The “attraction” or “repulsion” of animals by the light had been explained by the biologists in an anthropomorphic way by ascribing to the animals a “fondness” for light or for darkness. Thus Graber, who had made the most extensive experiments, gave as a result the statement that animals which are fond of light are also fond of blue while they hate the red, and those which are fond of the “dark” are fond of red and hate the blue.[218] In 1888 the writer published a paper in which he pointed out that the so-called fondness of animals for light and blue and for dark and red was simply a case of an automatic orientation of animals by the light comparable to the turning of the tips of a plant towards the window of the room in which the plant is raised.[219] [218] Graber, V., _Grundlinien zur Erforschung des Helligkeits- und Farbensinnes der Tiere_. Prag, 1884. [219] Loeb, J., _Sitzungsber. d. physik.-med. Gesellsch_. Würzburg, 1888. _Der Heliotropismus der Tiere und seine Übereinstimmung mit dem Heliotropismus der Pflanzen._ Würzburg, 1889. _Arch. f. d. ges. Physiol._, 1897, lxvi., 439. The phenomenon of a plant bending or growing to the source of light is called positive heliotropism (while we speak of negative heliotropism in all cases in which the plant turns away from the light, as is observed in many roots). The writer pointed out that animals which go to the light are positively heliotropic (or phototropic) and do so because they are compelled automatically by the light to move in this direction, while he called those animals which move away from the light negatively heliotropic; they are automatically compelled by the light to move away from it. What the light does is to direct the motions of the animals and to explain this the following theory was proposed. Animals possess photosensitive elements on the surface of their bodies, in the eyes, or occasionally also in epithelial cells of their skin. These photosensitive elements are arranged symmetrically in the body and through nerves are connected with symmetrical groups of muscles. The light causes chemical changes in the eyes (or the photosensitive elements of the skin). The mass of photochemical reaction products formed in the retina (or its homologues) influences the central nervous system and through this the tension or energy production of the muscles. If the rate of photochemical reaction is equal in both eyes this effect on the symmetrical muscles is equal, and the muscles of both sides of the body work with equal energy; as a consequence the animal will not be deviated from the direction in which it was moving. This happens when the axis or plane of symmetry of the animal goes through the source of light, provided only one source of light be present. If, however, the light falls sidewise upon the animal, the rate of photochemical reaction will be unequal in both eyes and the rate at which the symmetrical muscles of both sides of the body