Spatially Determined Reactions 179 



be purely passive. It cannot operate as a stimulus to 

 active response on the animal's part, for a stimulus is 

 always a change in environment, and gravity is a constant 

 force (742). This ignores the fact that the animal's rela- 

 tions to gravity may change though gravity does not, and 

 also the fact that the continuous action of light is a stimulus. 

 According to Verworn's theory, the geotropic orientation 

 of a single-celled organism takes place through a series of 

 "little falls" whereby the heavier end is directed downward. 

 Massart opposed this view on the basis of observations 

 which showed that the actual movements of the organisms 

 did not correspond to it, but were the result of active 

 orientation. If response to gravity is passive, then dead 

 animals should fall through the water in the same position 

 as that assumed by living animals when oriented to gravity. 

 Massart experimented with various Protozoa by killing 

 them and studying their positions in sinking, which he 

 found not always the same as the attitudes assumed in 

 response to gravity (461). There is always the possibility, 

 however, that the methods employed to kill may change the 

 specific gravity of some part of the body. Jensen offered 

 the theory that reaction to gravity may be due to' the differ- 

 ence in the water pressure on the two ends of the animal. 

 He asserted that when the air pressure on the water was 

 reduced by exhausting the air above, there was an increase 

 in the geotropism, indicating a relative rather than an 

 absolute sensibility to pressure (382), but Lyon points out 

 that this process may affect the animals in various other 

 ways besides altering the air pressure. Increasing the air 

 pressure, or protecting the surface with oil, has no effect 

 upon geotropism, Lyon finds, and he urges that Jensen's 

 theory requires enormous sensibility to pressure differences 

 on the organism's part, as great as that needed by a human 



