270 Animal Instincts and Tropisms 



the legs of the normal animal) no longer flows symmetri- 

 cally to the steering wheel, and the machine turns 

 towards the light. In this way the machine follows 

 a lantern in a dark room in a way similar to that of a 

 positively heliotropic animal. Here we have a model 

 of the heliotropic animal whose purely mechanistic 

 character is beyond suspicion, and we may be sure 

 that it is not ; 'fondness' for light or for brightness 

 nor will-power nor a method of "trial and error" which 

 makes the machine follow the light. 



6. It may also be of interest to know that in helio- 

 tropism the motions of the legs are automatically 

 controlled by the chemical changes taking place in 

 symmetrical elements of the retina. In order to prove 

 this point we will turn to the phenomenon of gal- 

 vanotropism. The galvanic current forces certain 

 animals to move in the direction of one of the two 

 electrodes just as the light forces the heliotropic animals 

 to move towards (or from) the source of light. The 

 change in the concentration of the ions at the 

 boundary of the various organs, especially the nerves, 

 determines the galvanotropic reactions. When the 

 shrimp Pal&monetes is put into a trough with dilute 

 salt solution through which a current of a certain 

 intensity flows, the animal is compelled to move 

 towards the anode. 1 It can walk forwards, back- 

 wards, or sidewise. Here we can observe directly 



1 Loeb, J., and Maxwell, S. S., Arch.f. d. ges. Physiol., 1896, Ixiii., 121. 



