The Dancing Mouse: A Study in Animal Behavior

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Title: The Dancing Mouse: A Study in Animal Behavior
Author: Yerkes, Robert Mearns
Language: English
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[Illustration: DANCING MICE--SNIFFING AND EATING.]

THE ANIMAL BEHAVIOR SERIES. VOLUME I

THE DANCING MOUSE

A Study in Animal Behavior

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 this LibraryBlog Digital Book "The Dancing Mouse: A Study in Animal Behavior" ***

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