84 Studies in Animal Behavior 



an important role in producing those orienting move- 

 ments known as compensatory motions where the 

 latter are, as in many insects, dependent upon the 

 organs of vision. Mast, who is an opponent of the 

 view that phototaxis is the result of light acting as 

 a constant stimulus, is inclined to attribute orienta- 

 tion very largely to intensity changes. In speaking 

 of orientation in many lower forms, he says: "In 

 many of these forms orientation is undoubtedly, and 

 in all it is probably, a response to change of light 

 intensity on some part of the organism. At any 

 rate it has in no instance been demonstrated that 

 it is, as Loeb states, 'a function of constant intensity,' 

 that orientation to light is like orientation to an elec- 

 tric current." There is much, I believe, that indi- 

 cates that light stimulates quite apart from the 

 shocks due to variations in its intensity, but the 

 question as to the relative potency of the two factors 

 involved, which Mast has done well to bring into 

 greater prominence, is one that can be answered 

 only by experiment. In the ordinary movements of 

 animals to and from the light both these factors 

 are free to come into play. The natural method 

 of attacking the problem, therefore, is to exclude 

 one of the possible agencies, and then to observe 

 the effect of the other alone. 



Recently a series of experiments was carried on 

 by Miss McGraw and myself, in which animals were 

 exposed to illumination which was rendered constant 

 so far as this could be done in the case of an ac- 



