256 LIGHT AND THE BEHAVIOR OF ORGANISMS 



stant intensity, direction through the tissue, or change of in- 

 tensity. We have however clearly demonstrated in Part II 

 of this volume that there is no experimental evidence 

 proving that direction of the rays through the tissue or 

 difference of light intensity on different parts of the body 

 is functional in the orientation of any organism, except- 

 ing in so far as it may cause changes of intensity on the 

 organism. 



Photopathy, or Unterschiedsempfindlichkeit, as Loeb 

 calls it, is due to change of intensity according to Loeb, to 

 difference of intensity on the organism according to Daven- 

 port, and to difference of intensity in the field according to 

 Yerkes. It is at once evident however that there may be 

 agreement in these apparently different statements. If an 

 organism is so illuminated that the light intensity on dif- 

 ferent parts of the body differs, every moment is almost 

 certain to cause changes of intensity on some part of the 

 organism, and if the intensity is not uniform in the field an 

 organism, of course, cannot move about without causing 

 changes of intensity on its surface. It may be, then, that 

 the fundamental factor involved in photopathy, according 

 to all of these authors, is change of intensity. 



It is thus evident that the reactions grouped under photo- 

 tropism and photopathy by the authors mentioned may all 

 depend upon changes of intensity, and that the two phe- 

 nomena may be fundamentally the same. If this be true, 

 then the classification of reactions to light as photopathic 

 and phototropic is without a foundation. Are there, then, 

 no differences in these reactions which will serve as a basis 

 for a classification? 



6. Reclassification of Reactions to Light 



Reactions to light may conveniently be classified (i) on 

 the basis of the character of the stimulus, and (2) on the 

 basis of the fundamental causes of the response. 



(i) On the basis of the character of the stimulus we 



