852 TEXTBOOK OF ZOOLOGY 



are often called chain reflexes or instincts. Psychologists are inclined 

 to drop the word instinct, because of the disagreement in definition. 

 The term chain reflex seems to describe better the reaction of animals. 

 A chain reflex, like a simple reflex, is an inherited type of reaction 

 and may appear so complicated and purposeful in character that the 

 inexperienced observer is often led to describe it as intelligent experi- 

 ence or reason. It differs from a simple reflex, furthermore, in that 

 it involves generally the reaction of the animal as a whole rather than 

 that of single parts of organs. In its most complex forms it often de- 

 fies analysis, as in the case of migration of birds, or can be only in- 

 completely analyzed, as in nest-building, care of young, and many 

 others. 



Habitual Behavior 



The terms instinct and habit are usually applied to relatively com- 

 plex types of behavior — the one native and the other acquired. Ber- 

 nard has shown that psychologists find it difficult to separate complex 

 behavior into the two categories. This is true because most instinctive 

 acts involve a considerable amount of practice in becoming estab- 

 lished. Practice is the earmark of habit. The capacity to form habits 

 is Dunlap's definition of instinct. Habits are also defined as the 

 modification of response tendencies which show probabilities of cer- 

 tain high degree, that particular responses will occur in particular 

 circiunstances. 



As we have seen, the animal has a set of native reactions to external 

 stimuli which we caU reflexes. A habit is an acquired response that 

 somehow came to be associated with a stimulus to which it is not ordi- 

 narily attached. Formerly the animal, learning this new association, 

 was said to possess associative memory and this was thought to be indic- 

 ative of consciousness in such animals. Associative memory was ac- 

 credited only to higher animals. Today in many of the lower animals 

 experimentalists find a reaction at first directly called forth by a cer- 

 tain stimulus may later be provoked by another, even an unrelated 

 stimulus. Ahnost every modern student of psychology or physiology 

 knows of Pavlov's experiment with the dog whose mouth watered 

 when a bell was rung as a signal of feeding time. This Pavlov called 

 a conditioned reflex. It has been produced experimentally in a great 

 many kinds of animals. Conditioned reflexes years ago were called 

 associations and were taken to indicate the presence of associative 

 memory. The term associative memory implies a certain subjective 



