278 BRADLEY M. PATTEN 



tensity. The reaction to light of constant intensity follows the 

 Bunsen-Roscoe Law. 



11. The evolution of phototaxis is the result of the develop- 

 ment, in connection with photokinesis, of certain factors which 

 modify the action of light on the organism, or indirectly dis- 

 tribute its effects. 



12. The critical factors of phototaxis are: 



a. A distribution of the stimulant in the field such that a 

 change in axial position on the- part of the animal involves a 

 change in the distribution, or intensity, of the stimulant acting 

 on the animal or on its sensitive surfaces. 



b. The presence, within the organism of a mechanism adapted 

 to the reception of differential stimulation and a transmitting 

 and motor apparatus that produces definite locomotor movements 

 proportional to the intensity of the stimulation. 



13. The 'response factor' may be present in the fonn of a bilat- 

 eral mechanism, or in the form of a unilateral mechanism that 

 reacts to both sides of the environment because of a rotational 

 method of locomotion. 



14. If we include under phototaxis any reaction which involves 

 a definite axial orientation with reference to light, the tropism 

 may be regarded as a special form of phototaxis, in which the 

 response depends on the bilateral structure of the mechanism 

 of response. 



