686 THE AMERICAN NATURALIST. [VoL. XXXV. 
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 sub- or 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 head toward 
or away from the source of light (7.4, whether it is positively or 
negatively phototactic) depends upon the physiological condition 
of the animal and the intensity of the light. By sub-optimal inten- 
sities organisms are supposed to be directed toward the light through 
the expansion of those motion-producing 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 supra- 
optimal 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 is the case 
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 “ direc 
tion" 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 to 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 unconditioned 
statement that it does not 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 Yerkes 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. M. Y. 
Notes. — The chief defects of laboratory guides arise from the 
difficulty of giving sufficient directions to the student without sup- 
plying him with information that he should get from laboratory 
