190 STUDIES IN GENERAL PHYSIOLOGY 
its ventral side toward the center of the earth, and keeps its 
longitudinal axis almost horizontal; but in every other posi- 
tion of the animal the altered distribution of strain and pres- 
sure upon the ends of the auditory nerves forces the animal 
again to assume its customary angle with the horizontal. 
This force is maximal when the animal lies on its back. If 
one of the auditory nerves is cut the animal is compelled to 
assume an oblique position in which the injured side is 
directed downward more than usual; if both auditory nerves 
are divided, no force exists to compel the animal to assume 
any definite geotropic position.’ 
I will give a single example to illustrate what may be the 
further biological significance of geotropism. The reader is 
acquainted with the marked difference in the pigmentation 
of the ventral and dorsal sides of many animals, especially 
that of the fishes. This difference is to a large extent inde- 
pendent of light and is determined by conditions which accom- 
pany development. In part, however, it is directly dependent 
upon light; the back, which is permanently directed toward 
the source of light, must become richer in pigment than 
the ventral side. Cunningham has, indeed, been able to 
show recently that flatfish develop pigment upon their lower 
surfaces in an abnormal way when these are illuminated by 
the help of mirrors, as strongly as their upper surfaces.” 
1T no longer believe that the direction of the waves of sound has any effect, 
which I considered possible in my preliminary paper on geotropism. 
2J. T. CUNNINGHAM, Zoologische Anzeiger, Vol. XIV (1891). 
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