Page:Popular Science Monthly Volume 79.djvu/111

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107
THE SIGNIFICANCE OF TROPISMS

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 movement 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 movement toward the light is definitely determined—provided that the animals 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.[1] 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 influence of light is as follows: Two factors govern the progressive move-

  1. Loeb, "Der Heliotropismus der Tiere und seine Ubereinstimmung mit dem Heliotropismus der Pflanzen," Würzburg, 1890 (Erschienen, 1889).