282 Animal Instincts and Tropisms 



The few young leaves on top of a twig are quickly 

 eaten by the caterpillar. The light, which saved its 

 life by making it creep upward w^here it finds food, 

 would cause it to starve could it not free itself from the 

 bondage of positive heliotropism. The animal, after 

 having eaten, is no longer a slave of the light, but can 

 and does creep downward. It can be shown that a 

 caterpillar, after having been fed, loses its positive 

 heliotropism almost completely and permanently. If 

 we submit unfed and fed caterpillars of the same nest 

 contained in two different test-tubes to the same 

 artificial or natural source of light, the unfed will 

 creep to the light and stay there until they die, while 

 those that have eaten will pay little or no attention 

 to the light. Their sensitiveness to light has dis- 

 appeared ; after having eaten they become independent 

 of light and can creep in any direction. The restlessness 

 which accompanies the condition of hunger makes the 

 animal creep downward — which is the only direction 

 open to it — where it finds new young leaves on which 

 it can feed. The wonderful hereditary instinct, upon 

 which the life of the animal depends, is its positive 

 heliotropism in the unfed condition and its loss of this 

 heliotropism after having eaten. The latter pheno- 

 menon is in harmony with the experiments which show 

 that the heliotropism of certain species of Daphnia 

 disappears when the water becomes neutral. 



And finally it may be pointed out that the majority 



