34 LIGHT AND THE BEHAVIOR OF ORGANISMS 



Hydra, however, show that in their case also the relation 

 is the same; at least it seems to me that Trembley's experi- 

 ments cannot be interpreted unless we assume that the 

 progressive movements of Hydra are determined by the 

 direction of the rays of light." 



I have quoted Loeb rather freely in trying to present his 

 views, mainly because he and others have repeatedly main- 

 tained that critics have failed to understand his work, 

 particularly that referring to the cause of orientation and 

 aggregation in regions of certain intensity. These quota- 

 tions together with the discussion presented seem to warrant 

 the following summary statements concerning his work on 

 reactions to light. 



(i) His object was to give a mechanical explanation of 

 behavior in opposition to so-called anthropomorphic ex- 

 planations of Bert, Graber and others. 



(2) He proposed to do this by showing that the reactions 

 in animals, especially those due to stimulation by light, are 

 governed by the same law as those in plants. 



(3) He accepted the explanation of orientation in plants 

 given by Sachs and states his theory correctly. Loeb's 

 conclusions however do not support this theory. He 

 confuses ray direction through the tissue with ray direction 

 in the field and difference of intensity on the surface of the 

 organism with diversity of intensity in the field. 



(4) Loeb failed to consider the effect of difference in 

 sensitiveness to light between the posterior and anterior 

 ends of animals and the effect of change in axial position on 

 the relative illumination of these ends. 



(5) His experimental evidence does not prove that the 

 direction of light rays functions in orientation except in so 

 far as it may produce difference of intensity on the surface 

 of the organism; nor does it prove the absence of sensation 

 in orientation. 



(6) In 1888 Loeb held that orientation in animals is 

 controlled by the direction in which the rays of light pass 

 through the tissue. From 1889 to 1903 he advocated the 



