3
+
+NUMBER OF WHIRLS TO THE RIGHT AND TO THE LEFT DURING
+  FIVE-MINUTE INTERVALS AS DETERMINED BY COUNTS MADE AT
+  SIX DIFFERENT HOURS, FOR EACH OF TEN FEMALE DANCERS
+
+
+NUMBER           9 A.M.            11 A.M.            2 P.M.
+OF
+ANIMAL      RIGHT    LEFT      RIGHT    LEFT     RIGHT    LEFT
+
+ 29            9       18         17      30        7       22
+ 33          287        0        329       1      352        3
+ 35           48       15        198      46      208       14
+151           13       88          7      75        3      167
+157           57        6         50      45       53       12
+211          218       21         31      55       66        5
+215           67      216         33     105       37      226
+225           46       39         72      49      143       44
+415           23        0        156       0       34        3
+425           43      296         12     201       12      210
+
+Averages     81.1     69.9       90.5    60.7     91.5     70.6
+
+Gen. Av.         151.0              151.2              162.1
+
+NUMBER            4 P.M.           6 P.M.           8 P.M.
+OF
+ANIMAL        RIGHT    LEFT    RIGHT    LEFT    RIGHT   LEFT
+
+ 29             33      114      31       36      45      99
+ 33            436        7     408        3     364       2
+ 35            279        6     165       24     353      10
+151              3        8       2      285       2     217
+157             52       15      19      125      51     104
+211            190        7      86       31      67     250
+215             15      292      45      336     150     232
+225            133       86      48       39     177      81
+415            268        3     437        7     382       8
+425             12      242      19      210       4     192
+
+Averages      142.1     78.0  126.0    109.6   159.5   119.5
+
+Gen. Av.           220.1           235.6            279.0
+
+
+
+The most important results of this statistical study of turning are the
+demonstration of the existence of individual tendencies to turn in a
+particular direction, and of the fact that the whirling increases in
+amount from morning to evening.
+
+In order to discover whether the distribution of the dancers among the
+three groups which have been designated as right, left, and mixed whirlers
+agrees in general with that indicated by Table 4 (approximately the same
+number in each group) I have observed the direction of turning in the case
+of one hundred dancers, including those of the foregoing tables, and have
+classified them in accordance with their behavior as is indicated below.
+
+
+
+               RIGHT          LEFT         MIXED
+              WHIRLERS      WHIRLERS      WHIRLERS
+
+Males            19            19            12
+Females          12            23            15
+
+      Totals     31            42            27
+
+
+
+The left whirlers occur in excess of both the right and the mixed
+whirlers. This fact, together with the results which have already been
+considered in connection with the counts of turning, suggests that a
+tendency to whirl in a certain way may be inherited. I have examined my
+data and conducted breeding experiments for the purpose of ascertaining
+whether this is true. But as the results of this part of the investigation
+more properly belong in a special chapter on the inheritance of behavior
+(XVIII), the discussion of the subject may be closed for the present with
+the statement that the preponderance of left whirlers indicated above is
+due to a strong tendency to turn to the left which was exhibited by the
+individuals of one line of descent.
+
+
+
+TABLE 4
+
+AVERAGE NUMBER OF WHIRLS TO THE RIGHT AND TO THE LEFT FOR
+THE SIX INTERVALS OF TABLES 2 AND 3, WITH A CHARACTERIZATION
+OF THE ANIMALS AS RIGHT WHIRLERS, LEFT WHIRLERS, OR
+MIXED WHIRLERS.
+
+
+                   AVERAGE NO.    AVERAGE NO.
+MALES     AGE       OF WHIRLS      OF WHIRLS      CHARACTERIZATION
+
+    2    12 mo.       136.7             3.3        Right whirler
+   30     2 mo.       109.7             2.5        Right whirler
+   34     2 mo.         4.3            96.5        Left  whirler
+   36     2 mo.       197.5            23.5        Right whirler
+  152     6 mo.         6.8           137.0        Left  whirler
+  156     1 mo.        73.2            12.8        Right whirler
+  210     3 mo.        78.7            62.0        Mixed whirler
+  220     4 mo.        74.3            40.2        Mixed whirler
+  410     3 mo.        10.3           139.0        Left  whirler
+  420     3 mo.        12.2           241.3        Left  whirler
+
+    Average            70.4            75.8      4 Right whirlers
+                                                 4 Left  whirlers
+                                                 2 Mixed whirlers
+
+
+FEMALES
+
+   29     2 mo.        23.7            53.2        Left  whirler
+   33     2 mo.       362.7             2.7        Right whirler
+   35     2 mo.       208.5            19.2        Left  whirler
+  151     6 mo.         5.0           140.0        Right whirler
+  157     1 mo.        47.0            51.2        Left  whirler
+  211     3 mo.       109.7            61.5        Right whirler
+  215     3 mo.        57.8           234.5        Mixed whirler
+  225     4 mo.       103.2            56.3        Mixed whirler
+  415     3 mo.       216.7             3.5        Left  whirler
+  425     3 mo.        17.0           225.2        Left  whirler
+
+    Average           115.1            84.7      3 Right whirlers
+                                                 4 Left  whirlers
+                                                 3 Mixed whirlers
+
+
+
+The tendency of the dancer's activity to increase in amount toward
+evening, which the results of Tables 2, 3, and 4 exhibit, demands further
+consideration. Haacke (7 p. 337) and Kishi (21 p. 458) agree that the
+dancing is most vigorous in the evening; but Alexander and Kreidl (i p.
+544) assert, on the contrary, that the whirling of the individuals which
+they observed bore no definite relation to the time of day and apparently
+was not influenced in intensity thereby. Since the results of my own
+observations contradict many of the statements made by the latter authors,
+I suspect that they may not have watched their animals long enough to
+discover the truth. The systematic records which I have kept indicate that
+the mice remain quietly in their nests during the greater part of the day,
+unless they are disturbed or come out to obtain food. Toward dusk they
+emerge and dance with varying intensity for several hours. I have seldom
+discovered one of them outside the nest between midnight and daylight. The
+period of greatest activity is between 5 and 10 o'clock P.M.
+
+Zoth states that he has observed the adult dancer whirl 79 times without
+an instant's interruption, and I have counted as many as 110 whirls. It
+seems rather absurd to say that an animal which can do this is weak.
+Evidently the dancer is exceptionally strong in certain respects, although
+it may be weak in others. Such general statements as are usually made fail
+to do justice to the facts.
+
+The supposition that light determines the periodicity of dancing is not
+borne out by my observations, for I have found that the animals continue
+to dance most vigorously toward evening, even when they are kept in a room
+which is constantly illuminated. In all probability the periodicity of
+activity is an expression of the habits of the mouse race rather than of
+the immediate influence of any environmental condition. At some time in
+the history of the dancer light probably did have an influence upon the
+period of activity; but at present, as a result of the persistence of a
+well-established racial tendency, the periodicity of dancing depends to a
+greater extent upon internal than upon external conditions. During its
+hours of quiescence it is possible to arouse the dancer and cause it to
+whirl more or less vigorously by stimulating it strongly with intense
+light, a weak electric current, or by placing two individuals which are
+strangers to one another in the same cage; but the dancing thus induced is
+seldom as rapid, varied, or as long-continued as that which is
+characteristic of the evening hours.
+
+One of the most interesting results of this study of the direction of
+turning, from the observer's point of view, is the demonstration of the
+fact that the truth concerning even so simple a matter as this can be
+discovered only by long and careful observation. The casual observer of
+the dancer gets an impression that it turns to the left more often than to
+the right; he verifies his observation a few times and then asserts with
+confidence that such is the truth about turning. That such a method of
+getting knowledge of the behavior of the animal is worse than valueless is
+clear in the light of the results of the systematic observations which
+have just been reported. But, however important the progress which we may
+have made by means of systematic observation of the phenomenon of turning,
+it must not for one moment be supposed that the whole truth has been
+discovered. Continued observation will undoubtedly reveal other important
+facts concerning circling, whirling, and the periodicity of dancing, not
+to mention the inheritance of peculiarities of dancing and the
+significance of the various forms of activity.
+
+
+
+CHAPTER IV
+
+BEHAVIOR: EQUILIBRATION AND DIZZINESS
+
+
+Quite as interesting and important as the general facts of behavior which
+we have been considering are the results of experimental tests of the
+dancer's ability to maintain its position under unusual spatial
+conditions--to climb, cross narrow bridges, balance itself on high places.
+Because of its tendency to circle and whirl, to dart hither and thither
+rapidly and apparently without control of its movements, the study of the
+mouse's ability to perform movements which demand accurate and delicate
+muscular coördination, and to control its expressions of activity, are of
+peculiar scientific interest.
+
+That observers do not entirely agree as to the facts in this field is
+apparent from the following comparison of the statements made by Cyon and
+Zoth (31 p. 174).
+
+Cyon states that the dancer
+
+
+Cannot run in a straight line,
+Cannot turn in a narrow space,
+Cannot run backward,
+Cannot run up an incline,
+Cannot move about safely when above the ground, because of
+  fear and visual dizziness,
+Can hear certain tones.
+
+
+Zoth, on the contrary, maintains that the animal
+
+
+Can run in a straight line for at least 20 cm.,
+Can and repeatedly does turn in a narrow space,
+Can run backward, for he has observed it do so,
+Can run up an incline unless the surface is too smooth for it to
+  gain a foothold,
+Can move about safely when above the ground, and gives no
+  signs of fear or dizziness,
+Cannot hear, or at least gives no signs of sensitiveness to sounds.
+
+
+
+Such contradictory statements (and unfortunately they are exceedingly
+common) stimulated me to the repetition of many of the experiments which
+have been made by other investigators to test the dancer's behavior in
+unusual spatial relations. I shall state very briefly the general
+conclusions to which these experiments have led me, with only sufficient
+reference to methods and details of results to enable any one who wishes
+to repeat the tests for himself to do so. For the sake of convenience of
+presentation and clearness, the facts have been arranged under three
+rubrics: equilibrational ability, dizziness, and behavior when blinded. To
+our knowledge of each of these three groups of facts important
+contributions have come from the experiments of Cyon (9 p. 220), Alexander
+and Kreidl (1 p. 545), Zoth (31 p. 157), and Kishi (21 p. 482), although,
+as has been stated, in many instances their results are so contradictory
+as to demand reexamination. All in all, Zoth has given the most
+satisfactory account of the behavior and motor capacity of the dancer.
+
+If the surface upon which it is moving be sufficiently soft or rough to
+furnish it a foothold, the dancer is able to run up or down inclines, even
+though they be very steep, to cross narrow bridges, to balance itself at
+heights of at least 30 cm. above the ground, and even to climb up and down
+on rods, as is shown by certain of Zoth's photographs which are reproduced
+in Figure 4. Zoth himself says, and in this I am able fully to agree with
+him on the basis of my own observations, "that the power of equilibration
+in the dancing mouse, is, in general, very complete. The seeming reduction
+which appears under certain conditions should be attributed, not to visual
+dizziness, but in part to excitability and restlessness, and in part to a
+reduced muscular power" (31 p. 161). The dancer certainly has far less
+grasping power than the common mouse, and is therefore at a disadvantage
+in moving about on sloping surfaces. One evidence of this fact is the
+character of the tracks made by the animal. Instead of raising its feet
+from the substratum and placing them neatly, as does the common mouse
+(Figure 5), it tends to shuffle along, dragging its toes and thus
+producing on smoked paper such tracks as are seen in Figure 6. From my own
+observations I am confident that these figures exaggerate the differences.
+My dancers, unless they were greatly excited or moving under conditions of
+stress, never dragged their toes as much as is indicated in Figure 6.
+However, there can be no doubt that they possess less power of grasping
+with their toes than do common mice. The animal is still further
+incapacitated for movement on inclined surfaces or narrow places by its
+tendency to move in circles and zigzags. The results of my own experiments
+indicate that the timidity of the adult is greater than that of the
+immature animal when it is placed on a bridge 1 or 2 cm. wide at a
+distance of 20 cm. from the ground. Individuals three weeks old showed
+less hesitation about trying to creep along such a narrow pathway than did
+full-grown dancers three or four months old; and these, in turn, were not
+so timid apparently as an individual one year old. But the younger animals
+fell off more frequently than did the older ones.
+
+[Illustration: FIGURE 4.--Zoth's photographs of dancers crossing bridges
+and climbing rods. Reproduced from _Pfluger's Archiv_, Bd. 86.]
+
+[Illustration: FIGURE 5--Tracks of common mouse Reproduced from Alexander
+and Kreidl's figure in _Pfluger's Archiv_, Bd 82]
+
+[Illustration: FIGURE 6--Tracks of dancing mouse Reproduced from Alexander
+and Kreidl's figure in _Pfluger's Archiv_ Bd 82]
+
+Additional support for these statements concerning equilibrational ability
+is furnished by the observations of Kishi (21 p. 482). He built a wooden
+bridge 60 cm. long, 1 cm. wide at one end, and 1/2 cm. at the other, and
+supported it at a height of 30 cm. above the ground by posts at the ends.
+On this bridge ten dancers were tested. Some attempted to move sidewise,
+others began to whirl and fell to the ground; only one of the ten
+succeeded in getting all the way across the bridge on the first trial. The
+second time he was tested this individual crossed the bridge and found the
+post; and the third time he crossed the bridge and climbed down the post
+directly. The others did not succeed in descending the post even after
+having crossed the bridge safely, but, instead, finally fell to the floor
+from awkwardness or exhaustion. On the basis of these and other similar
+observations, Kishi says that the dancer possesses a fair degree of
+ability to orient and balance itself.
+
+Inasmuch as equilibration occurs similarly in darkness and in daylight,
+Zoth thinks that there is neither visual dizziness nor fear of heights.
+But it is doubtful whether he is right concerning fear. There is no doubt
+in my mind, in view of the way the mice behave when placed on an elevated
+surface, that they are timid; but this is due probably to the
+uncomfortable and unusual position rather than to perception of their
+distance from the ground. That they lack visual dizziness seems fairly
+well established.
+
+When rotated in a cyclostat[1] the dancer, unlike the common mouse, does
+not exhibit symptoms of dizziness. The following vivid description of the
+behavior of both kinds of mice when rotated is given by Alexander and
+Kreidl (1 p. 548). I have not verified their observations.
+
+[Footnote 1: An apparatus consisting of a glass cylinder with a mechanism
+for turning it steadily and at different speeds about its vertical axis.]
+
+The common mouse at first runs with increasing rapidity, as the speed of
+rotation of the cyclostat cylinder is increased, in the direction opposite
+to that of the cylinder itself. This continues until the speed of rotation
+has increased to about 60 revolutions per minute. As the rotation becomes
+still more rapid the mouse begins to crawl along the floor, its body
+stretched out and clinging to the floor. At a speed of 250 revolutions per
+minute it lies flat on the floor with its limbs extended obliquely to the
+movement of rotation, and at times with its back bent against the axis of
+the cylinder; in this position it makes but few and feeble efforts to
+crawl forward. When the rotation is suddenly stopped, the animal pulls
+itself together, remains for some seconds with extended limbs lying on the
+floor, and then suddenly falls into convulsions and trembles violently.
+After several attacks of this kind, cramps appear and, despite its
+resistance, the animal is thrown about, even into the air at times, as if
+by an external force. This picture of the position assumed during rapid
+rotation, and of cramps after the cessation of rotation (the typical
+picture of rotation dizziness), is repeated with great uniformity in the
+case of the common mouse. Within fifteen minutes after being returned to
+its cage the animal recovers from the effects of its experience. This
+description of the symptoms of rotation dizziness in the common mouse
+applies equally well to the blinded and the seeing animal.
+
+In sharp contrast with the behavior of the common mouse in the cyclostat
+is that of the dancer. As the cylinder begins to rotate the dancer runs
+about as usual in circles, zigzags, and figure-eights. As the speed
+becomes greater it naturally becomes increasingly difficult for the mouse
+to do this, but it shows neither discomfort nor fear, as does the common
+mouse. Finally the centrifugal force becomes so great that the animal is
+thrown against the wall of the cylinder, where it remains quietly without
+taking the oblique position. When the cyclostat is stopped suddenly, it
+resumes its dance movements as if nothing unusual had occurred. It
+exhibits no signs of dizziness, and apparently lacks the exhaustion which
+is manifest in the case of other kinds of mice after several repetitions
+of the experiment. The behavior of the blinded dancer is very similar.
+
+If these statements are true, there is no reason to believe that the
+dancer is capable of turning or rotation dizziness. If it were, its daily
+life would be rendered very uncomfortable thereby, for its whirling would
+constantly bring about the condition of dizziness. Apparently, then, the
+dancer differs radically from most mammals in that it lacks visual and
+rotational dizziness. In the next chapter we shall have to seek for the
+structural causes for these facts.
+
+The behavior of the blinded animal is so important in its bearings upon
+the facts of orientation and equilibration that it must be considered in
+connection with them. Cyon insists that the sense of vision is of great
+importance to the dancer in orienting and equilibrating itself. When the
+eyes are covered with cotton wads fastened by collodion, this writer
+states (9 p. 223) that the mice behave as do pigeons and frogs whose
+semicircular canals have been destroyed. They perform violent forced
+movements, turn somersaults forward and backward, run up inclines and fall
+over the edges, and roll over and over. In a word, they show precisely the
+kind of disturbances of behavior which are characteristic of animals whose
+semicircular canals are not functioning normally. Cyon, however, observed
+that in certain dancers these peculiarities of behavior did not appear
+when they were blinded, but that, instead, the animals gave no other
+indication of being inconvenienced by the lack of sight than do common
+white mice. This matter of individual differences we shall have to
+consider more fully later.
+
+No other observer agrees with Cyon in his conclusions concerning vision,
+or, for that matter, in his statements concerning the behavior of the
+blind dancer. Alexander and Kreidl (1 p. 550) contrast in the following
+respects the behavior of the white mouse and that of the dancer when they
+are blinded. The white mouse runs less securely and avoids obstacles less
+certainly when deprived of vision. The dancer is much disturbed at first
+by the shock caused by the removal of its eyes, or in case they are
+covered, by the presence of the unusual obstruction. It soon recovers
+sufficiently to become active, but it staggers, swerves often from side to
+side, and frequently falls over. It moves clumsily and more slowly than
+usual. Later these early indications of blindness may wholly disappear,
+and only a slightly impaired ability to avoid obstacles remains.
+
+It was noted by Kishi (21 p. 484), that the dancer when first blinded
+trembles violently, jumps about wildly, and rolls over repeatedly, as Cyon
+has stated; but Kishi believes that these disturbances of behavior are
+temporary effects of the strong stimulation of certain reflex centers in
+the nervous system. After having been blinded for only a few minutes the
+dancers observed by him became fairly normal in their behavior. They moved
+about somewhat more slowly than usually, especially when in a position
+which required accurately coordinated movements. He therefore fully agrees
+with Alexander and Kreidl in their conclusion that vision is not so
+important for the guidance of the movements of the dancer as Cyon
+believes.
+
+In summing up the results of his investigation of this subject Zoth well
+says (31 p. 168), "the orientation of the positions of the body with
+respect to the horizontal and vertical planes seems to take place without
+the assistance of the sense of sight." And, as I have already stated, this
+excellent observer insists that the ability of the dancer to place its
+body in a particular position (orientation), and its ability to maintain
+its normal relations to its surroundings (equilibration) are excellent in
+darkness and in daylight, provided only the substratum be not too smooth
+for it to gain a foothold.
+
+It must be admitted that the contradictions which exist in the several
+accounts of the behavior of the dancer are too numerous and too serious to
+be explained on the basis of careless observation. Only the assumption of
+striking individual differences among dancers or of the existence of two
+or more varieties of the animal suffices to account for the discrepancies.
+That there are individual or variety differences is rendered practically
+certain by the fact that Cyon himself worked with two groups of dancers
+whose peculiarities he has described in detail, both as to structure and
+behavior.
+
+In the case of the first group, which consisted of three individuals, the
+snout was more rounded than in the four individuals of the second group,
+and there were present on the head three large tufts of bristly black hair
+which gave the mice a very comical appearance. The animals of the second
+group resembled more closely in appearance the common albino mouse. They
+possessed the same pointed snout and long body, and only the presence of
+black spots on the head and hips rendered them visibly different from the
+albino mouse.
+
+In behavior the individuals of these two groups differed strikingly. Those
+of the first group danced frequently, violently, and in a variety of ways;
+they seldom climbed on a vertical surface and when forced to move on an
+incline they usually descended by sliding down backwards or sidewise
+instead of turning around and coming down head first; they gave no signs
+whatever of hearing sounds. Those of the second group, on the contrary,
+danced very moderately and in few ways; they climbed the vertical walls of
+their cage readily and willingly, and when descending from a height they
+usually turned around and came down head first; two of the four evidently
+heard certain sounds very well. No wonder that Cyon suggests the
+possibility of a different origin! It seems not improbable that the
+individuals of the second group were of mixed blood, possibly the result
+of crosses with common mice.
+
+As I shall hope to make clear in a subsequent discussion of the dancer's
+peculiarities of behavior, in a chapter on individual differences, there
+is no sufficient reason for doubting the general truth of Cyon's
+description, although there is abundant evidence of his inaccuracy in
+details. If, for the present, we accept without further evidence the
+statement that there is more than one variety of dancer, we shall be able
+to account for many of the apparent inaccuracies of description which are
+to be found in the literature on the animal.
+
+As a result of the examination of the facts which this chapter presents we
+have discovered at least six important peculiarities of behavior of the
+dancer which demand an explanation in terms of structure. These are: (1)
+the dance movements--whirling, circling, figure-eights, zigzags; (2)
+restlessness and the quick, jerky movements of the head; (3) lack of
+responsiveness to sounds; (4) more or less pronounced deficiency in
+orientational and equilibrational power; (5) lack of visual dizziness; (6)
+lack of rotational dizziness.
+
+Naturally enough, biologists from the first appearance of the dancing
+mouse in Europe have been deeply interested in what we usually speak of as
+the causes of these peculiarities of behavior. As a result, the structure
+of those portions of the body which are supposed to have to do with the
+control of movement, with the phenomena of dizziness, and with the ability
+to respond to sounds, have been studied thoroughly. In the next chapter we
+shall examine such facts of structure as have been discovered and attempt
+to correlate them with the facts of behavior.
+
+
+
+
+CHAPTER V
+
+
+STRUCTURAL PECULIARITIES AND BEHAVIOR
+
+The activities of an animal are expressions of changes which occur in its
+structure, and they can be explained satisfactorily only when the facts of
+structure are known. Such peculiarities of activity as are exhibited by
+the dancing mouse, as contrasted with the common mouse, suggest at once
+that this creature has a body which differs in important respects from
+that of the ordinary mouse. In this chapter I shall present what is known
+concerning the structural bases for the whirling, the lack of
+equilibrational ability and of dizziness, the quick jerky head movements,
+the restlessness, and the partial or total deafness of the dancing mouse.
+
+Comparative physiologists have discovered that the ability of animals to
+regulate the position of the body with respect to external objects and to
+respond to sounds is dependent in large measure upon the groups of sense
+organs which collectively are called the ear. Hence, with reason,
+investigators who sought structural facts with which to explain the forms
+of behavior characteristic of the dancer turned their attention first of
+all to the study of the ear. But the ear of the animal is not, as might be
+supposed on superficial examination, a perfectly satisfactory natural
+experiment on the functions of this group of sensory structures, for it is
+extremely uncertain whether any one of the usual functions of the organ is
+totally lacking. Dizziness may be lacking, and in the adult hearing also,
+but in general the functional facts lead the investigator to expect
+modifications of the sense organs rather than their absence.
+
+I shall now give an account of the results of studies concerning the
+structure of the ear and brain of the dancer. Since the descriptions given
+by different anatomists contradict one another in many important points,
+the several investigations which have been made may best be considered
+chronologically.
+
+Bernhard Rawitz (25 p. 239) was the first investigator to describe the
+structure of the ear of the Japanese or Chinese dancers, as he calls them.
+The definite problem which he proposed to himself at the beginning of his
+study was, what is the structural basis of the whirling movement and of
+the deafness of the mice?
+
+In his first paper Rawitz described the form of the ears of five dancers.
+His method of work was to make microscopic preparations of the ears, and
+from the sections, by the use of the Born method, to reconstruct the ear
+in wax. These wax models were then drawn for the illustration of the
+author's papers (Figures 8, 9, 10).
+
+The principal results of the early work of Rawitz are summed up in the
+following quotation from his paper: "The Japanese dancing mice have only
+one normal canal and that is the anterior vertical. The horizontal and
+posterior vertical canals are crippled, and frequently they are grown
+together. The utriculus is a warped, irregular bag, whose sections have
+become unrecognizable. The utriculus and sacculus are in wide-open
+communication with each other and have almost become one. The utriculus
+opens broadly into the scala tympani, and the nervous elements of the
+cochlea are degenerate.
+
+"The last-mentioned degeneration explains the deafness of the dancing
+mice; but in my opinion it is a change of secondary nature. The primary
+change is the broad opening between the utriculus and the scala tympani
+from which results the streaming of the endolymph from the semicircular
+canals into the cochlea. When, as a consequence of the rapid whirling
+movements, a great part of the endolymph is hurled into the scala tympani,
+the organ of Corti in the scala vestibuli is fixed and its parts are
+rendered incapable of vibration. The condition of atrophy which is
+observable in the sense cells and in the nerve elements is probably due to
+the impossibility of functional activity; it is an atrophy caused by
+disuse "(25 p. 242).
+
+
+Ampulla externa
+Ampulla anterior
+Ramus utriculi
+
+Membrana basilaris
+
+Lagena
+
+Canalis utriculo-saccularis
+
+Membrana basilaris
+Ampulla posterior
+Macula acustica sacculi
+
+[Illustration: FIGURE 7.--The inner ear of the rabbit. Reproduced from
+Selenka after Retzius.]
+
+
+To render the terms which occur in this and subsequent descriptions of the
+ear of the dancer somewhat more intelligible to those who are not familiar
+with the general anatomy of the vertebrate ear, a side view of the inner
+ear of the rabbit is reproduced from a drawing by Retzius (Figure 7). I
+have chosen the ear of the rabbit for this purpose, not in preference to
+that of the common mouse, but simply because I failed to find any reliable
+description of the latter with drawings which could be reproduced. The
+rabbit's ear, however, is sufficiently like that of the mouse to make it
+perfectly satisfactory for our present purpose.
+
+This drawing of the rabbit's ear represents the three semicircular canals,
+which occur in the ear of all mammals, and which are called, by reason of
+their positions, the anterior vertical, the posterior vertical, and the
+horizontal. Each of these membranous canals possesses at one end, in an
+enlargement called the ampulla, a group of sense cells. In Figure 7 the
+ampullae of the three canals are marked respectively, ampulla anterior,
+ampulla posterior, and ampulla externa. This figure shows also the
+cochlea, marked lagena, in which the organ of hearing of mammals (the
+organ of Corti) is located. The ear sac, of which the chief divisions are
+the utriculus and the sacculus, with which the canals communicate, is not
+shown well in this drawing.
+
+Within a few months after the publication of Rawitz's first paper on the
+structure of the dancer's ear, another European investigator, Panse (23
+and 24) published a short paper in which he claimed that previous to the
+appearance of Rawitz's paper he had sectioned and mounted ears of the
+common white mouse and the dancing mouse side by side, and, as the result
+of careful comparison, found such slight differences in structure that he
+considered them unworthy of mention. Panse, therefore, directly
+contradicts the statements made by Rawitz. In fact, he goes so far as to
+say that he found even greater differences between the ears of different
+white mice than between them and the ears of the dancer (23 p. 140).
+
+In a somewhat later paper Panse (24 p. 498) expresses his belief that,
+since there are no peculiarities in the general form, sensory structures,
+or nerve supply of the ear of the dancer, which serve to explain the
+behavior of the animal, it is probable that there are unusual structural
+conditions in the brain, perhaps in the cerebellum, to which are due the
+dance movements and the deafness. The work of Panse is not very
+convincing, however, for his figures are poor and his descriptions meager;
+nevertheless, it casts a certain amount of doubt upon the reliability of
+the descriptions given by Rawitz.
+
+[Illustration: FIGURE 8.--The membranous labyrinth of the dancer's ear.
+Type I. This figure, as well as 9 and 10, are reproduced from Rawitz's
+figures in the _Archiv für Anatomie und Physiologie, Physiologische
+Abtheilung_, 1899. _C.s._, anterior vertical canal; _C.p._, posterior
+vertical canal; _C.e._, horizontal canal; _U._, utriculus.]
+
+The unfavorable light in which his report was placed by Panse's statements
+led Rawitz to examine additional preparations of the ear of the dancer.
+Again he used the reconstruction method. The mice whose ears he studied
+were sent to him by the physiologist Cyon.
+
+As has been noted in Chapter IV, Cyon discovered certain differences in
+the structure and in the behavior of these dancers (11 p. 431), which led
+him to classify them in two groups. The individuals of one group climbed
+readily on the vertical walls of their cages and responded vigorously to
+sounds; those of the other group could not climb at all and gave no
+evidences of hearing. After he had completed his study of their behavior,
+Cyon killed the mice and sent their heads to Rawitz; but unfortunately
+those of the two groups became mixed, and Rawitz was unable to distinguish
+them. When he examined the structure of the ears of these mice, Rawitz did
+find, according to his accounts, two structural types between which very
+marked differences existed. Were it not for the carelessness which is
+indicated by the confusion of the materials, and the influence of Cyon's
+suggestion that there should be different structures to account for the
+differences in behavior, Rawitz's statements might be accepted. As matters
+stand there can be no doubt of individual differences in behavior,
+external appearance, and the structure of the ear; but until these have
+been correlated on the basis of thoroughgoing, careful observation, it is
+scarcely worth while to discuss their relations.
+
+[Illustration: FIGURE 9.--The membranous labyrinth of the dancer's ear.
+Type II.]
+
+[Illustration: FIGURE 10.--The membranous labyrinth of the dancer's ear.
+Type III.]
+
+To his previous description of the conditions of the ear sacs, sense
+organs, and nerve elements of the dancer's ear, Rawitz adds nothing of
+importance in his second paper (26 p. 171). He merely reiterates his
+previous statements concerning the form of the canals, on the basis of his
+findings in the case of six additional dancers. Figures 8, 9, and 10 are
+reproduced from Rawitz to show the anatomical conditions which he claims
+that he found. As these figures indicate, the canals were found to be
+extremely variable, as well as unusual in form, and the sacs distorted. In
+the ears of some specimens there were only two canals, and in all cases
+they were more or less reduced in size, distorted, or grown together.
+
+[Illustration: FIGURE 11.--Photograph of a wax model of the membranous
+labyrinth of the ear of the dancer. Reproduced from Baginsky's figure in
+the _Centralblatt für Physiologie_, Bd. 16.]
+
+The work of Rawitz was unfavorably criticised by Alexander and Kreidl (2),
+Kishi (21), and Baginsky (4), as well as by Panse (23 and 24). To their
+criticisms Rawitz replied by insisting that the other investigators could
+not with right attack his statements because they had not used the
+reconstruction method. In order to test the value of this contention, and
+if possible settle the question of fact, Baginsky had a model of the ear
+of the dancer constructed by a skilled preparator (Herr Spitz) from
+sections which had been prepared by the best neurological methods. This
+model was made eighty times the size of the ear. It was then reduced in
+the process of photographic reproduction to sixteen times the natural size
+of the ear in the mouse. Figure 11 is a photograph of Baginsky's model. It
+shows beyond question the presence of three canals of the same general
+form and relations as those of the common mouse and of other mammals.
+Baginsky's paper is brief and to the point. His criticisms of the work of
+both Cyon and Rawitz are severe, but they are justified in all probability
+by the carelessness of these investigators in the fixation of their
+materials. Of the five skilled histologists who have examined the ear of
+the dancer, Rawitz alone found markedly abnormal canals. It is highly
+probable, therefore, that the canals in his preparations in some way
+became distorted before the ears were sectioned. He doubtless described
+accurately the conditions which he found, but the chances are that those
+conditions never existed in the living animals.
+
+The conflicting statements of Rawitz and Panse stimulated interest, and as
+a result two other investigators, without knowledge of one another's work,
+began careful researches on the dancer's ear. One, Alexander (2 and 3),
+worked in coöperation with the physiologist Kreidl; the other, Kishi (21),
+worked independently. The anatomical papers of Alexander and Kishi
+appeared at about the same time, and since neither contains a reference to
+the other, it is evident that the investigations were carried on almost
+simultaneously. Alexander's descriptions are more detailed than those of
+Rawitz and Panse, and in certain respects Kishi's are even more
+thoroughgoing. The first paper published by Alexander and Kreidl (1)
+contains the results of observations on the habits and behavior of the
+dancers. Having examined the chief facts of function, these investigators
+attempted to discover the structural conditions for the peculiarities of
+behavior which they had observed.
+
+As material for their anatomical work they made use of four dancers, one
+albino mouse, and four common gray mice. The ears of these individuals
+were fixed, sectioned, and examined microscopically in connection with
+parts of the brain. In all, eight dancer ears and six common mouse ears
+were studied.
+
+Very extensive descriptions of these preparations, together with
+measurements of many important portions of the ear, are presented in their
+paper, the chief conclusions of which are the following:--
+
+1. The semicircular canals, the ampullae, the utriculus, and the cristae
+acusticae of the canals are normal in their general form and relations to
+one another as well as in their histological conditions (2 p. 529). This
+is contradictory of the statements made by Rawitz.
+
+2. There is destruction of the macula sacculi (2 p. 534).
+
+3. There is destruction also of the papilla basilaris cochleae, with
+encroachment of the surrounding tissues in varying degrees.
+
+4. There is diminution in the number of fibers of the branches and roots
+of the ramus superior and ramus medius of the eighth nerve, and the fiber
+bundles are very loosely bound together.
+
+5. Similarly the number of fibers in the inferior branch (the cochlear
+nerve) of the eighth nerve is very much reduced.
+
+6. There is moderate reduction in the size of the two vestibular ganglia
+as a result of the unusually small number of nerve cells.
+
+7. The ganglion spirale is extremely degenerate.
+
+There is therefore atrophy of the branches, ganglia, and roots of the
+entire eighth nerve, together with atrophy and degeneration of the pars
+inferior labyrinthii. The nerve endings are especially degenerate (2 p.
+534).
+
+The above structural deviations of the ear of the dancer from that of the
+common mouse may be considered as primary or secondary according as they
+are inherited or acquired. Since, according to Alexander and Kreidl, the
+dancers' peculiarities of behavior and deafness are directly and uniformly
+inherited, it is obvious that certain primary structural deviations must
+serve as a basis for these functional facts. But it is equally clear, in
+the opinion of Alexander and Kreidl (2 p. 536), that other structural
+peculiarities of the dancer are the result of the primary changes, and in
+no way the conditions for either the dancing or the deafness. These
+authors feel confident that the facts of behavior which are to be
+accounted for are almost certainly due to the pathological changes which
+they have discovered in the nerves, ganglia, and especially in the
+peripheral nerve endings of the ear of the mouse (2 p. 537).
+
+It is further claimed by Alexander and Kreidl that there are very marked
+individual differences among the dancers in the structure of the ear. In
+some cases the otoliths and the sensory hairs are lacking; in others, they
+are present in the state of development in which they are found in other
+varieties of mouse. Sometimes the cochlea is much reduced in size; at
+other times it is found to be of normal size (2 p. 538). These variations
+in structure, if they really exist, go far toward justifying the tendency
+of Cyon and Alexander and Kreidl, as well as many other investigators, to
+regard the dancer as abnormal or even pathological.
+
+The functions of the ear as at present known to the comparative
+physiologist are grouped as the acoustic and the non-acoustic. The cochlea
+is supposed on very good grounds to have to do with the acoustic
+functions, and the organs of the semicircular canals on equally good
+evidence are thought to have to do with such of the non-acoustic functions
+as equilibration and orientation. Just what the functions of the organs of
+the ear sacs are is not certainly known. These facts are of importance
+when we consider the attempts made by Alexander and Kreidl to correlate
+the various peculiarities of behavior shown by the dancer with the
+structural facts which their work has revealed. This correlation is
+indicated schematically below. The physiological facts to be accounted for
+in terms of structure are presented in the first column, and the
+anatomical facts which are thought to be explanatory, in the second (2 p.
+539).
+
+
+FUNCTION
+
+1 Lack of sensitiveness to auditory stimuli. {Structure 1,2,3 below}
+
+2 Defective equilibrational ability. {Structure 4,5,6 below}
+
+3 Lack of turning dizziness. {Structure 4,5,6 below}
+
+4 Normal reactions to galvanic stimulation. (not related in table to any
+Structure)
+
+
+STRUCTURE
+
+1 Destruction of the papilla basilaris cochleae, etc.
+
+2 Diminution of the inferior branch of the eighth nerve.
+
+3 Marked degeneration of the ganglion spirale.
+
+4 Destruction of the macula sacculi.
+
+5 Diminution of the branches and roots of the superior and middle branches
+of the eighth nerve.
+
+6 Diminution of both ganglia vestibulii and of the nerve cells.
+
+
+Alexander and Kreidl themselves believe that the partial deafness of the
+dancers (for they admit that the total lack of hearing has not been
+satisfactorily proved) is due to the defective condition of the cochlea.
+They account for the imperfect equilibrational ability of the animals by
+pointing out the structural peculiarities of the sacculus, the vestibular
+ganglia, and the peripheral nerves. Similarly, the lack of dizziness they
+suppose to be due to the diminution of the fibers of the nerves which
+supply the canal organs, the atrophied condition of the vestibular
+ganglia, and a disturbance of the peripheral sense organs. Furthermore,
+there are no anatomical facts which would indicate a lack of galvanic
+dizziness (2 p. 552).
+
+Despite the fact that they seem to explain all the functional
+peculiarities of the dancer, the statements made by Alexander and Kreidl
+are neither satisfying nor convincing. Their statements concerning the
+structure of the ear have not been verified by other investigators, and
+their correlation of structural with functional facts lacks an
+experimental basis.
+
+In this connection it may be worth while to mention that a beautiful
+theory of space perception which Cyon (9) had constructed, largely on the
+basis of the demonstration by Rawitz that the dancers have only one normal
+canal, is totally destroyed by Panse, Baginsky, Alexander and Kreidl, and
+Kishi, for all of these observers found in the dancer three canals of
+normal shape. Cyon had noted that the most abnormal of the voluntary as
+well as of the forced movements of the dancer occur in the plane of the
+canal which Rawitz found to be most strikingly defective. This fact he
+connected with his observation that the fish Petromyzon, which possesses
+only two canals, moves in only two spatial dimensions. The dancer with
+only one functional canal in each ear moves in only one plane, and neither
+it nor Petromyzon is able to move far in a straight line (11 p. 444). From
+these and similar surmises, which his eagerness to construct an ingenious
+theory led him to accept as facts quite uncritically, Cyon concluded that
+the perception of space depends upon the number and arrangement of the
+semicircular canals, and that the dancer behaves as it does because it
+possesses canals of unusual shape and relations to one another. The
+absurdity of Cyon's position becomes obvious when it is shown that the
+structural conditions of which he was making use do not exist in the
+dancer.
+
+The results obtained by Kishi in his study of the ear of the dancer differ
+in many important respects from those of all other investigators, but
+especially from those of Rawitz and Alexander and Kreidl.
+
+Kishi's work was evidently done with admirable carefulness. His methods in
+the preparation of his materials, so far as can be judged from his report,
+were safe and satisfactory, and his descriptions of results are minute and
+give evidence of accuracy and conscientious thoughtfulness. The material
+for his histological work he obtained from three different animal dealers.
+It consisted of fifteen adult and nineteen young dancers, and, as material
+for comparison, ten common gray mice. The animals were studied first
+biologically, that their habits and behavior might be described accurately
+and so far as possible accounted for in the light of whatever histological
+results might be obtained subsequently; then they were studied
+physiologically, that the functional importance of various organs which
+would naturally be supposed to have to do with the peculiarities of the
+mouse might be understood; and, finally, they were killed and their ears
+and portions of their brains were studied microscopically, that structural
+conditions for the biological and physiological facts might be discovered.
+
+The ear, which was studied by the use of several series of sections, as
+well as in gross dissections, is described by Kishi under three
+headings:--
+
+(1) The sound-receiving apparatus (auditory organs).
+
+(2) The static apparatus (equilibrational organs).
+
+(3) The sound-transmitting apparatus (ear drum, ear bones, etc.).
+
+The chief results of his structural investigation may be stated briefly
+under these three headings. In the sound-receiving or auditory apparatus,
+Kishi failed to find the important deviations from the usual structure of
+the mammalian ear which had been described by Rawitz. The latter
+distinctly says that although the organ of Corti is present in all of the
+whirls of the cochlea, the auditory cells in it are noticeably degenerate.
+Kishi does not agree with Panse's statement (21 p. 476) that the auditory
+organ of the dancer differs in no important respects from that of the
+common mouse, for he found that in certain regions the hair cells of the
+organ of Corti were fewer and smaller in the dancer. He therefore
+concludes that the auditory organ is not entirely normal, but at the same
+time he emphasizes the serious discrepancy between his results and those
+of Rawitz. In not one of the ears of the twelve dancers which he studied
+did Kishi find the direct communication between the utriculus and the
+scala tympani which Rawitz described, and such differences as appeared in
+the organ of Corti were in the nature of slight deviations rather than
+marked degenerations.
+
+In the outer wall of the ductus cochlearis of the dancer the stria
+vasculosa is almost or totally lacking, while in the gray mouse it is
+prominent. This condition of the stria vasculosa Kishi was the first to
+notice in the dancer; Alexander and Kreidl had previously described a
+similar condition in an albino cat. If, as has been supposed by some
+physiologists, the stria vasculosa is really the source of the endolymph,
+this state of affairs must have a marked influence on the functions of the
+auditory apparatus and the static apparatus, for pressure differences
+between the endolymph and the perilymph spaces must be present. And, as
+Kishi points out, should such pressure differences be proved to exist, the
+functional disturbance in the organ of hearing which the lack of responses
+to sounds seems to indicate might better be ascribed to them than to the
+streaming of the endolymph from the canals into the cochlea as assumed by
+Rawitz (21 p. 477). Kishi merely suggests that the condition of the stria
+may account for the deafness of the mouse; he does not feel at all
+confident of the truth of his explanation, and he therefore promises in
+his first paper a continuation of his work in an investigation of the
+functions of the stria. This, however, he seems not to have accomplished
+thus far.
+
+[Illustration: FIGURE 12.--The inner ear of the dancer. Reproduced from
+Kishi's figure in the _Zeitschrift für wissenschaftliche Zoölogie_, Bd.
+71. _c.c._ crus simplex; o.b. anterior vertical canal; _h.b._ posterior
+vertical canal; _a.b._ horizontal canal.]
+
+The static apparatus Kishi describes as closely similar in form to that of
+the gray mouse. In none of his twelve preparations of the ear of the
+dancer did he find such abnormalities of form and connections in the
+semicircular canals as Rawitz's figures and descriptions represent. Rawitz
+states that the anterior canal is normal except in its lack of connection
+with the posterior and that the posterior and horizontal are much reduced
+in size. Kishi, on the contrary, insists that all of the three canals are
+normal in shape and that the usual connection between the anterior and the
+posterior canals, the crus simplex, exists. He justifies these statements
+by presenting photographs of two dancer ears which he carefully removed
+from the head. Comparison of these photographs (Figures 12 and 13) with
+Rawitz's drawings of the conditions of the canals and sacs as he found
+them (Figures 8, 9, and 10), and of both with the condition in the typical
+mammalian ear as shown by Figure 7, will at once make clear the meaning of
+Kishi's statements. That Rawitz's descriptions of the canals are not
+correct is rendered almost certain by the fact that Panse, Baginsky,
+Alexander and Kreidl, and Kishi all agree in describing them as normal in
+form.
+
+The only important respects in which Kishi found the membranous labyrinth,
+that is, the canals and the ear sacs, of the dancer to differ from that of
+the gray mouse are the following. In the dancer the cupola of the crista
+acustica is not so plainly marked and not so highly developed, and the
+raphae of the ampullae and canals, which frequently are clearly visible in
+the gray mouse, are lacking (21 p. 478).
+
+[Illustration: FIGURE 13.--The inner ear of the dancer, showing the spiral
+form of the cochlea. After Kishi.]
+
+The sound-transmitting apparatus of the dancer, according to Kishi,
+differs only very slightly from that of the gray mouse, and there is no
+reason to consider the differences which appear as important (21 p. 478).
+
+Almost as amusing as the way in which Cyon's theory of space perception
+disappears in the light of critical research is Panse's explanation of the
+deafness of the dancer. Failing to find any defects in the auditory
+apparatus of the inner ear which seemed adequate to account for the
+obvious lack of responsiveness to sounds, this investigator concluded that
+plugs of wax which he had noticed in the auditory meatus of the dancer
+excluded sounds or in some way interfered with the functioning of the
+tympanic membrane. Kishi reports that he found such plugs of wax in the
+ears of one gray mouse, but in none of the dancers which he examined did
+he discover them (21 p. 479). Panse's explanation of the defective hearing
+of the dancer neither needs nor deserves further comment.
+
+As one result of his investigation, Kishi is convinced that the dance
+movements are not due to peculiarities in the semicircular canals and
+their sense organs, as Rawitz claimed, for the general form and finer
+structure of these organs in the dancer is practically the same as in the
+common mouse. Kishi is just as certain that the whirling is not due to
+defects in the canal organs, as Rawitz is that it is due to such
+structural conditions! It is rather surprising that any one should feel
+confident of the power of the microscope to reveal all those structural
+conditions which are important as conditions of function. Probably there
+are histological differences between the ear of the dancer and that of the
+gray mouse, which, although undetectable by scientific means at present,
+furnish the structural basis for the marked differences in behavior. As
+has been set forth already (p. 9), Kishi accounts for the dance movements
+by assuming the inheritance of an acquired character of behavior. This
+inherited tendency to dance, he thinks, has been accentuated by the
+confinement of the mice in narrow cages and their long-continued movement
+in the wheels which are placed in the cages (21 p. 481).
