4 LIGHT AND THE BEHAVIOR OF ORGANISMS 



movement from a region of one intensity to that of another. 

 And all this is dependent upon the direction of the rays in 

 the field whereas ray direction through the organism could 

 have no such effect. It is evident then that the question 

 " intensity difference or ray direction" may mean any one 

 of several things. This loose way of stating the problems 

 has led to much confusion. 



Let us then, first of all, attempt to get a clear under- 

 standing of the questions involved in the reactions to light. 

 With this in view we shall consider the origin and develop- 

 ment of ideas concerning the movements in general of 

 plants and animals, and those induced through stimulation 

 by light in particular. 



