144 Journal of Comparative Neurology and Psychology. 



In Experiments 4 and 5, the animals moved from a region of 

 higher Hght intensity to one of lower. Now from this the author 

 concluded that difference in intensity does not cause orientation, 

 for if it did, the animals, being positive, would remain in the region 

 most highly illuminated. 



In discussing the effect of difference in light intensity it is 

 necessary to define the sense in which this is meant. There is a 

 vast difference between the difference in light intensity in a 

 given field and t,ie difference in intensity on different areas of 

 the surface of a particle in the field. For example, hold an 

 opaque piece of paper in direct sunlight so that the rays strike 

 it at right angles and you will find almost an infinite difference in 

 the light intensity on the two sides, but remove the paper and you 

 will find that the intensity difference in the field is actually infini- 

 tesimal. It is evident then that an organism can move from 

 regions of higher to regions of lower light intensity in a field pro- 

 duced by apparatus arranged as represented in Figs. 10 and 1 1, and 

 still have the anterior end constantly more highly illuminated than 

 the posterior. Loeb evidently did not recognize this in the experi- 

 ments cited above, for he accepts the theory of Sachs, who 

 ('87, p. 695) defines his position very clearly, as follows: "I 

 came to the conclusion that in heliotropic curvatures, the impor- 

 tant point is not at all that the one side of the part of the plant is 

 illuminated more strongly than the other, but that it is rather the 

 direction in which the rays pass through the substance of the plant." 



In moving toward the window in the test tubes arranged as 

 represented in Figs. 10 and 11, the anterior end of the animal 

 was very likely more highly illuminated than the posterior. On 

 the assumption that difference in intensity on the surface of the 

 organism causes orientation, the larvae would consequently be 

 expected to move toward the window. I can, therefore, see 

 nothing in these experiments which in any way indicates that 

 difference in light intensity on the surface of the body, regardless 

 of the direction of the light rays, is not the cause of orientation. 



9. ORIENTATION OF SEGMENTS. 



In working on Volvox it was noticed that colonies with various 

 portions missing still appeared to respond to stimulation by light. 

 Such colonies were most frequently found after heavy rain storms 

 or other rather violent disturbances. On July 28, 1905, a colony 



