QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 217 



jective point of work on light reactions. Interest no longer cen- 

 tered about the cataloguing of the positive or negative response 

 of various forms, but about the developing of a general theory of 

 light reactions. 



With the possible exception of Engelmann's work on swarm 

 spores, most of the work on behavior up to this time had dealt 

 with mass reactions rather than with the details of individual 

 reactions. In 1897 Jennings began a study of the aggregations 

 so commonly found in the cultures of Paramoecium, by observing 

 in the minutest detail the reactions that brought a single indi- 

 vidual into the aggregation and held it there. The work on 

 Paramoecium was followed by many other papers on the Protozoa 

 and some on the lower Metazoa, all characterized by the same 

 thoroughness in the observation of individual behavior. Such 

 methods brought out many facts of great interest, previously 

 overlooked. 



One of the points on which Jennings lays great emphasis is 

 that behavior does not depend on the external stimulus alone. 

 He believes that there are various internal factors which modify 

 the reactions to the same external conditions. Former stimuli 

 and the reactions of the organisms to them, as well as the meta- 

 bolic processes constantly going on within the animal, have their 

 effect on the physiological state. The physiological state in turn 

 determines to a large degree the reaction of the organism to ex- 

 ternal stimuli. 



Jennings also found that in the Protozoa on which he worked 

 there were no bilaterally located sensory areas and that the posi- 

 tion of orientation was not one in which the median plane of 

 structural symmetry was placed in a definite position w4th ref- 

 erence to the source of stimulation. The 'tropism theory,' as 

 put forward by Loeb, evidently did not apply to these organisms. 

 Jennings observed that changes in the direction of locomotion 

 were brought about by 'motor reflexes' directed toward a struc- 

 turally definite side of the organism. He characterizes this 

 method of orientation as one of 'trial and error.' The 'varied 

 movements' of locomotion involve contact with varying environ- 

 mental conditions, selection from among these conditions is 



