122 THE BEHAVIOR OF I.OWKR ORGANISMS. 



(1) We have seen that stimuli do unquestionably cause changes in 

 physiological condition. This is demonstrated by the fact that after a 

 stimulus has occurred and ceased to act, the organism reacts differently 

 to the same or other stimuli. (For examples see pages 112, 117-) 



(2) We have seen that the changes in physiological condition do un- 

 questionably cause definite movements, of exactly the sort that we are 

 accustomed to call reactions to stimuli. (Contraction of Hydra or Vor- 

 ticella, etc. ; see p. 120.) 



These two facts give a solid foundation for the above view of 

 reactions to stimuli, and, indeed, it seems to me, raise a presumption 

 that reactions to stimuli are, as a rule, brought about in the way 

 described. Further evidence in favor of this view is as follows : 



(3) In the paper which precedes the present one we have demon- 

 strated that, in the Infusoria and Rotifera at least, the action of stimuli 

 is not directly on the motor organs of that part of the body on which 

 the stimulus impinges. The organism reacts as a whole, and in a way 

 that is not explicable even on the assumption of a definite plan of ner- 

 vous interconnection between the regions stimulated and the motor 

 organs, an assumption that is, of course, in any case not allowable for the 

 Infusoria. Such reactions cannot be explained otherwise than as due 

 to changes in the physiological condition of the organism as a whole. 

 Further, evidence was given to show that the reactions of higher organ- 

 isms are in many cases equally inexplicable as a result of direct action 

 of the stimulus on the motor organs. 



Only in the reaction of some organisms to the constant electric cur- 

 rent did we find such conditions fulfilled as permit an explanation of a 

 part of the phenomena on the theory of the direct action of the agent 

 on that part of the body on which it impinges, in accordance with the 

 theory of tropisms. Other features of the reaction to this stimulus (in 

 many cases the determining ones) are only explicable on the theory 

 that they are due to the physiological state of the organisms as a 

 whole, induced by the stimulus (see p. 100). This shows that we may 

 find at any time these two methods of action mixed, or perhaps eithei 

 one separately. But the reaction to the electric current is the only one 

 out of the reactions to a multitude of agents, that, in the Infusoria, has 

 been shown to have this additional feature reactions of different parts 

 of the body in opposed ways. The reaction to the electric current is 

 thus of the very greatest interest, not because it stands as type for 

 reactions in general, but for exactly the opposite reason, because it 

 presents factors which are not known to occur in other reactions. 



(4) The view that reactions to stimuli take place through the inter- 

 mediation of changes in the physiological condition of the organism as 

 a whole is further reinforced by the fact, set forth above, that it is only 



