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. 1 



1 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. 2 



i 1 no longer believe that the direction of the waves of sound has any effect, 

 which I considered possible in my preliminary paper on geotropism. 



2 J. T. CUNNINGHAM, Zoologische Anzeiger, Vol. XIV (1891). 



