178 GENERAL BIOLOGY 



So, too, a puppy that has the vicious habit of snapping at pass- 

 ers-by, can be made to react differently by giving him a whipping sev- 

 eral times, immediately after he does the undesirable act. 



In both these cases memory enters, but only a simple sensory mem- 

 ory (association memory) which has little to do -with any thought The 

 impulse (inner stimulus) in the puppy to snap, is great, and so the 

 "snapping nerve-arc" carries the impulse and the snapping is done; but 

 the punishment which has been meted out has set up an impulse of an 

 opposing nature, and as soon as this latter becomes the stronger im- 

 pulse, the puppy has been trained. 



We may say in this case, that the puppy has the desire to snap, and 

 the nerve-arc which carries this snapping-impulse begins to function ; 

 but the whipping has caused a new nerve-arc to function at the same 

 moment, so that a third nerve-arc, that of inhibition, comes into play 

 and the animal does nothing. This is quite similar to the reaction of 

 persons in hypnosis. Here an individual is told he cannot bend his arm. 

 The impulse not to bend the arm is just as strong as the one to bend it, 

 and so no movement takes place. 



An impulse is denned as an inner stimulus. 



It is well to bear in mind the foregoing paragraphs as these show 

 the possibility of two opposing impulses and even two opposing reac- 

 tions taking place at one and the same time over different nerve-arcs. 

 Often we read of lower organisms possessing discriminating powers 

 of various kinds which can be interpreted in quite different ways from 

 what the writer of such an account would have us believe. We need 

 only remember that there can easily be one set of nerve-arcs function- 

 ing for the acceptance of food and another set for the rejection of it, so 

 that it depends on which set carries the stronger impulse as to whether 

 the animal accepts or rejects the food. It is by no means necessary to 

 assume any discriminating ability. 



There are also Complex Reflex Actions making use of several nerve- 

 arcs, sometimes often forming regular chains of reactions. In these 

 cases, the result of one stimulus sets up another, and so on. In fact 

 we call such continued setting up of stimuli reflex chains. As an exam- 

 ple we may refer back to the frog whose cerebral hemispheres have been 

 removed. If we place a fly on the tip of its nose, that stimulus sets the 

 ''snapping" nerve-arc functioning. Then as the fly is taken into the 

 'mouth, a new nerve-arc causes a swallowing impulse, which sets up re- 

 actions of still other nerve-arcs which in turn cause the digestion glands 

 to pour out digestive substances. 



It will be noted that such reflexes are quite useful to the animal, 

 and it will be remembered that our definition of instinct called attention 

 to the protective value of inherited nerve-arc actions. 



Instincts may be deferred. That is, they may not be observable at 



