104 THE BEHAVIOR OF LOWER ORGANISMS. 



with the requirements of the tropism theory, and this agreement is due 

 to an effect on the organism in the production of which the electric 

 stimulus is unique, so far as known. In none of the reactions which 

 have been thoroughly worked out, except partially in the reaction to 

 the electric current, are the phenomena to be explained on the view 

 that the result is due to the direct action of the stimulating agent on the 

 motor organs of the part of the body on which it impinges. In the 

 reactions to mechanical stimuli and to light, and in the reactions to 

 the electric current in some animals, this view is absolutely disproved. 

 The direction in which the organism turns is, in all the well known 

 reactions of unicellular organisms and rotifers (except in a portion of 

 the reactions to the electric current), determined by an internal factor, 

 and predictable from the structure of the organism without any know- 

 ledge of the direction from which the stimulating agent is to come. 



We should perhaps consider here a modification of the original form 

 of the tropism theory that has been proposed by some authors. This 

 is in regard to the assumption that the stimulating agent acts directly 

 on the motor organs upon which it impinges. For this it is sometimes 

 proposed to substitute the view that the action of the stimulating agent 

 is directly on the sense organs of the side on which the stimulus im- 

 pinges, and only indirectly on the motor organs through their nervous 

 connection with the sense organs. When thus modified the theory, of 

 course, loses its simplicity and its direct explaining power, which made 

 it so attractive. In order to retain any of its value for explaining the 

 movements of organisms, it would have to hold at least that the connec- 

 tions between the sense organs and motor organs are of a perfectly 

 definite character, so that when a certain sense organ is stimulated a 

 certain motor organ moves in a certain way. When we find, as we 

 do in the flatworm (see the following paper), that to the same stimulus 

 on the same part of the body, under the same external conditions, the 

 animal sometimes reacts in one way, sometimes in another, the tropism 

 theory, of course, fails to supply a determining factor for the behavior. 



But can we explain the reaction methods of the infusoria and other 

 animals which, as set forth above, are inconsistent with the tropism 

 theory in its original form, on the basis of the modification of this 

 theory, set forth in the last paragraph? While in the infusoria the 

 assumption of nervous connections, etc., is inadmissible, we may 

 waive that objection and answer the question proposed from an analysis 

 of the obsei-ved phenomena. In.Stentor or in Stylonychia, for example, 

 we find that the usual reaction to all classes of stimuli is by backing, 

 then turning toward the aboral side ; in some of the rotifers by turning 

 toward the aboral (dorsal) side. To simplify matters, we may take 

 into consideration only the turning toward the aboral side. This turn- 



