94 THE BEHAVIOR OF LOWER ORGANISMS. 



tropisms says that certain definite things happen in the change of 

 position undergone by organisms under the influence of stimuli ; that 

 the organisms perform certain acts in certain ways. The problem for 

 the investigator is, then, Do these things happen? Does the organism 

 perform these acts, in these particular ways? These questions are not 

 metaphysical ; they can be answered by observation. 



We have now before us a considerable body of exact observations 

 which permit us to answer these questions for a certain number of 

 organisms. We will here attempt to summarize these observations so 

 far as they bear upon the essential points in the theory of tropisms. In 

 particular, we will ask, (i) Is the observed behavior brought about 

 through orientation, in the way the theory of tropisms demands? (2) 

 Does the evidence show that the action of a stimulus is directly upon the 

 motor organs of that part of the body on which the stimulus impinges? 



REACTIONS TO MECHANICAL STIMULI. 



The reactions to simple mechanical stimuli, as when the organism 

 is touched or struck by a hard object over a certain definite area of the 

 body, of course do not as a rule present the conditions required for 

 the production of a tropism, including a definite orientation. Yet it is 

 important to bring out certain general relations shown in these reactions, 

 as they throw light on the reactions to stimuli of a different character. 



Most animals show in one way or another a tendency to avoid sources 

 of mechanical shock. In the higher organisms the reaction usually 

 takes the form of a turning away from the side stimulated. The point 

 which needs to be brought out here is that in ciliate infusoria the direc- 

 tion of turning depends, not upon the part of the body stimulated, but 

 upon an internal factor. Stylonychia turns to the right, whether stimu- 

 lated on the right side, on the left side, on the dorsal surface, on the 

 anterior end, or by a general unlocalized mechanical shock ; and parallel 

 statements can be made for other infusoria. (For details see Jennings, 

 1900.) We have proof, therefore, that the action of the stimulus is 

 on the organism as a whole, not merely upon the motor organs of 

 that region of the body stimulated. Further, it is clear that the 

 response is a reaction of the organism as a whole, not one brought 

 about as an indirect result of the fact that certain motor organs have 

 received a stimulus to contraction.* In these respects, therefore, the 

 reactions to mechanical stimuli are different in character from those 

 assumed to take place in the tropisms, and even in these unicellular 

 organisms the processes taking place must be more complex than the 



*This fact becomes still more striking when we recall that the reaction takes 

 place in the same way in pieces from any part of the body, from which any given 

 motor organs may have been removed. (Details in Jennings & Jamieson, 1902.) 



