50 LIGHT AND THE BEHAVIOR OF ORGANISMS 





3. Former reactions of the organism. 



4. Progressive internal changes (due to metabolic pro- 

 cesses, etc.). 



''5. The laws of the resolution of physiological states one 

 into another. 



" All these factors have been strictly demonstrated by 

 observation and experiment, even in unicellular organisms. 

 Any one of these alone, or any combination of these, may 

 determine the activity at a given moment." 



External factors (p. 299): "We may sum up the external 

 factors that produce or determine reactions as follows: 

 (i) The organism may react to a change, even though neither 

 beneficial nor injurious. (2) Anything that tends to inter- 

 fere with the normal current of life activities produces 

 reactions of a certain sort ('negative'). (3) Any change 

 that tends to restore or favor the normal life processes may 

 produce reactions of a different sort ('positive'). (4) 

 Changes that in themselves neither interfere with nor assist 

 the normal stream of life processes may produce negative 

 or positive reactions, according as they are usually followed 

 by changes that are injurious or beneficial. (5) Whether a 

 given change shall produce reaction or not, often depends 

 on the completeness or incompleteness of the performance 

 of the metabolic processes of the organism under the exist- 

 ing conditions. This makes the behavior fundamentally 

 regulatory." 



Reactions and change in the sense of reactions are, 

 therefore, according to Jennings, adaptive ; and if this be 

 true, an explanation of them must be looked for along the 

 same lines as an explanation of any other adaptive charac- 

 teristic in organisms, functional as well as structural. 



Finally we may refer to the "selection of random move- 

 ments" as a factor in orientation, as put forward by 

 Holmes (1905). He studied the reactions to light of earth- 

 worms and blow-fly larvae and found that when these 

 animals are stimulated they turn in many directions, 

 apparently feeling about until they become directed away 



