Produced by Juliet Sutherland, Michael Oltz,

Charles Franks and the Online Distributed Proofreading Team.

[Illustration: DANCING MICE—SNIFFING AND EATING.]

THE ANIMAL BEHAVIOR SERIES. VOLUME I

THE DANCING MOUSE

A Study in Animal Behavior

BY

ROBERT M. YERKES, Ph.D.
INSTRUCTOR IN COMPARATIVE PSYCHOLOGY IN HARVARD
UNIVERSITY

The Cartwright Prize of the Alumni Association of the College of
Physicians and Surgeons, Columbia University, was awarded, in 1907, for an
Essay which comprised the first twelve chapters of this volume.

1907

IN LOVE AND GRATITUDE THIS BOOK IS DEDICATED TO MY MOTHER

PREFACE

This book is the direct result of what, at the time of its occurrence, seemed to be an unimportant incident in the course of my scientific work— the presentation of a pair of dancing mice to the Harvard Psychological Laboratory. My interest in the peculiarities of behavior which the creatures exhibited, as I watched them casually from day to day, soon became experiment-impelling, and almost before I realized it, I was in the midst of an investigation of their senses and intelligence.

The longer I observed and experimented with them, the more numerous became the problems which the dancers presented to me for solution. From a study of the senses of hearing and sight I was led to investigate, in turn, the various forms of activity of which the mice are capable; the ways in which they learn to react adaptively to new or novel situations; the facility with which they acquire habits; the duration of habits; the roles of the various senses in the acquisition and performance of certain habitual acts; the efficiency of different methods of training; and the inheritance of racial and individually acquired forms of behavior.

In the course of my experimental work I discovered, much to my surprise, that no accurate and detailed account of this curiously interesting animal existed in the English language, and that in no other language were all the facts concerning it available in a single book. This fact, in connection with my appreciation of the exceptional value of the dancer as a pet and as material for the scientific study of animal behavior, has led me to supplement the results of my own observation by presenting in this little book a brief and not too highly technical description of the general characteristics and history of the dancer.

The purposes which I have had in mind as I planned and wrote the book are three: first, to present directly, clearly, and briefly the results of my investigation; second, to give as complete an account of the dancing mouse as a thorough study of the literature on the animal and long-continued observation on my own part should make possible; third, to provide a supplementary text-book on mammalian behavior and on methods of studying animal behavior for use in connection with courses in Comparative Psychology, Comparative Physiology, and Animal Behavior.

It is my conviction that the scientific study of animal behavior and of animal mind can be furthered more just at present by intensive special investigations than by extensive general books. Methods of research in this field are few and surprisingly crude, for the majority of investigators have been more deeply interested in getting results than in perfecting methods. In writing this account of the dancing mouse I have attempted to lay as much stress upon the development of my methods of work as upon the results which the methods yielded. In fact, I have used the dancer as a means of exhibiting a variety of methods by which the behavior and intelligence of animals may be studied. As it happens the dancer is an ideal subject for the experimental study of many of the problems of animal behavior. It is small, easily cared for, readily tamed, harmless, incessantly active, and it lends itself satisfactorily to a large number of experimental situations. For laboratory courses in Comparative Psychology or Comparative Physiology it well might hold the place which the frog now holds in courses in Comparative Anatomy.

Gratefully, and with this expression of my thanks, I acknowledge my indebtedness to Professor Hugo Münsterberg for placing at my command the resources of the Harvard Psychological Laboratory and for advice and encouragement throughout my investigation; to Professor Edwin B. Holt for valuable assistance in more ways than I can mention; to Professor Wallace C. Sabine for generous aid in connection with the experiments on hearing; to Professor Theobald Smith for the examination of pathological dancers; to Miss Mary C. Dickerson for the photographs of dancing mice which are reproduced in the frontispiece; to Mr. Frank Ashmore for additional photographs which I have been unable to use in this volume; to Mr. C. H. Toll for the drawings for Figures 14 and 20; to Doctors H. W. Rand and C. S. Berry for valuable suggestions on the basis of a critical reading of the proof sheets; and to my wife, Ada Watterson Yerkes, for constant aid throughout the experimental work and in the preparation of this volume.

R. M. Y.

CAMBRIDGE, MASSACHUSETTS,

August, 1907.

CONTENTS

LIST OF ILLUSTRATIONS

LITERATURE ON THE DANCING MOUSE

CHAPTER I

CHARACTERISTICS, ORIGIN, AND HISTORY

Peculiarities of the dancing mouse—Markings and method of keeping record of individuals—The dancer in China and Japan (Kishi, Mitsukuri, Hatai)— Theories concerning the origin of the race: selectional breeding; the inheritance of an acquired character; mutation, inheritance, and selectional breeding; pathological changes; natural selection—Instances of the occurrence of dancers among other kinds of mice—Results of crossing dancer with other kinds of mice.

CHAPTER II

FEEDING, BREEDING, AND DEVELOPMENT OF THE YOUNG

Methods of keeping and caring for dancers—Cages, nest-boxes, and materials for nest—Cleansing cages—Food supply and feeding—Importance of cleanliness, warmth, and pure food—Relations of males and females, fighting—The young, number in a litter—Care of young—Course of development—Comparison of young of dancer with young of common mouse— Diary account of the course of development of a typical litter of dancers.

CHAPTER III

BEHAVIOR: DANCE MOVEMENTS

Dancing—Restlessness and excitability—Significance of restlessness— Forms of dance: whirling, circling, and figure-eights—Direction of whirling and circling: right whirlers, left whirlers, and mixed whirlers— Sex differences in dancing—Time and periodicity of dancing—Influence of light on activity—Necessity for prolonged observation of behavior.

CHAPTER IV

BEHAVIOR: EQUILIBRATION AND DIZZINESS

Muscular coordination—Statements of Cyon and Zoth concerning behavior— Control of movements, orientation, equilibration, movement on inclined surfaces, climbing—The tracks of the dancer—Absence of visual dizziness—Comparison of the behavior of the dancer with that of the common mouse when they are rotated in a cyclostat—Behavior of blinded dancers (Cyon, Alexander and Kreidl, Kishi)—Cyon's two types of dancer— Phenomena of behavior for which structural bases are sought: dance movements; lack of response to sounds; deficiency in equilibrational ability; lack of visual and rotational dizziness.

CHAPTER V

STRUCTURAL PECULIARITIES AND BEHAVIOR

The functions of the ear—Structure of the ear of the dancer as described by Rawitz, by Panse, by Baginsky, by Alexander and Kreidl, and by Kishi— Cyon's theory of the relation of the semicircular canals to space perception—Condition of the auditory organs—Condition of the equilibrational organs—Condition of the sound-transmitting organs—The bearing of the results of anatomical investigations upon the facts of behavior.

CHAPTER VI

THE SENSE OF HEARING

Experiments on hearing in the dancer made by Rawitz, by Panse, by Cyon, by
Alexander and Kreidl, by Zoth, and by Kishi—Hearing and the voice—
Methods of testing sensitiveness to sounds—Results of tests with adults—
Importance of indirect method of experimentation—Results of tests with
young—The period of auditory sensitiveness—Individual differences.

