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, inasmuch 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 heliotropic 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 voluminuous literature upon this subject.
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 exception, 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 reaction, 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 aquarium 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
- Heliotropism is unusually common, namely, among the larvæ of marine animals and insects, but also not lacking in sexually mature individuals.
toward one of two sources of light, the movement in the latter case not being determined by heliotropism.