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BEHAVIOR MONOGRAPHS

Volume 3, Number 1, 1916 Serial Number 12

Edited by JOHN B. WATSON
The Johns Hopkins University

The Mental Life of Monkeys and Apes:
A Study of Ideational Behavior

ROBERT M. YERKES

Published
at Cambridge, Boston, Mass.
HENRY HOLT & COMPANY
34 West 33d Street, New York
G. E. STECHERT & CO., London, Paris and Leipzig, Foreign Agen :

The Journal of Animal Behavior

An organ for the publication of studies concerning the instincts,
habits and intelligence of organisms

The Journal contains a Department of Notes in which appear brief accounts of
especially interesting and valuable observations of behavior. 5

Published bi-monthly at Cambridge, Boston, Mass., by Henry
Holt and Company, New York. Each volume contains at least 450
pages with plates and text-figures.

The subscription price is $3.00 per volume (foreign subscription
$3.50) postpaid.

Manuscripts, subscriptions, and all business correspondence should
be addressed to

THE JOURNAL OF ANIMAL BEHAVIOR
Emerson Hall, Cambridge, Massachusetts

Behavior Monographs

For the publication of studies in behavior and intelligence which are
too lengthy or too expensive for acceptance
by The Journal of Anima} Behavior

Published at irregular intervals at Cambridge, Boston, Mass., in
connection with the Journal of Animal Behavior, by Henry Holt and
Company, New York. Each volume contains approximately 450
pages with plates and text-figures.

The subscription price is $3.00 per volume (foreign subscription
$3.50) postpaid.

Monographs may be purchased separately at prices varying with
the cost of manufacture.

Manuscripts and inquiries concerning terms of publication should
be addressed to the Editor of the Behavior Monographs,

JOHN B. WATSON,
The Johns Hopkins University,
Baltimore, Md.

Subscription to The Journal of Animal Behavior and the Behavior
Monographs should be sent to Emerson Hall, Cambridge, Massa-.
chusetts.

BEHAVIOR MONOGRAPHS

Volume 3, Number 1, 1916 Serial Number 12

Edited by JOHN B. WATSON
The Johns Hopkins University

The Mental Life of Monkeys and Apes:
A Study of Ideational Behavior

ROBERT M. YERKES

Harvard University

WITH SIX PLATES AND FIVE TEXT FIGURES

Published
at Cambridge, Boston, Mass.
HENRY HOLT & COMPANY
34 West 33d Street, New York
G. E STECHERT & CO., London, Paris and Leipzig, Foreign Agents

I

INTERESTS, OPPORTUNITY AND MATERIALS

Two strong interests come to expression in this report: the
one in the study of the adaptive or ideational behavior of the
monkeys and the apes; and the other in adequate and permanent
provision for the thorough study of all aspects of the lives
of these animals. The values of these interests and of the
tasks which they have led me to undertake are so widely
recognized by biologists that I need not pause to justify or define
them. I shall, instead, attempt to make a contribution of fact
on the score of each interest.

While recognizing that the task of prospecting for an anthro-
poid or primate station may in its outcome prove incomparably
more important for the biological and sociological sciences and
for human welfare than my experimental study of ideational
behavior, I give the latter first place in this report, reserving
for the concluding section an account of the situation regarding
our knowledge of the monkeys, apes, and other primates, and
a description of a plan and program for the thorough-going and
long continued study of these organisms in a permanent station
or research institute.

In 1915, a long desired opportunity came to me to devote
myself undividedly to tasks which I have designated above as
“prospecting ’’ for an anthropoid station and experimenting
with monkeys and apes. First of all, the interruption of my
academic duties by sabbatical leave gave me free time. But in
addition to this freedom for research, I needed animals and equip-
ment. These, too, happily, were most satisfactorily provided,
as I shall now describe.

When in 1913, while already myself engaged in seeking the
establishment of an anthropoid station, I heard of the founding
of such-an institution at Orotava, Tenerife, the Canary Islands,
I immediately made inquiries of the founder of the station,
Doctor Max Rothmann of Berlin, concérning his plans (Roth-

2 ROBERT M. YERKES

mann, 1912).1 As a result of our correspondence, I was invited
to visit and make use of the facilities of the Orotava station
and to consider with its founder the possibility of coéperative
work instead of the establishing of an American station. This
invitation I gratefully accepted with the expectation of spend-
ing the greater part of the year 1915 on the island of Tenerife.
But the outbreak of the war rendered my plan impracticable,
while at the same time destroying all reasonable ground for hope
of profitable codéperation with the Germans in the study of the
anthropoids. In August, 1915, Doctor Rothmann died. Pre-
sumably, the station still exists at Orotava in the interests of
certain psychological and physiological research. So far as I
know, there are as yet no published reports of studies made at
this station. It seems from every point of view desirable that
American psychologists should, without regard to this initial
attempt of the Germans to provide for anthropoid research,
further the establishment of a well equipped American station
for the study not only of the anthropoid apes but of all of the
lower primates.

In the early months of the war while I was making every
effort to obtain reliable information concerning conditions in the
Canary Islands, I received an urgent invitation from my friend
and former student, Doctor G. V. Hamilton, to make use of
his collection of aninnals and laboratory at Montecito, Cali-
fornia, during my leave of absence from Harvard. This invita-
tion I most gladly accepted, and in February, 1915, I established
myself in Santa Barbara, in convenient proximity to Doctor
Hamilton’s private laboratory where for more than six months
I was able to work uninterruptedly under nearly ideal conditions.

Doctor Hamilton without reserve placed at my disposal his
entire collection of animals, laboratory, and equipment, pro-
vided innumerable conveniences for my work, and in addition,
bore the entire expense of my investigation. I cannot ade-
quately thank him for his kindness nor make satisfactory ac-
knowledgment here of his generous aid. Thanks to his sympa-
thetic interest and to the courtesy of the McCormick family
on whose estate the laboratory was located, my work was done
under wholly delightful conditions, and with assistance from,
Ramon Jimenez and Frank Van Den Bergh, Jr., which was in-

1See bibliography at end of report.

PLATE I

9)

“a

FiGurE 1.—Orang utan, Julius, in grove of live oaks back of the laboratory.
FIGURE 2.—Portrait of same subject.

MENTAL LIFE OF MONKEYS AND APES 3

valuable. The former aided me most intelligently in the care
of the animals and the construction of apparatus; and the latter,
especially, was of very real service in connection with many of
my experiments.

The collection of animals which Doctor Hamilton placed at
my disposal consisted of ten monkeys and one orang utan.
The monkeys represented either Pithecus rhesus Audebert
(Macacus rhesus), Pithecus irus F. Cuvier (Macacus cynomolgos),
or the hybrid of these two species (Elliot, 1913). There were
two eunuchs, five males, and three females. All were thoroughly
acclimated, having lived in Montecito either from birth or for
several years. The orang utan was a young specimen of Pongo
pygmeus Hoppius obtained from a San Francisco dealer in
October, 1914 for my use. His age at that time, as judged by
his size and the presence of milk teeth, was not more than five
years. So far as I could discover, he was a perfectly normal,
healthy, and active individual. On June 10, 1915, his weight
was thirty-four pounds, his height thirty-two inches, and his
chest girt twenty-three inches. On August 18 of the same
year, the three measurements were thirty-six and one-half
pounds, thirty-three inches, and twenty-five inches.

For the major portion of my experimental work, only three
of the eleven animals were used. A growing male, P. rhesus
monkey, known as Sobke; a mature male, P. irus, called Skirrl;
and the young orang utan, which had been named Julius. Plates
I and II present these three subjects of my experiments in char-
acteristically interesting attitudes. In plate I, figure 1, Julius
appears immediately behind the laboratory seated on a rock,
against a background of live oaks. This figure gives one an
excellent idea of the immediate environment of the laboratory.
Figure 2 of the same plate is a portrait of Julius taken in the
latter part of August. By reason of the heavy growth of hair,.
he appeared considerably older as well as larger at this time
than when the photograph for figure 1 was taken. In plate II,.
figure 3, Julius is shown in the woods in the attitude of reaching
for a banana, while in figure 4 of the same plate he is represented
as walking upright in one of the cages.

Likenesses of Sobke are presented in figures 5 and 6 of plate
II. In the latter of these figures he is shown stretching his
mouth, apparently yawning but actually preparing for an attack

4 ROBERT M. YERKES

EXPLANATION OF PLATE II

Ficure 3.—Orang utan, Julius, reaching for banana.

Ficure 4.—Julius walking across his cage.

Ficure 5.—P. rhesus, Sobke.

FicurE 6.—Sobke stretching his jaws (yawn?) preparatory to
a fight.

Figure 7.—P. trus, Skirrl.

Ficure 8.—Skirrl using hammer and nail.

Ficure 9.—Skirrl using a saw.

PLATE II

MENTAL LIFE OF MONKEYS AND APES 5

on another monkey behind the wire screen. Figure 7 of this
plate indicates Skirrl in an interesting attitude of attention and
with an obvious lack of self-consciousness. The same monkey
is represented again in figures 8 and 9 of plate II, this time in
the act of using hammer and saw.

All of the animals except the orang utan had been used more
or less for experiments on behavior by Doctor Hamilton, but
this prior work in no way interfered with my own investigation.
Doctor Hamilton has accumulated a large mass of the most
valuable and interesting observations on the behavior of monkeys,
and he more thoroughly understands them than any other ob-
server of whom I have knowledge. Much to my regret and
embarrassment in connection with the present report, he has
thus far published only a small portion of his data (Hamilton,
1911, 1914). In his most recent paper on ‘‘A study of sexual
tendencies in monkeys and baboons,” he has given important
information concerning several of the monkeys -which I have
observed. For the convenience of readers who may make use
of both his reports and mine, I am designating the animals by
the names previously given them by Hamilton. The avail-
able and essential information concerning the individuals is
presented below.

List of animals in collection

Skirrl. Puithecus irus. Adult male.

Sobke. P. rhesus. Young adult male:

Gertie. P. irus-rhesus. Female. Born November, 1910.
Maud. P. rhesus. Young adult female.

Jimmy II. P.zirus. Adult male.

Scotty. P. irus (?). Adult male.

Tiny. P. irus-rhesus. Female. Born August, 1913.
Chatters. P.irus. Adult eunuch.

Daddy. P.irus. Adult eunuch.

Mutt. P.irus. Young adult male. Born August, 1911.
Julius. Pongo pygmeus. Male. Age, 4 years to 5 years.

When I arrived in Santa Barbara, Doctor Hamilton was
about to remodel, or rather reconstruct, his animal cages and
laboratory. This gave us opportunity to adapt both to the
special needs of my experiments. The laboratory was finally

ROBERT M. YERKES

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PLATE III

FIGURE 10.—Montecito laboratory and cages from the front.
FIGURE 11.—Same from the rear.

MENTAL LIFE OF MONKEYS AND APES 7

located and built in a grove of live oaks. From the front it is
well shown by figure 10 of plate III, and from the rear, by figure
11. Its location was in every way satisfactory for my work,
and in addition, the spot proved a delightful one in which to
spend one’s time.

Figure 12 is a ground plan, drawn to scale, of the laboratory
and the adjoining cages, showing the relations of the several
rooms of the laboratory among themselves and to the nine
cages. Although the construction was throughout simple,
everything was convenient and so planned as to expedite my ex-
perimental work. The large room A, adjoining the cages, was
used exclusively for an experimental study of ideational behavior
by means of my recently devised multiple-choice method. Addi-
tional, and supplementary, experiments were conducted in the
large cage Z. Room D served as a store-room and work-shop.

The laboratory was forty feet long, twenty-two feet wide,
and ten feet to the plate. Each small cage was six, by six, by
twelve feet deep, while the large compartment into which each
of the smaller cages opened was twenty-four feet long, ten feet
wide, and twelve feet deep. ,

II

OBSERVATIONAL PROBLEMS AND METHODS

My chief observational task in Montecito was the study of
ideational behavior, or of such adaptive behavior in monkeys
and apes as corresponds to the ideational behavior of man. It
was my plan to determine, so far as possible in the time at my
disposal, the existence or absence of ideas and the réle which
they play in the solution of problems by monkeys and apes.
I had in mind the behavioristic form of the perennial questions:
Do these animals think, do they reason, and if so, what is the
nature of these processes as indicated by the characteristics of
their adaptive behavior?

My work, although obviously preliminary and incomplete,
differs from most of the previous studies of the complex behavior
of the infrahuman primates in that I relied chiefly upon a spe-
cially devised method and applied it systematically over a
period of several months. The work was intensive and quantita=
tive instead of more or less incidental, casual, and qualitative as |
has usually been the case. Naturally, during the course of my
special study of ideational behavior observations were made
relative to various other aspects of the life of my subjects. Such,
for example, are my notes on the use of the hands, the instincts,
the emotions, and the natural aptitudes of individuals. It is,
indeed, impossible to observe any of the primates without noting
most interesting and illuminating activities. And although the
major portion of my time was spent in hard and monotonous
work with my experimental apparatus, I found time each day
to get into intimate touch with the free activities of my subjects
and to observe their social relations and varied expressions of
individuality. As a result of my close acquaintance with this
band of primates, I feel more keenly than ever before the neces-
sity of taking into account, in connection with all experimental
analyses of behavior, the temperamental characteristics, experi-
ence, and affective peculiarities of individuals.

MENTAL LIFE OF MONKEYS AND APES 9

The light which I have obtained on the general problem of
ideation has come, first, through a method which I have rather
inaptly named the multiple-choice method, and second, and
more incidentally, through a variety of supplementary methods.
which are described in Section IV of this report. These supple-
mentary methods are simple tests of ideation rather than sys-
tematic modes of research. They differ from my chief method,
among other respects, in that they have been used by various
investigators during the past ten or fifteen years. It was not
my aim to repeat precisely the observations made by others,
but instead to verify some of them, and more especially, to throw
additional light on my main problem and to further the analysis
of complex behavior.

What has been referred to as the multiple-choice method was
devised by me three years ago as a means of obtaining strictly
comparable objective data concerning the problem-solving abil-
ity of various types and conditions of animals. The method
was first tried with human subjects in the Psychopathic Hos-
pital, Boston, with a crude keyboard apparatus which, how-
ever, proved wholly satisfactory as a means of demonstrating
its value. It has since been applied by means of mechanisms
especially adapted to the structure and activities of the organ-
isms, to the study of the behavior of the crow, pig, rat, and ring-
dove (Yerkes, 1914; Coburn and Yerkes, 1915; Yerkes and Co-
burn, 1915). The method has also been applied with most
gratifying results to the study of the characteristics of idea-
tional behavior in human defectives,—children, and adults,—
and in subjects afflicted with various forms of mental disease.
It is at present being tried out as a practical test in connection
with vocational guidance and various forms of institutional ex-
amination, such as psychopathic hospital and court examinations.

As no adequate description of the method has yet been pub-
lished to which I can here refer, it will be necessary: to present
its salient characteristics along with a description of the special
form of apparatus which was found suitable for use with monkeys
and apes.

The method is so planned as to enable the observer to present
to any type or condition of organism which he wishes to study
any one or all of a series of problems ranging from the extremely
simple to the complex and difficultly soluble. All of the prob-

10 ROBERT M. YERKES

lems, however, are completely soluble by an organism of excellent
ideational ability. For the human subject, the solution of the
easiest problem of all requires almost no effort, whereas even
moderately difficult problems may require many repetitions
of effort and hours or days of application to the task. In each
case, the solution of the problem depends upon the perception
of a certain constant relation among a series of objects to which
the subject is required to attend and respond. Such relations
are, for example, secondness from one end of the group, middle-
ness, simple alternation of ends, or progressive movement by
constant steps from one end of a group to the other.

It is possible to present such relational problems by means
of relatively simple reaction-mechanisms. In their essential
features, all of the several types of multiple-choice apparatus
designed by the writer and used either by him or by his students
and assistants are the same. They consist of a series of pre-
cisely similar reaction-devices, any one or all of which may be
used in connection with a given observation. These reaction-
mechanisms are so chosen as to be suited to the structure and
action-system of the animal to be studied. For the human
being the mechanism consists of a simple key and the total
apparatus is a bank of keys, with such electrical connections
as are necessary to enable the observer to obtain satisfactory
records of the subject’s behavior. Let us suppose the bank of
keys, as was actually the case in my first form of apparatus,
to consist of twelve separate reaction-mechanisms; and let us
suppose, further, the constant relation (problem) on the basis
of which the subject is required to react to be that of middle-
ness. It is evident that in successive trials or experiments the
keys must be presented to the subject in odd groups, the possi-
bilities being groups of 3, 5, 7, 9, or 11. If for a particular ob-
servation the experimenter wishes to present the first three
keys at the left end of the keyboard, he pushes back the remain-
ing nine keys so that they cannot be operated and requires the
subject to select from the group of three keys the one which on
being pressed causes a signal to appear. It is of course the
clearly understood task of the subject to learn to select the cor-
rect key in the group on first trial. This becomes possible only
as the subject observes the relation of the key which produces
the desired effect to the other keys in the group. On the com-

MENTAL LIFE OF MONKEYS AND APES 11

pletion of a subject’s reaction to the group of three keys, a group
of seven keys at the opposite end of the keyboard may, for
example, be presented. Similarly, the subject is required to
discover with the minimum number of trials the correct reaction:
mechanism. Thus, time after time, the experimenter presents
a different group of keys so that the subject in no two successive
trials is making use of the same portion of the keyboard. It is
therefore impossible for him to react to spatial relations in the-
ordinary sense and manner, and unless he can perceive and ap-
propriately respond to the particular relation which constitutes
the only constant characteristic of the correct reaction-mechan-
ism for a particular problem, he cannot solve the problem, or
at least cannot solve it ideationally and on the basis of a small
number of observations or trials.

For the various infrahuman animals whose ideational behavior
has been studied by means of this method, it has been found
eminently satisfactory to use as reaction-mechanisms a series
of similar boxes, each with an entrance and an exit door. An
incentive to the selection of the right box in a particular test
is supplied by food, a small quantity of which is placed in a
covered receptacle beyond the exit door of each of the boxes,
Each time an animal enters a wrong box, it is punished for its
mistake by being confined in that box for a certain period, rang-
ing from five seconds to as much as two minutes with various
individuals or types of organism. This discourages random,
hasty, or careless choices. When the right box is selected, the
exit door is immediately raised, thus uncovering the food, which
serves as a reward. After eating the food thus provided, the
animal, according to training, returns to the starting point and
eagerly awaits an opportunity to attempt once more to find the
reward which it has learned to expect. With this form of the
apparatus, the boxes among which choice may be made are
indicated by the raising (opening) of the front door.

Since with various birds and mammals the box form of appa-
ratus had proved most satisfactory, I planned the primate ap-
paratus along similar lines, aiming simply to adapt it to the
somewhat different motor equipment and destructive tendencies
of the monkeys. I shall now briefly describe this apparatus
as it was constructed and used in the Montecito laboratory.

The apparatus was built in room A (figure 12), this room

12 ROBERT M. YERKES

EXPLANATION OF PLatTe IV

Ficure13.—Multiple-choice apparatus, showing observer’s bench
and writing stand.

Ficure 14.—Apparatus as seen from observer’s bench.

Ficure 15.—Entrances to multiple-choice boxes as seen from
the response-compartment.

Ficure 16.—Apparatus as seen from the rear, showing exit
doors, food receptacles, and covers for same.

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MENTAL LIFE OF MONKEYS AND APES 13

having been especially planned for it with respect to lighting
as well as dimensions and approaches. It was unfortunately
impossible to obtain photographs showing the whole of the
apparatus, but it is hoped that the four partial views of plate
IV may aid the reader who is unfamiliar with previously described
similar devices to grasp readily the chief points of construction.
In. this plate, figure 13 shows the front of the complete apparatus,
with the alleyway and door by way of which the experimenter
could enter. The investigator’s observation-bench and record-
table also appear in this figure, together with weighted cords
used to operate the various doors and the vertically placed levers
by means of which each pair of doors could be locked. Figure
14 is the view presented to the observer as he stood on the bench
or observation stand of figure 13 and looked over the entire ap-
paratus. Three of the entrance doors are shown at the right of
this figure as raised, whereas the remainder of the nine entrance
doors of the apparatus are closed. Figure 15 is a view of the
entrance doors from below the wire roof of the apparatus.
Again, two of the doors are shown as raised, and three additional
ones as closed. The rear of the apparatus appears in figure 16,
in which some of the exit doors are closed and gthers open. In
the latter case, the food receptacles appear, and on the lower
part of the raised doors of the corresponding boxes may be
seen metal covers for the food receptacles projecting at right
angles to the doors, while on the lower edge of each door is an
iron staple used to receive a sliding bar which could be operated
from the observer’s bench as a means of locking the doors after
they had been closed. The space beyond the exit doors was used
as an alleyway for the return of the animals to the starting
point.

It will be necessary at various points in later descriptions to
refer to these several figures. But further description of them
will be more readily appreciated after a careful examination of
the ground plan of the apparatus presented as figure 17. In
accordance with the labelling of this figure, the experimenter
enters the apparatus room through doorway 16, passes thence
through doorways 17 and 10 to the large cage Z, from which he
has direct access to the animals and can bring them into the
apparatus. The multiple-choice mechanism proper, consisting
of nine similar boxes (nine were used instead of twelve as a matter

14 ROBERT M. YERKES

of convenience of construction, not because this smaller number
is otherwise preferable) is labelled F. These boxes are numbered
1 to 9, beginning at the left. This numbering was adhered to
in the recording of results throughout the investigation. The
other important portions of the apparatus are the runway D,
from which the subject at the experimenter’s pleasure could be
admitted through doorway 12 to the large response-chamber .E;
the alleyways G, H, and I, by way of which return to the start-
ing point was possible; the observation bench C, with its approach
step B; and the observer’s writing table A.

In the construction of this large apparatus, it was necessary
to make provision for the extremely destructive tendencies of
monkeys and anthropoid apes,—hence the apparent cumber-
someness of certain portions. It was equally necessary to pro-
vide for the protection of the observer and the prevention of
escape of the subjects by completely covering the apparatus
and alleyways with a heavy wire netting.

Each of the eighteen doors of the multiple-choice. boxes, and
in addition doors 11, 12, and 15 of the runway D, were operated
by the observer from his bench C by means of weighted window
cords which were carried by pulleys appropriately placed above
the apparatus. Each weight was so chosen as to be just suffi-
cient to hold its door in position after the experimenter had
raised it. For the convenience of the experimenter in the rapid
operation of the twenty-one doors, the weights for the doors of
runway D were painted gray, those for the entrance doors,
white, and those for the exit doors, black.

In each entrance door, as is shown in figure 15 of plate IV,
a window was cut so that the experimenter might watch the
animal after it had entered a given box, and especially note when
it left the box after having received its reward. This window
was covered with wire netting. No such windows were necessary
in the exit doors, but to them were attached heavy galvanized
iron flanges which served to cover the food receptacles. One of
these flanges is labelled o in figure 17. The food receptacles
were provided by boring holes in a 2 by 4 inch timber securely
nailed to the floor immediately outside of the exit doors. Into
these holes aluminum cups fitted snugly, and the iron flanges,
when the doors were closed, fitted so closely over the cups that
it was impossible for the animals to obtain food from them.

MENTAL LIFE OF MONKEYS AND APES 15

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FiGurRE 17,—Ground plan of moulbiple-aiutice apparatus in experiment room A.

Scale —-
60

A, record stand; C, bench for observer; B, step as approach to C; D, alleyway
leading to E, response-compartment; F, one of the nine (1-9) similar multiple-
choice boxes; G, H, alleyways leading from boxes to starting point at D; I, alley-
way used by experimenter as approach to rear of apparatus; W, W, windows;
P, alleyway; Z, large cage; 16, entrance to room A; 17, entrance to apparatus and
thence via_10 to cages; 18, entrance to alleyway I; 11, 15, entrances to D; 12, en-
trance to E; 13, entrance door of box 5; 14, exit door of box 5; 0, cover for food
receptacle.

16 ROBERT M. YERKES

As originally constructed, no provision was made in the apparatus
for locking the entrance and exit doors of the several boxes when
they were closed. But as two of the subjects after a time learned
to open the doors from either outside or inside the boxes, it
became necessary to introduce locking devices which could be
operated by the experimenter from the observation bench. This
was readily accomplished by cutting holes in the floor, which
permitted an iron staple, screwed to the lower edge of each door,
to project through the floor. Through these staples by means
of a lever for each of the nine boxes, the observer was able to
slide a wooden bar, placed beneath the floor of the room, thus
locking or unlocking either the entrance door, the exit door, or
both, in the case of any one of the nine boxes.

Since figure 17 is drawn to scale, it will be needless to give
more than a few of the dimensions of the apparatus: Each of
the boxes was 42 inches long, 18 inches wide, and 72 inches deep,
inside measurements. The alleys D, I, and H were 24 inches,
and G 30 inches wide, by 6 feet deep. The doors of the several
boxes were 18 inches wide, by 5 feet high, while those in the
alleyways were 24 inches wide by 6 feet high. The response-
compartment E of figure 17 was 14 feet 4 inches, by 8 feet, by
6 feet in depth. In order that the apparatus might be used with
adult human subjects conveniently, if such use should prove
desirable, the depth throughout was made 6 feet, and it was
therefore possible for the experimenter to walk about erect in it.

The experimental procedure was briefly as follows: A small
quantity of food having been placed in each of the food cups
and covered by the metal flanges on the exit doors, the experi-
menter raised door 11 of figure 17 and then opened door 10 and
the door of the cage in which the desired subject was confined.
After the latter, in search of food, had entered the runway D,
the experimenter lowered door 11 to keep it in this runway, and
immediately proceeded to set the reaction-mechanisms for an
experiment (trial). Let us suppose that the first setting to be
tried involved all of the nine boxes. Each of the entrance doors
would therefore be raised. Let us further suppose that the right
door is defined as the middle one of the group. With the appa-
ratus properly set, the experimenter next raises door 12, thus
admitting the animal to the response-compartment E. Any
one of the nine boxes may now be entered by it. But if any

MENTAL LIFE OF MONKEYS AND APES 17

except number 5, the middle member of the group, be entered,
the entrance door is immediately lowered and both the exit and
entrance doors locked in position so that the animal is forced
to remain in the box for a stated period, say thirty seconds.
At the expiration of this time the entrance door is raised and
the animal allowed to retrace its steps and make another choice.
When the middle box is chosen, the entrance door is lowered and
the exit door immediately raised, thus uncovering the food,
which the animal eats. As a rule, by my monkeys and ape
the reward was eaten in the alleyway G instead of in the multiple-
choice box. As soon as the food has been eaten, the exit door is
lowered by the experimenter, and the animal returns by way of
G and H to runway D, where it awaits its next trial.

As rewards, bananas and peanuts were found very satisfac-
tory, and although occasionally other foods were supplied in
small quantities, they were on the whole less constantly desired
than the former.

Four problems which had previously been presented to other
organisms were in precisely the same form presented to the three
primates. These problems may be described, briefly, by defini-
tion of the right reaction mechanism, thus: problem 1, the first
mechanism at the subject’s left; problem 2, the second mechan-
ism at the subject’s right (that is, from the end of the series at
the subject’s right); problem 3, alternately, the first mechanism
at the subject’s left and the first at its right; problem 4, the mid-
dle mechanism of the group.