CHAPTER VII

THE SENSE OF SIGHT: BRIGHTNESS VISION

What is known concerning sight in the dancer—Brightness vision and color vision—Methods of testing brightness vision, the visual discrimination apparatus—Motives for discrimination and choice—Punishment versus reward as an incentive in animal experiments—Hunger as an incentive—An electric stimulus as an incentive—Conditions for brightness vision tests— White-black vision—Evidence of preference—Check experiments—Conclusion.

CHAPTER VIII

THE SENSE OF SIGHT: BRIGHTNESS VISION (Continued)

The delicacy of brightness discrimination—Methods of testing the dancer's ability to detect slight differences in brightness—Results of tests with gray papers—Relation of intensity of visual stimuli to the threshold of discrimination—Weber's law apparatus and method of experimentation— Results of Weber's law tests—Practice effects, the training of vision— Description of the behavior of the dancer in the discrimination box experiments—Modes of choice: by affirmation; by negation; by comparison— Evidence of indiscriminable visual conditions.

CHAPTER IX

THE SENSE OF SIGHT: COLOR VISION

Does the dancer see colors?—The food-box method of testing color vision— Waugh's food-box method—Results of tests—Tests by the use of colored papers in the visual discrimination box—Yellow-red vision—Blue-orange vision—Brightness vision versus color vision—Brightness check tests—Green-blue vision—Violet-red vision—Conclusions.

CHAPTER X

THE SENSE OF SIGHT: COLOR VISION (Continued)

The use of color filters—Testing color vision by the use of transmitted light—Green-blue vision—Green-red vision—Blue-red vision—Stimulating value of different portions of the spectrum—Does red appear darker to the dancer than to us?—Conclusions concerning color vision—Structure of the retina of the dancer and its significance.

CHAPTER XI

THE ROLE OF SIGHT IN THE DAILY LIFE OF THE DANCER.

Sight and general behavior—Behavior of blinded dancers—Experimental tests of ability to perceive form—Visual guidance in mazes—Following labyrinth paths in the dark—The relative importance of visual, olfactory, and kinaesthetic stimuli—Conditions for the acquisition of a motor habit—Conditions for the execution of an habitual act.

CHAPTER XII

EDUCABILITY: METHODS OF LEARNING

The modifiability of behavior—Educational value of experimental studies of modifiability—Methods: the problem method; the labyrinth method; the discrimination method—Relation of method to characteristics of animal— Simple test of the docility of the dancer—Lack of imitative tendency— Persistence of useless acts—Manner of profiting by experience—Individual differences in initiative.

CHAPTER XIII

HABIT FORMATION: THE LABYRINTH HABIT

The labyrinth method—Problems—Preliminary tests—Comparison of the behavior of the dancer in a maze with that of the common mouse—Evolution of a labyrinth method—Records of time and records of errors—Simple and effective method of recording the path—Curves of habit formation—Regular and irregular labyrinths—Points for a standard labyrinth—Values and defects of the labyrinth method.

CHAPTER XIV

HABIT FORMATION: THE DISCRIMINATION METHOD

Quantitative versus qualitative results—Motives—Precautions— Preference—Results of systematic habit-forming experiments—Curves of habit formation—Meaning of irregularity in curve—Individual differences—Comparison of curves for discrimination habits with those for labyrinth habits—Averages—The index of modifiability as a measure of docility—Reliability of the index.

CHAPTER XV

THE EFFICIENCY OF TRAINING METHODS

Importance of measuring the efficiency of educational methods—Rapidity of learning and permanency of modifications wrought by training—Results of a study of the efficiency of discrimination methods—Comparison by means of indices of modifiability—Number of tests per series versus number of series—Efficiency as measured by memory tests.

CHAPTER XVI

THE DURATION OF HABITS: MEMORY AND RE-LEARNING

Measures of the permanency of modifications in behavior—The duration of brightness and color discrimination habits—The relation of learning to re-learning—Can a habit which has been lost completely be re-acquired with greater facility than it was originally acquired?—Relation of special training to general efficiency—Does the training in one form of labyrinth aid the dancer in acquiring other labyrinth habits?

CHAPTER XVII

INDIVIDUAL, AGE, AND SEX DIFFERENCES IN BEHAVIOR

Individual peculiarities in sensitiveness, docility, and initiative—The relation of docility to age—The individual result and the average—How averages conceal facts—Sex differences in docility and initiative— Individual differences of motor capacity which seem to indicate varieties—Is the dancer pathological?

CHAPTER XVIII

THE INHERITANCE OF FORMS OF BEHAVIOR

Characteristics of the race—Inheritance of the tendency to whirl in a particular way—Tests of the inheritance of individually acquired forms of behavior.

INDEX

ILLUSTRATIONS

Dancing Mice—sniffing and eating Frontispiece

FIGURE

1. Color patterns of dancers. Record blanks

2. Double cage, with nest-boxes and water dishes

3. Double cages in frame

4. Photographs of dancers climbing (After Zoth)

5. Tracks of common mouse (After Alexander and Kreidl)

6. Tracks of dancer (After Alexander and Kreidl)

7. The inner ear of the rabbit (Retzius)

8. The membranous labyrinth of the ear of the dancer (After Rawitz)

9. Same

10. Same

it. Model of the ear of the dancer (After Baginsky)

12. Ear of the dancer (After Kishi)

13. Ear of the dancer (After Kishi)

14. Discrimination box

15. Ground plan of discrimination box

16. Nendel's gray papers

17. Weber's law apparatus

18. Food-box apparatus

19. Waugh's food-box apparatus

20. Color discrimination apparatus

21. Ground plan of color discrimination apparatus

22. Cards for form discrimination

23. Labyrinth B

24. Labyrinth B on electric wires

25. Labyrinth A

26. Curves of habit formation for labyrinth B

27. Plan of labyrinth C, and path records

28. Labyrinth D

29. Curve of learning for white-black discrimination, twenty individuals

30. Curve of learning for white-black discrimination, thirty individuals

31. Curve of habit formation for labyrinth D

32. Curves of learning and re-learning

33. Plasticity curves

LITERATURE ON THE DANCING MOUSE

1. ALEXANDER, G. UND KREIDL, A. "Zur Physiologie des Labyrinths der Tanzmaus." Archiv für die gesammte Physiologie, Bd. 82: 541-552. 1900.

2. ALEXANDER, G. UND KREIDL, A. "Anatomisch-physiologische Studien über das Ohrlabyrinth der Tanzmaus." II Mittheilung. Archiv für die gesammte Physiologie. Bd. 88: 509-563. 1902.

3. ALEXANDER, G. UND KREIDL, A. "Anatomisch-physiologische Studien über das Ohrlabyrinth der Tanzmaus." III Mittheilung. Archiv für die gesammte Physiologie, Bd. 88: 564-574. 1902.

