IX 



BEHAVIOR AND FORM 



'T^HE idea has been recently emphasized that in 

 •*• many cases the structure of an organism is, to 

 a considerable extent, the effect of its behavior. We 

 have, of course, long been familiar with the fact 

 of functionally produced modifications, but it is 

 only of late that behavior has been brought forward 

 as a factor of importance in development, regenera- 

 tion and other modes of form regulation. If the 

 way in which an organism acts is determined by its 

 structure, it may also be said that its structure is 

 to a certain degree determined by the way in which 

 it acts. 



The experiments of Child on the regeneration 

 of planarians, nemerteans, and other forms have 

 led him to the view that one very important ele- 

 ment determining the way in which a part differen- 

 tiates is the kind of movements it is called upon 

 to perform during the period of its formation. A 

 posterior cut end of a planarian, for instance, de- 

 velops a tail because owing to the movements of 

 the animal, the regenerating tissue is subjected to 

 the same impacts and tensions to which the tail end 

 of the entire animal is normally subjected. These 



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