42 The Mechanistic Conception of Life 



this law for what the metaphysician calls the will of these 

 animals. 



Ill 



The winged aphids serve as an example, because they fulfil 

 the above-mentioned requirement, namely, that all individuals, 

 without exception, move toward the light. For mechanistic 

 science it is a methodological postulate that the same law acts 

 without exception, or that the exception must be satisfactorily 

 explained. It was soon found, as was to be expected, that not 

 all organisms in their natural condition are equally suitable 

 for these experiments. Many animals show no heliotropism 

 at all ; many show only a slight reaction, while others show it in 

 a degree as pronounced as do the winged aphids. The problem 

 therefore presented itself of producing heliotropism artificially 

 in animals which, under natural conditions, show no positive 

 heliotropism. If small crustaceans of a fresh-water pond or 

 lake are taken with a plankton net at noontime or in the after- 

 noon and placed in an aquarium which is illuminated from one 

 side only, it is often found that these animals move about in 

 the vessel pretty much at random and distribute themselves 

 irregularly. Some seem to go more toward the source of light, 

 others in the opposite direction, and the majority perhaps pay 

 no attention to the light. 



This condition changes instantly if we add to the water 

 some acid, preferably carbonic acid, which easily penetrates the 

 cells of the animal. To every 50 c.c. of the fresh water a few 

 cubic centimeters of water charged with carbon dioxide are 

 slowly added. If the correct amount is added all the individuals 

 become actively positively heliotropic and move in as straight 

 a line as the imperfection of their swimming movements per- 

 mits, toward the source of light, and remain there closely 

 crowded together on the illuminated side of the vessel. If the 

 vessel is turned 180°, they go directly back again to the lighted 

 side of the vessel. Every other acid acts like carbonic acid and 



