46 The Mechanistic Conception of Life 



explanation he will wind up with the suggestion of ''trial and 

 error" which is no more chemical or scientific than the explana- 

 tions of metaphysicians in general. 



Some authors have, it seems, worked only with animals 

 which were not pronouncedly heliotropic and whose photo- 

 sensitiveness wavered about the threshold of stimulation in the 

 manner described above. Such animals are not suitable for 

 experiments in heliotropism and it is necessary to first increase 

 their photosensitiveness if the laws of the action of light upon 

 them are to be investigated. 



I also believe that observations upon animals which are 

 not sufl&ciently photosensitive have caused many writers to 

 assert that heliotropic animals do not place themselves directly 

 in the line of the rays of light,^ but that they first have to learn 

 the right orientation. A very striking experiment contradicts 

 this assertion. The larvae of Balanus perforatus develop 

 entirely in the dark. If the ovary filled with mature larvae 

 is placed in a watch crystal filled with sea-water in the dark, 

 the larvae emerge at once and, if they are brought into the light, 

 they move at once to the side of the watch crystal nearest to 

 the window. They were, therefore, pronouncedly positively 

 heliotropic before they came under the influence of the light. 



In experiments with winged aphids I often found that 

 after having gone through the heliotropic reactions a few times 

 they react much more quickly to light than at the beginning. 

 This might be interpreted as a case of ''learning." In so far 

 as it is not a case of a lessening of the stickiness of the feet or 

 the removal of some other purely mechanical factor which 

 retards the rate of movement, it may be brought about by the 

 carbonic or lactic acids produced through the muscular activity .^ 



1 Provided that only a single source of light is present. 



2 The so-called " staircase " phenomenon of stimulation of a muscle is ascribed, 

 probably rightly, also to the formation of acid. This phenomenon, that is, the 

 increase of the amount of contraction with every new stimulus, is, however, com- 

 parable to or identical with the increase in the rate of reactions in the experiments 

 described here. 



