UNIVERSITY OF MISSOURI BULLETIN 



Its essence is the interpretation of the stimulus as directly 

 and locally affecting the parts of the organism, so that it 

 responds by an automatism having no intermediate links be- 

 tween stimulation and motor response. There is no suggestion 

 of the stimulus affecting something in the organism, which 

 in turn affects the locomotor organs. In the protozoon, it 

 affects these organs directly and locally, making them beat 

 at relatively different rates so long as they are unequally 

 stimulated. For the behavior of the more complex organisms, 

 we know this explanation to be inadequate. For the protozoon, 

 with its small bulk and absence of sense organs and nervous 

 system, it is not necessarily so. The determination of its 

 adequacy in the case of the protozoon would seem to rest with 

 an examination of the exact manner in which the animal 

 behaves during the period of orientation. As a result of such 

 examination, we have a rival explanation of the nature of 

 tropic response which is most actively supported by Professor 

 Jennings and which he designates by the term used for what 

 he believes to be essentially the same kind of reaction in 

 higher organisms, namely, reaction by the method of "trial 

 and error." 



According to the latter theory, careful study shows the 

 response of organisms like the protozoa to be so indirect as to 

 preclude interpretation after the manner of the tropism theory. 

 The protozoon, when affected by a stimulus like light, goes 

 through a varying number of random movements which in 

 higher organisms we should call reaction by the method of 

 "trial and error." In some cases it may happen to swing 

 directly into an orientation that will carry it toward the light, 

 but it usually executes a number of false moves, "trials," 

 which if they do not result in the proper orientation are not 

 continued, being "errors," so to speak. If enough of these 

 are made, the organism will come by a process of exclusion 

 to the position in which progression toward the light may be 



