CHAPTER XVII 



ANALYSIS OF BEHAVIOR (Continued) 

 B. The External Factors in Behavior 



(1) As we have seen in the foregoing chapter, external agents produce I 

 reactions through the intermediation of changes in the internal physio- 

 logical condition of the organism. This proposition is, perhaps, a truism, 

 yet it needs to be kept in mind if behavior is to be understood. In the 

 following discussion it will be unnecessary to mention specifically in each 

 case the intermediate step in the process. 



(2) The most general external cause of a reaction is a change in the I 

 conditions affecting the organism. This has been illustrated in detail 

 in the descriptive portions of the present work. In most cases the change 

 which induces a reaction is brought about by the organism's own move- 

 ments. These cause a change in the relation of the organism to the 

 environment; to these changes the organism reacts. The whole be- 

 havior of free-moving organisms is based on the principle that it is the 

 movements of the 6rganism that have brought about stimulation ; the 

 regulatory character of the reactions induced is intelligible only on this 

 basis. Reactions due to stimulation produced in this manner are seen 

 when an organism progresses from a cooler to a warmer region, or vice 

 versa; when it moves into or out of a chemical in solution; when it 

 strikes in its course against a hard object ; when the unoriented infuso- 

 rian shows lateral movements while subjected to light coming from one 

 side. In all these cases it is the movement of the organism which causes 



a change in its relation to the external agent, and this change produces 

 reaction. In most, if not all, cases the change is one in the intensity of 

 some agent acting on the organism. 



But an active change in the environmental conditions, not produced 

 by movement of the organism, may likewise produce reaction ; this is, 

 of course, most frequently the case in fixed organisms, such as the sea 

 anemone. Responses produced in this way are seen in the reactions of 

 organisms when heated or cooled from outside, or when a chemical 

 or a solid object is brought in contact with them, or when the source 

 of light changes in intensity or position, or when the direction of a water 



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