296 BEHAVIOR OF THE LOWER ORGANISMS 



to be traced to this as the primitive type (see the following chapter). 

 That is, while the negative reaction is impelled by new unfavorable con- 

 ditions, tending to retain the more favorable old condition, the positive 

 reaction is impelled by the old unfavorable condition, tending to retain 

 the new more favorable one. 



(3) Sometimes change of behavior occurs without change in the 

 environment, the external conditions remaining uniform. As a rule, 

 we have found that change of behavior occurs under uniform conditions 

 only when these are decidedly injurious to the organism. If the water 

 containing infusoria or the flat worm is heated to about 37 degrees, the 

 animals react not merely to the change in temperature; they continue 

 to react violently, with frequent alternations in the behavior, until they 

 die. Many examples could be given of such reactions. Under uniform 

 conditions a change in behavior also occurs at times owing to internal 

 changes. The commonest cases of this sort are the changes in behavior 

 due to hunger. In almost all cases of reaction under uniform conditions 

 we find that the reaction is due to some interference with the normal 

 life processes. But reactions under uniform conditions play only a 

 small part in the behavior, as compared with reactions to changes. 



We have then two main results as regards the external causes of 

 changes in behavior: (1) change alone may produce reaction; (2) inter- 

 ference with the normal life processes or release from such interference 

 may produce reaction. The usual cause of a change in behavior is a 

 combination of both these factors — a change that hinders or helps the 

 normal life processes. In the lowest organisms it is chiefly interfering 

 changes that cause reaction. 



(4) Reactions to Representative Stimuli. — In the reactions due to 

 change, one further point is of much importance. The organism may 

 react to changes that in themselves neither favor nor interfere with the 

 normal life activities, but which do lead to such favor or interference. 

 The reaction given is then positive or negative in correspondence with 

 the benefit or injury to which the change leads. Thus, Stentor may 

 bend toward a small solid body when touched by it (Fig. 83), this reac- 

 tion aiding it to procure food, though there is no indication that the 

 touch itself is directly beneficial. Or it may contract away from a 

 light touch, this enabling it to escape from a possible approaching enemy, 

 though the touch itself is not injurious. Euglena reacts negatively 

 when its colorless anterior end alone is shaded, yet it is only when the 

 shadow affects its chlorophyll bodies that it interferes with metabolism. 

 The flatworm may turn toward a weak stimulus of any sort. This 

 leads in the long run to its obtaining food, though sometimes the 

 stimulus does not come from a food body. In such cases the animal 



