42 LIGHT AND THE BEHAVIOR OF ORGANISMS 



course the rays take through the organism " (p. 210). He 

 assumes that changes of Hght intensity do not result in 

 orientation but that stimulation caused by such changes 

 may determine the position of organisms in the field in 

 some such way as described by Engelmann. He says 

 (p. 211), " Two kinds of effects are produced by light: 

 one by the direction of its ray — phototactic; the other by 

 the difference in illumination of parts of the organism — 

 photopathic." 



Holt and Lee (1901) studied the behavior of Stentor 

 coeruleus in an aquarium receiving light through a prism 

 similar to the one used by Oltmanns, and found that the 

 animals collected at the darker end. In conclusion they 

 support Verworn's theory; but from the preceding dis- 

 cussion of the effect of the prism on the direction of 

 rays it is evident that the validity of this conclusion is 

 questionable. 



Radl's work on reactions to light was almost entirely 

 confined to the Crustacea and insects. In 1903 after a 

 rather extensive review and criticism of the results and 

 theories of others, and an exposition of his own work, he 

 arrived at two conclusions which are of interest in this 

 connection. One has reference to the mechanics of orien- 

 tation, the other to the explanation of negative reactions. 



His theory of orientation is based on the conception that 

 change in the direction of motion is brought about by 

 unequal stimulation of symmetrical points on the surface 

 of the organism, a conception which lies at the foundation 

 of all the theories thus far presented, excepting that of 

 Sachs and the first one of Loeb. While all of these differ 

 in some respects, they are alike in that they assume the 

 external agent to act through the effect of chemical changes 

 in the organism. Radl proposes to explain orientation as 

 the direct effect of light on the organism. He says (1903, 

 p. 151) : " Alle Autoren, welche bisher dieses Thema beriihrt 

 haben, haben an indirekte Wirkungen des Lichtes gedacht, 

 dass namlich durch dasselbe chemische Veranderungen 