It was my intention to present these four problems, in order,
to each of the three animals, proceeding with them as rapidly
as they were solved. But as it happened, only one of the three
subjects got as far as the fourth problem. When observations
had to be discontinued, Sobke was well along with the last, or
fourth problem; Skirrl was at work at the third problem; and
Julius had failed to solve the second problem.

For each of the problems, a series of ten different settings of
the doors was determined upon in advance. These settings
differ from those employed in a similar investigation with the
pig only in that the numbering of the doors is reversed. In the
present apparatus, the boxes as viewed from the front (entrance)
are numbered from the left to the right end, whereas those of the
pig apparatus were numbered from the right end to the left end.

18

ROBERT M. YERKES

Below are presented for each of the several problems (1) the
numbers of the settings presented in series; (2) the numbers of
the doors open; (3) the number of doors open in each setting and
for the series of ten settings; and (4) the number of the right door.

Settings

WO vescssrneors surg tes mat

Settings

VO beset gusaeaeci santo aoa

PROBLEM 1.

PROBLEM 2.

First mechanism at left of group

No. of
doors open

No. of
doors open

C2WOTWW Ol OTN

ALPORWUIAMAAW

PROBLEM 3. Alternately the first mechanism at the left and the first at

Settings

2
a
NI

WWNNRAL EO
AR WWOTINND
COR RAO WON

the right end of the group

No. of
-doors open

NN EP OOID MH WO

_No. of
right door

on

MENTAL LIFE OF MONKEYS AND APES 19

PROBLEM 4. Middle mechanism of the group

Doors No. of No. of
Settings open doors open right door
ils haeiateennesavuouactinell GAN 2s MAME Sola Ravana sence iinet Niivoaoranauaaclesonhes 3
LASTER aver DO COO os tecovevdrinis sénane aiedyhocesod DE sovsceneiorneucadierstenvlanae 7
Doe incidniecioaes koe L282 4b oO! Cs oo wesiuaeoe Mas Braves aes. haaistonake aes 4
A tad sant tinG Meee anaes STi Oe Osis Hentai can cuay slanetan doce Suen SD esrrarenoetnncine ureters 8
Dab ines acted iceman 415 Be lBnar swan eimai Bie sede urentae maid 6
Gisrieemen actin sone sone 132:324.5:65°7:8. 904 saa265s cron tseccirnseeyiin met 5
(et ee Vi scesachcen desiarastue cade We Cet: Bs esse ge acnnegearnanat 2
8 ss iccominann Gaonnann sets DOIG co ocseetcaisidsativiuaeieaun’ HD ceseonsictehimedeunde teksts 4
IG as iciiuiesdanninonnae BA D6. 7 Bu Dsccssiin csuane arapsiaare'e tea Nodensiaca te unnane 6
OQ istire ccrcavnd tenis aes Gees Oli ned cason Secnsteaanirena wees vote By Meyicievans sata Si cuca 7
Total 5

It was found desirable after a problem had been solved to
present a new and radically different series of settings in order
to determine to what extent the subject had learned to choose
the correct door by memorizing each particular setting. These
supplementary observations may be known as control experi-
ments, and the settings as supplementary settings. In case of
these, as for the original settings, the essential facts are pre-
sented in tabular arrangement.

Settings for Control Experiments
PROBLEM 1. First at left end

Doors No. of No. of
Settings open doors open right door
p Reena ere eee Di Btls gece caeiianuascdshakans a SB iccccal sation Rane iiaes ited
De td ste baba sey ate Gs VB cosas & soeose dubs tee sien Bw seco ine dv axaeaegiinctersie de 6
Oh Sabattus asentanmstoeiaee SB iA eset Secec ttc pat Yaretsncsinastadd cane Sitepanideactdad araspnecite 3
eteesriel ts catuorennins beets A 5 Gel Sicd iw micsainante aoa aires On eanclsing austen yas 4
Oiikeain an wert annua es GATEBi Oss wes s.ince ines ee SLA Ah cay areravincectanants icon atti 6
Gs ctisncre cacmenaess Ds DBA Ses ware areas sear cheers Dy cen ee eee ees Bes 1
Maa Na tcate dees ecaaee tag 235436 Fics ite Sea ned bets wr cach ass SOE e ios 2
Be nich Grane nat eaaccieacn B45. 6.7 Bios bis cae ein ewes Gest ancanrageneaunsactouens 3
Sc sae Abcaua dae anea nase BO sccyacostesasuts cus tecomauenionaene Sb sanedaueneGntncane 5
WO Sed ecaivacacestesd dese’ “oe od BD OAT Oi aeesecare axes Days sar ovacoslesevtv anda ouvsart 1
PROBLEM 2. Second from right end
Doors No. of No. of
Settings open doors open right door
Loscagiensoraredt goes BAGS Ts Bi she a coaietatenn nents Bae a eee Oc weir les tw ace chaste 7
Dass Brats DES REOOS 223A 5 Onn yan ageing eae ee Disa eine anu ee ne yas 5
Sy avsnareaiiad seus srteeusocsiit 1,2.33405:6: 75859) v0 cee wes Ce eee re ee 8
A ct hos Qs nica ee ay sat ISO. a cecencoasanseandaie uwastioeareanued Dadstenst cu ecealeapncauenee 6
is taeseornttvanieelsn aldol D223) (Aan cvincocdunutesmanctrasatanacaconand Ds codes sciiuere saeevacivhunasd 3
IG esdecducirn unsees inne see AIO 3 Aussi acs se tovaseh saeteel avai saat BD anlseaantherdioaniitersesvers 5
UT sccosarar sacs dnasransrey ANG DB SAL ssccuccnienatcie Sid daeseisia OA aba Ae, 2 tient ha tentarorstenslees 4
Bec srttiaietnaneniotiens MDB sovrcutn trict dian coaahoguecs Dias: ascihial glaienoaraaiee lates 2
Os siay cae tecanernmationtnn ates Wh tD Bis Bin S Od vane sista tere sieeom itt i set ens andthe eau sete od 6
lO snacks nae eee es 23 AD Ot GOs 9 wie em incusiaitereaets Shs er eistarmediti awa 8

20 ROBERT M. YERKES

PROBLEM 3. Alternate left and right ends

Doors No. of _No. of
* Setting: open doors open right door
lis ipagiieteeteanedea ame 4 BuiGuiciae (a aiavan + Alain sdb etmentes De ese css is tadegrans Gains’ 5
aces cdi ihetpeecks certo BO rare piclbatnde ateeles binunare Dh song iesre anaes 6
Sov aseeniemarn enon ASD Of JOO accrue cqaiamnas Ge rice awa 6 aa eeton 4
Ans Sats hyetapage ena BS; Oink BO essa. ena re case: cides Giz araheninncdeens oa eae 9
Dye sae aeneninaee aaa DDS A scien vii cone eangethide® ac Marcus Poaiier ene Soe st 1
Gaccrxecwen nee caus s Dt? VB AND tac. core neat cain eda De cies a age pemrewne Glee. hod 5
(Pe eee DOA Or Net ccca waclietcces aang ke G iudornclana dag anon 2
Sookie aa hee bare Deo ADO asian wide Sie ts reat ei O raph gantseiiae epi aiac 7
Oo ae ih ocemmaaraarektn 3.4.5: 6, 7: 8x ses wease agen es Garces are eile aloha pers 3
VO’ saints a spnciconyec heaton’ 3:4 DuOs TsO. /2ecnen gees emends Gawor nay pee nes eeeavers 8
PROBLEM 4. Middle
Doors No. of No. of
Settings open doors open right door
leva dornet any aarnaceacts 4 OT dB snes ae bacine Wawa oo D catia ted Asta eae eat ee 6
Din x esece corer a ery TREC ED lips getntel date? ghia ak a teae na ee ee ee renee 2
Bice sea tleeeon oi mesic ae 1.2,.3.:425,0;78. 9 o0s cmeans Oe A aaee Scheme cme tae ce 5
Bessa see cng Ps awe a 2 Bile OO ae bray Hide k wah icy ada Dy evo tas eo rcnaensseaa A 4
son itcaste Meena vee at pen a8 Ge Cine Maes a econ s Bs wen em Suds same nd a aeees q
Gado Sen nate ADS 34.5.6) 7.80902 5052253 ease LT sae ean Oras ie aaa ceed 6
Ss ioe evonag A Se ig ee TBs Oa easels cosecae hutin Bah a Hie Duwiden pend tek Gua see 8
Bh aaah cunacaivelsietns Le 2 Bis Big GF ars esas dooce toactuens easter Suid esas igi Gesued ae 4
Qi anne tindeen th wae: GA i carseac Syst ausutdatedsion Guar ted Si esitnd hea hae uae 3
DOS Aboatscnene een Wane ne Diet e Dia Oe Minaeib a aa aamiavsaboe i Acnleaseded ID ijciee cod Gaelic Dele tae 5

It was my aim so far as possible to present to a given subject
each day the ten settings under a given problem in order, with-
out interruption. If for any reason the series of observations
had to be interrupted, it was resumed at the same point subse-
quently. Occasionally it was found desirable or necessary to
present only five of the series of ten settings in succession and
then to interrupt observations for an interval of a few minutes
or even several hours. But as a rule it was possible to present
the series of ten settings. All things being considered, it proved
more satisfactory to give only ten trials a day to each subject.
Frequently twenty and rarely thirty trials were given on the
same day. In such cases the series of settings was simply re-
peated. The only pause between trials was that necessary for
resetting the entrance doors and replenishing the food which
served as a reward for success.

Ill

RESULTS OF MULTIPLE-CHOICE EXPERIMENTS

1. Skirrl, Pithecus irus

Problem 1. First at the Left End

Systematic work with the multiple-choice apparatus and
method described in the previous section was undertaken early
in April with Skirrl, Sobke, and Julius. The results for each
of them are now to be presented with such measure of detail as
their importance seems to justify.

Skirrl had previously been used by Doctor Hamilton in an
experimental study of reactive tendencies. He proved so re-
markably inefficient in the work that Doctor Hamilton was led
to characterize him as feeble-minded, and to recommend him
to me for further study because of his mental peculiarities.
With me he was from the first frank, aggressive, and inclined
“to be savage. It was soon possible for me to go into the large
cage, Z, with him and allow him to take food from my hand.
He was without fear of the experimental apparatus and it proved
relatively easy to accustom him to the routine of the experi-
ment. Throughout the work he was rather slow, inattentive,
and erratic.

Beginning on April 7, I sought to acquaint him with the
multiple-choice apparatus by allowing him to make trips through
the several boxes, with the reward of food each time. Thus;,
for example, with the entrance and exit doors of box 7 raised,
the monkey was allowed to pass into the reaction-compartment

’ E.and thence through box 7 to the food cup. As soon as he had
finished eating, he was called back to D by the experimenter
and, after a few seconds, allowed, similarly, to make a trip by
way of one of the other boxes. By reason of this preliminary
training he soon came to seek eagerly for the reward of food.

On April 10 the apparatus was painted white in order to in-
crease the lightness and thus render it easier for the experimenter

22 ; ROBERT M. YERKES

to observe the animal’s movements, and when on April 12 Skirrl
was again introduced to it for further preliminary training, he
utterly refused to enter the boxes, giving every indication of
extreme fear of the white floors and even of the sides of the
boxes. Finally, the attempts to induce him to enter the boxes
had to be given up, and he was returned to his cage unfed. The
following day I was equally unsuccessful in either driving or
tempting him with food into the apparatus. But on April 14
he was so hungry that he was finally lured in by the use of food.
He cautiously approached the boxes and attempted to climb
through on the sides instead of walking on the floor. It was
perfectly evident that he had an instinctive or an acquired fear
of the white surfaces. As the matter was of prime importance
for the success of my work, I inquired of Doctor Hamilton, and
of the men in charge of the cages, for any incident which might
account for this peculiar behavior, and I learned that some three
months earlier, while the animal cages were being whitewashed,
Skirrl had jumped at one of the laborers who was applying a
brush to the framework of one of the cages and had shaken
some lime into his eyes. He was greatly frightened and enraged.
Evidently he experienced extreme discomfort, if not acute pain,
and there resulted an association with whiteness which was
quite sufficient to cause him to avoid the freshly painted
apparatus.

Having obtained an adequate explanation of this monkey’s
peculiar behavior, I proceeded with my efforts to induce him to
work smoothly and rapidly, and on April 15, by covering the
floor with sawdust, I so diminished the influence of the white-
ness as to render the preliminary training fairly satisfactory.
At the end of two more days everything was going so well that
it seemed desirable to begin the regular experiment.

On the morning of April 19, Skirrl was introduced to the
apparatus and given his first series of ten trials on problem 1.
This problem demanded the selection of the first door at the
left in any group of open doors. The procedure was as pre-
viously described in that the experimenter raised the entrance
doors of a certain group of boxes, admitted the animal to the
reaction-chamber, punished incorrect choices by confining the
animal for thirty seconds, and rewarded correct choices by rais-
ing the exit door and thus permitting escape and the obtaining

MENTAL LIFE OF MONKEYS AND APES 23

of food. The trials were given in rapid succession, and the total
time required for this first series of ten trials was thirty-five min-
utes. Skirrl worked faithfully throughout this interval and
exhibited no marked discouragement. When confined in a box
he showed uneasiness and dissatisfaction by moving about con-
stantly, shaking the doors, and trying to raise them in order
to escape.

For the series of settings used in connection with problem 1,
the reader is referred to page 18. In the first setting, the doors
numbered 1, 2, and 3, were opened. As it happened, the animal
when admitted to the reaction-chamber immediately chose box
1. Having received the reward of food, he was called back to
D, and doors 8 and 9 having been raised in preparation for the _
next trial, he was again admitted to the reaction-chamber.
This time he quickly chose box 9 and was confined therein for
thirty seconds. On being released, he chose after an interval of
four minutes, box 8, thus completing the trial.

As it is highly important, not only in connection- with the
present description of behavior, but also for subsequent com-
parison of the reactions of different types of organism in this
experiment, to present the detailed records for each trial, tables
have been constructed which offer in brief space the essential
data for every trial in connection with a given problem.

Table 1 contains the results for Skirrl in problem 1. It is
constructed as follows: the date of a series of trials appears in
the first vertical column; the numbers (and number) of the trials
for the series or date appear in column 2; the following ten columns
present respectively the results of the trials for each of the ten
settings. Each number, in these results, designates a box en-
tered. At the extreme right of the table are three columns
which indicate, first, the number of trials in which the right box
was chosen first, column headed R; and second, the number of
trials in which at least one incorrect choice occurred, column
headed W. In the last column, the daily ratio of these first.
choices appears.

Taking the first line of table 1 below the explanatory headings,
we note on April 19 ten trials, numbered 1 to 10, were given to
Skirrl. In trial 1, with setting 1, he chose correctly the first
time, and the record is therefore simply 1. In trial 2, setting
2, he incorrectly chose box 9, the first time. At his next 9ppor-

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MENTAL LIFE OF MONKEYS AND APES ‘ 25

tunity, he chose box 8, which was the right one. The record
therefore reads 9.8. In trial 3, setting 3, he chose incorrectly
twice before finally selecting the right box. The record reads
6.7.3, and so on throughout the ten trials which constitute a
series. The summary for this series indicates three right and
seven wrong first choices, that is, three cases in which the right
box was entered first. The ratio of right to wrong first choices
is therefore 1 to 2.33. Since the total number of doors open
in the ten settings is thirty-five, and since in each of the ten
settings one door is describable as the right door, the probable
ratio, apart from the effects of training, of right to wrong first
choices is 1 to 2.50. It is evident, therefore, that Skirrl in his
first series of trials closely approximated expectation in the num-
ber of mistakes.

By reading downward in any particular column of results,
one obtains a description of the changes in the animal’s reaction
to a particular setting of the doors. Thus, for instance, in the
case of setting 1, which was presented to the animal in trials
numbered 1, 11, 21, and so on to 141, it is clear from the records
that no definite improvement occurred. But oddly enough,
in the case of setting 10, which presented the same group of
open doors, almost all of the reactions are right in the lower half
of the column. For setting 2, it is evident that mistakes soon
disappeared.

Comparison of the data of table 1 indicates that the number
of correct first choices ‘is inversely proportional to the number
of doors in use, while the number of choices made in a given
trial is directly proportional to the number of doors in use.

During the first week of work on this problem, Skirrl im-
proved markedly. His performance was somewhat irregular
and unpredictable, but on the whole the experiment seemed
fairly satisfactory. Cold, cloudy, or rainy days tended to dimin-
ish steadiness and to increase the number of mistakes. Simi-
larly, absence of hunger was unfavorable to continuous effort to
find the right box.

The period of confinement, as punishment for wrong choices,
was increased from thirty seconds to sixty seconds on April 26.
But there is no satisfactory evidence that this favored the solu-
tion of the problem. Work on May 4 was interrupted by a
severe storm, the noise of which so distracted the njonkey ‘that

26 ROBERT M. YERKES

he ceased to work. Consequently, observations were inter-
rupted on the completion of trial 132, and on May 5S, the series
was begun with setting 3. On this date, eighteen trials were
given in succession, and in only one of them did a mistake occur.
Since the-ten trials numbered 133 to 142 were correct, Skirrl
was considered to have solved problem 1, and systematic train-
ing was discontinued.

On the following day, as a measure of the extent to which the
animal had learned to select the first door at the left no matter
what its position or the number of doors in the group presented,
a control series was given in which the settings differed from the
regular series of settings. These supplementary settings are
presented at the bottom of table 1 together with the records of
reaction in ten trials.

Since in only six of these ten control settings was the first
choice correct, it is scarcely fair to insist that the animal was
reacting on the basis of an ideational solution of the problem.
Rather, it would seem that he had learned to react to particular
settings. A careful study of all of the data of response, together
with notes on the varied behavior of the animal during the
experiments, justifies the statement that Skirrl’s solution of
problem 1 was incomplete and unreliable. It was highly de-
pendent upon the particular situation, or even the particular
door at the left end of the group, and slightly if at all dependent
upon anything comparable to the human idea of first at the
left of the group.

This particular series of observations has been described and
discussed in some detail in order to make the chief points of
method clear. It will be needless, hereafter, to refer explicitly
to many of the characteristics of reaction or to the important
points in the construction of tables which have been mentioned.

A graphic representation of Skirrl’s learning process in problem
1 is presented in figure 18. The irregularities are most strik-
ing, and fairly indicate the erraticness of the animal. The
curve is based upon the data in next to the last column of table
1, that is, the column presenting the errors or wrong first choices
in each series of trials.

Unquestionably, the form of such a curve of learning should
be considered in connection with the method or methods of
selecting the right box employed by the animal during the course

MENTAL LIFE OF MONKEYS AND APES 27

Sobke

Skirrl

6 i Julius

2h

C0 a a ec aa a OTE OO eC ran ree 1

Trials 50 100 150 200 250 300

FIGURE 18.—Error curves of learning for the solution of problem 1
(first box at left end).

of experimentation. It appears from an analysis of the behavior
of Skirrl in problem 1 that there developed a single definite and
persistent method, namely, that of going to one box in the group,
“and in case it happened to be a wrong one, of choosing, on
emergence from it, the next toward the right end of the group,
and so on down the line. Having reached the extreme right
end, the tendency was to follow the side of the reaction-chamber
around to the opposite end and to enter the first box at the
left end of the group, which was, of course, the right one.
This method appears, with certain slight variations, in approxi-
mately ninety per cent of the trials which involved incorrect

choices. Thus, in the case of trials 121 to 130, of which eight

28 ROBERT M. YERKES

exhibit right first choices, the remaining two exhibit the method
described above except that the final member at the ere end of
the group was in each case omitted.

On the whole, Skirrl’s behavior in connection with this prob-
lem appears to indicate a low order of intelligence. He per-
sisted in such stupid acts as that of turning, after emergence
from the right box, toward the right and passing into the blind
alley I, instead of toward the left, through G and H,to D. In
contrast with the other animals, he spent much time before the
closed doors of the boxes, instead of going directly to the open
doors, some one of which marked the box in which the reward of
food could be obtained. It is, moreover, obvious that his re-
sponses, as they appear in table 1, are extremely different from
those of a human being who is capable of bringing the idea of
first at the left end to bear upon the problem in question.

Problem 2. Second from the Right End

Following the series of control trials of problem 1 given to
Skirrl on May 6, a period of four days was allowed during
which the animal was merely fed in the boxes each day. This
was done in order that he should partially lose the effects of his
previous training to choose the first box at the left before being
presented with the second problem, the second box from the
right.

On May 11 regular experimentation was begun with problem
2. Naturally the situation presented unusual difficulties to the
monkey because of his previously acquired habit, and on the
first day it was possible to give only five trials, in all except the
first of which Skirrl had to be aided by the experimenter to find
the right box. He persistently, as appears in the first line of
records of table 2, entered the first box at the left. The series
was continued on May 13, but with very unsatisfactory results,
since he apparently had been greatly discouraged by the unusual
difficulties previously met. Only four trials could be given,
and in these the showing made was very poor. It is noteworthy,
however, that in trials 6, 7, and 8, May 13, there was no marked
tendency to choose the first box at the left. Thus quickly had
the force of the previous habit been broken.

For problem 2, the total number of open doors in the ten set-
tings is ifty, as appears from the data on page 18, and as ten of

‘MENTAL LIFE OF MONKEYS AND APES 29

these fifty open doors may be defined as right ones, the expected
ratio of right to wrong first choices in the absence of previous
training is 1 to 4. The actual ratio for the first series given in
problem 2 is 1 to 8, while in the second series it is 0 to 10.

On the morning of May 13, work was interrupted in the ninth
trial by what seemed at the moment a peculiarly unfortunate
accident, but in the light of later developments, an incident most
fruitful of valuable results.

Skirrl, in trial 9, directly entered box 1. Since this was not
the right box, he was punished by being confined in it for ten
seconds. While in the box he howled and when the entrance
door was raised for him to retrace his steps, he came out with
a rush, showing extreme excitement and either rage or fear, I
could not be sure which. At intervals he. uttered loud cries,
which I am now able to identify as cries of alarm. Repeatedly
he went to the open door of box 1 and peered in, or peered down
through the hole in the floor which received the staple on the
door. He refused to enter any one of the open boxes and con-
tinued, at intervals of every half minute or so, his cries. For
thirty minutes I waited, hoping to be able to induce him to
complete the series of trials, but in vain. Although it was
obvious that he was eager to escape from the apparatus, he
would not enter any of the boxes even when the exit doors were
raised. Instead, he gnawed at the door (12 in fig. 17) to the
alleyway D and attempted to force his way through, instead of
taking the easy and clear route to the alleys, through one of the
boxes. His behavior was most surprising and puzzling. Finally,
I gave up the attempt to complete the series and returned him
to his cage by way of the entrance door to the response-compart-
ment E.

I then entered the apparatus to seek some explanation of the
animal’s behavior, and my search was rewarded by the finding of
two sharp pointed nails which protruded for an inch or more
in the middle of the floor of box 1. My assistant, who had been
charged with the task of installing the locks for the several doors,
had used nails instead of screws for attaching staples underneath
the floor and had neglected to clinch the nails. Skirrl, in the
dim light of the box, doubtless stepped upon one of the nails
and inflicted a painful, although not serious, injury upon him-
self. It was impossible for him to see clearly the source of his

30 ROBERT M. YERKES

injury. He was greatly frightened and expressed: the emotion
most vigorously. His behavior strongly suggested a super-
stitious dread of some unseen danger. It may be that the in-
stinctive fear of snakes, so strong in monkeys, was partly respon-
sible for his response.

The first result of this accident was that more than two weeks
were lost, for it was impossible, during the next few days, to
induce the animal to enter any of the multiple-choice boxes
voluntarily. From May 14 to May 24, I labored daily to over-
come his newly acquired fear. The usual procedure was to
coax him through one box after another by standing at the exit
door with some tempting morsel of food. After several days
of this treatment, he again trusted himself to the boxes, although
very circumspectly and only when both entrance and exit
doors were raised. Not until May 24 was it possible to resume
regular experimentation, and on that day it was found necessary
to indicate the right box by raising the exit door slightly and
then immediately lowering it. Trials in which this form of aid
was given are indicated in table 2 by a star following the last
choice.

Gradually, Skirrl regained his confidence in the apparatus
‘ and began to work more naturally. For a long time he would
not stand punishment, and it was necessary for the experimenter
to be very careful in locking the doors, since the sound of the
bar sliding beneath the floor often frightened and caused him to
quit work. Day after day the tendency to peer through the
‘holes in the floor at the entrance to the boxes rendered it clear
that the animal feared some danger from beneath the floor.
This behavior was so persistent that much time was wasted in
the experiments.

On the last day of May, punishment by confinement for ten
seconds in wrong boxes was introduced, but since this tended
to discourage the monkey, there was substituted for it on June
1 the punishment of forcing him to work his way out of each
wrong box by raising the entrance door which had been closed
behind him. This he could fairly readily do, and his stay in
a box rarely measured more than ten seconds.

As a variation in the mode of procedure, confinement for
thirty seconds was tried on June 5, but it worked unsatisfactorily
and had to be abandoned. During this series, the animal was

MENTAL LIFE OF MONKEYS AND APES 31

startled by the sound from one of the sliding bars under the
floor, and in the sixth trial he refused to work.

As improvement was very slow, varied modes of rewarding
and punishing the animal were tried in the hope of discovering
a means of facilitating the work. Among the former are the
use of banana, grapes, peanuts, and other eagerly sought foods
in varying quantities, and in the latter are included periods of
confinement ranging from ten seconds to sixty seconds. ‘In the
end, confinement of about thirty seconds, combined with a small
quantity of food which was much to the monkey’s taste, gave
most favorable results.

All this time Skirrl’s attention to the task in hand was seldom
good. He was easily diverted and even when extremely hungry,
often stopped work in the middle of an early trial, yawned re-
peatedly and finally sat down to wait for release from the ap-
paratus.

The results obtained during the long continued trials with
this animal in problem 2 are presented in table 2, which differs
from the previously described table, first, in that several of the
trials are followed by an asterisk to indicate that aid was given
by the experimenter, and second, in that two additional columns,
headed, respectively, R and W, are presented. These give the
right and wrong first choices for each day, whereas the two
columns preceding them give the same data for each series of
ten trials. Similarly, the ratio of right to wrong choices is pre-
sented for each day in table 2, instead of for each series of ten
trials as in table 1.

From the results of table 2, several peculiarly interesting
facts appear. In the first place the influence of the habit of
choosing the first box at the left disappears with surprising
suddenness, and in the second place, there are remarkable con-
trasts in the results for different settings as they appear in their
respective vertical columns. Thus, in the case of setting 1,
after the first trial mistakes became relatively infrequent, whereas
in setting 6, which involved the same number of doors, mistakes
continued to be the rule until nearly a thousand trials had been
given. The most likely explanation of this difference is that
for some reason the animal avoided box 9.

The reactive tendencies, or better, the methods of reaction which
manifested themselves during this long series of observations

ROBERT M. YERKES

32

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ROBERT M. YERKES

TABLE 2—Continued

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MENTAL LIFE OF MONKEYS AND APES, 37

may be described as follows: (a) choice of the first box at the
left; (b) random choice with tendency to choose first, a box
near the middle of the group; (c) choice of first box at the right
followed by the one next to it on the left; (d) direct choice of
the right box.