+
+Rawitz, Cyon, and Alexander and Kreidl felt themselves under the necessity
+of finding peculiarities of behavior in the dancer which could be referred
+to the various abnormalities of structure which they had either seen or
+accepted on faith; Kishi found himself in a very different predicament,
+for he had on his hands the commonly accepted statement that the animals
+are deaf, without being able to find any structural basis for this defect.
+To avoid the difficulty he questions the existence of deafness! If
+perchance they are deaf, he thinks that it is possibly because of the
+defect in the stria vasculosa. This suggestion Kishi makes despite the
+fact that our ignorance of the function of the stria renders it impossible
+for us to do otherwise than guess at its relation to hearing.
+
+We have now briefly reviewed the results of the various important
+investigations of the behavior and structure of the dancer.
+
+The observations of Cyon, Zoth, and the writer establish beyond doubt the
+existence of important individual differences in behavior if not of
+distinct divisions within the species of mouse, and the general results of
+the several anatomical investigations make it seem highly probable that
+the structure of the ear, as well as the externally visible structural
+features of the animals, vary widely. Unfortunately, the lack of agreement
+in the descriptions of the ear given by the different students of the
+subject renders impossible any certain correlation of structural and
+functional facts. That the whirling and the lack of dizziness and of
+hearing have their structural bases no one doubts, but whether it is in
+the brain itself, in the sense organs, or in the labyrinth, our knowledge
+does not permit us to say. With this statement Rawitz, Cyon, and Alexander
+and Kreidl would not agree, for they believe that they have discovered
+structural peculiarities which fully explain the behavior of the dancer.
+Panse and Kishi, on the other hand, contend that the ear gives no
+structural signs of such peculiarities as the dancing and deafness
+suggest; they therefore look to the cerebellum for the seat of the
+disturbance. With the same possibility in mind the author of "Fancy
+Varieties of Mice" writes: "These quaint little creatures make amusing
+pets for any one who is not scientific, or very fond of knowing 'the
+reason why.' In their case, the reason of the peculiarity which gives them
+their name is rather a sad one. It is now pretty conclusively established
+that they are no more Japanese than they are of any other country in
+particular, but that the originators of the breed were common fancy mice
+which were suffering from a disease of the brain analogous to the 'gid' in
+sheep. In the latter, the complaint is caused by a parasite in the brain;
+in the case of the Waltzing Mouse, it is probably due to an hereditary
+malformation therein. Be this as it may, the breed is now a firmly
+established one, and the children of waltzing mice waltz like their
+parents" (32 p. 45). Although it is quite possible that peculiarities in
+the central nervous system, rather than in the peripheral nervous system,
+may be responsible for the forms of behavior exhibited by the dancer, it
+must be remembered that no such peculiarities have been revealed by the
+examination of the central nervous system. The old fancier has neither
+better nor worse grounds for his belief than have Panse and Kishi.
+
+So far as the reliability of the anatomical work which has been discussed
+is in question, it would seem that Rawitz's results are rendered somewhat
+unsatisfactory by the carelessness of Cyon in fixing the materials; that
+Panse's descriptions and comparisons are neither careful nor detailed
+enough to be convincing; that the work of Alexander and Kreidl, as well as
+that of Kishi, gives evidence of accuracy and trustworthiness. The fact
+that the statements of Alexander and Kreidl frequently do not agree with
+those of Kishi proves that there are serious errors in the work of one or
+another of these investigators. Cyon's discussion of the anatomy of the
+dancer is not to be taken too seriously, for by his theory of space
+perception and of a sixth sense he was unduly biased in favor of the
+structural peculiarities described by Rawitz. Nevertheless, his discussion
+is not without interest, for the way in which he succeeded in making every
+structural fact which Rawitz suggested fit into his theories and help to
+account for the functional peculiarities which he had himself observed, is
+extremely clever and indicates a splendid scientific imagination.
+
+To sum up: All the facts of behavior and physiology which have been
+established lead us to expect certain marked structural differences
+between the dancer and the common mouse. The bizarre movements, lack of
+equilibrational ability, and the nervous shaking of the head suggest the
+presence of peculiar conditions in the semicircular canals or their sense
+organs; and the lack of sensitiveness to sounds indicates defects in the
+cochlea. Yet, strange as it may seem to those who are not familiar with
+the difficulties of the study of the minute structure of these organs, no
+structural conditions have been discovered which account satisfactorily
+for the dancer's peculiarities of behavior. That the ear is unusual in
+form is highly probable, since three of the four investigators who have
+studied it carefully agree that it differs more or less markedly from that
+of the common mouse. But, on the other hand, the serious lack of agreement
+in their several descriptions of the conditions which they observed
+renders their results utterly inconclusive and extremely unsatisfactory.
+The status of our knowledge of the structure of the central nervous system
+is even less satisfactory, if possible, than that of our knowledge of
+those portions of the peripheral nervous system which would naturally be
+supposed to have to do with such functional peculiarities as the dancer
+exhibits. So far as I have been able to learn, no investigator has
+carefully examined the brain and spinal cord in comparison with those of
+the common mouse, and only those who have failed to find any structural
+basis for the facts of behavior in the organs of the ear have attempted to
+account for the dancer's whirling and deafness by assuming that the
+cerebellum is unusual in structure. We are, therefore, forced to conclude
+that our knowledge of the nervous system of the dancing mouse does not at
+present enable us to explain the behavior of the animal.
+
+It seems highly probable to me, in the light of my observation of the
+dancer and my study of the entire literature concerning the animal, that
+no adequate explanation of its activities can be given in terms of the
+structure of the peripheral or the central nervous system, or of both, but
+that the structure of the entire organism will have to be taken into
+account. The dancer's physiological characteristics, in fact, suggest
+multitudinous structural peculiarities. I have confined my study to its
+behavior, not because the problems of structure seemed less interesting or
+less important, but simply because I found it necessary thus to limit the
+field of research in order to accomplish what I wished within a limited
+period.
+
+That there are structural bases for the forms of behavior which this book
+describes is as certain as it could be were they definitely known; that
+they, or at least some of them, are discoverable by means of our present-
+day histological methods is almost as certain. It is, therefore, obvious
+that this is an excellent field for further research. It is not an
+agreeable task to report inconclusive and contradictory results, and I
+have devoted this chapter to a brief account of the work that has been
+done by others on the structure of the ear of the dancer rather for the
+sake of presenting a complete account of the animal as it is known to-day
+than because of the value of the facts which could be stated.
+
+
+
+
+CHAPTER VI
+
+THE SENSE OF HEARING
+
+Repeatedly in the foregoing chapters mention has been made of the dancer's
+irresponsiveness to sounds, but it has not been definitely stated whether
+this peculiarity of behavior is due to deafness or to the inhibition of
+reaction. This chapter is concerned with the evidence which bears upon the
+problem of the existence of a sense of hearing. Again I may be permitted
+to call attention to the observations of other investigators before
+presenting the results of my own experiments and stating the conclusions
+which I have reached through the consideration of all available facts.
+
+By the results of various simple tests which he made, Rawitz (25 p. 238)
+was convinced that the adult dancer is totally deaf. He did not experiment
+with the young, but he says he thinks they may be able to hear, since the
+necessary structural conditions are present. This guess which Rawitz made
+on the basis of very indefinite and uncertain knowledge of the histology
+of the ear of the young dancer is of special interest in the light of
+facts revealed by my own experiments. Unfortunately the study of hearing
+made by Rawitz is casual rather than thorough, and although it may turn
+out that all of his statements are justified by his observations, the
+reader is not likely to get much satisfaction from his discussion of the
+subject.
+
+Inasmuch as he could discover no structural basis for deafness, Panse (23
+p. 140) expressed himself as unwilling to believe that the mice are deaf,
+and this despite the fact that he observed no responses to the sounds made
+by a series of tuning forks ranging from C5 to C8. He believes rather that
+they are strangely irresponsive to sounds and that their sensitiveness is
+dulled, possibly, by the presence of plugs of wax in the ears. Since
+another investigator, Kishi, has observed the presence of similar plugs of
+wax in the ears of common mice which could hear, there is but slight
+probability that Panse is right in considering the plugs of wax as the
+cause of the dancer's irresponsiveness to sounds.
+
+Far more thoroughgoing tests than those of Rawitz or Panse were made by
+Cyon (9 p. 218), who holds the unique position of being the only person on
+record who has observed the adult dancer give definite reactions to
+sounds. To a König Galton whistle so adjusted that it gave a tone of about
+7000 complete vibrations per second, which is said to be about the pitch
+of the voice of the dancer, some of the animals tested by Cyon responded
+unmistakably, others not at all. In one group of four mice, two not only
+reacted markedly to the sound of the whistle but apparently listened
+intently, for as soon as the whistle was blown they ran to the side of the
+cage and pressed their noses against the walls as if attempting to
+approach the source of the stimulus. The remaining two mice gave not the
+slightest indication that the sound acted as a stimulus. By the repetition
+of this sound from eight to twelve times Cyon states that he was able to
+arouse the mice from sleep. When thus disturbed, the female came out of
+the nest box before the male. Similarly when the mice were disturbed by
+the whistle in the midst of their dancing, the female was first to retreat
+into the nest box. There is thus, according to Cyon, some indication of
+sex, as well as individual, differences in sensitiveness to the sound of
+the whistle. Cyon's statement that in order to evoke a response the
+whistle must be held above the head of the dancer suggests at once the
+possibility that currents of air or odors instead of sounds may have been
+responsible for the reactions which he observed. The work of this
+investigator justifies caution in the acceptance of his statements.
+Neither the conditions under which the auditory tests were made nor the
+condition of the animals is described with sufficient accuracy to make
+possible the comparison of Cyon's work with that of other investigators.
+As will appear later, it is of the utmost importance that the influence of
+other stimuli than sound be avoided during the tests and that the age of
+the mouse be known. The conclusion reached by Cyon is that some dancers
+are able to hear sounds of about the pitch of their own cries.
+
+The fact, emphasized by Cyon, that the mice respond to tones of about the
+pitch of their own voice is of peculiar interest in its relation to the
+additional statements made by the same author to the effect that the
+female dancer is more sensitive to sounds than the male, and that the
+males either do not possess a voice or are much less sensitive to
+disagreeable stimuli than the females. In the case of the dancers which he
+first studied (9 p. 218), Cyon observed that certain strong stimuli evoked
+pain cries; but later in his investigation he noticed that four
+individuals, all of which were males, never responded thus to disagreeable
+stimulation (11 p.431). He asks, therefore, does this mean that the males
+lack a voice or that they are less sensitive than the females? The fact
+that he did not succeed in getting a definite answer to this simple
+question is indicative of the character of Cyon's work. My dancers have
+provided me with ample evidence concerning the matter. Both the males and
+the females, among the dancers which I have studied, possess a voice.
+
+The females, especially during periods of sexual excitement, are much more
+likely to squeak than the males. At such times they give their shrill cry
+whenever they are touched by another mouse or by the human hand. A slight
+pinching of the tail will frequently cause the female to squeak, but the
+male seldom responds to the same stimulus by crying out. The most
+satisfactory way to demonstrate the existence of a voice in the male is to
+subject him to the stimulating effect of an induced current, so weak that
+it is barely appreciable to the human hand. To this unexpected stimulus
+even the male usually responds by a sudden squeak. There can be no doubt,
+then, of the possession of a voice by both males and females. The males
+may be either less sensitive or less given to vocal expression, but they
+are quite able to squeak when favorable conditions are presented. The
+possession of a voice by an animal is presumptive evidence in favor of a
+sense of hearing, but it would scarcely be safe to say that the mice must
+be able to hear their own voices. Cyon, however, thinks that some dancers
+can. What further evidence is to be had?
+
+Although they obtained no visible motor reactions to such noises as the
+clapping of the hands, the snapping of the fingers, or to the tones of
+tuning forks of different pitches and the shrill tones of the Galton
+whistle, Alexander and Kreidl (1 p. 547) are not convinced of the total
+deafness of the dancer, for, as they remark, common mice which undoubtedly
+hear do not invariably respond visibly to sounds. Furthermore, the
+anatomical conditions revealed by their investigation of the ear of the
+dancer are not such as to render sensitiveness to sounds impossible. They
+recognize also that the existence of the ability to produce sounds is an
+indication of hearing. They have no confidence in the results reported by
+Cyon, for they feel that he did not take adequate precautions to guard
+against the action of other than auditory stimuli.
+
+Zoth (31 p. 170) has pointed out with reason and force that testing the
+sensitiveness of the mice is especially difficult because of their
+restlessness. They are almost constantly executing quick, jerky movements,
+starting, stopping, or changing the direction of movement, and it is
+therefore extremely difficult to tell with even a fair degree of certainty
+whether a given movement which occurs simultaneously with a sound is a
+response to the sound or merely coincident with it. With great care in the
+exclusion of the influence of extraneous stimuli, Zoth tried a large
+number of experiments to test the hearing of both young and adult dancers.
+Not once did he observe an indubitable auditory reaction. As he says, "I
+have performed numerous experiments with the Galton whistle, with a
+squeaking glass stopper, with caps and cartridges, without being able to
+come to any certain conclusion. With reference to the Galton whistle and
+particularly to the tone which was said to have been heard extremely well
+by Cyon's mice, I believe I am rather safe in asserting that my mice,
+young (12-13 days) as well as old, do not react to the König Galton
+whistle (7210 Vs.). They could not be awakened out of sleep by repetitions
+of the sound, nor enticed out of their nests, and their dancing could not
+be interrupted" (31 p. 170). Zoth's experiments appear to be the most
+careful and critical of those thus far considered.
+
+Last to be mentioned, but in many respects of greatest interest and value,
+is the work of Kishi (21 p. 482) on the problem of hearing. To this acute
+observer belongs the credit of calling attention emphatically to the ear
+movements which are exhibited by the dancer. Frequently, as he remarks,
+the ears move as if the animal were listening or trying to determine the
+direction whence comes a sound, yet usually the mouse gives no other sign
+of hearing. That the absence of ordinary reactions to sounds is due to
+deafness, Kishi, like Panse, is led to doubt because his anatomical
+studies have not revealed any defects in the organs of hearing which would
+seem to indicate the lack of this sense.
+
+This historical survey of the problem of hearing has brought out a few
+important facts. No one of the several investigators of the subject, with
+the exception of Cyon, is certain that the dancer can hear, and no one of
+them, with the exception of Rawitz, is certain that it cannot hear! Cyon
+almost certainly observed two kinds of dancing mice. Those of his dancers
+which exhibited exceptional ability to climb in the vertical direction and
+which also gave good evidence of hearing certain sounds may have been
+hybrids resulting from the crossing of the dancer with a common mouse, or
+they may have been exceptional specimens of the true dancer variety. A
+third possibility is suggested by Rawitz's belief in the ability of the
+young dancer to hear. Cyon's positive results may have been obtained with
+immature individuals. I am strongly inclined to believe that Cyon did
+observe two types of dancer, and to accept his statement that some of the
+mice could hear, whereas others could not. It is evident, in the light of
+our examination of the experimental results thus far obtained by other
+investigators, that neither the total lack of sensitiveness to sounds in
+the adult nor the presence of such sensitiveness in the young dancer has
+been satisfactorily proved.
+
+I shall now report in detail the results of my own study of the sense of
+hearing in the dancer. As the behavior of the young differs greatly from
+that of the adult, by which is meant the sexually mature animal, I shall
+present first the results of my experiments with adults and later, in
+contrast, the results obtained with mice from one to twenty-eight days
+old.
+
+My preliminary tests were made with noises. While carefully guarding
+against the interference of visual, tactual, temperature, and olfactory
+stimuli, I produced noises of varying degrees of loudness by clapping the
+hands together suddenly, by shouting, whistling, exploding pistol caps,
+striking steel bars, ringing an electric bell, and causing another mouse
+to squeak. To these sounds a common mouse usually responds either by
+starting violently, or by trembling and remaining perfectly quiet for a
+few seconds, as if frightened. The adult dancers which I have tested, and
+I have repeated the experiment scores of times during the last three years
+with more than a hundred different individuals, have never given
+unmistakable evidence of hearing. Either they are totally deaf or there is
+a most surprising lack of motor reactions.
+
+Precisely the same results were obtained in tests made with the Galton
+whistle throughout its range of pitches, and with Appuun whistles which,
+according to their markings, ranged from 2000 Vs. (C_4) to 48,000 (G_9),
+but which undoubtedly did not correspond at all exactly to this range, and
+with a series of König tuning forks which gave tones varying in pitch from
+1024 to 16,382 complete vibrations.
+
+I am willing to trust these experimental results the more fully because
+during all the time I have had adult dancers under observation I have
+never once seen a reaction which could with any fair degree of certainty
+be referred to an auditory stimulus. Never once, although I have tried
+repeatedly, have I succeeded in arousing a dancer from sleep by producing
+noises or tones, nor have I ever been able to observe any influence of
+sounds on the dance movements. All of Cyon's signs have failed with my
+mice. Occasionally what looked like a response to some sound appeared, but
+critical observation invariably proved it to be due to some other cause
+than the auditory stimulus. A sound produced above the animal is very
+likely to bring about a motor reaction, as Cyon claims; but I have always
+found it to be the result of the currents of air or odors, which usually
+influence the animal when the experimenter is holding any object above it.
+I do not wish to maintain that Cyon's conclusions are false; I merely
+emphasize the necessity for care in the exclusion of other stimuli. The
+mice are extremely sensitive to changes in temperature, such, for example,
+as are produced by the breath of the experimenter, and one must constantly
+guard against the misinterpretation of behavior.
+
+In a single experiment with mice over a month old, I observed what might
+possibly indicate sensitiveness to sound. While holding a mouse, thirty-
+five days old, in my hand I pursed my lips and made a very shrill sound by
+drawing in air; the mouse seemed to start perceptibly according to the
+indications given by my sense of touch. I repeated the stimulus several
+times and each time I could see and feel the animal start slightly. With
+two other individuals which I tested the reaction was less certain, and
+with several others I failed to get any indication of response. This would
+seem to prove that the three individuals which responded happened to be
+sensitive to that particular tone at the age of five weeks. The test is
+unsatisfactory because the vibrations from my own body may have brought
+about the reaction instead of the air vibrations produced by my lips, and
+I therefore merely mention it in the enumeration of the various
+experimental tests which I have made.
+
+If we should conclude from all the negative evidence that is available, or
+that could be obtained, that the dancer is totally deaf, it might fairly
+be objected that the conclusion is unsafe, since an animal does not
+necessarily respond to stimuli by a visible change in the position or
+relations of its body. Death feigning may fairly be considered a response
+to a stimulus or stimulus complex, yet there may be no sign of movement.
+The green frog when observed in the laboratory usually gives no indication
+whatever, by movements that are readily observable, that it hears sounds
+which occur about it, but I have been able to show by means of indirect
+methods of study that it is stimulated by these same sounds.[1] Its rate
+of respiration is changed by the sounds, and although a sound does not
+bring about a bodily movement, it does very noticeably influence movements
+in response to other stimuli which occur simultaneously with the sound. I
+discovered that under certain rather simple experimental conditions the
+green frog would regularly respond to a touch on the back by drawing its
+hind leg up toward the body. Under the same conditions the sound of an
+electric bell caused no visible movement of the leg, but if at the instant
+the back was touched the bell was rung, the leg movement was much greater
+than that brought about by the touch alone. This suggests at once the
+desirability of studying the sense of hearing in the dancer by some
+indirect method. The animal may be stimulated, and yet it may not give any
+visible sign of the influence of the auditory stimulus.
+
+[Footnote 1: "The Sense of Hearing in Frogs." _Journal of Comparative
+Neurology and Psychology_, Vol. XV, p. 288, 1905.]
+
+Were not the dancing so extremely variable in rapidity and duration, it
+might be used as an index of the influence of auditory stimuli. Cyon's
+statements would indicate that sounds interfere with the dancing, but as I
+obtained no evidence of this, I worked instead with the following indirect
+method, which may be called the method of auditory choice.
+
+The apparatus which was used is described in detail in Chapter VII.
+Figures 14 and 15 will greatly aid the reader in understanding its
+essential features. Two small wooden boxes, identical in form and as
+closely similar as possible in general appearance, were placed in a larger
+box in such positions that a mouse was forced to enter and pass through
+one of them in order to get to the nest-box. On the bottom of each of
+these small boxes was a series of wires through which an electric current
+could be made to pass at the will of the experimenter. The boxes could
+readily be interchanged in position. At one side of the large wooden box
+and beyond the range of vision of the mouse was an electric bell which
+could be caused to ring whenever the mouse approached the entrance to one
+of the small boxes. The point of the experiment was to determine whether
+the dancer could learn to avoid the box-which-rang when it was approached.
+The method of conducting the tests was as follows. Each day at a certain
+hour the mouse was placed in that part of the large box whence it could
+escape to the nest-box only by passing through one of the small boxes. If
+it approached the wrong box (whether it happened to be the one on the
+right or the one on the left depended upon the experimenter's decision),
+the bell began to ring as a warning against entering; if it approached the
+other box, all was silent. As motives for the choice of the box-which-did-
+not-ring both reward and punishment were employed. The reward consisted of
+freedom to return to the nest-box _via_ the passage which led from the
+box-which-did-not-ring; the punishment, which consisted of a disagreeable
+electric shock, was given whenever the mouse entered the wrong box, that
+is, the one which had the sound as a warning. Entering the wrong box
+resulted in a disagreeable stimulus and in the necessity of returning to
+the large box, for the exit to the nest-box by way of the passage from
+this box was closed. My assumption, on the basis of extended study of the
+ability of the dancer to profit by experience, was that if it could hear
+the sound of the bell it would soon learn to avoid the box-that-rang and
+enter instead the one which had no sound associated with it.
+
+Systematic tests were made with No. 4 from the 3d to the 12th of February,
+inclusive, 1906. Each day the mouse was permitted to find his way to the
+nest-box through one of the small boxes ten times in succession. Usually
+the experimenter rang the bell alternately for the box on the left and the
+box on the right. The time required for such a series of experiments
+varied, according to the rapidity with which the mouse made his choice,
+from ten to thirty minutes. If in these experiments the animal approached
+and entered the right, or soundless box, directly, the choice indicated
+nothing so far as ability to hear is concerned; if it entered the wrong,
+or sounding box, despite the ringing of the bell, it indicated either the
+lack of the influence of experience or inability to hear the sound; but if
+it regularly avoided the box-which-sounded it thus gave evidence of
+ability to hear the sound of the bell. The purpose of the test was to
+determine, not whether the mouse could learn, but whether it could hear.
+
+For ten successive days this experiment was carried on with No. 4 without
+the least indication of increasing ability to avoid the wrong box by the
+association of the sound of the bell with the disagreeable electric shock
+and failure to escape to the nest-box. In fact, the experiment was
+discontinued because it became evident that an impossible task had been
+set for the mouse. Day by day as the tests were in progress I noticed that
+the animal became increasingly afraid of the entrances to the small boxes;
+it seemed absolutely helpless in the face of the situation. Partly because
+of the definiteness of the negative results obtained with No. 4 and partly
+because of the cruelty of subjecting an animal to disagreeable conditions
+which it is unable to avoid, the experiment was not repeated with other
+individuals. I have never conducted an experiment which gave me as much
+discomfort as this; it was like being set to whip a deaf child because it
+did not learn to respond to stimuli which it could not feel.
+
+By a very similar method No. 18 was tested for his sensitiveness to the
+noise and jar from the induction apparatus which was used in connection
+with many of my experiments on vision and the modifiability of behavior.
+In this experiment the wrong box was indicated by the buzzing sound of the
+apparatus and the slight vibrations which resulted from it. Although No.
+18 was tested, as was No. 4, for ten successive days, ten trials each day,
+it gave no evidence of ability to avoid the box-which-buzzed.
+
+Since both direct and indirect methods of testing the hearing of the
+dancer have uniformly given negative results, in the case of mice more
+than five weeks old, I feel justified in concluding that they are totally
+deaf and not merely irresponsive to sounds.
+
+Rawitz's statements, and the fact that what may have been auditory
+reactions were obtained with a few individuals of five weeks of age,
+suggest that the mice may be able to hear at certain periods of life. To
+discover whether this is true I have tested the young of twenty different
+litters from the first day to the twenty-eighth, either daily or at
+intervals of two or three days. In these tests König forks, steel bars,
+and a Galton whistle were used. The results obtained are curiously
+interesting.
+
+During the first two weeks of life none of the mice which I tested gave
+any visible motor response to the various sounds used. During the third
+week certain of the individuals responded vigorously to sudden high tones
+and loud noises. After the third week I have seen only doubtful signs of
+hearing. I shall now describe in detail the method of experimentation, the
+condition of the animals, and the nature of the auditory reactions.
+
+Between the twelfth and the eighteenth day the auditory canal becomes open
+to the exterior. The time is very variable in different litters, for their
+rate of growth depends upon the amount of nourishment which the mother is
+able to supply. Without exception, in my experience, the opening to the
+ear appears before the eyes are open. Consequently visual stimuli usually
+are not disturbing factors in the auditory tests with mice less than
+sixteen days old. There is also a sudden and marked change in the behavior
+of the mice during the third week. Whereas, for the first fourteen or
+eighteen days they are rather quiet and deliberate in their movements when
+removed from the nest, some time in the third week their behavior suddenly
+changes and they act as if frightened when taken up by the experimenter.
+They jump out of his hand, squeak, and sometimes show fight. This is so
+pronounced that it has attracted my attention many times and I have
+studied it carefully to determine, if possible, whether it is due to some
+profound change in the nervous system which thus suddenly increases the
+sensitiveness of the animal or to the development of the sexual organs. I
+am inclined to think that it is a nervous phenomenon which is intimately
+connected with the sexual condition. Within a day or two after it appears
+the mice usually begin to show auditory reactions and continue to do so
+for three to five days.
+
+I shall now describe the results obtained with a few typical litters. A
+litter born of Nos. 151 and 152 gave uniformly negative results in all
+auditory tests up to the fourteenth day. On that day the ears were open,
+and the following observations were recorded. The five individuals of the
+litter, four females and one male, were taken from the nest one at a time
+at 7 A.M. and placed on a piece of paper in the bright sunlight. The
+warmth of the sun soon quieted them so that auditory tests could be made
+to advantage. As soon as an individual had become perfectly still, the
+Galton whistle was held at a distance of about four inches from its head
+in such a position that it could not be seen nor the currents of air
+caused by it felt, and suddenly blown. Each of the five mice responded to
+the first few repetitions of this stimulus by movements of the ears,
+twitchings of the body, and jerky movements of the legs. The most violent
+reactions resulted when the individual was lying on its back with its legs
+extended free in the air. Under such circumstances the four legs were
+often drawn together suddenly when the whistle was sounded. Similar
+responses were obtained with the lip sound already mentioned. Two other
+observers saw these experiments, and they agreed that there can be no
+doubt that the mice responded to the sound. The sounds which were
+effective lay between 5000 and 10,000 complete vibrations.
+
+On the fifteenth day the eyes were just beginning to open. Three of the
+mice responded definitely to the sounds, but the other two slightly, if at
+all. On the sixteenth day they were all too persistently active for
+satisfactory auditory tests, and on the seventeenth, although they were
+tested repeatedly under what appeared to be favorable conditions, no signs
+of sensitiveness were noted. Although I continued to test this litter, at
+intervals of three or four days, for two weeks longer, I did not once
+observe a response to sound.
+
+This was the first litter with which I obtained perfectly definite, clear-
+cut responses to sounds. That the reactive ability had not been present
+earlier than the fourteenth day I am confident, for I had conducted the
+tests in precisely the same manner daily up to the time of the appearance
+of the reactions. To argue that the mice heard before the fourteenth day,
+but were unable to react because the proper motor mechanism had not
+developed sufficiently would be short-sighted, for if the response
+depended upon the development of such a mechanism, it is not likely that
+it would disappear so quickly. I am therefore satisfied that these
+reactions indicate hearing.
+
+With another litter the following results were obtained. On the thirteenth
+day each of the eight members of the litter responded definitely and
+uniformly to the Galton whistle, set at 5 (probably about 8000 complete
+vibrations), and to a König steel bar of a vibration rate of 4096 Vs. The
+largest individuals, for almost always there are noticeable differences in
+size among the members of a litter, appeared to be most sensitive to
+sounds.
+
+On the fifteenth day and again on the seventeenth unmistakable responses
+to sound were observed; on the eighteenth the responses were indefinite,
+and on the nineteenth none were obtained. I continued the tests up to the
+twenty-eighth day without further indications of hearing.
+
+Certain individuals in this litter reacted so vigorously to the loud sound
+produced by striking the steel bar a sharp blow and also to the Galton
+whistle, during a period of five days, that I have no hesitation in saying
+that they evidently heard during that period of their lives. Other members
+of the litter seemed to be less sensitive; their reactions were sometimes
+so indefinite as to leave the experimenter in doubt about the presence of
+hearing.
+
+A third litter, which developed very slowly because of lack of sufficient
+food, first showed unmistakable reactions to sound on the twenty-first
+day. On this day only two of the five individuals reacted. The reactions
+were much more obvious on the twenty-second day, but thereafter they
+became indefinite.
+
+Still another litter, which consisted of one female and four males, began
+to exhibit the quick, jerky movements, already mentioned, on the
+fourteenth day. On the morning of the fifteenth day three members of the
+litter definitely reacted to the tone of the steel bar, and also to the
+hammer blow when the bar was held tightly in the hand of the experimenter.
+My observations were verified by another experimenter. Two individuals
+which appeared to be very sensitive were selected for special tests. Their
+reactions were obvious on the sixteenth, seventeenth, and eighteenth days;
+on the nineteenth day they were indefinite, and on the twentieth none
+could be detected. Some individuals of this litter certainly had the
+ability to hear for at least five days.
+
+A sixth litter of four females and two males first gave indications of the
+change in behavior which by this time I had come to interpret as a sign of
+the approach of the period of auditory sensitiveness, on the seventeenth
+day. I had tested them almost every day previous to this time without
+obtaining evidence of hearing. The tests with the steel bar and the Galton
+whistle were continued each day until the end of the fourth week without
+positive results. To all appearances the individuals of this litter were
+unable to hear at any time during the first month of life.
+
+Practically the same results were obtained with another litter of four
+females. The change in their behavior was obvious on the eighteenth day,
+but at no time during the first month did they give any satisfactory
+indications of hearing.
+
+In the accompanying table, I have presented in condensed form the results
+of my auditory tests in the case of twelve litters of young dancers.
+
+
+
+
+TABLE 5
+
+PERIOD OF AUDITORY REACTION IN YOUNG DANCERS
+
+PARENTS   No. in   Change in    Ears      Auditory Reactions
+          Litter    Behavior    Open       Appear    Disappear
+
+152+151      5     13th day    14th day   14th day   16th day
+152+15l      8        (?)      13th day   13th day   17th day
+152+151      5     13th day    13th day   13th day   17th day
+152+151      4     10th day    12th day   13th day   15th day
+410+415      5     14th day    15th day   15th day   19th day
+410+415      6     13th day    14th day   14th day   18th day
+420+425      2     12th day    14th day   14th day   17th day
+210+215      5     17th day    13th day   17th day   19th day
+210+215      6     11th day    14th day       No reactions
+220+225      6     16th day    14th day       No reactions
+220+225      6     17th day    13th day       No reactions
+212+211      4     15th day    14th day       No reactions
+
+
+
+
+Certain of the litters tested responded definitely to sounds, others gave
+no sign of hearing at any time during the first four weeks of life. Of the
+twelve litters for which the results of auditory tests are presented in
+Table 5, eight evidently passed through an auditory period. It is
+important to note that all except one of these were the offspring of Nos.
+151 and 152, or of their descendants Nos. 410 and 415 and Nos. 420 and
+425. In fact every one of the litters in this line of descent which I have
+tested, and they now number fifteen, has given indications of auditory
+sensitiveness. And, on the other hand, only in a single instance have the
+litters born of Nos. 210 and 215, or of their descendants, given evidence
+of ability to hear.
+
+These two distinct lines of descent may be referred to hereafter as the
+400 and the 200 lines. I have observed several important differences
+between the individuals of these groups in addition to the one already
+mentioned. The 200 mice were sometimes gray and white instead of black and
+white; they climbed much more readily and danced less vigorously than
+those of the 400 group. These facts are particularly interesting in
+connection with Cyon's descriptions of the two types of dancer which he
+observed.
+
+In criticism of my conclusion that the young dancers are able to hear
+certain sounds for a few days early in life, and then become deaf, it has
+been suggested that they cease to react because they rapidly become
+accustomed to the sounds. That this is not the case, is evident from the
+fact that the reactions often increase in definiteness during the first
+two or three days and then suddenly disappear entirely. But even if this
+were not true, it would seem extremely improbable that the mouse should
+become accustomed to a sudden and startlingly loud sound with so few
+repetitions as occurred in these tests. On any one day the sounds were not
+made more than five to ten times. Moreover, under the same external
+condition, the common mouse reacts unmistakably to these sounds day after
+day when they are first produced, although with repetition of the stimulus
+at short intervals, the reactions soon become indefinite or disappear.
+
+The chief results of my study of hearing in the dancer may be summed up in
+a very few words. The young dancer, in some instances, hears sounds for a
+few days during the third week of life. The adult is totally deaf. Shortly
+before the period of auditory sensitiveness, the young dancer becomes
+extremely excitable and pugnacious.
+
+
+
+
+CHAPTER VII
+
+THE SENSE OF SIGHT: BRIGHTNESS VISION
+
+The sense of sight in the dancer has received little attention hitherto.
+In the literature there are a few casual statements to the effect that it
+is of importance. Zoth, for example (31 p. 149), remarks that it seems to
+be keenly developed; and other writers, on the basis of their observation
+of the animal's behavior, hazard similar statements. The descriptions of
+the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and
+Kishi (p.47), apparently indicate that the sense is of some value; they do
+not, however, furnish definite information concerning its nature and its
+role in the daily life of the animal.
+
+The experimental study of this subject which is now to be described was
+undertaken, after careful and long-continued observation of the general
+behavior of the dancer, in order that our knowledge of the nature and
+value of the sense of sight in this representative of the Mammalia might
+be increased in scope and definiteness. The results of this study
+naturally fall into three groups: (1) those which concern brightness
+vision, (2) those which concern color vision, and (3) those which indicate
+the role of sight in the life of the dancer.
+
+Too frequently investigators, in their work on vision in animals, have
+assumed that brightness vision and color vision are inseparable; or, if
+not making this assumption, they have failed to realize that the same
+wave-length probably has markedly different effects upon the retinal
+elements of the eyes of unlike organisms. In a study of the sense of sight
+it is extremely important to discover whether difference in the quality,
+as well as in the intensity, of a visual stimulus influences the organism;
+in other words, whether color sensitiveness, as well as brightness
+sensitiveness, is present. If the dancer perceives only brightness or
+luminosity, and not color, it is evident that its visual world is
+strikingly different from that of the normal human being. The experiments
+now to be described were planned to show what the facts really are.
+
+[Illustration: Figure 14.--Discrimination box. _W_, electric-box with
+white cardboards; B, electric-box with black cardboards. Drawn by Mr. C.H.
+Toll.]
+
+As a means of testing the ability of the dancer to distinguish differences
+in brightness, the experiment box represented by Figures 14 and 15 was
+devised. Figure 14 is the box as seen from the position of the
+experimenter during the tests. Figure 15 is its ground plan. This box,
+which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as
+measured on the outside. The plan of construction and its significance in
+connection with these experiments on vision will be clear from the
+following account of the experimental procedure. A mouse whose brightness
+vision was to be tested was placed in the nest-box, A (Figure 15). Thence
+by pushing open the swinging door at _I_, it could pass into the entrance
+chamber, _B_. Having entered _B_ it could return to _A_ only by passing
+through one of the electric-boxes, marked _W_, and following the alley to
+_O_, where by pushing open the swing door it could enter the nest-box. The
+door at _I_ swung inward, toward _B_, only; those at _O_, right and left,
+swung outward, toward _A_, only. It was therefore impossible for the mouse
+to follow any other course than _A-I-B-L-W-E-O_ or _A-I-B-R-W-E-O_. The
+doors at _I_ and _O_ were pieces of wire netting of 1/2 cm. mesh, hinged
+at the top so that a mouse could readily open them, in one direction, by
+pushing with its nose at any point along the bottom. On the floor of each
+of the electric-boxes, _W_, was an oak board 1 cm. in thickness, which
+carried electric wires by means of which the mouse could be shocked in _W_
+when the tests demanded it. The interrupted circuit constituted by the
+wires in the two electric-boxes, in connection with the induction
+apparatus, _IC_, the dry battery, _C_, and the hand key, _K_, was made by
+taking two pieces of No. 20 American standard gauge copper wire and
+winding them around the oak board which was to be placed on the floor of
+each electric-box. The wires, which ran parallel with one another, 1/2 cm.
+apart, fitted into shallow grooves in the edges of the board, and thus, as
+well as by being drawn taut, they were held firmly in position. The coils
+of the two pieces of wire alternated, forming an interrupted circuit
+which, when the key _K_ was closed, was completed if the feet of a mouse
+rested on points of both pieces of wire. Since copper wire stretches
+easily and becomes loose on the wooden base, it is better to use phosphor
+bronze wire of about the same size, if the surface covered by the
+interrupted circuit is more than three or four inches in width. The
+phosphor bronze wire is more difficult to wind satisfactorily, for it is
+harder to bend than the copper wire, and it has the further disadvantage
+of being more brittle. But when once placed properly, it forms a far more
+lasting and satisfactory interrupted circuit for such experiments as those
+to be described than does copper wire. In the case of the electric-boxes
+under consideration, the oak boards which carried the interrupted circuits
+were separate, and the two circuits were joined by the union of the wires
+between the boxes. The free ends of the two pieces of wire which
+constituted the interrupted circuit were connected with the secondary coil
+of a Porter inductorium whose primary coil was in circuit with a No. 6
+Columbia dry battery. In the light of preliminary experiments, made in
+preparation for the tests of vision, the strength of the induced current
+received by the mouse was so regulated, by changing the position of the
+secondary coil with reference to the primary, that it was disagreeable but
+not injurious to the animal. What part the disagreeable shock played in
+the test of brightness vision will now be explained.
+
+[Illustration: FIGURE 15.--Ground plan of discrimination box. _A_, nest-
+box; _B_, entrance chamber; _W,W_, electric-boxes; _L_, doorway of left
+electric-box; _R_, doorway of right electric-box; _E_, exit from electric-
+box to alley; _I_, swinging door between _A_ and _B_; _O_, swinging door
+between alley and _A_; _IC_, induction apparatus; _C_, electric cell; _K_,
+key in circuit.]
+
+An opportunity for visual discrimination by brightness difference was
+provided by placing dead black cardboard at the entrance and on the inside
+of one of the electric-boxes, as shown in Figure 14, _B_, and white
+cardboard similarly in the other box. These cardboards were movable and
+could be changed from one box to the other at the will of the
+experimenter. The test consisted in requiring the mouse to choose a
+certain brightness, for example, the white cardboard side, in order to
+return to the nest-box without receiving an electric shock. The question
+which the experimenter asked in connection with this test really is, Can a
+dancer learn to go to the white box and thus avoid discomfort? If we
+assume its ability to profit by experience within the limits of the number
+of experiences which it was given, such a modification of behavior would
+indicate discrimination of brightness. Can the dancer distinguish white
+from black; light gray from dark gray; two grays which are almost of the
+same brightness? The results which make up the remainder of this and the
+following chapter furnish a definite answer to these questions.
+
+To return to the experimental procedure, the mouse which is being tested
+is placed by the experimenter in the nest-box, where frequently in the
+early tests food and a comfortable nest were attractions. If it does not
+of its own accord, as a result of its abundant random activity, pass
+through _I_ into _B_ within a few seconds, it is directed to the doorway
+and urged through. A choice is now demanded of the animal; to return to
+the nest-box it must enter either the white electric-box or the black one.
+Should it choose the white box, it is permitted to return directly to _A_
+by way of the doorway _E_, the alley, and the swinging door at _O_, and it
+thus gets the satisfaction of unobstructed activity, freedom to whirl, to
+feed, and to retreat for a time to the nest. Should it choose to attempt
+to enter the black box, as it touches the wires of the interrupted circuit
+it receives a shock as a result of the closing of the key in the circuit
+by the experimenter, and further, if it continues its forward course
+instead of retreating from the "stinging" black box, its passage through
+_E_ is blocked by a barrier of glass temporarily placed there by the
+experimenter, and the only way of escape to the nest-box is an indirect
+route by way of _B_ and the white box. Ordinarily the shock was given only
+when the mouse entered the wrong box, not when it retreated from it; it
+was never given when the right box was chosen. The box to be chosen,
+whether it was white, gray, or black, will be called the right box. The
+electric shock served as a means of forcing the animal to use its
+discriminating ability. But the question of motives in the tests is not so
+simple as might appear from this statement.
+
+The reader will wonder why the mouse should have any tendency to enter
+_B_, and why after so doing, it should trouble to go further, knowing, as
+it does from previous experiences, that entering one of the electric-boxes
+may result in discomfort. The fact is, a dancer has no very constant
+tendency to go from _A_ to _B_ at the beginning of the tests, but after it
+has become accustomed to the box and has learned what the situation
+demands, it shows eagerness to make the trip from _A_ to _B_, and thence
+by way of either the right or the left route to _A_. That the mouse should
+be willing to enter either of the electric-boxes, after it has experienced
+the shock, is even more surprising than its eagerness to run from _A_ to
+_B_. When first tested for brightness discrimination in this apparatus, a
+dancer usually hesitated at the entrance to the electric-boxes, and this
+hesitation increased rapidly unless it were able to discriminate the boxes
+by their difference in brightness and thus to choose the right one. During
+the period of increasing hesitancy in making the choice, the experimenter,
+by carefully moving from _I_ toward the entrances to the electric-boxes a
+piece of cardboard which extended all the way across _B_, greatly
+increased the mouse's desire to enter one of the boxes by depriving it of
+dancing space in _B_. If an individual which did not know which entrance
+to choose were permitted to run about in _B_, it would often do so for
+minutes at a time without approaching the entrance to the boxes; but the
+same individual, when confined to a dancing space 4 or 5 cm. wide in front
+of the entrances, would enter one of the electric-boxes almost
+immediately. This facilitation of choice by decrease in the amount of
+space for whirling was not to any considerable extent the result of fear,
+for all the dancers experimented with were tame, and instead of forcing
+them to rush into one of the boxes blindly and without attempt at
+discrimination, the narrowing of the space simply increased their efforts
+to discriminate. The common mouse when subjected to similar experimental
+conditions is likely to be frightened by being forced to approach the
+entrances to the boxes, and fails to choose; it rushes into one box
+directly, and in consequence it is as often wrong as right. The dancer
+always chooses, but its eagerness to choose is markedly increased by the
+restriction of its movements to a narrow space in front of the entrances
+between which it is required to discriminate. It is evident that the
+animal is uncomfortable in a space which is too narrow for it to whirl in
+freely. It must have room to dance. This furnished a sufficiently strong
+motive for the entering of the electric-boxes. It must avoid disagreeable
+and unfavorable stimuli. This is a basis for attempts to choose, by visual
+discrimination, the electric-box in which the shock is not given. It may
+safely be said that the success of the majority of the experiments of this
+book depended upon three facts: (1) the dancer's tendency to avoid
+disagreeable external conditions, (2) its escape-from-confinement-
+impelling need of space in which to dance freely, and (3) its abundant and
+incessant activity.
+
+Of these three conditions of success in the experiments, the second and
+third made possible the advantageous use of the first. For the avoidance
+of a disagreeable stimulus could be made use of effectively in the tests
+just because the mice are so restless and so active. In fact their
+eagerness to do things is so great that the experimenter, instead of
+having to wait for them to perform the desired act, often is forced to
+make them wait while he completes his observation and record. In this
+respect they are unlike most other animals.
+
+My experiments with the dancer differ from those which have been made by
+most students of mammalian behavior in one important respect. I have used
+punishment instead of reward as the chief motive for the proper
+performance of the required act. Usually in experiments with mammals
+hunger has been the motive depended upon. The animals have been required
+to follow a certain devious path, to escape from a box by working a
+button, a bolt, a lever, or to gain entrance to a box by the use of teeth,
+claws, hands, or body weight and thus obtain food as a reward. There are
+two very serious objections to the use of the desire for food as a motive
+in animal behavior experiments--objections which in my opinion render it
+almost worthless in the case of many mammals. These are the discomfort of
+the animal and the impossibility of keeping the motive even fairly
+constant. However prevalent the experience of starvation may be in the
+life of an animal, it is not pleasant to think of subjecting it to extreme
+hunger in the laboratory for the sake of finding out what it can do to
+obtain food. Satisfactory results can be obtained in an experiment whose
+success depends chiefly upon hunger only when the animal is so hungry that
+it constantly does its best to obtain food, and when the desire for food
+is equally strong and equally effective as a spur to action in the
+repetitions of the experiment day after day. It is easy enough to get
+almost any mammal into a condition of utter hunger, but it is practically
+impossible to have the desire for food of the same strength day after day.
+In short, the desire for food is unsatisfactory as a motive in animal
+behavior work, first, because a condition of utter hunger, as has been
+demonstrated with certain mammals, is unfavorable for the performance of
+complex acts, second, because it is impossible to control the strength of
+the motive, and finally, because it is an inhumane method of
+experimentation.