4. BAGINSKY, B. "Zur Frage über die Zahl der Bogengänge bei japanischen Tanzmäusen." Centralblatt für Physiologie, Bd. 16: 2-4. 1902.

5. BATESON, W. "The present state of knowledge of colour-heredity in mice and rats." Proceedings of the Zoölogical Society of London, Vol. 2: 71-99. 1903.

6. BREHM, A. E. "Tierleben." Dritte Auflage. Saugetiere, Bd. 2: 513-514. 1890.

7. BREHM, A. E. "Life of Animals." Translated from the third German edition of the "Tierleben" by G. R. Schmidtlein. Mammalia, p. 338. Marquis, Chicago. 1895.

8. CYON, E. DE. "Le sens de l'espace chez les souris dansantes japonaises." Cinquantenaire de la Société de Biologie (Volume jubilaire). p. 544-546. Paris. 1899.

9. CYON, E. VON. "Ohrlabyrinth, Raumsinn und Orientirung." Archiv für die gesammte Physiologie, Bd. 79: 211-302. 1900.

10. CYON, E. DE. "Presentation de souris dansantes japonaises." Comptes rendus du XIII Congrès International de Paris, Section de physiologie, p. 160-161. 1900.

11. CYON, E. VON. "Beiträge zur Physiologie des Raumsinns." I Theil. "Neue Beobachtungen an den japanischen Tanzmäusen." Archiv für die gesammte Physiologie, Bd. 89: 427-453. 1902.

12. CYON, E. DE. "Le sens de l'espace." Richet's "Dictionnaire de physiologie," T. 5: 570-571. 1901.

13. DARBISHIRE, A. D. Note on the results of crossing Japanese waltzing mice with European albino races. Biometrica, Vol. 2: 101-104. 1902.

14. DARBISHIRE, A. D. Second report on the result of crossing Japanese waltzing mice with European albino races. Biometrica, Vol.2: 165-173. 1903.

15. DARBISHIRE, A. D. Third report on hybrids between waltzing mice and albino races. Biometrica, Vol. 2: 282-285. 1903.

16. DARBISHIRE, A. D. On the result of crossing Japanese waltzing with albino mice. Biometrica, Vol 3: 1-51. 1904.

17. GUAITA, G. v. "Versuche mit Kreuzungen von verschiedenen Rassen der Hausmaus." Berichte der naturforschenden Gesellschaft zu Freiburg i. B., Bd. 10: 317-332. 1898.

18. GUAITA, G. v. "Zweite Mitteilung uber Versuche mit Kreuzungen von verschiedenen Hausmausrassen." Berichte der naturforschenden Gesellschaft zu Freiburg i. B., Bd. 11: 131-138. 1900.

19. HAACKE, W. "Ueber Wesen, Ursachen und Vererbung von Albinismus und Scheckung und über deren Bedeutung für vererbungstheoretische und entwicklungsmechanische Fragen." Biologisches Centralblatt, Bd. 15: 44-78. 1895.

19a. HUNTER, M. S. "A Pair of Waltzing Mice." The Century Magazine, Vol. 73: 889-893. April, 1907.

20. KAMMERER, P. "Tanzende Waldmaus und radschlagende Hausmaus." Zoölogische Garten, Bd. 41: 389-390. 1900.

21. KISHI, K. "Das Gehörorgan der sogenannten Tanzmaus." Zeitschrift für wissenschaftliche Zoölogie, Bd. 71: 457-485. 1902.

22. LANDOIS, H. "Chinesische Tanzmäuse." Jahresbericht des Westfähschen Provinzial-Vereins, Munster, 1893-1894: 62-64.

22a. LOSE, J. "Waltzing Mice." Country Life in America, September, 1904. p. 447.

23. PANSE, R. Zu Herrn Bernhard Rawitz' Arbeit: "Das Gehörorgan der japanischen Tanzmäuse." Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1901: 139-140.

24. PANSE, R. "Das Gleichgewichts- und Gehörorgan der japanischen Tanzmäuse." Münchener medicinische Wochenschrift, Jahrgang 48, Bd. I: 498-499. 1901.

25. RAWITZ, B. "Das Gehörorgan der japanischen Tanzmäuse." Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1899: 236-243.

26. RAWITZ, B. "Neue Beobachtungen über das Gehörorgan japanischer Tanzmäuse." Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1901, Supplement: 171-176.

27. RAWITZ, B. "Zur Frage über die Zahl der Bogengänge bei japanischen Tanzmäusen." Centralblatt für Physiologie, Bd. 15: 649-651. 1902.

28. SAINT-LOUP, R. "Sur le mouvement de manège chez les souris." Bulletin de la Société Zoölogique de France, T. 18: 85-88. 1893.

29. SCHLUMBERGER, C. "A propos d'un netzukè japonais." Memoires de la Société Zoölogique de France, T. 7: 63-64. 1894.

30. WELDON, W. F. R. Mr. Bateson's revisions of Mendel's theory of heredity. Biometrica, Vol. 2: 286-298. 1903.

31. ZOTH, O. "Ein Beitrag zu den Beobachtungen und Versuchen an japanischen Tanzmäusen." Archiv für die gesammte Physiologie, Bd. 86: 147-176. 1901.

32. ANONYMOUS. "Fancy Mice: Their Varieties, Management, and Breeding." Fourth edition. London: L. Upcott Gill. No date.

CHAPTER I

CHARACTERISTICS, ORIGIN, AND HISTORY

The variety of mouse which is known as the Japanese dancing or waltzing mouse has been of special interest to biologists and to lovers of pets because of its curious movements. Haacke in Brehm's "Life of Animals" (7 p. 337)[1] writes as follows concerning certain mice which were brought to Europe from China and Japan: "From time to time a Hamburg dealer in animals sends me two breeds of common mice, which he calls Chinese climbing mice (Chinesische Klettermäuse) and Japanese dancing mice (Japanische Tanzmäuse). It is true that the first are distinguished only by their different colors, for their climbing accomplishments are not greater than those of other mice. The color, however, is subject to many variations. Besides individuals of uniform gray, light yellow, and white color, I have had specimens mottled with gray and white, and blue and white. Tricolored mice seem to be very rare. It is a known fact that we also have white, black, and yellow mice and occasionally pied ones, and the Chinese have profited by these variations of the common mouse also, to satisfy their fancy in breeding animals. The Japanese, however, who are no less enthusiastic on this point, know how to transform the common mouse into a really admirable animal. The Japanese dancing mice, which perfectly justify their appellation, also occur in all the described colors. But what distinguishes them most is their innate habit of running around, describing greater or smaller circles or more frequently whirling around on the same spot with incredible rapidity. Sometimes two or, more rarely, three mice join in such a dance, which usually begins at dusk and is at intervals resumed during the night, but it is usually executed by a single individual."

[Footnote 1: The reference numbers, of which 7 is an example, refer to the numbers in the bibliographic list which precedes this chapter.]

