Animal Instincts and Tropisms 269 



But one wonders how it is possible on such an assump- 

 tion to account for the fact that the angle of deflection 

 of the larva of the fly when under the influence of two 

 h'ghts 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 re- 

 presents 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 elec- 

 tric energy (supplied by batteries inside the machine) 

 which makes the wheels turn (these take the place of 



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. ZooL, 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, 



