15. (a) Milk bottle (6); (b) Variation A. Under Tree 1. (a) (1) Feb. 4, 12:00, 15"; (2) 5, 12:30, 1'5".

(b) (1) Feb. 20, 1:10; (2) 21, 1:20; (3) 22, 1:06; (4) 23, 1:10; (5) 25, 1:25; (6) 26, 1:28; (7) 28, 12:38; (8) March 1, 11:30; (9) 2, 12:40; (10) 3, 12:15; (11) 4, 12:55; (12) 5, 1:01; (13) 6, 12:15; (14) 7, 12:10; (15) 8, 12:30; (16) 9, 12:00.

16. Slot box (new); (a) with hand, (b) with stick. Under Tree 3.

(a) (1) Feb. 8, 9:40, 6′ 30′′; (2) 9, 10:04, 47′′; (3) 11, 10:05, 5′′; (4) 13, 10:58, 1′; (5) 23, 9:35, 3'′ 10′′; (6) 26, 10:47, 14'; (7) 27, 11:20, 4' 30".

(b) (1) Feb. 18, 9:42, 30'; (2) 20, 9:40, 30′; (3) 21, 10:31, 30.

17. Delayed response (new). Under Tree 3.

Table of trials in this experiment, with essential details, is given on page 485.

18. Box and pole to pipe and rod transfer (new). Near Tree 4. (1) Feb. 18, 9:06, 28′; (2) 20, 11:25, 25′; (3) 21, 10:23, i'′ 18′′; (4) 22, 10:04, 1' 3"; complication experiment, March 8, 9:15, 15′ (see p. 423).

19. Box stacking with four boxes (new). Under Tree 4.

(1) Feb. 21, 12:05, 30'; (2) 22, 10:14, 30'; (3) 23, 9:45, 30′; (4) 25, 9:43, 30′; (5) 26, 11:05, 30'; (6) 27, 10:47, 30'; (7) March 2, 10:23, 30'; (8) 4, 10:00, 30′; (9) 5, 9:51, 60'; (10) 6, 9:27, 15'; (11) 7, 8:37, 15'; (12) 8, 9:33, 3′ 21′′; (13) 9, 9:30, 28'.

20. Buried food (like 7, but for different purpose). Specified locations.

(1) March 1, 9:45, to March 2, 9:20, 4'; March 2, 9:40, to March 4, 9:55, 5'; March 4, 11:00 to March 5, 1:05, 10'; March 8, 10:00, to March 10, 9:45, 15'.

21. Mirror (18). East of Tree 3.

(1) March 4, 12:00, 10'; (2) 5, 10:55, 10'; (3) 6, 9:47, 10'; (4) 7, 8:55, 10'; (5) 8, 9:45, 10'.

22. Round-about course, with stick (new). In cage.

(1) March 9, 8:32, 30', (2) 11:10, 30′; (3) 10, 8:33, 30'. The reader who for any reason desires to consider the experimental program of a given day, the work which preceded or followed a given trial, the number of trials in a given situation, the duration of a trial, the total amount of time. given to a certain problem, or similar information which may

prove important in evaluating results or in studying the relations of performance in one problematic situation to that in another, will find the information contained in the "list of problems" indispensable. The data have been assembled and presented in tabular form for the sake of condensation, and in reading the description of an experiment it is desirable that one may refer to and review the appropriate data given in the list of problems.

In the account of observations which follows, space is economized as far as possible by reference to the original report for descriptions of apparatus and arrangement of experimental situation. In most instances certain modifications of the original situation were necessitated by the changes in Congo, but for several of the experiments it was possible to use the appliances which had been prepared the previous winter.

V

USE OF STICK, OR FUNCTIONAL SUBSTITUTE, AS IMPLEMENT

Whereas when I first began to work with Congo in the winter of 1926, she exhibited no aptitude for using sticks or functionally similar objects as implements, at the beginning of the second period of work she used various objects freely, eagerly, and with notably greater facility. At least three assemblages of factors suggest themselves as possible conditions of this behavioral change: (1) previous experience in experiments which demanded the use of sticks; (2) effects of association with man, and (3) psycho-physiological growth and development. Although it is impossible to segregate and evaluate the contributory factors or conditions, it is almost certain that these three are important. It seems logical to place experience in experiments first, but it is possible that development is of even greater importance. My observations indicate the lesser and subordinate significance of association with man and observation of his actions.

