The Reactions of Planarians to Light 149 



sider 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 stim- 

 ulus impinges and only indirectly on the motor organs through 

 their nervous connection with the sense organs. When thus modi- 

 fied the theory of course loses its simphcity and its direct explain- 

 ing 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 connections between the sense organs 

 and the 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 

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

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

 behavior." 



It seems to me that the mechanism by means of which the 

 asymmetrical response is brought about is immaterial, so long as 

 that response can be shown to be the result of asymmetrical stimu- 

 lation. Asymmetrical response might occur either from direct 

 stimulation of the motor organs as was implied in the earlier 

 papers on the infusoria, or by means of a more complex method, 

 consisting of stimulation of the sense organ, transmission to the 

 central nervous system and thence to the motor organs. 



The outcome in either case would fulfill the demands of the trop- 

 ism theory, if asymmetrical response to asymmetrical stimulation 

 be taken as its criterion. In the quotation just cited, the objection 

 that such transmission compels stereotyped behavior is hardly 

 valid, since stereotyped reaction is by no means the only alterna- 

 tive of asymmetrical stimulation. That flatworms do not respond 

 uniformly to directive stimuli cannot be disputed, but that fact 

 does not exclude the possibility of all tropic reaction on their part. 

 The imperfection of response may be simply the result of imper- 



