148 Herbert Eugene Walter 



in question. Among the infusoria and rotifera, as Jennings has 

 shown in a masterly series of papers,* such attempts at orientation 

 are made by means of a "motor reflex," consisting in (i) a sudden 

 withdrawal from the stimulus, (2) a rotation toward a structurally 

 defined side of the asymmetrical organism, and (3), lastly, an 

 advance in a new direction. 



In the case of organisms which do not possess marked asym- 

 metry the trial and error method, as pointed out by Holmes ('05a), 

 resolves itself into a series of "random movements;" that is, a 

 number of apparently experimental movements are made, which 

 finally result in the best adjustment to the stimulus. 



In both of these methods the organism acts as a unit and not in 

 response to localized stimulation received asymmetrically. 



The tropism theory, on the contrary, is based upon asymmetrical 

 action as the result of asymmetrical stimulation. If an organism 

 receives a stronger stimulus on one side of its body than on the 

 other, the result, whether direct or indirect, is that it moves in such 

 a way that this asymmetrical stimulation becomes symmetrical. 

 In other words, orientation occurs. 



It is unfortunate that the tropism theory was made to apply to 

 the behavior of the infusoria, since it has been shown beyond 

 doubt by Jennings that exact observation of the behavior of these 

 organisms and an analysis of its details does not admit of the tropic 

 interpretation, but is, on the other hand, explained by the trial 

 and error theory of motor reflexes. It is also to be regretted that 

 the unquestionable rout of the tropism theory, as applied to cer- 

 tain protozoa and other asymmetrical forms, should have led to 

 an attempt to exclude it from the remainder of the animal king- 

 dom. 



In a paper on the tropism theory Jennings ('04a) names as an 

 essential criterion of tropism the direct unilateral stimulation of 

 the motor organs. After showing how inadequate such an assump- 

 tion is to explain the orientation of animals, particularly that of 

 Infusoria, he continues ('04a, p. 104), "We should perhaps con- 



* See bibliography in Contributions to the Study of the Behavior of Lower Organisms. Carnegie 

 Inst, of Washington. Publication No. i6. 256 pp. 1904. 



