294 BEHAVIOR OF THE LOWER ORGANISMS 



current changes. The general fact is that a change in the environment 

 produces a change in behavior. 



A. Change of conditions often produces a change of movement when 

 neither the preceding nor the following condition would, acting continu- 

 ously, produce any such effect. Thus when Euglena is swimming 

 toward the source of light, if the light is suddenly diminished, the organism 

 reacts by a change in its course ; it then returns to its course and continues 

 to swim toward the light as before. Its behavior before and after the 

 change is the same; but at the moment of change there is a reaction. 

 Paramecium may live and behave normally in water at 20 degrees or at 

 30 degrees, yet a change from one to the other, or a much less marked 

 change, produces a definite reaction. This relation could be illustrated 

 by many cases from the behavior of any of the organisms described in the 

 foregoing pages. Thus change simply as change may produce reaction. 



To constant conditions, on the other hand, unless differing very 

 greatly from the normal, the organism usually does not react. The 

 Paramecium placed in A^ per cent sodium chloride reacts at first, but 

 soon resumes its normal behavior. Euglena or Stentor when subjected 

 to changes in the illumination of the anterior end react till they come 

 into a position of orientation where these changes cease ; they then swim 

 forward in the normal manner. As a general rule, organisms soon be- 

 come acclimatized to a continuous condition, if it is not too intense. 

 Exceptions to this rule will be considered later. 



Of course a change must reach a certain amount before reaction is 

 produced; that is, there is a certain necessary threshold of stimulation. 

 In the best-known cases the amount of the change which produces re- 

 action is proportional to the intensity of the original condition ; in other 

 words, the relation of stimulus to reaction follows Weber's law (see pp. 

 38, 123). That is, it is relative change, not absolute change, that causes 

 reaction. 



B. But not every change, even if sufficiently marked, produces 

 reaction. It is usually not change alone that determines reaction, but 

 change in a certain direction. Of two opposite changes, one usually 

 produces a certain reaction, while the other either produces none or 

 brings about a reaction of opposite character. This point is one that 

 is of fundamental importance for an understanding of behavior. It 

 may be illustrated in its simplest aspect from the behavior of the infusoria, 

 where any reaction that is produced is usually of such a character as to 

 remove the organism from the source of stimulation (the "avoiding 

 reaction"). Paramecium at a temperature of 28 degrees reacts thus 

 negatively to a change to a higher temperature, not to the opposite change. 

 Paramecium at 22 degrees reacts to a decrease of temperature, not to 



