./ 



228 THEORETICAL BIOLOGY 



impulse, is called a reflex, whether it be perceived from the 

 standpoint of the ego or from that of an outside observer. In 

 both cases the reflex is regarded as a mechanical process that 

 owes its existence to a prepared framework. The obvious 

 consequence of this is that we try to refer to impulses every 

 action that is not a reflex. 



I have already indicated the different types of action that 



we can distinguish in the life of animals. These are the 



N^ reflex, the plastic action, the instinctive action, the action 



based on experience and, finally, the controlled action — 



which last, however, plays a part only in the highest animals. 



The plastic action has been thoroughly analysed by 

 Pawlow. He succeeded in demonstrating that, in dogs, the 

 secretion of saliva and of gastric juice (which are considered 

 reflex actions because in human beings they proceed without 

 an impulse of the will) are nevertheless regulatable or pla,stic. 

 Dogs whose salivary secretion appeared only when they 

 smelt food, learnt to secrete it also in presence of optical, 

 acoustic or thermal indications, where such invariably pre- 

 ceded feeding. 



Accordingly it is possible in this case so to modify the 

 fixed framework, which releases the secretion as a reflex only 

 when the olfactory nerves are excited, that it becomes focussed 

 on new indications. In this way the indicator is furnished 

 with new indications, and this means a considerable invasion 

 of the dogs' world-as-sensed. 



We must assume that in the mark-organ a change takes 

 place which converts the old reflex into a new one. This is 

 called plastic faculty. In the instance quoted, it remains a 

 reflex, which presupposes a prepared framework. But the 

 framework itself can be altered by experiment. The altera- 

 tion in the framework cannot be a reflex process, and accord- 

 ingly requires impulses. 



Instinctive actions afford an analogous case. We are 