The method of choosing the first box at the right end and
then the one next to it developed in the case of all except two
of the ten settings. The time of appearance is worth noting.
In setting 1, it failed to appear; in setting 2, it developed early,—
after about one hundred trials; in setting 3, after about one hun-
dred and fifty trials; in setting 4, after about one hundred and
fifty trials; in setting 5, after about one hundred and seventy
trials; in setting 6, after about one hundred trials; in setting 7,
after about fifty trials; in setting 8, it never developed; in setting
9, after about fifty trials; and in setting 10, it developed very
late,—after about four hundred and seventy trials.

This method of reaction, although inadequate, proved remark-
ably persistent, and it is doubtful whether it had been wholly
overcome at the conclusion of the experiment. In the case
of the series of trials given on June 8, numbered 191 to 200, the
method used was either that of the first at the right and then
the next, or direct choice of the right box.

Throughout the trials with this problem, the end boxes, num-
bers 1 and 9, were avoided. This is at least partially explained
by the fact that they never existed, and obviously never could
appear, in problem 2, as right boxes. In trials 601 to 610, given
on July 7, there occurred partial return to the formerly estab-
lished method of choosing the first door at the right. This
relapse was characteristic of what happened during the many
days which intervened between the definite appearance of this
habit and the final solution of the problem.

Especially in connection with such relapses, Skirrl showed
extreme fatigue or ennui and often would refuse to work and
simply sit before the open doors yawning. This happened even
when he was extremely hungry and evidently eager enough for
food.

From July 12 on the hunger motive was increased by feeding
the monkey only in the apparatus and by so regulating the
amount of food given in each trial that he should obtain barely
enough to keep him in good physical condition. An increase

38 ROBERT M. YERKES

in the number of correct choices promptly resulted, and con-
tinued until on July 14 the ratio of choices was 1 to .54. It
appeared from these data that a relatively small number of
choices, say not more than.ten a day, the rewards in connection
with which supplied the only food received by the animal, yielded
most favorable results.

On July 16, the period of confinement in wrong boxes was
increased to sixty seconds, and it was so continued for a number
of days. But in the end, it became clear that the period of
thirty seconds, combined with a liberal reward in the shape of
desired food and a single series of ten trials per day, was most
satisfactory. The detailed data of table 2 indicate that at
this time Skirrl was making his choices by memory of the par-
ticular setting.

Skirrl, on July 17 was evidently hungry and eager to locate
food, but seemingly unable to select the right box. In trial 5
(765th) of the series, he was punished by confinement in
box 8. When the doors were unlocked in order that the en-
trance door might be raised to release him, the lock-bar, sliding
under the floor, made a slight grating noise, and the instant the
entrance door was opened, he jumped out excitedly. He made
no outcry, but as soon as he was out of the box, sat down, and taking
up his right hind foot, examined it for a few seconds. Having
apparently assured himself that nothing serious had happened,
he went on unconcernedly about his task. The presumption
is that the sound of the lock-bar, associated as it was with his
painful experience in box 1, revived the strongly affective ex-
perience of stepping on the nail. Psychologically described, the
sound induced an imaginal complex equivalent to the earlier
painful experience. The behavior seems to the writer a most
important bit of evidence of imagery in the monkey.

Finally, on August 9, after ten hundred and seventy trials,
Skirrl succeeded in choosing correctly in the ten trials of a series,
and he was therefore considered to have solved the problem of
the second door from the right end of the group.

On the following day, he was given a control series with the
settings which are presented on page 19 and also at the bottom
of table 2. In this series he chose correctly five times,—in other
words, as often correctly as incorrectly. An analysis of the
choices indicates, however, that two of the five correct choices

MENTAL LIFE OF MONKEYS AND APES 39

were made in box 8, which, as it happened, had proved a
peculiarly easy one for him throughout the training, since from
the first he tended to avoid door 9. Consequently, it is only
fair to conclude, from the results for this control series and for
those given on August 11 and 12, that the animal chose not on
the basis of anything remotely resembling a general idea of
secondness from the right end, but instead on the basis.of gradu-
ally acquired modes of reaction to the particular settings. This
conclusion is strengthened by the fact that he had failed to
learn to react appropriately and readily to most of the settings
of the regular series.

The curve which represents the course of the learning process
in this problem is presented in figure 19. For this and all other
curves which involve more than a single series of observations
a day, the method of construction was as follows: The first series
for each day of training is, indicated on the curve by a dot, while
the second or third series on a given day, although space is allowed
for them, are not so indicated. Consequently, the form of the
curve is determined chiefly by the first series per day. The ex-
treme irregularities of this curve are most interesting and puzzl-
ing, as are also the variations in the daily ratios of right to wrong
first choices. Three times in the course of the training, this ratio
rose to 1 to 9, or higher. The causes for such extreme varia-
tions are not easily enumerated, but a few of the’ most obvious
contributory causes are variations in the weather, especially
cloudiness or fogginess, which rendered the apparatus dark;
variations in the degree of hunger or eagerness for food; differ-
ences in the activities of the animals in the cages outside of the
laboratory (sometimes they were noisy and distracted the sub-
ject), and finally, differences in the physical fitness and attitude
of the animal from day to day.

The more or less incidental behavior in connection with this
experiment more strongly than the statistical results of the work
on problem 2 indicate the existence of imagery. That ideas
played a part in the solution of the problem is probable, but
at best they functioned very ineffectively. The small number
of methods used in the selection of the right box, and the slight
variations from the chief method, that of choosing the first box
at the right end and then the one next to it, apparently justify
Doctor Hamilton’s characterization of this monkey as defective.

ROBERT M. YERKES

40

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MENTAL LIFE OF MONKEYS AND APES 41

Problem 3. Alternately First at’ Left and First at Right

Following the control series given in connection with problem
1, an interval of rest lasting from August 12 to August 19 was
allowed in order that Skirrl might in part at least lose the effects
of his training and regain his customary interest in the appa-
ratus by being allowed to obtain food easily instead of by dint
of hard labor,—labor which was harder by far, apparently, than
physical activity because it demanded of the animal certain
mental processes which were either lacking or but imperfectly
functional. The difficultness of the daily tasks appears to be
teliably indicated by the tendency to yawn.

Systematic work on problem 3, which has been defined as
alternately the first door at the left and the first door at the
right of the group, was begun August 19, and for nine days a
single series of ten trials per day was given. Work then had
to cease because of the experimenter’s return to Cambridge.

The results of the work on this problem demand but brief
analysis and comment. The expected ratio of one right to four
wrong choices per series appears (see table 3) for the first series
of trials, and this in spite of the fact that Skirrl had been trained
for several weeks to choose the second door from the right end. One
would or have predicted a much larger number of in-
correct choices. e right choices were due to the monkey’s
strong tendency to go first to the first door at the right and
thence to the one next/to it. Indeed in the series given on August
24; this method was followed without variation. In other words,
in every one of the ten trials Skirrl entered first the box at the
extreme right end of the group. This necessarily resulted in
as many right as wrong first choices. Consequently, the ratio
reads 1 to 1. But the method was not adhered to, and at no
time either before or after that date did he succeed in equalling
this achievement. There was, as a matter of fact, no steady.
improvement, and so far as one may judge from the records
which were obtained, the course of events in the solution of this
problem would have been similar to those in problem 2.

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ROBERT M. YERKES

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MENTAL LIFE OF MONKEYS AND APES 43

2. Sobke, Pithecus rhesus
Problem 1. First at the Left End

Sobke was somewhat afraid of the experimenter when the
investigation was undertaken, and instead of willingly coming
out of his cage when the door was raised, he often had to be
coaxed out and lured into the apparatus with food. Whereas
Skirrl was frank and rather aggressive, Sobke was stealthy in
his movements, furtive, and evidently suspicious of the experi-
menter as well as of the apparatus. He was perfectly safe to
approach, but would not permit anyone to touch him. After
a few days, he began to take food from the hands of the experi-
menter. ;

Preliminary work to acquaint this monkey with the routine
of the experiment was begun on April 13. As in the case of
Skirrl, he was lured into the apparatus and was taught the
route through the boxes to the starting point by being allowed
to obtain food once each day in each of the nine boxes. The
procedure was simple. The entrance door and the exit door
of a particular box were raised and the animal admitted to the
reaction-compartment and permitted to pass through the box
whose doors stood open, take its food, and return to the starting
point. Sobke very quickly learned the route perfectly and
came to work steadily and rapidly. After five days of prelim-
inary work of this sort, he was so thoroughly accustomed to
the apparatus that it was evidently desirable to begin with regu-
lar training experiments.

The first series of trials was given on-April 19. Both punish-
ment and reward were employed from the first. The punish-
ment consisted of confinement for thirty seconds in each wrong
box, and the reward of a small piece of banana, usually not
more than a tenth of a medium sized banana for each correct
choice. The total time for the first series of trials was fourteen
minutes. This indicates that Sobke worked rapidly. My
notes record that he worked quickly though shyly, wasted almost
no time, made few errors of choice, and waited quietly during
confinement in the boxes. In this, also, he differed radically
from Skirrl who was restless and always tried to escape from
confinement.

Throughout the work on problem 1, punishment and reward

44 ROBERT M. YERKES

were kept constant. Everything progressed smoothly; there were
no such irregularities of behavior as appeared in the case of
Skirrl, and consequently the description of results is a relatively
simple matter. Sobke invariably chose the end boxes. His
performance was in every way superior to that of Skirrl.,

As previously, the detailed results are presented in tabular
form (table 4). From this table it appears that, whereas the
expected ratio of right to wrong first choices for this problem
is 1 to 2.5, the actual ratio for Sobke’s first series was 1 to .67.
This surprisingly good showing is unquestionably due to his
marked tendency to choose the end box of a group; and this
tendency, in turn, may in part be the result of the preliminary
training, for during that only one box was open each time. But,
if the preliminary training were responsible for Sobke’s tendency,
it should be noted that it had very different effect upon Skirrl,
and, as will be seen later, upon Julius.

The results for the ten different settings of the doors for prob-
lem 1 as they appear in table 4 are of interest for a number of
reasons. Inthe first place, the setting 1. 2. 3 appearing twice,—
at the beginning of the series and again at the end—yielded
markedly different results in the two positions. For whereas
no mistakes were made in the case of setting 1, there were fifty
per cent of incorrect first choices for setting 10. Again, satis-
factory explanation is impossible. It is conceivable that fatigue
or approaching satiety may have had something to do with the
failures at the end of the séries, but as a rule, as is indicated by
settings 1, 2, and 6, if correct choices were made at the beginning,
they continued throughout the day’s work.

In this problem, Sobke’s improvement was steady and fairly
rapid, and in the eighth series, trials 71 to 80, only correct first
choices appear. Consequently, seventy trials were required
for the solution of the problem. This number is in marked
contrast with Skirrl’s one hundred and thirty-two trials.

Immediately following the first perfect series, Sobke was given
two series of control tests on April 28. Conditions were un-
favorable, since the day was stormy and the rain pattering on
the sheet-iron roof made a great din. Nevertheless, he worked
steadily and well up to the sixth trial, which was preceded by
a slight delay because of the necessity of refilling some of the
food boxes. After this interruption, wrong choices occurred in

45

MENTAL LIFE OF MONKEYS AND APES

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46 ROBERT M. YERKES

trial 6. And again after trial 9, there was brief interruption,
followed by wrong choices in trial 10. The ratio of right to
wrong choices for this first control series was therefore 1 to .25.

Six minutes after completion of the first control series, a sec-
ond was given under ‘slightly more favorable conditions, and
in this only a single wrong choice occurred, in that box 2 was first
chosen in trial 6 instead of box 1. From the results of these
two control series, it is evident that Sobke’s solution of problem
1 is reasonably adequate. He is easily diverted or disturbed
in his work by any unusual circumstances, but so long as every-
thing goes smoothly, he chooses with ease and certainty.
Whether it is fair to describe the behavior as involving an idea
of the relation of the right box to the other members of the
group would be difficult to decide. I hesitate to infer definite]
ideation from the available evidence, but I strongly suspect
the presence of images and relatively ineffective or inadequate
ideation.

It is perfectly evident that Sobke is much more intelligent
than Skirrl. In practically every respect, he adapted himself
more quickly to the experimental procedure and progressed more
steadily toward the solution of the problem than did Skirrl.
The contrast in the learning processes of the two monkeys could
scarcely be better exhibited than by the curves of learning which
are presented in figure 1892 he first, that for Sobke, is sur-
prisingly regular; the second, that for Skirrl, is quite as sur-
prisingly irregular. These results correlate perfectly with the
steadiness and predictability of the former’s responses and the
irregularity and erraticness of the latter’s.

Problem 2. Second from the Right End

On the completion of problem 1 Sobke was in perfect condi-
tion, as to health and training, for experimental work. He had
come to work quietly, fairly deliberately, and very steadily.
His timidity had diminished and he would readily come to the
experimenter for food, although still he was somewhat distrustful
at times and became timid when anything unusual occurred in
the apparatus.

As preparation for problem 2, a break in regular experimen-
tation covering four days followed the control series of problem
1. On each of these four days the monkey was allowed to get

MENTAL LIFE OF MONKEYS AND APES 47

food once from each of the nine boxes, both doors of a given box
being open for the trial and all other doors closed. For this
feeding experiment, the doors were opened in: acregtles order,
and this order was changed from day to day.

Systematic work with problem 2 began on May 3, with pun-
ishment of thirty seconds for mistakes and a liberal reward of
food for each success. Early in the series of trials it’ was dis-
-covered that Sobke was likely to become discouraged and waste
a great deal of time unless certain aid were given by the experi-
menter. On this account, after the first’two trials, the method
was adopted of punishing the animal by confinement for the
first ten mistakes in a trial, and of then, if need be, indicating
the right box by slightly and momentarily raising the exit’ door.
Every trial in which aid was thus given by the experimenter is
indicated in table 5 by an asterisk following the last choice. In
the first series of trials for this problem, aid had to be given in
seven of the ten trials, and even so the series occupied seventy-
one minutes. It is possible that had'no aid been given, the
work might have been continued successfully with a smaller
number of trials than ten per day. But under the circumstances
it seemed wiser to avoid the risk of discouraging and thus spoil-
ing the animal for use in the experiment. It should be stated,
also, that it proved impossible to adhere to the period of thirty
seconds as punishment in this series. For the majority of the
wrong choices confinement of not more than ten seconds was used.

For the second series, given on May 4, the conditions were
unfavorable in that it was dark and rainy, and the noise of the
rain on the roof frightened Sobke. He refused to work after
the fourth trial, and the series had to be completed on the fol-
lowing day. The total time required for this series was seventy-
eight minutes.

The work on May 6 was distinctly better, and the animal’s
behavior indicated, in a number of trials, definite recognition
of the right door. He might, for example, make a number of
incorrect choices, then pause for a few seconds to look steadily
at the doors, and having apparently found some cue, run directly
to the right box. No aid from the experimenter was needed in
this series.

On the following day improvement continued and athe ani-
mal’s method of choosing became definite and fairly precise. He

48 ROBERT M. YERKES

was deliberate, quiet, and extremely business-like. The time
for the series was thirty-one minutes.

The period of punishment was increased on May 12 to thirty
seconds. Previously, for the greater number of the trials, it
had been ten to fifteen seconds. This increase apparently did
not disturb the monkey, for he continued to work perfectly
throughout the series, although making many mistakes in spite of
deliberate choices and the refusal of certain boxes in each trial.

An interesting and significant incident occurred on May 13
when at the conclusion of trial 5, Doctor Hamilton came into
the experiment room for a few minutes. Sobke immediately
stopped working, and he could not be induced to make any
choices until Doctor Hamilton had left the room. This well
indicates his sensitiveness to his surroundings, and his inclina-
tion to timidity or nervousness even in the presence of conditions
not in themselves startling.

Work was continued thus steadily until May 28 when, because
of the failure of the animal to improve, it seemed wise to increase
the period of confinement as punishment to sixty seconds. In
the meantime, it had sometimes been evident that Sobke was
near to the solution of his problem. He would often make cor-
rect choices in three or four trials in succession and then appar-
ently lose his cue and fail utterly for a number of trials.

After June 1, in order to hasten the solution of the problem,
two series per day were given. In some instances the second
series was given almost immediately after the first, while in
others an interval of an hour or more intervened. It was fur-
ther found desirable to give Sobke all of his food in the appa-
tatus. When the rewards obtained in the several trials did not
satisfy his hunger, additional food was presented, on the com-
pletion of the series of experiments, in one or more of the food
cups. On days marked by unwillingness or refusal to work,
very little food was given. Thus, the eagerness of the monkey
to locate the right box was increased and, as a matter of obser-
vation, his deliberateness and care in choice increased corre-
spondingly. Sixty seconds punishment was found satisfactory,
and it was therefore continued throughout the work on this
problem.

It was evident, on June 9, from the behavior of the monkey
as well as from the score, that the perfect solution of the prob-

MENTAL LIFE OF MONKEYS AND APES 49

lem was near at hand. This fact the experimenter recorded
in his daily notes, and sure enough, on the following day Sobke
chose correctly throughout the series of ten trials. The time
for this series was only ten minutes. The choices were made
deliberately and readily. ;

An analysis of the data of table 5 reveals five methods or
reactive tendencies which appeared more or less definitely in
the following order: (a) Choice of first box at the left, because
of experience in problem 1. This tendency was very quickly
suppressed by the requirements in connection with problem 2.
Indeed one of the most significant differences which I have dis-
covered between the behavior of the primates and that of other
mammals is the time required for the suppression of such an
acquired tendency. The monkey seems to learn almost imme-
diately that it is not worth while to persist in a tendency which
although previously profitable no longer yields satisfaction,
whereas in the crow, pig, rat, and ring dove, the unprofitable
mode of response tends to persist during a relatively large.
number of trials. (b) The tendency to choose, first, a box near
the left end of the group, to go from that to the box at the
extreme right end of the group, thence to the one next in order,
which was, of course, the right box. This tendency appears
fairly clearly from May 7th on. (c) The box at the extreme
right was first chosen and then the one next to it. For example,
in setting 2, box 4 would be chosen first, then box 3. Or, if
this did not occur, the method previously described under (b)
was likely to be employed, as for example, in setting 8, where
such choices as 7.6.5.1.8 appear. (d) In certain series there
appeared. a marked preference for a particular box, usually
box 3 (see results for May 24). This was doubtless due in a
measure, if not wholly, to the fact that box 3 was the right box
twice in each series of ten settings. But it should be added that
the same is true of box 7, for which no preference was mani-
fested at any time. (e) Direct choice of the right box.

The five reactive methods or tendencies enumerated above
roughly appeared in the order named, but there were certain
irregularities and the order as well as the time of appearance
varied somewhat from setting to setting. In general, method
c was the most frequently used prior to the development of
method e, the direct choice of the right box.

50

TABLE 5
Results for Sobke, P. rhesus, in Problem 2

ROBERT M. YERKES

2 ~
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MENTAL LIFE OF MONKEYS AND APES

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51

enter.

rim

* Aided by expe

52 ROBERT M. YERKES

Examination of table 5 indicates that some of the settings
proved very easy for Sobke; others, extremely difficult. Con-
sequently, the number of methods which were tried and re-
jected for a given setting varies from two to five. Setting 2
proved a fairly simple one, and after the inhibition of the
tendency to choose the first box at the left, the only definite
tendency to appear was that of choosing the first box at the
right, and then the one next to it. After one hundred and
thirty trials, this method suddenly gave place to direct choice
of the right box, and during the following twenty-eight series,
no error occurred for this setting. Setting 4, on the contrary,
proved extremely difficult, and a variety of methods is more
or less definitely indicated by the records.

It is needless to lengthen the description by analyzing the
data for each setting, since the reader by carefully scanning the
columns of data in table 5 may observe for himself the various
tendencies and their mutual relations. yo

Sobke’s curve of learning (figure Loy problem 2, is extremely
irregular, as was that of Skirrl. Similar irregularities appear
in the daily ratios of right to wrong first choices presented in
the last column of table 5. Most of these irregularities were
due, I have discovered, to unfavorable external conditions.
Thus, dark rainy days and disturbing noises outside the labora-
tory were obviously conditions of poor work.

On the day following the final and correct series for problem
2, a control series was given. In this Sobke seemed greatly
surprised by the new situations which presented themselves.
Repeatedly he exhibited impulses to enter the box which would
have been the correct one in the regular series of settings. He
frequently inhibited such impulses and chose correctly, but at
other times he reacted quickly and made mistakes. It was
evident from his behavior that he was not guided by anything
like a definite idea of the relation of the right box to the other
members of the group.

In a second control series given on the following day, June 12,
confusion appeared, but less markedly. For the first setting,
a correct choice was made with deliberation. For the second
setting, box 3 was immediately chosen, as should have been the
case in the regular series of settings. Sobke seemed confused
when he emerged fronr this box and had difficulty in locating

MENTAL LIFE OF MONKEYS AND APES 53

the right one. Then followed direct correct choices for settings
3,4, and 5. For setting 6, there is recorded a deliberately made
wrong choice, and so on throughout the series, the choices being
characterized by deliberateness and definite search for the right
box. Uncertainty was plainly indicated, and in this the be-
havior of the animal differed markedly from that in the conclud-
ing series of the regular experiment. a
It seems safe to conclude from the results of these control’
series that Sobke has no free idea of the relation of secondness
from the right and is chiefly dependent upon memory of the
particular settings for cues which lead to correct choice.

Problem 3. Alternately First at Left and First at Right

For four successive days after the last control series in con-
nection with problem 2, Sobke was merely fed in the apparatus
according to previous description (p. 43). He exhibited a won-
derfully keen appetite and was well fed during this interval
between problems.

The method of experimentation chosen for problem 3 in the
light of previous experience was that of confining the monkey
for a short time, ten to fifteen seconds, in the wrong box, in each
of the first ten mistakes for a given trial, and of then aiding
him to find the right box by the slight and momentary raising
of the exit door. Aid proved necessary in a few of the trials
during the first four days. Then he worked independently.
As work progressed it was found possible and also desirable
to increase the period of confinement, and in the end, sixty sec-
onds proved satisfactory. It was also thought desirable to in-
crease the number of trials per day from a single series during
the early days to two or even three series from June 29 on.
Often three series could be given in succession without difficulty.
During the early trials on this problem Sobke worked remark-
ably well, but later his willingness diminished, evidently because
of his failure readily to solve the problem, and it became extremely
difficult to coax him into the apparatus. On days when he
entered only reluctantly and as it seemed against his will, he
was likely to be nervous, erratic, and often slow in making his
choices, but above all he tended to waste time by not returning
to the starting point, preferring rather to loiter in the alleyways
or run back and forth.

54

TABLE 6
Results for Sobke, P. rhesus, in Problem 3

9 20 No &
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56 ROBERT M. YERKES

The data of table 6 indicate for this problem only three pro-
nounced reactive tendencies: (a) As the initial tendency, the
choice of the second box from the right end. This proved sur-
prisingly weak, in view of the animal’s long training on problem
2, and it disappeared quickly. (b) Choice of the end boxes, and
(c) direct choice of the right box.

For this, as for the other problems, extreme differences in
method and in time and degree of success appear for the different
settings. Thus, while settings 1, 2, 3, 6, 7, and 10 proved to
be easy, settings 4, 5, 8, and 9 were evidently more difficult.

Trials 50 100 150 200 250 300 “350400 750

FiGuRE 20.—Error curve of learning for the solution of problem 3 (alternately
the first box at the left end and the first at the right end) by Sobke.

From the first this problem promised to be much easier for
Sobke than problem 2, and although the actual number of trials
necessary for the solution is greater by sixty for problem 3 than
for problem 2, comparison of the data of the tables justifies the
statement that the third problem was both easier and more
nearly adequately solved than the second. This is not surpris-
ing when the nature of the two problems is considered, for
whereas problem 2 requires choice by perception of the rela-
tionship of secondness from the right end of the group, problem
3 requires, instead, the choice of the end member of the group
each time, with the additional variation of alternation of ends.
Now as it happens, the end member is easily selected by the mon-
key, and it appears further that alternation was relatively easy
for Sobke to acquire. Consequently, the combination of end
and alternation proved easier than the choice of the second
from the right end of the group.

The above statements are supported by comparison of the
curves of learning. The curve for problem 2, figure 19, is ex-

MENTAL LIFE OF MONKEYS AND APES 57

tremely irregular; that for problem 3, figure 20, much more
regular. Similarly, the daily ratios of right to wrong choices
as exhibited in tables 5 and 6 indicate smaller variations for
the third problem than for the second.

Sobke made ten correct first choices in the third series for
July 17, but he was working very uncertainly and it seemed
rather a matter of good luck than good management that he
succeeded in presenting this perfect series: For this reason
and also because it did not seem feasible to have Sunday inter-
vene between the final and perfect regular series and the control
series, an additional regular series was given on July 19, in
which, as the table indicates, a single mistake occurred, in trial
5. The monkey was working perfectly. The series of trials
required only ten minutes, and it was evident that carelessness
and eagerness to obtain food were chiefly responsible for the
mistake. | :

The control series given on July 19 immediately after the
series just described resulted similarly in one failure and nine
successes. The choices were made easily and with certainty,
and the only mistake, that of setting 7, was apparently due to
carelessness.

This excellent showing for the control series wholly justifies
the comparison of problems 2 and 3 as to difficultness, made
above. Whereas in both problems 1 and 2 the control trials
caused confusion, in the case of problem 3, they did not essen-
tially alter the behavior of the animal. The fact seems to be
that for this problem the particular setting is of relatively little
importance; while turning alternately to the extreme left and
the extreme right is of prime importance. That Sobke had the
idea of alternation or of the end box, there seems no more reason
for insisting thari that he had the idea of secondness from the
right end in problem 2. It is possible, even probable, that these
ideas existed rather vaguely in his consciousness, but there is
obviously no necessity for insisting that the solution of the
problems depended upon them.

Problem 4. Middle
As the available time for the continuation of the experiment
was limited, it was decided to proceed with work on problem 4
immediately upon the completion of problem 3, and on July 20,

58 _ ROBERT M. YERKES

the problem of the middle door was presented to Sobke. Since
it was anticipated that this sudden change would confuse and
discourage him greatly, the only form of punishment admin-
istered was the momentary lowering of the entrance door ‘of
the wrong box. As in the previous problem, he was aided after
ten successive wrong choices. As might have been anticipated,
he persistently entered the end boxes of the groups, and this
in some instances probably would have been kept up for many
miriutes had not the experimenter lured him into the right box
by slightly raising the exit door. In the first series, he had to
be aided in five of the ten trials. The total time for the series
was forty-five minutes, the total number of choices, eighty-eight.
In the second series, he was aided’ in four of the trials. The
total time required was seventy-two minutes, and the total
number of choices was seventy-six.

Throughout the first series, Sobke worked hard, but with evi-
dently increasing dissatisfaction. He clung persistently to his
acquired tendency to choose the end boxes, and after each trial
he returned less willingly to the starting point.