+
+In general, the method of punishment is more satisfactory than the method
+of reward, because it can be controlled to a greater extent. The
+experimenter cannot force his subject to desire food; he can, however,
+force it to discriminate between conditions to the best of its knowledge
+and ability by giving it a disagreeable stimulus every time it makes a
+mistake. In other words, the conditions upon which the avoidance of a
+disagreeable factor in the environment depends are far simpler and much
+more constant than those upon which the seeking of an agreeable factor
+depends. Situations which are potentially beneficial to the animal attract
+it in varying degrees according to its internal condition; situations
+which are potentially disagreeable or injurious repel it with a constancy
+which is remarkable. The favorable stimulus solicits a positive response;
+the unfavorable stimulus demands a negative response.
+
+Finally, in connection with the discussion of motives, it is an important
+fact that forms of reward are far harder to find than forms of punishment.
+Many animals feed only at long intervals, are inactive, do not try to
+escape from confinement, cannot be induced to seek a particular spot, in a
+word, do not react positively to any of the situations or conditions which
+are employed usually in behavior experiments. It is, however, almost
+always possible to find some disagreeable stimulus which such an animal
+will attempt to avoid.
+
+As it happens, the dancer is an animal which does not stand the lack of
+food well enough to make hunger a possible motive. I was driven to make
+use of the avoiding reaction, and it has proved so satisfactory that I am
+now using it widely in connection with experiments on other animals. The
+use of the induction shock, upon which I depended almost wholly in the
+discrimination experiments with the dancer, requires care; but I am
+confident that no reasonable objection to the conduct of the experiments
+could be made on the ground of cruelty, for the strength of the current
+was carefully regulated and the shocks were given only for an instant at
+intervals. The best proof of the humaneness of the method is the fact that
+the animals continued in perfect health during months of experimentation.
+
+The brightness discrimination tests demanded, in addition to motives for
+choice, adequate precautions against discrimination by other than visual
+factors, and, for that matter, by other visual factors than that of
+brightness. The mouse might choose, for example, not the white or the
+black box, but the box which was to the right or to the left, in
+accordance with its experience in the previous test. This would be
+discrimination by position. As a matter of fact, the animals have a strong
+tendency at first to go uniformly either to the right or to the left
+entrance. This tendency will be exhibited in the results of the tests.
+Again, discrimination might depend upon the odors of the cardboards or
+upon slight differences in their shape, texture, or position. Before
+conclusive evidence of brightness discrimination could be obtained, all of
+these and other possibilities of discrimination had to be eliminated by
+check tests. I shall describe the various precautions taken in the
+experiments to guard against errors in interpretation, in order to show
+the lengths to which an experimenter may be driven in his search for
+safely interpretable results.
+
+To exclude choice by position, the cardboards were moved from one
+electric-box to the other. When the change was made regularly, so that
+white was alternately on the right and the left, the mouse soon learned to
+go alternately to the right box and the left without stopping to notice
+the visual factor. This was prevented by changing the position of the
+cardboards irregularly.
+
+Discrimination by the odor, texture, shape, and position of the cardboards
+was excluded by the use of different kinds of cardboards, by changing the
+form and position of them in check tests, and by coating them with
+shellac.
+
+The brightness vision tests described in this chapter were made in a room
+which is lighted from the south only, with the experiment box directed
+away from the windows. The light from the windows shone upon the
+cardboards at the entrances to the electric-boxes, not into the eyes of
+the mouse as it approached them. Each mouse used in the experiments was
+given a series of ten tests in succession daily. The experiment was
+conducted as follows. A dancer was placed in _A_, where it usually ran
+about restlessly until it happened to find its way into _B_. Having
+discovered that the swing door at _I_ could be pushed open, the animal
+seemed to take satisfaction in passing through into _B_ as soon as it had
+been placed in or had returned to _A_. In _B_, choice of two entrances,
+one of which was brighter than the other, was forced by the animal's need
+of space for free movement. If the right box happened to be chosen, the
+mouse returned to _A_ and was ready for another test; if it entered the
+wrong box, the electric shock was given, and it was compelled to retreat
+from the box and enter the other one instead. In the early tests with an
+individual, a series sometimes covered from twenty to thirty minutes; in
+later tests, provided the condition of discrimination was favorable, it
+did not occupy more than ten minutes.
+
+To exhibit the methods of keeping the records of these experiments and
+certain features of the results, two sample record sheets are reproduced
+below. The first of these sheets, Table 6, represents the results given by
+No. 5, a female,[1] in her first series of white-black tests. Table 7
+presents the results of the eleventh series of tests given to the same
+individual.
+
+[Footnote 1: It is to be remembered that the even numbers always designate
+males; the odd numbers, females.]
+
+In the descriptions of the various visual experiments of this and the
+following chapters, the first word of the couplet which describes the
+condition of the experiment, for example, white-black, always designates
+the visual condition which the animal was to choose, the second that which
+it was to avoid on penalty of an electric shock. In the case of Tables 6
+and 7, for example, white cardboard was placed in one box, black in the
+other, and the animal was required to enter the white box. In the tables
+the first column at the left gives the number of the test, the second the
+positions of the cardboards, and the third and fourth the result of the
+choice. The first test of Table 6 was made with the white cardboard on the
+box which stood at the left of the mouse as it approached from _A_, and,
+consequently, with the black cardboard on the right. As is indicated by
+the record in the "wrong" column, the mouse chose the black instead of the
+white. The result of this first series was choice of the white box four
+times as compared with choice of the black box six times. On the eleventh
+day, that is, after No. 5 had been given 100 tests in this brightness
+vision experiment, she made no mistakes in a series of ten trials (Table
+7).
+
+
+TABLE 6
+
+BRIGHTNESS DISCRIMINATION
+
+White-Black, Series 1
+
+Experimented on No. 5     January 15, 1906
+          POSITION OF
+TEST       CARDBOARDS      RIGHT      WRONG
+
+1      White left          --      Wrong
+2      White right         --      Wrong
+3      White left          --      Wrong
+4      White right         --      Wrong
+5      White left       Right         --
+6      White right      Right         --
+7      White left          --      Wrong
+8      White right      Right         --
+9      White left          --      Wrong
+10     White right      Right         --
+
+Totals                      4          6
+
+
+Before tests, such as have been described, can be presented as conclusive
+proof of discrimination, it must be shown that the mouse has no preference
+for the particular brightness which the arrangement of the test requires
+it to select. That any preference which the mouse to be tested might have
+for white, rather than black, or for a light gray rather than a dark gray,
+might be discovered, what may be called preference test series were given
+before the discrimination tests were begun. These series, two of which
+were given usually, consisted of ten tests each, with the white
+alternately on the left and on the right. The mouse was permitted to enter
+either the white or the black box, as it chose, and to pass through to the
+nest-box without receiving a shock and without having its way blocked by
+the glass plate. The conditions of these preference tests may be referred
+to hereafter briefly as "No shock, open passages." The preference tests,
+which of course would be valueless as such unless they preceded the
+training tests, were given as preliminary experiments, in order that the
+experimenter might know how to plan his discrimination tests, and how to
+interpret his results.
+
+
+
+TABLE 7
+
+BRIGHTNESS DISCRIMINATION
+
+White-Black, Series II
+
+Experimented on No. 5                  February 2, 1906
+
+           POSITION
+TEST     OF CARDBOARDS     RIGHT     WRONG
+
+ 1        White left       Right       --
+ 2        White left       Right       --
+ 3        White right      Right       --
+ 4        White right      Right       --
+ 5        White right      Right       --
+ 6        White left       Right       --
+ 7        White left       Right       --
+ 8        White left       Right       --
+ 9        White right      Right       --
+10        White right      Right       --
+
+Totals                       10         0
+
+
+
+The results given in the white-black preference tests by ten males and ten
+females are presented in Table 8. Three facts which bear upon the
+brightness discrimination tests appear from this table: (1) black is
+preferred by both males and females, (2) this preference is more marked in
+the first series of tests than in the second, and (3) it is slightly
+stronger for the first series in the case of females than in the case of
+males.
+
+
+TABLE 8
+
+WHITE-BLACK PREFERENCE TESTS
+
+MALES    FIRST SERIES    SECOND SERIES
+         WHITE  BLACK    WHITE  BLACK
+
+No. 10     3      7        3      7
+    18     5      5        5      5
+    20     2      8        4      6
+   152     4      6        6      4
+   210     4      6        4      6
+   214     6      4        3      7
+   220     5      5        3      7
+   230     4      6        2      8
+   410     4      6        5      5
+   420     4      6        9      1
+
+Averages  4.1    5.9      4.4    5.6
+
+
+FEMALES  FIRST SERIES    SECOND SERIES
+         WHITE  BLACK    WHITE  BLACK
+
+No.  11    5      5        4      6
+    151    6      4        5      5
+    215    2      8        2      8
+    213    2      8        5      5
+    225    4      6        2      8
+    227    4      6        6      4
+    235    6      4        4      6
+    415    2      8        4      6
+    425    5      5        8      2
+    229    2      8        5      5
+
+Averages  3.8    6.2      4.5    5.5
+
+
+That the dancers should prefer to enter the dark rather than the light box
+is not surprising in view of the fact that the nests in which they were
+kept were ordinarily rather dark. But whatever the basis of the
+preference, it is clear that it must be taken account of in the visual
+discrimination experiments, for an individual which strongly preferred
+black might choose correctly, to all appearances, in its first black-white
+series. Such a result would demonstrate preference, and therefore one kind
+of discrimination, but not the formation of a habit of choice by
+discrimination. The preference for black is less marked in the second
+series of tests because the mouse as it becomes more accustomed to the
+experiment box tends more and more to be influenced by other conditions
+than those of brightness. The record sheets for both series almost
+invariably indicate a strong tendency to continue to go to the left or the
+right entrance according to the way by which the animal escaped the first
+time. This cannot properly be described as visual choice, for the mouse
+apparently followed the previous course without regard to the conditions
+of illumination. We have here an expression of the tendency to the
+repetition of an act. It is only after an animal acquires considerable
+familiarity with a situation that it begins to vary its behavior in
+accordance with relatively unimportant factors in the situation. It is
+this fact, illustrations of which may be seen in human life, as well as
+throughout the realm of animal behavior, that renders it imperative that
+an animal be thoroughly acquainted with the apparatus for experimentation
+and with the experimenter before regular experiments are begun. Any animal
+will do things under most experimental conditions, but to discover the
+nature and scope of its ability it is necessary to make it thoroughly at
+home in the experimental situation. As the dancer began to feel at home in
+the visual discrimination apparatus it began to exercise its
+discriminating ability, the first form of which was choice according to
+position.
+
+Since there appears to be a slight preference on the part of most dancers'
+for the black box in comparison with the white box, white-black training
+tests were given to fifty mice, and black-white to only four. The tests
+with each individual were continued until it had chosen correctly in all
+of the tests of three successive series (thirty tests). As the
+reproduction of all the record sheets of these experiments would fill
+hundreds of pages and would provide most readers with little more
+information than is obtainable from a simple statement of the number of
+right and wrong choices, only the brightness discrimination records of
+Tables 6 and 7 are given in full.
+
+As a basis for the comparison of the results of the white-black tests with
+those of the black-white tests, two representative sets of data for each
+of these conditions of brightness discrimination are presented (Tables 9
+and 10). In these tables only the number of right and wrong choices for
+each series of ten tests appears.
+
+Tables 9 and 10 indicate--if we grant that the precautionary tests to be
+described later exclude the possibility of other forms of discrimination--
+that the dancer is able to tell white from black; that it is somewhat
+easier, as the preference tests might lead us to expect, for it to learn
+to go to the black than to the white, and that the male forms the habit of
+choosing on the basis of brightness discrimination more quickly than the
+female.
+
+
+
+TABLE 9
+WHITE-BLACK TESTS
+
+
+                      No. 210              No. 215
+                    AGE, 28 DAYS         AGE, 28 DAYS
+SERIES   DATE      RIGHT    WRONG       RIGHT    WRONG
+                  (WHITE)  (BLACK)     (WHITE)  (BLACK)
+  A     June 22      4        6           2        8
+  B          23      4        6           2        8
+
+  1          24      4        6           3        7
+  2          25      6        4           5        5
+  3          26      7        3           7        3
+  4          27      5        5           8        2
+  5          28      7        3           9        1
+  6          29      8        2           8        2
+  7          30      9        1           9        1
+  8     July  1     10        0          10        0
+  9           2     10        0           9        1
+ 10           3     10        0          10        0
+ 11           4     --       --          10        0
+ 12           5     --       --          10        0
+
+
+
+
+TABLE 10
+WHITE-BLACK TESTS
+
+                      No.  14              No.  13
+                    AGE, 32 DAYS         AGE, 32 DAYS
+SERIES   DATE      RIGHT    WRONG       RIGHT    WRONG
+                  (WHITE)  (BLACK)     (WHITE)  (BLACK)
+  1      May 13[1]   5        5           7        3
+  2          14      8        2           6        4
+  3          15      7        3           9        1
+  4          16      9        1           9        1
+  5          17     10        0          10        0
+  6          18     10        0           9        1
+  7          19     10        0          10        0
+  8          20     --       --          10        0
+  9          21     --       --          10        0
+
+[Footnote 1: No preference tests were given.]
+
+
+It is now necessary to justify the interpretation of these results as
+evidence of brightness discrimination by proving that all other conditions
+for choice except brightness difference may be excluded without
+interfering with the animal's ability to select the right box. We shall
+consider in order the possibility of discrimination by position, by odor,
+and by texture and form of the cardboards.
+
+The tendency which the dancer has in common with many, if not all, animals
+to perform the same movement or follow the same path under uniform
+conditions is an important source of error in many habit-formation
+experiments. This tendency is evident even from casual observation of the
+behavior of the dancer. The ease with which the habit of choosing the box
+on the left or the box on the right is formed in comparison with that of
+choosing the white box or the black box is strikingly shown by the
+following experiment. Five mice were given one series of ten trials each
+in the discrimination box of Figure 14 without the presence of cardboards
+or of other means of visual discrimination. The electric shock was given
+whenever the box on the left was entered. Thus without other guidance than
+that of direction, for the boxes themselves were interchanged in position,
+and, as was proved by additional tests, the animals were utterly unable to
+tell one from the other, the mouse was required to choose the box on its
+right. Only one of the five animals went to the box on the left after once
+experiencing the electric shock. The results of the series are given in
+Table 11.
+
+
+
+TABLE 11
+
+CHOICE BY POSITION
+                              Choices of           Choices of
+                             Box on Right          Box on Left
+First mouse                        9                     1
+Second mouse                       8                     2
+Third mouse                        9                     1
+Fourth mouse                       9                     1
+Fifth mouse                        9                     1
+
+
+
+This conclusively proves that the habit of turning in a certain direction
+or of choosing by position can be formed more readily than a habit which
+depends upon visual discrimination. A rough comparison justifies the
+statement that it takes from six to ten times as long for the dancer to
+learn to choose the white box as it does to learn to choose the box on the
+right. Since this is true, it is exceedingly important that the
+possibility of choice by position or direction of movement be excluded in
+the case of tests of brightness discrimination. To indicate how this was
+effectively accomplished in the experiments, the changes in the position
+of the cardboards made in the case of a standard set of white-black series
+are shown in Table 12. The number of the series, beginning at the top of
+the table with the two lettered preference series, is given in the first
+column at the left, the number of the tests at the top of the table, and
+the position of the white cardboard, left or right, is indicated below by
+the letters l (left) and r (right).
+
+
+TABLE 12
+
+POSITION OF WHITE CARDBOARDS FOR A SET OF 150 TESTS
+
+
+SERIES       1   2   3   4   5   6   7   8   9  10
+
+Preference
+   A         l   r   l   r   l   r   l   r   l   r
+   B         r   l   r   l   r   l   r   l   r   l
+
+   1         r   l   r   l   r   l   r   l   r   l
+   2         l   l   r   r   l   r   l   l   r   r
+   3         r   r   l   r   l   l   r   l   r   l
+   4         l   l   l   r   r   r   l   r   r   l
+   5         r   l   r   l   r   l   r   l   r   l
+   6         l   l   r   l   r   r   l   r   l   r
+   7         r   l   l   l   r   r   r   l   r   l
+   8         r   r   l   l   r   l   r   l   r   l
+   9         r   r   r   l   l   l   r   l   r   l
+  10         l   l   l   l   r   r   r   r   l   r
+  11         r   l   r   r   r   l   l   l   r   l
+  12         r   l   r   l   r   r   l   l   r   l
+  13         r   l   r   l   l   l   r   r   r   l
+  14         l   l   l   l   r   r   r   r   l   r
+  15         r   l   r   r   r   l   l   l   r   l
+
+
+It is to be noted that in the case of each series of ten tests the white
+cardboard was on the left five times and on the right five times. Thus the
+establishment of a tendency in favor of one side was avoided. The
+irregularity of the changes in position rendered it impossible for the
+mouse to depend upon position in its choice. It is an interesting fact
+that the dancer quickly learns to choose correctly by position if the
+cardboards are alternately on the left box and on the right.
+
+The prevalent, although ill-founded, impression that mice have an
+exceedingly keen sense of smell might lead a critic of these experiments
+to claim that discrimination in all probability was olfactory rather than
+visual. As precautions against this possibility the cardboards were
+renewed frequently, so that no odor from the body of the mouse itself
+should serve as a guiding condition, different kinds of cardboard were
+used from time to time, and, as a final test, the cardboards were coated
+with shellac so that whatever characteristic odor they may have had for
+the dancer was disguised if not totally destroyed. Despite all these
+precautions the discrimination of the boxes continued. A still more
+conclusive proof that we have to do with brightness discrimination, and
+that alone, in these experiments is furnished by the results of white-
+black tests made with an apparatus which was so arranged that light was
+transmitted into the two electric-boxes through a ground glass plate in
+the end of each box. No cardboards were used. The illumination of each box
+was controlled by changes in the position of the sources of light. Under
+these conditions, so far as could be ascertained by critical examination
+of the results, in addition to careful observation of the behavior of the
+animals as they made their choices, there was no other guiding factor than
+brightness difference. Nevertheless the mice discriminated the white from
+the black perfectly. This renders unnecessary any discussion of the
+possibility of discrimination by the texture or form of the cardboards.
+
+Since a variety of precautionary tests failed to reveal the presence, in
+these experiments, of any condition other than brightness difference by
+which the mice were enabled to choose correctly, and since evidence of
+ability to discriminate brightness differences was obtained by the use of
+both reflected light (cardboards) and transmitted light (lamps behind
+ground glass), it is necessary to conclude that the dancer possesses
+brightness vision.
+
+
+
+CHAPTER VIII
+
+THE SENSE OF SIGHT: BRIGHTNESS VISION (Continued)
+
+Since the ability of the dancer to perceive brightness has been
+demonstrated by the experiments of the previous chapter, the next step in
+this investigation of the nature of vision is a study of the delicacy of
+brightness discrimination, and of the relation of the just perceivable
+difference to brightness value. Expressed in another way, the problems of
+this portion of the investigation are to determine how slight a difference
+in brightness enables the dancer to discriminate one light from another,
+and what is the relation between the absolute brightnesses of two lights
+and that amount of difference which is just sufficient to render the
+lights distinguishable. It has been discovered in the case of the human
+being that a stimulus must be increased by a certain definite fraction of
+its own value if it is to seem different. For brightness, within certain
+intensity limits, this increase must be about one one-hundredth; a
+brightness of 100 units, for example, is just perceivably different from
+one of 101 units. The formulation of this relation between the amount of a
+stimulus and the amount of change which is necessary that a difference be
+noted is known as Weber's law. Does this law, in any form, hold for the
+brightness vision of the dancing mouse?
+
+Two methods were used in the study of these problems. For the first
+problem, that of the delicacy of brightness discrimination, I first used
+light which was reflected from gray papers, according to the method of
+Chapter VII. For the second, the Weber's law test, transmitted light was
+used, in an apparatus which will be described later. Either of these
+methods might have been used for the solution of both problems. Which of
+them is the more satisfactory is definitely decided by the results which
+make up the material of this chapter, Under natural conditions the dancer
+probably sees objects which reflect light more frequently than it does
+those which transmit it; it would seem fairer, therefore, to require it to
+discriminate surfaces which differ in brightness. This the use of gray
+papers does. But, on the other hand, gray papers are open to the
+objections that they may not be entirely colorless (neutral), and that
+their brightness values cannot be changed readily by the experimenter. As
+will be made clear in the subsequent description of the experiments with
+transmitted light, neither of these objections can be raised in connection
+with the second method of experimentation.
+
+To determine the delicacy of discrimination with reflected light it is
+necessary to have a series of neutral grays (colorless) whose adjacent
+members differ from one another in brightness by less than the threshold
+of discrimination of the animal to be tested. A series which promised to
+fulfill these conditions was that of Richard Nendel of Berlin. It consists
+of fifty papers, beginning with pure white, numbered 1, and passing by
+almost imperceptible steps of decrease in brightness through the grays to
+black, which is numbered 50. For the present we may assume that these
+papers are so nearly neutral that whatever discrimination occurs is due to
+brightness. The differences between successive papers of the series are
+perceptible to man. The question is, can they, under favorable conditions
+of illumination, be perceived by the dancer?
+
+On the basis of the fact that the dancer can discriminate between white
+and black, two grays which differed from one another in brightness by a
+considerable amount were chosen from the Nendel series; these were numbers
+10 and 20. It seemed certain, from the results of previous experiments,
+that the discrimination of these papers by brightness difference would be
+possible, and that therefore the use of papers between these two extremes
+would suffice to demonstrate the delicacy of discrimination. In Figure 16
+we have a fairly accurate representation of the relative brightness of the
+Nendel papers Nos. 10, 15, and 20.
+
+[Illustration: FIGURE 16. Three of Nendel's gray papers: Nos. 10, 15, and
+20. To exhibit differences in brightness.]
+
+Pieces of the gray papers were pasted upon cardboard carriers so that they
+might be placed in the discrimination box as were the white and black
+cardboards in the tests of brightness vision previously described. Mice
+which had been trained to choose the white box by series of white-black
+tests were now tested with light gray (No. 10) and dark gray (No. 20), my
+assumption being that they would immediately choose the brighter of the
+two if they were able to detect the difference. As a matter of fact this
+did not always occur; some individuals had to be trained to discriminate
+gray No. 10 from gray No. 20. As soon as an individual had been so trained
+that the ability to choose the lighter of these grays was perfect, it was
+tested with No. 10 in combination with No. 15. If these in turn proved to
+be discriminable, No. 10 could be used with No. 14, with No. 13, and so on
+until either the limit of discrimination or that of the series had been
+reached.
+
+That it was not necessary to use other combinations than 10 with 20, and
+10 with 15 is demonstrated by the results of Table 13. Mouse No. 420,
+whose behavior was not essentially different from that of three other
+individuals which were tested for gray discrimination, learned with
+difficulty to choose gray No. 10 even when it was used with No. 20. Two
+series of ten tests each were given to this mouse daily, and not until the
+twentieth of these series (200 tests) did he succeed in making ten correct
+choices in succession. Immediately after this series of correct choices,
+tests with grays No. 10 and No. 15 were begun. In the case of this amount
+of brightness difference twenty series failed to reveal discrimination.
+The average number of right choices in the series is slightly in excess of
+the mistakes, 5.8 as compared with 4.2.
+
+From the experiments with gray papers we may conclude that under the
+conditions of the tests the amount by which Nendel's gray No. 10 differs
+in brightness from No. 20 is near the threshold of discrimination, or, in
+other words, that the difference in the brightness of the adjacent grays
+of Figure 16 is scarcely sufficient to enable the dancer to distinguish
+them.
+
+
+TABLE 13
+
+GRAY DISCRIMINATION
+
+The Delicacy of Brightness Discrimination
+
+No. 420
+
+
+
+                   GRAYS NOS. 10              GRAYS NOS. 20
+                       AND 20                    AND 15
+SERIES    DATE                       DATE
+                   NO. 10   NO. 2             NO. 10   NO. 15
+                   (RIGHT) (WRONG)           (RIGHT)  (WRONG)
+
+   1     Jan. 26     5        5      Feb. 6     8        2
+   2          27     8        2           6     5        5
+   3          28     6        4           7     9        1
+   4          28     2        8           7     7        3
+   5          29     1        9           8     5        5
+   6          29     6        4           8     6        4
+   7          30     9        1           9     5        5
+   8          30     7        3           9     6        4
+   9          31     6        4          10     8        2
+  10          31     4        6          10     3        7
+  11     Feb.  1     7        3          11     4        6
+  12           1     8        2          11     4        6
+  13           2     7        3          12     7        3
+  14           2     8        2          12     7        3
+  15           3     9        1          13     6        4
+  16           3     9        1          13     4        6
+  17           4     6        4          14     4        6
+  18           4     9        1          14     5        5
+  19           5     6        4          15     5        5
+  20           5    10        0          15     8        2
+
+      Averages      6.6     3.4               5.8       4.2
+
+
+
+
+This result of the tests with gray papers surprised me very much at the
+time of the experiments, for all my previous observation of the dancer had
+led me to believe that it is very sensitive to light. It was only after a
+long series of tests with transmitted light, in what is now to be
+described as the Weber's law apparatus, that I was able to account for the
+meager power of discrimination which the mice exhibited in the gray tests.
+As it happened, the Weber's law experiment contributed quite as
+importantly to the solution of our first problem as to that of the second,
+for which it was especially planned.
+
+For the Weber's law experiment a box similar to that used in the previous
+brightness discrimination experiments (Figure 14) was so arranged that its
+two electric-boxes could be illuminated independently by the light from
+incandescent lamps directly above them. The arrangements of the light-box
+and the lamps, as well as their relations to the other important parts of
+the apparatus, are shown in Figure 17. The light-box consisted of two
+compartments, of which one may be considered as the upward extension of
+the left electric-box and the other of the right electric-box. The light-
+box was pivoted at A and could be turned through an angle of 180° by the
+experimenter. Thus, by the turning of the light-box, the lamp which in the
+case of one test illuminated the left electric-box could be brought into
+such a position that in the case of the next test it illuminated the right
+electric-box. The practical convenience of this will be appreciated when
+the number of times that the brightnesses of the two boxes had to be
+reversed is considered. The light-box was left open at the top for
+ventilation and the prevention of any considerable increase in the
+temperature of the experiment box. In one side of each of the compartments
+of the light-box a slit (B, B of the figure) was cut out for an
+incandescent lamp holder. A strip of leatherette, fitted closely into inch
+grooves at the edges of the slit, prevented light from escaping through
+these openings in the sides of the light-box. By moving the strips of
+leatherette, one of which appears in the figure, C, the lamps could be
+changed in position with reference to the bottom of the electric-box. A
+scale, S, at the edge of each slit enabled the experimenter to determine
+the distance of the lamp from the floor of the electric-box. The front of
+the light-box was closed, instead of being open as it appears in the
+figure.
+
+[Illustration: Figure l7.--Weber's law apparatus for testing brightness
+discrimination. Lower part, discrimination box similar to that of Figure
+14. Upper part, rotatory light-box, pivoted at A, and divided into two
+compartments by a partition P in the middle. L, L, incandescent lamps
+movable in slits, B, B, in which a narrow strip of leatherette, C, serves
+to prevent the escape of light. S, scale.]
+
+This apparatus has the following advantages. First, the electric-boxes,
+between which the mouse is expected to discriminate by means of their
+difference in brightness, are illuminated from above and the light
+therefore does not shine directly from the lamps into the eyes of the
+animal, as it approaches the entrances to the boxes. Choice is required,
+therefore, between illuminated spaces instead of between two directly
+illuminated surfaces. Second, the amount of illumination of each electric-
+box can be accurately measured by the use of a photometer. Third, since
+the same kind of lamp is used in each box, and further, since the lamps
+may be interchanged at any time, discrimination by qualitative instead of
+quantitative difference in illumination is excluded. And finally, fourth,
+the tests can be made expeditiously, conveniently, and under such simple
+conditions that there should be no considerable error of measurement or of
+observation within the range of brightness which must be used.
+
+It was my purpose in the experiment with this apparatus to ascertain how
+great the difference in the illumination of the two electric-boxes must be
+in order that the mouse should be able to choose the brighter of them.
+This I attempted to do by fixing an incandescent lamp of a certain known
+illuminating power at such a position in one compartment of the light-box
+that the electric-box below it was illuminated by what I call a standard
+value, and by moving the incandescent lamp in the other compartment of the
+light-box until the illumination of the electric-box below it was just
+sufficiently less than that of the standard to enable the dancer to
+distinguish them, and thereby to choose the brighter one. The light which
+was changed from series to series I shall call the _variable_, in contrast
+with the _standard_, which was unchanged.
+
+The tests, which were made in a dark-room under uniform conditions, were
+given in series of fifty each; usually only one such series was given per
+day, but sometimes one was given in the morning and another in the
+afternoon of the same day. To prevent choice by position the lights were
+reversed in position irregularly, first one, then the other, illuminating
+the right electric-box. For the fifty tests of each initial series the
+order of the changes in position was as follows: standard (brighter light)
+on the _l_ (left), _l, r_ (right), _r, l, l, r, r, l, r, l, r, l, l, r, r,
+l, l, r, r, l, l, l, r, r, r, l, r, l, r, r, r, l, l, l, r, r, r, l, l, r,
+l, r, l, r, l, r, l, r, l_. Twenty-five times in fifty the standard light
+illuminated the right electric-box, and the same number of times it
+illuminated the left electric-box. When a second series was given under
+the same conditions of illumination, a different order of change was
+followed.
+
+In order to discover whether Weber's law holds in the case of the
+brightness vision of the dancer it was necessary for me to determine the
+just perceivable difference between the standard and the variable lights
+for two or more standard values. I chose to work with three values, 5, 20,
+and 80 hefners, and I was able to discover with a fair degree of accuracy
+how much less than 5, 20, or 80 hefners, as the case might be, the
+variable light had to be in order that it should be discriminable from the
+other. For the work with the 5 hefner standard I used 2-candle-power
+lamps,[1] for the 20, 4-candle-power, and for the 80, 16-candle-power.
+
+[Footnote 1: I give merely the commercial markings of the lamps. They had
+been photometered carefully by two observers by means of a Lummer-Brodhun
+photometer and a Hefner amyl acetate lamp previous to their use in the
+experiment. For the photometric measurements in connection with the
+Weber's law tests I made use of the Hefner lamp with the hope of attaining
+greater accuracy than had been possible with a standard paraffine candle,
+in the case of measurements which I had made in connection with the
+experiments on color vision that are reported in Chapters IX and X. The
+Hefner unit is the amount of light produced by an amyl acetate lamp at a
+flame height of 40 mm. (See Stine's "Photometrical Measurements.") A
+paraffine candle at a flame height of 50 mm. is equal to 1.2 Hefner
+units.]
+
+For reasons which will soon appear, Weber's law tests were made with only
+one dancer. This individual, No. 51, had been thoroughly trained in white-
+black discrimination previous to the experiments in the apparatus which is
+represented in Figure 17. Having given No. 51 more than two hundred
+preliminary tests in the Weber's law apparatus with the electric-boxes
+sufficiently different in brightness to enable her to discriminate
+readily, I began my experiments by trying to ascertain how much less the
+value of the illumination of one electric-box must be in order that it
+should be discriminable from a value of 20 hefners in the other electric-
+box. In recording the several series of tests and their results hereafter,
+I shall state in Hefner units the value of the fixed or standard light and
+the value of the variable light, the difference between the two in terms
+of the former, and the average number of wrong choices in per cent.
+
+With the lamps so placed that the difference in the illumination of the
+two electric-boxes was .53 of the value of the standard, that is about one
+half, No. 51 made twenty wrong choices in one hundred, or 20 per cent.
+When the difference was reduced to .36 (one third) the number of errors
+increased to 36 per cent, and with an intermediate difference of .48 there
+were 26 per cent of errors (see Table 14).
+
+Are these results indicative of discrimination, or are the errors in
+choice too numerous to justify the statement that the dancer was able to
+distinguish the boxes by their difference in brightness? Evidently this
+question cannot be answered satisfactorily until we have decided what the
+percentage of correct choices should be in order that it be accepted as
+evidence of ability to discriminate, or, to put it in terms of errors,
+what percentage of wrong choices is indicative of the point of just
+perceivable difference in brightness. Theoretically, there should be as
+many mistakes as right choices, 50 per cent of each, when the two
+electric-boxes are equally illuminated (indiscriminable), but in practice
+this does not prove to be the case because the dancer tends to return to
+that electric-box through which in the previous test it passed safely,
+whereas it does not tend in similar fashion to reënter the box in which it
+has just received an electric shock. The result is that the percentage of
+right choices, especially in the case of series which have the right box
+in the same position two, three, or four times in succession, rises as
+high as 60 or 70, even when the visual conditions are indiscriminable.
+Abundant evidence in support of this statement is presented in Chapters
+VII and IX, but at this point I may further call attention to the results
+of an experiment in the Weber's law apparatus which was made especially to
+test the matter. The results appear under the date of May 27 in Table 14.
+In this experiment, despite the fact that both boxes were illuminated by
+80 hefners, the mouse chose the standard (the illumination in which it was
+not shocked) 59 times in 100. In other words the percentage of error was
+41 instead of 50. It is evident, therefore, that as low a percentage of
+errors as 40 is not necessarily indicative of discrimination. Anything
+below 40 per cent is likely, however, to be the result of ability to
+distinguish the brighter from the darker box. To be on the safe side we
+may agree to consider 25 wrong choices per 100 as indicative of a just
+perceivable difference in illumination. Fewer mistakes we shall consider
+indicative of a difference in illumination which is readily perceivable,
+and more as indicative of a difference which the mouse cannot detect. The
+reader will bear in mind as he examines Table 14 that 25 per cent of wrong
+choices indicates the point of just perceivable difference in brightness.
+
+
+
+TABLE 14
+
+RESULTS OF WEBER'S LAW EXPERIMENTS
+Brightness vision
+
+DATE    NUMBER   STANDARD  VARIABLE  DIFFERENCE % OF ERRORS
+       OF TESTS   LIGHT     LIGHT
+
+May 13   100       20        9.4       .53        20
+    15   100       20       12.8       .36        36
+    16   100       20       10.8       .46        26
+    20    50       80       37.6       .53         6
+    21    50       80       51.3       .36        10
+    22   100       80       71.1       .11        35
+    24   100       80       60.0       .25        21
+    25   100       80       65.0       .19        25
+    27   100       80       80           0        41
+    28    50        5        2.5       .50        18
+    29    50        5        4.0       .20        14
+    29   100        5        4.5       .10        25
+    31    50        5        4.25      .15        20
+June 1    50        5        4.85      .03        48
+     2    50       20       15.0       .25        16
+     3    50       20       17.4       .13        22
+     3   100       20       18.0       .10        22
+     4   100       80       72.0       .10        18
+     5   100        5        4.5       .10        12
+     7   100        5        4.67      .067       46
+     8    50       80       74.67      .067       56
+     9    50       20       18.67      .067       44
+
+
+
+If we apply this rule to the results of the first tests, reported above,
+it appears that a standard of 20 hefners was distinguished from a variable
+of 9.4 hefners (.53 difference), for the percentage of errors was only 20.
+But in the case of a difference of .36 in the illuminations lack of
+discrimination is indicated by 36 per cent of errors. A difference of .46
+gave a frequency of error so close to the required 25 (26 per cent) that I
+accepted the result as a satisfactory determination of the just
+perceivable difference for the 20 hefner standard and proceeded to
+experiment with another standard value.
+
+The results which were obtained in the case of this second standard, the
+value of which was 80 hefners, are strikingly different from those for the
+20 hefner standard. Naturally I began the tests with this new standard by
+making the differences the same as those for which determinations had been
+made in the case of the 20 standard. Much to my surprise only 6 per cent
+of errors resulted when the difference in illumination was .53. I finally
+discovered that about .19 difference (about one fifth) could be
+discriminated with that degree of accuracy which is indicated by 25 per
+cent of mistakes.
+
+So far as I could judge from the results of determinations for the 20 and
+the 80 hefner standards, Weber's law does not hold for the dancer. With
+the former a difference of almost one half was necessary for
+discrimination; with the latter a difference of about one fifth could be
+perceived. But before presenting additional results I should explain the
+construction of Table 14, and comment upon the number of experiments which
+constitutes a set.
+
+The table contains the condensed results of several weeks of difficult
+experimentation. From left to right the columns give the date of the
+initial series of a given set of experiments, the number of experiments in
+the set, the value of the standard light in hefners, the value of the
+variable light, the difference between the lights in terms of the standard
+(the variable was always less than the standard), and the percentage of
+errors or wrong choices. Very early in the investigation I discovered that
+one hundred tests with any given values of the lights sufficed to reveal
+whatever discriminating ability the mouse possessed at the time. In some
+instances either the presence or the lack of discrimination was so clear,
+as the result of 50 tests (first series), that the second series of 50 was
+not given. Consequently in the table the number of tests for the various
+values of the lights is sometimes 100, sometimes 50.
+
+After finishing the experiments with the 80 standard on May 27 (see table)
+I decided, in spite of the evidence against Weber's law, to make tests
+with 5 as the standard, for it seemed not impossible that the lights were
+too bright for the dancer to discriminate readily. I even suspected that I
+might have been working outside of the brightness limits in which Weber's
+law holds, if it holds at all. The tests soon showed that a difference of
+one tenth made discrimination possible in the case of this standard. If
+the reader will examine the data of the table, he will note that a
+difference of .20 gave 14 per cent of mistakes; a difference of .03, 48
+per cent. Evidently the former difference is above the threshold, the
+latter below it. But what of the interpretation of the results in terms of
+Weber's law? The difference instead of being one half or one fifth, as it
+was in the cases of the 20 and 80 standards respectively, has now become
+one tenth. Another surprise and another contradiction!
+
+Had these three differences either increased or decreased regularly with
+the value of the standard I should have suspected that they indicated a
+principle or relationship which is different from but no less interesting
+than that which Weber's law expresses. But instead of reading 5 standard,
+difference one tenth; 20 standard, difference one fifth; 80 standard,
+difference one half: or 5 standard, difference one half; 20 standard,
+difference one fifth; 80 standard, difference one tenth: they read 5
+standard, difference one tenth; 20 standard, difference one half; 80
+standard, difference one fifth. What does this mean? I could think of no
+other explanation than that of the influence of training. It seemed not
+impossible, although not probable, that the mouse had been improving in
+ability to discriminate day by day. It is true that this in itself would
+be quite as interesting a fact as any which the experiment might reveal.
+
+To test the value of my supposition, I made additional experiments with
+the 20 standard, the results of which appear under the dates June 2 and 3
+of the table. These results indicate quite definitely that the animal had
+been, and still was, improving in her ability to discriminate. For instead
+of requiring a difference of about one half in order that she might
+distinguish the 20 standard from the variable light she was now able to
+discriminate with only 22 per cent of errors when the difference was one
+tenth.
+
+As it seemed most improbable that improvement by training should continue
+much longer, I next gave additional tests with the 80 standard. Again a
+difference of one tenth was sufficient for accurate discrimination (18 per
+cent of errors). These series were followed immediately by further tests
+with the 5 standard. As the results indicated greater ease of
+discrimination with a difference of one tenth in the case of this standard
+than in the case of either of the others I was at first uncertain whether
+the results which I have tabulated under the dates June 3, 4, and 5 of the
+table should be interpreted in terms of Weber's law.
+
+Up to this point the experiments had definitely established two facts:
+that the dancer's ability to discriminate by means of brightness
+differences improves with training for a much longer period and to a far
+greater extent than I had supposed it would; and that a difference of one
+tenth is sufficient to enable the animal to distinguish two lights in the
+case of the three standard values, 5, 20, and 80 hefners. The question
+remains, is this satisfactory evidence that Weber's law holds with respect
+to the brightness vision of the dancer, or do the results indicate rather,
+that this difference is more readily detected in the case of 5 as a
+standard (12 per cent error) than in the case of 20 as a standard (22 per
+cent error)?
+
+For the purpose of settling this point I made tests for each of the three
+standards with a difference of only one fifteenth. In no instance did I
+obtain the least evidence of ability to discriminate. These final tests,
+in addition to establishing the fact that the limit of discrimination for
+No. 51, after she had been subjected to about two thousand tests, lay
+between one tenth and one fifteenth, proved to my satisfaction, when taken
+in connection with the results already discussed, that Weber's law does
+hold for the brightness vision of the dancer.
+
+In concluding this discussion of the Weber's law experiment I wish to call
+attention to the chief facts which have been revealed, and to make a
+critical comment. In my opinion it is extremely important that the student
+of animal behavior should note the fact that the dancer with which I
+worked week after week in the Weber's law investigation gradually improved
+in her ability to discriminate on the basis of brightness differences
+until she was able to distinguish from one another two boxes whose
+difference in illumination was less than one tenth[1] that of the brighter
+box. At the beginning of the experiments a difference of one half did not
+enable her to choose as certainly as did a difference of one tenth after
+she had chosen several hundred times. Evidently we are prone to
+underestimate the educability of our animal subjects.
+
+[Footnote 1: Under the conditions of the experiment I was unable to
+distinguish the electric-boxes when they differed by less than one
+twentieth.]
+
+ The reason that the experiments were carried out with only one mouse must
+now be apparent. It was a matter of time. The reader must not suppose that
+my study of this subject is completed. It is merely well begun, and I
+report it here in its unfinished state for the sake of the value of the
+method which I have worked out, rather than for the purpose of presenting
+the definite results which I obtained with No. 51.
+
+The critical comment which I wish to make for the benefit of those who are
+working on similar problems is this. The phosphor bronze wires, on the
+bottom of the electric-boxes, by means of which an electric shock could be
+given to the mouse when it chose the wrong box, are needless sources of
+variability in the illumination of the boxes. They reflect the light into
+the eyes of the mouse too strongly, and unless they are kept perfectly
+clean and bright, serious inequalities of illumination appear. To avoid
+these undesirable conditions I propose hereafter to use a box within a
+box, so that the wires shall be hidden from the view of the animal as it
+attempts to discriminate.
+
+A brief description of the behavior of the dancer in the brightness
+discrimination experiments which have been described may very
+appropriately form the closing section of this chapter. For the
+experimenter, the incessant activity and inexhaustible energy of the
+animal are a never-failing source of interest and surprise. When a dancer
+is inactive in the experiment box, it is a good indication either of
+indisposition or of too low a temperature in the room. In no animal with
+which I am familiar is activity so much an end in itself as in this odd
+species of mouse. With striking facility most of the mice learn to open
+the wire swing doors from either side. They push them open with their
+noses in the direction in which they were intended by the experimenter to
+work, and with almost equal ease they pull them open with their teeth in
+the direction in which they were not intended to work. In the rapidity
+with which this trick is learned, there are very noticeable individual
+differences. The pulling of these doors furnished an excellent opportunity
+for the study of the imitative tendency.
+
+When confronted with the two entrances of the electric-boxes, the dancer
+manifested at first only the hesitation caused by being in a strange
+place. It did not seem much afraid, and usually did not hesitate long
+before entering one of the boxes. The first choice often determined the
+majority of the choices of the preference series. If the mouse happened to
+enter the left box, it kept on doing so until, having become so accustomed
+to its surroundings that it could take time from its strenuous running
+from _A_ by way of the left box to the alley and thence to _A_, to examine
+things in _B_ a little, it observed the other entrance and in a seemingly
+half-curious, half-venturesome way entered it. In the case of other
+individuals, the cardboards themselves seemed to determine the choices
+from the first.
+
+The electric shock, as punishment for entering the wrong box, came as a
+surprise. At times an individual would persistently attempt to enter, or
+even enter and retreat from the wrong box repeatedly, in spite of the
+shock. This may have been due in some instances to the effects of fright,
+but in others it certainly was due to the strength of the tendency to
+follow the course which had been taken most often previously. The next
+effect of the shock was to cause the animal to hesitate before the
+entrances to the boxes, to run from one to the other, poking its head into
+each and peering about cautiously, touching the cardboards at the
+entrances, apparently smelling of them, and in every way attempting to
+determine which box could be entered safely. I have at times seen a mouse
+run from one entrance to the other twenty times before making its choice;
+now and then it would start to enter one and, when halfway in, draw back
+as if it had been shocked. Possibly merely touching the wires with its
+fore paws was responsible for this simulation of a reaction to the shock.
+The gradual waning of this inhibition of the forward movement was one of
+the most interesting features of the experiment. Could we but discover
+what the psychical states and the physiological conditions of the animal
+were during this period of choosing, comparative psychology and physiology
+would advance by a bound.
+
+If the conditions at the entrances of the two boxes were discriminable,
+the mouse usually learned within one hundred experiences to choose the
+right box without much hesitation. Three distinct methods of choice were
+exhibited by different individuals, and to a certain extent by the same
+individual from time to time. These methods, which I have designated
+_choice by affirmation_, _choice by negation_, and _choice by comparison_,
+are of peculiar interest to the psychologist and logician.
+
+When an individual runs directly to the entrance of the right box, and,
+after stopping for an instant to examine it, enters, the choice may be
+described as recognition of the right box. I call it choice by affirmation
+because the act of the animal is equivalent to the judgment--"this is it."
+If instead it runs directly to the wrong box, and, after examining it,
+turns to the other box and enters without pause for examination, its
+behavior may be described as recognition of the wrong box. This I call
+choice by negation because the act seems equivalent to the judgment--"this
+is not it." Further, it seems to imply the judgment--"therefore the other
+is it." In the light of this fact, this type of choice might appropriately
+be called choice by exclusion. Finally, when the mouse runs first to one
+box and then to the other, and repeats this anywhere from one to fifty
+times, the choice may be described as comparison of the boxes; therefore,
+I call it choice by comparison. Certain individuals choose first by
+comparison, and later almost uniformly by affirmation and negation.
