THEORY OF INSTINCTS 191 



animals living at the surface of the ocean are all per- 

 manently or transitorily positively heliotropic (and 

 also often negatively geotropic). Those among them 

 that carry out the daily depth-migrations described 

 above have some other peculiarities which we can 

 only understand if we go somewhat deeper into the 

 theory of animal heliotropism. We have already 

 mentioned that there is a negative as well as a posi- 

 tive heliotropism : negatively heliotropic animals bring 

 their median-plane into the direction of the rays of 

 light, but turn their aboral pole toward the source of 

 light. The difference in the behaviour of negatively 

 and positively heliotropic animals is as follows : If 

 light strikes one side of a positively heliotropic animal, 

 an increase takes place in the tension of those mus- 

 cles which turn the head to the source of light, while 

 in the negatively heliotropic animal under the influ- 

 ence of one-sided illumination a decrease takes place 

 in the tension of the same muscles. The result is 

 that the negatively heliotropic animal is forced to 

 move away from the source of light. Perhaps still 

 another possibility should be considered here, namely, 

 that the light aids the progressive movement when it 

 strikes the oral end of a positively heliotropic animal, 

 while it inhibits the progressive movement when it 

 strikes the aboral end. The opposite may be true of 

 negatively heliotropic animals. This would suggest 

 a further analogy between heliotropism and galvano- 

 tropism. 



Groom and I performed experiments on the larvae 



