374 LIGHT AND THE BEHAVIOR OF ORGANISMS 



support, especially in the fact that changes in metabolism 

 cause changes in reaction. " Hungry animals react 

 positively to possible food while satiated ones react nega- 

 tively to the same stimuli." Paramecium bursaria is 

 positive to light in solutions deficient in oxygen, whereas 

 it does not react under normal conditions. After Volvox 

 has been resting in darkness for some time it responds 

 to light in a manner very different from the response given 

 when it is active. Jennings (1906, pp. 251-253 and else- 

 where) cites several other similar instances indicating that 

 reactions depend upon physiological states, but he frankly 

 admits that ** it is rarely possible to observe them [physio- 

 logical states] directly," especially in the lower organisms, 

 in which "the real data of observation are the actions; 

 if we considered these alone, we could only state that a 

 given organism reacts under the same external conditions 

 sometimes in one way, sometimes in another. This would 

 give us nothing definite on which to base a formulation 

 and analysis of behavior, so that we are compelled to assume 

 the existence of changing internal states. This assump- 

 tion, besides being logically necessary, is, of course, sup- 

 ported by much positive evidence drawn from diverse 

 fields, and there is reason to believe that in time we shall 

 be able to study these states directly. Before we can 

 come to a full understanding of behavior, we shall have to 

 subject the physiological states of organisms to a detailed 

 study and analysis, as to their objective nature, causes, 

 and effects " (p. 251). 



And again, after giving a most excellent description of 

 the reactions of Stentor, in which he shows that these 

 creatures sometimes respond in at least five different ways 

 to the same stimulus, Jennings says (p. 177): "Since in 

 each of these cases the external conditions remain through- 

 out the same, the change in reaction must he due to a change 

 in the organism. The organism w^hich reacts to the carmine 

 grains by contracting or by leaving its tube must be differ- 

 ent in some way from the organism which reacted to the 