+Whenever the conditions are difficult to discriminate, choice by
+comparison occurs most frequently and persistently. If, however, the
+conditions happen to be absolutely indiscriminable, as was true, for
+example, in the case of the sound tests, in certain of the Weber's law
+tests, and in the plain electric-box tests, the period of hesitation
+rapidly increases during the first three or four series of tests, then the
+mouse seems to lessen its efforts to discriminate and more and more tends
+to rush into one of the boxes without hesitation or examination, and
+apparently with the expectation of a shock, but with the intention of
+getting it over as soon as possible. Now and then under such conditions
+there is a marked tendency to enter the same box each time.
+Indiscriminable conditions are likely to render the animals fearful of the
+experiment; instead of going from _A_ to _A_ willingly, they fight against
+making the trip. They refuse to pass from _A_ to _B_; and when in _B_,
+they fight against being driven toward the entrances to the electric-
+boxes.
+
+In marked contrast with this behavior on the part of the mouse under
+conditions which do not permit it to choose correctly is that of the
+animal which has learned what is expected of it. The latter, far from
+holding back or fighting against the conditions which urge it forward, is
+so eager to make the trip that it sometimes has to be forced to wait while
+the experimenter records the results of the tests. There is evidence of
+delight in the freedom of movement and in the variety of activity which
+the experiment furnishes. The thoroughly trained dancer runs into _B_
+almost as soon as it has been placed in _A_ by the experimenter; it
+chooses the right entrance by one of the three methods described above,
+immediately, or after whirling about a few times in _B_; it runs through
+_E_ and back to _A_ as quickly as it can, and almost before the
+experimenter has had time to record the result of the choice it is again
+in _B_ ready for another choice.
+
+
+
+
+CHAPTER IX
+
+THE SENSE OF SIGHT: COLOR VISION
+
+Is the dancing mouse able to discriminate colors as we do? Does it possess
+anything which may properly be called color vision? If so, what is the
+nature of its ability in this sense field? Early in my study of the mice I
+attempted to answer these and similar questions, for the fact that they
+are completely deaf during the whole or the greater part of their lives
+suggested to me the query, are they otherwise defective in sense
+equipment? In the following account of my study of color vision, I shall
+describe the evolution of my methods in addition to stating the results
+which were obtained and the conclusions to which they led me. For in this
+case the development of a method of research is quite as interesting as
+the facts which the method in its various stages of evolution revealed.
+
+Observation of the behavior of the dancer under natural conditions caused
+me to suspect that it is either defective in color vision or possesses a
+sense which is very different from human vision. I therefore devised the
+following extremely simple method of testing the animal's ability to
+distinguish one color from another. In opposite corners of a wooden box 26
+cm. long, 23 cm. wide, and 11 cm. deep, two tin boxes 5 cm. in diameter
+and 1.5 cm. deep were placed, as is shown in part I of Figure 18. One of
+these boxes was covered on the outside with blue paper (_B_ of Figure 18),
+and the other with orange[1] (_O_ of Figure 18). A small quantity of
+"force" was placed in the orange box. As the purpose of the test was to
+discover whether the animals could learn to go directly to the box which
+contained the food, the experiments were made each morning before the mice
+had been fed. The experimental procedure consisted in placing the
+individual to be tested in the end of the large wooden box opposite the
+color boxes, and then permitting it to run about exploring the box until
+it found the food in the orange box. While it was busily engaged in eating
+a piece of "force" which it had taken from the box and was holding in its
+fore paws, squirrel fashion, the color boxes were quickly and without
+disturbance shifted in the directions indicated by the arrows of Figure
+18, I. Consequently, when the animal was ready for another piece of
+"force," the food-box was in the corresponding corner of the opposite end
+of the experiment box (position 2, 18, II). After the mouse had again
+succeeded in finding it, the orange box was shifted in position as is
+indicated by the arrows in Figure 18, II. Thus the tests were continued,
+the boxes being shifted after each success on the part of the animal in
+such a way that for no two successive tests was the position of the food-
+box the same; it occupied successively the positions 1, 2, 3, and 4 of the
+figure, and then returned to 1. Each series consisted of 20 tests.
+
+[Footnote 1: These were the Milton Bradley blue and orange papers.]
+
+[Illustration: FIGURE 18.--Food-box apparatus for color discrimination
+experiments. _O_, orange food-box; _B_, blue food-box; 1, 2, 3, 4,
+different positions of the food-boxes, _O_ and _B_; I, II, III, IV,
+figures in which the arrows indicate the direction in which the food-boxes
+were moved.]
+
+[Illustration: FIGURE 19.--Food-box apparatus with movable partitions.
+_O_, orange food-box; _B_, blue food-box; _X_, starting point for mouse;
+_A_, point at which both food-boxes become visible to the mouse as it
+approaches them; 1, 2, two different positions of the food-boxes; _T_,
+_T_, movable partitions. (After Doctor Waugh.)]
+
+An improvement on this method, which was suggested by Doctor Karl Waugh,
+has been used by him in a study of the sense of vision in the common
+mouse. It consisted in the introduction, at the middle of the experiment
+box, of two wooden partitions which were pivoted on their mid-vertical
+axes so that they could be placed in either of the positions indicated in
+Figure 19. Let us suppose that a mouse to be tested for color vision in
+this apparatus has been placed at _X_. In order to obtain food it must
+pass through _A_ and choose either the orange or the blue box. If it
+chooses the former, the test is recorded as correct; if it goes to the
+blue box first, and then to the orange, it is counted an error. While the
+animal is eating, the experimenter shifts the boxes to position 1 of
+Figure 19, and at the same time moves the partitions so that they occupy
+the position indicated by the dotted lines. The chief advantage of this
+improvement in method is that the animal is forced to approach the color
+boxes from a point midway between them, instead of following the sides of
+the experiment box, as it is inclined to do, until it happens to come to
+the food-box. This renders the test fairer, for presumably the animal has
+an opportunity to see both boxes from _A_ and can make its choice at that
+point of vantage.
+
+Two males, A and B, of whose age I am ignorant, were each given seventeen
+series of tests in the apparatus of Figure 18. A single series, consisting
+of twenty choices, was given daily. Whether the animal chose correctly or
+not, it was allowed to get food; that is, if it went first to the blue
+box, thus furnishing the condition for a record of error, it was permitted
+to pass on to the orange box and take a piece of "force." No attempt was
+made to increase the animal's desire for food by starving it. Usually it
+sought the food-box eagerly; when it would not do so, the series was
+abandoned and work postponed. "Force" proved a very convenient form of
+food in these tests. The mice are fond of it, and they quickly learned to
+take a flake out of the box instead of trying to get into the box and sit
+there eating, for when they attempted the latter they were promptly pushed
+to one side by the experimenter and the box, as well as the food, was
+removed to a new position.
+
+The results of the tests appear in Table 15. No record of the choices in
+the first two of the 17 series was kept. The totals therefore include 15
+series, or 300 tests, with each individual. Neither the daily records nor
+the totals of this table demonstrate choice on the basis of color
+discrimination. Either the dancers were not able to tell one box from the
+other, or they did not learn to go directly to the orange box. It might be
+urged with reason that there is no sufficiently strong motive for the
+avoidance of an incorrect choice. A mistake simply means a moment's delay
+in finding food, and this is not so serious a matter as stopping to
+discriminate. I am inclined, in the light of result of other experiments,
+to believe that there is a great deal in this objection to the method.
+Reward for a correct choice should be supplemented by some form of
+punishment for a mistake. This conclusion was forced upon me by the
+results of these preliminary experiments on color vision and by my
+observation of the behavior of the animals in the apparatus. At the time
+the above tests were made I believed that I had demonstrated the inability
+of the dancer to distinguish orange from blue, but now, after two years'
+additional work on the subject, I believe instead that the method was
+defective.
+
+The next step in the evolution of a method of testing the dancer's color
+vision was the construction of the apparatus (Figures 14 and 15) which was
+described in Chapter VII. In connection with this experiment box the basis
+for a new motive was introduced, namely, the punishment of mistakes by an
+electric shock. Colored cardboards, instead of the white, black, or grays
+of the brightness tests, were placed in the electric-boxes.
+
+
+
+TABLE 15
+
+ORANGE-BLUE TESTS, WITH FOOD-BOX
+
+
+
+                       MOUSE A            MOUSE B
+SERIES    DATE
+          1904       RIGHT     WRONG    RIGHT     WRONG
+                    (ORANGE)  (BLUE)   (ORANGE)   (BLUE)
+
+   1     Dec.  6      --        --       --        --
+   2           7      --        --       --        --
+   3           8      12         8       12         8
+   4           9      10        10        9        11
+   5          10      15         5       10        10
+   6          11      10        10       12         8
+   7          12       9        11        9        11
+   8          13      10        10        9        11
+   9          14      12         8       12         8
+  10          15      13         7       12         8
+  11          16      13         7       10        10
+  12          17      12         8       10        10
+  13          18      11         9       10        10
+  14          19      13         7        8        12
+  15          20      13         7        9        11
+  16          22      14         6       12         8
+  17          23      10        10        9        11
+
+       TOTALS        177       123      153       147
+
+
+
+
+In preliminary tests, at the rate of four per day, the colored cardboards
+were placed only at the entrances to the boxes, not inside, and as was
+true also in the case of brightness tests under like conditions, no
+evidence of discrimination was obtained from ten days' training. This
+seemed to indicate that a considerable area of the colored surface should
+be exposed to the mouse's view, if discrimination were to be made
+reasonably easy.
+
+This conclusion was supported by the results of other preliminary
+experiments in which rectangular pieces of colored papers[1] 6 by 3 cm.,
+were placed on the floor at the entrances to the electric-boxes, instead
+of on the walls of the boxes. Mouse No. 2 was given five series of ten
+tests each with a yellow card to indicate the right box and a red card at
+the entrance to the wrong box. At first he chose the red almost uniformly,
+and at no time during these fifty tests did he exhibit ability to choose
+the right box by color discrimination. I present the results of these
+series in Table 16, because they indicate a fact to which I shall have to
+refer repeatedly later, namely, that the brightness values of different
+portions of the spectrum are not the same for the dancer as for us.
+Previous to this yellow-red training, No. 2, as a result of ten days of
+white-black training (two tests per day), had partially learned to go to
+the brighter of the two electric-boxes. It is possible therefore that the
+choice of the box in the case of these color experiments was in reality
+the choice of what appeared to the mouse to be the brighter box. If this
+were not true, how are the results of Table 16 to be accounted for?
+
+[Footnote 1: These were the only Hering papers used in my experiments.]
+
+TABLE 16
+
+YELLOW-RED TESTS
+
+In Color Discrimination Box with 6 by 3 cm. Pieces of Hering
+Papers at Entrances to Boxes
+
+No. 2
+
+  SERIES              DATE               RIGHT             WRONG
+                      1906             (Yellow)            (Red)
+    1              Jan. 16                 1                 9
+    2                   17                 3                 7
+    3                   18                 4                 6
+    4                   19                 5                 5
+    5                   20                 5                 5
+
+
+
+Without further mention of the many experiments which were necessary for
+the perfecting of this method of testing color vision, I may at once
+present the final results of the tests which were made with reflected
+light. These tests were made with the discrimination apparatus in
+essentially the same way as were the brightness discrimination tests of
+Chapter VII.
+
+In all of the color experiments, unless otherwise stated, a series of ten
+tests each day was given, until satisfactory evidence of discrimination or
+proof of the lack of the ability to discriminate had been obtained. The
+difficulties of getting conclusive evidence in either direction will be
+considered in connection with the results themselves. For all of these
+tests with reflected light the Milton Bradley colored papers were used.
+These colored papers were pasted on white cardboard carriers. I shall
+designate, in the Bradley nomenclature, the papers used in each
+experiment.
+
+With colored cardboards inside the electric-boxes as well as at their
+entrances (see Figure 14 for position of cardboards) blue-orange tests
+were given to Nos. 2 and 3 until they discriminated perfectly. The papers
+were Bradley's blue tint No. 1 and orange. Number 2 was perfect in the
+twelfth series (Table 17), No 3 in the fourteenth and again in the
+sixteenth. They were then tested with a special brightness check series
+which was intended by the experimenter to reveal any dependence upon a
+possible brightness difference rather than upon the color difference of
+the boxes.
+
+
+
+TABLE 17
+
+LIGHT BLUE-ORANGE TESTS IN COLOR DISCRIMINATION BOX
+
+SERIES       DATE              NO. 2               NO. 3
+             1906         RIGHT    WRONG      RIGHT    WRONG
+                         (LIGHT   (ORANGE)   (LIGHT   (ORANGE)
+                          BLUE)               BLUE)
+
+
+   1      Jan. 26           7        3          1         9
+   2           27           7        3          5         5
+   3           28           7        3          6         4
+   4           29           7        3          7         3
+   5           30           7        3          4         6
+   6           31          10        0          7         3
+   7      Feb.  1           9        1          7         3
+   8            2           8        2          6         4
+   9            3           9        1          9         1
+  10            5           7        3          5         5
+  11            6           8        2          5         5
+  12            7          10        0          5         5
+Special brightness check series (see Table 18)
+  13            8          10        0          7         3
+Special light blue-dark blue series
+  14            9           8        2         10         0
+  15           10           9        1          9         1
+Special light blue-dark blue series
+  16           11           9        1         10         0
+                                            Special brightness
+                                               check series
+  17           12          10        0          9         1
+
+
+
+
+TABLE 18
+
+LIGHT BLUE-ORANGE
+
+Brightness check series                     Mouse No. 2, Series 13
+                        Feb. 8, 1906
+
+TEST             CONDITION                       RIGHT     WRONG
+
+1         Light blue on right
+          Orange on left                         Right      ____
+
+2         Light blue on left
+          Orange on right                        Right      ____
+
+3         Light blue on right
+          Red substituted for orange             Right      ____
+
+4         Light blur on left
+          Red substituted for orange             Right      ____
+
+5         Dark blue on right
+          Orange on left                         Right      ____
+
+6         Dark blue on left
+          Orange on right                        Right      ____
+
+7         Dark blue on left
+          Orange on right                        Right      ____
+
+8         Dark blue on right
+          Red substituted for orange             Right      ____
+
+9         Dark blue on left
+          Red substituted for orange             Right      ____
+
+10        Dark blue on left
+          Red substituted for orange             Right      ____
+
+          Totals                                   10        0
+
+
+
+The nature of this brightness check series, as well as the results which
+No. 2 gave when tested by it, may be appreciated readily by reference to
+Table 18. Tint No. 1 of the blue, which is considerably brighter, in my
+judgment, than the Bradley blue, was replaced at intervals in this series
+by the latter. For it was thought that in case the mouse were choosing the
+blue of the series because it seemed brighter than the orange, this
+substitution might mislead it into choosing the orange. These blues are
+referred to in the table as light blue (tint No. 1) and dark blue
+(standard blue). Again a change in the opposite direction was made by
+substituting Bradley red for orange. As this was for the human eye the
+substitution of a color whose brightness was considerably less than that
+of the orange, it seemed possible that the mouse, if it had formed the
+habit of choosing the box which seemed the darker, might by this change be
+misled into choosing the red instead of the light blue. In a word, changes
+in the conditions of the experiments were made in such a way that now one
+color, now the other, appeared to be the brighter. But I did not attempt
+to exclude brightness discrimination on the part of the mouse by
+dependence upon the human judgment of brightness equality, for it is
+manifestly unsafe to assume that two colors which are of the same
+brightness for the human eye have a like relation for the eye of the
+dancer or of any other animal. My tests of color vision have been
+conducted without other reference to human standards of judgment or
+comparisons than was necessary for the description of the experimental
+conditions. In planning the experiments I assumed neither likeness nor
+difference between the human retinal processes and those of the dancer. It
+was my purpose to discover the nature of the mouse's visual discriminative
+ability.
+
+As is indicated in the tables, neither the substitution of dark blue for
+light blue, nor the replacement of the orange by red or dark blue rendered
+correct choice impossible, although certain of the combinations did render
+choice extremely difficult. In other words, despite all of the changes
+which were made in the brightness of the cardboards in connection with the
+light blue-orange tests, the mice continued to make almost perfect
+records. What are we to conclude from this? Either that the ability to
+discriminate certain colors is possessed by the dancer, or that for some
+reason the tests are unsatisfactory. If it be granted that the possibility
+of brightness discrimination was excluded in the check series, the first
+of these alternatives apparently is forced upon us. That such a
+possibility was not excluded, later experiments make perfectly clear. The
+fact was that not even in the check series was the brightness value of the
+orange as great as that of the blue. Consequently the mice may have chosen
+the brighter box each time while apparently choosing the blue.
+
+Although conclusive proof of the truth of this statement is furnished only
+by later experiments, the results of the light blue-orange series, as
+given in Table 17, strongly suggest such a possibility. Mouse No. 3 had
+not been experimented with previous to these color discrimination tests.
+Her preference for the orange, which in the case of the first series was 9
+to 1, consequently demands an explanation. If she had been trained
+previously to choose the white instead of the black, as was true of No. 2,
+it might be inferred that she went to the orange box because it appeared
+brighter than the blue. As this explanation is not available, we are
+driven back upon the results of the white-black preference tests in
+Chapter VII, which proved that many dancers prefer the black to the white.
+This may mean that they prefer the lower degree of brightness or
+illumination, and if so it might be argued, in turn, that the orange was
+chosen by No. 3 because it appeared darker than the blue. Since, as has
+already been stated, the orange was far brighter for me than the blue,
+this would also mean that the brightness values of different colors are
+not the same for man and mouse.
+
+Practically the same kind of color tests as those described for Nos. 2 and
+3 were given to Nos. 1000 and 5. The results appear in Table 19. These
+tests followed upon the formation of a habit to choose white instead of
+black (that is, the greater brightness). From the first both No. 1000 and
+No. 5 chose the light blue in preference to the orange or the red. It
+therefore seems probable that the former was considerably brighter than
+the latter. Number 1000, to be sure, was led into three erroneous choices
+by the brightness check series (series 7), but, on the other hand, No. 5
+was not at all disturbed in her choices by similar check tests. It seems
+natural to conclude from these facts that both of these mice chose the
+blue at first because of its relatively greater brightness, and that they
+continued to do so for the same reason. In other words, their behavior
+indicates that the brightness check tests were valueless because not
+enough allowance had been made for the possible differences between the
+vision of mouse and man.
+
+
+                 TABLE 19
+LIGHT BLUE-ORANGE AND DARK BLUE-RED TESTS
+                                         No. 1000           No. 5
+SERIES   DATE        Condition         Right   Wrong    Right  Wrong
+                                      (Light  (Orange   (Light (Orange
+                                       Blue or   or      Blue    or
+                                       Dark     Red)     Dark    Red)
+                                       Blue)             Blue)
+   1     Jan. 25     Blue-red            8       2        10       0
+   2          26     Blue-red or
+                     Light blue-orange  10       0        10       0
+   3          27     Light blue-orange  10       0         5       5
+   4          29     Light blue-orange   9       1         8       2
+   5          30     Light blue-orange  10       0         8       2
+   6          31     Light blue-orange  10       0        10       0
+   7    Feb.   1     Light blue-orange
+                     or Dark blue-red    7       3        10       0
+
+
+If only the final results of my experiments with the dancer and the
+conclusions to which they lead were of interest, all of this description
+of experiments which served merely to clear the ground and thus make
+possible crucial tests might be omitted. It has seemed to me, however,
+that the history of the investigation is valuable, and I am therefore
+presenting the evolution of my methods step by step. To be sure, not every
+detail of this process can be mentioned, and only a few of the individual
+results can be stated, but my purpose will have been fulfilled if I
+succeed in showing how one method of experimentation pointed the way to
+another, and how one set of results made possible the interpretation of
+others.
+
+As the results of my color vision experiments seemed to indicate that the
+red end of the spectrum appears much darker to the dancer than to us,
+tests were now arranged with colors from adjacent regions of the spectrum,
+green and blue. The papers used were the Bradley green and tint No. 1 of
+the blue. They were not noticeably different in brightness for the human
+eye. Green marked the box to be chosen. Three of the individuals which had
+previously been used in the light blue-orange series, and which therefore
+had perfect habits of going to the light blue, were used for the green-
+light blue tests. Of these individuals, No. 1000 became inactive on the
+fifth day of the experiment, and the tests with him were discontinued.
+Twenty series were given to each of the other mice, with the results which
+appear in Table 20. To begin with, both No. 4 and No. 5 exhibited a
+preference for the light blue, as a result of the previous light blue-
+orange training. As this preference was gradually destroyed by the
+electric shock which was received each time the light blue box was
+entered, they seemed utterly at a loss to know which box to enter.
+Occasionally a record of six, seven, or even eight right choices would be
+made in a series, but in no case was this unquestionably due to color
+discrimination; usually it could be explained in the light of the order of
+the changes in the positions of the cardboards. For example, series 9, in
+which No. 5 made a record of 8 right and 2 wrong, had green on the right
+for the first three tests. The animal happened to choose correctly in the
+first test, and continued to do so three times in succession simply
+because there was no change in the position of the cardboards. I have
+occasionally observed a record of seven right choices result when it was
+perfectly evident to the observer that the mouse could not discriminate
+visually. It was to avoid unsafe conclusions and unfair comparisons, as
+the result of such misleading series, that three perfect series in
+succession were required as evidence of a perfectly formed habit of
+discrimination.
+
+
+TABLE 20
+
+GREEN-LIGHT BLUE TESTS
+
+         Date       No. 1000          No. 4            No. 5
+SERIES   1906   RIGHT   WRONG     RIGHT   WRONG    RIGHT   WRONG
+               (GREEN)  (BLUE)   (GREEN)  (BLUE)  (GREEN)  (BLUE)
+
+1      Feb.3      2       8         3       7        3       7
+
+2          5      7       3         5       5        5       5
+
+3          6      5       5         6       4        5       5
+
+4          7      5       5         5       5        5       5
+
+5          8      2       8         5       5        4       6
+
+6          9                        7       3        7       3
+
+7         10                        4       6        3       7
+
+8         10                        6       4        4       6
+
+9         12                        6       4        8       2
+
+10        13                        6       4        6       4
+
+11        14                        5       5        3       7
+
+12        15                        6       4        7       3
+
+13        16                        5       5        7       3
+
+14        17                        3       7        6       4
+
+15        19                        6       4        6       4
+
+16        20                        7       3        5       5
+
+17        21                        4       6        8       2
+
+18        22                        3       7        4       6
+
+19        23                        6       4        4       6
+
+20        24                        6       4        5       5
+
+
+
+Twenty series, 200 tests for each of the individuals in the experiment,
+yielded no evidence whatever of the dancer's ability to tell green from
+blue. As it has already been proved that they readily learn to choose the
+right box under discriminable conditions, it seems reasonable to conclude
+either that they lack green-blue vision, or that they have it in a
+relatively undeveloped state.
+
+If it be objected that the number of training tests given was too small,
+and that the dancer probably would exhibit discrimination if it were given
+1000 instead of 200 tests in such an experiment, I must reply that the
+behavior of the animal in the tests is even more satisfactory evidence of
+its inability to choose than are the results of Table 20. Had there been
+the least indication of improvement as the result of 200 tests, I should
+have continued the experiment; as a matter of fact, the mice each day
+hesitated more and more before choosing, and fought against being driven
+toward the entrance to the experiment box. That they were helpless was so
+evident that it would have been manifestly cruel to continue the
+experiment.
+
+
+                            TABLE 21
+                       VIOLET-RED TESTS
+              With Odor of All Cardboards the Same
+
+SERIES           DATE            NO. 7            NO. 998
+                            RIGHT    WRONG    RIGHT    WRONG
+                           (VIOLET)  (RED)   (VIOLET)  (RED)
+   A          MAR. 7          8        2        5        5
+   B               7          3        7        2        8
+   1              14          3        7        6        4
+   2              15          4        6        4        6
+   3              16          5        5        5        5
+   4              19          4        6        4        6
+   5              20          5        5        6        4
+   6              21          4        6        8        2
+   7              22          8        2        4        6
+   8              23          4        6        6        4
+   9              24          6        4        4        6
+  10              25          4        6        6        4
+
+
+
+Further color tests with reflected light were made with violet and red.
+Two dancers, Nos. 998 and 7, neither of which had been in any experiment
+previously, were subjected to the ten series of tests whose results are to
+be found in Table 21. In this experiment the cardboards used had been
+coated with shellac to obviate discrimination by means of odor. It is
+therefore impossible to give a precise description of the color or
+brightness by referring to the Bradley papers.[1] Both the violet and the
+red were rendered darker, and apparently less saturated, by the coating.
+
+[Footnote 1: The violet was darker than Bradley's shade No. 2, and the red
+was lighter than Bradley's red.]
+
+These violet-red tests were preceded by two series of preference tests
+(_A_ and _B_), in which no shock was given and escape was possible through
+either electric-box. Although the results of these preference tests as
+they appear in Table 21 seem to indicate a preference for the red on the
+part of No. 998, examination of the record sheets reveals the fact that
+neither animal exhibited color preference, but that instead both chose by
+position. Number 998 chose the box on the right 15 times in 20, and No. 7
+chose the box on the left 15 times in 20.
+
+Ten series of tests with the violet-red cardboards failed to furnish the
+least indication of discrimination. The experiment was discontinued
+because the mice had ceased to try to discriminate and dashed into one or
+the other of the boxes on the chance of guessing correctly. When wrong
+they whirled about, rushed out of the red box and into the violet
+immediately. They had learned perfectly as much as they were able to learn
+of what the experiment required of them. Although we are not justified in
+concluding from this experiment that dancers cannot be taught to
+distinguish violet from red, there certainly is good ground for the
+statement that they do not readily discriminate between these colors.
+
+The experiments on color vision which have been described and the records
+which have been presented will suffice to give the reader an accurate
+knowledge of the nature of the results, only a few of which could be
+printed, and of the methods by which they were obtained.
+
+In brief, these results show that the dancer, under the conditions of the
+experiments, is not able to tell green from blue, or violet from red. The
+evidence of discrimination furnished by the light blue-orange tests is not
+satisfactory because the conditions of the experiment did not permit the
+use of a sufficiently wide range of brightnesses. It is obvious,
+therefore, that a method of experimentation should be devised in which the
+experimenter can more fully control the brightness of the colors which he
+is using. I shall now describe a method in which this was possible.
+
+
+
+
+CHAPTER X
+
+THE SENSE OF SIGHT: COLOR VISION (_Continued_)
+
+There are three well-known ways in which colors may be used as stimuli in
+experiments on animals: by the use of colored papers (reflected light); by
+the use of a prism (the spectrum which is obtained may be used as directly
+transmitted or as reflected light); and by the use of light filters
+(transmitted light). In the experiments on the color vision of the dancer
+which have thus far been described only the first of these three methods
+has been employed. Its advantages are that it enables the experimenter to
+work in a sunlit room, with relatively simple, cheap, and easily
+manipulated apparatus. Its chief disadvantages are that the brightness of
+the light can neither be regulated nor measured with ease and accuracy.
+The use of the second method, which in many respects is the most desirable
+of the three, is impracticable for experiments which require as large an
+illuminated region as do those with the mouse; I was therefore limited to
+the employment of light filters in my further tests of color
+discrimination.
+
+The form of filter which is most conveniently handled is the colored
+glass, but unfortunately few glasses which are monochromatic are
+manufactured. Almost all of our so-called colored glasses transmit the
+light of two or more regions of the spectrum. After making spectroscopic
+examinations of all the colored glasses which were available, I decided
+that only the ruby glass could be satisfactorily used in my experiments.
+With this it was possible to get a pure red. Each of the other colors was
+obtained by means of a filter, which consisted of a glass box filled with
+a chemical solution which transmitted light of a certain wave length.
+
+For the tests with transmitted light the apparatus of Figures 20 and 21
+was constructed. It consisted of a reaction-box essentially the same as
+that used in the brightness vision tests, except that holes were cut in
+the ends of the electric-boxes, at the positions _G and R_ of Figure 20,
+to permit the light to enter the boxes. Beyond the reaction-box was a long
+light-box which was divided lengthwise into two compartments by a
+partition in the middle. A slit in the cover of each of these compartments
+carried an incandescent lamp _L_ (Figure 20). Between the two lamps, _L,
+L_, and directly over the partition in the light-box was fastened a
+millimeter scale, _S_, by means of which the experimenter could determine
+the position of the lights with reference to the reaction-box. The light-
+box was separated from the reaction-box by a space 6 cm. wide in which
+moved a narrow wooden carrier for the filter boxes. This carrier, as shown
+in Figure 20, could be moved readily from side to side through a distance
+of 20 cm. The filter boxes, which are represented in place in Figures 20
+and 21, consisted of three parallel-sided glass boxes 15 cm. long, 5 cm.
+wide, and 15 cm. deep. Each box contained a substance which acted as a ray
+filter. Tightly fitted glass covers prevented the entrance of dust and the
+evaporation of the solutions in the boxes. Figures 20 and 21 represent the
+two end boxes, _R, R_, as red light filters and the middle one, _G_, as a
+green light filter. Three filters were used thus side by side in order
+that the position of a given color with reference to the electric-boxes
+might be changed readily. As the apparatus was arranged, all the
+experimenter had to do when he wished to change from green-left, red-right
+to green-right, red-left was to push the carrier towards the right until
+the green filter covered the hole on the right at the end of the electric-
+box. When this had been accomplished the red filter at the left end of the
+carrier covered the hole on the left at the end of the electric-box. Thus
+quickly, noiselessly, easily, and without introducing any other change in
+conditions than that of the interchange of lights, the experimenter was
+able to shift the positions of his colored lights at will.
+
+[Illustration: FIGURE 20.--Color discrimination apparatus. _A,_ nest-box;
+_B,_ entrance chamber; _R, R,_ red filters; _G,_ green filter; _L, L,_
+incandescent lamps in light-box; _S,_ millimeter scale on light-box; _I,_
+door between _A_ and _B; O, O,_ doors between alleys and _A_.]
+
+[Illustration: FIGURE 21--Ground plan of color discrimination apparatus.
+_E, E_, exits from electric-boxes. _LB_, light-box; _R, G, R_, filter
+boxes on carrier; _L_, left electric-box; _R_, right electric-box; _IC_
+induction apparatus; _C_, electric cell; _K_, key; _S_, millimeter scale.]
+
+In the tests which are now to be reported, three portions of the spectrum
+were used: the red end, the blue-violet end, and a middle region, chiefly
+green. The red light was obtained by the use of a filter which was made by
+placing two plates of ruby glass in one of the glass boxes, filling the
+box with filtered water and then sealing it to prevent evaporation. The
+blue-violet was obtained by the use of a filter box which contained a 5
+per cent solution of copper ammonium sulphate. The green, which, however,
+was not monochromatic, was obtained by the use of a filter box which
+contained a saturated solution of nickel nitrate. These three sets of
+filters were examined spectroscopically both before the experiments had
+been made and after their completion.[1] The red filters, of which I had
+two for shifting the lights, transmitted only red light. The blue-violet
+filters, two also, at first appeared to transmit only portions of the blue
+and violet of the spectrum, but my later examination revealed a trace of
+green. It is important to note, however, that the red and the blue-violet
+filters were mutually exclusive in the portions of the spectrum which they
+transmitted. Of all the filters used the green finally proved the least
+satisfactory. I detected some yellow and blue in addition to green in my
+first examination, and later I discovered a trace of red. Apparently the
+transmitting power of the solutions changed slightly during the course of
+the experiments. On this account certain solutions are undesirable for
+experiments on color vision, for one must be certain of the constancy of
+the condition of stimulation. It is to be understood, of course, that each
+of the three filters transmitted, so far as the eye is concerned, only the
+color named. I consider the red filter perfectly satisfactory, the blue-
+violet very good, and the green poor. Henceforth, in testing color vision
+in animals, I shall make use of colored glasses as filters, if it is in
+any way possible to obtain or have manufactured blue, green, and yellow
+glasses which are as satisfactory as the ruby.
+
+[Footnote 1: A Janssen-Hoffman spectroscope was used.]
+
+The apparatus needs no further description, as its other important
+features were identical with those of the reflected light experiment box.
+The use of artificial light for the illumination of the electric-boxes
+made it necessary to conduct all of the following tests in a dark-room.
+The method of experimentation was practically the same as that already
+described. A mouse which had been placed in _A_ by the experimenter was
+permitted to enter _B_ and thence to return to _A_ by entering one of the
+electric-boxes, the red or blue or green one, as the case might be.
+Mistakes in choice were punished by an electric shock. One further point
+in the method demands description and discussion before the results of the
+tests are considered, namely, the manner of regulating and measuring the
+brightness of the lights.
+
+Regulating brightness with this apparatus was easy enough; measuring it
+accurately was extremely difficult. The experimenter was able to control
+the brightness of each of the two colored lights which he was using by
+changing the position or the power of the incandescent lamps in the light-
+box. The position of a lamp could be changed easily between tests simply
+by moving it along toward or away from the electric-box in the slit which
+served as a lamp carrier. As the distance from the entrances of the
+electric-boxes to the further end of the light-box was 120 cm., a
+considerable range or variation in brightness was possible without change
+of lamps. Ordinarily it was not necessary to change the power of the
+lamps, by replacing one of a given candle power by a higher or lower,
+during a series of tests. Both the candle power of the lamps and their
+distance from the filters were recorded in the case of each test, but for
+the convenience of the reader I have reduced these measurements to candle
+meters[1] and report them thus in the descriptions of the experiments.
+
+[Footnote 1: The illuminating power of a standard candle at a distance of
+one meter.]
+
+But measuring the actual brightness of the red light or the green light
+which was used for a particular series of tests, and the variations in
+their brightnesses, was not so simple a matter as might appear from the
+statements which have just been made. The influence of the light filters
+themselves upon the brightness must be taken into account. The two red
+filters were alike in their influence upon the light which entered them,
+for they were precisely alike in construction, and the same was true of
+the two blue-violet filters. The same kind of ruby glass was placed in
+each of the former, and a portion of the same solution of copper ammonium
+sulphate was put into each of the filter boxes for the latter. But it is
+difficult to say what relation the diminution in brightness caused by a
+red filter bore to that caused by a blue-violet or a green filter. My only
+means of comparison was my eye, and as subjective measurement was
+unsatisfactory for the purposes of the experiment, no attempt was made to
+equalize the amounts of brightness reduction caused by the several
+filters. So far as the value of the tests themselves, as indications of
+the condition of color vision in the dancer is concerned, I have no
+apology for this lack of measurement, but I do regret my inability to give
+that accurate objective statement of brightness values which would enable
+another experimenter with ease and certainty to repeat my tests. The
+nearest approach that it is possible for me to make to such an objective
+measurement is a statement of the composition and thickness of the filters
+and of the candle-meter value of the light when it entered the filter. The
+distance from this point to the entrance to the electric-box was 20 cm.
+
+To sum up and state clearly the method of defining the brightness of the
+light in the following experiments: the candle-meter value of each light
+by which an electric-box was illuminated, as determined by the use of a
+Lummer-Brodhun photometer and measurements of the distance of the source
+of light from the filter, is given in connection with each of the
+experiments. This brightness value less the diminution caused by the
+passage of the light through a filter, which has been defined as to
+composition and thickness of the layer of solution, gives that degree of
+brightness by which the electric-box was illuminated.
+
+Tests of the dancer's ability to discriminate green and blue[1] in the
+transmitted light apparatus were made with four animals. An incandescent
+lamp marked 16-candle-power was set in each of the light-boxes. These
+lamps were then so placed that the green and the blue seemed to be of
+equal brightness to three persons who were asked to compare them
+carefully. Their candle-meter values in the positions selected were
+respectively 18 and 64, as appears from the statement of conditions at the
+top of Table 22.
+
+[Footnote 1: Hereafter the light transmitted by the blue-violet filter
+will be referred to for convenience as blue.]
+
+
+
+                            TABLE 22
+
+                        GREEN-BLUE TESTS
+
+                 Brightnesses Equal for Human Eye
+
+Green 18 candle meters             Blue 64 candle meters
+
+SERIES        DATE          NO. 10          NO. 11
+              1906      RIGHT   WRONG   RIGHT   WRONG
+                       (GREEN)  (BLUE) (GREEN)  (BLUE)
+A and B[1]   April 2     10       10      12      8
+   1               3      6        4       5      5
+   2               4      5        5       6      4
+   3               5      5        5       5      5
+   4               6      5        5       5      5
+   5               7      7        3       5      5
+   6               8      7        3       3      7
+   7               9      7        3       5      5
+   8              10      3        7       7      3
+   9              11      5        5       4      6
+  10              12      5        5       6      4
+[Footnote: A single preference series of twenty tests.]
+
+
+
+Numbers 10 and 11 exhibited no preference for either of these colors in
+the series of 20 tests which preceded the training tests, and neither of
+them gave evidence of ability to discriminate as the result of ten series
+of training tests. In this case, again, the behavior of the animals was as
+strongly against the inference that they can tell green from blue as are
+the records of choices which appear in the table. Granted, that they are
+unable to discriminate green from blue when these colors are of about the
+same brightness for the human eye, what results when they differ markedly
+in brightness? Table 23 furnishes a definite answer to this question.
+Numbers 5 and 12 were given eight series of green-blue tests with each
+light at 18 candle meters. Little, if any, evidence of discrimination
+appeared. Then, on the supposition that the difference was not great
+enough for easy discrimination, the blue light was reduced almost to 0,
+the green being left at 18. The tests (series 9) immediately indicated
+discrimination. For series 10 the green was made 64 candle meters, the
+blue 18, and again there was discrimination. These results were so
+conclusively indicative of the lack of color vision and the presence of
+brightness vision, that there appeared to be no need of continuing the
+experiment further.
+
+Accepting provisionally the conclusion that the dancers cannot tell green
+from blue except by brightness differences, we may proceed to inquire
+whether they can discriminate other colors. Are green and red
+distinguishable?
+
+Green-red discrimination now was tested by a method which it was hoped
+might from the first prevent dependence upon brightness. The light in the
+light-box on the left was so placed that it had a value of 18 candle
+meters, that in the light-box on the right so that it had a value of 1800
+candle meters. Neither light was moved during the first four series of the
+green-red tests which were given to Nos. 151 and 152.
+
+
+
+TABLE 23
+
+GREEN-BLUE TESTS
+
+Brightnesses Different for Human Eye
+
+Green 18 candle meters                Blue 18 candle meters
+
+
+                       No. 5              No. 12
+           DATE
+SERIES     1906     RIGHT   WRONG     RIGHT    WRONG
+                   (GREEN)  (BLUE)   (GREEN)   (BLUE)
+
+   1     April 10     6       4         5        5
+   2           11     5       5         7        3
+   3           12     6       4         7        3
+   4           13     4       6         7        3
+   5           14     7       3         5        5
+   6           15     4       6         6        4
+   7           16     6       4         8        2
+   8           17     5       5         4        6
+
+
+
+As it was now evident that the intensity difference was not sufficient to
+render discrimination easy, the blue was reduced to 0 and the green left
+at 18.
+
+
+   9           17     7       3         8        2
+
+
+Now the brightnesses were made, green 64, blue 18, just the reverse of
+those of series of Table 22.
+
+
+  10           17     8       2         8        2
+
+
+
+Each of these series consisted of 20 tests instead of 10. As a result of
+the arrangement of the lights just mentioned, the green appeared to me
+very much brighter than the red when it was on the right and very much
+darker when it was on the left. If this were true for the mouse also, it
+is difficult to see how it could successfully depend upon brightness for
+guidance in its choices. Such dependence would cause it to choose now the
+green, now the red.
+
+The first four series of green-red tests so clearly demonstrated
+discrimination, of some sort, that it was at once necessary to alter the
+conditions of the experiment. The only criticism of the above method of
+excluding brightness discrimination, of which I could think, was that the
+red at no time had been brighter than the green. In other words, that
+despite a value of 1800 candle meters for the red and only 18 candle
+meters for the green, the latter still appeared the brighter to the mouse.
+To meet this objection, I made the extreme brightness values 1 and 1800
+candle meters in some of the later series, of which the results appear in
+Table 24. From day to day different degrees of brightness were used, as is
+indicated in the second column of the table. Instead of having first one
+color and then the other the brighter, after the fourth series I changed
+the position of the lights each time the position of the filters was
+changed; hence, the table states a certain brightness value for each color
+instead of for each electric-box.
+
+Series 5 to 14 so clearly indicated discrimination, that it seemed
+necessary to devise some other means than that of changing the
+brightnesses of the colored lights themselves to test the assumption that
+the animals were choosing the brighter light. I therefore removed the
+light filters so that the colors which had been present as conditions of
+discrimination were lacking, and arranged the apparatus so that first one
+box, then the other, was illuminated the more brightly. The purpose of
+this was to discover whether as the result of their green-red training the
+mice had acquired the habit of choosing uniformly either the lighter or
+the darker box. One series was given under the conditions of illumination
+specified in Table 24 with the result that the brighter box was chosen
+eight times in ten by No. 151 and every time by No. 152. Since neither of
+these individuals had previously been trained by white-black tests to go
+to the white, and since, furthermore, the dancers usually manifest a
+slight preference for the lower instead of the higher illumination, this
+result may be interpreted as indicative of dependence upon brightness in
+the previous color tests. It looks very much indeed as if the green had
+been chosen, not because of its greenness, but on account of its
+relatively greater brightness.
+
+This test of brightness preference was followed by two series, 16 and 17,
+under conditions similar to those of the first four series of the table.
+For series 16 the value of the light in the left box was 1 candle meter,
+that of the light in the right box 1800 candle meters. Discrimination was
+perfect. For series 17 the value for the left remained at 1 candle meter,
+but that of the right box was decreased to 0. In this series No. 152 was
+entirely at a loss to know which box to choose. Of course this was an
+entirely new set of conditions for choice, namely, a colored box, the
+green or the red as the case might be, beside a dark box, the one which
+was not illuminated. If the mice really had been choosing correctly
+because of a habit of avoiding the red or of seeking the green, this
+method should bring out the fact, for the red box, since with it the
+disagreeable electric shock had always been associated, should be a box to
+be avoided. For No. 151 this seemed to be the case.
+
+Series 23 to 27 of Table 24 were given as final and crucial tests of the
+relation of brightness discrimination to color discrimination. As it is
+not possible to express in a simple formula the conditions of the tests, a
+sample series which indicates the brightness of the colors in each of the
+twenty tests of a series, and in addition the results given by No. 151 in
+the first of these final series, is reproduced in Table 25. For an animal
+which had presumably learned perfectly to choose green in preference to
+red, the record of 8 mistakes in 20 choices as a result of changes in
+relative brightness is rather bad, and it renders doubtful the existence
+of color discrimination in any of these experiments. No. 152 showed no
+ability whatever to choose the green in the first of the series (series 23
+of Table 24) of which that of Table 25 is a sample. His record, 10
+mistakes in 20 choices, was even poorer than that of No. 151. That both of
+these mice learned to choose fairly accurately in these final tests is
+shown by the results of series 24, 25, 26, and 27. I must admit, however,
+that these records indicate little ability on the part of the animals to
+discriminate colors.
+
+
+
+TABLE 24
+
+GREEN-RED TESTS
+
+Brightnesses Extremely Different for Human Eye
+Intensities are given in candle meters (c.m.)
+
+
+                                           NO. 151         NO. 152
+SERIES     DATE        CONDITIONS
+                                        RIGHT   WRONG   RIGHT   WRONG
+                                       (GREEN)  (RED)  (GREEN)  (RED)
+
+    1    April 26  18 c.m. on left
+                   1800 c.m. on right     11       9       7      13
+    2          27  Same                   16       4      16       4
+    3          28  Same                   20       0      17       3
+    4          29  Same                   19       1      19       1
+    5          30  Green 18 c.m.
+                   Red 18 c.m.             9       1      10       0
+    6          30  Green 64 c.m.
+                   Red 18 c.m.             9       1       8       2
+    7    May    1  Green 6 c.m.
+                   Red 1500 c.m.           7       3       9       1
+    8           1  Green 4 c.m.
+                   Red 1500 c.m.           8       2       7       3
+    9           2  Both varied from
+                     4 to 1500 c.m.        18      2      18       2
+   10           3  Green 2 c.m.
+                   Red 1800 c.m.            6      4       7       3
+   11           3  Same                    10      0      10       0
+   12           4  Same                     7      3       8       2
+   13           4  Same                     8      2       6       4
+   14           5  Green 1 c.m.
+                   Red 1800 c.m.           19      1      19       1
+
+
+
+Filters were now removed. An illumination of 15 c.m. was established on
+one side and an illumination of 0 on the other side, in order to ascertain
+whether the mice would choose the brighter box. This was done to test the
+assumption that the green in the previous tests had always appeared
+brighter to the mice than did the red, and that in consequence they had
+chosen the brighter box instead of the green box.
+
+
+TABLE 24--CONTINUED
+
+
+                                          No. 151         No. 152
+
+SERIES    DATE        CONDITIONS       RIGHT   WRONG   RIGHT   WRONG
+                                      (GREEN)  (RED)  (GREEN)  (RED)
+
+  15      May 5   Brighter 15 c.m.      8[1]    2[2]   10[1]    0[2]
+                  Darker 0 c.m.
+  16          5   1 c.m. on left
+                  1800 c.m. on right    10       0      10       0
+  17          5   1 c.m. on left
+                  0 c.m. on right        9       1       4       6
+  18          5   Green 18 c.m.
+                  Red 18 c.m.           19       1      17       3
+  19          9   Same                   9       1       9       1
+  20          9   Same                  10       0      10       0
+  21         10   Same                  10       0      10       0
+  22         11   Same                  10       0      10       0
+  23     June 1   Both varied from
+                    1 to 1800 c.m.      12       8      10      10
+  24          2   Same                  18       2      14       6
+  25     June 3   Both varied from
+                    2 to 1800 c.m.      19       1      17       3
+  26          4   Same                  17       3      17       3
+  27          5   Same                  18       2      18       2
+
+[Footnote 1: Brighter.]
