CHAPTER XIII 



BEHAVIOR 



By behavior is meant the sum total of an animal's movements. Noth- 

 ing else can be included in behavior, since only by the movement of the 

 body as a whole or of some part of it can an animal convey any indication 

 of change within it. This also holds true with human behavior, since 

 posture, facial expression, speaking, or any other mode of communication 

 all involve movement. The movement of animals when involving no 

 change in location of the animal is termed motion, but when change in 

 location occurs it is locomotion. 



84. Stimuli. — All movement is referred to some cause, either outside 

 the animal or within it, which is termed a stimulus. If the cause is 

 purely external the phenomenon is called external stimulation; if, on the 

 other hand, the cause exists within the body of the animal, it is recognized 

 as internal stimulation. The stimulus initiates a change, chemical and 

 physical, within a part or whole of the animal which results in either 

 motion or locomotion and which is called a response. External stimuli 

 are either continuous or discontinuous. The first application of a con- 

 tinuous stimulus produces a movement, while the continuation of that 

 stimulus, if it is maintained at the same intensity, has no effect. When 

 the operation of the stimulus ceases, movement is again observed. A 

 discontinuous stimulus may be looked upon as a series of similar stimuli 

 each of which produces the appropriate response on the part of the animal, 

 both when it begins and when it ends. Since the response takes a certain 

 length of time, discontinuous stimuli, if following each other with suffi- 

 ciently brief intervals between them, may produce a continuous effect. 

 An example is the production of a tetanic, or prolonged, muscular con- 

 traction by a discontinuous electrical current. 



85. Direct Response. — If the response which the animal gives immedi- 

 ately follows the application of the stimulus and seems to be determined 

 by the nature and force of the stimulus, the response is termed direct and 

 is called a troy ism. Two words have been used in this connection: 

 tropism, which means simply a turning, and taxis, which implies move- 

 ment of the animal as a whole in response to the stimulus. Since the 

 difference between these two is one determined by the locomotor ability 

 of the organism, the word taxis has given way to the more general term 

 tropism. 



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