192 The Animal Mind 



eyes have not opened, save themselves from falling when 

 they reach the edge of the object on which they have been 

 crawling (490, 683). Water-dwelUng animals, accustomed 

 to plunge off solid supports, lack this protective instinct; 

 Yerkes showed that among several species of tortoises, 

 some land-dwelling, some amphibious, and some aquatic, 

 the first mentioned were much more reluctant than the 

 second to crawl off the edge of a board, and the second more 

 reluctant than the third (810). 



§ 57. The Psychic Aspect of Orientation to Gravity 



Glancing back over these examples of the responses made 

 by animals to gravity, we note that while in some cases the 

 earth's attraction appears to act mechanically upon the 

 animal, causing the body passively to assume a certain 

 position, the common method of bringing about orientation 

 seems to be that some structure in the body, /placed in an 

 abnormal position, presents a stimulus which brings about 

 a compensatory movement. This structure may be heavier 

 particles of the body substance, as probably is the case in 

 Parameciimi ; it may be a statolith, or the fluid in the laby- 

 rinth ; it may be the eyes. In any case, what shall we say 

 about the sensation quality involved? Perhaps the re- 

 actions produced are wholly reflex. Perhaps the statolith 

 or the canal fluid produces a specific sensation quality. 

 Or perhaps, as Verworn thinks, the sensation quality is 

 merely that of pressure (741). Whatever its nature, 

 spatial perception, the perception of the spatial relations 

 between several stimuli simultaneously apprehended, plays 

 no part in the orientation of animals to gravity. 



