THE SIGNIFICANCE OF TROPISMS 107 



a roof to a seed lying in the street, we speak of an act of will, but 

 if a dead sparrow falls from the roof upon the seed this does not appear 

 to us to be an act of will. In the latter case purely physical forces are 

 concerned, while in the former chemical reactions are also taking place 

 in the sense organs, nerves and muscles of the animal. We speak of an 

 act of will, only when this latter complex, that is, the natural move- 

 ment of locomotion, plays its part also, and it is only with this sort of 

 reactions that we have to deal in the psychology of the will. 



II 



Some experiments on winged plant lice may serve as an introduction 

 to these methods of definitely prescribing to animals the direction of 

 their progressive movements. 



In order to obtain the material, potted rose-bushes or cinerarias 

 thickly infected with plant lice are brought into a room and placed in 

 front of a closed window. If the plants are allowed to dry out, the 

 aphids, previously wingless, change into winged insects. After this 

 metamorphosis the animals leave the plants, fly to the window and there 

 creep upward on the glass. They can then be easily collected by holding 

 a test-tube underneath and touching one animal at a time from above 

 with a pen or scalpal; the animals then drop into the test-tube. 

 In this way a sufficiently large number, perhaps twenty-five or fifty 

 suitable subjects for the experiment, may be quickly obtained. With 

 these animals it may be demonstrated that the direction of their move- 

 ment toward the light is definitely determined — provided that the ani- 

 mals are healthy and that the light is not too weak. The experiment is 

 arranged so that only a single source of light, e. g., artificial light, 

 is used. 



The animals place themselves with their heads toward the source of 

 light and move in as direct a line as the imperfectness of their locomotor 

 apparatus allows, toward it. If they are in a test-tube they go as far 

 toward the source of light as their prison allows. When they reach that 

 end of the test-tube which is directed toward the source of light, they 

 remain there, stationary, in a closely crowded mass. If the test-tube is 

 turned around 180° the animals again go straight toward the source of 

 light until the interference of the glass stops their further progressive 

 movements. 3 It can be demonstrated in these animals that the direction 

 of their progressive movement is just as unequivocally governed by the 

 source of light as the direction of the movement of the planets is 

 determined by the force of gravity. 



The theory of the compulsory movements of aphids under the influ- 

 ence of light is as follows : Two factors govern the progressive move- 



8 Loeb, ' ' Der Heliotropismus der Tiere und seine Ubereinstimmung mit dem 

 Heliotropismus der Pflanzen," Wurzburg, 1890 (Erschienen, 1889). 