Up to this time his attitude toward the experimenter had
been perfectly friendly, if not wholly trustful. But when on
July 21 he was brought into the apparatus for the second series,
he exhibited a wholly new form of behavior, for instead of at-
teriding diligently to the open doors and devoting his energies
to trying to find the right box, he instead, after gazing at them
for a few seconds, turned toward the experimenter and jumped
for him savagely, throwing himself against the wire netting
with great force. This was repeated a number of times during
the first two or three trials, and it occurred less frequently later
in the series. Since nothing unusual had happened outside of
the experiment room, the suggested explanation of this sudden
change in attitude and behavior is that the monkey resented and
blamed on the experimenter the difficulty which he was having
in obtaining food.

From this time on until the end of my work, Sobke was
always savage and both in and out-of the apparatus he was
constantly on the watch for an opportunity to spring upon me.
Previously, it had been possible for me to coax him into the
apparatus by offering him food and to return him to his cage
by walking after him. But on and after the twenty-first of

MENTAL LIFE OF MONKEYS AND APES 59

July, it was impossible for me to approach him without extreme
risk of being bitten.

Doctor Hamilton when told of this behavior, reported that
several times monkeys have shown resentment toward him when
they were having trouble in the experiment. I therefore feel
fairly confident that I have not misinterpreted Sobke’s behavior.

When on July 22 I gave Sobke an opportunity to enter the
apparatus, he refused, and it was impossible to lure him in with
food. Two hours later, having waited meantime for his break-
fast, he entered readily and worked steadily and persistently
through his third series of trials, but in no one of these trials
did he choose correctly. Neither on this day nor the following
did he exhibit resentment while at work. He apparently had
regained his affective poise and was able to attend as formerly
to the task of locating his rewards.

During these first three series, although the ratio of right to
wrong choices stood 0 to 10, there occurred a marked reduction
in the number of trials in which aid was necessary as well as
in the total number of choices, and on July 23 correct reactions
began to appear. Improvement during the next hundred trials
was steady. and fairly rapid, and on July 31, a record of seven
right to three wrong trials was obtained. This was surprising
to the experimenter, as well as gratifying, since he was eager to
have the animal complete this problem before work should have
to be discontinued.

Everything went smoothly until August 2, when my assistant,
who had been left in charge of the experimental work for a week,
attempted to increase the number of trials per day to two series.
Sobke apparently was not quite ready for this increase in the
amount of his day’s labor and refused to work at the end of the
first series. In this series he did less well than on the previous
day. The following day, August 3, unfortunately and contrary to
the wishes of the experimenter, the laboratory was painted and
there was necessarily considerable disturbance because of the
presence of the workmen, and in addition, the pervasive odor
of fresh paint. Sobke chose still less successfully on this date,
and on August 4, he refused to work after the eighth trial. It is
true that during these bad days the total number of choices
steadily diminished while the successes, also, diminished, or at
best, failed to increase. When on August 9, I returned to the

60

TABLE 7
Results for Sobke, P. rhesus, in Problem 4

ROBERT M. YERKES

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MENTAL LIFE OF MONKEYS AND APES

* Aided by experimenter.

61

62 ROBERT M. YERKES

laboratory to take charge, I found that Sobke was no longer
trying to solve the problem as when I had gone away. His
attitude had changed in that he had become indifferent, care-
less, and obviously discouraged with his task.

I immediately set about reinstating the former attitude by
lessening the number of trials and the punishment, and by in-
creasing the value of the reward, but my best efforts, continuing
up to August 28, failed markedly to improve the condition. The
number of correct choices did somewhat increase, but at no
time did the animal attain the degree of success which he had
achieved as early as July 31 in the eleventh series of trials.

During the last two weeks of experimentation, all possible
efforts were put forth to discover the best combination of re-
wards and punishments. Punishment was varied from 0 to
confinement of sixty seconds, and many kinds of food in different
amounts were tried as rewards, but in spite of everything
Sobke failed to improve markedly. From time to time, notably
on August 12 and 21, he exhibited peculiarly strong resentment
toward me and repeatedly attempted to attack me.

The outcome of my experiments with problem 4 is peculiarly
interesting in that it indicates the importance of a favorable
attitude toward the work and the extreme risk from disturbing
or discouraging conditions. It seems not improbable that had
the work progressed without change in experimenter, or method
of procedure, and above all without the disturbance of the paint-
ing, Sobke might have solved problem 4 within a few days.
This is by no means certain, however, for in problems 2 and 3
the ratio of right to wrong choices instead of increasing steadily
increased very irregularly.

The detailed results for this problem are given in table 7.
Reactive tendencies which appear are: (a) persistent choice of the
end boxes followed, subsequently, by (b) the tendency to locate the
middle box directly. This proved fairly easy when the number
of boxes involved was only three as in settings 1, 4, 7, and 10.
Setting 4 was most difficult of all, because box 9 was avoided
or ignored. When the number of open boxes was as great as
five, as in settings 2 and 8, ‘the task was obviously more difficult,
but whereas success in setting 2 appeared early, in setting 8 it
failed to appear during the course of experimentation. For
the settings 3, 6, and 9, involving either seven or nine open boxes,

MENTAL LIFE OF MONKEYS AND APES 63

the direct choice of the middle box was next to impossible, and
Sobke tended to choose, first of all, a particular box toward one
end of the series, for example, box 2, in setting 3, and box 7 in
setting 9. To the experimenter, as he watched the animal’s
behavior, it looked as though effort each time were being made
to locate the middle member of the group. This appeared
relatively easy for groups of three boxes, extremely difficult
for as many as five boxes, and almost impossible for seven or
nine.
3. Julius, Pongo pygmaeus
Problem 1. First at the Left End

The orang utan, Julius, was gentle, docile, and friendly with
the experimenter throughout the period of investigation. He
at no time showed inclination to bite and could be handled safely.
As contrasted with Skirrl and even with Sobke, he adapted
himself to the multiple-choice apparatus very promptly, and
only slight effort on the part of the observer was necessary to
prepare him, by preliminary trials, for the regular experiments.
But in order to facilitate work, he was familiarized with the
apparatus by means of regular route training and feeding in
the several boxes from April 5 to April 9.

On April 10 the apparatus was painted white as has been
stated previously, and on the following Monday, April 12,
Julius when again introduced to it gave no indications of fear,
uneasiness, or dislike, but worked as formerly, making his round
trips quickly and eagerly entering any box which happened to
be open, in order to obtain the reward of food.

The regular experimentation was undertaken on April 13,
and the results of the first series of trials with Julius are sharply
contrasted with those obtained with the monkeys in that fewer
choices were necessary. Instead of the expected ratio of right to
wrong: first choices, 1 to 2.5, the orang utan gave a ratio of I to
1. An additional markedly different result from that obtained
with the monkeys is indicated below in the total time required
for a series of trials. As examples, the data for the first, second,
fifth, and tenth series are presented.

TIME FOR SERIES OF TRIALS
lst series 2nd series 5th series 10th series

SHIT sie.cs sieeve tos 35 min. 20min, 14 min. 10 min,
Sobke............. 14 “ 17“ 10 3 . (8th series)

JUUS sce igexeees 12 5 14 “

64 ROBERT M. YERKES *

It is also noteworthy that Julius in the presence of visitors
or under other unusual conditions worked steadily and well,
whereas the monkeys, and especially Sobke, tended to be dis-
tracted and often refused to work at all.

Almost from the beginning of his work on problem 1, Julius
began to develop the tendency to enter immediately the open
door nearest the starting point. In case the group of open
doors lay to the right of the middle of the apparatus, this method
naturally yielded success; whereas if the group included doors
to the left of the middle, it resulted in failure. Obviously it
was a most unsatisfactory method, and although it enabled him
to make more right than wrong first choices, it prevented him
from increasing the number of right choices, and as table 1
indicates, it maintained the ratio of 1 right to .67 wrong first
choices for eight successive days.

On April 23 a break occurred in which the number of correct
choices was reduced from six to five. Julius worked very rapidly
and with almost no hesitation in choosing. My notes record
“he seems to miss‘the point wholly. It is doubtful whether
the punishment is sufficiently severe.” At this time he was
being punished by thirty seconds confinement in each wrong
box, the interval having been held fairly steadily from the first
series of experiments. On April 26 it was increased to sixty
seconds, in an effort to break him of the habit of choosing the
“nearest ’’ door. But he became extremely restless under the
longer confinement and tried his best to raise the entrance and
exit doors. Since there was at this time no mechanism for
locking them when closed, it was difficult for the experimenter
to prevent him from escaping by way of the entrance door or
from raising the exit door sufficiently to obtain the food. In-
deed, the longer confinement worked so unsatisfactorily that on
the following day I substituted for it the punishment of forcing
him to raise the entrance door of the wrong box in order to escape
for a new choice. He was rewarded with food in the alleyway
H, beside door 15 (figure 17), only when he chose correctly on
first attempt.

This method discouraged him extremely and proved wasteful
of time. Consequently, in a second series on the same date
return was made to the former method, and he was rewarded
with food whenever he found the right box. But on April 28,

MENTAL LIFE OF MONKEYS AND APES , 65

the two methods were again employed, the first in the initial
series and the second in a final series of trials. The animal’s
persistent attempts to raise the doors gave the experimenter
so much trouble that on April 29 barbed wire was nailed over
the windows of the entrance doors with the hope that it might
prevent him from working at them. But he quickly learned to
place his fingers between the barbs and raise the doors as effec-
tively as ever.

On April 30 the reward of food was given only when the first
choice was that of the right box and in that event it was placed
in the alleyway H as stated above.

As it seemed absolutely essential to break the unprofitable
habit of choosing the nearest door, on May 3 a new series of
settings was presented, in which only the doors to the left of
the middle of the row of nine boxes were used as right doors.
That is, in this new series, doors 1 to 4 occur as right doors; 5
to 9 do not. As punishment for wrong choices on this date,
Julius was confined in the wrong box from one to five minutes.
It was difficult to keep him in, but by means of cords which had
been attached to the doors, this was successfully accomplished.
Yet another and slightly different series of settings was employed
on May 4, and this, proving satisfactory, was continued in use
until the end of the experiment, with punishment ranging from
sixty to one hundred and twenty seconds for each mistake.

Naturally the modification of settings introduced May 3
greatly increased the proportion of wrong first choices. Indeed,
as appears in table 8, the ratio of right to wrong immediately
changed from 1:.67 to 1:4.00. Between May 3 and May 10,
no steady and consistent improvement in method or in the
number of correct first choices occurred, and on the last named
date, Julius chose correctly only three times in his ten trials.
At this time there was, as my notes record, no satisfactory indi-
cation of progress, and the status of the experiment seemed ex-
tremely unsatisfactory in as much as in spite of the experimenter’s
best efforts to break up the habit of choosing the nearest door,
the orang utan still persisted, to a considerable extent, in the
use of this method. The only encouraging feature of the results
was an evident tendency to choose somewhat nearer the left end
of a group than previously.

A series of correct first choices was obtained on May 11,

66

TABLE 8
Results for Orang utan in Problem 1

ROBERT M. YERKES

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Fos ANCWAOMOMOANGHRORS A OF
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g = LInwoh nN me

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P A BRS ASS RRANRARAS Ake -< os

MENTAL LIFE OF MONKEYS AND APES

r2O0QMEO fo)
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68 ROBERT M. YERKES

greatly to the surprise of the experimenter, for no indication had
previously appeared of this approaching solution of the problem.
It seemed possible, however, that the successes were accidental,
and it was anticipated that in a control series Julius would again
make mistakes. But on the following day, May 12, the presen-
tation of the original series of ten settings, which, of course,
differed radically from the settings used from May 4 to May 11
was responded to promptly, readily, and without a single mistake.

Julius had solved his problem suddenly and, in all probability,
ideationally.

Only three reactive tendencies or methods appeared during
Julius’s work on this problem: (a) choice of the open door near-
est to the starting point (sometimes the adjacent boxes were
entered); (b) a tendency to avoid the “‘ nearest ’’ door and select
instead one further toward the left end of the group; (c) direct
choice of the first door on the left.

The curve of learning plotted from the daily wrong choices
and presented in figure 18, had it been obtained with a human
subject, would undoubtedly be described as an ideational, and
possibly even as a rational curve; for its sudden drop from near
the maximum to the base line strongly suggests, if it does not
actually prove, insight. :

Never before has a curve of learning like this been obtained
from an infrahuman animal. I feel wholly justified in concluding
from the evidences at hand, which have been presented as ade-
quately as is possible without going into minutely detailed
description, that the orang utan solved this simple problem idea-
tionally. As a matter of fact, for the solution he required about
four times the number of trials which Sobke required and twice
as many as were necessary for Skirrl. Were we to measure the

intelligence of these three animals by the number of trials needed

in problem 1, Sobke clearly would rank first, Skirrl second, and
Julius last of all. But other facts clearly indicate that Julius
is far superior to the monkeys in intelligence. We therefore
must conclude that where very different methods of learning ap-
pear, the number of trials is not a safe criterion of intelligence.
The importance of this conclusion for comparative and genetic
psychology needs no emphasis.

MENTAL LIFE OF MONKEYS AND APES - 69

Problem 2. Second jrom the Right End

Julius was given four days’ rest before being presented with
problem 2. He was occasionally fed in the apparatus, but regular
continuation of training was not necessary to keep him in good
form. During this rest interval, locks were attached to the doors
of the apparatus so that the experimenter by moving a lever
directly in front of him could fasten either one or both of the
doors of a given box by a single movement. On May 13 Julius
was given opportunity to obtain food from each of the boxes
in turn, and trial of the locks was made in order to familiarize
him with the new situation. He very quickly discovered that
the doors could not be raised when closed, and after two days
of preliminary work, he practically abandoned his formerly per-
sistent efforts to open them. The locks worked satisfactorily
from a mechanical point of view as well as from that of the adapta-
tion of the animal to the modified situation.

Problem 2 was regularly presented for the first time on May
17, on which day a single series was given. The period of pun-
ishment adopted was twenty seconds, and for each successful
choice a small piece of banana was given as a reward. After
the first trial in this series, in which Julius repeatedly entered
the first box at the left, that is box 7, there was but slight ten-
dency to reénter the first box at the left of the group. Instead,
Julius developed the method of moving box by box toward the
right end of the group. The choices were made promptly, and
their systematic character enabled the animal to obtain his
reward fairly quickly, in spite of the large number of mistakes.

In the second series, the orang utan developed the interesting
trick of quickly dodging out of the wrong box before the experi-
menter could lower the door behind him. This he did only
after having been punished for many wrong choices to the point
of discouragement. The trick was easily broken up by the
_ sudden lowering of the entrance door as soon as he had passed

under it.

There appeared on May 21 an unfavorable physical condition
which manifested itself, first of all through the eyes which ap-
peared dull and bloodshot. On the following day they were
inflamed and the lids nearly closed. Julius refused to eat, and
experimentation was impossible. Until June 2 careful treat-
ment and regulation of diet was necessary. He passed through

70 ROBERT M. YERKES

what at the time seemed a rather startling condition, but rapidly
regained his usual good health, and on June 3, although some-
what weak and listless, he again worked fairly ‘steadily.

Since it was now possible to lock the doors and confine the
animal for any desired period, on June 5 the interval of punish-
ment was made sixty seconds, and a liberal quantity of banana,
beet, or carrot was offered as reward. No increase in the num-
ber of successful choices appeared, and Julius showed discourage-
ment. Sawdust had been strewn on the floor, and in the inter-
vals between trials as well as during confinement in wrong boxes,
he took to playing with the sawdust. He would take it up in
one hand and pour it from hand to hand until all had slipped
through his fingers, then he would scrape together another
handful and go through the same process. Often He became
so intent on this form of amusement that even when the exit
door was raised, he would not immediately go to get the food.

The reactive tendencies which appeared in the work on prob-
lem 2 will now be presented in order, since I shall have to refer
to them repeatedly, and the list will be more useful to the reader
at this point than at the conclusion of the presentation of daily
results. The following is not an exhaustive list but includes
only the most important and conspicuous tendencies or methods
together with the dates on which they were most apparent.

(a) May 17, choice of first box at left of group or near it, then
the next in order, and so on, until the second from the right was
reached. This method with irregularities and certain definite
skipping was used at various times, sometimes over periods of
several days, during the course of the work.

(b) June 3, preference for number 3 and number 4 developed
immediately after the orang utan’s illness and when he was work-
ing rather listlessly.

On June 9 and 10, the original tendency (a) reappeared and
persisted forxa number of series.

(c) June 14, a tendency to choose the box at or near the right
end of a group, and then the one next to it. In connection
with this tendency, which of course required only two choices
in any given trial, interest in playing with the sawdust on the
floor developed.

Again on June 21, the animal returned to the use of tendency

(a).

MENTAL LIFE OF MONKEYS AND APES . 71

(d) June 29, movement to box at right end of group, hesita-
tion before it, and turning through a complete circle so that the
second box from the right was faced. This, the correct box, was
often promptly entered. This method, if persisted in, would
obviously have yielded solution of the problem.

(e) July 5, approach to and pretense to enter the box next
to the right end (right one), and then choice of some other box.
This feint is peculiarly interesting, and its origin and persistence
are difficult to account for.

(f) In connection with the tendency to pretend that he was
going to enter the second box from the right end, Julius developed
also the tendency to turn around in front of the box at the right
end, starting sometimes to back into it, and then to enter, in-
stead, the box second from the end.

(g) July 6 and 7, a fairly definite tendency to take the one
next in order or, instead, to go directly to the right box.

(h) July 10, direct first choices without approach to other
boxes appeared for the first time on this date.

For this problem, it proved impossible to establish and main-
tain uniform conditions of experimentation. Instead, because
of the failure of the animal to improve and the tendency to
discouragement, both punishment and reward had to be altered
from time to time, and other and more radical changes were
oceasionally made in the experimental procedure. Below for
the sake of condensed and consecutive presentation, the most
important conditions from day to day are arranged in tabular

form:
CONDITIONS OF EXPERIMENT FROM Day TO Day FOR PROBLEM 2

Date Punishment Reward
May 17........... 20 sec. confinement.......... Food in right box for each
(Aid after 10 trials) trial
© VW tO 21s ses 30 sec. confinement.......... Food (banana) in right box

for each trial
“ 22 to June 2.. Illness, no experiments.......

Jiine: Bsa factou aes 15 sec. confinement.......... Food (banana) in right box
for each trial
OS Abbe ood aiaaetevactreds 30 « Haste aes tae an Food (banana) in right box
for each trial
© lO ic ceds 60 “ ar ee ee Beet, carrot and loquat, in
addition to banana
He A eco atte Wisse dies 10 to 30 sec. confinement..... Beet, carrot and loquat, in
addition to banana
© IZ 10 15 secs 60 sec. confinement.......... Beet, carrot and loquat, in
addition to banana
© Wiis iwevwes ve 60 “ Cepatpearasats Banana and sweet corn—for-

mer preferred

72 ROBERT M. YERKES

CONDITIONS OF EXPERIMENT FROM Day TO Day FOR PROBLEM 2—Continued

Date : Punishment Reward

June 17 (1st series). 60 sec. confinement.......... Food. ene as in early
series

“ 17 (2nd series). No confinement in wrong box; Food only for correct first
but instead, return to start- choices
ing point by way of alleys

AS tO 22 icine No confinement in wrong box; Food only for correct first
but instead, return to start- _ choices z
ing point by way of alleys

“22 (2nd series). No punishment; allowed to en- Food for.each trial
ter boxes until right one was
found

OD Dias keep ususeaste didn Return to starting point. After
five wrong choices of a given
box the animal was held for
60 secs. in one of the boxes
and was then released by
way of the exit door and
rewarded when the right one

was chosen

“ 23 (2nd series). No punishment. . , . Reward for each trial

« 24 (1st series). Return to starting ‘point. Sarin .-. Food _only for correct first
choices

“ 2A (2nd series). No punishment.............. ‘Reward for each trial

25-30......... Same as on 24th.............

July 1 (1st series). No punishment.............. £ oe «

“1 (2nd series). Return to starting point...... a bo only for correct first
choices

«2-8... Same ason July 1........... .

ot 26 (2nd series). No punishment.............. Reward for each trial

“8 (8rd series). Return to starting point...... Revard only for correct first
choices

« 9-10.. Same as for July 8 (8rd series) S

« 10 (2nd ‘series). Momentary confinement in Reward for each correct choice
wrong boxes

Ce WD coe eache Risawicen Return to starting point...... Reward for correct first choice
“ 12 (2nd series). 30 sec. confinement.......... Reward for each correct choice
“12 (8rd series), 5 “ He Lassa arasaeesnia & & 8 ie «

“ 13 30 “ BOY Nii dete wat « « “ “« “

se Return to starting point...... Reward for correct first choices
eS . 60 sec. confinement i bireses Buttle Reward for each correct choice
a“ 30 SE Ve ooette eat “ “ “« «& “

“ 10 a“ el a) Re oe “ «& “ “ «“
DT BO wee ncn cue Right box indicated by slight Reward in each right box

raising of exit door momen-
tarily. No eal or a

ae 2 (2nd series). Return to starting is : .. Reward for correct first choices
31 (2nd series)
to Aug. 10.. 10 to 60 sec. confinement..... Reward for each correct choice
Aug. 10 (2nd series). Threatened with ere ¢ a ec «
“11 (1st series). nate aise a ee fe tt
“ 11 (2nd series). 10 sec. Snes hob ech oet day a i> Ae i es
ME ND cxrereseuat bccncet Threatened with whip........ . oe Tee e
“ 12 (2nd series). 10 sec. confinement.......... a 1 tse o e
1 Ne sire era oars 10 Le ere re « BS: Ga See «

MENTAL LIFE OF MONKEYS AND APES 73

With the above reactive tendencies and modifications of
method in mind we may continue our description of results. On
June 9 there developed a tendency to increase the magnitude
of the original error by choosing nearer the left end of the groups.
This is odd, since one would naturally suppose that an animal
as intelligent as the orang utan would tend to avoid the general
region in which success was never obtained and to focus atten-
tion on the right, as contrasted with the wrong end of each group.
It obviously contradicts the law of the gradual elimination of use-
less activities. In other words, it is wholly at variance with the
principle of trial and error exhibited by many infrahuman organ-
isms. Julius, although making many mistakes, worked dili-
gently and, for the most part, fairly rapidly. The day’s work
proved most important because of the change in method and
also because of the appearance of hesitation, the rejection of cer-
tain boxes, and the definite choice of others. My notes record
“this is a most important day for Julius in problem 2;” but
subsequent results do not clearly justify this prophecy.

The method of choosing the first box at the left and then of
moving down the line until the right one was reached was so
consistently followed that during a number of days it was pos-
sible for me to predict almost every choice. Indeed, to satisfy
my curiosity in this matter during a number of series I guessed
in advance the box which would be chosen. The percentages
of correct guesses ranged from ninety to one hundred. June 10,
for example, yielded two series for which the ratio of right to
wrong first choices was 0 to 10, and in which the method described
above was used consistently throughout.

It was inevitable that punishment by confinement and the
discouragement resulting therefrom should interfere with the
regularity of work and make it extremely difficult to obtain
strictly comparable results from series to series and from day to
day. The data for this problem, as presented in table 9, have
values quite different from those for the monkeys, chiefly because
of the more variable conditions of observation.

It was occasionally noted that the disintegration of a definite
method and the disappearance of the tendency on which it
depended occurred rather suddenly. Frequently it happened
that having used an inadequate method fairly persistently on
a given day, the animal would on the following day exhibit a

M. YERKES

TABLE 9

Results for Orang utan in Problem 2

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MENTAL LIFE OF MONKEYS AND APES

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76

TABLE 9—Continued
Results for Orang utan in Problem 2

ROBERT M. YERKES

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Bu 9 ni a i ai = a A a
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MENTAL LIFE OF MONKEYS AND APES

77

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ROBERT M. YERKES

TABLE 9—Continued

Results for Orang utan in Problem 2

nN (Rey Te)
69 % c 8 Ss &
S a N ar N ite) a
a a = a a a a
rm
an s = AQ N a eS
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MENTAL LIFE OF MONKEYS AND APES 81

wholly different method. Even over night a new method might
develop. In the monkeys, although there was occasionally
something comparable with this, it was by no means so evident.

After two hundred and fifty trials on problem 2 had been given
Julius, it seemed desirable to introduce a radical change in method
in order to stimulate him to maximal effort. It was therefore
decided to force him to make a round trip through the apparatus
in connection with each choice, and to let this forced labor serve,
in the place of confinement, as punishment for mistakes. This
new method yielded peculiar and characteristic results. They
differ from those previously obtained largely because of the
orang utan’s remarkably strong tendency to reénter the box
through which he had just passed. This occurred so persist-
ently, as may be seen in table 9 (June 17, second series, June
18, etc.), that a further modification of method was introduced
in that after the same wrong box had been entered five times
in succession, the experimenter on the next choice of the box
confined the animal for a stated interval, say sixty seconds, in
it, and then allowed it to escape by way of the exit door and
choose repeatedly until it finally located the right box. Were
it not for this particular feature of the method, the number of
choices recorded after June 17 would unquestionably be very
much greater than the table indicates.

The new method proved a severe test of the orang utan’s
patience and perseverance, for he had to work much harder
than formerly for his reward, and often became much fatigued
before completing the regular series of ten trials. Early in the
use of this method, he developed the habit of rolling around
from exit door to starting point by a series of somersaults. When
especially discouraged he would often bump his head against
the floor so hard that I could hear the dull thud. As has been
noted, I found it desirable to vary the procedure repeatedly.
It proved especially interesting to give one series per day with
the round trip as punishment and another series with confine-
ment as punishment.

Day after day, as the experiment progressed, slight or great
fluctuations of the ratios of right to wrong choices appeared,
but without consistent improvement. There was, to be sure,
as the last column of table 9 shows, a radical improvement
during the first six hundred and fifty trials, for the number of

82 ROBERT M. YERKES

right choices per series increased from 0 to 8. But, as the obser-
vations were continued from day to day, it became more and
more evident that the animal was merely passing from tendency
to tendency—method to method—mixing tendencies, and occa-
sionally developing new ones, without approach to the solution
of the problem. This fact would have led me to discontinue
the work much earlier than I actually did had it not been for
the peculiarity of the results obtained with problem 1. It
seemed not improbable that at any time Julius might succeed
in perfectly solving this problem over night precisely as he had
solved the first problem.

A curiously interesting bit of behavior appeared for the first
time on June 29. Julius had gone to the first box at the right
end of the group, and instead of entering, he had wheeled around
toward his right, and turning a complete circle, faced the right
box, which he promptly entered. Subsequently, the tendency
developed and the method was used with increasing frequency.
On June 30, it appeared in the first series, four times, in the
second series, six times; on July 1, in the first series, three times,
and in the second series, four times; on July 2, in the first series,
five times, and in the second series, nine times. It was indeed
only by accident that the animal failed to fulfill the technical
requirement for perfect solution of the problem in this series.
Yet, had he done so, his subsequent trials would doubtless have
revealed the lack of any other idea than that of turning com-
pletely around before entering a box.

