Jacques Loeb has given the following interpretation of these phenomena: When the rays coming from a luminous center strike asymmetrically one side of the sensitive region (the head with the articulates) of a symmetrical animal endowed with positive phototropism, the muscles of that side of the body become more active than those of the opposite side, so that the axis of the body is finally turned in the same direction as the luminous rays. Then, "the luminous intensity being the same on both sides, there is no reason for the animal to turn from this direction either to the right or to the left. It therefore directs itself toward the source of light." When the animal is negatively phototropic the muscles of the illuminated side are less active, which has the result of turning the animal away from the source. The law common to the two cases may be expressed by saying that "with sensitive symmetrical animals, an asymmetry of lighting clearly provokes an asymmetrical muscular activity which results in a symmetry of activity and of lighting"".
If the explanation of Loeb is well founded, if, in other words, it is to the asymmetry of lighting that the asymmetrical muscular activity of sensitive animals is due, an insect deprived of one eye should move in a circle to the opposite side if it is positively phototropic, and, deprived of both eyes, it should cease to rea*ct to the light by a directive orientation. This is, in fact, what was shown by the experiments of Parker (1903) with a butterfly, the mourning-cloak (Vanessa antiopa), which has a very strong positive phototropism. When one of the eyes is covered with black paint, the butterfly turns in a circle to the opposite side and little by little approaches the light. The recent experiments of Dolley (1916), on the same species, largely confirm the preceding, while this author does not consider them favorable to the views of Loeb. Under normal lighting conditions, or where there is an open window, the Vanessas of the American biologist behave always like Parker's when one of its eyes are covered with asphalt; bnt under the influence of a powerful light ray, sometimes they turn in a circle, sometimes they move obliquely in a straight or sinuous line toward the side of the sound eye, which shows that the muscles are always more active on that side. Dolley's memoir will be read with profit, for it adds a good chapter to this discussion; but, as we shall shortly see (page 78), it does not sufficiently indicate the difference between tropisms and the phenomena of differential sensitiveness.