The Reactions of Planarians to Light 151 



from the stimulus. In the one case it is accomplished by direct 

 reflex without more ado; in the other, only after a considerable 

 waste of energy in inconsequential vermiculations" (Holmes, 

 '05a, p. no). 



It is at least conceivable that under the tropism schema, as the 

 nervous differentiation of an animal becomes more complete, the 

 ability of the organism to interfere with and modify its machine- 

 like responses to external stimuli might also increase, resulting 

 in a flexibility of behavior which would present quite as much 

 variation for natural selection to act upon as that evolved by the 

 trial and error method. This point of view by no means denies 

 that trial and error is the usual "method of intelligence" (C. L. 

 Morgan '00, p. 139). It is simply an attempt to recognize in the 

 method of tropism also one of the possibilities of evolutionary 

 progress in behavior and as such holding a higher position in the 

 scale of evolutionary methods than trial and error by motor reflexes. 



It has been shown (p. 143) that planarian responses of an appar- 

 ently asymmetrical character may occur as a result of symmetri- 

 cal stimulation. Similar instances in the case of planarians have 

 also been demonstrated by Mast ('03) with reference to thermal 

 stimuli. This, however, is no exception to the validity of the tro- 

 pism theory, in which asymmetrical responses result from asym- 

 metrical stimulation. Because a planarian may make an appar- 

 ently phototropic response when subjected to symmetrical stim- 

 ulation, is not evidence against the supposition that the usual 

 phototropic response is due to asymmetrical stimulation. 



The "wigwag" movements of planarians, to which repeated 

 reference has been made in the preceding pages, resemble super- 

 ficially the "random movements" of the earthworm as described 

 by Holmes. They do not, however, ordinarily appear to be the 

 basis of trial and error selection resulting in orientation, since 

 in a majority of cases, after a worm halts and makes wigwag 

 movements it continues on its way without a change of direction. 

 The movements of Bdelloura Candida, as shown in Fig. 12, 

 form an exception to ordinary planarian behavior in this respect. 



As a rule wigwag movements are probably occasioned by a 

 general disturbance arising from some stimulation which throws 



