THE THEORY OF TROPISMS. lOf 



reaction" consisting usually of a movement backward, followed by a 

 turning toward a structurally defined side. The direction of turning is 

 thus determined by internal factors. 



In the reaction to light orientation is a striking factor, but the orienta- 

 tion is not primary, being due to the production of the same " motor 

 reaction" described in the last paragraph. The method of orientation 

 is incompatible with the idea that orientation is due to the direct action 

 of the stimulus upon the motor organs of the part of the body on which 

 the light impinges, for orientation occurs by turning always toward a 

 certain structurally defined side, without regard to the part of the body 

 struck by the light. The turning may, therefore, be toward or away 

 from the source of light, or in any intermediate direction. In any case 

 it is continued or repeated until the anterior end is directed away from 

 the source of light, when it ceases. 



The exact method of reaction to gravity has not been worked out by 

 direct observation. 



In the reaction to the electric current the reaction method of the 

 rotifer is by a " motor reflex," and is hence inconsistent with the tro- 

 pism schema. In the Infusoria there is a partial (but only partial) 

 agreement with the requirements of the tropism theory. But this 

 partial agreement with the theory is due to certain peculiar effects of 

 the electric current which are not known to be produced by any other 

 stimulus. Hence the reaction to the electric current, far from being a 

 type for reactions in general, is a unique phenomenon, demanding 

 special explanation. 



The general conclusion is drawn that the theory of tropisms does 

 not go far in helping us to understand the behavior of the lower organ- 

 isms ; on the contrary their reactions, when accurately studied, are, as 

 a rule, inconsistent with its fundamental assumptions. The responses 

 to stimuli are usually reactions of the organisms as wholes, brought 

 about by some physiological change produced by the stimulus ; they 

 can not, on account of the way in which they take place, be interpreted 

 as due to the direct effect of stimuli on the motor organs acting more 

 or less independently. The organism reacts as a unit, not as the sum 

 of a number of independently reacting organs. 



