44 LIGHT AND THE BEHAVIOR OF ORGANISMS 



well known that contrary to Radl's conclusion, most of 

 the organisms which face the source of stimulation when 

 positive, turn and face in the opposite direction when 

 negative. 



2. Afore thorough Experimental Analysis Showing the Rela- 

 tive Importance of Internal and External Factors in Behavior 



None of the investigators thus far mentioned studied the 

 behavior of lower organisms in sufficient detail to be able 

 to tell from direct observation precisely what takes place 

 in the reactions. It was well known from direct observa- 

 tion that many of these organisms form dense aggregations 

 under certain conditions and that they frequently orient 

 when subjected to certain stimuli; but just what takes 

 place during the process of aggregation and orientation was 

 with a few exceptions known only theoretically. 



Jennings was the first to supply this deficiency in obser- 

 vation. He began his investigations on this subject in 

 1897 by working out in minutest detail precisely what 

 movements are involved in the formation of the dense 

 aggregations so frequently seen in cultures containing 

 paramecia. His work differs from that of his predecessors 

 in this line largely in that, while they, with the possible 

 exception of Engelmann, studied mass movements and end 

 results, he studied the individuals. He was interested 

 not so much in the aggregations as in the process of their 

 formation. How does each individual get there? and 

 why does it stay there? were prominent questions in his 

 mind. 



The observations on the formation of aggregations of 

 paramecia were followed by similar observations on the 

 reactions of representative species of the various groups of 

 protozoa and lower metazoa to various sorts of stimuli. 

 All of this work is characterized by unity of purpose, keen- 

 ness of observation and simplicity of method. 



The results of all of Jennings' work, published in nu- 