As a rule the dancing mouse is considerably smaller than the common mouse, and observers agree that there are also certain characteristic peculiarities in the shape of the head. One of the earliest accounts of the animal which I have found, that of Landois (22 p. 62), states, however, that the peculiarities of external form are not remarkable. Landois further remarks, with reason, that the name dancing mouse is ill chosen, since the human dance movement is rather a rhythmic hopping motion than regular movement in a circle. As he suggests, they might more appropriately be called "circus course mice" (22 p. 63).

Since 1903 I have had under observation constantly from two to one hundred dancing mice. The original pair was presented to the Harvard Psychological Laboratory by Doctor A.G. Cleghorn of Cambridge. I have obtained specimens, all strikingly alike in markings, size, and general behavior, from animal dealers in Washington, Philadelphia, and Boston. Almost all of the dancers which I have had, and they now number about four hundred, were white with patches, streaks, or spots of black. The black markings occurred most frequently on the neck, ears, face, thighs, hind legs, about the root of the tail, and occasionally on the tail itself. In only one instance were the ears white, and that in the case of one of the offspring of a male which was distinguished from most of his fellows by the possession of one white ear. I have had a few individuals whose markings were white and gray instead of white and black.

The method by which I was able to keep an accurate record of each of my dancers for purposes of identification and reference is illustrated in Figure 1. As this method has proved very convenient and satisfactory, I may briefly describe it. With a rubber stamp[1] a rough outline of a mouse, like that of Figure 1 A, was made in my record book. On this outline I then indicated the black markings of the individual to be described. Beside this drawing of the animal I recorded its number, sex,[2] date of birth, parentage, and history. B, C, and D of Figure 1 represent typical color patterns. D indicates the markings of an individual whose ears were almost entirely white. The pattern varies so much from individual to individual that I have had no trouble whatever in identifying my mice by means of such records as these.

[Footnote 1: For the use of the plate from which this stamp was made, I am indebted to Professor W.E. Castle, who in turn makes acknowledgment to Doctor G.M. Allen for the original drawing.]

[Footnote 2: I have found it convenient to use the even numbers for the males and the odd numbers for the females. Throughout this book this usage is followed. Wherever the sex of an individual is not specially given, the reader therefore may infer that it is a male if the number is even; a female if the number is odd.]

All of my dancers had black eyes and were smaller as well as weaker than the albino mouse and the gray house mouse. The weakness indicated by their inability to hold up their own weight or to cling to an object curiously enough does not manifest itself in their dancing; in this they are indefatigable. Frequently they run in circles or whirl about with astonishing rapidity for several minutes at a time. Zoth (31 p. 173), who measured the strength of the dancer in comparison with that of the common mouse, found that it can hold up only about 2.8 times its own weight, whereas the common white mouse can hold up 4.4 times its weight. No other accurate measurements of the strength, endurance, or hardiness of the dancer are available. They are usually supposed to be weak and delicate, but my own observations cause me to regard them as exceptionally strong in certain respects and weak in others.

[Illustration: FIGURE I.—Typical markings of dancers. A, blank outline of mouse for record. B, markings of No. 2 [symbol for male], born September 7, 1905, of unknown parents, died March 30,1907. C, markings of No 43 [symbol for female], born November 10, 1906, of 212 and 211. D, markings of No. 151 [symbol for female], born February 28, 1906, of 1000 and 5, died February 26, 1907.]

What the Japanese have to say about the dancing mouse is of special importance because Japan is rather commonly supposed to be its home. For this reason, as well as because of the peculiar interest of the facts mentioned, I quote at length from Doctor Kishi (21 p. 457). "The dancing mouse has received in Europe this name which it does not bear in its own home, because of the fact that the circular movements which it makes are similar to the European (human) dance. Sometimes it is also called the Japanese or Chinese mouse; originally, however, China must have been its home, since in Japan it is mostly called 'Nankin nesumi,' the mouse from Nankin. When this animal came from China to Japan I shall inquire at a later opportunity. There were originally in Japan two different species of mouse, the gray and the white; therefore in order to distinguish our dancing mouse from these it was necessary to use the name of its native city.

"In Japan, as in Europe, the animal lives as a house animal in small cages, but the interest which is taken in it there is shown in quite another way than in Europe, where the whirling movements, to which the name dancing mouse is due, are of chief interest. For this reason in Europe it is given as much room as possible in its cage that it may dance conveniently. In Japan also the circular movements have been known for a long time, but this has had no influence upon our interest in the animal, for the human fashion of dancing with us is quite different from that in Europe. What has lent interest to the creature for us are its prettiness, its cleverness in tricks, and its activity. It is liked, therefore, as an amusement for children. For this purpose it is kept in a small cage, usually fifteen centimeters square, sometimes in a somewhat broader wooden box one of whose walls is of wire netting. In this box are built usually a tower, a tunnel, a bridge, and a wheel. The wheel is rather broad, being made in the form of a drum and pierced with holes on one side through which the animal can slip in and out. Running around on the inside, the mouse moves the wheel often for hours at a time, especially in the evening. Moreover, there are found in the box other arrangements of different kinds which may be set in motion by the turning of the wheel. No space remains in the box in which the animal may move about freely, and therefore one does not easily or often have an opportunity to observe that the animal makes circular movements, whether voluntarily or involuntarily. This is the reason that in its home this interesting little animal has never been studied by any one in this respect."

It is odd indeed that the remarkable capacity of the dancer for the execution of quick, graceful, dextrous, bizarre, and oft-repeated movements has not been utilized in America as it has in Japan. The mice are inexhaustible sources of amusement as well as invaluable material for studies in animal behavior and intelligence.

Concerning the origin and history of this curious variety of mouse little is definitely known. I have found no mention of the animal in scientific literature previous to 1890. The fact that it is called the Chinese dancing mouse, the Japanese dancing mouse, and the Japanese waltzing mouse is indicative of the existing uncertainty concerning the origin of the race.

Thinking that Japanese literature might furnish more information bearing on the question of racial history than was available from European sources, I wrote to Professor Mitsukuri of the University of Tokyo, asking him whether any reliable records of the dancer existed in Japan. He replied as follows: "I have tried to find what is known in Japan about the history of the Japanese waltzing mice, but I am sorry to say that the results are wholly negative. I cannot find any account of the origin of this freak, either authentic or fictitious, and, strange as it may seem to you, no study of the mice in a modern sense has been made, so you may consider the literature on the mouse in the Japanese language as absolutely nil." In explanation of this somewhat surprising ignorance of the origin of the race in what is commonly supposed to be its native land, Professor Mitsukuri adds: "The breeders of the mice have mostly been ignorant men to whom writing is anything but easy."

