98 LIGHT AND THE BEHAVIOR OF ORGANISMS 



difference of Intensity between the light in the two beams 

 is not too great, orientation takes place just as described 

 above; but if it is the ventral surface which is exposed, it 

 frequently happens that the organism first becomes directed 

 away from the source of light and then toward it only after 

 repeated reactions. The first step in all these reactions, 

 regardless of how they are induced, is the same. It consists 

 of a bending of the anterior end toward the ventral surface. 

 It has been demonstrated (i) that this reaction can be 

 induced in positive Euglenae by reducing the light intensity 

 of the field without changing the direction of the rays, no 

 matter which surface is illuminated, and (2) that it can be 

 induced without any variation in the light intensity of the 

 field by changing the direction of the rays from one in 

 which the anterior end is illuminated to one in which the 

 dorsal surface is illuminated, or (3) it can be induced by the 

 rotation of the organism on the long axis from a position 

 in which the ventral surface is exposed to one in w^hich the 

 surface containing the eye-spot is exposed. Since the reac- 

 tion under the first condition can be due only to a change of 

 intensity on the whole or some part of the organism it is 

 evident that the reaction under the second and third con- 

 ditions is likewise due to a change of intensity. But since 

 the light intensity of the field is constant under these 

 conditions it is evident that the decrease of intensity must 

 be restricted to a portion of the body and that it must be 

 due to the shading of one part by another owing to the 

 movement of the organisms. Our observations show that 

 a change in the position of the organisms, from one in 

 which the ventral surface is illuminated to one in which 

 the dorsal surface is exposed, causes a reaction. Such a 

 change in position must therefore produce a change of 

 intensity on the sensitive parts of the organism. This may 

 be conceived to be due to the eye-spot's acting as an opaque 

 screen and casting a shadow when it faces the light on some 

 highly sensitive protoplasmic structure located near it 

 (see Fig. 11), or to the location of the more highly sensitive 



