The Reactions of Planarians to Light 37 



VI General Conclusions 138 



1 Direction or intensity 138 



A Historical 138 



B Conclusions with reference to planarians 140 



a The distinction between direction and intensity 140 



b The modifying influence of direction 141 



c Instances of behavior due to intensity alone 142 



d The modifying effect of other factors 144 



Summary 145 



2 Trial and error, or tropism 146 



Summary 153 



3 Adaptation 153 



Summary 155 



VII Bibliography 155 



I Introduction 



Light is one of the physical factors which influence the behavior 

 of organisms. The great majority of living things are normally 

 subjected to regular periodic changes in the amount of light to 

 which they are exposed during the alternation of day and night. 

 In addition to these constant periodic changes, there are innu- 

 merable irregular gradations in both the intensity and the character 

 of the light naturally acting upon any organism. An agent of 

 such wide range and almost universal influence as light ought, 

 therefore, when properly analyzed, to prove of material service in 

 interpreting the behavior of animals and plants. The depend- 

 ence upon light of animals provided with organs of sight, is self 

 evident. The direct bearing, too, of light upon chlorophyllaceous 

 plants in the manufacture of their food substance, is plain. But 

 how far light plays a direct part in the life of non-photosynthetic 

 plants and of animals which cannot "see," is less clear. 



Although possessing eyes, it is very probable that planarians 

 are unable to see in the sense of distinguishing shapes, and it is 

 questionable how far they can distinguish between even large 

 regions of different light intensity. 



The object of the following paper is to examine the relation of 

 light to animal behavior as applied to certain planarians. 



