no THE POPULAR SCIENCE MONTHLY 



animal, is turned toward the source of light. As soon as this happens, 

 the two retinas become illuminated equally. There is therefore no 

 longer an)' cause for the animal to turn in one direction or the other. 

 It is thus automatically guided toward the source of light. In this 

 instance the will of the animal which determines the direction of its 

 movement is light, just as it is gravity in the case of a falling stone 

 or the movement of a planet. Only the action of gravity upon the 

 direction of movement of the falling stone is direct, while the action of 

 light upon the direction of movement of the aphids is indirect, inas- 

 much as the animal only by means of an acceleration of photochemical 

 reactions is caused to move in a definite direction. 



"We will now designate as positively heliotropic those animals which 

 are forced to turn the head or the parts of the body which are foremost 

 during locomotion toward the source of light, and as negatively helio- 

 tropic those animals which are oriented in the opposite direction. 



The aphids serve here only as an example. The same phenomena 

 of positive heliotropism may be demonstrated with equal precision in a 

 great many animals, vertebrates as well as invertebrates, for instance in 

 young fishes. We can not, of course, give an account of all these cases 

 here. The reader who is interested in them must look into the vol- 

 uminuous literature upon this subject. 7 



Ill 



The winged aphids can serve as an example, because in their case the 

 above-mentioned requirement is fulfilled, namely, that all individuals, 

 without exception, move toward the light. For mechanistic science it is 

 a methodological postulate that the same law applies without excep- 

 tion, or that a sufficient reason must be given in case of an exception. 

 But it was soon found, as might 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 re- 

 action, while others show it to as pronounced a degree as do the winged 

 aphids. The problem therefore presented itself of making artificially 

 heliotropic those animals which show no positive heliotropism. Such 

 attempts give us a broad insight into the mechanism of acts of will. 

 If small crustaceans of a fresh-water pond or lake are taken with a 

 plankton net at noon-time or in the afternoon and placed in an aqua- 

 rium which is illuminated from one side only, it is generally found that 

 these animals move about in the vessel pretty much at random and 

 distribute themselves irregularly. Some seem to go more toward the 



toward one of two sources of light, the movement in the latter case not being 

 determined by heliotropism. 



1 Heliotropism is unusually common, namely, among the larvae of marine 

 animals and insects, but also not lacking in sexually mature individuals. 



