VIII 
THE ARTIFICIAL TRANSFORMATION OF POSITIVELY 
HELIOTROPIC ANIMALS INTO NEGATIVELY HELIO- 
TROPIC AND VICE VERSA! 
THE new facts contained in the following pages deal 
chiefly with the task of rendering positively heliotropic 
animals negatively heliotropic, and vice versa. I think also 
that I have discovered a difference in positively and in nega- 
tively heliotropic animals with regard to the liberation of 
energy. As both series of observations may give us some 
clue in regard to the nature of heliotropic phenomena in 
general, | have briefly repeated here the description of the 
simple facts of heliotropism, and have prefaced it with a 
short theoretical explanation. A later part in this paper 
treats of the behavior of animals, which, though not helio- 
tropic, still react to the light by movements. These I shall 
term photokinetic (unterschiedsempfindlich). In the con- 
cluding part of this paper are given the results of some further 
experiments bearing on the causes of depth-migration and 
depth-distribution in marine animals. 
I. THE SIMPLE FACTS OF HELIOTROPISM 
1. All former authors who have studied the behavior of 
animals toward light have, without exception, been of the 
opinion that animals ‘“‘preferred”’ either light or darkness, 
and correspondingly either sought the light places in space 
or shunned them. Five years ago I showed that there is a 
large number of animals which are oriented by the light, 
and in such a way that they are forced to place their axes 
or planes of symmetry in the direction of the rays of light. 
1 Pfltigers Archiv, Vol. LIV (1893), p. 81. 
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