ON INSTINCT AND WILL IN ANIMALS 113 



larvae of Musca vomitoria are energetically negatively helio- 

 tropic only when fully grown, etc. In ants sensitiveness to 

 light is, as I have shown, connected with sexuality. The 

 males are more sensitive than the females ; at the time of the 

 nuptial flight the males and females become energetically 

 heliotropic, while the so-called workers remain practically 

 uninfluenced by the light. There must also be mentioned 

 the change which occurs in the sense of heliotropism of many 

 animals in different stages of their development. The full- 

 grown larva of Musca vomitoria is negatively heliotropic; 

 yet the sexually mature insect is positively heliotropic. 

 Such a behavior is quite widely distributed. 



Finally, it is not infrequently possible to change at will, 

 through the influence of light, positively heliotropic animals 

 to negatively heliotropic animals, and the reverse. The 

 larvse of Balanus perforatus, the larvre of certain worms, and 

 indeed a large number of other animals, become positively 

 heliotropic when they are left in the dark for a long time. 

 If they are brought into light of sufficient intensity, they 

 become negatively heliotropic after a time, and this the more 

 quickly the more intense the light. 



We do not, therefore, always meet with simple conditions 

 in analyzing the causes which determine the "voluntary" 

 movements of an animal; but, however complicated they 

 may be, the "voluntary" movements of animals are never- 

 theless, as our experience indicates, always unequivocally 

 determined only by such circumstances as determine also 

 the movements of bodies in inanimate nature. 



5. To be sure, many of the authors who oppose my con- 

 clusions would protest if it were said of them that they hold 

 the "will" to be something which cannot be explained on 

 physical or chemical grounds. But if some physical agency is 

 pointed out which prescribes unequivocally the orientation 

 of an animal body or the direction of its movements, which 