In response to similar inquiries, I received the following letter, confirmatory of Professor Mitsukuri's statements, from Doctor S. Hatai of Wistar Institute, Philadelphia: "If I remember rightly the so-called Japanese dancing mouse is usually called by us Nankin-nedzumi. Nankin means anything which has been imported from China, and nedzumi means rat-like animal, or in this case mouse, or Chinese mouse. I referred to one of the standard Japanese dictionaries and found the following statement: 'The Nankin-nedzumi is one of the varieties of Mus spiciosus (Hatszuka-nedzumi), and is variously colored. It was imported from China. These mice are kept in cages for the amusement of children, who watch their play.' Mus spiciosus, if I remember correctly, is very much like Mus musculus in color, size, and several other characteristics, if not the same altogether."

In Swinhoe's list of the mammals of China, which appeared in the Proceedings of the Zoological Society of London for 1870, Mus musculus L. is mentioned as occurring in houses in South China and in Formosa. It is further stated that black and white varieties which are brought from the Straits are often kept by the Chinese (p. 637).

The statements of Kishi, Mitsukuri, and Hatai which have been quoted, taken in connection with the opinions expressed by various European scientists who have studied the dancer, make it seem highly probable that the race appeared first in China, and was thence introduced into Japan, from which country it has been brought to Europe and America. Accepting for the present this conclusion with reference to the place of origin of the dancer, we may now inquire, how and when did this curious freak, as Professor Mitsukuri has called it, come into existence? Concerning these matters there is wide divergence of opinion.

Haacke (6 p. 514), as quoted in Brehm's "Tierleben," says that an animal dealer with whom he discussed the question of the possible origin of the dancer maintained that it came from Peru, where it nests in the full cotton capsules, arranging the cotton fibers in the form of a nest by running about among them in small circles. Hence the name cotton mouse is sometimes applied to it. Haacke himself believes, however, that the race originated either in China or Japan as the result of systematic selectional breeding. Of this he has no certainty, for he states that he failed to find any literature on the "beautiful mice of China and Japan." Whether Haacke's description of the dancing mouse was published elsewhere previous to its appearance in Brehm's "Tierleben" I am unable to state; I have found nothing written on the subject by him before 1890. Zoth (31 p. 176) also thinks that the race was developed by systematic breeding, or in other words, that it is a product of the skill of the Asiatic animal breeders.

Another account of the origin of the race is that accepted by Kishi (21 p. 481) and some other Japanese biologists. It is their belief that the forms of movement acquired by the individual as the result of confinement in narrow cages are inherited. Thus centuries of subjection to the conditions which Kishi has described (p. 6) finally resulted in a race of mice which breed true to the dance movement. It is only fair to add, although Kishi does not emphasize the fact, that in all probability those individuals in which the dancing tendency was most pronounced would naturally be selected by the breeders who kept these animals as pets, and thus it would come about that selectional breeding would supplement the inheritance of an acquired character. Few indeed will be willing to accept this explanation of the origin of the dancer so long as the inheritance of acquired characters remains, as at present, unproved.

Still another mode of origin of the mice is suggested by the following facts. In 1893 Saint Loup (28 p. 85) advanced the opinion that dancing individuals appear from time to time among races of common mice. The peculiarity of movement may be due, he thinks, to an accidental nervous defect which possibly might be transmissible to the offspring of the exceptional individual. Saint Loup for several months had under observation a litter of common mice whose quick, jerky, nervous movements of the head, continuous activity, and rapid whirling closely resembled the characteristic movements of the true dancers of China. He states that these mice ran around in circles of from 1 to 20 cm. in diameter. They turned in either direction, but more frequently to the left, that is, anticlockwise. At intervals they ran in figure-eights ([Symbol: figure eight]) as do the true dancers. According to Saint Loup these exceptional individuals were healthy, active, tame, and not markedly different in general intelligence from the ordinary mouse. One of these mice produced a litter of seven young, in which, however, none of the peculiarities of behavior of the parents appeared.

In view of this proof of the occurrence of dancing individuals among common mice, Saint Loup believes that the race of dancers has resulted from the inheritance and accentuation of an "accidental" deviation from the usual mode of behavior. It is scarcely necessary to say that this opinion would be of far greater weight had he observed, instead of postulating, the inheritance of the peculiarities of movement which he has described. It might be objected, to the first of his so-called facts, that the litter resulted from the mating of mice which possessed dancer blood. Until the occurrence of dancers among varieties of mice which are known to be unmixed with true dancers is established, and further, until the inheritance of this peculiar deviation from the normal is proved, Saint Loup's account of the origin of the dancing mouse race must be regarded as an hypothesis.

The occurrence of dancing individuals among common mice has been recorded by several other observers. Kammerer (20 p. 389) reports that he found a litter of young wood mice (Mus sylvaticus L.) which behaved much as do the spotted dancers of China. He also observed, among a lot of true dancers, a gray individual which, instead of spinning around after the manner of the race, turned somersaults at frequent intervals. It is Kammerer's opinion, as a result of these observations, that the black and white dancers of China and Japan have been produced by selectional breeding on the basis of this occasional tendency to move in circles. Among albino mice Rawitz (25 p. 238) has found individuals which whirled about rapidly in small circles. He states, however, that they lacked the restlessness of the Chinese dancers. Some shrews (Sorex vulgaris L.) which exhibited whirling movements and in certain other respects resembled the dancing mouse were studied for a time by Professor Häcker of Freiburg in Baden, according to a report by von Guaita (17 p. 317, footnote). Doctor G. M. Allen of Cambridge has reported to me that he noticed among a large number of mice kept by him for the investigation of problems of heredity[1] individuals which ran in circles; and Miss Abbie Lathrop of Granby, Massachusetts, who has raised thousands of mice for the market, has written me of the appearance of an individual, in a race which she feels confident possessed no dancer blood, which whirled and ran about in small circles much as do the true dancers.

[Footnote 1: Allen, G.M. "The Heredity of Coat Color in Mice." Proc. Amer.
Academy, Vol. 40, 59-163, 1904.]

Although it is possible that some of these cases of the unexpected appearance of individuals with certain of the dancer's peculiarities of behavior may have been due to the presence of dancer blood in the parents, it is not at all probable that this is true of all of them. We may, therefore, accept the statement that dancing individuals now and then appear in various races of mice. They are usually spoken of as freaks, and, because of their inability to thrive under the conditions of life of the race in which they happen to appear, they soon perish.

Another and a strikingly different notion of the origin of the race of dancers from those already mentioned is that of Cyon (11 p. 443) who argues that it is not a natural variety of mouse, as one might at first suppose it to be, but instead a pathological variation. The pathological nature of the animals is indicated, he points out, by the exceptionally high degree of variability of certain portions of the body. According to this view the dancing is due to certain pathological structural conditions which are inherited. Cyon's belief raises the interesting question, are the mice normal or abnormal, healthy or pathological? That the question cannot be answered with certainty off-hand will be apparent after we have considered the facts of structure and function which this volume presents.

Everything organic sooner or later is accounted for, in some one's mind, by the action of natural selection. The dancing mouse is no exception, for Landois (22 p. 62) thinks that it is the product of natural selection and heredity, favored, possibly, by selectional breeding in China. He further maintains that the Chinese dancer is a variety of Mus musculus L. in which certain peculiarities of behavior appear because of bilateral defects in the brain. This author is not alone in his belief that the brain of the dancer is defective, but so far as I have been able to discover he is the only scientist who has had the temerity to appeal to natural selection as an explanation of the origin of the race.

