46 LIGHT AND THE BEHAVIOR OF ORGANISMS 



(2) In place of getting into regions of a given light in- 

 tensity by mere wandering movements, organisms may 

 orient and move directly toward such regions, and the 

 avoiding reaction may keep them in this region just as 

 described above, or they may remain because it is illumi- 

 nated by light of optimum intensity. If they get into 

 light of lower intensity they become positive and return 

 to the optimum directly after becoming oriented. If they 

 get into light of higher intensity they become negative 

 and orient in the opposite direction, which again causes 

 them to return to the optimum intensity. The organ- 

 ism usually tries numerous positions before it becomes 

 oriented. Many errors are made before the successful posi- 

 tion is attained; many directions of motion are tried; one 

 is selected. Jennings has designated this method of orien- 

 tation as orientation by " trial and error," or more recently 

 merely by ''trial." Some seem to be of the opinion that the 

 trial movements are haphazard movements, that they are 

 not definitely determined. In answer to this Jennings says 

 (1906a, p. 452): ''The behavior may perhaps be most 

 accurately characterized as ' selection from among the 

 conditions produced by varied movements.' In general we 

 find that many organisms are so constituted that internal 

 conditions (permanent or temporary) will produce under 

 stimulation movements that are varied in precisely such 

 a way as to subject the creature to as varied environmental 

 conditions as possible, and thus give it an opportunity to 

 select what is nearest the optimum. Every one of these 

 movements is, of course, as absolutely determined as the 

 most orthodox tropism, only the determining factor is not 

 the localization of the stimulus (or other external factor) 

 alone. 



" Certain recent writers have seemed to imply that there 

 is a contrast between the 'trial and error' method, and 

 behavior that is definitely determined by structural and 

 other internal conditions. It needs to be emphasized, 

 perhaps, that the behavior which I and others have char- 