+[Footnote 2: Darker.]
+
+
+
+These long-continued and varied tests with Nos. 151 and 152 revealed three
+facts: that the mice depend chiefly upon brightness differences in visual
+discrimination; that they probably have something which corresponds to our
+red-green vision, although their color experience may be totally unlike
+ours; and that the red end of the spectrum seems much darker to them than
+to us, or, in other words, that the least refrangible rays are of lower
+stimulating value for them than for us.
+
+
+
+TABLE 25
+
+GREEN-RED TESTS
+
+June 1, 1906                                             No. 151
+
+                       BRIGHTNESS VALUE IN CANDLE    RIGHT    WRONG
+TEST      POSITION              METERS              (GREEN)   (RED)
+  1    Green on left        Green    4, Red  448      Right     --
+  2    Green on right       Green  448, Red    4      Right     --
+  3    Green on right       Green    4, Red  448      Right     --
+  4    Green on left        Green  448, Red    4      Right     --
+  5    Green on left        Green    3, Red 1800        --    Wrong
+  6    Green on right       Green 1800, Red    3        --    Wrong
+  7    Green on right       Green    3, Red 1800        --    Wrong
+  8    Green on left        Green 1800, Red    3      Right     --
+  9    Green on right       Green    5, Red   34      Right     --
+ 10    Green on left        Green   34, Red    5      Right     --
+ 11    Green on right       Green    6, Red   74      Right     --
+ 12    Green on left        Green   74, Red    6      Right     --
+ 13    Green on left        Green    4, Red  448        --    Wrong
+ 14    Green on right       Green  448, Red    4      Right     --
+ 15    Green on right       Green    4, Red  448        --    Wrong
+ 16    Green on left        Green  448, Red    4      Right     --
+ 17    Green on right       Green    3, Red 1800        --    Wrong
+ 18    Green on left        Green 1800, Red    3        --    Wrong
+ 19    Green on right       Green 1800, Red    3        --    Wrong
+ 20    Green on left        Green    3, Red 1800      Right     --
+
+         Totals                                         12       8
+
+
+
+So many of the results of my color experiments have indicated the all-
+important role of brightness vision that I have hesitated to interpret any
+of them as indicative of true color discrimination. But after I had made
+all the variations in brightness by which it seemed reasonable to suppose
+that the mouse would be influenced under ordinary conditions, and after I
+had introduced all the check tests which seemed worth while, there still
+remained so large a proportion of correct choices that I was forced to
+admit the influence of the quality as well as of the intensity of the
+visual stimulus.
+
+The first of the facts mentioned above, that brightness discrimination is
+more important in the life of the mouse than color discrimination, is
+attested by almost all of the experiments whose results have been
+reported. The second fact, namely, that the dancer possesses something
+which for the present we may call red-green vision, also has been proved
+in a fairly satisfactory manner by both the reflected and the transmitted
+light experiments. I wish now to present, in Table 26, results which
+strikingly prove the truth of the statement that red appears darker to the
+dancer than to us.
+
+The brightness conditions which appeared to make the discrimination
+between green and red most difficult were, so far as my experiments permit
+the measurement thereof, green from 1 to 4 candle meters with red from
+1200 to 1600. Under these conditions the red appeared extremely bright,
+the green very dark, to the human subject.
+
+According to the description of conditions in Table 26, Nos. 2 and 5 were
+required to distinguish green from red with the former about 3 candle
+meters in brightness and the latter about 1800 candle meters. In the
+eighth series of 20 tests, each of these animals made a perfect record. As
+it seemed possible that they had learned to go to the darker of the two
+boxes instead of to the green box, I arranged the following check test.
+The filters were removed, the illumination of one electric-box was made 74
+candle meters, that of the other 3, and the changes of the lighter box
+from left to right were made at irregular intervals. In February, No. 2
+had been trained to go to the black in black-white tests, and at the same
+time No. 5 had been trained to go to the white in white-black tests. The
+results of these brightness check tests, as they appear in the table,
+series 8 _a_, are indeed striking. Number 2 chose the darker box each
+time; No. 5 chose it eight times out of ten. Were it not for the fact that
+memory tests four weeks after his black-white training had proved that No.
+2 had entirely lost the influence of his previous experience (he chose
+white nine times out of ten in the memory series), it might reasonably be
+urged that this individual chose the darker box because of his experience
+in the black-white experiment. And what can be said in explanation of the
+choices of No. 5? I can think of no more reasonable way of accounting for
+this most unexpected result of the brightness tests than the assumption
+that both of these animals had learned to discriminate by brightness
+difference instead of by color.
+
+
+
+TABLE 26
+
+GREEN-RED TESTS
+
+Brightnesses Different for Human Eye
+
+                                      No. 2           No. 5
+
+SERIES    DATE      BRIGHTNESS     RIGHT   WRONG   RIGHT   WRONG
+                      VALUES      (GREEN)  (RED)  (GREEN)  (RED)
+
+   1     May  7   Green
+                  Red 1800 c.m.      10      10      12       8
+   2          8   Same               12       8      11       9
+   3          9   Same               15       5      14       6
+   4         10   Same               18       2      12       8
+   5         11   Same               18       2      14       6
+   6         12   Same               19       1      16       4
+   7         13   Same               19       1      18       2
+   8         14   Same               20       0      20       0
+
+
+Brightness tests without colors were now given to determine whether the
+mice had been choosing the brighter or the darker instead of the green.
+
+
+TABLE 26--CONTINUED
+
+
+                                          NO. 2           NO. 5
+
+ SERIES   DATE   BRIGHTNESS VALUES    RIGHT   WRONG   RIGHT   WRONG
+                                     (GREEN)  (RED)  (GREEN)  (RED)
+
+    8a     14    Brighter 74 c.m.      0[1]   10[2]    2[1]    8[2]
+                 Darker 3 c.m.
+    9      15    3 c.m. on left
+                 1800 c.m. on right      8      12      16       4
+   10      16    4 c.m. on left
+                 36 c.m. on right        5       5       7       3
+   11      16    Green 4 c.m.
+                 Red 36 c.m.             9       1       8       2
+   12      17    11 c.m. on left
+                 1800 c.m. on right      7       3       6       4
+   13      17    Green 11 c.m.
+                 Red 1800 c.m.           9       1       8       2
+   14      18    Mixed values
+                 3 to 1800 c.m.          7       3       8       2
+   15      19    Same                    7       3       7       3
+   16      20    Same                    7       3       7       3
+   17      21    Same                    7       3       9       1
+   18      22    Same                    9       1       8       2
+   19      23    Same                    7       3       9       1
+   20      24    Same                   10       0       8       2
+   21      25    Same                   10       0       9       1
+   22      26    Same                    9       1      10       0
+
+
+[Footnote 1: Brighter]
+[Footnote 2: Darker]
+
+
+
+Immediately after the brightness series, the influence of making first one
+color, then the other, the brighter was studied. Throughout series 9 the
+brightness value of the left box remained 3 candle meters, that of the
+right side 1800 candle meters. Number 2 was so badly confused by this
+change that his mistakes in this series numbered 12; No. 5 made only 4
+incorrect choices. Then series after series was given under widely
+differing conditions of illumination. The expression "mixed values," which
+occurs in Table 26 in connection with series 14 to 22 inclusive, means
+that the brightnesses of the green and the red boxes were changed from
+test to test in much the way indicated by the sample series of Table 25.
+In view of the results of these 22 series, 320 tests for each of two mice,
+it is evident that the dancer is able to discriminate visually by some
+other factor than brightness. What this factor is I am not prepared to
+say. It may be something akin to our color experience, it may be distance
+effect. No other possibilities occur to me.
+
+Table 26 shows that discrimination was relatively easy for Nos. 2 and 5
+with green at 3 candle meters and red at 1800. That their discrimination
+was made on the basis of the greater brightness of the red, instead of on
+the basis of color, is indicated by the results of the brightness check
+series 8a. Increase in the brightness of the green rendered discrimination
+difficult for a time, but it soon improved, and by no changes in the
+relative brightness of the two colors was it possible to prevent correct
+choice.
+
+In addition to giving point to the statement that red appears darker to
+the dancer than to us, the above experiment shows that the animals depend
+upon brightness when they can, and that their ability to discriminate
+color differences is extremely poor, so poor indeed that it is doubtful
+whether their records are better than those of a totally color blind
+person would be under similar conditions. Surely in view of such results
+it is unsafe to claim that the dancer possesses color vision similar to
+ours.
+
+Perfectly trained as they were, by their prolonged green-red tests, to
+choose the green, or what in mouse experience corresponds to our green,
+Nos. 2 and 5 offered an excellent opportunity for further tests of blue-
+green discrimination. For in view of their previous training there should
+be no question of preference for the blue or of a tendency to depend upon
+brightness in the series whose results constitute Table 27.
+
+
+                          TABLE 27
+                       BLUE-GREEN TESTS
+
+                                         NO. 2           NO. 5
+
+SERIES   DATE     BRIGHTNESS VALUES  RIGHT  WRONG    RIGHT   WRONG
+                                     (BLUE) (GREEN)  (BLUE)  (GREEN)
+
+   1    June 1    Blue 74 c.m.
+                  Green 36 c.m.        3      7        3        7
+   2         2    Same                 5      5        4        6
+   3         3    Same                 5      5        6        4
+   4         4    Same                 6      4        3        7
+   5         5    Same                 6      4        5        5
+   6         6    Blue 21 c.m.
+                  Green 21 c.m.        6      4        7        3
+   7         7    Same                 2      8        3        7
+   8         8    Same                 5      5        4        6
+   9         9    Same                 3      7        6        4
+  10        10    Same                 2      8        4        6
+  11        12    Same                 6      4        3        7
+  12        13    Blue 36 c.m.
+                  Green 21 c.m.        3      7        4        6
+  13        14    Same                 5      5
+  14        15    Blue 62 c.m.
+                  Green 21 c.m.        4      6
+  15        16    Same                 5      5
+  16        17    Same                 5      5
+  17        18    Same                 6      4
+
+
+
+
+Now, as a final test, blue and green glasses were placed over the
+electric-boxes, the brightness of the two was equalized for the human eye,
+and the tests of series 18 and 19 were given to No. 2:--
+
+
+TABLE 27--CONTINUED
+
+                                        NO. 2
+SERIES   DATE   BRIGHTNESS VALUES
+                                     RIGHT  WRONG
+                                    (Blue) (Green)
+
+  18      18    Blue 62 c.m.
+                Green 21 c.m           4      6
+  19      19    Same                   6      4
+  20      20    Blue 21 c.m.
+                Green 88 c.m.          2      8
+
+
+The green was now made much the brighter.
+
+
+  21      21    Blue 21 c.m.
+                Green 18  c.m.         7      3
+  22      23    Same                   8      2
+
+
+To begin with, the blue and the green were made quite bright for the human
+subject, blue 74 candle meters, green 36. Later the brightness of both was
+first decreased, then increased, in order to ascertain whether
+discrimination was conditioned by the absolute strength of illumination.
+No evidence of discrimination was obtained with any of the several
+conditions of illumination in seventeen series of ten tests each.
+
+On the supposition that the animals were blinded by the brightness of the
+light which had been used in some of the tests, similar tests were made
+with weaker light. The results were the same. I am therefore convinced
+that the animals did justice to their visual ability in these experiments.
+
+Finally, it seemed possible that looking directly at the source of light
+might be an unfavorable condition for color discrimination, and that a
+chamber flooded with colored light from above and from one end would prove
+more satisfactory. To test this conjecture two thicknesses of blue glass
+were placed over one electric-box, two plates of green glass over the
+other; the incandescent lamps were then fixed in such positions that the
+blue and the green within the two boxes appeared to the experimenter, as
+he viewed them from the position at which the mouse made its choice, of
+the same brightness.
+
+Mouse No. 2 was given two series of tests, series 18 and 19, under these
+conditions, with the result that he showed absolutely no ability to tell
+the blue box from the green box. The opportunity was now taken to
+determine how quickly No. 2 would avail himself of any possibility of
+discriminating by means of brightness. With the blue at 21 candle meters,
+the green was increased to about 1800. Immediately discrimination
+appeared, and in the second series (22 of Table 27) there were only two
+mistakes.
+
+The results of the blue-green experiments with light transmitted from in
+front of the animal and from above it are in entire agreement with those
+of the experiments in which reflected light was used. Since the range of
+intensities of illumination was sufficiently great to exclude the
+possibility of blinding and of under illumination, it is necessary to
+conclude that the dancer does not possess blue-green vision.
+
+Again I must call attention to the fact that the behavior of the mice in
+these experiments is even more significant of their lack of discriminating
+ability than are the numerical results of the tables. After almost every
+series of tests, whether or not it came out numerically in favor of
+discrimination, I was forced to add the comment, "No satisfactory evidence
+of discrimination."
+
+We have now examined the results of green-red, green-blue, and blue-green
+tests. One other important combination of the colors which were used in
+these experiments is possible, namely, blue-red. This is the most
+important of all the combinations in view of the results already
+described, for these colors represent the extremes of the visible
+spectrum, and might therefore be discriminable, even though those which
+are nearer together in the spectral series were not.
+
+
+TABLE 28
+BLUE-RED TESTS
+
+
+
+                                      No. 2         No. 205
+SERIES DATE    BRIGHTNESS VALUES
+                                     RIGHT   WRONG  RIGHT  WRONG
+                                     (BLUE)  (RED)  (BLUE) (RED)
+
+1      July 31  1800 c.m. on left
+                  24 c.m. on right      5      5      6      4
+2      Aug. 1   21 c.m. on left
+                  1800 c.m. on right    6      4      6      4
+3           2   1800 c.m. on left
+                  21 c.m. on right      8      2      6      4
+4           3   19 c.m. on left
+                  1800 c.m. on right    9      1      6      4
+5           4   1800 c.m. on left
+                  7 c.m. on right       7      3      5      5
+6           5   6 c.m. on left
+                  1800 c.m. on right   10      0      7      3
+7           6   18 c.m. on left
+                  74 c.m. on right     10      0      9      1
+8           7   1800 c.m. on left
+                  7 c.m. on right       8      2      8      2
+9           8   7 c.m. on left
+                  1800 c.m. on right    7      3      8      2
+10          9   Mixed values
+                  6 to 1800 c.m.        8      2      9      1
+11         10   Blue 3 c.m.
+                  Red 1800 c.m.         7      3      6      4
+
+
+Brightness tests were now made, without the use of colors.
+
+11a        10                           4      6      5      5
+
+12         10   Blue 3 c.m.
+                  Red  8 c.m.           4      6      6      4
+13         11   Blue 3 c.m.
+                  Red  7200 c.m.        8      2      5      5
+14         13   Mixed values
+                  3 to 7200 c.m.        7      3      7      3
+15         13   Same                    7      3      9      1
+16         14   Blue 3 to 6 c.m.
+                  Red 112 to 3650 c.m. 10      0     10      0
+
+
+Series were now given to test the assumption that red appears dark to the
+dancer.
+
+
+
+17         14   Darkness on one side
+                  Red 3 c.m.            5      5      7     3
+18         14   Blue 3 to 3650 c.m.
+                  Red 3 to 3650 c.m.   10      0     10     0
+19         15   Darkness on one side
+                  Red 3 c.m.            5      5      4     6
+20         15   Blue 3 to 3650 c.m.
+                  Red 3 to 3650 c.m.   10      0      9     1
+21         16   Darkess on one side
+                  Red 72 c.m.           5      5      7     3
+22         16   Darkness on one side
+                  Red 1800 c.m.         6      4     10     0
+
+
+As is shown by the results in Table 28, no combination of brightnesses
+rendered correct choice impossible in the case of the blue-red tests which
+are now to be described. Choice was extremely difficult at times, even
+more so perhaps than the table would lead one to suppose, and it is quite
+possible that color played no part in the discrimination. But that
+brightness difference in the colors was not responsible for whatever
+success these mice attained in selecting the right box is proved by the
+brightness-without-color series which follows series II of the table.
+Neither No. 2 nor No. 205 showed preference for the lighter or the darker
+box. At the end of the sixteenth blue-red series, I was convinced that one
+of two conclusions must be drawn from the experiment: either the dancers
+possess a kind of blue-red vision, or red is of such a value for them that
+no brightness of visible green or blue precisely matches it.
+
+The latter possibility was further tested by an experiment whose results
+appear in series 17 to 22 inclusive, of Table 28. The conditions of series
+17 were a brightness value of 0 in one box (darkness) and in the other red
+of a brightness of 3 candle meters. Despite the fact that they had been
+perfectly trained in _blue-red tests_ to avoid the red, neither of the
+mice seemed able to discriminate the red from the darkness and to avoid
+it. This was followed by a series in which the brightness of both the blue
+and the red was varied between 3 and 3650 candle meters, with the striking
+result that neither mouse made any mistakes. In series 19 red was used
+with darkness as in series 17, and again there was a total lack of
+discrimination. Series 20 was a repetition of series 18, with practically
+the same result. I then attempted to find out, by increasing the
+brightness of the red, how great must be its value in order that the
+dancers should distinguish it readily from darkness. For the tests of
+series 21 it was made 72 candle meters, but discrimination did not clearly
+appear. At 1800 candle meters, as is shown in series 22, the red was
+sufficiently different in appearance from total darkness to enable No. 205
+to discriminate perfectly between the two electric-boxes. For No. 2
+discrimination was more difficult, but there was no doubt about his
+ability. It would appear from these tests that the dancers had not learned
+to avoid red. Therefore we are still confronted with the question, can
+they see colors?
+
+
+
+TABLE 29
+
+VISUAL CHECK TESTS
+With the Electric-boxes Precisely Alike Visually
+
+
+                     No. 151        No. 152
+SERIES   DATE
+                   RIGHT  WRONG   RIGHT  WRONG
+
+1        Sept. 29    6      4       4      6
+2              30    5      5       6      6
+3          Oct. 1    3      7       4      6
+4               2    5      5       3      7
+5               3    3      7       5      5
+6               4    6      4       5      5
+7               5    5      5       5      5
+8               6   --     --       3      7
+9               7   --     --       6      4
+10              8   --     --       4      6
+
+Averages            4.7     5.3    4.5    5.5
+
+
+
+The account of my color vision experiments is finished. If it be objected
+that other than visual conditions may account for whatever measure of
+discriminating ability, apart from brightness discrimination, appears in
+some of the series, the results of the series of Table 29, in which all
+conceivable visual means of discrimination were purposely excluded, and
+those of the several check tests which have been described from time to
+time in the foregoing account, should furnish a satisfactory and definite
+answer. I am satisfied that whatever discrimination occurred was due to
+vision; whether we are justified in calling it color vision is quite
+another question.
+
+I conclude from my experimental study of vision that although the dancer
+does not possess a color sense like ours, it probably discriminates the
+colors of the red end of the spectrum from those of other regions by
+difference in the stimulating value of light of different wave lengths,
+that such specific stimulating value is radically different in nature from
+the value of different wave lengths for the human eye, and that the red of
+the spectrum has a very low stimulating value for the dancer. In the light
+of these experiments we may safely conclude that many, if not most, of the
+tests of color vision in animals which have been made heretofore by other
+investigators have failed to touch the real problem because the
+possibility of brightness discrimination was not excluded.
+
+Under the direction of Professor G. H. Parker, Doctor Karl Waugh has
+examined the structure of the retina of the dancing mouse for me, with the
+result that only a single type of retinal element was discovered.
+Apparently the animals possess rod-like cells, but nothing closely similar
+to the cones of the typical mammalian retina. This is of peculiar interest
+and importance in connection with the results which I have reported in the
+foregoing pages, because the rods are supposed to have to do with
+brightness or luminosity vision and the cones with color vision. In fact,
+it is usually supposed that the absence of cones in the mammalian retina
+indicates the lack of color vision. That this inference of functional
+facts from structural conditions is correct I am by no means certain, but
+at any rate all of the experiments which I have made to determine the
+visual ability of the dancer go to show that color vision, if it exists at
+all, is extremely poor. It is gratifying indeed to learn, after such a
+study of behavior as has just been described, that the structural
+conditions, so far as we are able to judge at present, justify the
+conclusions which have been drawn.
+
+
+
+
+CHAPTER XI
+
+THE ROLE OF SIGHT IN THE DAILY LIFE OF THE DANCER
+
+Darting hither and thither in its cage, whirling rapidly, now to the left,
+now to the right, running in circles, passing through holes in the nest
+box quickly and neatly, the dancer, it would seem, must have excellent
+sight. But careful observation of its behavior modifies this inference.
+For it appears that a pair of mice dancing together, or near one another,
+sometimes collide, and that it is only those holes with which the animal
+is familiar that are entered skillfully. In fact, the longer one observes
+the behavior of the dancer under natural conditions, the more he comes to
+believe in the importance of touch, and motor tendencies. Sight, which at
+first appears to be the chief guiding sense, comes to take a secondary
+place. In this chapter it is my purpose to show by means of simple
+experiments what part sight plays in the dancer's life of habit formation.
+
+The evidence on this subject has been obtained from four sources: (1)
+observation of the behavior of dancers in their cages; (2) observation of
+their behavior when blinded; (3) observation of their behavior in a great
+variety of discrimination experiments, many of which have already been
+described; and (4) observation of their behavior in labyrinth experiments
+which were especially planned to exhibit the importance of the several
+kinds of vision which the dancer might be supposed to possess. The
+evidence from the first three of these sources may be presented summarily,
+for much of it has already appeared in earlier chapters. That from the
+fourth source will constitute the bulk of the material of this chapter.
+
+My observation of the behavior of the mice has furnished conclusive
+evidence of their ability to see moving objects. But that they do not see
+very distinctly, and that they do not have accurate perception of the form
+of objects, are conclusions which are supported by observations that I
+have made under both natural and experimental conditions. In Chapters VII,
+VIII, IX, and X, I have presented an abundance of evidence of brightness
+vision and, in addition, indications of a specific sensitiveness to wave
+length which may be said to correspond to our color vision. It is
+noteworthy, however, that all of the experimental proofs of visual ability
+were obtained as the result of long periods of training. Seldom, indeed,
+in my experience with them, have the dancers under natural conditions
+exhibited forms of activity which were unquestionably guided by vision.
+
+It is claimed by those who have experimented with blinded dancers that the
+loss of sight decreases the amount and rapidity of movement, and the
+ability of the animals to avoid obstacles.
+
+By means of the discrimination method previously used in the preliminary
+experiments on color vision, a full description of which may be found in
+Chapter IX, p. 133, the dancers' ability to perceive form was tested.
+Immediately after the two males _A_ and _B_ had been given the "food-box"
+tests, whose results appear in Table 15, they were tested in the same
+apparatus and by the same method for their ability to discriminate a
+rectangular food-box from a round one. In the case of the color
+discrimination tests, it will be remembered that the circular tin boxes 5
+cm. in diameter by 1.5 cm. in depth, one of which was covered with blue
+paper, the other with orange, were used. For the form discrimination tests
+I used instead one of the circular boxes of the dimensions given above and
+a rectangular box 8.5 cm. long, 5.5 cm. wide and 2.5 cm. deep. "Force" was
+placed in the circular box. The tests were given, in series of 20, daily.
+
+
+
+TABLE 30
+
+VISUAL FORM TESTS
+
+SERIES    DATE            MOUSE A                MOUSE B
+                      RIGHT     WRONG        RIGHT     WRONG
+                   (CIRCULAR  (RECTANGU-  (CIRCULAR  (RECTANGU-
+                      BOX)     LAR BOX)      BOX)     LAR BOX)
+   1     Jan. 5        10         10           9         11
+   2          7        12          8          13          7
+   3         10         6         14          10         10
+   4         11         7         13          10         10
+   5         12         9         11          10         10
+   6         13        11          9          11          9
+   7         14        13          7           9         11
+   8         15        10         10          11          9
+   9         16        10         10          11          9
+  10         17        11          9           9         11
+  11         18        11          9          12          8
+  12         19        12          8          10         10
+  13         20        10         10          12          8
+  14         21        10         10           8         12
+  15         22        10         10          10         10
+
+     Totals           152        148         155        145
+
+
+
+The results of 15 series of these tests, as may be seen by the examination
+of Table 30, are about as definitely negative, so far as form
+discrimination is in question, as they possibly could be. From the first
+series to the last there is not one which justifies the inference that
+either of the dancers depended upon the form of the boxes in making its
+choice. In view of the general criticisms I have made concerning the use
+of hunger as a motive in experiments on animal behavior, and in view of
+the particular criticisms of this very method of testing the
+discriminating powers of the mouse, it may seem strange that space should
+be given to a report of these tests. I sympathize with the feeling, if any
+one has it, but, at the same time, I wish to call attention to the fact
+that almost any mammal which is capable of profiting by experience, and
+which, under the same conditions, could distinguish the rectangular box
+from the circular one, would have chosen the right box with increasing
+accuracy as the result of such experience. The results are important in my
+opinion, not because they either prove or disprove the ability of the
+dancer to discriminate these particular forms, the discrimination of which
+might fairly be expected of any animal with an image-forming eye, but
+because they demonstrate an important characteristic of the dancing mouse,
+namely, its indifference to the straightforward or direct way of doing
+things.
+
+Most mammals which have been experimentally studied have proved their
+eagerness and ability to learn the shortest, quickest, and simplest route
+to food without the additional spur of punishment for wandering. With the
+dancer it is different. It is content to be moving; whether the movement
+carries it directly towards the food is of secondary importance. On its
+way to the food-box, no matter whether the box be slightly or strikingly
+different from its companion box, the dancer may go by way of the wrong
+box, may take a few turns, cut some figure-eights, or even spin like a top
+for seconds almost within vibrissa-reach of the food-box, and all this
+even though it be very hungry. Activity is pre-eminently important in the
+dancer's life.
+
+In passing I may emphasize the importance of the fact that at no time did
+the brightness or color discrimination tests furnish evidence of attempts
+on the part of the dancers to choose by means of slight differences in the
+form of the cardboards or the cardboard carriers. Several times form
+differences, which were easily perceivable by the human subject, were
+introduced in order to discover whether the mice would detect them and
+learn to discriminate thereby instead of by the visual conditions of
+brightness or color. As these experiments failed to furnish evidences of
+form discrimination, the following special test in the discrimination box
+was devised.
+
+[Illustration: FIGURE 22.--Cards used for tests of form discrimination.]
+
+The color discrimination box of Chapter X was arranged so that the light
+at the entrance to each electric-box had a value of 20 candle meters, less
+the diminution caused by a piece of ground glass which was placed over the
+end of the electric-boxes to diffuse the light. The windows through which
+the light entered the electric-boxes were covered with pieces of black
+cardboard; in one of these cardboards I had cut a circular opening 4 cm.
+in diameter, and in the other an opening of the same area but markedly
+different shape. These openings are shown in Figure 22. As the mouse
+approached the entrance to the electric-boxes, it was confronted by these
+two equally illuminated areas, whose chief difference was one of form.
+Difference in the amount of light within the boxes was excluded so far as
+possible. The question which I asked was, can the dancer discriminate by
+means of this difference in visual form?
+
+For the purpose of settling this point and of gaining additional knowledge
+of the role of vision, two individuals were tested in the discrimination
+box under the conditions which have just been described. During the first
+ten days of the experiment each of these mice, Nos. 420 and 425, was given
+a series of ten tests daily. At the end of this period experimentation
+with No. 425 had to be discontinued, and the number of daily tests given
+to No. 420 was increased to twenty.
+
+Instead of taking space for the presentation of the daily records, I may
+state the general results of the tests. Neither of the mice learned to
+choose the right box by means of form discrimination. In fact, there was
+absolutely no sign of discrimination at any time during the tests. This
+result is as surprising as it is interesting. I could not at first believe
+that the mice were unable to perceive the difference in the lighted areas,
+but assumed that they were prevented from getting the outlines of the
+areas by the blinding effect of the light. However, decreasing the
+intensity of the illumination did not alter the result. According to the
+indications of this experiment, the dancer's ability to perceive visual
+form is extremely poor.
+
+Thus far the purpose of our experiments has been to ascertain what the
+dancer is enabled to do by sight. Suppose we now approach the problem of
+the role of this sense by trying to find out what it can do without sight.
+
+[Illustration: FIGURE 23.--Labyrinth B. _I_, entrance; _O_, exit; 1, 2, 3,
+doorways between alleys.]
+
+For the investigation of this matter the labyrinth method seemed eminently
+suitable. The first form of labyrinth which was used in these visual tests
+appears in ground plan in Figure 23. It was made of 1-1/2 cm. boards. The
+length was 52 cm., the width 17 cm., the depth 10 cm. Each of the
+doorways, _I_ (the entrance), 1, 2, 3, and _O_ (the exit), was 5 by 5 cm.
+The alleys were 2-1/2 cm. wide. For this width the necessity is obvious
+from what has already been said of the animal's propensity to whirl on all
+occasions. As the mice almost never tried to climb up the walls, no cover
+for the labyrinth was needed. The direct route is indicated by the symbols
+_I_-1-2-3-_O_. If an error be defined as a choice of the wrong path as the
+animal progressed toward the exit, five mistakes were possible in the
+forward course: the first by turning to the left at the entrance; the
+second by failing to pass through doorway 1; the third by turning to the
+right after passing through doorway 1; the fourth by failing to pass
+through doorway 3, and the fifth by turning to the left after passing
+through 3. In case the mouse retraced its course, any mistakes made as it
+again progressed towards _O_ were counted, as at first, no matter how many
+times it went over the same ground. Thus an individual might make the same
+mistake several times in the course of a single test in the labyrinth.
+
+With this labyrinth Nos. 7, 998, 15, 16, 151, and 152 were tested. At
+first a record was kept of the time which elapsed from the instant the
+animal entered _I_ to the instant it emerged at _O_, of the path which it
+followed, and of the number of errors which it made; but later only the
+number of errors was recorded.
+
+
+TABLE 31
+
+THE ROLE OF SIGHT
+
+Labyrinth-B Experiments
+
+
+                   NO. 7            NO. 998
+
+TEST  DATE      TIME    ERRORS   TIME   ERRORS
+  1   June 16    66"      8      127"     19
+  2        16    11       0       94      12
+  3        16    15       2       18       3
+  4        16     7       0       13       2
+  5        16     5       0       10       1
+  6        18    61      15       12       3
+  7        18    13       3       14       4
+  8        18    14       5        8       1
+  9        18    24       9       16       2
+ 10        18    10       1        9       1
+ 11        19    36      13       80      17
+ 12        19     8       3       10       1
+ 13        19     6       1        7       1
+ 14        19     9       1        8       0
+ 15        19    12       2        7       0
+ 16        20    14       1       25       0
+ 17        20    28       3
+ 18        20   No efforts       No efforts
+                 to escape        to escape
+
+
+
+
+
+                    TABLE 32
+
+              LABYRINTH-B EXPERIMENTS
+
+                      with
+
+Electric Shock given as Punishment for Mistakes
+
+                    No. 7                No. 998
+TEST   DATE   CONDITION   ERRORS   CONDITION   ERRORS
+
+
+1    June 29   Light         4      Light         9
+2         29   Light         1      Light         3
+3         29   Light         1      Light         2
+4         29   Light         0      Light         0
+5         29   Light         0      Light         0
+6         29   Light         0      Light         0
+7         29   Light         1      Light         0
+8         29   Light         0      Light         0
+9         29   Light         1      Darkness      0
+10        29   Light         1      Light         0
+11        29   Light         1      Darkness      0
+12        29   Light         0      Light         0
+13        29   Light         0      Light         0
+14        29   Light         0      Light         0
+15        29   Light         0      Light         0
+16        29   Light         0      Light         0
+17        29   Darkness      2      Darkness      0
+18        29   Light         2      Light         0
+                                      with paper
+19        29   Light         0      Light         0
+20        29   Darkness      0      Light         0
+                                      with paper
+21        29   Light         0      Light         0
+22        29   Light         0      Darkness      0
+23        29   Light         0      Odorless      0
+24   June 29   Light         0      Darkness      0
+25        29   Light         0
+26        29   Darkness      4
+27        29   Light with
+                 paper       1
+28        29   Light         0
+29        29   Light with
+                 paper       1
+30        29   Darkness      0
+31        29   Odorless      2
+32        29   Darkness      4
+
+
+
+
+As the results in Table 31 show, the time and number of errors rapidly
+diminished. Number 7, for example, made no errors in the second test. The
+chiefly significant fact which appeared in these preliminary experiments,
+however, was that the mice soon ceased to care whether they got out of the
+labyrinth or not. After they knew the path perfectly, they would enter the
+wrong passages repeatedly and wander about indefinitely. It was obvious,
+therefore, that the labyrinth could not be used to reveal the role of
+sight unless some sufficiently strong motive for continuous effort to
+escape from it could be discovered. Naturally I looked to the electric
+shock for aid.
+
+The labyrinth of Figure 23, which for convenience in distinguishing it
+from several other forms to be described later I have designated as
+labyrinth B, was placed upon a board 90 cm. long and 30 cm. wide about
+which had been wound two pieces of phosphor bronze wire after the manner
+described on p. 94. At _O_, Figure 24, there was an opening closed by a
+swinging door which led into a box 40 by 24 cm. In one corner of this box
+was a small nest-box. The significance of this rearrangement of the
+labyrinth is apparent. As in the preliminary tests, the dancer was started
+at I, but instead of being allowed to wander about without any other
+result than delay in escape, it was given a shock each time it made an
+error. The satisfaction of escaping from the narrow bounds of the
+labyrinth's passages, which alone was not strong enough to impel a dancer
+constantly to do its best to escape, was thus supplemented by the powerful
+and all-controlling tendency to avoid the disagreeable stimulus which
+resulted from entering certain of the passages. The result of this
+modification of method is strikingly exhibited by the data of Table 32.
+
+[Illustration: Figure 24.--Labyrinth B on an interrupted circuit board.
+_I_-1-2-3-_O_, labyrinth path; _B_, nest-box; _N_, nest; _EW_, board wound
+with phosphor bronze wire; _IC_, induction apparatus; _C_ electric cell;
+_K_, key.]
+
+This table was constructed for the purpose of exhibiting the principal
+features of the results obtained with labyrinth _B_ in certain preliminary
+experiments in which the conditions were changed in various ways. Chief
+among the important facts which appear in the illustrative data (for Nos.
+7 and 998) which are presented, are the following. The dancers readily
+learn the path of labyrinth B so that they can follow it quickly and with
+perfect accuracy. After familiarity with the direct path from entrance to
+exit has been gained, they become indifferent about escaping and tend to
+wander aimlessly. The introduction of the electric shock as punishment for
+the choice of the wrong passage impels them to do their best to avoid
+errors. The path once learned can be followed in total darkness with few
+or no errors. Table 32 indicates marked differences in the behavior of No.
+7 and No. 998. The latter learned the path readily and was little
+disturbed by any of the changes in conditions. In total darkness he
+followed the path rapidly and accurately, as was indicated by the time of
+the trip and the path that he left on a sheet of smoked paper that had
+been placed on the floor of the labyrinth as a means of obtaining a record
+of the errors made. The presence of the smoked paper did not seem to
+interfere at all with his behavior, nor did the thorough washing of the
+labyrinth and the resultant removal of its odors. In the case of No. 7 the
+opposite was true. She did not learn the path readily, was confused by any
+change in conditions, had great difficulty in finding her way in darkness,
+made errors when the smoked paper was placed on the floor and after the
+odors of the labyrinth had been removed by washing. Of the six dancers
+which were observed in these preliminary tests, No. 7 alone gave
+convincing evidence of the importance of sight.
+
+I think we may say in the light of the results of the table that such
+errors as appear in the darkness tests are due rather to the disturbing
+influence of a change in the conditions of the experiment than to the
+exclusion of visual data, for as many or more errors were sometimes caused
+simply by changing the position of the labyrinth, placing smoked paper on
+the floor, or by introducing a new odor at some point. The exclusion of
+the possibility of guidance by smell and touch did not seriously interfere
+with the animal's ability to follow the path.
+
+The results which have just been considered seemed to be of sufficient
+interest and importance to justify the further use of the labyrinth method
+in the investigation of the role of vision. A series of experiments with
+labyrinth B was therefore planned so that the importance of sight, touch,
+and smell in connection with this form of habit should be more
+satisfactorily exhibited. Does the dancer follow the path by sight, touch,
+smell, by all, or by no one of them?
+
+This series of tests with labyrinth B, whose several purposes may best be
+explained in connection with the various kinds of tests enumerated below,
+consisted of:
+
+I. A preliminary test in which the dancer was permitted to wander about in
+the labyrinth, without being shocked, until it finally escaped to the
+nest-box by way of the exit. Thus the animal was given an opportunity to
+discover that escape from the maze was possible.
+
+II. This was immediately followed by a series of tests at the rate of
+about one per minute, with an electric shock as punishment for every
+mistake. This was continued without interruption until the path had been
+followed without error five times in succession.
+
+III. The labyrinth was now moved about 3 cm. to one side so that it
+covered a new floor area, and a test was given for the purpose of
+ascertaining whether the mouse had been following a trail on the floor.
+
+IV. Tests with smoked paper on the floor were now alternated with tests in
+which the floor was plain. The alternation was rendered necessary by the
+fact that the paper was laid over the electric wires and therefore
+prevented the punishment of mistakes. The purpose of these tests was to
+discover whether the smoked paper, which was an essential condition for
+the next test, was itself a disturbing condition. These tests were
+continued until the animal had followed the path correctly, despite the
+smoked paper, twice in succession.
+
+V. The electric lights were now turned out and tests were given in total
+darkness, with smoked paper on the floor as a means of obtaining a record
+of the number of errors. These tests were continued until the path had
+been followed once correctly.
+
+VI. The labyrinth was now thoroughly washed with warm water, to which a
+little kerosene had been added, and quickly dried over a steam radiator.
+This usually necessitated a delay of about five minutes. As soon as the
+labyrinth was dry, tests were given to discover whether the odors of the
+various passages had been serving as important guiding conditions. These
+tests were continued until the path had been followed once without error.
+
+VII. A final test in darkness completed the series.
+
+As it was not possible for the observer to watch the animal and thus to
+count the number of mistakes which it made in total darkness, the simple
+method of placing a piece of smoked paper on the floor of the labyrinth
+was used. The mouse left a graphic record of its path on the paper and
+from this the number of errors could be ascertained. In the tests now to
+be described the smoked paper was placed upon the electric wires, but
+later a form of electric labyrinth was devised in which it was underneath
+and therefore did not interfere with the electric shock.
+
+The above series of tests was given under the same external conditions in
+a dark-room to six pairs of dancers. In all cases, two individuals, a male
+and a female, which had been kept in the same cage, were experimented with
+at the same time, _i.e._ one was permitted to rest in the nest-box while
+the other was being put through a test. This was done in order that the
+comparison of the results for males and females should be perfectly fair.
+
+The detailed results of this long series of tests may be presented for
+only two individuals, Nos. 210 and 215, Table 33. In this table lines
+separate the results of the seven different kinds of tests.
+
+
+TABLE 33
+
+THE ROLE OF SIGHT, TOUCH, AND SMELL IN LABYRINTH EXPERIMENTS
+
+
+            No. 210                      No. 215
+
+ TEST     CONDITION       ERRORS      CONDITION      ERRORS
+
+ I.   1     No shock         9      I. No shock         2
+
+II.   2     Shock            5     II. Shock            3
+      3     Shock            4         Shock            1
+      4     Shock            2         Shock            0
+      5     Shock            3         Shock            0
+      6     Shock            0         Shock            0
+      7     Shock            0         Shock            0
+      8     Shock            0         Shock            0
+
+      9     Shock            0    III. Labyrinth        0
+                                         moved
+
+     10     Shock            0     IV. Paper on floor   4
+
+III. 11     Labyrinth        0         No paper (shock) 0
+              moved
+
+IV.  12     Paper on         0                          0
+              floor
+     13     No paper         0         No paper         0
+             (shock)
+     14     Paper            1         Paper            1
+     15     No paper         0         No paper         0
+     16     Paper            7         Paper            4
+     17     No paper         0         No paper         0
+     18     Paper            0         Paper            0
+     19     No paper         0         No paper         0
+     20     Paper            4         Paper            0
+     21     No paper         0         No paper         0
+     22     Paper            2      V. Darkness         0
+     23     No paper         2     VI. Labyrinth        2
+     24     Paper            1           washed         0
+     25     No paper         0    VII. Darkness         2
+     26     Paper            0
+     27     No paper         0
+     28     Paper            0
+     29     No paper         0
+  V. 30     Darkness         0
+ VI. 31     Labyrinth        2
+              washed
+     32                      0
+VII. 33     Darkness         0
+
+
+
+
+The average results for the twelve individuals (six of each sex) which
+were subjected to the tests, I have arranged in Table 34. The Roman
+numerals at the top of the table designate the seven groups of tests, and
+the figures under each, the numerical results of the tests. I may explain
+and comment upon the averages of the several columns of this table in
+turn.
+
+Column I gives the number of errors made in the preliminary test.
+Curiously enough, the males made many more errors than the females.
+
+For the second group of tests (II) two results have been tabulated: the
+number of the first correct test, and the total number of tests before the
+path was followed correctly five times in succession. The first correct
+trip came usually after not more than five or six tests, but five
+successive correct trips demanded on the average at least fourteen
+training tests.
+
+Destruction of the floor path by movement of the labyrinth to one side,
+without changing its relations to the points of the compass, disturbed the
+mice very little. Only four of the twelve individuals made any mistakes as
+a result of the change in the tactual conditions, and the average error as
+it appears in Column III is only .3.
+
+
+TABLE 34
+
+ROLE OF SIGHT, TOUCH, AND SMELL IN LABYRINTH EXPERIMENTS
+
+
+                              II.                            IV.
+                        TRAINING TESTS                     SMOKED
+            I.        NO OF TESTS BEFORE       III.       PAPER ON
+MALES   PRELIMINARY        CORRECT          LABYRINTH      FLOOR
+           TEST.     _____________________    MOVED.     NO OF TIMES
+          ERRORS     FIRST TIME FIVE TIMES    ERRORS     BEFORE COR-
+                                                         RECT TWICE
+
+ 210           9            5          9           0            9
+ 212           2            3          8           1            3
+ 214           6           10         28           0           22
+ 220          25            4          8           0           14
+ 410          11            6         20           0           10
+ 420          14            6         14           1            7
+
+AVERAGES    11.2          5.7       14.5         .3          10.8
+
+FEMALES
+
+ 211          16            6         10           1            5
+ 213           7            5         14           1           21
+ 215           2            3          7           0            6
+ 225          14            6         18           0           14
+ 415           6            6         13           0            3
+ 425          10            7         13           0            8
+
+AVERAGES     9.2          5.5       12.5          .3          9.5
+
+
+
+                       V.
+                    DARKNESS                   VI.
+ MALES                                      LABYRINTH       VII.
+            ERRORS IN      NO. OF TESTS      WASHED.      DARKNESS.
+           FIRST TEST     BEFORE COR'CT      ERRORS        ERRORS
+
+  210           0              1                 2            0
+  212           2              2                 0            0
+  214           0              1                --            0
+  220           2              4                 2            0
+  410           1              3                 2            1
+  420           2              4                 1            4
+
+Averages      1.2            2.5               1.2          0.8
+
+FEMALES
+
+  211           2              2                 0            0
+  213           2              2                --            3
+  215           0              1                 2            2
+  225           3              2                 0            0
+  415           1              3                 2            1
+  425           1              7                 0            0
+
+Averages      1.5            2.8               0.7          1.0
+
+
+
+
+That covering the floor with smoked paper forced the mice to relearn the
+path, in large measure, is evident from the results of Column IV. An
+average of ten tests was necessary to enable the mice to follow the path
+correctly. It is almost certain, however, that the interference with the
+perfectly formed labyrinth habit which this change in the condition of the
+floor caused was not due to the removal of important tactual sense data.
+
+As Column V shows, the number of errors in total darkness is very small.
+Some individuals gave no sign of being disturbed by the absence of visual
+guidance, others at first seemed confused. I have given in the table the
+number of errors in the first darkness test and the number of the first
+test in which no mistakes occurred.
+
+No more disturbance of the dancer's ability to follow the path which it
+had learned resulted from washing the labyrinth thoroughly than from
+darkening the room. Indeed it is clear from Column VI that the path was
+not followed by the use of smell. However, the test in darkness, after the
+odor of the box had been removed, proved conclusively that in most cases
+the mice could follow the path correctly without visual or olfactory
+guidance.
+
+The behavior of 18 individuals as it was observed in labyrinth B makes
+perfectly evident three important facts, (1) In following the path which
+it has learned, the dancer in most instances is not guided to any
+considerable extent by a trail (odor or touch) which has been formed by
+its previous journeys over the route; (2) sight is quite unnecessary for
+the easy and perfect execution of the labyrinth habit, for even those
+individuals which are at first confused by the darkening of the experiment
+room are able after a few tests to follow the path correctly; (3) and,
+finally, smell, which according to current opinion is the chiefly
+important sense of mice and rats, is not needful for the performance of
+this habitual act.
+
+At this point we may very fittingly ask, what sense data are necessary for
+the guidance of the series of acts which constitutes the labyrinth habit?
+I answer, probably none. A habit once formed, the senses have done their
+part; henceforth it is a motor process, whose initiation is conditioned by
+the activity of a receptive organ (at times a sense receptor), but whose
+form is not necessarily dependent upon immediate impressions from eye,
+nose, vibrissae, or even from internal receptors. These are statements of
+my opinion; whether they express the truth, either wholly or in part, only
+further experimentation can decide.
