644 BRAIN MECHANISMS AND LEARNING 



Thus, for instance, in the rook at the hatching stage any external stinuihis 

 strengthens and speeds up the act of pecking. Whilst in the young bird which has 

 already freed itself from the egg the same stimulus initiates the feeding reaction bv 

 the opening of the beak. 



In this way the heredity-fixing, heterochrony growth of the nervous structures 

 and the oecological factors of a given animal determine the perfection and accuracy 

 ot its inborn behaviour. 



Thorpe. Obviously an animal is continually dependent on a groat many things 

 in the environment (the oxygen supply, for instance), and if this is changed the 

 whole behaviour pattern may go wrong. But I think we can rule out that kind of 

 environmental factor as an adequate 'explanation' for the complexity of instinctive 

 behaviour. I do think, however, that you have touched upon an important point 

 in distinguishing behaviour arising from stimuli from within the nervous system, 

 without being dependent on interoceptors, from behaviour which is dependent on 

 interoceptors. Take behaviour which is emitted at regular intervals (a 24-hour 

 rhythm, for instance). It seems to have been shown that such rhythms can arise in 

 a single cell. A probable instance is provided by the cardiac ganglion of the lobster. 

 This seems to be an important example of behaviour which is dependent neither 

 upon interoceptors nor exteroceptors. 



On the other hand, a great many instinctive behaviour patterns are undoubtedly 

 dependent on interoceptors. I think ethologists were to blame at one stage 

 for using the word 'endogenous' very loosely, sometimes meaning 'within the 

 animal' and sometimes 'within the CNS', and that gave rise to much misunder- 

 standing. 



A further point: I think the distinction between instinct and reflex referred to is 

 an important one. I think it is perfectly possible to define reflexes in such a way as 

 to include all behaviour. But it we do that we have abandoned a very useful term 

 which is most valuable for denoting the 'partial reflexes' of physiology. If we 

 expand the term 'reflex' to include everything then we tend to speak so loosely 

 that we get involved in much sterile argument about definition. 



Fessard. I think so too, in view of the importance we should give to the notion 

 of 'trigger'. If the trigger is under close control of the environmental factors so 

 that yoti can make a correspondence between the responses and the variations in 

 the environment, then you can speak of a reHex or of a succession of reflexes. But if 

 the trigger is not under external control and you cannot therefore anticipate what 

 the reaction will be like then we may consider the reaction to be instinctive. 



Gerard. X incorporates into itself that part of the experience of the organism 

 which has been fixed, and therefore the reaction of the system is different at each 

 time from what it was before. To some extent, then, any line of division is mean- 

 ingless: we have a spectrum, not a series of black and white divisions. We would 

 all agree that the inborn, congenital, and reflex mechanism, and I would think a 

 good many of the others also, are set by the original growth — of something — 

 and learning leaves behind a change in the permanent structural substrate, which 



