168 BIOLOGY: GENERAL AND MEDICAL 



The suddenness and violence of the irritation has much 

 to do with the reflexes, for connected with many of the 

 reflex - circuits of the higher animals are inhibitors that 

 prevent their occurrence or greatly modify them. How 

 awkward it would be to find that the hand was jerked 

 away from every object it touched! Yet such might be 

 the case were there no inhibiting influences governing the 

 reflex circuits. 



If the brain of a pigeon be destroyed, the bird sits 

 quietly with ruffled feathers, and never moves unless dis- 

 turbed. If it be taken into the hand and thrown into the 

 air, it immediately spreads its wings and flies to some 

 nearby object upon which it perches and remains quietly 

 for an indefinite period. 



^ How different are the brainless frog and brainless pigeon 

 from the normal frog and the normal pigeon. If the 

 normal frog be touched it jumps into the water, swims 

 away, and hides under the lily-pads; if the pigeon be 

 approached too closely it flies away. Here we say we see 

 manifestations of a higher sort, viz., an instinct that 

 prompts the creature to self-preservation, and as we did 

 not see it in either of the creatures when their brains were 

 destroyed, we conclude that we have to do with a function 

 whose localization was in that part of the nervous system 

 that we had destroyed, the brain, which is undoubtedly 

 the case. 



However, we saw in tropisms the flight from injurious 

 agents, we saw in the simple reflex the withdrawal from 

 an injurious agent, and we see but an exaggeration of the 

 phenomenon in what are now to be discussed as instincts. 

 When shall it be said that tropisms and end reflexes begin? 

 When shall reflexes end and instincts begin? 



Instincts. Sherrington and Parmelee both believe in 

 making a clear separation between reflexes and other 

 forms of behavior, but we find upon examining a series 

 of animals, with the lowest at one end and the highest 

 at the other, that their nervous reactions pass from the 

 most simple to the most complex, without thresholds suffi- 