+
+In considering the results of these labyrinth tests it is important that
+we distinguish clearly those which have to do with the conditions of habit
+formation from those which instead have to do with the conditions of habit
+performance. Sense data which are absolutely necessary for the learning of
+a labyrinth path may be of little or no importance for the execution of
+the act of following the path after the learning process has been
+completed. Thus far in connection with the labyrinth tests we have
+discussed only the relations of sight, touch, and smell to what I have
+called habit performance. We may now ask what part these senses play in
+the formation of a labyrinth habit.
+
+A very definite answer to this question is furnished by observation of the
+behavior of the dancers in the tests. Most of them continuously made use
+of their eyes, their noses, and their vibrissae. Some individuals used one
+form of receptive organ almost exclusively. I frequently noticed that
+those individuals which touched and smelled of the labyrinth passages most
+carefully gave least evidence of the use of sight. It is safe to say,
+then, that under ordinary conditions habit formation in the dancer is
+conditioned by the use of sight, touch, and smell, but that these senses
+are of extremely different degrees of importance in different individuals.
+And further, that, although in the case of some individuals the loss of
+sight would not noticeably delay habit formation, in the case of others it
+would seriously interfere with the process. When deprived of one sense,
+the dancer depends upon its remaining channels of communication with
+environment. Indeed there are many reasons for inferring that if deprived
+of sight, touch, and smell it would still be able to learn a labyrinth
+path; and there are reasonable grounds for the belief that a habit once
+formed can be executed in the absence of all special sense data.
+Apparently the various receptive organs of the body furnish the dancer
+with impressions which serve as guides to action and facilitate habit
+formation, although they are not necessary for habit performance.
+
+The reader may wonder why I have not carried out systematic experiments to
+determine accurately and quantitatively the part which each sense plays in
+the formation of a labyrinth habit instead of basing my inferences upon
+incidental observation of the behavior of the dancers. The reason is
+simply this: the number and variety of experiments which were suggested by
+the several directions in which this investigation developed rendered the
+performance of all of them impossible. I have chosen to devote my time to
+other lines of experimentation because a very thorough study of the
+conditions of habit formation has recently been made by Doctor Watson.[1]
+
+[Footnote 1: Watson, J. B., _Psychological Review_, Monograph Supplement,
+Vol. 8, No. 2, 1907.]
+
+What is the role of sight in the dancing mouse? How shall we answer the
+question? The evidence which has been obtained in the course of my study
+of the animal indicates that brightness vision is fairly acute, that color
+vision is poor, that although form is not clearly perceived, movement is
+readily perceived. My observations under natural conditions justify the
+conclusion that sight is not of very great importance in the daily life of
+the dancer, and my observations under experimental conditions strongly
+suggest the further conclusion that movement and changes in brightness are
+the only visual conditions which to any considerable extent control the
+activity of the animal.
+
+
+
+
+CHAPTER XII
+
+EDUCABILITY: METHODS OF LEARNING
+
+Nearly all of the experiments described in earlier chapters have revealed
+facts concerning the educability of the dancer. In order to supplement the
+knowledge of this subject thus incidentally gained and to discover the
+principles of educability, the specially devised experiments whose results
+appear in this and succeeding chapters were arranged and carried out with
+a large number of mice. In the work on the modifiability of behavior I
+have attempted to determine (1) by what methods the dancer is capable of
+profiting by experience, (2) the degree of rapidity of learning, (3) the
+permanency of changes wrought in behavior, (4) the effect of one kind of
+training upon others, (5) the relation of re-training to training, and (6)
+the relation of all these matters to age, sex, and individuality.
+
+As it is obvious that knowledge of these subjects is a necessary condition
+for the intelligent appreciation of the capacities of an animal, as well
+as of the choice of methods by which it may be trained advantageously,
+perhaps it is not too much to expect that this investigation of the nature
+and conditions of educability in the dancing mouse may give us some new
+insight into the significance of certain aspects of human education and
+may serve to suggest ways in which we may measure and increase the
+efficiency of our educational methods.
+
+Merely for the sake of convenience of description I shall classify the
+methods which have been employed as problem methods, labyrinth methods,
+and discrimination methods. That these names are not wholly appropriate is
+suggested by the fact that discrimination necessarily occurs in connection
+with each of them. As problem methods we may designate those tests of
+initiative and modifiability which involve the opening of doors by pushing
+or pulling them, and the climbing of an inclined ladder. An example of the
+labyrinth method has been presented in Chapter XI. The name discrimination
+method I have applied to those tests which involve the choice of one of
+two visual, tactual, or olfactory conditions. The white-black
+discrimination tests, for example, served to reveal the rapidity and
+permanency of learning as well as the presence of brightness vision.
+
+In the case of most mammals whose educability has been studied
+experimentally, problem methods have proved to be excellent tests of
+docility and initiative. The cat, the raccoon, the monkey, in their
+attempts to obtain food, learn to pull strings, turn buttons, press
+latches, slide bolts, pull plugs, step on levers. The dancer does none of
+these things readily. Are we therefore to infer that it is less
+intelligent, that it is less docile, than the cat, the raccoon, or the
+monkey? Not necessarily, for it is possible that these methods do not suit
+the capacity of the animal. As a matter of fact, all of the tests which
+are now to be described in their relation to the educability of the dancer
+bear witness to the importance of the selection of methods in the light of
+the motor equipment and the habits of the animal which is to be tested.
+Judged by ordinary standards, on the basis of results which it yields in
+problem and labyrinth tests, the dancer is extremely stupid. But that this
+conclusion is not justified is apparent when it is judged in the light of
+tests which are especially adapted to its peculiarities.
+
+Problems which are easy for other mammals because of their energetic and
+persistent efforts to secure food in any way which their motor capacity
+makes possible are useless as tests of the dancer's abilities, because it
+is not accustomed to obtain its food as the result of strenuous and varied
+activities. There are problems and problems; a condition or situation
+which presents a problem to one organism may utterly lack interest for an
+organism of different structure and behavior. What is a problem test in
+the case of the cat or even of the common mouse, is not necessarily a
+problem for the dancer. Similarly, in connection with the labyrinth
+method, it is clear that the value of the test depends upon the desire of
+the organism to escape from the maze. The cat, the rat, the tortoise do
+their best to escape; the dancer is indifferent. Clearly, then, methods of
+training should be chosen on the basis of a knowledge of the
+characteristics of the animal whose educability is to be investigated.
+
+The simplest possible test of the intelligence of the dancer which I could
+devise was the following. Beside the cage in which the mice were kept I
+placed a wooden box 26 cm. long, 23 cm. wide, and 12 cm. deep. Neither
+this box nor the cage was covered, for the animals did not attempt to
+climb out. As a way of passing from one of these boxes to the other I
+arranged a ladder made of wire fly-screen netting. This ladder was about 8
+cm. broad and it extended from the middle of one side of the wooden box
+upward at an angle of about 30° to the edge of the box and then descended
+at the same angle into the cage.
+
+A dancer when taken from the nest-box and placed in the wooden box could
+return to its cage and thus find warmth, food, and company by climbing the
+ladder. It was my aim to determine, by means of this apparatus, whether
+the dancers can learn such a simple way of escape and whether they learn
+by watching one another. As it turned out, a third value belonged to the
+tests, in that they were used also to test the influence of putting the
+mice through the act.
+
+In the first experiment three dancers, Nos. 1000, 2, and 6, were together
+placed in the wooden box. At the end of 15 minutes not one of them had
+succeeded in returning to the cage. They were then driven to the bottom of
+the ladder and started upward by the experimenter; with this assistance
+all escaped to the nest-box. At the expiration of 5 minutes they were
+again placed in the wooden box, whence the chilly temperature (about 60°
+F.) and the lack of food made them eager to return to their cage. No
+attempt to climb up the ladder was made by any of them within 15 minutes,
+so the experimenter directed them to the ladder and started them upward as
+in the first test. This completed the experiment for the day. The
+following day two tests were given in the same way. In the second of these
+tests, that is, on its fourth trial, No. 1000 climbed over of his own
+initiative in 5 minutes. The others had to be assisted as formerly. On the
+third day No. 1000 found his way back to the nest-box quickly and fairly
+directly, but neither No. 2 or No. 6 climbed of its own initiative in the
+first test. When their movements were restricted to the region of the box
+about the base of the ladder, both of them returned to the cage quickly.
+And on the second test of the third day all the mice climbed the ladder
+directly.
+
+In Table 35 I have given the time required for escape in the case of 40
+tests which were given to these 3 individuals at the rate of 2 tests per
+day.
+
+When the time exceeded 15 minutes the mice were helped out by the
+experimenter; a record of 15 minutes, therefore, indicates failure.
+Naturally enough the motives for escape were not sufficiently strong or
+constant to bring about the most rapid learning of which the dancer is
+capable. Sometimes they would remain in the wooden box washing themselves
+for several minutes before attempting to find a way of escape. On this
+account I made it a rule to begin the time record with the appearance of
+active running about. The daily average time of escape as indicated in the
+table does not decrease regularly and rapidly. On the fourth day, which
+was the first on which all three of the dancers returned to the cage by
+way of the ladder of their own initiative in both tests, the average is
+214 seconds. In contrast with this, on the twentieth day the time was only
+5 seconds. It is quite evident that the dancers had learned to climb the
+ladder.
+
+At the end of the twentieth day the experiment was discontinued with Nos.
+2 and 6, and after two weeks they were given memory tests, which showed
+that they remembered perfectly the ladder-climbing act, for when placed in
+the wooden box, with Nos. 4 and 5 as controls, they returned to the cage
+by way of the ladder immediately and directly.
+
+
+
+TABLE 35
+
+LADDER CLIMBING TEST
+
+Time in Minutes and Seconds
+
+
+No. of      Date       No. 1000   No. 2     No. 6  Average  Daily Av.
+ Exp.       1905                                   For All  For All
+
+  1       Nov. 14         15'      15'        15'    --       --
+  2                       15'      15'        15'    --       --
+
+  3            15         15'      15'        15'    --       --
+  4                      300"      15'        15'    --       --
+
+  5            16        480"      15'        15'    --       --
+  6                      180"     300"       420"   300"     300"
+
+  7            17        450"     240"       540"   410"
+  8                       20"      15"        18"    18"     214"
+
+  9            18         90"     180"       135"   135"
+ 10                      135"     105"       165"   135"     135"
+
+ 11            19        480"     240"       330"   350"
+ 12                       30"     120"        90"    80"     143"
+
+ 13            20        360"      75"       120"   185"
+ 14                        5"       6"         8"     6"      95"
+
+ 15            21        105"     450"       120"   192"
+ 16                        8"      80"        20"    54"     123"
+
+ 17            22        255"     300"       180"   245"
+ 18                       10"      30"       270"   103"     174"
+
+ 19            23        300"     660"       450"   470"
+ 20                       90"     120"       150"   120"     295"
+
+ 21            24        240"     125"       225"   197"
+ 22                        4"       6"       168"    59"     128"
+
+ 23       Nov. 25       305"       85"       130"   173"
+ 24                       5"        6"       118"    43"     108"
+
+ 25            26         3"        8"        44"    18"
+ 26                      19"        1"       176"    98"      58"
+
+ 27            27       150"       79"       269"   166"
+ 28                      26"        3"        31"    20"      93"
+
+ 29            28       214"       18"       267"   166"
+ 30                      40"        3"         4"    16"      91"
+
+ 31            29       130"       45"       250"   142"
+ 32                      12"        3"        25"    13"      77"
+
+ 33       Dec.  2        61"       35"        44"    47"
+ 34                      50"        5"        24"    26"      36"
+
+ 35             3       66"       18"         2"     29"
+ 36                      8"        5"        10"      8"      19"
+
+ 37             4        9"        4"         3"      5"
+ 38                     10"        5"         6"      7"       6"
+
+ 39             5        5"        3"         5"      4"
+ 40                     10"        4"         3"      6"       5"
+
+
+
+
+One of the most interesting and important features of the behavior of the
+dancer in the ladder experiment was a halt at a certain point on the
+ladder. It occurred just at the edge of the wooden box at the point where
+the ladder took a horizontal position, and led over into the cage. Every
+individual from the first test to the last made this halt. Although from
+the point of view of the experimenter the act was valueless, it may have
+originated as an attempt to find a way to escape from the uncomfortable
+position in which the animal found itself on reaching the top of the
+ladder. Its persistence after a way of escape had been found is an
+indication of the nature of habit. Day after day the halt became shorter
+until finally it was little more than a pause and a turn of the head
+toward one side of the ladder. I think we may say that in this act we have
+evidence of the persistence of a particular resolution of physiological
+states which is neither advantageous nor disadvantageous to the organism.
+Had the act resulted in any gain, it would have become more marked and
+elaborate; had it resulted in injury or discomfort, it would have
+disappeared entirely. I have observed the same kind of behavior in the
+frog and in other animals. What the animal begins to do it persists in
+unless the act is positively harmful or conflicts with some beneficial
+activity. The only explanation of certain features of behavior is to be
+found in the conditions of their original occurrence. They persist by
+sheer force of conservatism. They have value only in the light of the
+circumstances under which they first appeared. Although this is merely a
+fact of habit formation, it suggests that many of the problems which have
+puzzled students of behavior for ages may be solved by a study of the
+history of activity.
+
+That there are marked individual differences in intelligence in the
+dancing mice is apparent from the results of the ladder-climbing
+experiment. No. 1000 learned to climb quickly, and largely by his own
+initiative; Nos. 2 and 6, on the contrary, learned only by reason of
+tuition (being put through the required act by the experimenter). It
+occurred to me that this experiment, since it was difficult for some
+individuals and easy for others, might be used to advantage as a test of
+imitation. If a dancer which knows how to escape to the cage by way of the
+ladder be placed in the wooden box with one which, despite abundant
+opportunity, has proved unable to form the habit on his own initiative,
+will the latter profit by the activity of the former and thus learn the
+method of escape?
+
+On November 20, Nos. 4 and 5 were placed in the wooden box and left there
+for half an hour. As they had failed to escape at the end of this
+interval, they were taken out of the box by the experimenter and returned
+to the nest-box. November 21 and 22 this test of their ability to learn to
+climb the ladder was repeated with the same result. On November 23 they
+were placed in the box with the three mice which had previously been
+trained to climb the ladder. The latter escaped at once. Apparently the
+attention of Nos. 4 and 5 was drawn to the ladder by the disappearance of
+their companions, for they approached its foot and No. 5 climbed up a
+short distance. Neither succeeded in escaping, however, and they made no
+further efforts that day. On the 24th, and daily thereafter until the
+29th, these two dancers were placed in the box for half an hour, with
+negative results. At the end of the half hour on the 29th, Nos. 2 and 6
+were placed in the box and permitted to go back and forth from one box to
+the other repeatedly within sight of Nos. 4 and 5. The latter made no
+attempts to follow them, although at times they seemed to be watching
+their movements as they ascended the ladder.
+
+To render the results of this test of imitation still more conclusive No.
+5 was given further opportunity to learn from No. 1000. Beginning December
+2, the following method of experimentation was employed with these two
+individuals. They were placed in the wooden box together. No. 1000 usually
+climbed out almost immediately. Sometimes No. 5 apparently saw him
+disappear up the ladder; sometimes she paid no attention whatever either
+to the presence or absence of her companion. After he had been in the
+nest-box for a few seconds, No. 1000 was returned to the wooden box by the
+experimenter and again permitted to climb out for the benefit of No. 5.
+This mode of procedure was kept up until No. 1000 had made from three to
+ten trips. No. 5 was left in the box for half an hour each day. This test
+was repeated on 18 days within a period of 3 weeks. No. 5 showed no signs
+of an imitative tendency, and she did not learn to climb the ladder.
+
+To this evidence of a lack of an imitative tendency in the dancer I may
+here add the results of my observations in other experiments. In the
+discrimination tests and in the labyrinth tests I purposely so arranged
+conditions, in certain instances, that one individual should have an
+opportunity to imitate another. In no case did this occur. Seldom indeed
+did the animals so much as follow one another with any considerable degree
+of persistence. They did not profit by one another's acts.
+
+Excellent evidence in support of this conclusion was furnished by the
+behavior of the mice in the discrimination experiments. Some individuals
+learned to pull as well as to push the swinging wire doors of the
+apparatus and were thus enabled to pass through the doorways in either
+direction; other individuals learned only to pass through in the direction
+in which the doors could be pushed open. Naturally I was interested to
+discover whether those which knew only the trick of opening the doors by
+pushing would learn to pull the doors or would be stimulated to try by
+seeing other individuals do so. At first I arranged special tests of
+imitation in the discrimination box; later I observed the influence of the
+behavior of one mouse upon that of its companion in connection with visual
+discrimination experiments. This was made possible by the fact that
+usually a pair of individuals was placed in the discrimination box and the
+tests given alternately to the male and to the female. Both individuals
+had the freedom of the nest-box and each frequently saw the other pass
+through the doorway between the nest-box, _A_, and the entrance chamber,
+_B_ (Figure 14), either from _A_ to _B_ by pushing the swing door or from
+_B_ to _A_ by pulling the door.
+
+Although abundant opportunity for imitation in connection with the opening
+of the doors in the discrimination box was given to twenty-five
+individuals, I obtained no evidence of ability to learn by imitation. The
+dancers did not watch the acts which were performed by their companions,
+and in most instances they did not attempt to follow a mate from nest-box
+to entrance chamber.
+
+These problem tests, simple as they are, have revealed two important facts
+concerning the educability of the dancer. First, that it does not learn by
+imitation to any considerable extent, and, second, that it is aided by
+being put through an act. Our general conclusion from the results of the
+experiments which have been described in this chapter, if any general
+conclusion is to be drawn thus prematurely, must be that the dancing mouse
+in its methods of learning differs markedly from other mice and from rats.
+
+
+
+
+CHAPTER XIII
+
+HABIT FORMATION: THE LABYRINTH HABIT
+
+The problem method, of which the ladder and door-opening tests of the
+preceding chapter are examples, has yielded interesting results concerning
+the individual initiative, ingenuity, motor ability, and ways of learning
+of the dancer; but it has not furnished us with accurate measurements of
+the rapidity of learning or of the permanency of the effects of training.
+In this chapter I shall therefore present the results of labyrinth
+experiments which were planned as means of measuring the intelligence of
+the dancer.
+
+The four labyrinths which have been used in the investigation may be
+designated as _A, B, C,_ and _D_. They differ from one another in the
+character of their errors, as well as in the number of wrong choices of a
+path which the animal might make on its way from entrance to exit. In the
+use of the labyrinth method, as in the case of the discrimination method
+of earlier chapters, the steps by which a satisfactory form of labyrinth
+for testing the dancer was discovered are quite as interesting and
+important for those who have an intelligent appreciation of the problems
+and methods of animal psychology as are the particular results which were
+obtained. For this reason, I shall describe the various forms of labyrinth
+in the order in which they were used, whether they proved satisfactory or
+not. At the outset of this part of my investigation, it was my purpose to
+compare directly the capacity for habit formation in the dancer with that
+of the common mouse. This proved impracticable because the same labyrinth
+is not suited to the motor tendencies of both kinds of mice.
+
+[Illustration: FIGURE 25.--Labyrinth A. _I_. entrance; _O_, exit; 1, 2, 3,
+4, blind alleys.]
+
+The first of the four labyrinths, A, appears in ground plan in Figure 25.
+It was constructed of wood, as were the other labyrinths also, and
+measured 60 cm. in length and width, and 10 cm. in depth. The outside
+alleys were 5 cm. wide. In the figure, _I_ marks the starting point or
+entrance to the maze, and _O_ the exit through which the mouse was
+permitted to pass into its nest-box. Any turn in the wrong direction which
+the animal made in its progress from entrance to exit was recorded as an
+error. The four errors, exclusive of the mistake of turning back, which
+were possible in this labyrinth, are indicated in the figure by the
+numerals 1, 2, 3, and 4. By retracing its steps a mouse might repeat any
+one or all of these errors, and add to them the error of turning back.
+
+In the experiments a mouse was permitted to enter the maze from a small
+box which had been placed by the experimenter at _I_, and an accurate
+record was kept of the number of errors which it made in finding its way
+from entrance to exit, and of the time occupied. Each of five dancers was
+given 31 tests in this labyrinth. The number of tests per day varied, as
+is indicated in Table 36, from 1 to 4. The results of the tests, so far as
+errors and times are in question, appear in the table. _T_ at the head of
+a column is an abbreviation for time, _E_ for errors.
+
+The dancers did not learn to escape from this labyrinth easily and
+quickly. In fact, the average time of the thirty-first test (198") is
+considerably longer than that of the first (130"). The number of errors
+decreased, it is true, but even for the last test it was 6.6 as compared
+with only a little more than twice that number for the first test. The
+last column of the table furnishes convincing proof of the truth of the
+statement that the animals did not acquire a perfect labyrinth-A habit.
+Was this due to inability to learn so complex a path, or to the fact that
+the method is not adapted to their nature? Observation of the behavior of
+the mice in the experiments enables me to say with certainty that there
+was no motive for escape sufficiently strong to establish a habit of
+following the direct path. Often, especially after a few experiences in
+the maze, a dancer would wander back and forth in the alleys and central
+courts, dancing much of the time and apparently exploring its surroundings
+instead of persistently trying to escape. This behavior, and the time and
+error results of the accompanying table, lead me to conclude that the
+labyrinth method, as it has been employed in the study of the intelligence
+of several other mammals, is not a satisfactory test of the ability of the
+dancer to profit by experience. That the fault is not in the labyrinth
+itself is proved by the results which I obtained with common mice.
+
+
+
+TABLE 36
+
+RESULTS OF LABYRINTH A TESTS WITH DANCERS
+
+
+                                                                 AVERAGE
+TEST  DATE   No. 1000    No. 2     No. 6     No. 4     No. 5     FOR ALL
+      1905
+               T   E     T   E     T   E     T   E     T   E     T    E
+
+  1  Nov 23   130" 14   100"  8   170" 13    60"  6   190" 26   130" 13.4
+  2      24   140  19    78   7    60   8   149   6   211  25   128  13.0
+  3      25   392  31    87   1    98   5   185  13   120   9   176  11.8
+  4      26   448  38    38   3    47   2    50   3   121  12   141  11.3
+  5      27   142   8    21   2    27   3    27   2    17   1    47   3.2
+  6      28    45   2    61   7    63   5   102   8    33   4    61   5.2
+  7      29   303  17    64   7    36   3    42   2    57   4   100   6.6
+  8      30   222  15    26   2    37   5    42   3     7   0    67   5.0
+  9  Dec  1   185   9    36   5    48   3    63   3    94   8    85   5.6
+ 10       2    52   2    71   4    19   0   196   5    95  11    87   4.4
+ 11       3   180   8    32   2   107   4    52   3    38   4    82   4.2
+ 12       4   310  10   133  11    65   3   242   6   125   6   175   7.2
+ 13       4   153   9   335  55   130  10   195  15   154  18   193  21.4
+ 14       5   330   7    69   2    42   2   201   6   130  10   154   5.4
+ 15       5   287   7    34   4    61   4   136   7    25   2   109   4.8
+ 16       5   455  15    65   4    25   0   110   8   160  15   183   8.4
+ 17       6   120  15   280   9    33   0   168   4    39   2   128   6.0
+ 18       6   120   4   164  10    81   4   101   5    85   4   110   5.4
+ 19       6   132  12    78   7   110   6    40   2   151  12   102   7.8
+ 20       7   258  10   223  16    33   1    92   5    37   1   129   6.6
+ 21       7   110   7    23   3    44   4    20   4   305  23   100   8.2
+ 22       7   100   4    60   8   167  15    44   7    58   4    86   7.6
+ 23       8    43   1   179   7   356   6    34   3    65   3   135   4.0
+ 24       8    92   5    56   5    42   3    17   1    23   1    46   3.0
+ 25       9    85   5   114   3    62   3   129   8    31   0    84   3.8
+ 26       9    30   2    36   4   109  15    12   1    34   2    44   4.8
+ 27       9    69   5    40   4    85   6    36   3    16   1    49   3.8
+ 28      10   169   7    80   3    28   0   142   5    35   2    89   3.4
+ 29      10   155   5   266   8    91   5    27   0    37   2   115   4.0
+ 30      10    29   1    25   2   124  14    83   6   111  12    74   7.0
+ 31      10   465   6   208   8    95   3    65   3   159  13   198   6.6
+
+
+
+
+On the basis of two tests per day, two common mice, a white one and a gray
+one, quickly learned to escape from labyrinth _A_ by the shortest path.
+The time of escape for the gray individual (Table 37) decreased from 180"
+in the first test to 21" in the tenth, and the number of errors from 6 to
+1. Similarly in the case of the white individual, the time decreased from
+122" to 8", and the errors from 5 to 1. A fraction of the number of tests
+to which the dancer had been subjected sufficed to establish a habit of
+escape in the common mouse. It is evident, therefore, that the dancer
+differs radically from the common mouse in its behavior in a maze, and it
+is also clear that the labyrinth method, if it is to be used to advantage,
+must be adapted to the motor tendencies of the animal which is to be
+tested.
+
+
+
+TABLE 37
+
+RESULTS OF LABYRINTH A TESTS WITH COMMON MICE
+
+        GREY MOUSE          WHITE MOUSE
+TEST     T       E           T       E
+
+  1     180"     6          122"     5
+  2      26      2           80      6
+  3      37      1           56      4
+  4      18      0           27      1
+  5      68      2           33      2
+  6      10      1           19      1
+  7      11      1           17      1
+  8      13      1           17      1
+  9      10      0            8      1
+ 10      21      1            8      1
+
+
+
+The behavior of the dancer made obvious two defects in labyrinth A. Its
+passages are so large that the mouse is constantly tempted to dance, and
+it lacks the basis for a strong and constant motive of escape by the
+direct path. To obviate these shortcomings labyrinth B was constructed, as
+is shown in Figures 23 and 24, with very narrow passages, and a floor
+which was covered with the wires of an interrupted electric circuit so
+that errors might be punished. The length of this labyrinth was 52 cm. and
+the passages were 2.5 cm. wide and 10 cm. deep. Dancing in these narrow
+alleys was practically impossible, for the mice could barely turn around
+in them. In the case of all except the common mice and two dancers, a
+depth of 10 cm. was sufficient to keep the animals in the maze without the
+use of a cover.
+
+As an account of the behavior of the dancer in labyrinth B has already
+been given in Chapter XI, I may now state the general results of the
+experiments. In all, thirty individuals were trained in this labyrinth.
+Each individual was given tests at the rate of one per minute until it had
+succeeded in following the correct path five times in succession. The weak
+electric shock, which was given as a punishment for mistakes, provided an
+activity-impelling motive for escape to the nest-box.
+
+An idea of the extreme individual difference in the rapidity with which
+the labyrinth-B path was learned by these dancers may be obtained by an
+examination of Table 38, from which it appears that the smallest number of
+training tests necessary for a successful or errorless trip through the
+maze was one and the largest number fourteen. It is to be remembered that
+each mouse was given an opportunity to pass through the labyrinth once
+without punishment for errors, and thus to discover, before the training
+tests were begun, that a way of escape existed. This first test we may
+designate as the preliminary trial. Table 38 further indicates that the
+females acquired the labyrinth habit more quickly than did the males.
+
+
+
+TABLE 38
+
+RESULTS OF LABYRINTH-B EXPERIMENTS, WITH TWENTY DANCERS
+
+
+            MALES                               FEMALES
+
+NO. OF  NO. OF FIRST NO. OF LAST OF  NO. OF  NO. OF FIRST NO. OF LAST OF
+ MOUSE     CORRECT    FIVE CORRECT    MOUSE     CORRECT    FIVE CORRECT
+            TEST          TESTS                  TEST          TESTS
+
+   76         8            14           75         4            15
+   78         5            20           77         7            11
+   86        13            22           87        12            22
+   58         2            14           49         1             5
+   50         6            23           57         3            20
+   60        13            37           59        14            28
+  410         6            20          415         4            13
+  220         4             8          225         6            18
+  212         3             7          211         6            10
+  214        10            28          213         5            14
+
+  AV.         7.0          19.3        AV.         6.2          15.6
+
+
+
+
+A graphic representation of certain of the important features of the
+process of formation of the labyrinth-B habit is furnished by Figure 26 in
+which the solid line is the curve of learning for the ten males of Table
+38, and the broken line for the ten females. These two curves were plotted
+from the number of errors made in the preliminary trial (P in the figure)
+and in each of the subsequent tests up to the sixteenth. In the case of
+both the males and the females, for example, the average number of errors
+in the preliminary trial was 11.3, as is indicated by the fact that the
+curves start at a point whose value is given in the left margin as 11.3.
+In the second training test the number of errors fell to 3.3 for the males
+and 2.7 for the females. The number of the test is to be found on the base
+line; the number of errors in the left margin. If these two curves of
+learning were carried to their completion, that for the males would end
+with the thirty-seventh test, and that for the females with the twenty-
+eighth.
+
+[Illustration: FIGURE 26.--Curves of habit formation, plotted from the
+data of labyrinth-B tests with ten males and ten females. The figures in
+the left margin indicate the number of errors; those below the base line
+the number of the test. _P_ designates the preliminary test. Males
+____[solid line]; Females ----[broken line].]
+
+Time records are not reported for these and subsequent labyrinth tests
+because they proved to be almost valueless as measures of the rapidity of
+habit formation. At any point in its progress through a labyrinth, the
+dancer may suddenly stop to wash its face, look about or otherwise examine
+its surroundings; if a shock be given to hurry it along it may be
+surprised into an error. It is my experience, and this is true of other
+animals as well as of the dancing mouse, that a long trip, as measured in
+time units, does not necessarily indicate the lack of ability to follow
+the labyrinth path correctly and rapidly. Hence, whenever it is possible
+(and the experimenter can always plan his tests so that it shall be
+possible), the number of errors should be given first importance and the
+time of the test second place. I have presented in Table 38 the number of
+the first correct test, and the number of the last of five successive
+correct tests. Space cannot be spared for records of the errors made in
+the several tests by each individual.
+
+In general, labyrinth B proved very satisfactory as a means of testing the
+ability of the dancer to learn a simple path. The narrow passages
+effectively prevented dancing, and the introduction of the electric shock
+as a punishment for mistakes developed a motive for escape which was
+uniform, constant, and so strong that the animals clearly did their best
+to escape from the labyrinth quickly and without errors. This maze was so
+simple that it did not tend to discourage them as did the one which is
+next to be described. It must be admitted, however, that, though labyrinth
+B is perfectly satisfactory as a test of the dancer's ability to learn to
+follow a simple path, it is not an ideal means of measuring the rapidity
+of habit formation. This is due to the fact that the preliminary trial and
+the first training test play extremely different roles in the case of
+different individuals. A dancer which happens to follow the correct path
+from entrance to exit in the preliminary trial may continue to do so, with
+only an occasional error, during several of the early training tests, and
+it may therefore fail for a considerable time to discover that there are
+errors which should be avoided. The learning process is delayed by its
+accidental success. On the other hand, an individual which happens to make
+many mistakes to begin with immediately attempts to avoid the points in
+the maze at which it receives the electric shock. I was led to conclude,
+as a result of the labyrinth-B experiments, that the path was too easy,
+and that a more complex labyrinth would, in all probability, furnish a
+more satisfactory means of measuring the rapidity of habit formation.
+
+[Illustration: FIGURE 27--A record sheet, showing the plan of labyrinth C
+(as made on the sheet by means of a rubber stamp) on which the
+experimenter recorded the path followed by the mouse. This sample sheet
+presents the path records for the first, fifth, tenth, and eleventh tests
+of No. 2 in labyrinth C. 1, 2, 3, 4, 5 designate the several errors of the
+labyrinth.]
+
+On the basis of the supposition that a maze whose path was so complex that
+the animal would not be likely to follow it correctly in the early trials
+would be more to the purpose than either A or B, labyrinth C was devised.
+As is shown in the plan of this maze, Figure 27, five mistakes in choice
+of path were possible on the forward trip. These errors, as a rule, were
+more difficult for the dancers to avoid than those of labyrinths A and B.
+Those which are designated by the numerals 2, 3, and 4 were especially
+difficult. Error 4 was much more troublesome for left whirlers than for
+right whirlers because, after turning around abruptly at the entrance to
+the blind alley, the former type of dancer almost always followed the side
+wall of the maze so far that it missed the correct path. Undoubtedly the
+various errors are not of the same value for different individuals; but it
+would be extremely difficult, if not impossible, to devise a maze which
+should be equally difficult for several normal individuals.
+
+In order that records of the path followed by a mouse in test after test
+might be kept with ease and accuracy by the experimenter, the plan of this
+labyrinth, and also that of labyrinth D, were cast in rubber. The outlines
+of labyrinths C and D which appear in Figures 27 and 28 respectively were
+made with the rubber stamps which were thus obtained. Figure 27 is the
+reproduction of a record sheet which presents the results of the first,
+the fifth, the tenth, and the eleventh tests of No. 2 in labyrinth C. The
+path followed by this individual in the first test was far too complex to
+be traced accurately on the record sheet. The record therefore represents
+merely the number of errors which was made in each region of the maze. For
+the fifth test, and again for the tenth and the eleventh, the path was
+recorded accurately. This simple device for making record blanks which can
+readily be filled in at the time of the experiment should recommend itself
+to all students of animal behavior.
+
+In labyrinth C ten pairs of dancers were given continuous training tests
+at the rate of one test per minute until they were able to follow the
+direct path correctly. Because of the difficulty in learning this maze
+perfectly, it was not demanded of the mice that they should follow the
+path correctly several times in succession, but instead the training was
+terminated after the first successful trip.
+
+
+
+TABLE 39
+
+RESULTS OF LABYRINTH-C EXPERIMENTS, WITH TWENTY DANCERS
+
+
+           MALES                      FEMALES
+
+  NO. OF     NO. OF FIRST     NO. OF     NO. OF FIRST
+  MOUSE      CORRECT TEST     MOUSE      CORRECT TEST
+
+     2            11            29            15
+    30            33            49            34
+    50            49            57            15
+    52            22            59            15
+    58            16           215            10
+    60            17           415            10
+    76             3            75             8
+    78             6            77            11
+    86             5            87             9
+    88            25            85            11
+
+    AV.           18.7          AV.           13.8
+
+
+
+
+The results of the experiments with this labyrinth as they are presented
+in Table 39 indicate that its path is considerably more difficult for the
+dancer to learn than that of labyrinth B, that the females learn more
+quickly than the males, and finally, that individual differences are just
+as marked as they were in the case of the simpler forms of labyrinth. It
+therefore appears that increasing the complexity of a labyrinth does not,
+as I had supposed it might, diminish the variability of the results.
+Certain of the individual differences which appear in Table 39 are due,
+however, to the fact that in some cases training in labyrinth B had
+preceded training in labyrinth C, whereas in the other cases C was the
+first labyrinth in which the animals were tested. But even this does not
+serve to account for the wide divergence of the results given by No. 2 and
+No. 50, for the latter had been trained in B previous to his training in
+C, and the former had not been so trained. Yet, despite the advantage
+which previous labyrinth experience gave No. 50, he did not learn the path
+of C as well in fifty tests as No. 2 did in eleven. The facts concerning
+the value of training in one form of labyrinth for the learning of
+another, as they were revealed by these experiments, may more fittingly be
+discussed in a later chapter in connection with the facts of memory and
+re-learning.
+
+[Illustration: FIGURE 28.--Plan of Labyrinth _D_, as reproduced from a
+print made with a rubber stamp. _I_, entrance; _O_, exit; numerals 1 to
+13, errors.]
+
+Labyrinth C is a type of maze which might properly be described as
+irregular, since the several possible errors are extremely different in
+nature. In view of the results which this labyrinth yielded, it seemed
+important that the dancer be tested in a perfectly regular maze of the
+labyrinth-D type. The plan which I designed as a regular labyrinth has
+been reproduced, from a rubber stamp print, in Figure 28. As is true also
+of the mazes previously described, it provides four kinds of possible
+mistakes: namely, by turning to the left (errors 1, 5, 9, and 13), by
+turning to the right (errors 3, 7, and 11), by moving straight ahead
+(errors 2, 4, 6, 8, 10, and 12), and by turning back and retracing the
+path just followed. The formula for the correct path of _D_ is simple in
+the extreme, in spite of the large number of mistakes which are possible,
+for it is merely "a turn to the right at the entrance, to the left at the
+first doorway, and thereafter alternately to the right and to the left
+until the exit is reached." This concise description would enable a man to
+find his way out of such a maze with ease. Labyrinth D had been
+constructed with an exit at 10 so that it might be used as a nine-error
+maze if the experimenter saw fit, or as a thirteen-error maze by the
+closing of the opening at 10. In the experiments which are now to be
+described only the latter form was used.
+
+Can the dancer learn a regular labyrinth path more quickly than an
+irregular one? Again, I may give only a brief statement of results. Each
+of the twenty dancers, of Table 40, which were trained in labyrinth D had
+previously been given opportunity to learn the path of C, and most of them
+had been trained also in labyrinth B. All of them learned this regular
+path with surprising rapidity. The numerical results of the tests with
+labyrinths B, C, and D, as well as the behavior of the mice in these
+several mazes, prove conclusively that the nature of the errors is far
+more important than their number. Labyrinth D with its thirteen chances of
+error on the forward trip was not nearly as difficult for the dancer to
+learn to escape from as labyrinth C with its five errors. That the
+facility with which the twenty individuals whose records are given in
+Table 40 learned the path of D was not due to their previous labyrinth
+experience rather than to the regularity of the maze is proved by the
+results which I obtained by testing in D individuals which were new to
+labyrinth experiments. Even in this case, the number of tests necessary
+for a successful trip was seldom greater than ten. If further evidence of
+the ease with which a regular labyrinth path may be followed by the dancer
+were desired, it might be obtained by observation of the behavior of an
+individual in labyrinths C and D. In the former, even after it has learned
+the path perfectly, the mouse hesitates at the doorways from time to time
+as if uncertain whether to turn to one side or go forward; in the latter
+there is seldom any hesitation at the turning points. The irregular
+labyrinth is followed carefully, as by choice of the path from point to
+point; the regular labyrinth is followed in machine fashion,--once
+started, the animal dashes through it.
+
+
+
+TABLE 40
+
+RESULTS OF LABYRINTH-D EXPERIMENTS, WITH TWENTY DANCERS
+
+
+            MALES                              FEMALES
+
+ NO. OF NO. OF FIRST NO. OF LAST OF  NO. OF NO. OF FIRST NO. OF LAST OF
+ MOUSE     CORRECT     TWO CORRECT   MOUSE     CORRECT     TWO CORRECT
+            TEST          TESTS                 TEST          TESTS
+
+    2         3             7          29        10            11
+   58         7            10          49         7             8
+   30         9            10          57         3             6
+   60        10            14         215         6            10
+  402        10            11         415         7             8
+   76         4             7          75         4            13
+   78         4             5          77        11            12
+   86         3             9          87         4             9
+   88         4             8          85         3             4
+   90         7             8          83         4             7
+
+  Av.         6.1           8.9       Av.         5.9           8.8
+
+
+
+
+From the results of these labyrinth experiments with dancers I am led to
+conclude that a standard maze for testing the modifiability of behavior of
+different kinds of animals should be constructed in conformity with the
+following suggestions. Errors by turning to the right, to the left, and by
+moving forward should occur with equal frequency, and in such order that
+no particular kind of error occurs repeatedly in succession. If we should
+designate these three types of mistake by the letters _r, l_, and _s_
+respectively, the error series of labyrinth C would read _l-l-r-s-l_. It
+therefore violates the rule of construction which I have just formulated.
+In the case of labyrinth D the series would read _l-s-r-s-l-s-r-s-l-s-r-s-
+l_. This also fails to conform with the requirement, for there are three
+errors of the first type, four of the second, and six of the third. Again,
+in a standard maze, the blind alleys should all be of the same length, and
+care should be taken to provide a sufficiently strong and uniform motive
+for escape. In the case of one animal the desire to escape from
+confinement may prove a satisfactory motive; in the case of another, the
+desire for food may conveniently supplement the dislike of confinement;
+and in still other cases it may appear that some form of punishment for
+errors is the only satisfactory basis of a motive for escape. Readers of
+this account of the behavior of the dancing mouse must not infer from my
+experimental results that the electric shock as a means of forcing
+discrimination will prove satisfactory in work with other animals or even
+with all other mammals. As a matter of fact it has already been proved by
+Doctor G. van T. Hamilton that the use of an electric shock may so
+intimidate a dog that experimentation is rendered difficult and of little
+value. And finally, in connection with this discussion of a standard
+Labyrinth, I wish to emphasize the importance of so recording the results
+of experiments that they may be interpreted in terms of an animal's
+tendency to turn to the right or to the left. My work with the dancer has
+clearly shown that the avoidance of a particular error may be extremely
+difficult for left whirlers and very easy for right whirlers.
+
+I hope I have succeeded in making clear by the foregoing account of my
+experiments that the labyrinth method is more satisfactory in general than
+the problem method as a means of measuring the rapidity of habit formation
+in the dancer, and I hope that I have made equally clear the fact that it
+is very valuable as a means of discovering the roles of the various senses
+in the acquirement of a habit (Chapter XI). From my own experience in the
+use of the labyrinth with the dancer and with other animals, I am forced
+to conclude that its chief value lies in the fact that it enables the
+experimenter so to control the factors of a complex situation that he may
+readily determine the importance of a given kind of sense data for the
+formation or the execution of a particular habit. As a means of measuring
+the intelligence of an animal, of determining the facility with which it
+is capable of adjusting itself to new environmental conditions, and of
+measuring the permanency of modifications which are wrought in its
+behavior by experimental conditions, I value the labyrinth method much
+less highly now than I did previous to my study of the dancer. It is
+necessarily too complex for the convenient and reasonably certain
+interpretation of results. Precisely what is meant by this statement will
+be evident in the light of the results of the application of the
+discrimination method to the dancer, which are to be presented in the next
+chapter. The labyrinth method is an admirable means of getting certain
+kinds of qualitative results; it is almost ideal as a revealer of the role
+of the senses, and it may be used to advantage in certain instances for
+the quantitative study of habit formation and memory. Nevertheless, I
+think it may safely be said that the problem method and the discrimination
+method are likely to do more to advance our knowledge of animal behavior
+than the labyrinth method.
+
+
+
+
+CHAPTER XIV
+
+HABIT FORMATION: THE DISCRIMINATION METHOD
+
+Discrimination is demanded of an animal in almost all forms of the problem
+and labyrinth methods, as well as in what I have chosen to call the
+discrimination method. In the latter, however, discrimination as the basis
+of a correct choice of an electric-box is so obviously important that it
+has seemed appropriate to distinguish this particular method of measuring
+the intelligence of the dancer from the others which have been used, by
+naming it the discrimination method.
+
+It has been shown that neither the problem nor the labyrinth method proves
+wholly satisfactory as a means of measuring the rapidity of learning, or
+the duration of the effects of training, in the case of the dancer. The
+former type of test serves to reveal to the experimenter the general
+nature of the animal's capacity for profiting by experience; the latter
+serves equally well to indicate the parts which various receptors (some of
+which are sense organs) play in the formation and execution of habits. But
+neither of them is sufficiently simple, easy of control, uniform as to
+conditions which constitute bases for activity, and productive of
+interpretable quantitative results to render it satisfactory. The problem
+method is distinctly a qualitative method, and, in the case of the dancing
+mouse, my experiments have proved that the labyrinth method also yields
+results which are more valuable qualitatively than quantitatively. I had
+anticipated that various forms of the labyrinth method would enable me to
+measure the modifiability of behavior in the dancer with great accuracy,
+but, as will now be made apparent, the discrimination method proved to be
+a far more accurate method for this purpose.
+
+Once more I should emphasize the fact that my statements concerning the
+value of methods apply especially to the dancing mouse. Certain of the
+tests which have proved to be almost ideal in my study of this peculiar
+little rodent would be useless in the study of many other mammals. An
+experimenter must work out his methods step by step in the light of the
+daily results of patient and intelligent observation of the motor
+capacity, habits, instincts, temperament, imitative tendency,
+intelligence, hardihood, and life-span of the animal which he is studying.
+The fact that punishment has proved to be more satisfactory than reward in
+experiments with the dancer does not justify the inference that it is more
+satisfactory in the case of the rat, cat, dog, or monkey. Methods which
+yielded me only qualitative results, if applied to other mammals might
+give accurate quantitative results; and, on the other hand, the
+discrimination method, which has proved invaluable for my quantitative
+work, might yield only qualitative results when applied to another kind of
+animal.
+
+The form of the discrimination method whose results are to be presented in
+this chapter has already been described as white-black discrimination. In
+the discrimination box (Figures 14 and 15, p. 92) the two electric-boxes
+which were otherwise exactly alike in appearance were rendered
+discriminable for the mouse by the presence of white cardboards in one and
+black cardboards in the other. In order to escape from the narrow space
+before the entrances to the two electric-boxes, the dancer was required to
+enter the white box. If it entered the black box a weak electric shock was
+experienced. After two series of ten tests each, during which the animal
+was permitted to choose either the white or the black box without shock or
+hindrance, the training was begun. These two preliminary series serve to
+indicate the natural preference of the animal for white or black previous
+to the training. An individual which very strongly preferred the white
+might enter, from the first, the box thus distinguished, whereas another
+individual whose preference was for the black might persistently enter the
+black box in spite of the disagreeable shocks. First of all, therefore,
+the preliminary tests furnish a basis for the evaluation of the results of
+the subsequent training tests. On the day succeeding the last series of
+preliminary tests, and daily thereafter until the animal had acquired a
+perfect habit of choosing the white box, a series of training tests was
+given. These experiments were usually made in the morning between nine and
+twelve o'clock, in a room with south-east windows. The entrances to the
+electric-boxes faced the windows, consequently the mouse did not have to
+look toward the light when it was trying to discriminate white from black.
+All the conditions of the experiment, including the strength of the
+current for the shock, were kept as constant as possible.