This odd bit of behavior proved peculiarly interesting and
significant in that the tendency to turn became dissociated from
the position (in front of the first box at the right end of the group)
in connection with which it originally developed. After a few
days, Julius would enter the reaction-chamber and instead of
proceeding directly to the right end of the group, would stop
suddenly wherever he happened to be, turn toward his right in
a complete circle, and hasten into the box nearest to him which,
as often as not, proved to be the wrong one. Thus the idea of
turning completely about, which had it continued its associa-
tion with the idea of facing the first box at the right, would have
yielded success, instead became useless because of its dissociation.

That the orang utan is capable of using free ideas seems clear
enough in the light of this behavior. That he proved incapable

MENTAL LIFE OF MONKEYS AND APES 83

of getting the idea of second from the right end is as clearly
shown by the detailed results of table 9,—the fruits of weeks
of experimenting.

Certain other interesting tricks developed in Julius’s behavior.
Thus, on July 5, there appeared the tendency to move as though
about to enter the right box (feint), then to stop suddenly and
promptly enter another box, which was, of course, a wrong one.
The reason for the development of this tendency could not be
discovered, but in connection with it, there appeared another
tendency which possibly can be explained. Julius took to back-
ing into the chosen box so that he could face the experimenter.
He would then, after a period of hesitation, come out and
promptly enter one of the other boxes. This tendency was
apparently due to the fact that during one or two series the
experimenter growled at the orang utan every time he made a
mistake. The growl startled him and caused him to look around.
He evidently felt the need of keeping his eyes on the experi-
menter,—hence the backing into the open box. The tendency
disappeared shortly after the experimenter gave up the use of
the growl as a method of punishing the animal for what were
suspected to be careless choices.

Curiously enough, it was not until July 10 that direct choice
of the right box was made at all frequently. Previously, selec-
tion of it had been made almost invariably after approach to
other boxes. But in the second series for July 10 there was an
extraordinary improvement in method. This developed in the
presence of two visitors, and it is therefore all the more. sur-
prising. The choices were made not only directly, but with
decision and evident certainty that was quite at variance with
the previous behavior of the animal.

All the while through variation of methods, I was seeking to
discover the best means of holding the orang utan to his maxi-
mumi effort and care in attempting to select the right box. One
day it would seem as though forcing him to make round trips
with rewards only for correct first choices proved most satis-
factory, and the next it might seem equally clear that punish-
ment by confinement for thirty seconds or sixty seconds, with
reward for correct choice in every trial, yielded better results.
In the end I had to admit that no best method had been demon-
strated and that I had failed to develop conditions which served

84 ROBERT M. YERKES

to compel the animal’s attention to the problem and to lead him
to work without discouragement. There were, it is true, days
on which it seemed practically certain that the problem would
be solved, but as it turned out, Julius never succeeded in choos-
ing correctly throughout a series of ten trials.

As a last resort, in order to make perfectly sure that the orang
utan was doing his best, I decided to introduce corporal punish-
ment in a mild form. For this purpose, I placed my assistant
in charge of the apparatus and the series of trials, and stationed
myself in one corner of the reaction-chamber with a whip in
my hand. Whenever Julius entered a wrong box, I approached
him with the whip and struck at him, being careful not to injure
him and rarely striking him at all, for the threat was more effec-
tive than a blow. He was extremely afraid of the whip and
would begin to whine and attempt to get out of the way as soon
as he saw it.

This method was introduced on August 10, but no improve-
ment resulted, and in the end there was no reason to consider
it more satisfactory than the other procedures. I am now
wholly convinced that Julius did his best to choose correctly
in the majority of the numerous series which were given him in
connection with problem 2.

From trials 1001 to 1100, a radical departure from the pre-
vious methods was introduced in that the right box was indi-
cated to the animal by the slight and momentary raising of its
exit door. Of course no records of the choices for this group
.of one hundred trials appear in table 9, for the simple reason
that the animal inevitably and immediately entered the right
box. It was thought that this method might serve to break
up the previously developed tendencies toward inadequate
forms of response and so encourage the animal that he would
later solve the problem when given opportunity to select the
right box without aid from the experimenter. But as a matter
of fact, while the ratio of right to wrong first choices was 1 to
.67 in the series preceding this change of method, it was 1 to 1.50
in the first series following its use. There is no satisfactory
evidence that Julius profited by this experience, though as a
matter of fact he did succeed in making his best daily record,
eight right to two wrong choices, on August 4, after 1190 trials.

The curve of learning for this problem has been plotted and

MENTAL LIFE OF MONKEYS AND APES 85

is presented in figure 19. It is of course incomplete and it is
offered only to indicate the extreme irregularity in performance.

Problem la. First at the Right End

It was decided on August 19 that the further continuation of
the work of Julius on problem 2 was not worth while. He had
become much discouraged, and although willing to work for
food, gave no indications whatever of improvement and seemed
to have exhausted his methods. It seemed wise instead of
giving up work with him in the multiple-choice method to return
to a form of problem 1. We may designate it as problem la.
The right box is definable as the first at the right end of the
series instead of the first at the left end as in the original problem
1. It was thought possible that Julius might quickly solve
this problem by a process similar to that used for problem 1.

Work was begun on problem 1a, August 20, and for six suc-
cessive days two series of trials per day were given, the settings
for which as well as the resulting choices are given in table 10.
Most notable in these results is the large number of cases in which
Julius chose first the second box from the right end of the series,
or in other words that box which had been the right one in prob-
lem 2. Contrary to expectation, he showed no inclination to
abandon this tendency to choose the second from the right end,
and the ratio of right to wrong choices changed in the direction
opposite from expectation, beginning with 1 to 4 and ending on
the sixth day with 0 to 20.

It was obviously useless to continue the experiment further
since Julius had given up his attempts to locate the right box
in the first choice and was apparently satisfied to discover it
by a process of trial and error. He had, it would seem, satisfied
himself that the problem was it insoluble. These results obtained
in “problem 1a constitute a most interesting comment on the
effects of problem-2 on the orang utan. Behavior similar to |
that which he developed well might have been obtained from
a child of three to four years placed in a like situation and forced
to strive, day after day, to solve a problem beyond its ideational
capacity.

In many respects the most interesting and to the experimenter
the most surprising result of this long series of observations with
Julius was the lack of consistent improvement. It seemed

86

TABLE 10
Results for Orang utan in Problem 1a

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MENTAL LIFE OF MONKEYS AND APES 87

almost incredible that he should continue, day after day, to
make incorrect choices in a particular setting while choosing
correctly in some other setting which from the standpoint of
the experimenter was not more difficult. :

The evidence suggests that in this young orang utan idea-
tionalTearning tended to replace the simpler mode of problem
solution by -triat~and error. Seemingly incapable of solving
his problems by the-tower grade process, he strove persistently,
and often vainly, to_gain-insight.—“He_used ideas n_effectively.
Animals far lower in intelligence (e. g., the pig), surpass him in
ability to solve these relational problems becatise they use the
method of elimination by t trial consistently and effectively.
Julius, in these experiments, made a poor showing because his
substitute for trial and error is only slightly developed. Would
he have succeeded better with the same problems if mentally
mature?

There are many important features of the results which, for
lack of space, have not been indicated or discussed. They can
be developed from later comparative studies of the data, for
in the tables appear all of the essential facts of response apart
from those mentioned in the text.

IV

RESULTS OF SUPPLEMENTARY TESTS OF IDEATIONAL
BEHAVIOR

1. Julius, Pongo pygmeus

Box Stacking Experiment

In addition to the multiple-choice experiments which have
been described in detail in the previous section, it was possible
to conduct certain less systematic tests of ideational behavior
in the monkeys and the orang utan. From the technical stand-
point these tests were relatively unsatisfactory because only
inexactly describable. But their results are in many respects
more interesting, if not also more important, in the light which
they throw on ideation than are those previously presented.

First, in order of time, comes a test which may be desig-
nated as the box stacking experiment. The method will now
be described in connection with an account of the behavior
of Julius as contrasted with that of a child of three years and
four months of age. ~

In the large central cage labelled Z, figure 12, which was
twenty-four feet long, ten feet wide, and ten to twelve feet
deep, the following situation was arranged. From the center
of the wire covering of the cage, a banana was suspended on
a string so that it was approximately six feet from the floor,
five feet from either side of the cage, and twelve feet from either
end. From all approaches it was far beyond the reach of
Julius, since it was impossible for him to climb along the wire
roof and thus reach the string. Two boxes were placed on
the floor of the cage several feet from the point directly under
the banana. The one of these boxes was heavy and irregular
in shape, as is shown in figures 21, 23 and 24 of plate V. Its
greatest height was twenty-one inches; its least height, eighteen
inches; its other dimensions, twelve and sixteen inches respec-
tively. The smaller and lighter box measured twenty-two by
twelve by ten inches. According to the experimenter’s calcu-

MENTAL LIFE OF MONKEYS AND APES 89

lations, the only way in which Julius could obtain the banana
was by placing the smaller box upon the larger and then climb-
ing upon them.

At 10 a. m. on March 5, Julius was admitted to the large
cage, and the banana was pointed out to him by the experi-
menter. He immediately set about trying to get it, and worked
diligently during the whole of the period of observation, which,
because of the unfinished condition of some of the cages, was
limited to slightly over ten minutes. Within this period he
made upward of a dozen fairly well directed attempts to obtain
the food. Chief among them were three attempts to reach the
banana from different positions on the left wall of the cage
(as the experimenter faced the laboratory); two attempts to
reach it from different positions on the right wall; two from
the large box in positions nearly under the banana; two from the
large box with the aid of the experimenter’s hand; and one
from the distant end of the cage(?). There occurred, also, less
definite and easily describable efforts to get at the reward.

On account of the unfinished condition of the cages, the
experimenter had to remain in the large cage with Julius during
the test. This interfered with the experiment because the
animal tended both to try to escape and to get the experimenter
to help him with his task. Particularly interesting is the latter
sort of behavior. After the orang utan had made two or three
futile attempts to obtain the food he came to the experimenter,
who was standing in one corner of the cage, took him by the
hand, and led him to a point directly under the banana. He
then looked up toward the banana, grasped the experimenter’s
arm, raised it, and then tried to pull himself up. He was not
allowed to get the food by climbing up on the experimenter.
A few minutes later, he again led the experimenter toward the
banana, but receiving discouragement in this activity, he pro-
ceeded to devote himself to other methods.

Apart from the distractions which have been mentioned
above, Julius’s attention to the food was surprisingly constant.
Whatever his position with respect to it, he seemed not for
an instant to lose his motive, and to whatever part of the cage
he went and whatever he did during the interval of observation
was evidently guided by the strong desire to obtain the banana.
Frequently he would look directly at it for a few seconds and

90 ROBERT M. YERKES

EXPLANATION OF PLATE V

Orang utan, Julius, obtaining banana by piling boxes
or by using pole

Ficure 21.—Julius in act of setting larger box on end.

Figure 22.—Placing smaller box on larger.

Ficure 23.—Balancing on larger box preparatory to reaching for
banana.

Ficure 24.—Balancing and reaching to the utmost.

Ficure 25.—Standing on three boxes (after stacking them) and
reaching for reward.

Ficure 26.—Lifting smaller box up toward banana.

Ficure 27.—The act of stacking the boxes.

Ficure 28.—Sequel to figure 27.

Ficure 29.—Box and pole experiment. Pushing the second pole
into the box.

Ficure 30.—Pushing pole into box.

Ficure 31.—Enjoying the reward of success.

PLATE V

MENTAL LIFE OF MONKEYS AND APES 91

then try some new method of reaching it. His gaze was delib-
erate and in the handling of the boxes he accurately gauged
distances. Several times he succeeded in placing the larger box
almost directly under the banana, and repeatedly he located
that portion of the side wall from which he could most nearly
reach the coveted prize.

From my notes I quote the following comment on the results
of the initial experiment: ‘‘ Despite all that has been written
concerning the intelligent behavior of the orang utan, I was
amazed by Julius’s behavior this morning, for it was far more
deliberate and apparently reflective as well as more persistently
directed toward the goal than I had anticipated. I had looked
for sporadic attempts to obtain the banana, with speedy dis-
couragernent and such fluctuations of attention as would be
exhibited by a child of two to four years. But in less than ten
minutes Julius made at least ten obvious and well directed
attempts to reach the food. There were also wanderings, efforts
to obtain aid from the experimenter, and varied attempts to
escape from the cage.”

Before proceeding further with the description of the behavior
of Julius in the box stacking test, I shall describe for contrast
the behavior of a boy three years four months of age when
confronted with a situation practically identical with that which
the ape was given an opportunity to meet. For the child, the
banana was suspended, as previously described, from the roof
of the cage. The same two boxes were placed on the floor at
considerable distances from the banana, and in addition, a light
stick, about six feet long, and a piece of board, the latter by
accident, were on the floor. The child was asked to get the
banana for Julius, and he eagerly and confidently volunteered
to do so.

His behavior may best be described by enumeration of the
several attempts made. They include (1) placing the larger
box nearly under the banana and reaching from it. (2) Stand-
ing of the larger box on end with resulting failure because the
child could not stand on the sloping edges of the top of the
box. (3) The larger box was turned on its side and the lighter
box drawn up opposite it and stood on end. The child then
mounted the larger box and from it stepped to the top of the
smaller. But the boxes had not been placed beneath the banana,

92 ROBERT M. YERKES

and when the child reached for it, he found himself several feet
away from his prize. (4) The boxes were moved to a position
nearly under the banana and another futile attempt was made
to reach it without placing the smaller box on top of the larger
one, the only position from which the child could readily obtain
it. (5) The piece of board was placed on top of the larger box
and from this height the child again reached upward. (6) The
six-foot stick was taken up and an attempt was made to strike
the banana and thus dislodge it, but it was too securely fastened
to be obtained thus. (7) Attention shifted to other things,
and the child played for a time with the board. Reminded of
the banana by the experimenter, he again tried method (3).
(8) He again used the stick on the banana. (9) The effort to
knock the prize to the floor having failed, he became discouraged
and said that he must go home. (10) When told that Julius
was very hungry and wanted the banana, he repeated efforts
similar to those described in (3) and (6).

Up to this time the observations had covered a period of
twenty minutes. The child was now taken from the cage and
allowed to play about for fifteen minutes. Asked then whether
he would go back and try to get the banana, he replied, ‘‘ No,
‘cause I don’t want to get it,’ thus indicating his discourage-
ment with the situation. When taken into the cage, he, never-
theless, made the additional attempts indicated below: (11) Use
of one of the boxes. (12) He remarked, ‘‘ Now I know, I'll
get it,” and after so saying, repeated (3). (13) Failing, he
turned to me and said, ‘I could get it if I was on your head,”
but he did not, as Julius had done, lead me to the proper place
and try to reach the banana by climbing up or by urging me to
lift him. (14) Later, he played in the boxes, apparently forget-
ful of his task. Finally he remarked: ‘I'll get the banana,”
but he made no attempt to do so, and instead, watched the
monkeys intently. Thereafter, he showed no further interest
in the solution of the problem, and the experiment, after a total
period of fifty-five minutes, was discontinued.

Comparison of the behavior of the ape with that of the child
indicates a greater variety of ideas for the latter. Julius gauged
his distances much more accurately than the child, attended
more steadily, and worked more persistently to obtain the
reward, but he did not so nearly approach the idea of stacking

MENTAL LIFE OF MONKEYS AND APES 93

the boxes as did the child, for the latter, in placing the board
on one of the boxes, exhibited in ineffective form the idea which
should have yielded the solution of the problem.

The child was given no further opportunity to work at the
problem, whereas Julius, as I shall now describe, continued his
efforts on subsequent days under somewhat different conditions.

On Wednesday, March 10, the banana was suspended as
formerly, and three boxes, all of them small and light enough
to be readily handled by the ape, were placed in distant parts
of the cage. The six-foot stick which had been present in the
test with the child, but not in the first test with Julius, was
also placed in the cage.

Julius was allowed to work for about an hour. As formerly,
he was sufficiently hungry to be eager to get the food and evi-
dently tried all of the possible ways which occurred to him.
Chief among these were (1) the use of the various boxes sepa-
rately or in pairs in very varied positions but never with one
upon another,—the only way in which the banana could be
reached; (2) climbing to various points on the sides of the cage,
with infrequent attempts to reach the banana. Usually his
eyes saved him the vain effort.

Unlike the child, Julius paid little attention to the six-foot
stick. Two or three times he took it up and seemingly reached
for the banana, but in no case did he try persistently to strike
it and knock it from the string. It is but fair, however, to
remark that such an act is very difficult for the young orang
uttan, as compared with the child, because of the weakness of
the legs and the awkwardness of striking from a sitting posture.

As previously, the steadiness of attention and the persistence
of effort toward the end in view were most surprising. At one
time Julius walked to the end of the cage and there happened
to see one of the monkeys eating. He watched intently a few
seconds and then hastened back to the banana as if his task
had been suggested to him by the sight of the feeding animal.

Most interesting and significant in this behavior was the
suddenness with which he would turn to a new method. It
often looked precisely as though a new idea had come to him,
and he was all eagerness to try it out.

On March 11, Julius was given another opportunity to obtain
the banana by the use of the three boxes. Although he used

94 ROBERT M. YERKES

them together he made no effort to place one upon another.
Certain of his methods are shown in plate V, especially by
figures 21, 23 and 24.

This experiment was continued on April 2 under yet different
conditions, for this time only two boxes were placed in the
cage, the one of them the heavy, irregularly shaped box and
the other the smaller, lighter one originally used. On the end
of the heavier box had been nailed a two by two inch wooden

. block in order to increase the difficulty in using this box alone.
‘As previously, Julius made varied attempts to obtain the banana,
but on the whole his interest and attention seemed somewhat
weaker than previously and there were indications of discourage-
ment because of repeated failures.

He handled the boxes conspicuously well, and it seemed at
times that he would certainly succeed in placing the one upon
the other and in reaching the food.

After one series of attempts from the sides of the cage and
from the large box, he deliberately turned away from the box
and neatly exectitted a somersault on the floor of the cage, as
much as to say, ‘““I am disgusted with the whole situation.”
Again, later on the same day, after falling from the top of the
larger box, which tilted over very easily, he rolled himself into
a ball, and childlike, played with his feet. An additional evi-
dence of his changed affective attitude toward his task, especially
in connection with definite failures, appeared in his rough hand-
ling and biting of the boxes. When most impatient, he worked
very roughly.

Julius was allowed to work for the reward from thirty to
ninety minutes, or, as a rule, until he had become completely
discouraged on April 3, 5, 6, 7, 8, 9 and 13. His behavior was
interesting and significant, but nothing new appeared except
that his willingness to work gradually disappeared, and on
April 13, although previously hungry, he made only a single
attempt to obtain the banana and then paid no further atten-
tion to it.

The prolonged and varied efforts to obtain the banana were
due in a measure at least to three accidental successes. Thus
on April 2, 3 and again on the 5th, by fortunate combinations
of circumstances, he succeeded in getting the banana, contrary
to the intention of the experimenter.

MENTAL LIFE OF MONKEYS AND APES 95

Although active at first on April 6, he soon wearied of his
task and quit work. The same was true on April 7, and again
on the 8th and 9th. On these days, although hungry, he did
not care to enter the large cage and worked only a few minutes
each day, seldom making more than two or three half-hearted
attempts to obtain the banana. His attitude toward the task
had changed completely, in that hopelessness had taken the
place of eager expectancy. By the 13th of April he had so
nearly given up voluntary efforts to solve the problem that it
seemed worth while to test his ability to get the idea by watch-
ing the experimenter. For this purpose the following test of
imitation was made.

On the morning of April 14, having placed a banana in the
usual position, I took Julius into the large cage, dragged the
two boxes to the proper position beneath the banana, placed
the smaller one upon the larger one and then climbed up on
them to show the ape that I could reach the banana. I then
stepped down and gave him a chance to climb on the boxes. He
did so immediately and obtained the food.

Another piece of banana was supplied, the boxes were placed
in distant corners of the cage, and fifteen minutes were allowed
Julius so to place them that he could obtain his reward. He
gave no indications of having profited by my demonstration,
but worked with the boxes singly, usually with the larger one.

On April 16, with the banana in position and the two boxes
also in the cage, Julius was admitted and allowed to work for
five minutes, but again without success. I then placed the
boxes properly for him and he immediately climbed up and
got the banana. While he was eating, the boxes were carried
to distant corners of the cage and another banana placed in posi-
tion. Now thirty minutes were allowed him for unaided work
on the problem. As formerly, the larger box was used repeatedly
and attempts to reach from the side ofthe cage appeared, but
there was no tendency to try to use the two boxes together.
He worked fairly persistently, however, and showed clearly the
stimulating and encouraging effect of aid from the experimenter.

Once more, on April 17, Julius was taken into the cage and
allowed to watch me place the boxes in proper position. He
then climbed up and obtained the desired food. After the bait
had been renewed and the boxes displaced, he immediately tried

96 ROBERT M. YERKES

to use the larger one, then he reached for the small one as though
to use both together. But the impulse died out and he turned
again to the larger box as usual, standing it on end, and per-
sistently trying to balance himself on it. Nothing else of special
interest’ happened during the interval of unaided effort.

Similarly, I placed the boxes for the ape on April 19, allowed
him to get the banana and then gave him opportunity to try
for himself after the boxes had been displaced. This time he
immediately reached for the smaller box and moved it about
a little, thus indicating a new association. He next turned to
the larger box and worked with it persistently. Later, he once
more worked with the smaller box in an unusual manner. He
repeatedly stood on it, but made no attempt to lift it or to place
it on the larger box. Clearly the usually neglected smaller box
had become associated with the satisfaction of obtaining the
banana. The same method was carried out on April 20. As
I placed the boxes in position beneath the banana, Julius watched
with unusual intentness, and when it came his turn to try to
obtain the food by the use of the boxes, he began at once to
work with the smaller box, but as on April 19, he soon aban-
doned it and turned to the other. While I was making note of
this particular feature of his behavior, he suddenly seized the
smaller box by two corners with his hands and by one edge
with his teeth, and after a few attempts placed it on top of
the larger box, climbed up, and obtained the banana.

Because of bad weather on April 21, the next test was made
on April 22, with everything as usual. Unaided, the ape was
given an opportunity to obtain the coveted reward, while I
stood ready to obtain records of his behavior with my camera.
He wasted no time, but piled the smaller box on top of the
larger one immediately, and obtained his reward. As soon as
opportunity was offered, he repeated the performance. The same
thing happened on April 23 and several succeeding dates.

Julius had got the idea, and the only further improvement
possible was in skill in manipulating the boxes.

One of the curious performances which appeared during the
imitative period is pictured in figure 26, plate V, where the
ape is seen lifting the smaller box into the air. This he did three
or four times one day, raising it toward the banana each time
as though he expected thus to obtain the reward. As he did

MENTAL LIFE OF MONKEYS AND APES 97

not go up. with the box (according to his expectation?), he aban-
doned this method, and looking about, discovered the larger .
box in a distant corner. Thereupon, he promptly pulled the
boxes to their proper position beneath the banana, stacked them,
and obtained his food.

After considerable skill had been acquired in the placing of
the boxes, the one upon the other, the height of the banana
above the floor was increased so that three boxes were neces-
sary. Figure 25 of plate V shows him standing on three boxes
and reaching upward, and figures 22, 27 and 28 show various
modes of handling the boxes and of reaching from them. He
was not at all particular as to the stability of his perch, and
often mounted the boxes when it seemed to the experimenter
inevitable that they should topple over and precipitate him to
the floor. Only once, however, during the several days of
experimentation did he thus fall.

Obviously important is the evident change in the animal’s
attention on April 20. He watched with a keenness of interest
which betokened a dawning idea. Before he had succeeded in
stacking the boxes, I had written in my note-book, “‘ He seemed
much interested today, in my placing of the boxes.’’ Interest-
ing, and important also, is the ease and efficiency with which
he met the situation time after time, after this first success.
“Trial and error’ had no obvious part in the development of
the really essential features of the behavior. The ape had the
idea and upon it depended for guidance.

Except for the fact that Julius was immature, probably under
five years of age, it is likely that he would have stacked the
boxes spontaneously instead of by suggestion from the experi-
menter or imitatively.

No unprejudiced psychologist would be likely to interpret the
activities of the orang utan in the box-stacking experiment as
other than imaginal or ideational. He went directly, and in the
most business-like way from point to point, from method to
method, trying in turn and more or less persistently or repeat-
edly, almost all of the possible ways of obtaining the coveted
food. The fact that he did not happen upon the only certain
road to success is surprising indeed in view of the many ineffec-
tive methods which he used. It seemed almost as though he
avoided the easy method. ;

98 ROBERT M. YERKES

It is especially important, in connection with these results,
, to point out the risk of misinterpretation of observations on the
anthropoid apes. If they can imitate human activities as readily
and effectively as Julius did in this particular experiment, we
can never be sure of the spontaneity of their ideational behavior
unless we definitely know that they have had no opportunity
to see human beings perform similar acts.

Of all the methods of eliciting ideational or allied forms of
behavior used in my study of the monkeys and ape, none yielded
such illuminating results as the box stacking test, and although
from the technical standpoint, it has many shortcomings, as a
means to qualitative results it has proved invaluable.

Other Methods of Obtaining the Reward

Some weeks later, I tried to discover how Julius would obtain
the much desired banana when the boxes were absent. I placed
in the large cage a stick about six feet long and an old broom.
When admitted, he looked about for the boxes, but not seeing
them, picked up the broom and placing it with the splints down,
beneath the banana, he tried to climb it, but as it fell over with
him, he abandoned this after a few trials, went to his cage,
and picking up some old bags which he used at night as covers,
he dragged them out and placed them on the floor beneath the
banana. He next put the broom upon them and tried to climb
up. This general type of behavior persisted for several min-
utes, everything within reach being used as were the bags, as
a means of raising him in the desired direction. Finally, he
placed his feet on the broom where the handle joins the splints,
seized the handle near the top with his hands, drew himself up
as far as possible, and then launched himself in the air and
tried to seize the banana. On the third attempt he succeeded.

Later, he was given a plain stick about five feet long. Figure
32 of plate VI shows him using this to obtain the banana in
the manner described above. He would grasp it with one or
both feet, usually one, ten to fifteen inches from the floor of
the cage, meanwhile holding with his hands near the top of
the stick. He would then, with all his strength, draw himself
up suddenly and jump toward the banana. Often he came down
rather hard on the cement floor, much to his disgust.

Yet another method of obtaining the reward developed a day

MENTAL LIFE OF MONKEYS AND APES 99

or two later. A light red-wood stick about five feet long and
an inch in its other dimensions was the only object in the cage
which could possibly be of use in obtaining the banana. The
aim of the experimenter was to discover whether Julius would
use this as a club.

Previously, in connection with the use of the boxes, he had
taken up the same stick two or three times and reached for the
banana with it, but in no case had he struck at it or clearly
tried to knock it from the string, as did the child most readily
and naturally. When provided with this same stick, and it
alone, as a means of obtaining the food, he hit upon the fol-
lowing interesting method. Placing one end of the stick be-
tween a wooden brace and the wire side of the cage, he climbed
up to a level with the banana as is shown in figure 33 of plate
VI. Then holding with one hand and one foot to a timber of
the cage and to the stick with his other foot, he swung outward
as far as possible and reached the banana with his free hand.
Having once succeeded by the method, he used it whenever
given an opportunity. It was impossible for him to make the
reach without the use of the small stick, while with it he suc-
ceeded fairly easily and regularly.

