132 Inside the Living Cell 



of the Eumenes is broken, the wasp may mend it, but it will not 

 place the egg inside. Its power of modifying the action to suit the 

 circumstances is very limited; but cases in which the action can be 

 varied have been noted. 



LEARNING TO ACT 



Although instinct is very remarkable, it obviously has great dis- 

 advantages as a basis of animal life. It does not, as a rule, offer the 

 possibility of learning from previous experiences and therefore it is 

 at a loss when presented with a new set of circumstances. 



The 'intelligent* animals have developed a very different method 

 of finding actions which are appropriate to the circumstances — the 

 method of learning by trial and error. One feature of these actions 

 is that they are first of all performed very badly. The movements 

 of a young mammal are at first disorganized and it only gradually 

 learns how to use its limbs effectively. It will try to repeat actions 

 which produce satisfying results, and after many repetitions they 

 become easy. This means that the nerve connections which enable 

 them to be carried out are not initially present. They become estab- 

 lished by being used, but when established the action can be repeated 

 with very little trouble. Thus when a young kitten has iearnt' to 

 run, all the muscle actions occur in the correct sequence, without 

 any attention being given to the details. Animals thus acquire a 

 whole repertory of muscle sequences which they are able to perform — 

 and accurately — in order to bring about desired actions. 



To be useful these actions have to be controlled by the information 

 which the sense impressions give the animal about his surroundings. 

 We have seen that from this information he constructs a 'picture' 

 of the surroundings — a picture which, as we have already seen, is 

 not just a momentary photograph but an interpretation based on 

 previous knowledge and experience. A way in which muscular 

 actions were related to the 'picture' which an animal or person makes 

 of his surroundings was suggested by Dr K. J. N. Craik, a Cambridge 

 psychologist, who died in 1945. He suggested how human beings 

 (and possibly animals) may make use of the 'model' or 'picture' of 

 their surroundings in planning actions. The suggestion was that the 

 brain keeps on trying out possible actions within the 'model' or 

 'picture' of the outside world, which it has constructed. It attempts 

 to forecast the results of possible actions within the model and when 

 one is found which appears to give a satisfactory result, the action 

 may follow. For example, a man using a cricket ball must predict 

 the result of his action. This prediction is made within the picture he 



