Significance of Tropisms for Psychology 57 



specific gravity and can study their behavior, since in this case 

 they are of different color. 



In animals it has been observed that orientation toward the 

 center of gravity of the earth often becomes less compulsory 

 when the inner ear has been removed. Mach first pointed 

 out the possibility that the otoliths are responsible for this. 

 He believed that they might press upon the end-organs of the 

 sensory nerves and every change of pressure might cause a cor- 

 rection of the position of the animal. It is generally assumed 

 that this view has been verified by experiment but I cannot 

 entirely agree with it although I once described experiments 

 which seemed to support Mach's otolith theory. I had found 

 that when the otoliths of the inner ear of the shark are scraped 

 out with a sharp spoon the normal orientation of the animal 

 suffers; but if the otoliths are simply washed out from the 

 inner ear by a weak current of sea-water the orientation does 

 not so easily suffer. 



In the latter case, it is doubtful whether all the otolith 

 powder has been removed from the ear. The problem was 

 solved by experiments on flounders, which have only a single 

 large otolith that can easily be removed from the ear. E. P. 

 Lyon carried out these experiments, which showed that no 

 disturbance of the orientation resulted from this operation. We 

 may conclude, therefore, that in my experiments of scraping 

 out the otoliths a disturbance of the orientation occurred, 

 because in so doing the nerve endings in the ears were injured. 

 We have, therefore, no right to maintain that the orientation of 

 animals in relation to the center of gravity of the earth is regu- 

 lated by the pressure of the otoliths upon the nerve endings, 

 but that this regulation takes place in the nerve endings them- 

 selves, and probably, indeed, as a result of the existence there 

 of two different phases of different specific gravity which react 

 upon one another. Through the change of orientation of the 

 cells in relation to the center of gravity of the earth, the two 



