54 The Mechanistic Conception of Life 



In my opinion it is merely a case where a metabolic product either 

 alters the photochemical action or so influences the central 

 nervous system that the excitation of the retina by the light 

 weakens the tonus of the muscles, instead of strengthening it. 



Some of the other mistakes have perhaps also arisen because 

 the writers worked with complicated experimental conditions 

 instead of with simple ones, for instance, because they used a 

 hollow prism filled with ink in order to produce a gradual 

 decrease in the light intensity. In the semidarkness thus pro- 

 duced, the intensity of light often remains beneath or near 

 the threshold of stimulation, and the writers fall victims to 

 that class of errors which we have already pointed out in 

 speaking of the influence of lesser intensities of light. 



VIII 



Heliotropic phenomena are determined by the relative 

 rates of chemical reactions occurring simultaneously in sym- 

 metrical surface elements of an animal. There is a second class 

 of phenomena which is determined by a sudden change in the 

 rate of chemical reactions in the same surface elements. Reac- 

 tions to a sudden change in the intensity of light are shown most 

 clearly in marine tube-worms, whose gills are exposed to light. 

 If the intensity of the light in the aquarium is suddenly dimin- 

 ished the worms withdraw quickly into their tubes. A sudden 

 increase in the intensity of light has no such effect. With other 

 forms, for instance, with planarians, a sudden decrease in the 

 intensity of the light causes a decrease in movement. Such ani- 

 mals gather chiefly in parts of the space where the intensity of 

 light is relatively small. I have designated such reactions as 

 the expression of sensitiveness to changes in the intensity of a 

 stimulus C'Unterschiedsempfindlichkeit") differential sensi- 

 bility, in order to distinguish them from tropisms.^ 



1 Loeb, " Ueber die Umwandliing positiv heliotropischer Tiere, u.s.w.," Pflugera 

 Archiv, 1893. See also the recent investigations of Georges Bohxi, La naissance de 

 V intelligence, Paris, 1909; "Les essais et les erreurs chez les etioles de mer," Bull. 



