Il6 THE BEHAVIOR OF LOWER ORGANISMS. 



4. After the worm in the excited condition has been stimulated 

 repeatedly on one side, so that it turns its head steadily in the opposite 

 direction, after a time it suddenly changes its method of reaction. It 

 jerks backward, then turns the anterior end quickly through a consider- 

 able arc, usually toward the side from which the stimulus is coming, 

 so that the head now points in an entirely new direction. Since the 

 stimulus and other external conditions remain the same, the organism 

 must have passed into a new physiological condition, or it would not 

 now react in a different way. We may call this for convenience the 

 condition of over-stimulation. 



5. Sometimes individuals are found which for a brief period (two or 

 three hours) seem in a much more active condition than usual. They 

 move about rapidly, but do not conduct themselves like the excited 

 individuals. As they move they keep the anterior end raised and wave 

 it continually from side to side as if searching. Specimens in this con- 

 dition react to almost all mechanical stimuli, whether weak or strong, 

 by the positive reaction, turning toward the point stimulated. Experi- 

 mentation failed to show that this condition was due to hunger. We 

 may speak of this fifth physiological condition as the state of heightened 

 activity. 



In addition to the effect of these five well-defined physiological states 

 on the method of reaction to mechanical stimuli at the anterior part of 

 the body. Pearl finds that other less easily definable internal conditions 

 affect the reactions. At times an individual will give the positive reac- 

 tion to a stimulus of a certain strength a few times, then cease to give 

 it. On account of these and other complications due to varying internal 

 conditions, Pearl concludes : 



It is almost an absolute necessity that one should become familiar, or perhaps 

 better, intimate, with an organism, so that he knows it in something the same 

 way that he knows a person, before he can get even an approximation of the 

 truth regarding its behavior. 



We have taken up above only the physiological conditions influ- 

 encing the reactions to simple mechanical stimuli in the anterior region 

 of the body. We find the condition of affairs even here somewhat 

 involved. When other more complex stimuli are taken into considera- 

 tion the results of the interplay of change of physiological condition 

 and variations in the stimuli become, of course, much more complicated. 



Thus we find in the bilateral metazoan Planaria, as in the unsymmet- 

 rical protozoan Stentor, that we can by no means predict the behavior 

 of the individual from a knowledge of the anatomical structure and of 

 the strength of the stimulus. The anatomical structure limits the possi- 

 bilities of reaction to several methods, which are, however, entirely 

 different or opposite in their effects on the relation of the organism to 



