ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 285 



impulse, and because movement in this direction is not prevented. 

 This possibility must be considered in all cases. 



Further, when the action of a stimulus actually changes the direction 

 of movement in an organism, persistence in this new direction by no 

 means demands persistence in stimulation. The new direction once 

 attained may be followed, from the internal impulse to movement, 

 merely because there is nothing to change this direction, or because 

 stimulation does occur when this direction is changed, bringing the 

 organism back to it. This is apparently the case, as we have seen, in 

 the reactions of infusoria to gravity, to water currents, and to light com- 

 ing from a certain direction. 



Often, of course, stimulation does rouse an organism to increased 

 activity. But even in this case the activity is due to the release of in- 

 ternal energy. It may, therefore, continue long after the stimulation 

 which inaugurated the release has ceased to act. Such continuance 

 thus does not necessarily imply continued action of the stimulus. In 

 many cases the specific stimulus to action is only the change of conditions. 

 Thus, if light or a chemical acts upon an organism, the only stimulus 

 may be the sudden change, even though the organism continues to 

 move after the conditions have become constant. Whether the effective 

 stimulation actually continues, must be determined by experiment; it 

 cannot be simply assumed. 



In general, when an organism is moving in a certain way — even 

 when toward or from a certain agent — careful analytical experimenta- 

 tion is necessary to determine whether this movement is due to present 

 stimulation, or to the simple outflow of the stored-up energy of the or- 

 ganism through the channels provided by its structure. In most cases, 

 apparently, the latter is true. 



The spontaneous activities of the organism — those not due directly 

 to present specific external stimulation — are, perhaps, the most im- 

 portant factors in its behavior. 



(2) Activity may change without External Cause. — If we watch a 

 specimen of Vorticella under uniform conditions, we find that its behavior 

 does not remain uniform. At first the animal is outstretched, its cilia 

 bringing a current of water to the mouth. After a certain period its 

 stalk contracts, its peristome folds inward, and its cilia cease moving. 

 Soon it extends and resumes its normal activity. These alternations of 

 different ways of behaving occur at rather regular intervals, though the ex- 

 ternal conditions remain unchanged. Hydra shows parallel changes of 

 behavior at intervals, under uniform external conditions (p. 189) ; the 

 medusa contracts at intervals, though there is no change in the outer con- 

 ditions, and similar examples could be given for many other organisms. 



