I20 THE BEHAVIOR OF LOWER ORGANISMS. 



this matter, describing especially the interference of contact with the 

 reaction to heat and to electricity. In the second of these contributions 

 (p. 32), we have seen that attached Stentors do not react at all to 

 light. Physically considered, there is no necessary opposition between 

 the action of contact and the action of the other stimuli named. We 

 must conclude then that contact with solids so alters the physiological 

 condition of the organism that it no longer reacts to the other stimuli. 



SPONTANEOUS MOVEMENTS. 



Further, we find that alterations in physiological condition may cause 

 definite movements, which take place without external stimulus. Such 

 are the movements which we call spontaneous. As an example of this 

 we may take the case of Hydra. If an undisturbed green Hydra is 

 observed continuously, it is found to contract and again to extend with- 

 out visible cause every i^ to 2 minutes. Thus, it remains at rest for 

 a period of, say, i^ minutes. Its physiological condition at this time 

 we may call X. At the end of this period it contracts. Since the 

 external conditions have not changed the Hydra itself must have 

 changed, otherwise it would continue at rest. The physiological condi- 

 tion A^ passes into the condition T^ and the Hydra as a result contracts. 

 This contraction is, of course, exactly the reaction given as a response 

 to most stimuli in Hydra. In Vorticella we find similar spontaneous 

 contractions at intervals, essentially as in Hydra. Cases of movements 

 in the lower organisms that are inaugurated by internal changes in 

 condition could, of course, be multiplied indefinitely. For our present 

 point of view it is of importance to recognize clearly the fact that a 

 change in physiological condition may, by itself, cause exactly the same 

 behavior that at other times appears as a response to external stimuli. 



METHODS OF CAUSING CHANGES IN PHYSIOLOGICAL 

 CONDITION. 



Changes in physiological condition are thus evidently brought about 

 in a number of different ways. We may attempt to summarize here 

 the different methods which appear to exist in the lower organisms. 



(i) A single simple stimulus may bring about a change in physiologi- 

 cal condition. This is proved by the fact that the organism after it has 

 received a single stimulus may react differently from its previous 

 method. Thus, Stentor reacts to a single touch, but after this single 

 touch it may no longer react when touched in the same way again ; 

 or it may react in a different manner. It is probable, further, that the 

 first reaction to a single simple stimulus is to be considered due to a 

 change in physiological condition produced by this stimulus. 



(2) Repetition of the same stimulus may cause a change in physio- 

 logical condition such as is not produced by a single stimulus. This 



