Such constancy of quantitative results is only possible where we are dealing with purely physicochemical phenomena or where life phenomena are unequivocally determined by purely physicochemical conditions.
5. It seems difficult for some biologists, even with the validity of the Bunsen-Roscoe law proven, to imagine that the movements of the animals under the influence of light are not voluntary (or not dictated by the mysterious "trial and error" method of Jennings).[231] But one wonders how it is possible on such an assumption to account for the fact that the angle of deflection of the larva of the fly when under the influence of two lights of different intensities should be always the same for a given difference in intensity; or why the time for curvature in _Eudendrium_ should vary inversely with the intensity of illumination. It is, however, possible to complete the case for the purely physicochemical analysis of these instincts. John Hays Hammond, Jr., has succeeded in constructing heliotropic machines which in the dark follow a lantern very much in the manner of a positively heliotropic animal. The eyes of this heliotropic machine consist of two lenses in whose focus is situated the "retina" consisting of selenium wire. The two eyes are separated from each other by a projecting piece of wood which represents the nose and allows one eye to receive light while the other is shaded. The galvanic resistance of selenium is altered by light; and when one selenium wire is shaded while the other is illuminated, the electric energy (supplied by batteries inside the machine) which makes the wheels turn (these take the place of the legs of the normal animal) no longer flows symmetrically 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.
[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_.