Milne-Edwards, as quoted by Schlumberger (29 p. 63), is of the opinion that the Chinese dancer is not a natural wild mouse race, but instead the product of rigid artificial selection. And in connection with this statement Schlumberger describes a discovery of his own which seems to have some bearing upon the problem of origin. In an old Japanese wood carving which came into his possession he found a group of dancing mice. The artist had represented in minute detail the characteristics of the members of the group, which consisted of the parents and eight young. The father and mother as well as four of the little mice are represented as white spotted with black. Of the four remaining young mice, two are entirely black and two entirely white. The two pure white individuals have pink eyes, as has also the mother. The eyes of all the others are black. From these facts Schlumberger infers that the dancer has resulted from the crossing of a race of black mice with a race of albinos; the two original types appear among the offspring in the carving.

Experimental studies of the inheritance of the tendency to dance are of interest in their bearing upon the question of origin. Such studies have been made by Haacke (19), von Guaita (17, 18), and Darbishire (13, 14, 15, 16), and the important results of their investigations have been well summarized by Bateson (5).

By crossing dancing mice with common white mice both Haacke and von Guaita obtained gray or black mice which are very similar to the wild house mouse in general appearance and behavior. The characteristic movements of the dancers do not appear. As the result of a long series of breeding experiments, Darbishire (16 pp. 26, 27) says: "When the race of waltzing mice is crossed with albino mice which do not waltz, the waltzing habit disappears in the resulting young, so that waltzing is completely recessive in Mendel's sense; the eye-color of the hybrids is always dark; the coat-color is variable, generally a mixture of wild-gray and white, the character of the coat being distinctly correlated with characters transmitted both by the albino and by the colored parent." When hybrids produced by the cross described by Darbishire are paired, they produce dancers in the proportion of about one to five.

Bateson (5 p. 93, footnote), in discussing the results obtained by Haacke, von Guaita, and Darbishire, writes: "As regards the waltzing character, von Guaita's experiments agree with Darbishire's in showing that it was always recessive to the normal. No individual in F1 [thus the first hybrid generation is designated] or in families produced by crossing F1 with the pure normal, waltzed. In Darbishire's experiments F1 x F1 [first hybrids mated] gave 8 waltzers in 37 offspring, indicating 1 in 4 as the probable average. From von Guaita's matings in the form DR x DR the totals of families were 117 normal and 21 waltzers…. There is therefore a large excess of normals over the expected 3 to 1. This is possibly due to the delicacy of the waltzers, which are certainly much more difficult to rear than normals are. The small number in von Guaita's litters makes it very likely that many were lost before such a character as this could be determined."

Bateson does not hazard a guess at the origin of the dancer, but merely remarks (5 p. 86) that the exact physiological basis of the dancing character is uncertain and the origin of this curious variation in behavior still more obscure. "Mouse fanciers have assured me," he continues, "that something like it may appear in strains inbred from the normal type, though I cannot find an indubitable case. Such an occurrence may be nothing but the appearance of a rare recessive form. Certainly it is not a necessary consequence of inbreeding, witness von Guaita's long series of inbred albinos." (von Guaita (17 p. 319) inbred for twenty-eight generations.)

From the foregoing survey of the available sources of information concerning the origin and history of the race of dancing mice the following important facts appear. There are four theories of the origin of the race: (1) origin by selectional breeding (Haacke, Zoth, Milne- Edwards); (2) origin through the inheritance of an acquired character (Kishi); (3) origin by mutation, inheritance, and selectional breeding (Saint Loup, Kammerer, Cyon); (4) origin by natural selection, and inheritance, favored by selectional breeding (Landois). Everything indicates that the race originated in China. It is fairly certain that individuals with a tendency to move in circles appear at rare intervals in races of common mice. It seems highly probable, in view of these facts, that the Chinese took advantage of a deviation from the usual form of behavior to develop by means of careful and patient selectional breeding a race of mice which is remarkable for its dancing. Even if it should be proved that the mutation as it appears among common mice is not inherited, the view that slight deviations were taken advantage of by the breeders would still be tenable. The dancing tendency is such in nature as to unfit an individual for the usual conditions of mouse existence, hence, in all probability human care alone could have produced and preserved the race of dancers.

In answer to the question, how and when did the race of dancers originate, it may be said that historical research indicates that a structural variation or mutation which occasionally appears in Mus musculus, and causes those peculiarities of movement which are known as dancing, has been preserved and accentuated through selectional breeding by the Chinese and Japanese, until finally a distinct race of mice which breeds true to the dance character has been established. The age of the race is not definitely known, but it is supposed to have existed for several centuries.

CHAPTER II

FEEDING, BREEDING, AND DEVELOPMENT OF THE YOUNG

In this chapter I shall report, for the benefit of those who may wish to know how to take care of dancing mice, my experience in keeping and breeding the animals, and my observations concerning the development of the young. It is commonly stated that the dancer is extremely delicate, subject to diseases to an unusual degree and difficult to breed. I have not found this to be true. At first I failed to get them to breed, but this was due, as I discovered later, to the lack of proper food. For three years my mice have bred frequently and reared almost all of their young. During one year, after I had learned how to care for the animals, when the maximum number under observation at any time was fifty and the total number for the year about one hundred, I lost two by disease and one by an accident. I very much doubt whether I could have done better with any species of mouse. There can be no doubt, however, that the dancer is delicate and demands more careful attention than do most mice. In March, 1907, I lost almost all of my dancers from what appeared to be an intestinal trouble, but with this exception I have had remarkably good luck in breeding and rearing them.

My dancers usually were kept in the type of cage of which Figure 2 is a photograph.[1] Four of these double cages, 70 cm. long, 45 cm. wide, and 10 cm. deep in front, were supported by a frame as is shown in Figure 3. The fact that the covers of these cages cannot be left open is of practical importance. A similar type of cage, which I have used to some extent, consists of a wooden box 30 by 30 cm. by 15 cm. deep, without any bottom, and with a hinged cover made in part of 1 cm. mesh wire netting. Such a cage may be placed upon a piece of tin or board, or simply on a newspaper spread out on a table. The advantage of the loose bottom is that the box may be lifted off at any time, and the bottom thoroughly cleansed. I have had this type of cage constructed in blocks of four so that a single bottom and cover sufficed for the block. If the mice are being kept for show or for the observation of their movements, at least one side of the cages should be of wire netting, and, as Kishi suggests, such objects as a wheel, a tower, a tunnel, a bridge, and a turntable, if placed in the cage, will give the animals excellent opportunity to exhibit their capacity for varied forms of activity.

[Footnote 1: This cage was devised by Professors W.E. Castle and E.L.
Mark, and has been used in the Zoological Laboratories of Harvard
University for several years.]

[Illustration: FIGURE 2.—Double cage, with nest boxes and water dishes.]

