PHYSIOLOGICAL STATES AS DETERMINING FACTORS. 117 



the stimulus. Just which of these reactions shall be given as a response 

 to any particular stimulus depends on the physiological Condition of 

 the organism. This physiological condition depends largely, as we 

 shall note later, on the history of the individual. Thus no single fixed 

 schema, such as we have in the tropism theory, can ever possibly 

 explain or define the essential points in the behavior of an animal. 



Stentor and Planaria may be taken as typical examples of the higher 

 Protozoa and of the lower Metazoa, respectively. It is true that we are 

 not so well informed as to changes in physiological condition in other 

 lower organisms as in these two cases, but this is unquestionably due 

 merely to the fact that investigation has not been directed especially to 

 this point. There are, however, many cases in the literature which 

 explicitly or implicitly show the importance of physiological conditions 

 in determining the behavior of lower organisms. A number of these 

 cases may be brought together here. 



CHANGES IN THE SENSE OF " TROP1SMS " AND OTHER 



REACTIONS. 



Loeb (1893) and Nagel (1894) have called attention to the fact that 

 certain worms and mollusks respond to a shadow by suddenly with- 

 drawing into their tubes, but that after the first reaction has been thus 

 produced the worms may no longer react. In this " after effect of the 

 stimulus" (Loeb) we have, of course, a case of changed physiological 

 condition. 



Changes in the sense of "tropisms" belong here. Groom & Loeb 

 (1890) found that larvae of Balanus are at certain times of the day 

 positively phototactic ; at other times negatively phototactic. This 

 difference, in so far as it is independent of changed external conditions, 

 is, of course, due to differences in the physiological condition of the 

 organism. Loeb (1893) found that the larvae of the moth Porthesia 

 are positively phototactic when hungry ; not so after eating. Here we 

 have a well-defined physiological condition determining the nature of 

 the reaction ; hunger is one of the most important conditions in many 

 of the lower animals. Sosnowski (1899) and Moore (1903) show that 

 the geotropism of Paramecium changes from negative to positive under 

 various conditions. Towle (1900) and Yerkes (1900) have shown that 

 the sense of the phototactic reaction in Entomostraca is dependent on 

 the preceding treatment of the organism, mere transference with the 

 pipette often changing the sense of the reaction from positive to nega- 

 tive or vice versa. Such instances could doubtless be multiplied 

 indefinitely. Each case taken by itself seems perhaps of comparatively 

 little significance. We may look upon them, however, as indications of 

 an extensive dependence of behavior on physiological conditions, such 



