128 DYNAMICS OF LIVING MATTER 



cally effective rays. Consequently the heads of the animals are turned 

 automatically toward the room side of the test-tube, just as would be 

 the case with the tip of a positively heliotropic stem under similar cir- 

 cumstances. They move toward the room side, but cannot go far, 

 because, as soon as the unfiltered light from the window again strikes 

 them, the latter being stronger, the head is now turned automatically 

 toward the window again, and they move toward the latter until they 

 get under the red glass; and now the whole process repeats itself. It 

 thus happens that the animals gather in front of a at the limit between 

 the uncovered part of the test-tube and the red glass, where they keep 

 on moving in a narrow circle. Similar results are obtained if an opaque 

 body is substituted for the red glass. 



If the whole test-tube be covered with red glass, the animals still 

 show a slight tendency to move toward the window side of the tube; 

 but their motions are no longer in a straight line as before, but more 

 irregular. They finally, however, gather at the window side of the 

 tube; but it requires much more time before they gather there than 

 if the test-tube is covered with blue glass. Red light acts upon these 

 animals like weak light ; this can be shown directly by experiments 

 with daylight towards sunset, or in a comparatively dark room. 



It seems, therefore, that these phenomena are indeed the same as 

 those in positively heliotropic sessile animals and plants; and we may 

 designate such animals whose heads are turned automatically toward 

 the light, when the light strikes them from one side, as positively helio- 

 tropic. It should be observed that the essential feature in these re- 

 actions is the compulsory turning of the head by the light, which leaves 

 the animal no choice, making all the caterpillars of Porthesia or all 

 the plant lice of the same culture behave exactly alike, just as in the 

 case of a magnet all the pieces of iron are compelled to behave alike. 

 This compulsory character of heliotropic reactions seems to have been 

 overlooked by those anthropomorphic opponents of the theory of animal 

 heliotropism, who offer the objection that we can turn toward the win- 

 dow voluntarily. This objection is about as absurd as if we should 

 argue against the existence of magnetism because we can turn and move 

 toward a magnet without being made of iron. 



We not only find animals whose heads bend or turn toward the 

 light, which consequently must move toward the source of light, if 

 they move at all, but also animals whose heads bend or turn away 

 from the source of light. We call such animals negatively heliotropic. 

 Such negatively heliotropic animals are, e.g. Gammarus pulex, a fresh- 

 water Crustacean, the larvae of the house fly, when fully grown and 

 ready to go into the pupa stage, the larvae of Limulus in a certain stage, 



