RHEOTAXIS IN ISOPODA 271 



sider 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 on which to base a formulation 

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

 of changing internal states. This assumption besides being logically 

 necessary, is of course, supported 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. 



In commenting on Jennings' position, Mast ('11, pp. 375-378) 

 placed some emphasis on another factor. He said in part : 



It is evident, that while there is some evidence bearing on physiological 

 states we know but little about their nature and regulation. For all 

 that is known to the contrary, subjective factors, entelechies, or psy- 

 choids, factors foreign to inorganics, may have a hand in controlling 



physiological changes and consequently the reactions 



Whether or not there are any such phenomena is the question at issue. 

 But until this question is settled .... those who 

 maintain .... there are no entelechies are certainly no more 

 scientific than those who maintain the opposite. 



The analysis of the relationship existing between the physio- 

 logical state of the isopods used and their rheotactic reaction has 

 been carried on in accordance with the statements of Jennings 

 just quoted. The work has proceeded far enough at present to 

 enable one to predict with certainty the action that will result 

 from a physiological state experimentally produced, so that in 

 this case the behavior of the animals becomes a check on the pro- 

 duction of a physiological state as well as an indicator that a 

 change must have occurred. While it is as yet impossible to 

 control with certainty all the minor details of the reaction, yet 

 sufficiently complete control has been maintained to show that in 

 the rheotactic' response with these animals there is no necessity 

 to call in any 'factor foreign to inorganics' in order to explain the 

 changes in physiological states. 



1 Italics mine. 



