ORGANIC MOVEMENTS 



here by a specified movement of another specified part. 

 And the same holds for some, though not very many, 

 movements of Invertebrates. 



It is the invariability, the absolute fixation of the 

 relation between a simple cause and a simple motor effect 

 or reaction, with regard to quality as well as to localisation, 

 that characterises this type of the simple reflex ; a indeed, 

 a simple reflex occurs with the precision of machinery. 

 Nothing in fact speaks against the real existence of such 

 machinery : we therefore may assume hypothetically that 

 true simple reflexes are machine-like in every respect, and 

 with this assumption we may now leave this type of 

 organic movement, which affords us no theoretical problems 

 of a complicated kind. 



fi. THE DIRECTIVE MOVEMENTS 



In the simple free directive movement or " taxis " it is 

 the typical relation between the direction of the stimulus 

 and the direction of the effect, with regard to the main axis 

 or the plane of symmetry of the organism, which separates 

 this type of motion from others. The significance of this 

 will best be illustrated by certain phenomena which do not 

 properly belong to the class of free movements we are 

 dealing with here, but which more correctly belong to the 

 physiology of growth : the so-called " tropisms." 



Tropism 



Let us first devote a few words to the chief characteristics 

 of these " tropisms." We did not discuss them whilst 



1 Of course the general type of a simple reflex is not changed, if the 

 locality of the cause and of the effect is the same. 



