250 THE BEHAVIOR OF LOWER ORGANISMS. 



When the starfish is placed on its back, the physiological state existing 

 induces all the arms to initiate feeling movements ; the animal tries to 

 reach a solid with its tube feet. As soon as one or two arms have suc- 

 ceeded, this success is recognized by the cessation of effort on the part 

 of the other arms. Their physiological state has changed to one cor- 

 responding to success. 



We may sum up our discussion on these points as follows : The 

 method of trial and error involves some way of distinguishing error, 

 and also, in some cases at least, some method of distinguishing success. 

 The problem as to how this is done is the same for man and for the 

 infusorian. We are compelled to postulate throughout the series cer- 

 tain physiological states to account for the negative reactions under 

 error, and the positive reactions under success. In man these physio- 

 logical states are those conditioning pain and pleasure. 



The " method of trial and error," as this phrase is used in the 

 present paper, is evidently the same as reaction by " selection of over- 

 produced movements," which plays so large a part in the theories of 

 Spencer and Bain and especially in the recent discussions of behavior 

 by J. Mark Baldwin. To this aspect of the matter the present writer 

 will return in the future. 



This method of trial and error, which forms the most essential feature 

 of the behavior of these lower organisms, is in complete contrast with 

 the tropism schema, which has long been supposed to express the 

 essential characteristics of their behavior. The tropism was conceived 

 as a fixed way of acting, forced upon the organism by the direct action 

 of external agents upon its motor organs. Each class of external agents 

 had its corresponding tropism ; under its action the organism performed 

 certain forced movements, usually resulting in its taking up a rigid 

 position with reference to the direction from which the stimulus came. 

 Whether it then moved toward or away from the source of stimulus 

 was determined by accidental conditions, and played no essential part 

 in the reaction. There was no trial of the conditions; no indication 

 of anything like what we call choice in the higher organisms ; the 

 behavior was stereotyped. Doubtless such methods of reaction do 

 exist. In the reactions of infusoria to the electric current (an agent 

 with which they never come into relation in nature), there are cer- 

 tain features which fit the tropism schema, and in the instincts the 

 "Triebe" of animals there are features of this stereotyped character. 

 The behavior of animals is woven of elements of the most diverse kind. 

 But certainly in the lower organisms which we have taken chiefly into 

 consideration the behavior is not typically of the stereotyped character 

 expressed in the tropism schema. The method of trial and error is 

 flexible ; indeed, plasticity is its essential characteristic. Working in 



