REVIEWS OF RECENT LITERATURE. 
PHYSIOLOGY. 
Heliotropism. — In this paper Holt and Lee! are chiefly concerned 
with a theoretic discussion of the nature of the responses of organisms 
to light ; but they present, also, in support of their theory, the results 
of some experiments with Stentor Cceruleus, a species of Lynceus, 
and some fresh-water planarians. The title of the paper, “ The 
Theoty of Phototactic Response," is misleading, since the term 
* phototactic" is used, not in the ordinary sense, but as including 
those responses which have been called photopathic. In the present 
state of our terminology **heliotropic " would be a better term. 
Intensity of light and direction of ray have been championed for 
half a century as the important factors of light as a. stimulus: at 
one moment * direction" has been emphasized, at another “ inten- 
sity" The present writers maintain that light acts only by its 
intensity, the direction of the ray being a secondary factor and one, 
furthermore, of an entirely different nature. Their theoretic discus- 
sion centers about the views of two prominent physiologists, Loeb 
and Verworn; and Verworn's theory is accepted as adequate for the 
explanation of all responses to light. Loeb is characterized as a 
believer in the importance of “direction,” and Verworn in that of 
"intensity," but it is observed that their theories are not contra- 
dictory. So far as the exposition of these two theories, as given 
in the text, is concerned one seems just as applicable to the facts 
in question as the other, the only difference being that Loeb has 
sometimes used the term * direction " when intensity is evidently 
the determining factor, thus in appearance giving emphasis to the 
Importance of direction, while Verworn has in all cases expressed 
himself in terms of intensity. i 
The facts of response to light, admitted as such by Holt and Lee, 
are thrée : (1) orientation in the axis of the ray, (2) positive or 
negative movement, (3) random movement. In explanation it is 
assumed that animals which show these three phases of reaction have 
‘Holt, Edwin B, and Lee, Frederic S. The Theory of Phototactic Response, 
Amer. Journ. Physiol., vol. vi, pp. 460-481, January, 1901. 
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