Spatially Determined Reactions 155 



ments of Jthe 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 methods of sinking, which 

 he found not always the same as the attitudes assumed in 

 response to gravity (259). 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 difference 

 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 (215), 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 being to note the 

 difference between the air pressure on the head and that on 

 the feet (255). Another suggestion was offered by Daven- 

 port, namely, that negatively geotropic organisms swim in 

 the direction where the greatest resistance to their progress 

 is offered. This is like one theory put forward to explain 

 rheotropism, or the tendency of animals to swim against 

 currents, and anemotropism, or the "head against wind" 

 movement of insects; and as Rdl (355) first and Lyon (254) 

 afterward pointed out, it assumes the fact to be explained, 

 for only if an animal actively opposes a force, will that force 

 exert more pressure at one point of its body than at another. 



