THEORY OF INSTINCTS 189 



to move. But the direction of their movements is 

 determined by the light. Out-of-doors, where the 

 diffused light strikes the animal on all sides, every ray 

 of light can be resolved into a horizontal and a verti- 

 cal component. The horizontal components destroy 

 each other, and only the effect of the vertical compo- 

 nents remains. Hence the animals are forced, as a re- 

 sult of their positive heliotropism, to crawl upwards 

 until they reach the tip of a branch. They are held 

 there by the light. The chemical stimuli which are 

 transmitted to the animal by the young buds produce 

 the eatino- movements. In this instinct, which is 



o 



necessary for the preservation of life, we have an- 

 other instance of simple positive heliotropism, and the 

 central nervous system plays only the role of a proto- 

 plasmic connection between the skin and contractile 

 tissue, which in plants is performed just as success- 

 fully by undifferentiated protoplasm. 



We have seen, however, that these same caterpil- 

 lars leave the tips of the branches as soon as they 

 have eaten and crawl downward. Why does the 

 light not hold them on the highest point permanently ? 

 My experiments showed that these caterpillars are 

 only positively heliotropic as long as they remain un- 

 fed ; after having eaten they lose their positive helio- 

 tropism. This is not the only instance of this kind, 

 for I have found a series of facts which show that 

 chemical changes influence the irritability of the ani- 

 mal toward light. We can imagine that the taking 

 up of food leads to the destruction of the substances 



