xviii INSTINCTS AND TROPISMS 349 



tion of shaded regions at a distance and a tendency to 

 make for them, which is not merely a matter of photopathy 

 as that term is usually employed. It is possible that the 

 latter peculiarity is not a primary instinctive response, but a 

 habit acquired by experience. It is known that the frog is 

 capable of forming simple associations, and it may learn in 

 the course of its experiences with light and shade to connect 

 moving toward the latter with a sense of comfort or respite 

 from the disagreeable effects of strong light. 



The reactions of the frog to light are influenced by temper- 

 ature to a marked degree. At 25 C. the positive response 

 becomes considerably accelerated, the frog moving more 

 quickly and more directly toward the light end of the box. 

 Above 30 C. the movements become irregular, owing to 

 the predominant effect of heat. When the temperature is 

 lowered, the positive reaction becomes less decided, and 

 according to Miss Torelle, when a temperature of 8° C. is 

 reached, the animal becomes negatively phototactic both in 

 air and in water. The evidence cited in favor of this con- 

 clusion is, however, not convincing, inasmuch as other reac- 

 tions, such as the tendency to dive downward and to crawl 

 under objects, are evoked when the temperature is lowered 

 to this point. These relations were brought about both 

 when light came in from above the aquarium, and when the 

 top and the upper two thirds of the aquarium were covered 

 by an opaque cloth. 



As in all animals thus far investigated, it is the blue and 

 violet rays that are the most influential in evoking the photo- 

 tactic response ; the effectiveness of the other colors of the 

 spectrum diminishes in order from blue to red. If frogs are 

 placed in a box illuminated through one end with blue light 

 and through the other with red, they soon gather at the blue 

 end. If they have the choice between yellow and green, 



