CHAPTER XIII 

 ON THE THEOR Y OF ANIMAL INSTINCTS 



i. The discrimination between reflex and instinctive 

 actions is chiefly conventional. In both cases we have 

 to deal with reactions to external stimuli or conditions. 

 But while we speak of reflex actions when only a 

 single organ or a group of organs react to an external 

 stimulus, we generally speak of instincts when the 

 animal as a whole reacts. In such cases the reactions 

 of the animal, although unconscious, seem often to be 

 directed towards a certain end. A fly acts instinct- 

 ively when it lays its egg on objects which serve 

 the hatching larvae as food. We call the periodical 

 migrations of animals instinctive. We call it instinct- 

 ive when certain animals conceal themselves in cracks 

 and crevices where they are safe from persecution. 

 But the purposeful character of instincts cannot be 

 used to distinguish them from reflexes, as a great 

 many of the reflexes are also purposeful, for instance, 

 the closing of the eyelid if the conjunctiva be touched 

 or the wiping off of acetic acid that is put on the 

 skin of a decapitated frog. On the other hand, it 

 cannot be said that every instinctive action is purpose- 



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