Box and Pole Experiment

Following the box stacking test, Julius was given an oppor-
tunity to exhibit ideation in another typeof experiment. This
may be designated the box and pole test. The conditions are
describable thus. A strong wooden box eighty-four inches long,
by four inches wide, by four inches deep, with open ends, was
built with one side hinged. Hasps and padlocks enabled the
experimenter to lock this “id” after food had been placed in
the center of the box. This box could be placed in the center of
the large cage and there fastened by means of cross bars. It is
well shown in position in figure 29, plate V. Two poles each
eight feet long and approximately one and a half inches in
their other dimensions were the only additional materials in
the experiment.

On May 1, Julius was allowed to see the experimenter place
a half banana in this box, close the lid, lock it in position, and
securely fasten the box by means of the cross bars. He was
then given opportunity to try to get the banana. The two

100 ROBERT M. YERKES

poles lay on opposite sides of the box and near the edges of the
cage. Doctor Hamilton and the writer were in the cage watch-
ing. Julius looked into the box through one end, and seeing
the banana, reached for it. He could not obtain it in this way,
so he began to bite at the box and to pull at it with all his strength.
During the fifteen minutes allowed him, he worked at the box
in a great variety of ways, fooling with the locks which had been
attached to the hasps as well as with the cross bars and con-
tinually reaching in at the one or the other end. He was some-
what distracted by the presence of the two observers and attended
rather unsatisfactorily to the task in hand. Not once did he
touch the poles, and it is doubtful whether he even noticed them.
He was not very hungry at this time, and after a few minutes
active work he virtually gave up trying to get the food.

Two days later, on May 3, the box was once more placed in
position, this time with a half banana in the middle and a small
piece of banana near each open end. The two poles lay on the
floor of the cage, each several feet distant from the box. Julius
was eager for food. When released he went immediately to the
box, reached in and obtained a piece of banana from the end
nearer the laboratory. He then looked in and saw the piece
near the middle of the box. His next move was to pick up the
eight foot pole and push it into the box, but before pushing it
all the way through, he stopped and began to pull at the box
in various ways. Shortly he returned to the pole and twice
thrust it in as far as he could reach. The first time, after thrust-
ing it all the way through, he pulled it out and examined the end
as though expecting the banana to come out with it. After a
third attempt he looked into the box, presumably seeing the
banana, then turned a backward somersault, came to the end
of the cage, and looked at me. Had it been at all possible, he
would have taken me by the hand and led me to the box as
a helper. After a few seconds, he returned to the pole, pried
the lid of the box with it, then gnawed at the pole. For about
five minutes he worked fairly rapidly and steadily, using the
poles, pulling, gnawing, and walking about.

His next move was to go to the opposite end of the box,
look in, take the piece of banana which was near the opening,
then pick up the second pole, which had not previously been
noticed, and after a number of attempts, push it into and through

PLATE VI

; 34
FIGURE 32.—Julius obtaining banana by using pole to climb up on and spring from.

FIGURE 33.—Using pole to swing out on so that banana could be grasped.
FiGuRE 34,—Using stick to draw carrot within reach.

bzz

MENTAL LIFE OF MONKEYS AND APES 101

the box, looking after it and then pulling it out and looking
into the box. Having done this he again came to my end of
the cage, and from there returned to try once more with the
pole which he had first used. He pushed this pole all the way
through, then walked to the other end of the box, looked in
and reaching in, obtained the banana which had been pushed
far enough along to be within his grasp. Figures 29, 30 and 31
of plate V show stages of this process.

Julius had worked twenty-four minutes with relatively little
lost time before succeeding. He had shown almost from the
start the idea of using the pole as an instrument, and his sole
difficulty was in making the pole serve the desired purpose.

The experiment was rendered still more crucial on May 5
by the placing of the two poles upright in opposite corners of
the large cage. For a few minutes after he entered the cage,
Julius did not see them, and his time was spent pulling and
gnawing at the box. Then he discovered one of the poles,
seized it, and pushed it into the box. He tried four times, then
went and got the other pole and pushed it into the opposite

\ end of the box. Twice he did this, then he returned to the
| Original pole, bringing the second one with him. He pushed
| it in beside the first, and as it happened, shoved the banana out
of the opposite end of the box. But he did not see this, and
_ only after several seconds when he happened to walk to that
\ end of the box did he discover the banana. The total time
\ until success was fifteen minutes.

\ Subsequently the ape became very expert in using the pole
‘o obtain the banana, and often only a minute or two sufficed
r success. It was not possible for him to direct the stick
ry accurately, for when he was in such a position that he
cduld look through the box, he could not work the stick itself.
It was, therefore, always a matter of chance whether he obtained

thebanana immediately or only after a number of trials.

Athough it is possible that the use of the poles in this experi-
men\ was due to observation of human activities, it seems prob-
able n the light of what we know of the natural behavior of the
anthrypoid apes that Julius would have solved this problem
indepé&idently of human influence. It was the expectation of
the experimenter that the pole would be used to push the banana

through the box, but as a matter of fact the ape used it, first

\

102 ROBERT M. YERKES

of all, to pull the food toward him, thus indicating a natural
tendency which is important in connection with the statements
just made. Subsequently he learned that the banana must be
pushed through and obtained.at the farther end of the box.
I am not prepared to accept the solution of this problem as
satisfactory evidence of ideation, but I do know that few ob-
servers could have watched the behavior of the orang utan with-
out being convinced that he was acting ideationally.

Draw-in Experiment

An interesting contrast with the box and pole test is fur-
nished by what may be called the draw-in experiment. This
was planned as a simple test of Julius’s ability to use a stick
to draw things into his cage from beyond the wire side. A
board was placed, as is shown in figure 34 of plate VI, with
sides to hold a banana, carrot, or some other bit of food, in
position. In the actual test either a carrot or a banana was
placed about two feet from the wire netting and a stick two
feet long was then put into the cage with the ape.

When this situation was first presented to Julius, he looked
at the banana, reached for it, and failing, picked up a bag from
the floor of the cage and tried to push it through the wire mesh
toward the banana. He also used a bit of wire in the same
way, but was unable thus to get the food. As soon as a stick
was placed in his cage, he grasped it and used it in a very defi-
nite, although unskillful, way to pull the banana toward him.
He was extremely eager and impatient, but nevertheless per-
sistent in his efforts, and within five minutes from the beginning
of the first trial, he had succeeded in getting two pieces of
banana, using always his left hand to manipulate the stick.
This test was repeated a nuniber of times with similar results.
He had from the first the ability to use a stick in this way, end
the only difficulty with the test as a means of obtaining evidence
of ideational behavior is that the possibility of imitatio1 of
man cannot be certainly excluded. j

Lock and Key Test

By my assistant it was reported on May 5 that. the orang
utan had been seen to place a splinter of wood in a 2adlock
which was used on the cages and to work with it persstently.

4

MENTAL LIFE.OF MONKEYS AND APES 103

It looked very much like imitation of the human act of using
the key, and I therefore planned a test to ascertain whether
Julius could readily and skillfully use a key or could learn
quickly to do so by watching me.

The first test was made on May 15 with a heavy box whose
hinged lid was held securely in position by means of a hasp
and a padlock. The key, which was not more than an inch in
length, was fastened to a six inch piece of wire so that Julius
could not readily lose it. With the animal opposite me, I
placed a piece of banana in the box, then closed the lid and
snapped the padlock. I next handed Julius the key. He im-
mediately laid it on the floor opposite him and began biting the
box, rolling it around, and occasionally biting also at the lock
and pulling at it. During these activities he had pulled the
box toward his cage. Now he suddenly looked up to the posi-
tion where the banana had been suspended in the box experi-
ment. Evidently the box had suggested to him the banana.
For thirty. minutes he struggled with the box almost contin-
wously, chewing persistently at the hinges, the hasp, or the-
lock. Then he took the key in his teeth and tried to push it
into one of the hinges, then into the crack beneath the lid of
the box.

Subsequently I allowed him to see me use the key repeatedly,
and as a result, he came to use it himself now and then on the
edge of the box, but he never succeeded in placing it in the
lock, and the outcome of the experiment was total failure on
the part of the animal to unfasten the lock of his own initiative
or to learn to use the key by watching me do so. I did not
make any special attempt to teach him to use the key, but
merely gave him opportunity to imitate, and it is by no means
impossible that he would have succeeded had the key been
larger arid had the situation required less accurately coérdinated
movements. However, it is fair to say that the evidence of
the idea of using the key in the lock was unconvincing. My
assistant’s observation was, perhaps, misleading in so far as it
suggested that idea. It may and probably was purely by ac-
cident that the animal used the splinter on the padlock.

104 ROBERT M. YERKES

2. Skirrl, Pithecus irus
Box Stacking Experiment

The monkey Skirrl was tested by means of the box stacking
experiment much as Julius had been. On August 23, with a
carrot suspended six feet from the floor of the large cage and
three boxes in distant corners, the animal was admitted and
his behavior noted.

The boxes, which were made of light, thin material, ranged
in size from one six inches in its several dimensions to one
twenty inches long, thirteen inches wide, and eleven inches
deep. Only by using at least two of these boxes was it pos-
sible for the animal to reach the carrot.

Immediately on admission to the cage, Skirrl began to gnaw
at the boxes, trying with all his might to tear them to pieces.
After some thirty minutes of such effort, interrupted by wander-
ings about the cage and attempts to get at the other monkeys,
he suddenly went to the largest box of all, set it up on end
almost directly under the carrot, mounted it, and looked up
at the food. It was still beyond his reach and he made no
effort to get it, but instead, he reached from his perch on the
big box for the next smaller box, which was approximately
sixteen inches, by fourteen, by twelve. This he succeeded in
pulling toward him, at the same time raising it slightly from
the floor, but his efforts caused the large box to topple over
and he quit work. The experiment was discontinued after a
few minutes, the total period of observation having been thirty-
five minutes.

Skirrl handled the boxes with ease and with evident pleasure
and interest. He also noticed the carrot at various times during
the interval, but his attention was fixed on it only for short
periods.

The test was continued on August 24 when, instead of a
carrot, a half banana was used as bait. It was placed only
five feet from the floor, and three boxes were as formerly placed
in distant corners of the cage. When admitted, Skirrl looked at
the banana, then pulled one of the boxes toward it, but in-
stead of mounting, he went to the smallest box and began to
gnaw it. Shortly, he mounted the middle sized box and looked
up toward the banana, but the box was not directly under the

MENTAL LIFE OF MONKEYS AND APES 105

bait, and in any event, it would have been impossible for him
to reach it. He next went to the largest box, gnawed it vigor-
ously, turned it over several times, and then abandoned it for
the middle sized box, from which by skillful use of his teeth
and hands, he quickly tore off one side.

By this time, apparently without very definitely directed
effort on the part of the monkey, all three of the boxes were
in the center of the cage and almost directly beneath the banana.
Skirrl climbed up on the largest box and made efforts to pull
the middle sized one up on to it, the while looking at the
banana every few seconds. He did not succeed in getting the
boxes properly placed, and after a time began moving them
about restlessly.

His behavior plainly indicated that hunger was not his chief
motive. He was more interested in playing with things or in
working with them than in eating, and the satisfaction of tear-
ing a box to pieces seemed even greater than that of food. It
is especially noteworthy that when Skirrl attempts to dismember
a box, instead of starting at random, he searches carefully for
a favorable starting point, a place where a board is slightly
loosened or where a slight crack or hole enables him to insert
his hand or use his teeth effectively. Many times during this
experiment he was observed to examine the boxes on all sides
in search of some weak point. If no such weak point were
found, he shortly left the box; but if he did find a favorable
spot, he usually succeeded, before he gave up the attempt, in
doing considerable damage to the box.

Following the behavior described above, Skirrl returned to
the middle sized box, placed it on end under the banana, mounted,
and looked upward at the bait, but as it was a few inches beyond
his reach, he made no attempt to get it, but instead, after a
few seconds, went to the smallest box, and finding a weak point,
began to tear it to pieces.

Later he rolled what was left of the smallest box close to
the other two boxes, nearly under the banana, and the remainder
of his time was spent gnawing at the boxes and playing with
pieces which he had succeeded in tearing from them. During
the remainder of the thirty minute interval of observation, no
further attention was given the bait.

Again, on August 25, the test was tried, but this time with

106 ROBERT M. YERKES

boxes whose edges had been bound with tin so that it was
impossible for the monkey to destroy them. He spent several
minutes searching for a starting point on the middle sized box,
but finding none, he dragged it under the banana, looked up,
mounted the box, but, as previously, did not reach for the bait
because it was beyond his reach. He then played with the
boxes for several minutes. Finally he worked the two smaller
boxes to a position directly under the banana, put the middle
sized one on end, mounted it, and looked at the bait, but again
abandoned the attempt without reaching.

During the thirty minutes of observation he made no definite
effort to place one box upon another. Three times he mounted
one or another of the boxes when it was under the banana or
nearly so, but in no case was it possible for him to reach the bait.

From the above description of this monkey’s behavior, it
seems fairly certain that with sufficient opportunity, under
strong hunger, he would ultimately succeed in obtaining the
bait by the use of two or more boxes. For his somewhat abor-
tive and never long continued efforts to drag two boxes together
or to place the one upon the other clearly enough indicate a
tendency which would ultimately yield success. The possibility
of imitation is not excluded, for Skirrl had opportunities to see
Julius and the experimenter handle the boxes.

Because of the other work which seemed more important at
the time, this experiment was not continued further. The re-
sults obtained suggest the desirability of testing thoroughly the
ability of monkeys to use objects as only the anthropoid apes
and man have heretofore been thought capable of using them.

Box and Pole Experiment

Skirrl was first tested with the box and pole experiment on
August 12. As in the case of Julius, a half banana was placed
in the middle of the long box and the attention of the monkey
was attracted to the bait by small pieces of carrot placed near
each open end. Two poles were placed near the box on the
floor of the cage. When admitted to the cage Skirrl went
almost directly to the ends of the box, took the pieces of carrot
which were in sight, but apparently failed to perceive the bait
in the middle of the box. For a while he played with the locks
on the box, shoved it about, and amused himself with it, showing

MENTAL LIFE OF MONKEYS AND APES 107

no interest in obtaining the food. Later he looked through the
box and saw the banana. He then dragged the box about,
apparently trying to get it into his cage, but he gave no atten-
tion to the poles nor did he make any evident effort to obtain
the banana which was easily visible in the center of the box.
The period of observation was only twelve minutes.

On August 24 this experiment was repeated with an import-
ant modification of the apparatus in that the wooden lid of
the long box had been replaced by a wire cover through which
the animal could see the bait. Two poles were as formerly
on the floor of the cage, not far from the box. Skirrl almost
immediately noticed the banana and tried to get it by gnawing
at the box. He did not once reach in at the ends of the box,
but he did handle the poles, throwing them about and pound-:
ing with them. There was not the slightest attempt to use
them in obtaining the bait.

This experiment was later repeated three times at intervals
of a number of days, but in no case did Skirrl show any ten-
dency to use the poles as means of obtaining the food.

Draw-in Experiment

This also was arranged in the same manner as for Julius, and
on each of five days Skirrl was allowed at least thirty minutes
to work for the bait. Either a banana or a carrot was each
day placed on the board well beyond his reach, and one or
two, usually two, small sticks were put into his cage. Not
once during the several periods of observation did Skirrl make
any attempt to use a stick or any other object as a means of
drawing the food to him. Instead, he reached persistently with
his arm, pulled and gnawed at the wires which were in his way,
and occasionally picked up and gnawed or pounded with the
sticks in the cage. His attention every now and then would
come back to the food, but it tended to fluctuate rather rapidly,
and in the regular period of observation, thirty minutes, it is
unlikely that he attended to the bait itself for as much as five
minutes. In this respect as well as many others; Skirrl’s be-
havior contrasts sharply with that of the orang utan.

The results of this experiment indicate the lack in the monkey
of any tendency or ability, apart from training, to use objects
as means of obtaining food. Ways of using objects as tools

108 ROBERT M. YERKES

which apparently are perfectly natural to the anthropoid apes
and to man are rarely employed by the lower primates.

Hammer and Nail Test :

One day I happened to observe Skirrl playing with a staple
in his cage. He had found it on the floor where it had fallen
and was intently prodding himself with the sharp points, ap-
parently enjoying the unusual sensations which he got from
sticking the staple into the skin in various portions of his body,
and especially into the prepuce.

A few days later I saw him playing in similar fashion with a
nail which he had found, and still later he was seen to be using
a stick to pound the nail with. This suggested to me the hammer
and nail test.

A heavy spike was driven into an old hammer to serve as
an indestructible handle. This hammer, along with a number
of large wire nails and a piece of redwood board, was then
placed in the monkey’s cage. Skirrl immediately took up the
hammer, grasping the middle of the handle with his left hand,
and with his right hand taking up a nail. He then sat down
on the board, examined the nail, placed the pointed end on the
board, and with well directed strokes by the use of the head of
the hammer drove the nail into the board for the distance of
at least an inch. He then tried to pull it out, but was forced
to knock it several times with the hammer before he could do so.

This performance, during the next few minutes, was repeated
several times with variations. Often the side of the hammer
was used instead of the head, and occasionally, as is shown
in figure 8 of plate II, he seized the hammer well up toward
the juncture of the same with the spike. This figure does
justice to the performance. At the moment the picture was
taken, Skirrl’s attention had been attracted by a monkey in
an adjoining cage, and he had momentarily looked up from
his task, the while holding nail and hammer perfectly still.

This test was repeated on various days, and almost uniformly
Skirrl showed intense interest in hammer and nails and used them
more or less persistently in the manner described. Occasionally,
apparently for the sake of variety, he would put the blunt end
of the nail on the board and hammer on the point. Again, he
would try persistently to drive the nail into the cement floor,

MENTAL LIFE OF MONKEYS AND APES 109

and once by accident, when hammer and nails were left in his
cage over night, he succeeded in making several holes in the
bottom of his sheet iron water pan. There was no doubting
the keen satisfaction which the animal took in this form of
activity.

It is impossible to say that the behavior was not imitative
of man, for Skirrl, along with all of the other monkeys, had had
abundant opportunity to see carpenters working. But this much
can be said against the idea of imitation,—no one of the other
animals, not excepting the orang utan, showed any interest what-
ever in hammer and nails. Occasionally they would be played
with momentarily or pushed about, but Sobke, Jimmie, Gertie,
Julius, although given several opportunities to exhibit any ability
which they might have to drive.nails, made not the least attempt
to do so. Evidently we must either conclude that Skirrl had
a peculiarly strong imitative tendency in this direction, or in-
stead, a pronounced disposition or instinct for the use of objects
as tools. It would seem fair to speak of it as an instinct for
mechanical activity.

Under this same heading may be described Skirrl’s reactions
to such objects as a handsaw, a padlock, and a water faucet.
The saw was given to him in order to test his ability to use it in
human fashion, for if he could so expertly imitate the carpenter
driving nails, it seems likely that he might also imitate the use of
the saw.

As a matter of fact, he showed no tendency to use the saw
as we do. Instead, he persistently played with it in various
ways, at first using it as a sort of plane to scrape with, later
often rubbing the teeth over a board so that they cut fairly well,
but never as effectively as in the hands of a man. After two
or three days’ practice with the saw, Skirrl hit upon a method
which is, as I understand, used by man in certain countries,
namely, that of placing the saw with the teeth up, holding it
rigid, and then rubbing the object which is to be sawed over
it. This Skirrl succeeded in doing very skillfully, for he would
sit down on the floor of the cage, grip with both feet the handle
of the saw, with the.teeth directed upward, then holding either
end in his hands, he would repeatedly rub a stick over the
teeth. In this way, of course, he could make the saw cut fairly
well. But still more to his liking was the use of a spike instead

110 ROBERT M. YERKES

of a stick as an object to rub over the teeth, for with this he
was able to make a noise that would have satisfied even a
small boy.

Further light is shed on the force of the tendency to imitate
man by the saw test. After Skirrl had been given an oppor-
tunity to show what he could do with the tool spontaneously,
I demonstrated to him the approved human way of sawing.
Often he would watch my performance intently as though fas-
cinated by the sound and motion, but when given the tool he
invariably followed his own methods. Although I repeated this
test of imitation several times on three different days, the re-
sults were wholly negative.

Other Activities

One day as Skirrl was being returned to his own cage by
way of the larger cage, he picked up an unfastened padlock
and carried it into the cage with him. For more than an hour
he amused himself almost without interruption by playing with
this lock. The things which he did with it during that time
would require pages to describe. His interest in it was very
similar to that which he had exhibited in hammer and nails,
saw, and indeed any objects which he could play with. The
lock was pounded in various ways, bitten, poked with nails,
hooked into the wires of the cage, used to pull on, pounded with
a stick, used to hammer on the floor of the cage with, and in
fine, manipulated in quite as great a variety of ways as a human
being could have discovered. Finally it was hooked to the
side of the cage and snapped shut, and as Skirrl was unable
to dislodge it from this position, he shortly gave up playing
with it.

At the end of the large cage and just outside the wire netting
was a faucet to which a hose was usually attached. The valve
could be opened by turning a wheel-shaped hand piece. Both
Skirrl and Julius learned to turn this wheel in order to get
water to play with, but usually the former’s strength was not
sufficient to turn on the water. The latter could do it readily.
The indications are that both animals profited by seeing human
beings turn on the water. This unquestionably attracted their
attention to the faucet, and probably by playing with it they
accidentally happened upon the proper movement. At any

MENTAL LIFE OF MONKEYS AND APES 111

rate, Skirrl’s behavior was significant in this connection, for
he would pick up the hose to see if water were flowing, and if
it were not, he would throw it down, go directly to the faucet,
and try to turn the wheel. The association of the wheel with
the desired flow of water was therefore definitely established.
Shall we describe the act as ideational? It seems the natural
thing to do.

3. Sobke, Pithecus rhesus

Box Stacking Experiment

For this test, in the case of Sobke, three light boxes made
of redwood about one-third of an inch thick were used. The
smallest, box 1, was six inches in each direction, the next larger,
box 2, was twelve inches, and the third, box 3, eighteen inches.
As in the case of the other animals, bait, either banana or carrot,
was suspended from the middle of the roof of the large cage
at such distance from the floor as to be reached by the animal
only by the use of the boxes.

The first observations on Sobke were made on June 14. The
three boxes had been placed in the form of a pyramid directly
under the banana, which hung about eighteen inches above the
uppermost box. Sobke’s attention while in his cage had been
attracted to the bait by seeing me fastening it in position, but
when admitted to the large cage, he simply glanced at it and
then wandered about the cage, picking up bits of food and
struggling to get at the other monkeys. This he did for about
five minutes. He then went to the boxes, placed his hands
on top of the bottom one, but did not climb up on it. A few
minutes later he returned to the box again, climbed up, and
readily reached the food, which he ate while resting on boxes
1 and 2.

I now replaced the bait and gave the monkey a second chance
to obtain it.. Almost immediately he climbed up as far as the
second box, but although he could reach the banana only from
the uppermost box, he deliberately shoved it off to the ground
and sat down upon box 2. As he was unable to obtain the
banana from this, he soon began to gnaw and pull at it, and as
he was succeeding all too well in his efforts to tear the box to
pieces, he had to be returned to his cage.

The most important features of his behavior were, first, his

112 ROBERT M. YERKES

stealthy and indirect manner, and second, his failure to use
‘other means of obtaining the bait than that supplied by the
observer. Instead of looking straight at the experimenter, or
at the object which he wished to obtain, he apparently looked
and attended elsewhere. For this reason it was often difficult
to decide whether or not he had noticed the bait or the boxes.
Finally I was led to conclude that he usually knew exactly what
was going on and had in his furtive way noted all of the essential
features of the situation, and that his manner was extremely
indicative of his mental attitude of limited trust. Both Julius
and Skirrl went to the opposite extreme in the matter of direct-
ness, or as we should say in human relations, frankness. They
would look the experimenter directly in the eye, and they usu-
ally gazed intently at anything, such for example as the bait,
that interested them. Sobke, even when very hungry, instead
of going directly toward the bait, and trying to obtain it, usually
did various other things as though pretending that he had no
interest in food.

On the following day, June 15, the three boxes were again
placed nearly under the banana, but this time the two smaller
boxes, numbers 1 and 2, were pushed to the extreme end of
the lower box and so far from the bait that it could not be
reached from box 1. It was necessary then for the animal to
push boxes 1 and 2 along on box 3 until they were nearer the bait.

Sobke, when admitted to the cage, evidently noticed the
banana, but as formerly, he made no immediate effort to obtain
it. After wandering in search of food and quarreling with the
other monkeys for several minutes, he went to the boxes, pushed
the topmost one, number 1, off on to the floor, and then carried
it into his cage where he quickly tore one side off. He next
returned to the large cage, climbed up on box 2, and he was
able, by jumping, to reach and obtain the banana.

As Sobke was very good at jumping, his new method ren-
dered the box stacking experiment of uncertain value, since it
was next to impossible so to arrange the spatial relations of bait
and boxes that he should be neither discouraged by too great
a distance nor encouraged to jump by too small a distance.
Evidently it would be more satisfactory to simplify the con-
ditions by trying to discover, first of all, whether he would use
a single box as a means of reaching the reward.

MENTAL LIFE OF MONKEYS AND APES 113

In pursuance of this idea, I suspended a piece of bread five
feet from the floor of the cage, and a few feet to one side of it,
I placed a box from which it could be reached, or at least easily
seized by jumping. Sobke shortly walked to a point beneath the
bait and leaping into the air, seized it.

I then replaced the bait, raising it to a height of five feet
ten inches from the floor of the cage. When I had retired,
Sobke placed himself in the proper position beneath, looked
up at it, but went away without jumping for it. During the
remaining ten minutes of observation, he paid no further atten-
tion to the bait, having satisfied himself evidently that it was
beyond his reach.

My use of this test was concluded on June 16 when once
more I suspended a piece of bread six feet from the floor and
placed a few feet to one side the eighteen inch box, number 3,
from which had the monkey pushed it to a point directly under
the bread, he could have obtained the food easily.

Sobke noticed the food promptly, and from time to time
as he wandered about, he glanced at it out of the corner of his
eye, but not once did he sit down and look at it steadily and
directly as Julius and Skirrl might have done.

In the first twenty minutes of observation the monkey made
no attempt either to use the box or to reach the food by jump-
ing. I then placed the box directly under the bait, and scarcely
had I withdrawn from the cage before Sobke climbed up on it
and looked toward the food. He could not reach it without
jumping, and he made no effort to get it. I had left a second
box in the cage,—one which I had been using as a seat. Sobke
now went to this box, placed his hands on it, looked toward the
bait, and then went to a distant part of the cage. No further
indications were obtained during the remainder of the period
of observation of interest in the boxes as possible means of
obtaining the desired food.

