an optimum intensity in which they are not directed by the light but move at random. Below this is the sub-optimal and above it the supra-optimal. An animal in either the subor supra-optimal intensity is oriented by the rays so that symmetrical points of the body are equally stimulated; this is, of course, accomplished by the placing of the longitudinal axis of the organism in the axis of the ray. And whether an animal orients itself with bead toward or away from the source of light (i-.C., whether it is positively or negatively phototactic) depends upon the physiological condition of the animal and the intensity of the light. By sub-optimal intensities organisms are supposed to be directed toward the lioht through the expansion of those motion-producinog elements which are on the side most strongly stimulated; hence there results from this kind of orientation a positive reaction. The same organism, if in a supraoptimal intensity, will be oriented with head away from the light, because in this case contraction instead of expansion is caused, and the reaction will be negative. The orientation theory has been very clearly stated by both Loeb and Verworn. Several different kinds of reactions, representatively selected, are explained by the writers by their theory; among them-i is the case of a positively phototactic animal moving toward the source of light into a less intensely illuminated region. This reaction has been taken heretofore as evidence of the independent influence of " clirection ' of ray. It is clearly shown in the present paper, however, that such a contention is probably false, for difference in intensity and the angle at which the organism strikes the side of the vessel are sufficient o explain the observed courses taken under such conditions. That light acts through intensity alone is a conclusion which this paper makes plausible, but it scarcely justifies the uncondlitionedl statement that it does ;iot act by the course which the rays take through the organism. Such a reaction as the reversal of response observed by Towle in Cypridopsis and by Verkes in Daphnia and Cypris is not easily explained by the hypothesis under consideration. The paper is valuable in that it makes clear the importance of intensity and at the same time indicates the danger of confusion in using direction as a causal term, although intensity does in part depend upon it. R. AI. Y.