508 'Journal of Comparative Neurology and Psychology. 



single movement of the key, so that the cold water would flow 

 through the one and the hot water through the other. Thus the 

 distribution of temperature in the capillary tube could be reversed 

 at will. By cold water is meant water at normal temperature 

 to which the animal gives no reaction. 



As soon as the capillary tube is heated at one end by contact 

 with the hot support the Paramoecia at that end dart about at 

 random until they are headed toward the cool end of the tube and 

 even then do not swim to the cool end at first but often turn back 

 to the hot end several times before finally swimming over to the 

 cool water. Once arrived at the cool end the Paramoecia do not 

 stay there but turn and start back to the hot water. In this way 

 a Paramoecium may traverse the length of the tube a dozen times 

 or more before coming to rest at the cool end of the tube. If 



V 



zs 



Fig. 3. T, L-shaped tube containing Paramoecia. R, Brass rod heated and in contact with T. B, 

 Bunsen burner used to heat R. 



now the switch (fig. 2) is turned so that the cool end becomes hot, 

 the animal will dart about much as it did when first stimulated and 

 only after many trials will it reach the cool water. As the animals 

 leave their resting place at the warm meniscus in obedience to the 

 repeated reversing of temperatures in the tube, their movements 

 become slower and more regulated and they seldom turn more than 

 once toward the cold water before swimming in that direction. It 

 is not significant to express this modification of behavior in terms 

 of time for, although the time involved in getting away from the 

 heated end of the tube is somewhat reduced as the stimulus recurs, 

 it is the suitability of the movement to accomplish the result which 

 characterizes the later reactions. In these the actual locomotion 

 is slower but the random movements give place to more determined 

 ones. 



The influence of an associated past experience upon the reaction 

 to a given stimulus. Although in the following experiments the 