The floors of the cages were covered with a thin layer of sawdust for the sake of cleanliness, and in one corner of each cage a nest box of some sort was placed. During the warm months I found it convenient and satisfactory to use berry boxes, such as appear in Figure 2, with a small entrance hole cut in one side; and during the cold months cigar boxes, with an entrance hole not more than 5 cm. in diameter at one end. In the nest box a quantity of tissue paper, torn into fragments, furnished material for a nest in which the adults could make themselves comfortable or the female care for her young. Cotton should never be used in the nest boxes, for the mice are likely to get it wound about their legs with serious results. Apparently they are quite unable to free themselves from such an incumbrance, and their spinning motion soon winds the threads so tightly that the circulation of the blood is stopped.

[Illustration: FIGURE 3.—Double cages in frame.]

The cages and nest boxes were emptied and thoroughly cleaned once a week with an emulsion made by heating together one part of kerosene and one part of water containing a little soap. This served to destroy whatever odor the cages had acquired and to prevent vermin from infesting the nests. In hot weather far greater cleanliness is necessary for the welfare of the mice than in cold weather. The animals attend faithfully to their own toilets, and usually keep themselves scrupulously clean.

For water and food dishes I have used heavy watch glasses[1] 5 cm. in diameter and 1/2 cm. deep. They are convenient because they are durable, easily cleaned, and not large enough for the young mice to drown in when they happen to spin into one which contains water. It is said that mice do not need water, but as the dancers seem very fond of a little, I have made it a rule to wash the watch glasses thoroughly and fill them with pure fresh water daily. The food, when moist, may be placed in the cages in the same kind of watch glass.

[Footnote 1: Minot watch glasses.]

There is no need of feeding the animals oftener than once a day, and as they eat mostly in the evening and during the night, it is desirable that the food should be placed in the cage late in the afternoon. For almost a year I kept a pair of dancers on "force"[1] and water. They seemed perfectly healthy and were active during the whole time, but they produced no young. If the animals are kept as pets, and breeding is not desired, a diet of "force," "egg-o-see,"[1] and crackers, with some bird-seed every few days, is likely to prove satisfactory. As with other animals, a variety of food is beneficial, but it appears to be quite unnecessary. Too much rich food should not be given, and the mice should be permitted to dictate their own diet by revealing their preferences. They eat surprisingly little for the amount of their activity. I have had excellent success in breeding the mice by feeding them a mixture of dry bread- crumbs, "force," and sweet, clean oats slightly moistened with milk. The food should never be made soppy. A little milk added thus to the food every other day greatly increases fertility. About once a week a small quantity of some green food, lettuce for example, should be given. It is well, I have found, to vary the diet by replacing the bread and "force" at intervals with crackers and seeds. Usually I give the food dry every other day, except in the case of mice which are nursing litters. One person to whom I suggested that lettuce was good for the dancers lost four, apparently because of too much of what the mice seemed to consider a good thing. This suggests that it should be used sparingly.

[Footnote 1: A cereal food.]

Success in keeping and breeding dancing mice depends upon three things: cleanliness, warmth, and food supply. The temperature should be fairly constant, between 60° and 70° Fahr. They cannot stand exposure to cold or lack of food. If one obtains good healthy, fertile individuals, keeps them in perfectly clean cages with soft nesting materials, maintains a temperature of not far above or below 65°, and regularly supplies them with pure water and food which they like, there is not likely to be trouble either in keeping or breeding these delicate little creatures. Several persons who have reported to me difficulty in rearing the young or in keeping the adults for long periods have been unable to maintain a sufficiently high or constant temperature, or have given them food which caused intestinal trouble.

The males are likely to fight if kept together, and they may even kill one another. A male may be kept with one or more females, or several females may be kept together, for the females rarely, in my experience, fight, and the males seldom harm the females. Unless the male is removed from the cage in which the female is kept before the young are born, he is likely to kill the newborn animals. When a female is seen to be building a nest in preparation for a litter, it is best to place her in a cage by herself so that she may not be disturbed.

The sex of individuals may be determined easily in most cases, at the age of 10 to 12 days, by the appearance of teats in the case of females.

The period of gestation is from 18 to 21 days. The maximum number born by my dancers in any single litter was 9, the minimum number 3. In 25 litters of which I have accurate records, 135 individuals were born, an average of 5.4. The average number of males per litter was precisely the same, 2.7, as the number of females.

On the birth of a litter it is well to see that the female has made a nest from which the young are not likely to escape, for at times, if the nest is carelessly made, they get out of it or under some of the pieces of paper which are used in its construction, and perish. Several times I have observed nests so poorly built that almost all of the young perished because they got too far away to find their way back to the mother. It is surprising that the female should not take more pains to keep her young safe by picking them up in her mouth, as does the common mouse, and carrying them to a place where they can obtain warmth and nourishment. This I have never seen a dancing mouse do. For the first day or two after the birth of a litter the female usually remains in the nest box almost constantly and eats little. About the second day she begins to eat ravenously, and for the next three or four weeks she consumes at least twice as much food as ordinarily. Alexander and Kreidl (3 p. 567) state that the female does not dance during the first two weeks after the birth of a litter, but my experience contradicts their statement. There is a decreased amount of activity during this period, and usually the whirling movement appears but rarely; but in some cases I have seen vigorous and long-continued dancing within a few hours after the birth of a litter. There is a wide range of variability in this matter, and the only safe statement, in the light of my observations, is that the mother dances less than usual for a few days after a litter is born to her.

The development of the young, as I have observed it in the cases of twenty litters, for ten of which (Table I) systematic daily records were kept, may be sketched as follows. At birth the mice have a rosy pink skin which is devoid of hair and perfectly smooth; they are blind, deaf, and irresponsive to stimulation of the vibrissae on the nose. During the first week of post-natal life the members of a litter remain closely huddled together in the nest, and no dance movements are exhibited. The mother stays with them most of the time. On the fourth or fifth day colorless hairs are visible, and by the end of the week the body is covered with a coat which rapidly assumes the characteristic black and white markings of the race. For the first few days the hind legs are too weak to support the body weight, and whatever movements appear are the result of the use of the fore legs. As soon as the young mice are able to stand, circling movements are exhibited, and by the end of the second week they are pronounced. Somewhere about the tenth day the appearance of the teats in the case of the females serves to distinguish the sexes plainly. Between the tenth and fifteenth days excitability, as indicated by restless jerky movements in the presence of a disturbing condition, increases markedly; the auditory meatus opens, and, in the case of some individuals, there are signs of hearing. On or after the fifteenth day the eyes open and the efforts to escape from the nest box rapidly become more vigorous. About this time the mother resumes her dancing with customary vigor, and the young, when they have opportunity, begin to eat of the food which is given to her. They now dance essentially as do the adults. From the end of the third week growth continues without noteworthy external changes until sexual maturity is attained, between the fourth and the sixth week. For several weeks after they are sexually mature the mice continue to increase in size.