It is of course obvious that this experiment was not long
enough continued to justify the conclusion that either Sobke
or Skirrl could not use the boxes or even learn to place one
box upon another in order to obtain the bait. The experiment,
like several others which are being described briefly, was used
to supplement the multiple-choice experiment, and the experi-
menter’s chief interest was to discover the number and variety

114 ROBERT M. YERKES

of methods which would be used by the animal in the first few
presentations of a situation. It is practically certain that both
of these monkeys would have succeeded ultimately in solving
the problem of obtaining the food had they been left in the
cage with a number of boxes, for Skirrl very early indicated
interest in moving the boxes about, and Sobke showed a ten-
dency in that direction which perhaps was inhibited partially
by his distrust of the experimenter.

Draw-in Experiment

For Sobke, as for Julius and Skirrl, the draw-in test was
made by putting food on a shelf outside the cage, beyond the
reach of the animal, and placing in the cage with the animal
one or two sticks long enough to be used for drawing in the bait.

Sobke was first given this test on July 24. He tried per-
sistently to reach the banana with his hand, seized the box
which supported the bait, shook it, picked up one or other of
the sticks, and chewed at it repeatedly, but not once did he
make any move to use a stick to draw the food toward him.

This experiment was repeated on July 27, 29, 30 and 31, a
period of thirty minutes being allowed on each day for obser-
vation. At no time did Sobke show any inclination to use
either a stick or any other object as a means of reaching the
bait. Instead, he confined himself strictly to the use of hands
and teeth.

This test makes it fairly certain that Sobke had no natural
tendency to use objects as tools. In so far as he attended to
things about the cage or laboratory, it seemed to be rather to
play with them in a general way than to use them ideationally
or otherwise for definite purposes.

The definitely negative result of the draw-in experiment ren-
dered needless prolonged observation with the box and pole
test, whose results are now to be presented.

Box and Pole Experiment

The eighty-four inch box, previously used for a similar test
with Julius, was presented to Sobke on August 24, the wooden
cover having been replaced by a wire one so that the monkey
could readily see the bait in the middle of the box. Sobke,

MENTAL LIFE OF MONKEYS AND APES 115

when admitted to the large cage, went directly to the box and
at once discovered the banana which was midway between the
ends. He evidently desired it. Shortly, he went to one end
of the box and. looked in. This he repeated later. He also
shook the box and tried to pull it about and tear it with his
teeth, but to the two poles lying nearby on the floor of the cage
he gave not the slightest attention during a thirty minute period
of observation. ;

The experiment was not repeated because of more important
work.

Other Activities

In more respects than I have taken time to enumerate in the
above descriptions of behavior, the relations of Sobke to objects
differed from those of Skirrl, and still more from those of Julius.
Hammer, nails, saw, stones, sticks, locks, and various other
objects received relatively little attention from Sobke unless
they happened to come in his way; then they were usually pushed
aside with but scant notice. Rarely he would carry something
to the shelf of his cage with him, but as a rule only to lay it
down and attend to something else. Skirrl, on the contrary,
attended persistently to anything new in the shape of a movable
object. He was extremely partial to objects which could be
manipulated by him in various ways, and especially to any-
thing with which he could make a noise. His interest in hammer
and nails, saw, locks, etc., seemed never to wane. I have seen
him play for an hour almost uninterruptedly with a hammer
and a nail, or even with a big spike which he could use to pry
about his cage. In the absence of anything more interesting,
even a staple or a small nail might receive his undivided atten-
tion for minutes at a time. How important is the species
difference in this connection, I have no means to judge, but if we
may not consider these different modes of behavior character-
istic of P. rhesus as contrasted with P. irus, we must conclude
that remarkable individual differences exist among monkeys,
for whereas Skirrl is by nature a mechanical genius, Sobke has
apparently no such dispositiqn. I can imagine no more fasci-
nating task than the careful analytical study of the tempera-
ments of these two animals. Skirrl’s behavior has importantly
modified my conception of genius.

Vv
MISCELLANEOUS OBSERVATIONS

1. Right- and left-handedness

Several years ago Doctor Hamilton reported to me observa-
tions which he had made on preference for the right or left paw
in dogs. He has not, I believe, published an account of his
work. Subsequently, Franz observed a similar preference in
monkeys which, according to his report, exhibit marked tendency
to be right-handed, left-handed, or ambidextrous.

My own observations, although they are wholly incidental to
my other work, seem worthy of description at this point.

I noted, first of all, that the orang utan Julius tended to use
his left hand. He by no means limited himself to this, but
in difficult situations he almost invariably reached for food or
manipulated objects in connection with food getting with the
left hand. Figures 23 and 24 of plate V. show him reaching
for a banana with the left hand. Likewise, figure 34 exhibits
the use of the left hand in the draw-in experiment.

So marked was Julius’s preference for his left hand that I
became interested in observing similar phenomena in the monkeys.

Skirrl, when driving nails, held the hammer with his left
hand and the nail with his right hand. The fact that he never
was observed to reverse the use of the hands is surprising, for
other observations indicate that he preferred the right hand
for certain acts.

Stimulated by the obvious left-handedness, in certain con-
nections, of Julius and Skirrl, I tested the preference of several
of the monkeys in the following simple way. Standing outside
the cage I would hold out a peanut to a hungry animal, keeping
it so far from the cage that the monkey could barely reach it
with its fingers. I noted the hand which was used to grasp the
food. Next I varied the procedure by placing the peanut on
a board in order to make sure that I was not definitely directing
the animal’s attention.

e

MENTAL LIFE OF MONKEYS AND APES 117

With Sobke the following results were obtained. In forty
trials given on two different days, he reached for and obtained
the food each time with his left hand. Only by holding the
bait well toward the right side of his body was it possible to
induce him to use the right hand. So far as may be judged
from these observations and from others in connection with
the experiments, this animal is definitely left-handed.

With Skirrl the results are strikingly different. As stated
above, he used the hammer consistently with his left hand, but
in twenty attempts to obtain food by reaching, he used his
right hand seventeen times and his left only three times. It
was quite as difficult to induce him to use his left hand for this
purpose as it was to induce Sobke to use his right. We must
therefore conclude that Skirrl is right-handed in connection with
certain movements and left-handed in others.

The monkey named Gertie in the reaching experiment con-
sistently used her left hand, never once using the right.

Jimmie, so far as it was possible to make tests with him,
also used his left hand, but it should be said that the results
are unsatisfactory because he was at the time extremely pug-
nacious and paid attention to the experimenter rather than
to the food.

Scotty, in the first series of ten trials, used his right hand
eight times, his left twice. In the second series, given the
following day, he used the right hand three times and the left
seven times. From this we should have to infer that he is
ambidextrous.

A female rhesus monkey which had been brought to the labo-
ratory only a few days previously showed a preference for the
right hand by the use of it fourteen times to six.

In connection with these data which are, I should repeat,
too scanty to be of any considerable value, I wish to describe
my own experience. Although naturally left-handed, I am by
training right-handed to the extent of having been able to use
my hands in writing and in various other activities equally well
at the age of twelve. I am at present ambidextrous in that
there are many things which I do with equal readiness and
skill with either hand. Delicate, exact, and finely coérdinated
movements, such as those of writing and using surgical instru-
ments, I perform always with my left hand, while grosser move-

118 ROBERT M. YERKES

ments involving the whole hand or arm, I am rather likely to
perform with my right hand.

It seems not improbable in the light of my own experience
that we shall find some specialization among the lower animals
with respect to preference for right and left hand or arm. I
should not be ‘at all surprised to discover that it is the rule for
animals to possess or to develop readily definite preference for
one hand in connection with a given act of skill and to have quite
as definite a preference for the other hand in connection with a
radically different kind of act.

ca

2. Instinct and emotion

Of the many presumably instinctive modes of behavior which
were observed, only those which have to do with social relations
seem especially worth reporting.. From among them I shall
select for description a few which have already been referred
to in connection with the experimental observations.

Maternal Instinct

Aspects of the maternal instinct I had opportunity to observe
in Gertie, who on February 27 gave birth to a male infant, I
present below the substance of a previously published note on
her behavior (Yerkes, 1915).

“On February 27 one of the monkeys of our collection gave
birth, in the cages at Montecito, to a male infant. The mother
is a Macacus cynomolgus rhesus (P. irus rhesus) who has been
described by Hamilton (1914, p. 298) as ‘Monkey 9, Gertie,
M. cynomolgus rhesus (P. irus rhesus). Age, 3 years 2 months.
(She is now, May 1, 1915, 4 years and 6 months.) Daughter
of monkeys 3 and 10. First pregnancy began September, 1913.’
The result of this pregnancy was, I am informed, a still-birth.

“The second pregnancy, which shall now especially concern
us, resulted likewise in a still-birth. Parturition occurred Satur-
day night, and the writer first observed the behavior of the
mother the following Monday morning. In the meantime the
laboratory attendant had obtained the data upon which I base
the above statements.

“At the time of parturition Gertie was in a 6 by 6 by 12 foot
out-door cage containing a small shelter box, with an excep-

MENTAL LIFE OF MONKEYS AND APES 119

tionally quiet and gentle male (not the father of the infant)
who is designated in Hamilton’s paper as Monkey 28, Scotty.

‘““My notes record the following exceptionally interesting and
genetically important behavior. On March 1, when I approached
her cage, Gertie was sitting on the floor with the infant held in
one hand while she fingered its eyelids and eyes with the other.
Scotty sat close beside her watching intently. When disturbed
by me the mother carried her infant to a shelf at the top of the
cage. Repeatedly attempts were made to remove the dead baby,
but they were futile because Gertie either held it in her hands or
sat close beside it ready to seize it at the slightest disturbance.

“Especially noteworthy on this, the second day after the
birth of the infant, are the male’s, as well as the female’s, keen
interest in the body and their frequent examinations of the
eyes, as if in attempts to open them. Often, also, the mother
searched the body for fleas.

“Observations were made from day to day, and each day
opportunity was sought to remove the body without seriously
frightening or exciting the female. No such opportunity came,
and during the second week the corpse so far decomposed that,
with constant handling and licking by the adults, it rapidly
wore away. By the third week there remained only the shriv-
eled skin covering a few fragments of bone, and the open skull
from the cavity of which the brain had been removed. This
the mother never lost sight of: even when eating she either
held it in one hand or foot, or laid it beside her within easy. reach.

“Gradually this remnant became still further reduced until
on March 31 there existed only a strip of dry skin about four
inches long with a tail-like appendage of nearly the same length.

“The male, Scotty, on this date was removed to another cage.
Gertie made a great fuss, jumping about excitedly and uttering
plaintive cries when she discovered that her mate was gone.
Whenever I approached her cage she scurried into the shelter
box and stayed there while I was near. This behavior I never
before had observed. It continued for two days. On April 2,
it was noted that she had lost her recently acquired shyness
and she no longer made any attempts to avoid me. As usual,
on this date, she was carrying the remnant about with her.

‘The following day, April 3, Gertie was lured from her cage
to a large adjoining compartment for certain experimental obser-

120 ROBERT M. YERKES

vations. After she had been returned to her own cage the
remnant was noticed on the floor of the large cage. I picked it
up. Gertie evidently noticed my act, for although at a distance
of at least ten feet from me, she made a sharp outcry and
sprang to the side of the cage nearest me. I held the piece of
skin (it looked more like a bit of rat skin than the remains of
a monkey) out to her and she immediately seized it and rushed
with it to the shelf at the top of the cage.

“Two days later the remnant was missing, and careful search
failed to discover it in the cage. It is probable that Gertie had
carelessly left it lying on the floor whence it was washed out
when the cages were cleaned. On this date Gertie seemed
quieter than for weeks previously.

‘“ Thus it appears that during a period of five weeks the in-
stinct to protect her offspring impelled this monkey to carry its
gradually vanishing remains about with her and to watch over
them so assiduously that it was utterly impossible to take them
from her except by force.

“After reading this note in manuscript, Doctor Hamilton in-
formed me that Gertie had behaved toward her first still-birth
as toward her second. And, further, that Grace, a baboon,
also carried a still-birth about for weeks.

““T am now heartily glad that my early efforts to remove
the corpse were futile, for this record of the persistence of ma-
ternal behavior seems to me of very unusual interest to the
genetic psychologist.”

Fear

In connection with the multiple-choice experiments Skirrl
exhibited what seemed to be instinctive fear as a result of his
unfortunate experience with nails in the floor of box 1. He
seemingly referred his misadventure to some unseen enemy under
the floor, and this in spite of the fact that he was given abundant
opportunity to examine the floor of the box, but not until after
the dangerous nails had been clinched. His long continued
avoidance of the experiment boxes and his still more persis-
tent hesitancy in entering them, coupled with his almost ludi-
crous efforts to see beneath the floor through the holes cut for
the staples on the doors, gave me the impression of supersti-
tious fear of the unseen. As I watched and recorded his beha-
vior day after day during the period of most pronounced fear,

°

MENTAL LIFE OF MONKEYS AND APES 121

I could not avoid the thought that the instinctive fear of snakes
had something to do with his peculiar actions, and although
I have never studied either the natural or the acquired re-
sponses of monkeys to snakes, I suspect that lacking such in-
stinctive equipment, Skirrl would have behaved differently as
a result of the pricks which he received from the nails.

It is needless to redescribe his acquired fear of whiteness as
it manifested itself in the freshly painted apparatus.

Accompanying these instructive modes of response and their
emotions are suggestions of peculiarly interesting problems as
well as of modes of attacking them. As a matter of fact, Skirrl’s
fear-reactions did much to alter my conception of the constitu-
tion of his mind. I should not have been surprised by the
features of behavior exhibited, but I was by no means prepared
for their persistence, and for the highly emotional attitude
toward the particular situation. Only an organism of complesly
constituted nervous system and fairly highly developed affec-
tive life could be expected to respond as did this monkey. As
has been suggested above, I find the appeal to instinct, modi-
fied by experience, a natural mode of accounting for the unex-
pected features of Skirrl’s behavior.

Sympathy

The instinctive playfulness of the young monkey Tiny con-
trasted most strikingly with the more serious, if not more sedate,
modes of behavior of the older individuals.

During the greater part of my period of observation Tiny was
cage-mate of Scotty, the most calm and apparently lazy of all
the monkeys. Tiny delighted in teasing Scotty, and her varied
modes of mildly tormenting him and of stirring him to pursuit
or to retaliation were as interesting as they were amusing. Her
most common trick was to steal up behind him and pull the
hair of his back, or seize his tail with her hands or teeth.
Often when he was asleep she would suddenly run to him, give
a sudden jerk at a handful of hairs, and leap away. He was
surprisingly patient, and I never saw him treat her roughly
in retaliation.

Another of Tiny’s favorite forms of amusement was that of
trying to stir up the other monkeys to attacks on one another.
She very cleverly did this by pretending that she herself was

a

122 ROBERT M. YERKES

being attacked. The instant the older animals began to show
hostility toward one another she would leap out of the way and
watch the disturbance with evident satisfaction. It was this
mode of behavior in the little animal which ultimately pro-
vided opportunity for the observations which I wish now to
report as indicative of sympathetic, possibly I may say al-
truistic, emotions.

Tiny was confined with Scotty in a cage adjoining the one
in which Jimmie and Gertie were being kept. The cages were
separated by wire netting of half-inch mesh.

One morning as I was watching the behavior of the animals
in the several cages, I noticed Tiny dressing with her teeth a
wounded finger. It had evidently been bitten by one of the
other animals, in all probability either by Jimmie or Gertie.
Tiny was trimming away the loose bits of skin very neatly and
cleansing the wound. After working at this task for a few
minutes, she quickly climbed up to the shelf near the top of
her cage, and by rushing to the partition wire between the
two cages, she lured Gertie to an attempted attack on her.
Gertie sprang up to the partition, placed her hands on it, with
the fingers projecting through the meshes, and attempted to seize
Tiny’s fingers with her teeth. But the latter was too quick
for her, and withdrawing her hands, like a flash seized in her
teeth the middle finger of Gertie’s left hand. She then bit it
severely and with all her might, at the same time pulling and
twisting violently, often placing the entire weight of her body
on the finger. Her sharp teeth cut to the bone, and it was
impossible for the larger and stronger monkey to tear away.
For several seconds this continued, then Gertie succeeded in
escaping, whereupon she at once retreated to the opposite end
of her shelf and proceeded to attend to her injured finger. She
cried, wrung her hands, and from time to time placed the finger
in her mouth as though in an effort to relieve the pain. By
this time Jimmie’s attention had been attracted by the dis-
turbance and he rushed up to the shelf, and facing Gertie, watched
her intently for a few seconds. The look of puzzled concern on
his face was most amusing. Apparently he felt dimly that
something in which he should have intelligent interest was
going on, but was unable wholly to understand the situation.
After watching Gertie for a time and trying to discover what

MENTAL LIFE OF MONKEYS AND APES 123

she was doing, which was rendered difficult by her tendency
to turn away from him, in order to shield her injured finger,
he rushed over to the wire partition and made strenuous efforts
to seize Tiny with his hands and teeth. But although she
continued close to the partition and often crowded against it
with face and hands flattened on the wires, he was not able to
get hold of her, and after a few vain attempts he returned to
his mate, and again with evident solicitousness and the most
troubled expression, watched her wringing her hands and chew-
ing or sucking at her injured finger. Shortly he again returned
to the partition and renewed his attempts to seize the young
monkey. Thus he went back and forth from one place of in-
terest to the other several times, but being unable to achieve
anything at either point, he finally gave up and returned to
his breakfast on the floor of the cage.

I report this incident fully because the behavior of Jimmie
was in marked contrast with the usual behavior of the mon-
keys. Selfishness seemed everywhere dominant, while clear in-
dications of sympathetic emotions were rare indeed. The above
is undoubtedly the best evidence of anything altruistic that
I obtained.

It is possible that Tiny’s action was retaliatory, but although
it is practically certain that either Gertie or Jimmie inflicted
the wound on her finger, I of course cannot be sure that the
spirit of revenge stirred her to punish Gertie so severely. Jim-
mie’s part in the whole affair is, however, perfectly intelligible
from our human point of view, and there seems no reason to
doubt that he did experience something like a feeling of sym-
pathy with his mate, coupled with a feeling of resentment or
anger against Tiny.

VI

HISTORICAL AND CRITICAL DISCUSSION OF IDEATIONAL
BEHAVIOR IN MONKEYS AND APES

It is my purpose in this section to indicate the relations of
my work on monkeys and apes to that of other investigators.

Although throughout the report I have used freely the psy-
chological terms idea and ideation, it has been my aim to describe
the behavior of my animals rather than to interpret it or
speculate concerning its accompaniments. Certain acts I have
designated as ideational simply because they seemed to exhibit
the essential features of what we call ideational behavior in man.
Further study may, and probably will, modify my opinion
concerning this matter. It is of prime importance to analyze
ideational behavior so that it may be accurately described
and satisfactorily defined in terms of its distinguishing charac-
teristics. J had hoped to be able to present a tentative
analysis in this report, but the results of my efforts are so
unsatisfactory that I do not feel justified in publishing them.

The terms idea and ideation have been used to designate
contents of consciousness which are primarily representative.
Nowhere have I attempted to indicate different types or grades
of ideational behavior and nowhere have I found it necessary
to emphasize differences between image and idea. In general,
the acts which I have called ideational have been highly adap-
tive, and the learning processes in connection with which they
have appeared have differed strikingly from those of the selec-
tive sort in their abruptness of appearance.

Extremely interesting and valuable definitions of ideation and
discussions of the characteristics of different sorts of ideas in
the light of original observations on monkeys have been pre-
sented by Thorndike (1901, pp. 1, 2; 1911, p. 174); Kinnaman
(1902, p. 200); and Hobhouse (1915, p. 270). As these authors
have contributed importantly to our knowledge of the behavior
of monkeys, their discussions of the meaning of terms are especi-
ally valuable. Serviceable definitions are to be found, also, in

S,

MENTAL LIFE OF MONKEYS AND APES 125

Romanes (1900), Morgan (1906), Washburn (1908), and Holmes
1911).
Evidences of Ideation in Monkeys

Aside from anecdotal and traveller’s notes on the behavior
of monkeys and apes we have only a scanty literature. In
fact, the really excellent articles on the behavior and mental
life of these animals may be counted on one’s fingers; and not
more than half of these are experimental studies. I shall, in
this brief historical sketch, neglect entirely the anecdotal litera-
ture, since my own work is primarily experimental, and since
its results should naturally be compared with those of other
experimenters.

Thorndike (1901), the American pioneer in the application of
the experimental method to the study of mind in animals, pub-
lished the first notable paper on the psychology of monkeys.
His results force the conclusion that “‘free ideas’ seldom appear
in the monkey mind and have a relatively small part in beha-
vior. That the species of Cebus which he observed exhibits
various forms of ideation he is willing to admit. But he insists
that it is of surprisingly little importance in comparison with
what the general behavior of monkeys as known in captivity
and as described by the anecdotal writers have led us to expect.
It is important to note, however, that Thorndike’s observations
were limited to Cebus monkeys which, as contrasted with various
old world types, are now considered of relatively low intelligence.

In many respects the most thoroughgoing and workmanlike
experimental study of monkeys is that of Kinnaman (1902),
who has reported on the study of various forms of response
in P. rhesus. He presents valuable data concerning the learn-
ing processes, sensory discrimination, reaction to number, and
to tests of imitation. His results indicate a higher level of
intelligence than that discovered by Thorndike, but this is
almost certainly due to difference in the species observed. Kin-
naman goes so far as to. say ‘‘We have found evidence, also,
of general notions and reasoning, both of low order” (p. 211).

The contribution of Hobhouse (1915) to our knowledge of the
mental life of monkeys, although in a measure experimental, is
based upon relatively few and unsystematic observations as
contrasted with those of Thorndike and Kinnaman. It ap-
pears, however, that Hobhouse’s experiments were admirably

126 ROBERT M. YERKES

planned to test the ideational capacity of his subjects, and one
can not find a more stimulating discussion of ideation than
that contained in his ‘‘ Mind in Evolution.” The results of
his tests made with a P. rhesus monkey are similar to those
of Kinnaman, for almost all of them indicate the presence and
importance of ideas.

Watson (1908) in tests of the imitative ability of P. rhesus
saw relatively little evidence of other than extremely simple
forms of ideation. But in contrast with his results, those ob-
tained by Haggerty (1909), in a much more extended investiga-
tion in which several species of monkey were used, obtained
more numerous and convincing evidences of ideation in imita~-
tive behavior. Although this author wholly avoids the use of
psychological terms, seeking to limit himself to a strictly objec-
tive presentation of results, it is clear from an unpublished
manuscript (thesis for the Doctorate of Philosophy, deposited
in the Library of Harvard University) that he would attribute
to monkeys simple forms of ideational experience.

Witmer (1910) reports, in confirmation of Haggerty’s results,
intelligently imitative behavior in P. irus.

The work of Shepherd (1910) agrees closely, so far as evi-
dences of ideation are concerned, with that of Thorndike. He
obviously strives for conservatism in his statements concerning
the adaptive intelligence of his monkeys, all of which belonged
to the species P. rhesus. At one point he definitely states that
they exhibit ideas of a low order, or something which corres-
ponds to them. Satisfactory evidences of reasoning he failed
to obtain.

Franz’s (1907, 1911) studies of monkeys, unlike those men-
tioned above, have for their chief motive not the accurate de-
scription of various features of behavior but instead knowledge
of the functions of various portions of the brain. His results,
therefore, although extremely interesting and of obvious value
to the comparative psychologist, throw no special light upon
the problem of ideation.

The investigation by Hamilton (1911) of reactive tendencies
in P. rhesus and irus yielded preéminently important data con-
cerning complex behavior. For the ingenious quadruple-choice
method devised by this observer showed that mature monkeys
exhibit fairly adequate types of response. As Hamilton’s in-

MENTAL LIFE OF MONKEYS AND APES 127

_terest centered in behavior, he did not discuss ideation, but
‘this does not prevent the comparison of his data with those of
the present report, and the agreement of his findings with my
own is obvious.

My work contrasts sharply with that briefly mentioned above
in that I applied systematically and over a period of several
months an experimental method suited to reveal problem solv-
ing ability. Previously, the so-called problem or puzzle-box
method had been used as a means of testing for the presence
of ideas. For this I substituted the multiple-choice method.
One of the chief advantages of this new method is the possi-
bility of obtaining curves of learning for the solution or attempted
solution of relational problems of varying difficultness. I am
confident that these curves of learning will prove far more
valuable than such data as are yielded by the puzzle-box method.

The Pithecus monkeys, which I studied intensively, ‘yielded
relatively abundant evidences of ideation, but with Thorndike
I must agree that of ‘free ideas’ there is scanty evidence; or
rather, I should prefer to say, that although ideas seem to be
in play frequently, they are rather concrete and definitely at-
tached than ‘‘free.”” Neither in my sustained multiple-choice
experiments nor from my supplementary tests did I obtain
convincing indications of reasoning. What Hobhouse has called.

“articulate ideas, I believe to appear infrequently in these ani-
mals. But on the whole, I believe that the general conclusions
of previous experimental observers have done no injustice to
the ideational ability of monkeys. It is clearly important, how-
ever, that we always should take into account the species of
animal observed, for unquestionably there are extreme differ-

ences in mental development among the monkeys.

As I view my results in the light of their relations to earlier |
work, I am strongly impressed with the importance of the use |
of improved methods for the study of complex behavior. The
delayed reaction method of Hunter, the quadruple-choice method
of Hamilton, and my multiple-choice method offer new and
promising approaches to forms of activity which thus far have
been only superficially observed.

The ability exhibited by Skirrl to try a method out and then
to abandon it suddenly is characteristic of animals high in
intelligence. Most of the problems which I presented to my

128 ROBERT M. YERKES

animals would be rated as difficult by psychologists, for as a
rule they involved definite relations and demanded on the part
of the subject both perception of a particular relation and the
ability to remember or re-present it on occasion.

I was greatly surprised by the slow progress of the monkeys
toward the solution of these problems. It had been my suppo-
sition that they would solve them more quickly than any lower
type of mammal, but as a matter of fact they succeeded less
well than did pigs. Their behavior throughout the work proved
that of far greater significance for the experimenter than the
solution of a problem is definite knowledge of the modes of
behavior exhibited from moment to moment, or day to day.
This is true especially of those incidental or accidental modes
of response which so frequently appeared in connection with
my work that I came to look upon them, the surprises of each
day, as my chief means of insight.

Evidences of Ideation in Apes

Reliable literature of any sort concerning the behavior and
mental life of the anthropoid apes is difficult to find, and still
more rare are reports concerning experimental studies of these
-animals. There are, it is true, a few articles descriptive of tests
of mental ability, but even these are scarcely deserving of being
classed as satisfactory experimental studies of the psychology
of the ape. I have the satisfaction of being able to present in
the present report the first systematic experimental study of
any feature of the behavior of an anthropoid ape.

Among the most interesting and valuable of the descriptions
which may be classed among accounts of tests of mental ability
is Hobhouse’s (1915) study of the chimpanzee. The subject was
an untrained animal, so far as stated, of somewhat unsatisfac-
tory condition because of timidity. Nevertheless, Hobhouse was
able to obtain from him numerous and interesting responses to
novel situations, some of which may be safely accepted as evi-
dences of ideation of a fairly high order.

