234 LIGHT AND THE BEHAVIOR OF ORGANISMS 



in orientation of any organisms without image-forming eyes 

 except in so far as it may produce difference of intensity on 

 different parts of the organism. Those organisms which 

 orient by sight are of course guided by the direction of the 

 rays reflected from the object toward which they go. 



(10) There is no evidence indicating that symmetrically 

 located points on the surface must be struck by light at the 

 same angle when organisms are oriented, as Loeb maintains 

 (1905). The only organisms which do not travel toward or 

 from a point located anywhere between two sources of light 

 are those with highly developed image-forming eyes. Such 

 forms, as far as experimental evidence indicates, always go 

 toward one or the other of the two sources of light. With 

 but one functional eye these animals still orient fairly 

 accurately, although under such conditions symmetrically 

 located points are unequally stimulated. 



(11) There is no conclusive evidence, except perhaps in 

 animals with image-forming eyes, showing that light acts 

 continuously as a directive stimulus, that symmetrically 

 located sides are continuously stimulated, — equally when 

 the light intensity on them is equal, unequally when it is 

 not, and that this regulates orientation by regulating the 

 rate of motion of the locomotor apparatus on the two sides 

 as is demanded by the theories of De Candolle, Loeb, Ver- 

 worn, Davenport and Radl. 



(12) There is no conclusive evidence showing that orien- 

 tation in light is ever due to tropic reactions in any organ- 

 isms, if the definitions of tropisms given by Loeb, Verworn, 

 or Radl are used as criteria. 



(13) All organisms that respond to light at all respond 

 to changes of intensity. In some the response to such 

 changes results in orientation, in others it does not. They 

 are all tinterschiedsempfindlich, in accord with Loeb's defi- 

 nition of this term. They all respond to time rate of 

 change in light intensity. The idea of reactions to change 

 of intensity is however not original with Loeb, as is some- 

 times assumed. The explanations of reactions to light 



