22 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



determined, the problem at once arises, by what factors or 

 conditions the actual performance of a particular movement 

 in a particular case is actually determined as such. 



Let us first remark that motion in itself by no means 

 requires a separate external cause for each of its single 

 phases. On the contrary, not only can periodic movements 

 like those in medusae or in the heart of animals be said to 

 be due to innate causes or stimuli, and to be, so to say, the 

 normal permanent state of the animal or the organ, but 

 changes of the specific type of random-movements may also 

 occur from within. In Hydra such an innate change of 

 different contingent motions may be studied with the 

 greatest advantage. 



This possibility of a change of single random-motions 

 from within now gives us the key to an understanding of 

 their change as occurring in response to an external stimulus. 

 It is always the interior general state of the organism that 

 determines which particular motor performance is to go on, 

 whether the state of rest is to be changed into a state of 

 some possible movement, or whether permanent motion is to 

 change its type. 



Yet we may speak of motions occurring " at random ' 

 although we know that they are determined, provided that 

 we know nothing specific about the general state of the 

 organism in question. In fact, the movements of an 

 animal which otherwise would not move at all, or the 

 changes of motion in a permanently moving organism, may 

 properly be called " random," if they do not follow any 

 specific law with regard to their sequence, if they go on 

 until the stimulus from without, that has caused them, is 

 escaped quite accidentally during and by the moving. 