Similar in method and result to the work of Hobhouse is
that of Haggerty (unpublished thesis for the Doctorate of Philos-
ophy, deposited in the Library of Harvard University). Hag-
gerty’s tests of the ability of young orang utans and chim-

MENTAL LIFE OF MONKEYS AND APES 129

panzees to solve simple problems and to use tools in various
ways yielded results which contrast most strikingly with those
obtained in his experimental study of the imitative tendency
in monkeys. His observations, had he committed himself to
anything approaching interpretation, doubtless would have led
him to conclusions concerning the ideational life of these animals
very similar to those of Hobhouse.

Koehler, working in the Canary Islands, has, according to
information which I have received from him by letter, made
certain experiments with orang utans and chimpanzees similar
to those of Hobhouse and Haggerty. His results I am unable
to report as I have scanty information concerning them. They
are, presumably, as yet unpublished.

In his laboratory at Montecito, California, Hamilton has from
time to time kept anthropoid apes, but without special effort
to investigate their ideational behavior. He has most inter-
esting and valuable data concerning certain habits and instincts,
all as yet unpublished.

To a congress of psychologists Pfungst (1912) briefly reported
on work with anthropoid apes in certain of the German zodlo-
gical gardens. His preliminary paper does not enable one to
make definite statements concerning either his methods or such
results as he may have obtained concerning ideational behavior.
So far as I know, he has not as yet published further concerning
his investigation.

Mobius (1867) has described interesting observations concern-
ing the mental life of the chimpanzee. But this, like all of
the work previously mentioned, is rather in the nature of casual
testing than thoroughgoing, systematic, and analytic study.

In addition to the above reports, there are a few concerning
the behavior of apes which have been especially trained for
purposes of exhibition. Most interesting of these is that of
Witmer (1909), who studied in exhibitions and in his own labo-
ratory the behavior of the chimpanzee Peter. The varied forms
of intelligently adaptive behavior exhibited by this ape con-
vinced Witmer of ideational experience and even of an approach
to reasoning. In his brief report he expresses especial interest
in the possibility of educating this ‘‘genius among apes” to the

use of language.
A chimpanzee named Consul was observed several years ago

130 ROBERT M. YERKES

by Hirschlaff (1905), and his tricks were interestingly described
from the pedagogical standpoint.

Similar in character is Shepherd’s (1915) brief description of
the stage behavior of Peter and Consul, both chimpanzees. It
is impossible to determine from the account whether these
animals are the same as were observed by both Witmer and
Hirschlaff. As no reference is made in Shepherd’s paper
to other descriptions of the behavior of these animals and as
he adds nothing to what had already been presented, the reader
obtains no additional light on ideation.

I have mentioned only samples of the articles on trained
anthropoids. All are necessarily descriptions of the behavior
of individuals who had been trained. not for psychological pur-
poses but for the vaudeville stage, and although such observa-
tions unquestionably have certain value for comparative psy-
chology, it is well known that unless an observer knows the
history of an act, he is not able to evaluate it in terms of intel-
ligence and is especially prone to overestimate its value as
evidence of ideation.

There remain studies of the apes, dealing primarily with
behavior and mental characteristics, which are slightly if at all
experimental and deserve to be ranked as naturalistic accounts.
Such is, for example, the book of Sokolowski (1908), in which
attention is given to the characteristics of young as well as
fairly mature specimens of the gorilla, chimpanzee and orang utan.

The various publications of Garner (1892, 1896, 1900) deal _
especially with the language habits of monkeys and apes, but
observations bearing on ideation are reported.

Wallace (1869) describes certain features of the behavior of
an infant orang utan whose mother he shot in Borneo. He
also reports observations concerning the behavior of adult orang
utans, many specimens of which were shot by him during his
travels.

Early in the last: century, Cuvier (1810) interested himself
in studies of the intellectual characteristics of the orang utan,
and his data, taken with those of Wallace, Sokolowski, and
others similarly interested in the natural history of mind, give
one a valuable glimpse of the life of the anthropoid ape.

Finally, the data brought together by Brehm (1864, 1875,
1888) in his famous Tierleben; by Darwin (1859, 1871) in ‘‘ The

MENTAL LIFE OF MONKEYS AND APES 131

Origin of Species,’’ and other works, by Romanes (1900), especi-
ally in his books on mental evolution, by C. Llo¥d Morgan
(1906) in his several works on comparative psychology, and by
Holmes (1911) in his discussion of the evolution of. intelligence,
contribute not unimportantly to our all too meagre knowledge
of the mental life of the anthropoid apes.

My own results, viewed in the light of what one may learn
from the literature, stand out as unique because of the method
of research. Never before, so far as I have been able to learn,
Has any ape been subjected to observation under systematically
controlled conditions for so long a period as six months. More-
over, my multiple-choice method has the merit of having yielded
the first curve of learning for an anthropoid ape. This fact
is especially interesting when one considers the nature of the
particular curve. For so far as one may say by comparing it
with the curves for various learning processes exhibited by other
mammals, it is indicative of ideation of a high order, and possibly
of reasoning. I do not wish to exaggerate the importance of
my results, for as contrasted with what might be obtained by
further study, and with what must be obtained if we are ade-
quately to describe the mind of the orang utan, they are
meager indeed.

Especially noteworthy, as evidences of ideation, in the results
yielded by the multiple-choice method are (1) the use by the
orang utan of several different methods in connection with each
problem; (2) the suddenness of transition from method to method,
(3) the final and perfect solutien-of problem T without diminu-
tion of the initial errors; (4) the dissociation of the act of turn-
ing in a circle from that of standing in front of a particular box.
_ To ‘these features of behavior others of minor importance
might be added. But as they have been sufficiently empha-
sized in the foregoing detailed descriptions, I need only repeat
my conclusion, from the summation of evidence, that this young
orang utan exhibited numerous free ideas and simple thought
processes in connection with the multiple-choice experiment.
His ultimate failure to solve the second problem is peculiarly
interesting, although in the light of other features of his beha-
vior by no means indicative of inferior intelligence.

The various supplementary experimental tests which I em-
ployed are in no wise importantly distinguished from those

132 ROBERT M. YERKES

used by qther observers. The box stacking experiment has,
according to my private information, been used by Koehler.
It is obviously important that such tests be applied in the same
manner to individuals not only of the different genera of anthro-
poid apes, but of different ages, sex, and condition of training.

The box stacking experiment, although it yielded complete
success only as a result of suggestion on my part, proved far
more interesting during its progress than any other portion of
my work. In connection with it, the orang utan exhibited
surprisingly diverse and numerous efforts to meet the demands
of the situation. It is fair to characterize him as inventive,
for of the several possible ways of obtaining the banana which
were evident to the experimenter, the ape voluntarily used all
but two or three, and one of these he subsequently used on
the basis of imitation.

-Had Julius been physically and mentally mature, my results
would undoubtedly have been much more impressively indica-
tive of ideas, but even as matters stand, the survey of my ex-
perimental records and supplementary notes force me to con-
clude that as contrasted with the monkeys and other mammals,
the orang utan is capable of expressing free ideas in considerable
number and of using them in ways highly indicative of thought
processes, possibly even of the rational order. But contrasted
with that of man the ideational life of the orang utan seems
poverty stricken. Certainly in this respect Julius was not above
the level of the normal three-year-old child.

In common with other observers, 1 have had the experience
of being profoundly impressed by the versatility of the ape,
and however much I might desire to disprove the presence of
free ideas and simple reasoning processes in the orang utan,
I should feel bound to accept many of the results of my tests
as evidences of such experience.

I have attempted to indicate briefly the historical setting of
my investigation. I propose, now, in the concluding section,
to look forward from this initial research and to indicate as
well as I may in a few words the possibilities of results im-
portant for mankind from the thorough study of the monkeys
and anthropoid apes.

VIL

PROVISION FOR THE STUDY OF THE PRIMATES, AND ESPECIALLY
THE MONKEYS AND ANTHROPOID APES!

I should neglect an important duty as well as waste an oppor-
tunity if in this report I did not call attention to the status of
our knowledge concerning the monkeys and apes and present
the urgent need of adequate provision for the comparative
study of all of the primates.

Although for centuries students of nature have been keenly
interested in the various primates, the information which has
been accumulated is fragmentary and wholly inadequate for
generally recognized scientific and practical needs. There is a
voluminous literature on many aspects of the organization and
lives of the monkeys and apes, but when one searches in it for
reasonably connected and complete descriptions of the organisms
from any biological angle, one is certain to meet disappointment’

Concerning their external characteristics we know much; and
our classifications, if not satisfactory to all, are at least emi-
nently useful. But when one turns to the morphological sciences
of anatomy, histology, embryology, and pathology, one dis-
covers great gaps, where knowledge might reasonably be ex-
pected. Even gross anatomy has much to gain from the care-
ful, systematic examination of these organisms. With still
greater force this statement applies to the studies of finer struc-
tural relations. Little is known concerning the embryological
development and life history of certain of the primates, and
almost nothing concerning their pathological anatomy.

Clearly less satisfactory than our knowledge of structure is
the status of information concerning those functional processes
which are the special concern of physiology and pathology.
Certain important experimental studies have been made on the
nervous system, but rarely indeed have physiologists dealt sys-
tematically with the functions of other systems of organs. There

1 Much of the material of this section was published originally in Science (Yerkes,
1916).

134 ROBERT M. YERKES

SUB-DIVISIONS OF THE ORDER PRIMATES

Order Sub-orders Families
a. PRosIMII (Lemurs and Aye-Ayes)

i. Hapalidae (Marmosets)

PRIMATES. .. ii. Cebidae (Howling Monkeys, Tee Tees,
Squirrel Monkeys, Spider Monkeys,
and Capuchin Monkeys)

b. ANTHROPOIDEA..... iii. Cercopithecidae (Baboons and Macaques)

iv. Simiidae (Gibbons, Orangs, Chimpanzees,
and Gorillas)

| v. Hominidae (Man)

are almost no satisfactory physiological descriptions of the
monkeys, anthropoid apes, or lower primates.

When we turn to the science of genetics we meet a similar
condition, for the literature reveals only scattered bits of infor-
mation concerning heredity in the primates. No important
experimental studies along genetic lines have been made with
them, and such general observations from nature as are on
record are of extremely uncertain value. Were one to insist
that we know nothing certainly concerning the relation of her-
edity in other primates than man, the statement could not well
be disputed.

Occasionally in recent years students of human diseases have
employed monkeys or apes for experimental tests, but aside
from the isolated results thus obtained, extremely little is known
concerning the diseases peculiar to the various types of infra-
human primates or the significant relations of their diseases to
those of man.

Next in order of extent to our morphological knowledge of
these organisms is that of their behavior, mental life, and social
relations. But certainly no one who is conversant with the
behavioristic, psychological and sociological literature could do
otherwise than emphasize its incompleteness and inadequacy.
For our knowledge of behavior has come mostly from natural-
istic observation, scarcely at all from experimentation; our
knowledge of social relations is obviously meager and of un-
certain value; and finally, our knowledge of mind is barely
more than a collection of carelessly drawn inferences.

MENTAL LIFE OF MONKEYS AND APES 135

This picture of the status of scientific work on the primates,
although not overdrawn, will doubtless surprise many readers,
and even the biologist may find himself wondering why we are
so ignorant concerning the lives of the organisms most nearly
akin to us, and naturally of deepest interest to us. The reasons
are not far to seek. Most scientific investigators are forced by
circumstances to work with organisms which are readily ob-
tained and easily kept. The primates have neither of these
advantages, for many, if not most of them, are expensive to
get and either difficult or expensive to keep in good condition.
Clearly, then, our ignorance is due not to lack of appreciation
of the scientific value of primate research but instead to its
difficultness and costliness. =

Strangely enough, the practical importance of knowledge of
the primates has seldom been dwelt upon even by those biolo-
gists who are especially interested in it. It is, therefore, appro-
priate to emphasize the strictly human value of the work for
which I am seeking provision.

During the past few years it has been abundantly and con-
vincingly demonstrated that knowledge of other organisms may
aid directly in the solution of many of the problems of experi-
mental medicine, of physiology, genetics, psychology, sociology,
and economics. In the light of these results, it is obviously
desirable that all studies of infrahuman organisms, but especi-
ally those of the various primates, should be made to contri-
bute to the ‘solution of our human problems.

To me it seems that thoroughgoing knowledge of the lives
of the infrahuman primates would inevitably make for human
betterment. Through the science of genetics, as advanced by
experimental studies of the monkeys and anthropoid apes, prac-
tical eugenic procedures should be more safely based and our
ability to predict organic phenomena greatly increased. Sim-
ilarly, intensive knowledge of the diseases of the other primates
in their relations to human diseases should contribute import-
antly to human welfare. And finally, our careful studies of
the fundamental instincts, forms of habit formation, and social
relations in the monkeys and apes should lead to radical im-
provements in our educational methods as well as in other
forms of social service.

Along theoretical lines, no less than practical, systematic

136 ROBERT M. YERKES

research with the primates should rapidly justify itself, for upon
its results must rest the most significant historical or genetic
biological descriptions. It is beyond doubt that genetic psy-
chology can best be advanced to-day by such work, and what
is obviously true of this science is not less true of all the biolo-
gical sciences which take account of the developmental or genetic
relations of their events.

In view of the probable values of increasingly complete ac-
counts of primate life, it seems far from extravagant to insist
that the securing of adequate provision for systematic and long
continued research is the most important task for our genera-
tion of biologists and the one which we shall be least excusable
for neglecting. Indeed, when one stops to reflect concerning
the situation, it seems almost incredible that the task has not
been accomplished.

Some ten years ago Professor John B. Watson (1906) entered
a plea for the founding of a station for the experimental study
of behavior. He made no special mention of work with the
monkeys and apes, but it is clear from the problems which
he enumerates that he would consider them most important
subjects for observation. Professor Watson’s plea has appar-
ently been forgotten by American biologists, and it seems not
inappropriate to revive it at this time. For surely we have
advanced sufficiently along material and scientific lines during
the last ten years to render possible the realization of his hope.

To my knowledge, only one definite attempt has thus far
been made to gain special provision for the study of the pri-
mates. Somewhere about the year 1912 there was established
on Tenerife, one of the Canary Islands, a modest station for
the study of the anthropoid apes. I have already referred to
it briefly on page 1. The plan and purpose of this station,
which is of German origin, have been presented briefly by Roth-
mann (1912). From personal communications I know that a
single investigator has been in residence at the station since
its founding and that psychological and physiological results of
value have been obtained, but no published reports have come
to my attention.

When I first heard of the existence of the German anthropoid
station I naturally thought of the possibility of coéperative
work, but the events of the past two years have rendered the

MENTAL LIFE OF MONKEYS AND APES 137

chances of coédperation so remote that it now seems wholly
desirable and indeed imperative to seek the establishment of
.an American station, which, unlike the German station, shall
provide adequately not only for the study of the anthropoid
apes but for that of all of the lower primates. It should be the
function of such a station or research institute (1) to bring
together and correlate all the information at present available;
(2) to fill in existing gaps observationally and thus complete
and perfect our knowledge of these organisms; (3) to seek to
bring all available information to bear upon the problems of
human life.

Hitherto the unsatisfactoriness of progress has been due to
the lack of a definite plan and program. Every investigator
has gone his own way, doing what little his personal means and
opportunity rendered possible. The time has at last come when
concerted action seems feasible as well as eminently desirable.
I am therefore offering a plan and program which, if wisely
developed, should lead ultimately to fairly complete and prac-
tically invaluable knowledge of the lives of all of the primates.

There should be provided in a suitable locality a station or
research institute which should offer adequate facilities (1) for
the maintenance of various types of primate in normal, healthy
condition; (2) for the successful breeding and rearing of the
animals, generation after generation; (3) for systematic and
continuous observation under reasonably natural conditions;
(4) for experimental investigations -from every significant bio-
logical point of view; (5) for profitable coéperation with existing
biological institutes or departments of research throughout the
world.

The station should be located in a region whose climate is
highly favorable to the life of many of the lower primates as
well as to that of man. Such a location is by no means easy
to find. Because of my intense interest in the subject, I have,
during the past five years, prospected in various parts of the
world for a satisfactory site. I shall now attempt to indicate
the chief requirements and also the foremost advantages and
disadvantages of several regions which have been considered.

It is first of all requisite that the climate be such as to agree
with the organisms to be studied and such, also, as to render
their breeding normal and dependable. Second in importance

138 ROBERT M. YERKES

is its satisfactoriness for the life and scientific productiveness
of the observer. While certain tropical localities would meet
the first requirement perfectly, they would prove extremely un-
satisfactory for research activity. It therefore seems essential
to find a region whose climate shall reasonably meet the needs
of the experimenter while adequately meeting those of the
animals to be studied.

A further factor which has important bearing upon the pro-
ductiveness of the observer is the degree of isolation from civili-
zation and from other scientific work. No scientist can long
work effectively, even in a reasonably healthy and stimulating
climate, if entirely cut off from similar interests and activities.
It is therefore desirable, if at all possible, to discover a location
in the midst of civilization and with reasonably good oppor-
tunities for scientific associations.

With these several desiderata before us, I shall call attention
to a number of possible sites for a station, several of which I
have visited. Southern California, and especially the portion
of the State between Santa Barbara and San Diego, promises
fairly well. It is definitely known that certain, if not all, spe-
cies of monkey will breed there fairly satisfactorily, and al-
though it has not yet been demonstrated, there is no reason
to suppose that in certain regions the anthropoid apes might
not also be kept in perfect health and successfully bred. The
main advantages of this general region are (a) a climate which
promises to be reasonably satisfactory for many if not all of
the primates; (b) admirable climatic conditions for investiga-
tors; (c) wholly satisfactory scientific and cultural environment
for the staff of a station. The most significant disadvantages
are (a) a temperature, which is at times a trifle too low for the
comfort of certain of the monkeys and apes. It is by no means
certain, however, that they would not usually adapt themselves
to it. (b) The necessity of importing all of the animals and of
having to rely upon successful acclimatization. Of course it
is to be assumed that importation would be necessary only at
the outset of such work, since the animals later should replenish
themselves within the confines of the station.

Florida offers possibilities somewhat similar to those of south-
ern California, but as I have not had opportunity to examine
the conditions myself, I can say only that in view of such infor-

-

MENTAL LIFE OF MONKEYS AND APES 139

mation as is available the advantage seems to be greatly in
favor of the latter.

Cuba, Jamaica, Porto Rico, and for that matter, several of
the West Indies, offer possible sites for a successful station. I
have reasonably intimate personal knowledge only of the con-
ditions in Jamaica. The major advantages in the West Indies
are (a) suitable climatic conditions and food supply for the
animals; and (b) reasonably satisfactory climatic conditions for
the staff. These are, however, more than counterbalanced in
my opinion by the following serious disadvantages: (a) the
relative isolation of the investigators from their fellow scien-
tists; (b) the necessity of importing all of the animals originally
used; (c) the risk of destruction of the station by storms.

It is definitely known that anthropoid apes as well as mon-
keys can be successfully kept, bred, and reared in the West
Indies. During: the past year, on the estate of Dofia Rosalia
Abreu, near Havana, Cuba, a chimpanzee was born in cap-
tivity. A valuable account of this important event and of the
young ape has been published by Doctor Louis Montané (1915).
It therefore seems practically certain that regions could be found
readily on Jamaica, Porto Rico, or smaller islands, which would
be eminently satisfactory for the breeding of apes.

There are obvious reasons why an American station for the
study of the primates should be located on territory controlled
by the United States Government, and if a tropical location
proves necessary, it would probably be difficult to find more
satisfactory regions, aside from the inconveniences and risk of
importation and the relative isolation of the investigators, than
_are available on Porto Rico.

I have not seriously considered the possibility of locating an
American station on the continent of Africa, for although two
of the most interesting and important of the anthropoid apes,
the gorilla and the chimpanzee, are African forms, while many
species of monkey are either found there or could readily be
imported, it has seemed to me that the islands of the West and
East Indies and the portions of the United States referred to
above are much to be preferred over anything available in
Africa.

In the East, Borneo, the Philippine Islands, and Hawaii are
well worth considering. Borneo is the home of the gibbon and

140 : ROBERT M. YERKES

of at least one species of orang utan, and in addition to these
important assets, it presents the advantages of (a) a wholly
suitable climate and food supply for monkeys and apes; and
(b) climatic conditions for investigators which, I am informed ,
by scientific friends, are nearly ideal. For investigators the
most serious disadvantage here, as in all other parts of the East,
would be the isolation from other scientific work and workers.

The possibilities of Central America I considered several years
ago when it seemed to me possible that work might profitably
be done with monkeys and apes on the Canal Zone. The advan-
tages are (a) a climate which promises fairly well for the animals;
and (b) reasonable accessibility from the United States. The
disadvantages are (a) a far from ideal climate for long con-
tinued scientific work; and (b) an environment which from the
cultural and scientific point of view leaves much to be desired.

Were a permanent psycho-biological station for the study of
the primates to be established in southern California, it would,
even though wholly satisfactory conditions for the breeding,
rearing, and studying of the animals were maintained, furnish
more or less inadequate opportunity for the observation of the
animals under free, natural conditions. It would therefore be
necessary.to supplement the work of such a station’ by field
work in Borneo, Sumatra, Africa, India, South America, and
such other regions as the species of organism under considera-
tion happen to inhabit.

Considering equally the needs of the experimenter and the
demands of the animals, it seems to me reasonable to conclude
that southern California should be definitely proved unsuitable
before a more distant site were selected. For the information
which I have been able to accumulate convinces me that it
would in all probability be possible successfully to breed and
keep the primates there, and it is perfectly clear that in such
event the output of a station would be enormously greater
because of the more favorable conditions for research than in
any tropical region or in a more isolated location.

Assuming that satisfactory provision in the shape of a scien-
tific establishment for the study of the primates in their rela-
tions to man were available, the following program might be
followed: (1) Systematic and continuous studies of important
forms of individual behavior, of social relations, and of mind;

MENTAL LIFE OF MONKEYS AND APES 141

(2) experimental studies of physiological processes, normal and
pathological, and especially of the diseases of the lower pri-
mates, in their relations to those of man; (3) studies of heredity,
embryology, and life history; (4) research in comparative anat-
omy, including gross anatomy, histology, neurology, and path-
ological anatomy.

Each of these several kinds of research should be in progress
almost continuously in order that no materials or opportuni-
ties for observation be needlessly wasted. Because of the nature
of the work, it would be necessary to provide, first of all, for
those functional studies which demand healthy and normally
active organisms, whose life history is intimately and com-
pletely.known. This is true of all studies in behavior; whether
physiological, psychological, or sociological. Simultaneously with
behavioristic observations and often upon the same individuals,
genetic experiments might be conducted. This would be ex-
tremely desirable because of the relatively long periods between
generations. After the usefulness of an animal in behavioristic
or genetic inquiries had been exhausted, it might be made to
render still further service to science in various experimental
physiological, or medical inquiries. And finally, the same indi-
vidual might ultimately be used for various forms of anatomical
research. Thus, it is clear that the scientific usefulness of a
lemur, a monkey, or an ape might be maintained at a high level
throughout and even beyond the period of its life history.

The program thus briefly sketched would provide either
directly or indirectly for work on every aspect of primate life.
Especially important would be the intimacy of interest and
coéperation among investigators, for the comparative method
should be applied consistently and to the limit of its value.
The results of various kinds of observation should be correlated
so that there should ultimately emerge a unitary and prac-
tically valuable account of primate life, to replace the patch-
work of information which we now possess.

Because of the costliness of maintaining and breeding the
monkeys and apes, it is especially desirable that the several
kinds of research mentioned above should be conducted. In-
deed, it would seem inexcusably wasteful to attempt to main-
tain a primate or anthropoid station for psychological observa-
tions alone, or for any other narrowly limited biological inquiry.

142 °¢ ROBERT M. YERKES

Furthermore, the station should be permanent, since for many
kinds of work it would be essential to have intimate knowledge
of the life history and descent of an individual. With the
lower primates, a generation might be obtained in from two to
five years; with the higher, not more frequently, probably, than
from ten to fifteen years. It therefore seems not improbable
that the value of the work done in such a station would con-
tinue to increase for many years and would not reach its max-
imum short of fifty or even one hundred years.

A staff of several highly trained and experienced biologists
would be needed. The following organization is suggested as
desirable, although, as indicated below, not necessarily essential
in the beginning: (1) An expert especially interested in the
problems of behavior, psychology, and sociology, with keen Aap-
preciation of practical as well as of theoretical problems; (2) an
assistant trained especially in comparative physiology; (3) an
expert in genetics and experimental zodlogy; (4) an assistant
with training and interests in comparative anatomy, histology,
and embryology; (5) an expert in experimental medicine, who
could conduct and direct studies of the diseases of man as well
as of the lower primates and of measures for their control; (6) an
assistant trained especially in pathology and neurology.

To this scientific staff of six highly trained individuals there
should be added a business manager, a clerical force of three
individuals, a skilled mechanician, a carpenter, and at least
four laborers.

The annual expenditures of an institute with such a working
staff, would in southern California, approximate fifty thousand
dollars. It would therefore be necessary that it have an en-
dowment of approximately one million dollars.

In the absence of this foundation it would, of course, be pos-
sible to make a reasonably satisfactory beginning on the work
which has been outlined in the following less expensive manner.
A working plant might be established, on ground rented or
purchased at a low figure, for about ten thousand dollars; the
salary of a director, assistants, a clerical helper, and combined
mechanic and laborer might be estimated at the same figure;
the cost of animals and of maintenance of the plant would
approximate five thousand dollars. Thus, we should obtain as
an estimate of the expenditures for the first year twenty-five

VIII

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1871. The descent of man, and selection in relation to sex.
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Hrpvicka, A. Anatomical observations on a collection of orang skulls from west-
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MENTAL LIFE OF MONKEYS AND APES 143

thousand dollars. Without expansion, the work might be con-
ducted during the second year for fifteen thousand dollars,
and subsequently it might be curtailed or expanded, resources
permitting, according as results achieved and in prospect justi-
fied.

An institute established on such a modest basis as this still
might render largely important scientific service through its
own research and through .organized coéperation with other
existing research establishments. Thus, for example, suppos-
ing that behavioristic, psychological, sociological, and genetic
inquiries were conducted in the institute itself, animals might
be supplied on a mutually satisfactory basis to institutes for
experimental medicine, for physiological research, and for anat-
omical studies. Under such conditions, it is conceivable that
extremely economical and good use might be‘ made of all the
available primate materials. But it is not improbable that even
coéperative research would prove on the whole more profitable,
except possibly in the case of morphological work, if investiga-
tors could conduct their studies in the institute itself rather
than in distant laboratories. In any event, the idea of coépera-
tion should be prominent in connection with the organization
of a research station for the study of the primates. For thus,
evidently, scientific achievement in connection with these im-
portant types of animal might be vastly increased over what
would be possible in a single relatively small institution with
a limited and necessarily specialized staff of workers.

Despite the fact that biologists generally recognize the import-
ance of the work under consideration and are eager to have it
done, it is perfectly certain that we shall accomplish nothing un-
less we devote ourselves confidently, determinedly and unitedly,
with faith, vision, and enthusiasm, to the realization of a definite
plan. Our vision is clear,—if we are to gather and place at the
service of mankind adequate comparative knowledge of the life
of the primates and if we are to make this possible harvest of
scientific results: count for human betterment, we must bend
all our efforts to the establishment of a station or institute for
research.

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185-225. » 5,

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