140 Herbert Eugene Walter 



B Conclusions with Reference to Planarians 



The behavior of planarians may in general be more satisfac- 

 torily explained by regarding, with Loeb, the intensity rather than 

 the direction of the light as the principal operative factor in light 

 reactions. At the same time there is much evidence that the inten- 

 sity utilized by the organism, is intimately associated v^ith, and 

 powerfully modified by the direction of the light. As a basis for 

 these conclusions the following points will be considered. First, 

 the distinction between direction and intensity; secondly, the way 

 in which directive light modifies the intensity with reference to 

 planarians; thirdly, the action of intensity without the modifying 

 efi^ect of direction, and finally, modifying effects of factors other 

 than light. 



a. The Distinction Between Direction and Intensity 



Theoretically it is plain that light per se with respect to any fixed 

 point, may be regarded in two distinct aspects, namely, that of 

 intensity and that of direction. The intensity of light under ordi- 

 nary circumstances varies inversely as the square of the distance 

 and is independent of the position of the source of light. That 

 is to say, at any points equidistant from its source, light has the 

 same intensity, but the more remote the less is the intensity at 

 any given point. The direction of light, on the contrary, is depend- 

 ent solely upon the position of the source of the light and in no 

 way upon the distance. When intensity and direction are con- 

 sidered with reference not to a fixed point but to an organism 

 presenting three dimensions and made up of differentiated pro- 

 toplasm, the basis of light relations becomes more complex. 

 Light cannot here be treated as a phenomenon per se but must be 

 considered in relation to a differentiating organism. 



It is true that intensity in the case of the organism, as in the case 

 of a fixed point, varies with the distance from the source of the 

 light. A decided difference, however, appears in the case of the 

 organism inasmuch as, owing to its structure, the intensity received 

 by it varies also in accordance with the position of the light. This 



