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. [231] According to this theory the animal is not directly oriented by the outside force, _e. g._ the light, but selects among its random movements the one which is most “suited” and keeps on moving in this direction. This idea is untenable for most if not all the cases of tropisms and has been refuted by practically all the workers in this field, _e. g._, Parker and his pupils, Bohn, H. B. Torrey, Holmes, Bancroft, Ewald, and others. It is only upheld by Jennings and Mast; and is accepted among those to whom the idea of a physicochemical explanation of life phenomena does not appeal. Torrey and Bancroft (for the literature the reader is referred to Bancroft’s paper, _Jour. Exper. Zoöl._, 1913, xv., 383) have shown directly that the theory of trial and error is not even correct for the organism for which Jennings has developed this idea; namely _Euglena_. 6. It may also be of interest to know that in heliotropism 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 galvanotropism. 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 _Palæmonetes_ 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.[232] It can walk forwards, backwards, or sidewise. Here we can observe directly that the effect of the current consists in altering the tension of the muscles of the legs in such a way as to make it easy for the animal to move toward the anode and difficult to move toward the cathode. Thus if the current be sent sidewise through the animal, say from left to right (Fig. 45), the legs of the left side assume the flexor position, those of the right the extensor position. With this position of its legs the animal can easily move to the left, _i. e._, the anode, and only with difficulty to the right, _i. e._, the cathode. This change in the position of the legs occurs when the animal is not moving at all, thus showing that the galvanotropic movements take place not because the animal intends to go to the anode, but that the animal goes to the anode because its legs are practically prevented by the galvanic current from working in any other way. This is exactly what happens in the heliotropic motions of animals.[233] [232] Loeb, J., and Maxwell, S. S., _Arch. f. d. ges. Physiol._, 1896, lxiii., 121.