70 Studies in Animal Behavior 



very plausible interpretation in accordance with this 

 scheme. 



The studies of Jennings on the behavior of the 

 simpler animals revealed many cases of apparent 

 tropisms to which the tropism scheme of Loeb did 

 not apply. Infusorians were found to collect in 

 regions of dilute acid, not because they were ori- 

 ented to the lines of the diffusing substance, but be- 

 cause when they accidentally swam into the region 

 of the dilute acid they remained there. The col- 

 lections thus formed were the result of a sort of 

 selection of chance movements. The organisms 

 were found not to be able to orient themselves at 

 all, although the aggregations formed resembled 

 those which in other organisms are the result of 

 orientation. 



Although the formation of collections by the in- 

 fusoria and other asymmetrical organisms may occur 

 without orientation, Jennings believes that in sym- 

 metrical forms in which the body of the organism 

 is obviously oriented to the stimulus the process 

 does not occur in accordance with the tropism 

 scheme of Loeb. "In the symmetrical Metazoa," 

 he says, ".we of course find many cases in which the 

 animal turns directly toward or away from a source 

 of stimulation, without anything in the nature of 

 preliminary trial movements. This is a simple fact 

 of observation, which leaves open the possibility of 

 many different explanations. Is the simple expla- 

 nation given by the local action theory of tropisms 



