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The Dancing Mouse by Robert M. Yerkes

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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

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

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.



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

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


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 fuer 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 fuer 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 cooeperation 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.

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.


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


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

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

(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 fuer wissenschaftliche Zooelogie_, 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

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

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.



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

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 Koenig 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 Koenig 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

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 Koenig 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 Koenig forks, steel bars,
and a Galton whistle were used. The results obtained are curiously

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 Koenig 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

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

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.



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

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.



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.

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

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

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

[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



White-Black, Series 1

Experimented on No. 5 January 15, 1906

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.



White-Black, Series II

Experimented on No. 5 February 2, 1906


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




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


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

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


No. 210 No. 215
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


No. 14 No. 13
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.


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).



SERIES 1 2 3 4 5 6 7 8 9 10

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.



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

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