On INSTINCT AND WILL IN ANIMALS 113 
‘larvee 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 
larvee of Balanus perforatus, the larvee 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 
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