+
+Choice by position was effectively prevented, as a rule, by shifting the
+cardboards so that now the left now the right box was white. The order of
+these shifts for the white-black series whose results are quantitatively
+valuable appear in Table 12 (p. III). That the order of these changes in
+position may be criticised in the light of the results which the tests
+gave, I propose to show hereafter in connection with certain other facts.
+The significant point is that the defects which are indicated by the
+averages of thousands of tests could not have been predicted with
+certainty even by the most experienced investigator in this field.
+
+In Table 41 are to be found the average number of errors in each series of
+ten white-black discrimination tests for five males and for five females
+which were trained by being given ten tests per day, and similarly for the
+same number of individuals of each sex, trained by being given twenty
+tests per day. Since the results for these two conditions of training are
+very similar, the averages for the twenty individuals are presented in the
+last column of the table. For the present we may neglect the interesting
+individual, sex, and age differences which these experiments revealed and
+examine the significant features of the general averages, and of the
+white-black discrimination curve (Figure 29).
+
+
+
+                      TABLE 41
+
+WHITE BLACK DISCRIMINATION TESTS. NUMBER OF ERRORS IN
+                  THE VARIOUS SERIES
+
+                MALES                      FEMALES
+
+       AVERAGES AVERAGES GENERAL  AVERAGES AVERAGES GENERAL  AVERAGES
+SERIES FOR 5,   FOR 5,   AVERAGES FOR 5,   FOR 5,   AVERAGES FOR ALL
+       10 TESTS 20 TESTS FOR 10   10 TESTS 20 TESTS FOR 10   (20) MALES
+       PER DAY  PER DAY           PER DAY  PER DAY           AND FEMALES
+
+   A     5.8      6.0      5.9      5.8      5.8      5.8       5.85
+   B     5.6      6.2      5.9      5.8      5.6      5.7       5.8
+   1     5.0      5.0      5.0      5.6      4.6      5.1       5.05
+   2     2.6      4.6      3.6      4.4      5.0      4.7       4.15
+   3     3.0      3.4      3.2      3.4      3.4      3.4       3.3
+   4     2.6      3.8      3.2      2.4      2.2      2.3       2.75
+   5     2.4      2.0      2.2      2.6      1.8      2.2       2.2
+   6     1.6      1.6      1.6      1.0      2.2      1.6       1.6
+   7     1.0      1.4      1.2      2.0      0.4      1.2       1.2
+   8     0.2      0.6       .4      1.4      1.6      1.5        .95
+   9     0.2      1.0       .6      0.6      0.8       .7        .65
+  10       0       .8       .4      1.0      0.8       .9        .65
+  11       0       .8       .4      0.8        0       .4        .40
+  12       0       .6       .3      0.4        0       .2        .25
+  13       0        0        0        0        0        0       0
+  14       0        0        0        0                 0       0
+  15       0        0        0        0                 0       0
+
+
+
+
+[Illustration: FIGURE 29.--Error curve plotted from the data given by
+twenty dancers in white-black discrimination tests. The figures in the
+left margin indicate the number of errors; those below the base line, the
+number of the series. _A_ and _B_ designate the preference series.]
+
+ The preference series, _A_ and _B_, reveal a constant tendency to choose
+the black box, whose strength as compared with the tendency to choose the
+white box is as 5.8 is to 4.2. In other words, the dancer on the average
+chooses the black box almost six times in ten. The first series of
+training tests reduced this preference for black to zero, and succeeding
+series brought about a rapid and fairly regular decrease in the number of
+errors, until, in the thirteenth series, the white was chosen every time.
+Since I arbitrarily define a perfect habit of discrimination as the
+ability to choose the right box in three successive series of ten tests
+each, the tests ended with the fifteenth series.
+
+The discrimination curve, Figure 29, is a graphic representation of the
+general averages of Table 41.--It is an error curve, therefore. Starting
+at 5.85 for the first preliminary series, it descends to 5.8 for the
+second series, and thence abruptly to 5.05 for the first training series.
+This series of ten tests therefore served to reduce the black preference
+very considerably. The curve continues to descend constantly until the
+tenth series, for which the number of errors was the same as for the
+preceding series, .65. This irregularity in the curve, indicative, as it
+would appear, of a sudden cessation in the learning process, demands an
+explanation. My first thought was that an error in computation on my part
+might account for the shape of the curve. The error did not exist, but in
+my search for it I discovered what I now believe to be the cause of the
+interruption in the fall of the error curve. In all of the training series
+up to the tenth the white cardboard had been on the right and the left
+alternately or on one side two or three times in succession, whereas in
+the tenth series, as may be seen by referring to Table 12 (p.111), it was
+on the left for the first four tests, then on the right four times, and,
+finally, on the left for the ninth test and on the right for the tenth.
+This series was therefore a decidedly more severe test of the animal's
+ability to discriminate white from black and to choose the white box
+without error than were any that had preceded it. If my interpretation of
+the results is correct, it was so much more severe than the ninth series
+that the process of habit formation was obscured. It would not be fair to
+say that the mouse temporarily ceased to profit by its experience; instead
+it profited even more than usually, in all probability, but the
+unavoidably abrupt increase in the difficultness of the tests was just
+sufficient to hide the improvement.
+
+As I have suggested, the plan of experimentation may be criticised
+adversely in the light of this irregularity in the error curve. Had the
+conditions been perfectly satisfactory the curve would not have taken this
+form. I admit this, but at the same time I am glad that I chose that
+series of shifts in the position of the cardboards which, as it happens,
+served to exhibit an important aspect of quantitative measures of the
+modifiability of behavior that otherwise would not have been revealed. Our
+mistakes in method often teach us more than our successes. I have taken
+pains, therefore, to describe the unsatisfactory as well as the
+satisfactory steps in my study of the dancer.
+
+[Illustration: FIGURE 30.--Error curve plotted from the data given by
+thirty dancers, of different ages and under different conditions of
+training, in white-black discrimination tests.]
+
+The form of the white-black discrimination curve of Figure 29 is more
+surprising than disappointing to me, for I had anticipated many more
+irregularities than appear. What I had expected, as the result of training
+five or even ten pairs of mice, was the kind of curve which is presented,
+for contrast with the one already discussed, in Figure 30. This also is an
+error curve, but, unlike the previous one, it is based upon results which
+were got from individuals of different ages which were trained according
+to the following different methods. Ten of these individuals were given
+two or five tests daily, ten were given ten tests daily, and ten were
+given twenty tests daily. The form of the curve serves to call attention
+to the importance of uniform conditions of training, in case the results
+are to be used as accurate measures of the rapidity of learning.
+
+Examination of the detailed results of the white-black discrimination
+tests as they appear in the tables of Chapter VII will reveal the fact
+that some individuals succeeded in choosing correctly in a series of ten
+tests after not more than five series, whereas others required at least
+twice as many tests as the basis of a perfect series. In very few
+instances, however, was a perfect habit of discrimination established by
+fewer than one hundred tests. As the averages just presented in Table 41
+indicate, fifteen series, or one hundred and fifty tests, were required
+for the completion of the experiment. One might search a long time,
+possibly, for another mammal whose curve of error in a simple
+discrimination test would fall as gradually as that of the dancer. It is
+fair to say that this animal learns very slowly as compared with most
+mammals which have been carefully studied. It is to be remembered,
+however, that quantitative results such as are here presented for the
+dancer are available for few if any other animals except the white rat.
+Neither in the form of the curve of learning nor in the behavior of the
+animal as it makes its choice of an electric-box is there evidence of
+anything which might be described as a sudden understanding of the
+situation. The dancer apparently learns by rote. It exhibits neither
+intelligent insight into an experimental situation nor ability to profit
+by the experience of its companions. That the selection of the white box
+occurs in various ways in different individuals, and even in the same
+individual at different periods in the training process, is the only
+indication of anything suggestive of implicit reasoning. Naturally enough
+comparison of the two boxes is the first method of selection. It takes the
+dancers a surprisingly long time to reach the point of making this
+comparison as soon as they are confronted by the entrances to the two
+electric-boxes. The habit of running from entrance to entrance repeatedly
+before either is entered, once having been acquired, is retained often
+throughout the training experiments. But in other cases, an individual
+finally comes to the point of choosing by what appears to be the immediate
+recognition of the right or the wrong box. In the former case the mouse
+enters the white box immediately; in the latter, it rushes from the black
+box into the white one without hesitation. So much evidence the
+discrimination tests furnish of forms of behavior which in our fellow-men
+we should interpret as rational.
+
+[Illustration: FIGURE 31.--Curve of habit formation, plotted from the data
+of labyrinth-D tests with ten males and ten females.]
+
+Comparison of the error curves for the labyrinth tests (Figures 26 and 31)
+with those for the discrimination tests (Figures 29 and 30) reveals
+several interesting points of difference. The former fall very abruptly at
+first, then with decreasing rapidity, to the base line; the latter, on the
+contrary, fall gradually throughout their course. Evidently the labyrinth
+habit is more readily acquired by the dancer than is the visual
+discrimination habit. Certain motor tendencies can be established quickly,
+it would seem, whereas others, and especially those which depend for their
+guidance upon visual stimuli, are acquired with extreme slowness. From
+this it might be inferred that the labyrinth method is naturally far
+better suited to the nature of the dancer than is any form of the
+discrimination method. I believe that this inference is correct, but at
+the same time I am of the opinion that the discrimination method is of
+even greater value than the labyrinth method as a means of discovering the
+capacity of the animal for modification of behavior.
+
+Inasmuch as my first purpose in the repetition of white-black
+discrimination tests with a number of individuals was to obtain
+quantitative results which should accurately indicate individual, age, and
+sex differences in the rapidity of learning, it is important to consider
+the reliability of the averages with which we have been dealing. Possibly
+two groups of five male dancers each, chosen at random, would yield very
+different results in discrimination tests. This would almost certainly be
+true if the animals were selected from different lots, or were kept before
+and during the tests under different environmental conditions. But from my
+experiments it has become apparent that the average of the results given
+by five individuals of the same sex, age, and condition of health, when
+kept in the same environment and subjected to the same experimental tests,
+is sufficiently constant from group to group to warrant its use as an
+index of modifiability for the race. This expression, index of
+modifiability, is a convenient mode of designating the average number of
+tests necessary for the establishment of a perfect habit of white-black
+discrimination. Hereafter I shall use it instead of a more lengthy
+descriptive phrase.
+
+As an indication of the degree of accuracy of measurements of the rapidity
+of learning which are obtained by the use of 5 individuals I may offer the
+following figures. For one of two directly comparable groups of 5 male
+dancers which were chosen from 16 individuals which had been trained, the
+number of tests which resulted in a perfect habit of white-black
+discrimination was 92; for the other group it was 96. These indices for
+strictly comparable groups of 5 individuals each differ from one another
+by less than 5 per cent. Similarly, in the case of two groups of females,
+the indices of modifiability were 94 and 104. These figures designate the
+number of tests up to the point at which errors ceased for at least three
+successive series (30 tests).
+
+The determination of the probable error of the index of modifiability
+further aids us in judging of the reliability of the measure of the
+rapidity of learning which is obtained by averaging the results for 5
+individuals. For a group of 5 males (Table 43, p. 243) the index was 72 ±
+3.5; and for a group of 5 females of the same age as the males and
+strictly comparable with respect to conditions of white-black training, it
+was 104 ± 2.9. A probable error of ± 3.5 indicates the reliability of the
+first of these indices of modifiability; one of ± 2.9, that of the second.
+
+I do not doubt that 10 individuals would furnish a more reliable average
+than 5, but I do doubt whether the purposes of my experiments would have
+justified the great increase in work which the use of averages based upon
+so large a group would have necessitated.
+
+Further discussion of the index of modifiability may be postponed until
+the several indices which serve as measures of the efficiency of different
+methods of training have been presented in the next chapter.
+
+From the data which constitute the materials of the present chapter it is
+apparent that the results of the discrimination method are amenable to
+much more accurate quantitative treatment than are those of the problem
+method or the labyrinth method. But I have done little more as yet than
+describe the method by which it is possible to measure certain dimensions
+of the intelligence of the dancer, and to state some general results of
+its application. In the remaining chapters it will be our task to discover
+the value of this method and of the results which it has yielded.
+
+
+
+
+CHAPTER XV
+
+
+THE EFFICIENCY OF TRAINING METHODS
+
+The nature of the modifications which are wrought in the behavior of an
+organism varies with the method of training. This fact is recognized by
+human educators, as well as by students of animal behavior (makers of the
+science of comparative pedagogy), but unfortunately accurate measurements
+of the efficiency of our educational methods are rare.
+
+Whatever the subject of investigation, there are two preeminently
+important aspects of the educative process which may be taken as
+indications of the value of the method of training by which it was
+initiated and stimulated. I refer to the rapidity of the learning process
+and its degree of permanency, or, in terms of habit formation, to the
+rapidity with which a habit is acquired, and to its duration. Of these two
+easily measurable aspects of the modifications in which training results,
+I have chosen the first as a means to the special study of the efficiency
+of the training to which the dancing mouse has been subjected in my
+experiments.
+
+The reader who has followed my account of the behavior of the dancer up to
+this point will recall that in practically all of the discrimination
+experiments the number of tests in a series was ten. Some readers
+doubtless have wondered why ten rather than five or twenty tests was
+selected as the number in each continuous series. I shall now attempt to
+answer the question. It was simply because the efficiency of that number
+of tests, given daily, when taken in connection with the amount of time
+which the conduct of the experiments required, rendered it the most
+satisfactory number. But this statement demands elaboration and
+explanation.
+
+Very early in my study of the dancer, I learned that a single experience
+in a given experiment day after day had so little effect upon the animal
+that a perfect habit could not be established short of several weeks or
+months. Similarly, experiments in which two tests per day were given
+proved that even a simple discrimination habit cannot be acquired by the
+animal under this condition of training with sufficient rapidity to enable
+the experimenter to study the formation of the habit advantageously. Next,
+ten tests in succession each day were given. The results proved
+satisfactory, consequently I proceeded to carry out my investigation on
+the basis of a ten-test series. After this method had been thoroughly
+tried, I decided to investigate the efficiency of other methods for the
+purpose of instituting comparisons of efficiency and discovering the
+number of tests per day whose efficiency, as measured by the rapidity of
+the formation of a white-black discrimination habit, is highest.
+
+For this purpose I carefully selected five pairs of dancers of the same
+age, descent, and previous experience, and gave them white-black tests in
+series of two tests per day (after the twentieth day the number was
+increased to five) until they had acquired a perfect habit of
+discriminating. Similarly other dancers were trained by means of series of
+ten tests, twenty tests, or one hundred tests per day. Since it was my aim
+to make the results of these various tests strictly comparable, I spared
+no pains in selecting the individuals, and in maintaining constancy of
+experimental conditions. The order of the changes in the position of the
+cardboards which was adhered to in these efficiency tests was that given
+in Table 12.
+
+At the beginning of the two-test training I thought it possible that the
+animals might acquire a perfect habit with only a few more days' training
+than is required by the ten-test method. This did not prove to be the
+case, for at the end of the twentieth day (after forty tests in all) the
+average number of mistakes, as Table 42 shows, was 3.2 for the males and
+3.0 for the females. Up to this time there had been clear evidence of the
+formation of a habit of discriminating white from black, but, on the other
+hand, the method had proved very unsatisfactory because the first test
+each day usually appeared to be of very different value from the second.
+On account of the imminent danger of the interruption of the experiment by
+the rapid spread of an epidemic among my mice, I decided to increase the
+number of tests in each series to five in order to complete the experiment
+if possible before the disease could destroy the animals. On the twenty-
+first day and thereafter, five-test series were given instead of two-test.
+Unfortunately I was able to complete the experiment up to the point of
+thirty successive correct tests with only six of the ten individuals whose
+numbers appear at the top of Table 42. That the results of this table are
+reliable, despite the fact that some of the individuals had to be taken
+out of the experiment on account of bad condition, is indicated by the
+fact that all the mice continued to do their best to discriminate so long
+as they were used. Possibly the habit would have been acquired a little
+more quickly by some of the individuals had they been stronger and more
+active.
+
+It should be explained at this point that the results in all the
+efficiency-of-training tables of this chapter are arranged, as in the
+previous white-black discrimination tables, in tens, that is, each figure
+in the tables indicates the number of errors in a series of ten tests. In
+all cases _A_ and _B_ mark preliminary series of tests which were given at
+the rate of ten tests per series. The numbers in the first column of these
+tables designate groups of ten tests each, and not necessarily daily
+series. In Table 42, for example, 1 includes the results of the first five
+days of training, 2, of the next five days, and so on. The table shows
+that No. 80 made seven wrong choices in the first five series of two tests
+each. This method of grouping results serves to make the data for the
+different methods directly comparable, and at the same time it saves space
+at the sacrifice of very little valuable information concerning the nature
+of the daily results. It is to be noted, with emphasis, that the two-five
+tests per day training established a perfect habit after four weeks of
+training. This method is therefore costly of the experimenter's time.
+
+
+
+TABLE 42
+
+EFFICIENCY OF TRAINING. WHITE-BLACK TESTS AT THE RATE
+OF 2 OR 5 PER DAY
+
+                 MALES                        FEMALES
+SETS      80  82  84  86  88   AV.     73  79  83  85  89   AV.
+OF 10
+
+ A         5   5   4   8   5   5.4      5   6   7   7   6   6.2
+ B         5   3   6   5   6   5.0      7   5   7   6   7   6.4
+
+ 1         7   7   6   6   6   6.4      7   6   9   4   6   6.4
+ 2         2   1   0   6   6   3.0      6   5   6   5   5   5.4
+ 3         4   5   5   1   2   3.2      6   5   2   4   1   3.6
+ 4         3   4   7   2   0   3.2      4   3   1   4   3   3.0
+ 5         2   3   3   2   4   2.8      -   3   4   3   1   2.7
+ 6         2   2   -   2   2   2.0      -   0   2   2   0   1.0
+ 7         -   1   -   0   1   0.7      -   1   0   2   1   1.0
+ 8         -   -   -   1   1   1.0      -   1   1   0   0   0.5
+ 9         -   -   -   0   1   0.5      0   1   1   0   0   0.5
+10         -   -   -   0   0    0       -   0   0   0   0    0
+11         -   -   -   0   0    0       -   0   0            0
+12         -   -   -       0    0       -   0   0            0
+
+
+
+
+TABLE 43
+
+EFFICIENCY OF TRAINING. WHITE-BLACK TESTS AT THE RATE OF
+10 PER DAY
+
+                 MALES                        FEMALES
+SETS     210 220 230 410 420  AV.     215 225 235 415 425  AV.
+OF 10
+
+ A        6   5   6   6   6   5.8      8   4   4   8   5   5.8
+ B        6   8   8   5   1   5.6      8   7   6   6   2   5.8
+
+ 1        6   7   6   2   4   5.0      7   6   5   6   4   5.6
+ 2        4   3   1   2   3   2.6      5   6   4   2   5   4.4
+ 3        3   1   4   3   4   3.0      3   3   4   2   5   4.4
+ 4        5   0   3   3   2   3.2      2   1   3   3   3   2.4
+ 5        3   0   4   1   4   2.4      1   3   3   3   3   2.6
+ 6        2   1   4   0   1   1.6      2   1   1   1   0   1.0
+ 7        1   0   3   1   0   1.0      1   1   2   3   3   2.0
+ 8        0   0   1   0   0   0.2      0   0   2   2   3   2.0
+ 9        0   0   0   1   0   0.2      1   0   0   1   1   0.6
+10        0       0   0        0       0   2   1   0   2   1.0
+11                0   0        0       0   3   0   1   0   0.8
+12                    0        0       0   0   0   2   0   0.4
+13                                         0   0   0   0    0
+14                                         0       0        0
+15                                                 0        0
+
+
+
+
+TABLE 44
+
+EFFICIENCY OF TRAINING. WHITE-BLACK TESTS AT THE RATE OF
+20 PER DAY
+
+                 MALES                        FEMALES
+SETS      72  74 208 240 402  AV.     217 230 245 403 407  AV.
+OF 10
+
+ A        4   6   7   7   6   6.0      5   4   7   7   6   5.8
+ B        6   4   6   8   7   6.2      7   3   5   8   5   5.6
+
+ 1        3   5   7   5   5   5.0      3   6   4   4   6   4.6
+ 2        4   3   7   5   4   4.6      7   3   5   4   6   5.0
+ 3        3   3   3   5   3   3.4      4   3   3   2   5   3.4
+ 4        6   3   1   4   5   3.8      5   0   1   2   3   2.2
+ 5        4   1   0   2   3   2.0      6   0   0   1   2   1.8
+ 6        3   1   0   2   2   1.6      4   1   1   0   6   2.2
+ 7        3   2   0   1   1   1.4      1   0   0   0   1   0.4
+ 8        2   0       1   1   0.6      0   3   3   0   2   1.6
+ 9        2   1       1   1   1.0      1   0   0       3   0.8
+10        1   2       1   0   0.8      0   1   1       2   0.8
+11        3   1       0   0   0.8      0   0   0       0    0
+12        1   2       0   0   0.6      0   0   0       0    0
+13        0   0       0        0           0   0       0    0
+14        0   0                0
+15        0   0                0
+
+
+The results of the ten-test training as they appear in Table 43 need no
+special comment, for quite similar data have already been examined in
+other connections. In the case of this table it is to be remembered that
+each figure represents the number of errors for a single day as well as
+for a series of ten successive tests. The results of Table 44, on the
+other hand, appear as subdivided series, since each daily series was
+constituted by two series of ten tests, or in all twenty tests.
+
+Finally, in Table 45 I have arranged the results of what may fairly be
+called the continuous training method. In connection with several of the
+labyrinth experiments of Chapter XIII continuous training proved very
+satisfactory. It therefore seemed worth while to ascertain whether the
+same method would not be more efficient than any other for the
+establishment of a white-black discrimination habit. That this method was
+not applied to ten individuals as were the two-five-test, the ten-test,
+and the twenty-test methods is due to the fact that it proved practically
+inadvisable to continue the tests long enough to complete the experiment.
+I have usually designated the method as one hundred or more tests daily. I
+applied this training method first to individuals Nos. 51 and 60. At the
+end of one hundred and twenty tests with each of these individuals I was
+forced to discontinue the experiment for the day because of the approach
+of darkness. In the table the end of a series for the day is indicated by
+a heavy line. The following day Nos. 51 and 60 succeeded in acquiring a
+perfect habit after a few more tests.
+
+
+
+TABLE 45
+
+EFFICIENCY OF TRAINING. WHITE-BLACK TESTS AT THE RATE OF
+100 OR MORE PER DAY
+
+ SETS   51[1]  60   87    Av.
+OF 10
+
+A        5     5     6    5.3
+B        5     3     7    5.0
+
+1        6     6     5    5.7
+2        3     2     5    3.3
+3        5     4     7    5.3
+4        7     4     5    5.3
+5        6     2     3    3.7
+6        1     1     3    1.7
+7        4     2     3    3.0
+8        3     3     0    2.0
+9        2     2     3    2.3
+10       5     0     2    2.3
+11       1     2     2    1.7
+12       2     1     1    1.3
+
+13       4     1     2    2.3
+14       1     2     1    1.3
+15       3     1     5    3.0
+16       3     3     2    2.7
+17       1     0     1    0.7
+18       2     0     1    1.0
+19       0     0     2    0.7
+20       0           0    0
+21       0           1    0.3
+22                   -
+23                   -
+24                   -
+
+[Footnote 1: Age of No. 51, 22 weeks. Age of No. 60, 17 weeks. Age of No.
+87, 8 weeks.]
+
+The results of the continuous training method for these two mice were so
+strikingly different from those yielded by the other methods that I at
+once suspected the influence of some factor other than that of the number
+of tests per day. The ages of Nos. 51 and 60 at the time of their tests
+were twenty-two and seventeen weeks, respectively, whereas all the
+individuals used in connection with the other efficiency tests were four
+weeks of age. It seemed possible that the slow habit formation exhibited
+in the continuous training experiments might be due to the greater age of
+the mice. I therefore selected a healthy active female which was only
+eight weeks old, and tried to train her by the continuous training method.
+With this individual, No. 87, the results were even more discouraging than
+those previously obtained, for she was still imperfect in her
+discrimination at the end of two hundred and ten tests. At that point the
+experiment was interrupted, and it seemed scarcely worth while to continue
+it further at a later date. The evidence of the extremely low efficiency
+of the continuous method in comparison with the other methods which we
+have been considering is so conclusive that further comment seems
+superfluous.
+
+We are now in a position to compare the results of the several methods of
+training which have been applied to the dancer, and to attempt to get
+satisfactory quantitative expressions of the efficiency of each method. I
+have arranged in Table 46 the general averages yielded by the four
+methods. Although these general results hide certain important facts which
+will be exhibited later, they clearly indicate that an increase in the
+number of tests per day does not necessarily result in an increase in the
+rapidity of habit formation. Should we attempt, on superficial
+examination, to interpret the figures of this table, we would doubtless
+say that in efficiency the two-five-test method stands first, the
+continuous-test method last, while the ten-test and twenty-test methods
+occupy intermediate positions.
+
+
+
+TABLE 46
+
+EFFICIENCY OF TRAINING
+
+Number of Errors in White-Black Series for Different Methods of
+Training
+
+SETS OF 10   2 OR 5 TESTS   10 TESTS   20 TESTS    100 OR MORE
+               PER DAY       PER DAY    PER DAY   TESTS PER DAY
+
+A                5.8           5.8        5.9         5.3
+B                5.7           5.7        5.9         5.0
+
+1                6.4           5.3        4.8         5.7
+2                4.2           3.5        4.8         3.3
+3                3.4           3.2        3.4         5.3
+4                3.1           2.5        3.0         5.3
+5                2.7           2.5        1.9         3.7
+6                1.5           1.3        1.9         1.7
+7                0.9           1.5        0.9         3.0
+8                0.7           0.8        1.1         2.0
+9                0.5           0.4        0.9         2.3
+10               0             0.5        0.8         2.3
+11               0             0.4        0.4         1.7
+12               0             0.2        0.3         1.3
+13                             0          0           2.3
+14                             0          0           1.3
+15                             0          0           3.0
+16                                                    2.7
+17                                                    0.7
+18                                                    1.0
+19                                                    0.7
+20                                                    0
+
+
+
+We may now apply to the results of our efficiency-of-training tables the
+method of measuring efficiency which was mentioned at the end of the
+preceding chapter as the _index of modifiability (that number of tests
+after which no errors occur for at least thirty tests)_. By taking the
+average number of tests for the several individuals in each of the Tables
+42, 43, 44, and 45 we obtain the following expressions of efficiency:--
+
+
+
+METHOD               INDEX OF MODIFIABILITY (EFFICIENCY)
+
+Two-five-test                 81.7 ± 2.7
+Ten-test                      88.0 ± 4.1
+Twenty-test                   91.0 ± 5.3
+Continuous-test              170.0 ± 4.8
+
+
+
+Since the difference between the indices for the ten-test and the twenty-
+test methods lies within the limits of their probable errors (±4.1 and
+±5.3) it is evident that it is not significant. Except for this, I think
+these indices may be accepted as indications of real differences in the
+value of the several methods of training.
+
+A somewhat different interpretation of our results is suggested by the
+grouping of individuals according to sex. In Table 47 appear the general
+averages for the males and the females which were tested by the several
+methods. The most striking fact exhibited by this table is that of the
+high efficiency of the twenty-test method for the females. Apparently they
+profited much more quickly by this method than by the ten-test method,
+whereas just the reverse is true of the males. I present the data of this
+table merely to show that general averages may hide important facts.
+
+
+
+TABLE 47
+
+EFFICIENCY OF TRAINING
+
+CONDITION                     MALES                  FEMALES
+                      INDEX OF MODIFIABILITY  INDEX OF MODIFIABILITY
+2 or 5 tests per day              85.0                 80.0
+10 tests per day                  72.0                104.0
+20 tests per day                  94.0                 88.0
+100 or more tests per day        160.0                180.0
+
+
+
+From all considerations that have been mentioned thus far the reader would
+be justified in concluding that I made a mistake in selecting the ten-test
+method for my study of the modifiability of the behavior of the dancer.
+That this conclusion is not correct is due to the time factor in the
+experiments. If the dancer could acquire a perfect habit as a result of
+twelve days' training, no matter whether two, five, ten, or twenty tests
+were given daily, it would, of course, be economical of time for the
+experimenter to employ the two-test method. But if, on the contrary, the
+two-test method required twice as many days' training as the five-test
+method, it would be economical for him to use the five-test method despite
+the fact that he would have to give a larger number of tests than the two-
+test method would have demanded. In a word, the time which the work
+requires depends upon the number of series which have to be given, as well
+as upon the number of tests in each series. As it happens, the ten-test
+method demands less of the experimenter's time than do methods with fewer
+tests per day. The twenty-test method is even more economical of time, but
+it has a fatal defect. It is at times too tiresome for both mouse and man.
+These facts indicate that a balance should be struck between number of
+tests and number of series. The fewer the tests per day, within the limits
+of two and one hundred, the higher the efficiency of the method of
+training, as measured in terms of the total number of tests necessary for
+the establishment of a perfect habit, and the lower its efficiency as
+measured in terms of the number of series given. The greater the number of
+tests per day, on the other hand, the higher the efficiency of the method
+in terms of the number of series, and the lower its efficiency in terms of
+the total number of tests. By taking into account these facts, together
+with the fact of fatigue, we are led to the conclusion that ten tests per
+day is the most satisfactory number.
+
+If my time and attention had not been fully occupied with other problems,
+I should have determined the efficiency of various methods of training in
+terms of the duration of habit, as well as in terms of the rapidity of its
+formation. As these two measures of efficiency might give contradictory
+results, it is obvious that a training method cannot be fairly evaluated
+without consideration of both the rapidity of habit formation and the
+permanency of the habit. A _priori_ it seems not improbable that slowness
+of learning should be directly correlated with a high degree of
+permanency. By the further application of the method which I have used in
+this study of the efficiency of training we may hope to get a definite
+answer to this and many other questions concerning the nature of the
+educative process and the conditions which influence it.
+
+
+
+
+CHAPTER XVI
+
+THE DURATION OF HABITS: MEMORY AND RE-LEARNING
+
+The effects of training gradually disappear. Habits wane with disuse. In
+the dancer, it is not possible to establish with certainty the existence
+of memory in the introspective psychological sense; but it is possible to
+measure the efficiency of the training to which the animal is subjected,
+and the degree of permanency of habits. The materials which constitute
+this chapter concern the persistence of unused habits, and the influence
+of previous training on the re-acquisition of a habit which has been lost
+or on the acquisition of a new habit. For convenience of description, I
+shall refer to certain of the facts which are to be discussed as facts of
+memory, with the clear understanding that consciousness is not necessarily
+implied. By memory, wherever it occurs in this book, I mean the ability of
+the dancer to retain the power of adaptive action which it has acquired
+through training.
+
+I first discovered memory in the dancer, although there was previously no
+reason for doubting its existence, in connection with the ladder-climbing
+tests of Chapter XII. In this experiment two individuals which had
+perfectly learned to escape from the experiment box to the nest-box by way
+of the wire ladder, when tested after an interval of two weeks, during
+which they had remained in the nest-box without opportunity to exercise
+their newly acquired habit, demonstrated their memory of the method of
+escape by returning to the nest-box by way of the ladder as soon as they
+were given opportunity to do so. As it did not lend itself readily to
+quantitative study, no attempts were made to measure the duration of this
+particular habit. At best the climbing of a wire ladder is of very
+uncertain value as an indication of the influence of training.
+
+Similarly, the persistence of habits has been forced upon my attention day
+after day in my various experiments with the mice. It is obvious, then,
+that the simple fact of memory is well established, and that we may turn
+at once to an examination of the facts revealed by special memory and re-
+learning experiments.
+
+The visual discrimination method, which proved invaluable as a means of
+measuring the rapidity of habit formation, proved equally serviceable in
+the measurement of the permanency or duration of habits. Memory tests for
+discrimination habits were made as follows. After a dancer had been
+trained in the discrimination box so that it could choose the correct
+electric-box, white, red, blue, or green as it might be, in three
+successive daily series of ten tests each, it was permitted to remain for
+a certain length of time without training and without opportunity to
+exercise its habit of visual discrimination and choice. At the expiration
+of the rest interval, as we may designate the period during which the
+habit was not in use, the mouse was placed in the discrimination box under
+precisely the same conditions in which it had been trained and was given a
+series of ten memory tests with the box to be chosen alternately on the
+right and on the left. In order that the entire series of ten tests, and
+sometimes two such series given on consecutive days, might be available as
+indications of the duration of a habit, the mouse was permitted to enter
+and pass through either of the electric-boxes without receiving a shock.
+Had the shock been given as punishment for a wrong choice, it is obvious
+that only the first test of the memory series would be of value as an
+indication of the existence of a previously acquired habit. Even under the
+conditions of no shock and no stop or hindrance the first test of each
+memory series is of preeminent importance, for the mouse tends to persist
+in choosing either the side or the visual condition (sometimes one,
+sometimes the other) which it chooses in the first test. If the wrong box
+is chosen to begin with, mistakes are likely to continue because of the
+lack of punishment; in this case the animal discriminates, but there is no
+evidence that it remembers the right box. Likewise, if the right electric-
+box is chosen in the first test, correct choices may continue simply
+because the animal has discovered that it can safely enter that particular
+box; again, the animal discriminates without depending necessarily upon
+its earlier experience. I have occasionally observed a series of ten
+correct choices, made on the basis of an accidental right start, followed
+by another series in which almost every choice was wrong, because the
+animal happened to start wrong.
+
+As the results of my tests of memory are of such a nature that they cannot
+advantageously be averaged, I have arranged in Table 48 a number of
+typical measurements of the duration of visual discrimination habits. In
+this table I have indicated the number and age of the individual tested,
+the habit of discrimination which had been acquired, the length of the
+rest interval, the result of the first test (right or wrong), and the
+number of errors made in each series of ten memory tests.
+
+
+
+TABLE 48
+
+MEASUREMENTS OF THE DURATION OF A HABIT
+
+Memory
+
+                                                          ERRORS
+NO.     AGE     NAME OF TEST        REST       FIRST   FIRST   SECOND
+                                    INTERVAL   CHOICE  SERIES  SERIES
+
+1000  25 weeks  White-black         4 weeks    Right     0
+   5  27        White-black         4          Right     5       7
+ 210  15        White-black         8          Right     5
+ 220  15        White-black         8          Right     4
+ 230  15        White-black         8          Wrong     5
+ 215  15        White-black         8          Right     5
+ 225  15        White-black         8          Right     2
+ 235  15        White-black         8          Right     7
+ 410  15        White-black         8          Wrong     4
+ 415  15        White-black         8          Wrong     6
+ 420  15        White-black         8          Wrong     3
+ 425  15        White-black         8          Right     3
+   2  28        Black-white         4          Wrong     9
+   7  17        Black-white         2          Wrong     1
+   7  21        Black-white         6          Right     1
+   7  27        Black-white        10          Right     1       6
+ 998  18        Black-white         2          Wrong     3
+ 998  22        Black-white         4          Right     0
+ 998  28        Black-white        10          Right     5       5
+  13  10        Black-white         4          Right     3
+  14  10        Black-white         4          Right     3
+  15  10        Black-white         4          Right     2
+  16  10        Black-white         4          Right     4
+1000  25        Light blue-orange   4          Right     4
+   2  28        Light blue-orange   2          Wrong     5
+   5  28        Light blue-orange   6          Wrong     4       6
+   3  25        Light blue-orange   4          Wrong     8
+  10  24        Light blue-orange   2          Right     8
+  10  26        Light blue-orange   2          Right     5
+  11  25        Light blue-orange   2          Right     6
+  11  27        Light blue-orange   2          Wrong     5
+ 151  13        Green-red           2          Right     1       0
+ 152  13        Green-red           2          Right     5       1
+
+
+
+This quantitative study of the duration of simple habits of choice showed
+that in the majority of cases a perfectly acquired habit persists for at
+least two weeks. To be perfectly fair to the animal I must restrict this
+statement to visual conditions other than colors, for the dancer exhibited
+little ability either to acquire or to retain a habit of distinguishing
+spectral colors. Altogether, I made a large number of white-black and
+black-white memory tests after rest intervals of four, six, eight, or ten
+weeks. The results for the four-week interval show extreme individual
+differences in memory. Number 1000, for example, was able to choose
+correctly every time in a series of white-black tests after a rest
+interval of four weeks, whereas No. 5 was wrong as often as she was right
+after the same interval. I have placed the results for these two
+individuals at the head of the table because they suggest the variations
+which render averages undesirable. Number 1000 had a perfect habit at the
+end of four weeks of disuse; No. 5 had no habit whatever. I shall reserve
+further discussion of age, sex, and individual differences in the
+permanency of habits for the next chapter.
+
+With Nos. 7 and 998 memory tests were made after three different rest
+intervals. At the end of two weeks the black-white habit was present in
+both individuals, although it was not perfect. After six and four weeks,
+respectively (see Table 48), it still persisted; in fact, it apparently
+had improved as the result of additional training after the earlier memory
+tests. At the expiration of ten weeks it had wholly disappeared. In her
+first series of memory tests after the ten-week interval No. 7 made only
+one error, but a chance choice of the black (right) in the first test and
+the subsequent choice of the box in which no shock had been received serve
+to account for results which at first appear to be indicative of memory.
+That this explanation is correct is proved by the fact that a second
+memory series, in which the first choice happened to be wrong, resulted in
+six mistakes. Evidently she had lost the habit.
+
+In no instance have memory tests definitely indicated the presence of a
+habit after a rest interval of more than eight weeks. It is safe,
+therefore, to conclude from the results which have been obtained that a
+white-black or black-white discrimination habit may persist during an
+interval of from two to eight weeks of disuse, but that such a habit is
+seldom perfect after more than four weeks.
+
+The measurements of memory which were made in connection with color
+discrimination experiments are markedly different from those which were
+obtained in the brightness tests. As might have been anticipated (?), in
+view of the extreme difficulty with which the dancer learns to
+discriminate colors, the habit of discriminating between qualitatively
+different visual conditions does not persist very long. I have never
+obtained evidence of a perfect habit after an interval of more than two
+weeks, and usually, as is apparent from Table 48, the tests indicated very
+imperfect memory at the end of that interval. It seems probable that even
+in these so-called color tests discrimination is partly by brightness
+difference, and that the imperfection of the habit and its short duration
+are due to the fact that the basis of discrimination is inadequate. This
+is the only explanation which I have to offer for the difference which has
+been demonstrated to exist between the duration of brightness
+discrimination habits and color discrimination habits.
+
+The duration of a discrimination habit having been measured with a fair
+degree of accuracy, I undertook the task of ascertaining whether training
+whose results have wholly disappeared, so far as memory tests are in
+question, influences the re-acquisition of the same habit. Can a habit be
+re-acquired with greater facility than it was originally acquired? Is re-
+learning easier than learning? To obtain an answer to the question which
+may be asked in these different forms, ten individuals were experimented
+with in accordance with a method whose chief features are now to be
+stated. In each of these ten individuals a perfect white-black habit was
+established by the use of the standard series of tests the order of which
+is given in Table 12. At the expiration of a rest interval of eight weeks
+precisely the same series of tests were repeated as memory and re-training
+tests. In this repetition, the preliminary series, _A_ and _B_, served as
+memory tests, and the subsequent training series, as re-training series.
+
+[Illustration: FIGURE 32.--Error curves plotted from the data given by ten
+dancers in white-black discrimination tests. The solid line ([Symbol:
+solid line]) is the error curve of the original learning process; the
+broken line (------) is that of the re-learning process, after an interval
+of eight weeks.]
+
+The striking results of this investigation of re-learning are exhibited in
+the curves of learning and re-learning of Figure 32. These curves make it
+appear that the mice re-acquired the white-black discrimination habit much
+more readily than they had originally acquired it. But in addition to
+furnishing the basis for some such statement as the foregoing, the curves
+suggest a serious criticism of the experiment.
+
+In the original tests, the preliminary series indicated a strong
+preference for black. In series _A_ it was chosen on the average 5.8 times
+in 10, and in series _B_, 5.7 times. This preference was rapidly overcome
+by the training series, and at the end of 130 tests discrimination was
+perfect. All this appears in the curve of learning (solid line of figure).
+On the other hand, these preliminary series when repeated as memory tests,
+after a rest-interval of eight weeks, gave markedly different results.
+Series _A_ indicated preference for white (5.6 times in 10) instead of
+black, and series _B_ indicated only a slight preference for black. In
+brief, series _A_ and _B_ show that the preference for black was
+considerably stronger at the beginning of the training than at the
+beginning of the re-training.
+
+In the light of these facts it is fair to claim that the effects of the
+white-black training had not wholly disappeared as the result of eight
+weeks of rest, and that the experiment therefore fails to furnish
+satisfactory grounds for the statement that re-learning occurs more
+rapidly than learning. I accept this criticism as pertinent, although not
+necessarily valid, and at the same time I freely admit that the results
+have a significance which I had not anticipated. But they are not less
+interesting or valuable on that account. Granting, then, that at least
+some of the ten individuals which took part in the experiment had not
+completely lost the memory of their white-black training at the end of
+eight weeks, it is still possible that an examination of the individual
+results may justify some conclusion concerning the question which was
+proposed at the outset of the investigation. Such an examination is made
+possible by Tables 49 and 50, in which I have arranged separately the
+results for the males and the females.
+
+
+
+
+TABLE 49
+
+WHITE-BLACK TRAINING. TEN TESTS PER DAY
+
+
+                               Males
+                TRAINING                    RETRAINING
+         210 220 230 410 420  AV.     210 220 230 410 420  AV.
+
+
+ A        6   5   6   6   6   5.8      5   4   5   4   3   4.2
+ B        6   8   8   5   1   5.6      8   4   5   4   6   5.4
+
+ 1        6   7   6   2   4   5.0      3   3   4   7   3   4.0
+ 2        4   3   1   2   3   2.6      2   4   2   5   3   3.2
+ 3        3   1   4   3   4   3.0      1   4   1   4   1   2.2
+ 4        5   0   3   3   2   2.6      0   1   0   1   2   0.8
+ 5        3   0   4   1   4   2.4      0   2   0   2   0   0.8
+ 6        2   1   4   0   1   1.6      0   1   0   0   2   0.6
+ 7        1   0   3   1   0   1.0          0       0   0   0
+ 8        0   0   1   0   0   0.2          0       0   1   0.2
+ 9        0   0   0   1   0   0.2          0           0   0
+10        0       0   0       0                        1   0.2
+11                0   0       0                        0   0
+12                    0       0                        0   0
+13                                                     0   0
+14
+15
+
+
+
+Only three of the ten individuals failed to re-acquire the habit of white-
+black discrimination more quickly than it had originally been acquired,
+and, in the case of these exceptions, No. 220 required exactly the same
+number of tests in each case, and No. 420 was placed at a slight
+disadvantage in the re-learning series by an interruption of the training
+between the seventh and the eighth series. Had his training been completed
+by the sixth series he too would have had the same number of tests in
+training and re-training. Moreover, and this is of preëminent importance
+for a fair interpretation of the results, in several instances even those
+individuals which exhibited as strong a preference for the black in the
+memory series as in the preliminary series re-learned more quickly than
+they had learned. Number 210, for example, although he gave no evidence of
+memory, and, in fact, chose the black more frequently in the memory series
+than he did in the preliminary series, re-acquired the discrimination
+habit in less than half the number of tests which had been necessary for
+the establishment of the habit originally.
+
+
+
+                             TABLE 50
+
+             WHITE-BLACK TRAINING. TEN TESTS PER DAY
+
+                             Females
+
+               TRAINING                    RE-TRAINING
+   215  225  235  415  425  Av.   215  225  235  415  425  Av.
+ A  8    4    4    8    5   5.8    5    2    7    6    3   4.6
+ B  8    7    6    6    2   5.8    8    5    6    4    3   5.2
+ 1  7    6    5    6    4   5.6    4    1    5    4    3   3.4
+ 2  5    6    4    2    5   4.4    1    1    1    2    3   1.6
+ 3  3    3    4    3    4   3.4    1    0    3    6    0   2.0
+ 4  2    1    3    3    3   2.4    0    0    3    3    1   1.4
+ 5  1    3    3    3    3   2.6    0    0   died  2    0   0.5
+ 6  2    1    1    1    0   1.0    0              1    0   0.2
+ 7  1    1    2    3    3   2.0                   0    0   0
+ 8  0    0    2    2    3   1.4                   1        0.2
+ 9  1    0    0    1    1   0.6                   0        0
+10  0    2    1    0    2   1.0                   0        0
+11  0    3    0    1    0   0.8                   0        0
+12  0    0    0    2    0   0.4
+13       0    0    0    0   0
+14       0         0        0
+15                 0        0
+
+
+
+The facts which have been presented thus far become more significant when
+the indices of modifiability for the learning and the re-learning
+processes are compared.
+
+
+                INDICES OF MODIFIABILITY
+
+                                    LEARNING   RE-LEARNING
+
+Females   .   .   .   .   .   .   .    104         42.5
+Males .   .   .   .   .   .   .   .     72         54
+
+
+The behavior of the mice in the experiments, the detailed results of
+Tables 49 and 50, and the indices of modifiability together justify the
+following conclusions. Most of the ten dancers, at the end of a rest
+interval of eight weeks, had so far lost the habit of white-black
+discrimination that memory tests furnished no conclusive evidence of the
+influence of previous training; a few individuals seemed to possess traces
+of the habit after such an interval. In the case of each group of
+individuals re-training brought about the establishment of a perfect habit
+far more quickly than did the original training. This suggests the
+existence of two kinds or aspects of organic modification in connection
+with training; those which constitute the basis of a definite form of
+motor activity, and those which constitute the bases or dispositions for
+the acquirement of certain types of behavior. There are several
+indications that further study of the modifiability of behavior will
+furnish the facts which are necessary to render this suggestion
+meaningful.
