When one packing-case did not suffice as a foothold for reach ing a banana which was hanging particularly high up, the apes placed one or more cases on top of the first. They managed to erect tower-like constructions of four cases. It is true that the boxes were placed on top of one another in such an inexact manner that it was only the inborn gymnastic dexterity of the apes that prevented the dangerous construction from collapsing in use. The problem of fetching food which had been thrown down behind a trellis out of reach, by circumventing the trellis, was solved by the chimpanzees without more ado. Dogs often and fowls always fail at the same task because they are so attracted by the sight of the near-by food that they take no notice of the round-about way leading to it, although they may know this route already. On the other hand, it was extremely difficult for the chimpanzees to shove away a box which prevented them from seizing a fruit lying in front of the bars of their cage. It was al ways apparently considerably easier for them to make a detour with their own body, i.e., to run or climb by a roundabout way to the object of their desires, than to achieve their purpose by means of a tool. This was the case when a small-meshed wire net was placed before the fruit and the net had to be pushed aside with a stick. The most difficult case was when it was necessary to push the fruit away first of all to drag it in afterwards. A com plete solution of this problem was arrived at by the two most intelligent of the apes. For the different chimpanzees showed a definite gradation in their capabilities. Such individual differ ences in the ability to adapt themselves to a given situation are found everywhere in the animal kingdom. These differences are present, too, when pure but modifiable instinctive actions are con cerned. Probably the highest accomplishment attained in the
Teneriffe experiments was the case of an animal which itself manufactured a tool. Only one chimpanzee did this. The sticks at its disposal proved all to be too short to drag in a fruit lying in front of the netting of the cage. After many efforts, the animal eventually succeeded in sticking a narrow tube inside a wider one and so lengthening the stick. After this had succeeded once, the animal used the same method every time there was a neces sity for it. In doing this he always held the wider tube in the left hand and pushed the end of the narrower tube into it with the right. Eventually, he perfected it to an instrument composed of three tubes, the narrowest being in the middle between the two wider ones.
An important element in the manner in which the chimpanzees mastered all of these tasks was the fact that the solution fre quently occurred suddenly, the animal as it were giving itself a jolt. In this respect the manner of solution differs notably from the solutions discovered by other trained animals purely by chance. Lower apes have been presented with the same prob lems and it has been found that these animals too can work with intermediary tools and intermediary actions. But the action is always towards the animal. A preliminary movement of the tool away from the animal is never carried out. Whereas it occurs to the anthropoids to push a rake behind a banana to drag it towards them, or to move a bolt which closed a case in the direc tion away from themselves, the lower apes abandon such tasks. Thus the actions of the anthropoids are actually the highest at tained in the behaviour of animals. In this respect, their conduct is a real intermediary between animal and human behaviour.