MOVKMENTS, ETC., OF FRESH-WATER I'LANARIANS. 587 



uirIlt discussion, that iu au auinial in a condition of liypor- 

 seusitivity I have been able to produce the reaction by the 

 weakest stimuli whicli I was practically able to give. Under 

 normal conditions of sensitivenes it takes a slightly stronger 

 stimulus. No absolute value can be given for the upper 

 limen of the reaction, beyond which it docs not appear, but 

 gives place to the negative reaction. This value varies 

 greatly with different individuals. The general statement 

 may be made that the positive reaction is the characteristic 

 response to stimuli of very low intensity, and its production 

 is very closely dependent on the proper gradation in the 

 intensity. This dependence is so close that it is possible to 

 obtain a part of both the negative and positive responses 

 combined in the same reaction by tlie use of a stimulus of 

 the proper intensity. I have been able in a few very favour- 

 able cases to produce by a single stimulus a pronounced 

 raising of the head, such as is characteristic of the positive 

 reaction, followed by a turning away from the source of the 

 stimulus. Now the raising of the bead is no part of the typical 

 negative reaction, and, furthermore, was done in the very 

 characteristic way in which it occurs in the positive reaction. 

 The stimulus whicli produced it was evidently about in- 

 termediate in intensity between what, in the case of tlie par- 

 ticular animal used, would have called forth either the positive 

 or negative reaction, as the case might be. This experiment 

 shows in a very striking and conclusive way that in both 

 the positive and negative reactions we are dealing with a 

 complex of reflexes, since here a part of one of the reactions 

 is associated with a part of the other. This point will be 

 alluded to again in another connection, and its significance 

 more fully pointed out. 



The evident purposeful character of the positive reaction is 

 plainly apparent. It is a reaction admirably suited, on the 

 whole, to bring the organism into contact with beneficial 

 things, such as food, etc. It seems to me that it nnist be by 

 far the most important reaction of the animal in the struggle 

 for existence. In the conditions under which planariaus 



