MOLLUSKS, ARTHROPODS AND VERTEBRATES 215 



tial response to localized stimulation has become so highly 

 developed that trial movements are largely eliminated. 

 Jennings (1906a, p. 453) makes the following characteris- 

 tically clear statement regarding this question: " It is, of 

 course, very true, as Harper ('05) remarks, that definitely 

 localized reaction methjods are developed as we rise higher 

 in the scale, yet it appears to be equally true that if we 

 mean by ' trial and error ' the performance of varied move- 

 ments, subjecting the organism to varied conditions, certain 

 of which are selected, then this also becomes more highly 

 developed and more used by organisms as we ascend the 

 scale. We must not forget that this expression ' trial and 

 error ' was originally based on the behavior of such highly 

 developed organisms as the cat, dog and monkey ; and doubt- 

 less there is no organism which uses this method to any such 

 extent as does man. Whenever the external conditions do 

 not furnish a precise determining factor for the movements 

 yet some sort of reaction is required, any organism is forced 

 to have recourse to this style of behavior, performing varied 

 movements till a condition is reached that relieves the organ- 

 ism of the necessity of continuing these movements. In its 

 highest form we call this experimentation." 



2. Circus Movements 



In Euglena, Stentor and some of the other lower forms 

 it was demonstrated that the orienting stimulus is due to 

 a change of light intensity. Is there any evidence as to how 

 the local stimulus which leads to orientation in the higher 

 forms is produced ? 



It has been found by a number of investigators working 

 on different forms that if one of two symmetrically located 

 sense organs is in any way prevented from functioning, the 

 organism no longer orients but continually turns toward 

 one side when stimulated. Loeb and others found this to 

 be the case in several different animals with one-half of 

 the brain destroyed. Holmes (1901, p. 220) found that the 



