Concerning the Theory of Tropurns 155 



relegated the earlier views to the limbo of discarded anthropo- 

 morphisms so-called. Without essaying any review of the pros 

 and cons of this problem it may be said that already a reaction 

 has taken place and frankness compels a reconsideration of some 

 of these discarded and discredited views. Such a review has 

 already been made by Jennings so far as it relates to the lower 

 organisms, and his conclusion must, it seems to me, be equally 

 true for many if not most higher animals as well" (Hargitt, p. 



313)- 



What Hargitt has done in one case, Jennings has done in a num- 

 ber of cases. He selects reactions of the type/ I I, shows th?.t 



these reactions do not conform with the theory of tropisms but 

 fails to inform his readers that I had pointed out the existence of 

 this type of reaction and their difference from tropisms long before 

 he did. To give an illustration: In a paper on the "Brain Phy- 

 siology of Worms" I described experiments on Planarians and 

 earthworms, showing that these animals are not or only slightly 

 heliotropic but react to sudden changes in the intensity of light. 

 Such animals become more quiet when the intensity of light is 

 rapidly diminished, become more active when the intensity of 

 light is suddenly increased. The consequence is that places of a 

 relative minimum in the intensity of light act like a trap upon them. 

 To illustrate this effect and the difference of this reaction from 

 thr.t of heliotropic animals I mentioned the following experiment: 

 One half of a glass vessel is covered with black paper, the other 

 half left uncovered. If Planarians or earthworms are put into 

 such a vessel they collect under the covered half. "They come 

 to rest in those regions which are more weakly illuminated than 

 the surrounding areas. The direction ot thfe rays of light is of 

 little consequence."^ On p. 130 of his recent book on the 

 "Behavior of the Lower Organisms," Jennings describes the same 

 experiment for Stentor and shows that these organisms will go 

 from the light into the dark but not in the reverse direction. 

 "The essential point is the running back into the shaded region 



*Loeb: Studies in General Physiology, vol. i; p. 360. Pfliiger's Archiv, vol. 56, p. 247, 1894. 



