I 
THE HELIOTROPISM OF ANIMALS AND ITS IDENTITY 
WITH THE HELIOTROPISM OF PLANTS! 
I. INTRODUCTION 
I inTEnD to show in the following pages that animal 
movements depend upon light in the same way as the move- 
ments of plants. 
It is a well-known fact that animals, when light falls on 
them, move toward the source of light, like the moth, or 
move away from it, like the earthworm. It is also well 
known that certain plant organs have a tendency to turn 
toward or from the source of light when illuminated from 
one side only. While the conditions which govern the 
behavior of plants toward light have been well analyzed, 
especially by Sachs, little has been done to investigate the 
conditions upon which depend the movements of animals 
toward a source of light. It is the purpose of this paper 
to fill this gap, and to enumerate the facts which show that 
in reality the animal motions called forth by light depend 
upon the same circumstances as the motions which light 
produces in plants. 
The effects of light which we intend to study are purely 
mechanical, inasmuch as they consist in changes in position, 
as well as in the direction and the sense of the progressive 
movements of living animals. Consequently we shall regard 
as essential such circumstances as can help to explain the 
mechanical effects of the light. These circumstances, as in 
the case of all stimulations, are of a double origin: first, 
those belonging to the stimulus—in this case the light; and, 
1 Pamphlet, Wurzburg, 1889. 
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