Problem 1. Shelf and stick. It was with great difficulty that Congo was at first induced to use a stick in securing food. Indeed, in the initial problematic situation, shelf and stick, which stands as problem 1 in the current list as well as in the previous one, she failed completely in 1926. In the repetition experiments this simple situation was presented first of all with reward thirty-six to thirty-eight inches from the feeding-door in end of cage. When on January 17, five minutes after a hard-wood stick twenty-four inches in length had been placed in her nest-room, the door giving access to the shelf had been opened and one-half of an apple thus exposed to view, Congo came, repeatedly reached for it, tried to squeeze through the doorway; then desisted, went to the grill and there sat for a few seconds. Soon she returned to the shelf with a bit of stick about six inches long which, while sitting at the

grill, she had picked up from the floor of the cage. With this she reached vainly for the apple. Finally, she threw it toward the food so that it was beyond her reach. Withdrawing from the doorway, she now walked first toward. the grill, then toward the nest-room. Near the porch of the latter lay the 24-inch stick where she had dropped it after playing with it in the room. Picking it up she walked quietly to the feeding-door and pushed it through the opening to use it on the shelf. Her first attempt was unsuccessful, but in her second, holding the stick with her right hand, she touched the apple by sweeping the stick toward her left and gradually pushed it along until it came into contact with the left side-rail of the shelf. Along this she carefully drew it until it was some twenty-eight inches from her, when with her right hand she reached it readily and carried it away to devour.

Although in this experiment she did not seem especially hungry, she nevertheless worked eagerly, and after obtaining the apple ate it. The total time of the trial was three minutes, but of this less than one minute was used in obtaining the reward after the stick had been carried to the shelf. The observation clearly demonstrates that ability to use a stick as implement, in the interim between the shelf and stick experiments of 1926 and the observation herein recorded, transferred from other problematic situations to this particular situation. Never previously had she succeeded in obtaining food from the shelf by use of a stick. She was given no aid whatever in this experiment, and her initiative and success surprised the observer, whose prediction it was that she would not use the stick as implement and under the conditions of experimentation could not succeed, even by brute strength, in obtaining the reward.

The shelf and stick experiment was repeated on January 18 with the difference that a large sweet potato was used as reward instead of piece of apple. Congo's behavior in this trial is, I think, clearly indicative of the developmental status of her ability to act ideationally. When the situation was

presented, she went to the feeding-door and reached for the potato with her left hand. Failing, she went to the grill, then to the nest-room, thence back to the shelf and reached with her right hand, then again with her left hand, with which she next pushed toward the potato a bit of straw which she had carried to the shelf. Two minutes after the beginning of the trial she walked from the feeding-door directly to the 24-inch stick, which lay on the floor of the cage, and taking up returned to the shelf where after a few seconds of awkward effort she succeeded in pushing it through the door and then, grasping it with her right hand, in sweeping the potato within reach. The total time for the second trial in the shelf and stick experiment was three minutes, but of this interval only a few seconds were actually required for success by use of the stick.

it

Image or other representation of the stick as an essential feature of the situation apparently was not at first in her mind. Indeed, she spent the first two minutes of the trial in the vain use of methods which she had persistently tried during the previous winter. To the observer it seemed as though she had failed in the experiment before she turned to the stick, but on the other hand it may be that she actually turned away from the door in search of the stick instead of merely going in its direction and happening to observe it. From the totality of my observations I am of the opinion that she is somewhere between the concrete stage of adaptive ability which would require presentation of the stick as condition of use and the definite representative stage in which it might be sought in response to mental content. Although her behavior in this experiment is by no means so strongly suggestive of the functioning of images or other representational processes as is that of the chimpanzee in comparable problematic situations, this may be largely because of the more leisurely way in which the gorilla proceeds and her apparent lack of intense interest. and eagerness.

As no obvious reason for additional repetitions of this ex