272 BRADLEY M. PATTEN 



5. Orientation to light from two sources depends on the relative 

 amount of stimulation received by symmetrically located sensitive 

 areas. This is shown by : perpendicular orientation to equal lights ; 

 deflection toward the weaker of two unequal lights; circus move- 

 ments when one side of the sensitive areas is blackened; the 

 presence of a natural asymmetry of response which may be coun- 

 terbalanced by a corresponding inequality in bilateral stimulation. 



6. An arrangement of bilateral sensitive areas may be postu- 

 lated whereby stimulation on opposite sides of the animal may be 

 equalized by a change of axial position. This arrangement ac- 

 cords with the facts so far as they are known. 



7. The phototactic response of the blowfly larva depends to a 

 large extent on the stimulating effect of constant intensity. 

 The reaction to light of constant intensity follows the Bunsen- 

 Roscoe law. 



c. Analysis of factors involved in phototaxis. In the preliminary 

 discussion of the relation of directive to non-directive light re- 

 actions, the interpretation was advanced that phototaxis had 

 been evolved from photokinesis by the development, in connec- 

 tion with the latter, of certain factors which modify the action 

 of the light on the organism, or indirectly distribute its effects. 

 In the following section, an attempt has been made to analyze 

 more closely certain of the factors involved in photokinesis and 

 phototaxis, and to ascertain, as far as possible, their relative 

 effectiveness. 



The distribution of the stimulus in the field and on the sensitive 

 surfaces of the animal, to a large extent determines the nature 

 of the response. In a field uniformly illuminated from above 

 there is, so far as we know, but one type of reaction, undirected 

 activity which is maintained till muscular fatigue ensues or 'ac- 

 climatization' results. But if the field is not of uniform intensity 

 and there are regions in it where stimulation does not take place, 

 the organisms will sooner or later gather in those regions simply 

 because they are not stimulated enough to move away from 

 them. Under either mode of illumination, the reaction of the 

 animal is of the photokinetic type; the condition which deter- 

 mines whether or not the aggregation takes place, lies not in the 



