THEORY OF INSTINCTS 179 



continued long enough, all the animals collect at the 

 positive pole. When this process is observed with- 

 out a careful analysis, it seems as though these Crus- 

 taceans possessed the instinct to move toward the 

 anode, just as the moths possess the instinct to move 

 into the flame. The flight of the moth into the flame 

 is in reality only the result of a tropism, heliotropism, 

 which differs from galvanotropism chiefly in that the 

 rays of light take the place of the curves of the current. 

 The reader knows that certain plants when exposed 

 to the light on one side, for instance, when cultivated 

 at a window, bend their tip toward the window until 

 the tip of the stem is in the direction of the rays 

 of light. The tip then continues to grow in the 

 direction of the rays. We call this dependence of 

 orientation on light heliotropism. We speak of 

 positive heliotropism when the organ bends towards 

 the source of light, of negative heliotropism when 

 the organ bends away from it. It is generally 

 assumed that the light has a chemical effect in these 

 cases. 



The relations of symmetry in plants and animals 

 play an important part in these phenomena. We 

 will take, by way of illustration, the stem of a 

 hydroid, Eudendrium that is being raised near a win- 

 dow. I have found that it bends toward the window 

 like a positively heliotropic plant under the same 

 conditions. The process may be described as follows : 

 The light strikes the Eudendrium-stem from the 

 side. A contraction of the protoplasm on that side 