+
+Closely related to the facts which have been revealed by the re-training
+experiments are certain results of the labyrinth experiments. For the
+student of animal behavior, as for the human educator, it is of importance
+to learn whether one kind of training increases the efficiency of similar
+forms of training. Can a dancer learn a given labyrinth path the more
+readily because it has previously had experience in another form of
+labyrinth?
+
+The answer to this question, which my experimental results furnish, is
+given in Table 51. In the upper half of the table have been arranged the
+results for six individuals which were trained first in labyrinth B, then
+in labyrinth C, and finally in labyrinth D. Below, in similar fashion, are
+given the results for six individuals which were trained in the same three
+labyrinths in the order C, B, D, instead of B, C, D. My purpose in giving
+the training in these two orders was to ascertain whether labyrinth C,
+which had proved to be rather difficult for most individuals, would be
+more easily learned if the training in it were preceded by training in
+labyrinth C.
+
+
+
+TABLE 51
+
+THE INFLUENCE OF ONE LABYRINTH HABIT UPON THE FORMATION
+OF ANOTHER
+         LABYRINTH B             LABYRINTH C             LABYRINTH D
+     NO. OF      NO. OF      NO. OF      NO. OF      NO. OF   NO. OF
+NO.  FIRST COR-  LAST OF     FIRST COR-  LAST OF     FIRST COR- LAST OF
+     RECT TEST   FIVE COR-   RECT TEST   FIVE COR-   RECT TEST  FIVE COR-
+                 RECT TESTS              RECT TESTS             RECT TESTS
+
+76       8          14           3          19           4          7
+78       5          20           6          14           4          5
+86      13          22           5          12           3          9
+75       4          15           8          19           4         13
+77       7          11          11          29          11         12
+87      12          22           9          20           4          9
+
+AV.      8.2        17.3         7.0        18.8         5.0        9.2
+
+
+         LABYRINTH C             LABYRINTH B             LABYRINTH D
+
+58      16          --           2          14           7         10
+60      17          --          13          37          10         14
+88      25          35           9          22           4          8
+49      34          --           1           5           7          8
+57      15          --           3          20           3          6
+85      11          18           2          11           3          4
+
+AV.     19.7        26.5         5.0        18.2         5.7        8.3
+
+
+
+The results are sufficiently definite to warrant the conclusion that
+experience in B rendered the learning of C easier than it would have been
+had there been no previous labyrinth training. Those individuals whose
+first labyrinth training was in C made their first correct trip as the
+result of 19.7 trials, whereas those which had previously been trained in
+labyrinth B were able to make a correct trip as the result of only 7.0
+trials. Similarly the table shows that training in C rendered the
+subsequent learning of B easier. To master B when it was the first
+labyrinth required 8.2 trials; to master it after C had been learned
+required only 5 trials. In addition to proving that the acquisition of one
+form of labyrinth habit may facilitate the acquisition of others,
+comparison of the averages of Table 51 furnishes evidence of the truth of
+the statement that no results of training can be properly interpreted in
+the absence of knowledge of the previous experience of the organism.
+
+
+
+
+CHAPTER XVII
+
+
+INDIVIDUAL, AGE, AND SEX DIFFERENCES IN BEHAVIOR
+
+All dancers are alike in certain important respects, but to the trained
+observer of animal behavior their individual peculiarities are quite as
+evident, and even more interesting than their points of resemblance.
+Omitting consideration of the structural marks of individuality, we shall
+examine the individual, age, and sex differences in general behavior,
+rapidity of learning, memory, and discrimination, which have been revealed
+by my experiments. Observations which bear on the subject of differences
+are scattered through the preceding chapters, but in no case have they
+been given sufficient prominence to force them upon the attention of those
+who are not especially interested in individual peculiarities. It has
+seemed worth while, therefore, to assemble all the available material in
+this chapter for systematic examination and interpretation.
+
+In the pages which follow, individual, age, and sex peculiarities are
+discussed in turn. Within each of these three groups of differences I have
+arranged in order what Royce has appropriately named the facts of
+discriminating sensitiveness, docility, and initiative. Individuals of the
+same age and sex no less than those which differ in sex or age exhibit
+important differences in ability to discriminate among sense impressions
+("discriminative sensitiveness"), in ability to profit by experience
+("docility"), and in ability to try new kinds of behavior ("initiative").
+
+Individual differences in sensitiveness to visual, auditory, tactual, and
+olfactory stimuli have been revealed by many of my experiments. The
+brightness discrimination tests conclusively proved that a degree of
+difference in illumination which is easily detectable by one dancer may be
+beyond the discriminating sensitiveness of another. Both the tests with
+gray papers and those with the Weber's law apparatus furnished striking
+evidence of individual differences in the kind of visual sensitiveness
+which throughout this book has been called brightness vision. I suspect
+that certain of the differences which were observed should be referred to
+the experience of the individuals rather than to the capacity of the
+visual organs, for training improves visual discrimination to a much
+greater extent than would ordinarily be thought possible. To the truth of
+this statement the results of the Weber's law experiments with No. 51 bear
+witness. Likewise in color discrimination there are individual
+differences, examples of which may be discovered by the examination of the
+results given in Chapters IX and X.
+
+No differences in auditory sensitiveness appeared in my adult dancers, for
+in none of them was there definite response to sounds, but among the young
+individuals differences were prominent. I may call attention to the data
+on this subject which Table 5, p. 89, contains. The mice in four out of
+twelve litters gave no indications of hearing any sounds that I was able
+to produce; the remaining individuals responded with varying degrees of
+sensitiveness. I made no attempt to measure this sensitiveness, but it
+obviously differed from mouse to mouse. I feel justified, therefore, in
+stating that the young dancers exhibit extreme individual difference in
+sensitiveness to sounds.
+
+My observations of differences in sensitiveness to other forms of
+stimulation were made in connection with training tests, and although they
+are not quantitative, I venture to call attention to them. Indeed, I am
+led by the results of my study of various aspects of the dancer's behavior
+to conclude that the race exhibits individual differences in
+discriminating sensitiveness to a far greater extent than do most mammals,
+not excepting man. The importance of this fact (for I am confident that
+any one who carefully examines the detailed results of the various
+experiments which are described in this book will agree that it is an
+established fact) cannot be overlooked. It alters our interpretation of
+the results of training, memory, heredity, and discrimination experiments,
+and it leads us to suspect that the dancing race is exceedingly unstable.
+I do not venture to make comparison of my own observations of the dancer's
+sense equipment with those of Cyon, Rawitz, Zoth, and Kishi, for the
+differences are too great in many instances to be thought of as other than
+species or variety peculiarities. It has seemed fairer to compare only
+individuals of the same breed, or, as I have done and shall continue to do
+throughout this chapter, of two lines of descent.
+
+With respect to docility individual differences are prominent. We need
+only turn to the various tables of results to discover that in
+modifiability of behavior, in memory, in re-learning, not to mention other
+aspects of docility, dancers of the same sex and age differed strikingly.
+Let me by way of illustration cite a few cases of difference in docility.
+Number 1000 learned to discriminate white from black more quickly and
+retained his habit longer than any other dancer with which I have
+experimented. I should characterize him as an exceptionally docile
+individual. Table 44 offers several examples. Numbers 403 and 407, though
+they were born in the same litter and were alike in appearance and in
+conditions of life, acquired the white-black habit with a difference in
+rapidity which is expressed by the indices of modifiability 50 and 100. In
+other words, it took No. 407 twice as long to acquire this habit as it
+took No. 403. Similarly the ladder-climbing tests revealed important
+individual differences in ability to profit by experience. In the tables
+of labyrinth tests (38, 39, 40) individual differences are too numerous to
+mention. It required forty-nine tests to establish in No. 50 a labyrinth-C
+habit which was approximately equal in degree of perfection to that which
+resulted from twenty-two tests in the case of No. 52. The figures in this
+and other instances do not exaggerate the facts, for repeatedly I have
+tested individuals of the same litter, the same sex, and, so far as I
+could judge, of the same stage of development, and obtained results which
+differ as markedly as do those just cited. If space limits permitted, I
+could present scores of similar differences in docility which the problem,
+labyrinth, and discrimination methods have revealed.
+
+In examining the detailed individual results of the various tables for
+differences of this sort, it is important to bear in mind that sex, age,
+and descent should be taken into account, for with each of them, as will
+be shown clearly later in this chapter, sensitiveness, docility, and
+initiative vary. I have therefore based my statements concerning
+individual differences in docility upon the results of comparison of mice
+of the same litter, sex, and age. It is safe to say that human beings
+similarly selected for comparison do not exhibit greater differences in
+ability to profit by experience than did these dancing mice.
+
+The facts concerning individual differences in initiative which I have
+discovered are not less definite than those of the preceding paragraphs.
+From the beginning of my study of the dancer I observed that what one
+individual would readily learn of his own initiative another never
+learned. For example, in the ladder-climbing experiment No. 1000
+distinguished himself for his initiative, whereas Nos. 4 and 5 never
+acquired the habit of escaping from confinement by using the ladder. I
+noticed, in this test of the animal's ability to learn, that while one
+individual would be scurrying about trying all ways of escape,
+investigating its surroundings, looking, sniffing, and dancing by turns,
+another would devote all its time to whirling, circling, or washing
+itself. One in the course of its activity would happen upon the way of
+escape, the other by reason of the limited scope of its activity, not the
+lack of it, would fail hour after hour to discover even the simplest way
+of getting back to its nest, to food, and to its companions. Hundreds of
+times during the past three years I have noticed important individual
+differences in initiative in connection with the discrimination
+experiments. The swinging wire doors which one dancer learned to push open
+before he had been in the box five minutes, another might not become
+familiar with through his own initiative for hours or days. In fact, it
+was not seldom that I had to teach an individual to pass from one
+compartment to the other by gently pushing him against the door until it
+opened sufficiently to allow him to squeeze through. Occasionally a mouse
+learned to pull the doors open so that he could pass through the openings
+in either direction with facility. This was a form of individual
+initiative which I had not anticipated and did not especially desire, so I
+did not encourage its development, but, nevertheless, at least one fourth
+of the mice which I experimented with in the discrimination box learned
+the trick. The other three fourths, although they were used in the box day
+after day sometimes for weeks, never discovered that they might return to
+the nest-box by pulling the swing-door through which they had just passed
+as well as by entering one of the electric-boxes.
+
+Another indication of individual initiative in action appeared in the
+tendency of certain mice to climb out of the experiment boxes or
+labyrinths. It would have been extremely easy for any of the mice to
+escape from the labyrinths by scaling the walls of the alleys, for they
+were only 10 cm. in height, and when a dancer stood on its hind legs it
+could easily reach the top with its nose. But, strange though it will seem
+to any one who has not worked with the dancer, not more than one in ten of
+the animals which I observed made any attempt to escape in this manner.
+They lacked initiative. That it was not due to a lack of the power to
+climb, I abundantly demonstrated by teaching a few individuals that a
+scramble in one corner meant easy escape from the maze of paths. I do not
+think any one of the mice was physically incapable of climbing, but I am
+confident that they differed markedly, not only in the willingness to try
+new modes of action, but in the readiness with which they could climb. I
+have already said that individuals differ noticeably in the scope of their
+activity. By this statement I mean that they try a varying number of kinds
+of activity. As in the case of men, so in mice, one individual will do a
+greater number of things in a few hours than another will in weeks or
+months. The dancers differ in versatility, in individual initiative, as do
+we, albeit not so markedly.
+
+Important differences which may with certainty be described as age
+differences are not so obvious as are such marks of individuality as have
+been set forth in the preceding pages. I have noted few changes in
+discriminative sensitiveness, other than those with regard to auditory
+sensitiveness, which could be correlated with age. In certain instances
+adults appeared to be able to discriminate more accurately and more easily
+than young mice, but it is difficult to say whether this change belongs
+under sensitiveness or docility. I have not made an ontogenetic study of
+the senses, and I am therefore unable to describe in detail the course of
+their development and decline. Of one important fact I am certain, that
+discriminative sensitiveness increases up to a certain point with age and
+with training.
+
+Differences in docility which are obviously to be correlated with age
+abound. In the prime of its life (from the second to the tenth month) the
+dancer is active, full of energy, quick to learn; in its senility (during
+the second year) it is inactive, but at times even more docile than during
+the period of greatest physical development. Frequently I have noticed in
+connection with labyrinth tests that individuals of the age of a year or
+more learn much more quickly than do individuals of the age of two or
+three months. But, on the other hand, I have contradictory observations,
+for now and then I obtained just the opposite result in experiments to
+test docility. Evidently this is a matter which demands systematic,
+quantitative investigation. Casual observation may suggest conclusions,
+but it will not justify them.
+
+Early in my investigation of the behavior of the dancer I conceived the
+idea of determining the relation of modifiability of behavior (docility)
+to age. The question which was foremost in my mind and for which I first
+sought an answer may be stated thus: can the dancer acquire a given habit
+with the same facility at different ages? Since the visual discrimination
+experiment seemed to be well suited for the investigation of this problem
+I planned to train, in the white-black discrimination experiment, five
+pairs of dancers at the age of one month, and the same number for each of
+the ages four, seven, ten, thirteen, sixteen, and nineteen months.[1]
+
+[Footnote 1: I have not been able thus far to determine the average length
+of the dancer's life. The greatest age to which any of my individuals has
+attained is nineteen months.]
+
+To test the same individuals month after month would be the ideal way of
+obtaining an answer to our question, but I could devise no satisfactory
+way of doing this. The effects of training last so long, as the results of
+the previous chapter proved, and the uncertainty of their entire
+disappearance is so serious, that the same training process cannot be used
+at successive ages. The use of different methods of training is even more
+unsatisfactory because it is extremely difficult to make accurate
+quantitative comparison of their results. It was these considerations that
+forced me to attempt to discover the relation of docility to age by
+carrying out the same experiments with groups of individuals of different
+ages.
+
+As my plan involved the execution of precisely the same set of tests with
+at least seventy individuals whose age, history, and past experience were
+accurately known, and of which some had to be kept for nineteen months
+before they could be trained, the amount of labor and the risk of mishap
+which it entailed were great. To make possible the completion of the
+investigation within two years, I accumulated healthy individuals for
+several months without training any of them. In March, 1907, I had
+succeeded in completing the tests for the age of one month, and I had on
+hand for the remaining tests almost a hundred individuals, whose ages
+ranged from a few days to eighteen months. Had everything gone well, the
+work would have been finished within six months. Suddenly, and without
+discoverable external cause, my mice began to die of an intestinal
+trouble, and despite all my efforts to check the disease by changing food
+supply and environment, all except a single pair died within a few weeks.
+Thus ended a number of experiments whose final results I had expected to
+be able to present in this volume. However, the work which I have done is
+still of value, for the single pair of survivors have made possible the
+continuance of my tests with other individuals of the same line of descent
+as those which perished, and I have to regret only the loss of time and
+labor.
+
+As I have on hand results for ten individuals of the age of one month, and
+for four individuals of the age of four months, it has seemed desirable to
+state the problem, method, and incomplete results of this study of the
+relation of modifiability to age. The indices of modifiability for these
+two groups of dancers differ so strikingly that I feel justified in
+persisting in my efforts to obtain comparable data for the seven ages
+which have been mentioned.
+
+
+
+                          TABLE 52
+
+       PLASTICITY (RELATION or MODIFIABILITY TO AGE)
+
+Number of Errors in Successive Daily Series of Ten White-Black
+            Tests, with Dancers Four Months Old
+
+
+
+  SERIES     MALES                 FEMALES
+
+          NO. 76 NO. 78   AV.   NO. 75 NO. 77   AV.  GENERAL AV.
+
+    A       7      7      7.0     4      8     6.0     6.50
+    B       8      6      7.0     6      5     5.5     6.25
+
+    1       5      5      5.0     5      5     5.0     5.00
+    2       5      4      4.5     2      2     2.0     3.25
+    3       4      5      4.5     2      5     3.5     4.00
+    4       3      4      3.5     1      1     1.0     2.25
+    5       5      2      3.5     0      1     0.5     2.00
+    6       3      2      2.5     1      0     0.5     1.50
+    7       2      1      1.5     1      2     1.5     1.50
+    8       5      1      3.0     0      0     0       1.50
+    9       1      3      2.0     0      0     0       1.00
+   10       1      2      1.5     1      0     0.5     1.00
+   11       1      1      1.0     0            0       0.50
+   12       1      1      1.0     0            0       0.50
+   13       0      0      0       0            0       0
+   14       0      0      0                            0
+   15       0      0      0                            0
+
+
+
+[Illustration: FIGURE 33.--Plasticity curves. In the left margin are given
+the indices of modifiability (the number of tests necessary for the
+establishment of a perfect habit). Below the base line the age of the
+individuals is given in months. Curve for males, --•--•--•--; curve for
+females, - - - -; curve for both males and females,----. When these three
+plasticity curves are completed, they will represent the indices of
+modifiability as determined for ten individuals at the age of 1 month, and
+similarly for the same number of individuals at each of the ages, 4, 7,
+10, 13, 16, and 19 months.]
+
+The detailed results for the one-month old individuals appear in Table 43;
+those for the four-month individuals in Table 52. The general averages for
+the former are to be found in the third column of Table 46, under the
+heading "10 tests per day"; those for the latter in the last column of
+Table 52. Mere inspection of these tables reveals the curious sex
+difference which goes far towards justifying the presentation of this
+uncompleted work. The index of modifiability for the ten one-month
+individuals is 88 (that is, 88 tests were necessary for the establishment
+of a habit); for the four-month individuals it is 102.5. The heavy solid
+line of Figure 33 joins the points on the ordinates at which these values
+are located. Apparently, then, the dancer acquires the white-black
+discrimination habit less readily at the age of four months than at the
+age of one month.
+
+Further analysis of the results proves that this statement is not true.
+When the averages for the two sexes are compared, it appears that the
+males learned much less quickly at four months than at one month, whereas
+just the reverse is true of the females. The dash and dot line of the
+figure extends from the index of modifiability of the one-month males (72)
+to that of the four-month males (120); and the regularly interrupted line
+similarly joins the indices of the one-month (104) and the four-month (85)
+females. In seeking to discover age differences in docility or ability to
+profit by experience we have stumbled upon what appears to be an important
+sex difference. Perhaps I should add to this presentation of partial
+results the following statement. Since there are only four individuals in
+the four-month group, two of each sex, the indices are not very reliable,
+and consequently too much stress should not be laid upon the age and sex
+differences which are indicated.
+
+In view of this impressive instance of the way in which averages may
+conceal facts and lead the observer to false inferences, I wish to remark
+that my study of the dancer has convinced me of the profound truth of the
+statement that the biologist, whether he be psychologist, anthropologist,
+physiologist, or morphologist, should work with the organic individual and
+should first of all deal with his results as individual results. Averages
+have their place and value, but to mass data before their individual
+significance has been carefully sought out is to conceal or distort their
+meaning. Too many of us, in our eagerness for quantitative results and in
+our desire to obtain averages which shall justify general statements, get
+the cart before the horse.
+
+Figure 33 presents the beginning of what I propose to call plasticity
+curves. When these three curves are completed on the basis of experiments
+with five dancers of each sex for each of the ages indicated on the base
+line of the figure, they will indicate what general changes in plasticity,
+modifiability of behavior, or ability to learn (for all of these
+expressions have been used to designate much the same capacity of the
+organism) occur from the first month to the nineteenth in the male and the
+female dancer, and in the race without respect to sex. So far as I know,
+data for the construction of plasticity curves such as I hope in the near
+future to be able to present for the dancing mouse have not been obtained
+for any mammal.
+
+At present it would be hazardous for me to attempt to state any general
+conclusion concerning the relation of docility to age.
+
+The initiative of the dancer certainly varies with its age. In scope the
+action system rapidly increases during the first few months of life, and
+if the animal be subjected to training tests, this increase may continue
+well into old age. The appearance of noticeable quiescence does not
+necessarily indicate diminished initiative. Frequently my oldest mice have
+shown themselves preëminent in their ability to adjust their behavior to
+new conditions. However, I have not studied individuals of more than
+eighteen months in age. One would naturally expect initiative to decrease
+in senility. All that I can say is that I have seen no indications of it.
+
+We may now briefly consider the principal sex differences which have been
+revealed by the experiments. In sensitiveness I have discovered no
+difference, but it should be stated that no special attention has been
+given to the matter. In docility the males usually appeared to be superior
+to the females. This was especially noticeable early in my visual
+discrimination tests. The males almost invariably acquired a perfect habit
+quicker than the females. I may cite the following typical instances.
+Number 14 acquired the black-white habit with 40 tests; No. 13, with 60
+(Table 10, p. 109). Of the five pairs of individuals whose records in
+white-black training appear in Table 43, not one contradicts the statement
+which has just been made. It is to be noted, however, that under certain
+conditions of training, for example, 20 tests per day, the female is at an
+advantage. Recently I have with increasing frequency obtained measures of
+docility which apparently favor the female. That this difference in the
+results is due to a difference in age is probable.
+
+In labyrinth tests the female is as much superior to the male as the male
+is to the female in discrimination tests. From the tables of Chapter XIII
+I may take a few averages to indicate the quantitative nature of this
+difference. A degree of proficiency in labyrinth B attained by the males
+after 7.0 trials was equaled by the females after 6.2 trials. In labyrinth
+C the males acquired a habit as a result of 18.7 trials; the females, as a
+result of 13.8. And similarly in labyrinth D, 6.1 trials did no more for
+the males than 5.9 did for the females.
+
+That at the age of about one month the male dancer should be able to
+acquire a visual discrimination habit more rapidly than the female,
+whereas the female can acquire a labyrinth habit more readily than the
+male, suggests an important difference in the nature of their equipment
+for habit formation. One might hazard the suggestion that the male depends
+more largely upon discrimination of external conditions, whereas the
+female depends to a greater extent than does the male upon the internal,
+organic changes which are wrought by acts. At any rate the female seems to
+follow a labyrinth path more mechanically, more accurately, more easily,
+and with less evidence of sense discrimination than does the male.
+
+Finally, in concluding this chapter, I may add that in those aspects of
+behavior which received attention in the early chapters of this volume the
+dancers differ very markedly. Some climb readily on vertical or inclined
+surfaces to which they can cling; others seldom venture from their
+horizontally placed dance floor. Some balance themselves skillfully on
+narrow bridges; others fall off almost immediately. My own observations,
+as well as a comparison of the accounts of the behavior of the dancer
+which have been given by Cyon, Zoth, and other investigators, lead me to
+conclude that there are different kinds of dancing mice. This may be the
+result of crosses with other species of mice, or it may be merely an
+expression of the variability of an exceptionally unstable race.
+
+I can see no satisfactory grounds for considering the dancer either
+abnormal or pathological. It is a well-established race, with certain
+peculiarities to which it breeds true; and no pathological structural
+conditions, so far as I have been able to learn, have been discovered.
+
+I have presented in this chapter on differences a program rather than a
+completed study. To carry out fully the lines of work which have been
+suggested by my observations and by the presentation of results would
+occupy a skilled observer many months. I have not as yet succeeded in
+accomplishing this, but my failure is not due to lack of interest or of
+effort.
+
+
+
+
+CHAPTER XVIII
+
+THE INHERITANCE OF FORMS OF BEHAVIOR
+
+In a general way those peculiarities of behavior which suggested the name
+dancing mouse are inherited. Generation after generation of the mice run
+in circles, whirl, and move the head restlessly and jerkily from side to
+side. But these forms of behavior vary greatly. Some individuals whirl
+infrequently and sporadically; others whirl frequently and persistently,
+at certain hours of the day. Some are unable to climb a vertical surface;
+others do so readily. Some respond to sounds; others give no indications
+of ability to hear. I propose in this chapter to present certain facts
+concerning the inheritance of individual peculiarities of behavior, and to
+state the results of a series of experiments by which I had hoped to test
+the inheritance of individually acquired forms of behavior.
+
+My study of the nature of the whirling tendency of the dancer has revealed
+the fact that certain individuals whirl to the right almost uniformly,
+others just as regularly to the left, and still others now in one
+direction, now in the other. On the basis of this observation, the animals
+have been classified as right, left, or mixed whirlers. Does the dancer
+transmit to its offspring the tendency to whirl in a definite manner?
+
+Records of the direction of whirling of one hundred individuals have been
+obtained. For twenty of these mice the determination was made by counting
+the number of complete turns in five-minute intervals at six different
+hours of the day. For the remaining eighty individuals the direction was
+discovered by observation of the activity of the animals for a brief
+interval at five different times. Naturally, the former results are the
+more exact; in fact, they alone have any considerable quantitative value.
+But for the problem under consideration all of the determinations are
+sufficiently accurate to be satisfactory.
+
+The distribution of the individuals which were examined as to direction of
+whirling is as follows.
+
+
+
+      RIGHT WHIRLERS LEFT WHIRLERS MIXED WHIRLERS TOTAL
+
+Males       19            19             12        50
+Females     12            23             15        50
+
+
+The frequency of occurrence of left whirlers among the females is
+unexpectedly high. Is this to be accounted for in terms of inheritance? In
+my search for an answer to this question I followed the whirling tendency
+from generation to generation in two lines of descent. These two groups of
+mice have already been referred to as the 200 line and the 400 line. The
+former were descended from Nos. 200 and 205, and the latter from Nos. 152
+and 151. Individuals which resulted from the crossing of these lines will
+be referred to hereafter as of mixed descent. There were some striking
+differences in the behavior of the mice of the two lines of descent. As a
+rule the individuals of the 200 line climbed more readily, were more
+active, danced less vigorously, whirled less rapidly and less
+persistently, and were in several other respects much more like common
+mice than were the individuals of the 400 line. It is also to be noted
+(see Table 5) that few of the litters of the 200 line exhibited auditory
+reactions, whereas almost all of the litters of the 400 line which were
+tested gave unmistakable evidence of sensitiveness to certain sounds.
+These differences at once suggest the importance of an examination of the
+whirling tendency of each line of descent.
+
+The results for the several generations of each line which I had
+opportunity to examine are unexpectedly decisive so far as the question in
+point is concerned.
+
+
+
+                 INDIVIDUALS OF THE 200 LINE
+
+                              MALES                    FEMALES
+
+First generation      No. 200, ?               No. 205, ?
+Second generation     No. 210, Mixed whirler   No. 215, Left whirler
+Third generation      No. 220, Mixed whirler   No. 225, Mixed whirler
+Fourth generation     No. 230, Right whirler   No. 235, Mixed whirler
+Fifth generation      No. 240, Right whirler   No. 245, Left whirler
+
+                 INDIVIDUALS OF THE 400 LINE
+
+                               MALES              FEMALES
+
+First generation      No. 152, Left whirler    No. 151, Left whirler
+Second generation     No. 410, Left whirler    No. 415, Right whirler
+Third generation      No. 420, Left whirler    No. 425, Left whirler
+
+
+
+One line of descent exhibited no pronounced whirling tendency; the other
+exhibited a strong tendency to whirl to the left. Are these statements
+true for the group of one hundred individuals whose distribution among the
+three classes of whirlers has been given? In order to obtain an answer to
+this question I have reclassified these individuals according to descent
+and direction of whirling.
+
+
+
+                INDIVIDUALS OF THE 200 LINE
+
+          RIGHT WHIRLERS    LEFT WHIRLERS    MIXED WHIRLERS   TOTAL
+
+Males             7              6                 8            21
+Females           5              8                 8            21
+                 12             14                16            42
+
+
+
+
+                INDIVIDUALS OF THE 400 LINE
+
+          RIGHT WHIRLERS   LEFT WHIRLERS   MIXED WHIRLERS   TOTAL
+
+Males           4                9                1           14
+Females         6                9                4           19
+               10               18                5           33
+
+               INDIVIDUALS OF MIXED DESCENT
+
+                9                10                6           25
+
+
+Three interesting facts are indicated by these results: first, the
+inheritance of a tendency to whirl to the left in the 400 line of descent;
+second, the lack of any definite whirling tendency in the 200 line; and
+third, the occurrence of right and left whirlers with equal frequency as a
+result of the crossing of these two lines of descent.
+
+It is quite possible, and I am inclined to consider it probable, that the
+pure dancer regularly inherits a tendency to whirl to the left, and that
+this is obscured in the case of the 200 line by the influences of a cross
+with another variety of mouse. It is to be noted that the individuals of
+the 200 line were predominantly mixed whirlers, and I may add that many of
+them whirled so seldom that they might more appropriately be classed as
+circlers.
+
+
+
+THE INHERITANCE OF INDIVIDUALLY ACQUIRED FORMS OF
+BEHAVIOR
+
+The white-black discrimination experiments which were made in connection
+with the study of vision and the modifiability of behavior were so planned
+that they should furnish evidence of any possible tendency towards the
+inheritance of modifications in behavior. The problem may be stated thus.
+If a dancing mouse be thoroughly trained to avoid black, by being
+subjected to a disagreeable experience every time it enters a black box,
+will it transmit to its offspring a tendency to avoid black?
+
+Systematic training experiments were carried on with individuals of both
+the 200 and 400 lines of descent. For each of these lines a male and a
+female were trained at the age of four weeks to discriminate between the
+white and the black electric-boxes and to choose the former. After they
+had been thoroughly trained these individuals were mated, and in course of
+time a male and female, chosen at random from their first litter, were
+similarly trained. All the individuals were trained in the same way and
+under as nearly the same conditions as could be maintained, and accurate
+records were kept of the behavior of each animal and of the number of
+errors of choice which it made in series after series of tests. What do
+these records indicate concerning the influence of individually acquired
+forms of behavior upon the behavior of the race?
+
+
+
+TABLE 53
+
+THE INHERITANCE OF THE HABIT OF WHITE-BLACK DISCRIMINATION
+
+Number of Errors in Daily Series of Ten Tests
+
+
+                    MALES                          FEMALES
+
+SERIES  FIRST  SECOND   THIRD  FOURTH   FIRST  SECOND   THIRD  FOURTH
+       GENERA- GENERA- GENERA- GENERA- GENERA- GENERA- GENERA- GENERA-
+        TION    TION    TION    TION    TION    TION    TION    TION
+
+       No. 210 No. 220 No. 230 No. 240 No. 215 No. 225 No. 235 No. 245
+
+   A      6       5       6       7       8       4       4       7
+   B      6       8       8       8       8       7       6       5
+
+   1      6       7       6       5       7       6       5       4
+   2      4       3       1       5       5       6       4       5
+   3      3       1       4       5       3       4       4       3
+   4      5       0       3       4       2       1       3       1
+   5      3       0       4       2       1       3       3       0
+   6      2       1       4       2       2       1       1       1
+   7      1       0       3       1       1       1       2       0
+   8      0       0       1       0       0       0       2       3
+   9      0       0       0       1       1       0       0       0
+  10      0               0       1       0       2       1       1
+  11                      0       0       0       3       0       0
+  12                              0       0       0       0       0
+  13                              0               0       0       0
+  14                                              0
+
+
+
+I have records for four generations in the 200 line and for three
+generations in the 400 line.[1] As the results are practically the same
+for each, I shall present the detailed records for the former group alone.
+In Table 53 are to be found the number of errors made in successive series
+of ten tests each by the various individuals of the 200 line which were
+trained in this experiment. The most careful examination fails to reveal
+any indication of the inheritance of a tendency to avoid the black box.
+No. 240, in fact, chose the black box more frequently in the preference
+series than did No. 210, and he required thirty more tests for the
+establishment of a perfect habit than did No. 210. Apparently descent from
+individuals which had thoroughly learned to avoid the black box gives the
+dancer no advantage in the formation of a white-black discrimination
+habit. There is absolutely no evidence of the inheritance of this
+particular individually acquired form of behavior in the dancer.
+
+[Footnote 1: This experiment was interrupted by the death of the animals
+of both lines of descent.]
+
+
+
+
+
+INDEX
+
+
+Abnormal dancers.
+Acquired forms of behavior.
+Act, useless, repeated.
+Activity, periods of.
+Affirmation, choice by.
+Age, peculiarities;
+  maximum age;
+  and intelligence.
+Albino cat;
+  dog.
+Alexander and Kreidl, young dancer;
+  behavior;
+  tracks of mice;
+  behavior in cyclostat;
+  behavior of white mouse and dancer;
+  structure of ear;
+  deafness.
+Allen, G. M., drawing of dancer;
+  heredity in mice.
+Alleys, width of, in labyrinths.
+Amyl acetate for photometry.
+Anatomy of dancer.
+Animals, education of.
+Appuun whistles.
+Audition. _See_ Hearing.
+Averages, dangers in.
+
+
+Baginsky, B., model of ear of dancer.
+Bateson, W., breeding experiments.
+Behavior, of dancer;
+  inheritance of;
+  when blinded;
+  equilibration;
+  dizziness;
+  structural bases of;
+  of young;
+  changes in;
+  useless acts;
+  under experimental conditions;
+  in indiscriminable conditions;
+  value of sight;
+  in labyrinth experiments;
+  modifiability of;
+  history of;
+  explanations of;
+  individual differences in.
+Blinded dancers, behavior of.
+Blue-orange tests;
+  blue-red tests;
+  blue-green tests;
+  blue-green blindness.
+Bradley papers.
+Brain, structure of.
+Breeding of dancers.
+Brehm, A. E., "Tierleben".
+Brightness vision;
+  preference;
+  check experiments;
+  relation to color vision.
+
+
+Cages for dancers.
+Candle meter.
+Candle power.
+Cardboards, for tests of vision;
+  positions of.
+Care of dancer.
+Castle, W. E., drawing of mouse;
+  cages.
+Cat, albino;
+  training of.
+Cerebellum of dancer.
+Characters, acquired.
+Check experiments.
+China, dancers of.
+Choice, exhibition of;
+  by affirmation;
+  by negation;
+  by comparison;
+  methods of.
+Circling, a form of dance.
+Circus course mice.
+Cleghorn, A. G.
+Climbing of dancer.
+Cochlea, functions of.
+Color blindness.
+Color discrimination apparatus.
+Colored glasses.
+Colored papers.
+Color patterns of dancers.
+Color vision, problem;
+  methods of testing;
+  tests with colored papers,
+  tests with ray filters,
+  orange-blue tests,
+  yellow-red tests,
+  light blue-orange tests,
+  dark blue-red tests,
+  green-light blue tests,
+  violet-red tests,
+  green-blue tests,
+  green-red tests,
+  blue-green tests,
+  blue-red tests,
+  structure of the retina,
+  conclusions,
+  of different animals,
+Comparative pedagogy,
+Comparison, choice by,
+Cones, lacking in eye of dancer,
+Corti, organ of, in dancer,
+Cotton mouse,
+Curves, of habit formation,
+  irregularities of,
+  of labyrinth habit,
+  of discrimination habit,
+  of learning and re-learning,
+  of plasticity,
+Cyclostat, behavior of dancer in,
+Cyon, E. de, dancer pathological,
+  behavior,
+  behavior of blinded dancers,
+  varieties of dancer,
+  space perception,
+  individual differences,
+  anatomy of dancer,
+  hearing of dancer,
+  pain cries.
+
+
+Dancers, occurrence among common mice,
+  varieties of,
+  hybrid,
+Dancing,
+  forms of dance movement,
+  whirling, circling, figure-eights, manège movements, solo
+    dance, centre dance,
+  direction of,
+  periods of,
+  amount of,
+  causes of,
+  sex differences in,
+  individual differences in,
+Darbishire, A. D., breeding experiments with dancers,
+Deafness of dancer,
+  causes of,
+Descent, lines of,
+Development of young dancer,
+Differences, individual,
+  sex,
+Direction of movement, choice by,
+Direction of whirling,
+Discrimination, visual, box,
+  of brightness,
+  white-black and black-white,
+  of grays,
+  habits,
+  by odor,
+  by form,
+  method,
+  habit defined,
+Diseases of dancer,
+Dizziness,
+  visual,
+  rotational,
+Docility,
+Dog, albino,
+  training of,
+  fear of electric shock.
+
+
+Ear, structure of,
+  structural types,
+  model of,
+  of rabbit,
+  functions of,
+  movements of,
+Educability of dancer,
+Education, human,
+  methods of,
+  of vision,
+Efficiency of training,
+Electric-box for visual tests,
+Electric-labyrinth for habit experiments,
+Electric-shock as punishment for mistakes,
+Epidemic among dancers,
+Equilibration in dancer,
+Error curves,
+  form of,
+Error records versus time records,
+Errors, in labyrinths,
+  nature of,
+  types of,
+  value of,
+  number of,
+Even numbers to designate males,
+Excitability of dancer,
+Experience, value of,
+  influence of,
+Eyes, of dancer
+  opening of
+  retina of
+
+
+Fear, in dancer
+Females,
+  designated by odd numbers
+  dancing of
+  voice of
+  _See_ Sex
+Fighting of dancers
+Figure eight dance
+Filters for obtaining colored light
+Food of dancer
+Form discrimination
+Frog,
+  reactions of
+  repetition of act by
+Functions of eye
+
+
+Galton whistle
+Gestation, period of, in dancer
+Gray papers
+Green blue tests
+Green-red tests
+Grouping for averages
+Guaita, G von,
+  breeding experiments with dancers
+
+
+Haacke, W,
+  description of dancer,
+  origin of dancer
+  breeding experiments
+Habit,
+  of dancing,
+  discrimination,
+  useless
+  labyrinth,
+  duration of,
+  reacquisition of,
+  relations of,
+Habit formation,
+  and the senses,
+  _versus_ habit performance,
+  in the dancer and in the common mouse,
+  curves of,
+  speed of
+Habituation to sounds
+Hacker, dancing shrews
+Hair, appearance of
+Hamilton, G V, experiments with dog
+Hatai, S, the dancer
+Head, shape of, in dancer
+Hearing,
+  in dancer
+  in young
+  in adult
+  methods of testing,
+  in frog
+Hefner unit of light
+Heredity
+  _See_ Inheritance
+Hering, E, colored papers
+History,
+  of dancer
+  of acts
+Hunger as motive in experiments
+Hybrid dancers
+
+
+Imitation in dancer
+Index of modifiability
+Individuality
+Inheritance
+Inhibition of an act
+Initiative of dancer
+Insight of dancer
+Intelligence,
+  measures of,
+  comparisons
+Interrupted circuit for experimental use
+Irregular labyrinths
+
+
+Janssen-Hoffman spectroscope
+Japan, dancers in
+Judgment in dancer
+
+
+Kammerer, P, dancing wood mice,
+Kishi, K,
+  dancer in Japan
+  origin of race,
+  equilibration,
+  blinded dancer,
+  structure of ear,
+  wax in ears,
+  tests of hearing
+König tuning forks,
+  steel bars
+Kreidl, A
+  _See_ Alexander
+
+
+Labyrinth,
+  forms of,
+  labyrinth A,
+  errors in,
+  tests,
+  labyrinth B,
+  tests,
+  labyrinth C,
+  labyrinth D,
+  a standard labyrinth,
+  regular and irregular labyrinths
+Labyrinth errors and individual tendencies
+Labyrinth habits,
+Labyrinth method,
+Labyrinth path, formula,
+  method of recording,
+Ladder climbing tests,
+Landois, H, account of dancer,
+Lathrop, A, dancers,
+Learning, process,
+  methods of in dancer,
+  by being put through act,
+  by imitation,
+  by rote,
+  rapidity of,
+  permanency of,
+  learning and relearning,
+  curves of,
+Left whirlers,
+Life span of dancer,
+Light, reflected,
+  transmitted, unit of measurement,
+  control of,
+Litter, size of, in dancer,
+Lummer-Brodhun photometer.
+
+Males, dancing of,
+  fighting and killing young, designation of,
+  voice of, _See_ Sex
+Manège movements,
+Mark, E L, cages,
+Maze _See_ Labyrinth
+Measurements, of light,
+  of rapidity of habit formation,
+  of intelligence,
+  of efficiency of training,
+Memory, defined,
+  for ladder climbing,
+  tests of,
+  measurements of,
+  span of,
+  for brightness,
+  for color,
+Method, of studying dance,
+  for testing hearing,
+  indirect,
+  for testing vision,
+  motives,
+  for brightness vision,
+  for color vision,
+  of shifting filters,
+  of testing form discrimination,
+  of testing Weber's law,
+  development of methods,
+  of choice,
+  food box,
+  labyrinth,
+  of recording errors,
+  of training
+  problem method, labyrinth method, discrimination method,
+  of recording labyrinth path,
+  qualitative versus quantitative,
+  of studying senses,
+  values of methods,
+  of measuring intelligence,
+  quantitative,
+  comparisons of,
+Milne-Edwards, origin of dancer,
+Mitsukun, K, the dancer in Japan,
+Mixed whirlers,
+Modifiability, of behavior,
+  of useless acts,
+  index of,
+Motives, for activity,
+  for choice,
+  avoidance of discomfort,
+  in labyrinths,
+  desire to escape, to get food, to avoid pain,
+Motor, tendencies,
+  ability,
+  capacity
+Movements,
+  of ears,
+_Mus musculus L_,
+_Mus spiciosus L_,
+_Mus sylvaticus L_.
+
+_Nankin nesumi_, name for dancer,
+Negation, choice by,
+Nendel, R, gray papers,
+Nerve, eighth,
+Nervous system,
+Nest materials,
+Noises, effects of,
+Numbers, odd for females, even for males,
+  reference,  _See_ Bibliographic List.
+
+Odors, discrimination by,
+Old Fancier's description of dancer,
+Olfactory sense _See_ Smell
+Orange-blue tests,
+Orientation of dancer,
+Origin of dancer;
+  by selectional breeding;
+  by inheritance of an acquired character;
+  by mutation;
+  by pathological changes;
+  by natural selection.
+
+
+Panse, R.,
+  structure of ear;
+  explanation of deafness.
+Papers, Nendel's grays;
+  Bradley's colored;
+  Hering's colored.
+Parker, G. H.
+Path in labyrinth, record of.
+Pathological condition of dancer.
+Pedagogy, comparative.
+Perception,
+  of brightness;
+  of color;
+  of movement;
+  of form.
+Peru, dancers in.
+Petromyzon, semicircular canals of.
+Photometer, Lummer-Brodhun.
+Plasticity of dancer;
+  curves of.
+Position choice by,
+  of cardboards.
+Preference for brightness,
+  tests of.
+Preliminary tests.
+Probable error.
+Problems, of structure;
+ of method.
+Punishment versus reward.
+Putting-through, training by.
+
+
+Qualitative methods.
+Quantitative methods.
+
+
+Rabbit, ear of.
+Rawitz, B., behavior of dancer;
+ structure of ear;
+ deafness of dancer;
+ hearing in young.
+Ray filters.
+Reactions, to sounds;
+  to disagreeable stimuli;
+  valueless.
+Reasoning, implicit.
+Reconstruction method.
+Records, of markings of dancers;
+  of time;
+  of errors;
+  of path.
+Red, stimulating value of;
+ vision.
+Reference numbers to literature.
+ _See_ Literature on Dancer.
+Reflected light.
+Refrangibility and vision of dancer.
+Regular labyrinth.
+Re-learning, relation to learning;
+  curves of.
+Reliability of averages.
+Repetition of useless acts.
+Rest-interval.
+Restlessness, of dancer;
+  cause of.
+Retina of dancer.
+Retzius, ear of rabbit.
+Reward, for performance of act;
+  versus punishment.
+Right whirlers;
+  behavior in labyrinth;
+  occurrence of;
+  inheritance of tendency.
+Rods of retina.
+Rotational dizziness.
+Rubber stamps of labyrinths.
+
+
+Saint-Loup, R.
+Schlumberger, C.;
+ wood carving with dancers.
+Selenka, ear of rabbit.
+Semicircular canals.
+Sense organs.
+Senses, and habit formation;
+ differences in.
+Sensitiveness
+Sex, recognition of, designation
+  of, peculiarities
+Shellac to coat cards
+Shrews, dancing
+Sight, role of, _See_ Vision,
+  Brightness Vision, and Color Vision
+Smell sense of, in labyrinth habits
+Sniffing by dancer
+Solutions as ray filters
+_Sorex vulgaris L_
+Sound, reactions to
+Space perception
+Spectroscope
+Spectrum, stimulating value of
+Standard, candle, light,
+  labyrinth
+Stine, W M, photometrical measurements
+Strength of dancer
+Structure, of brain,
+  of ear, of eye
+Swinhoe, mice in China
+
+Temperament of animal
+Temperature sense
+Tests, visual, number of,
+  per day,
+Threshold of discrimination
+Time records
+Touch, and labyrinth habits
+Training conditions of, Weber's
+  law, methods of,
+  and retraining, in labyrinths,
+  efficiency of, two test,
+  ten-test, twenty-test,
+  continuous, relation to
+  methods, spread of
+Transmitted light.
+
+Variability of dancer,
+Variable light.
+Varieties of dancer
+Violet red tests
+Vision, brightness
+  vision, color vision,
+  training of, importance
+  of, conclusions concerning
+Visual dizziness
+Voice of dancer
+
+Watson, J B, habit formation
+Waugh, K, color vision apparatus,
+  retina of mouse
+Wax, plugs of, in ear of mouse
+Weber's law, tests of, apparatus
+Weldon, W F R, breeding experiments,
+Whirling of dancer
+
+Yellow Red tests
+Young dancers, killing of, by male,
+  description of, development
+  of, hearing of, intelligence
+  of, size of
+
+Zoth, O, origin of dancer, size
+  of young mice, the senses of
+  dancer, behavior, dancing,
+  equilibration, climbing
+  dancers, individual differences,
+  tests of hearing,
+  vision
+
+
+
+
+
+End of the Project Gutenberg EBook of The Dancing Mouse, by Robert M. Yerkes
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