256 Animal Instincts and Tropisms 



test-tube illuminated from one side only, they will rush 

 to the side from which the light comes and will continue 

 to do this whenever the orientation of the trough 

 or test-tube to the light is changed; while they will be 

 held at the window side of the vessel if the Hght or 

 the position of the vessel remains unchanged. This 

 instinct to get to the source of light is so strong that, 

 e, g., the caterpillars of Porthesia chrysorrhosa die of 

 starvation on the window side of the vessel, with plenty 

 of food close behind. This powerful ''instinct" is, 

 as we intend to show, in the last analysis, the expres- 

 sion of the Bunsen-Roscoe law of photochemical re- 

 actions. A large number of chemical reactions are 

 induced or accelerated by light, and the Bunsen-Roscoe 

 law shows that the chemical effect is in these cases, 

 within certain limits, equal to the product of the in- 

 tensity into the duration of illumination. 



The "attraction" or "repulsion" of animals by the 

 light had been explained by the biologists in an anthro- 

 pomorphic way by ascribing to the animals a "fond- 

 ness" for light or for darkness. Thus Graber, who had 

 made the most extensive experiments, gave as a result 

 the statement that animals which are fond of light 

 are also fond of blue while they hate the red, and 

 those which are fond of the "dark" are fond of red and 

 hate the blue.* In 1888 the writer published a paper 



^ Graber, V., Grundlinien zur Erforschung des Helligkeits- und 

 Farbensinnes der Tiere. Prag, 1884. 



