1 1 2 Journal of Comparative Neurology and Psychology. 



in others by the chance success of a random movement. There 

 will doubtless occur many cases difficult to classify where trials 

 are not perfectly random movements but where the stimulus 

 may have a certain directive effect which is in large measure ob- 

 scured. A tropism of the 'direct sort is not necessarily a per- 

 fectly fixed and rigid affair. It may be a tendency more or 

 les obscured by a lot of random movements arising from inter- 

 nal causes. An organism may be drawn to a certain point 

 through a direct orienting reflex, but if there be at the same 

 time a large element of random activity in its behavior it may 

 seem to reach that point by the method of trial and error. In 

 the trial and error method the random character of the move- 

 ments impresses us most; in the tropisms, the element of direct 

 determination by the environment. Both of these factors run 

 through the behavior of all animals, but they are mingled in 

 various proportions in different forms. In the lives of most, if 

 not all, animals both are essential elements in the adjustment of 

 the organism to its conditions of existence. 



University of Michigan, Ann Arbor, Mich., Dec. 6, 1^04. 