TABLE I

DEVELOPMENT OF THE YOUNG

NUMBER JERKY REACT IN HAIR TEATS MOVE- EARS TO EYES PARENTS LITTER VISIBLE VISIBLE MENTS OPEN SOUND OPEN M F APPEAR

152+151 5 0 4th day — 13th day 14th day 14th day 16th day 152+151 1 3 4th day 9th day 10th day 12th day 13th day 15th day 410+415 4 1 5th day 11th day 14th day 15th day 15th day 17th day 410+415 2 4 5th day 10th day 13th day 14th day 14th day 16th day 420+425 0 2 4th day 10th day 12th day 14th day 14th day 16th day 210+215 4 1 — — 17th day 13th day 17th day 15th day 210+215 3 3 5th day 11th day 11th day 14th day No 16th day 212+211 1 3 4th day 10th day 15th day 14th day No 15th day 220+225 2 4 4th day 10th day 16th day 14th day No 15th day 220+225 3 3 4th day 10th day 17th day 13th day No 15th day

A course of development very similar to that just described was observed by Alexander and Kreidl (3 p. 565) in three litters of dancing mice which contained 3, 5, and 7 individuals respectively. These authors, in comparing the development of the dancer with that of the common mouse, say that at birth the young in both cases are about 24 mm. in length. The young common mouse grows much more rapidly than the dancer, and by the ninth day its length is about 43 mm. as compared with 31 mm. in the case of the dancer. According to Zoth (31 p. 148) the adult dancer has a body length of from 7 to 7.5 cm., a length from tip of nose to tip of tail of from 12 to 13 cm., and a weight of about 18 grams. The movement of the dancer from the first tends to take the form of circles toward the middle of the nest; that of the common mouse has no definite tendency as to direction. When the common mouse does move in circles, it goes first in one direction, then in the other, and not for any considerable period in one direction as does the true dancer. Neither the young dancer nor the common mouse is able to equilibrate itself well for the first few days after birth, but the latter can follow a narrow path with far greater accuracy and steadiness than the former. The uncertain and irregular movements of the common mouse are due to muscular weakness and to blindness, but the bizarre movements of the young dancer seem to demand some additional facts as an explanation.

A brief account of the development of the dancer given by Zoth (31 p. 149) adds nothing of importance to the description given by Alexander and Kreidl. As my own observations disagree with their accounts in certain respects, I shall now give, in the form of a diary, a description of the important changes observed from day to day in a normal litter. The litter which I have selected as typical of the course of development in the dancer grew rapidly under favorable conditions. I have observed many litters which passed through the various stages of development mentioned in this description anywhere from a day to a week later. This was usually due to some such obviously unfavorable condition as too little food or slight digestive or bowel troubles. According to the nature of the conditions of growth the eyes of the dancer open anywhere from the fourteenth to the twentieth day. This statement may serve to indicate the degree of variability as to the time at which a given stage of development is reached by different litters.

On July 14, 1906, No. 151 (female) and No. 152 (male) were mated, and on August 3 a litter of six was born to them. The course of the development of this litter during the first three weeks was as follows:—

First day The skin is pink and hairless, several vibrissae are visible on the nose and lips, but there is no definite response when they are touched. The mice are both blind and deaf, but they are able to squeak vigorously. The mother was not seen to dance or eat during the day.

Second day. There is a very noticeable increase in size. The vibrissae are longer, but touching them still fails to cause a reaction. No hairs are visible on the body. The mother danced rapidly for periods of a minute several times while the record was being made. She ate very little to-day.

Third day. Scales began to appear on the skin to-day. The animals are rapidly increasing in strength; they can now crawl about the nest easily, but they are too weak to stand, and constantly roll over upon their sides or backs when they are placed on a smooth surface. Because of their inability to progress it is impossible to determine with certainty whether they have a tendency to move in circles. The mother was seen out of the nest dancing once to-day. She now eats ravenously.

Fourth day. One of the six young mice was found under a corner of the nest this morning dead, and the others were scattered about the nest box. I gathered them together into a nest which I made out of bits of tissue paper, and the mother immediately began to suckle them. They are very sensitive to currents of air, but they do not respond to light or sound and seldom to contact with the vibrissae.

Fifth day. When placed on a smooth surface, they tend to move in circles, frequently rolling over. When placed on their sides or backs, they immediately try to right themselves. They do not walk, for their legs are still too weak to support the weight of the body; instead they drag themselves about by the use of the fore legs. Fine colorless hairs are visible over the entire body surface. When the vibrissae are touched, the head is moved noticeably. The mother dances a great deal and eats about twice as much as she did before the birth of the litter.

Sixth day. Certain regions of the skin, which were slightly darker than the remainder on the fourth and fifth days, are now almost black. It is evident that they are the regions in which the black hair is to appear. The movement in circles is much more definite today, although most of the individuals are still too weak to stand on their feet steadily for more than a few seconds at a time. Most of their time, when they are first taken from the nest, is spent in trying to maintain or regain an upright position. The hair is now easily visible, and the skin begins to have a white appearance as a result.

Seventh day. Although they are strong enough to move about the nest readily, none of the young has attempted to leave the nest. They huddle together in the middle of it for warmth. The epidermal scales, which have increased in number since the third day, are dropping off rapidly. Contact with the vibrissae or with the surface of the body, frequently calls forth a motor reaction but neither light nor sound produces any visible change in behavior. The black and white regions of the skin are sufficiently definite now to enable one to distinguish the various individuals by their markings. The mother was seen to dance repeatedly today, and she ate all the food that was given to her.

Eighth day. A fold is plainly visible where later the eyelids will separate. The black pigment in the skin has increased markedly.

Ninth day. The eyelids are taking form rapidly, but they I have not separated. The body is covered with a thick coat of hair which is either pure white or black. Standing on the four legs is still a difficult task.

Tenth day. To-day teats are plainly visible in the case of four of the five individuals of the litter. Up to this time I had thought, from structural indications, that there were three males and two females; it is now evident that there are four females and one male. The external ear, the pinna, is well formed, and has begun to stand out from the head, but no opening to the inner portion of the ear is present. The eyelids appear to be almost fully formed.

Eleventh day. There are no very noticeable changes in appearance except in size, which continues to increase rapidly. They are able to regain their normal upright position almost immediately when they happen to roll over. The mother dances as usual.

Twelfth day. It appears to-day as if the eyes were about to open. The ears are still closed, and there is no evidence of a sense of hearing. They squeaked considerably when in the nest, but not at all when I took them out to note their development. The mother stays outside of the nest box much of the time now, probably to prevent the young ones from sucking continuously.

Thirteenth day. One of the little mice came out of the nest box while I was watching the litter this morning, and was able to find his way back directly despite the lack of sight. The mice are still dependent upon the mother for nourishment. I have not seen any of them attempt to eat the food which is given to the mother. They are extremely neat and clean. I watched one of them wash himself this morning. Each foot was carefully licked with the tongue. There seems to be special care taken to keep the toes perfectly clean.