278 Animal Instincts and Tropisms 



that there are two groups of heliotropic substances, one 

 with a maximum of photosensitiveness in the blue, the 

 other in the yellowish-green; and that the latter group 

 may or may not be related or identical with the visual 

 purple which is most rapidly bleached by light of a 

 wave length near X = 520 530^. 



The ophthalmologist Hess 1 has utilized the helio- 

 tropic reactions of animals in an attempt to prove that 

 all animals from the lowest invertebrates up to the 

 fishes inclusive suffer from total colour-blindness. This 

 statement was based on the observation that for most 

 positively heliotropic animals the region in the yellowish- 

 green near X = 520 ^ seems the most efficient. Since 

 this region of the spectrum appears also as the brightest 

 to a totally colour-blind man, he concluded that all 

 these animals are totally colour-blind. There is no 

 reason why heliotropic reactions should be used as an 

 indicator for colour sensations; if totally colour-blind 

 human beings were possessed of an irresistible impulse 

 to run into a flame Hess's assumption might be con- 

 sidered, but no such phenomenon exists in colour- 

 blind man. Moreover, v. Frisch 2 has shown by ex- 

 periments on the influence of the background on the 

 colouration of fish as well as by experiments on bees and 



T Hess, C., "Gesichtssinn," Winterstein's Handb. d. vergl. PhysioL, 

 1913, iv. 



a v. Frisch, K., "Der Farbensinn "und Formensinn der Biene," Zool. 

 Jahrb. Abt.f. allg. Zool. u. PhysioL, 1914, xxxv. See also Ewald, W. F., 

 Ztschr. f. SinnesphysioL, 1914, xlviii., 285. 



