144 THEORETICAL BIOLOGY 



thereby formed, only through the influence of the water - 

 pressure on the lateral-Hne organs, and these indications 

 correspond to our quaUties of density. But it is quite un- 

 certain whether a fish brings to its mark-organ in any sort 

 of form the function-rule that governs its swimming 

 movement. 



This entire field awaits exploration. But it is extremely 

 important that the road to true knowledge should not be 

 blocked by arbitrary experiment and amateurish formulation 

 of the problem. 



Fortunately, Koehler has succeeded in showing that a 

 chimpanzee which sees a banana lying on the far side of the 

 bars of its cage and out of its reach, will use a stick lying at 

 hand to pull the banana nearer. If the stick is taken away, 

 the ape will use any movable object that it can push through 

 the bars, in order to secure the fruit. The form, colour and 

 consistency of the thing it uses is a matter of indifference ; 

 the only consideration is the possibility of its efficacy in doing 

 what is required of it. So here also the function forms the 

 object. Koehler speaks, in a descriptive way, of a " stick 

 metamorphosis " of a bundle of straw, of an old shoe and of 

 other things of the kind. 



Later on, the optical image also comes in to form imple- 

 ments, when the ape pulls the leaves off a twig in order to 

 make it look like a stick. 



It is very interesting to find that at first the goal must 

 be in view, if the stick is to be used. If the stick lies behind 

 the monkey, it will not be used in the first set of experiments. 

 As long as the monkey sees the banana, it does not see the 

 stick ; and conversely, if it is looking at the stick, the banana 

 disappears from his field of view, and with it goes the motive 

 for making a stick. Later on, the indication of the banana 

 outlasts the momentary impression, and the stick, wherever 

 it be lying, is picked up and used. 



