332 BEHAVIOR OF THE LOWER ORGANISMS 



All the way up the scale, from Amoeba and bacteria to man, we find 

 that organisms react negatively to powerful and injurious agents. In 

 man and higher animals such reactions are usually said to be due to 

 pain. In the lower organisms the objective facts are parallel, and natu- 

 rally lead to the assumption of a physiological state similar to what we 

 have in the higher forms. As to subjective accompaniments of such 

 a state we of course know nothing in animals other than ourselves. The 

 essential cause of the states corresponding to pain is interference with 

 any of the processes of which the organism is the seat, and the correlate 

 in action of these states is a change in movement. This point will 

 be developed in our final chapter. 



A similar basis exists for distinguishing throughout the organic 

 series a physiological state corresponding to that accompanying pleasure 

 in man. This is correlated with a relief from interference with the life 

 processes, or with the uninterrupted progression of these processes. 



In man and higher animals we often find a negative reaction to that 

 which is not in itself injurious, but which is usually followed by some- 

 thing injurious. The sight of a wild beast is not injurious, considered 

 by itself, but as preceding actual and injurious contact with this beast, 

 it leads to powerful negative reactions. Such reactions are said to be 

 due to fear. In fear there is then a negative reaction to a representative 

 stimulus one that stands for a really injurious stimulation. In lower 

 organisms we find the objective indications of a parallel state of affairs. 

 The infusoria react negatively to solutions of chemicals that are not, 

 so far as we can determine, injurious, though they would naturally, 

 under ordinary circumstances, be immediately followed by a solution 

 so strong as to be injurious. Euglena reacts negatively when darkness 

 affects only its colorless anterior end, though we have reason to believe 

 that it is only the green part of the body which requires the light for the 

 proper discharge of its functions. A much clearer case is seen in the 

 sea urchin, which reacts by defensive movements when a shadow falls 

 upon it, though shade is favorable to its normal functions. Objectively, 

 fear has at its basis the fact that a negative reaction may be produced 

 by a stimulus which is not in itself injurious, provided it leads to an 

 injurious stimulation ; this basis we find throughout organisms. 



Sometimes higher animals and man are thrown into a "state of 

 fear," such that they react negatively to all sorts of stimuli, that under 

 ordinary circumstances would not cause such a reaction. A similar 

 condition of affairs we have seen in Stentor and the flatworm. After 

 repeated stimulation, they react negatively to all stimuli to which they 

 react at all. 



The general fact of which the reactions through fear are only a special 



