HELIOTROPIC REACTIONS — ANIMALS AND PLANTS 219 



retinas are replaced by selenium wire, these machines following 

 a lantern in the dark in the same way as a positively heliotropic 

 animal. 



It was easy to interpret the phenomena found by Graber from 

 this heliotropic viewpoint. The botanists had long ago shown 

 that positively heliotropic plants bend readily to the light when 

 behind a blue screen, while they do not do so or only very slowly 

 when behind a red screen; from which they concluded that the 

 rays going through a blue screen had a higher heliotropic efficiency 

 than the rays passing through a red screen. 



If the animals were, as Loeb stated, merely positively or nega- 

 tively heliotropic the light going through blue glass should act 

 like more intense light than that going through red glass, and 

 hence negatively heliotropic animals should gather in the red, 

 positively heliotropic animals in the blue. He could show by a 

 series of experiments that this statement was correct. In this 

 way, purely objective methods and explanations were given for 

 the arbitrary assumption of Graber and the other anthropo- 

 morphic biologists that animals moved to or from the light 

 because they were 'fond' of light. 



The advantage of this change in viewpoint lies in the fact that 

 it opened this field to the methods of exact experiments and meas- 

 urements without which no progress is possible; while the at- 

 tempt to explain reactions l)v a hypothetical 'fondness' of animals 

 for light or by hypothetical light sensations barred the wa}' to 

 the exact type of investigation. 



Recently, however, the old anthropomorphic viewpoint has 

 been resumed by the ophthalmologist Hess,^ who has tried to 

 show that all the animals from fish downward suffer from a 

 visual deficiency, namely total color blindness. As a criterion for 

 the presence oi- absence of color sensations Hess uses (very arbi- 

 trarily in our opinion) the heliotropic reactions of animals. 

 Thus in 1909 he confirmed Bert's observation that Daphnia col- 

 lect in the yellow-green part of the spectrum but gave it a dif- 

 ferent interpretation. By calling attention to the fact that the 

 yellowish-green, which is heliotrojiically most efficient for Daph- 



5 Hess. Gesirhtssiiiii. Haiidl). d. vvv<^\v\rh. T'hysiol., 191.3, 4, ."wo. 



THE JOURNAL OF EXPEKI \I t.NT \r, ZOOI.OCV, VOL. 20, NO. 2 



