242 LIGHT AND THE BEHAVIOR OF ORGANISMS 



The organisms simply do not orient but swim about in an 

 apparently aimless way. If in their wandering they get 

 out of the optimum they orient and return. Under certain 

 conditions the avoiding reaction serves to prevent them 

 from leaving the optimum, just as described under (i) above, 

 but this reaction functions mainly in the process of orienta- 

 tion, as set forth in detail for many different forms in the 

 part of this volume devoted to that subject. 



Practically all the lower motile forms which react to light 

 at all make use of this method of getting into the optimum 

 and remaining there. In this group we may put a few 

 rhizopods, numerous flagellates, some ciliates, several colo- 

 nial forms, a few rotifers, at least one coelenterate, a num- 

 ber of vermes, and some insect larvae. 



The advantage of orientation over random movement in 

 getting to the optimum is evident, for it usually guides the 

 organisms directly there. Conditions can however be so 

 arranged that the greatest amount of light does not come 

 from the portion of the field most highly illuminated. On 

 the floor in front of a window for instance the region of 

 highest illumination is some distance from the window. 

 From this region toward the window the intensity decreases, 

 but the window is still the source of strongest illumination. 

 Under such conditions the organisms may be led astray. 

 If they are positive they proceed in the direction from which 

 the strongest light comes and may thus be carried directly 

 from the optimum. This was the case in a number of 

 Loeb's experiments. It led him and others to the erro- 

 neous conclusion that difference of intensity is of no impor- 

 tance in regulating reactions to light in many forms. They 

 failed to realize that the process of orientation is in itself 

 an attempt on the part of the organism to attain optimum 

 environmental conditions. Its purpose is essentially the 

 same in all organisms without eyes, plants as well as ani- 

 mals, sessile organisms as well as motile. When an organ- 

 ism is oriented it is in its most favorable light conditions 

 as far as the immediate surrounding is concerned, for in 



