180 The Animal Mind 



being to note the difference between the air pressure on 

 the head and that on the feet (449). Another suggestion 

 was offered by Davenport (175), 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 anemot- 

 ropism, or the "head against wind" movement of insects; 

 and as Radl (622) first and Lyon (448) 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. The 

 theory cannot explain why an animal at rest should be 

 oriented. Another argument that tells against it is offered 

 by experiments showing that animals placed in solu- 

 tions of the same density as their own bodies, in which, 

 therefore, they have no weight, still display negative 

 geotropism, and that the direction of the response is 

 not reversed when the fluid is made heavier than the 

 animals (449). Lyon's own theory, accepted by Jennings, 

 is that the stimulus for geotropism is furnished by the 

 action of gravity within the body of the organism, upon 

 substances of different weight which exert varying pressures 

 and take up different positions according to the position of 

 the body (449). 



Harper (289) in 1911 revived the mechanical theory of the 

 geotropism of Paramecium. He argued that an animal 

 which, like this protozoon, moved in a spiral could hardly 

 use the changes of position of internal particles as effective 

 stimuli. The reaction of Paramecium can be altered by 

 altering the specific gravity of its body, as by causing it to 

 absorb particles of iron or paraffin. When it has ingested 

 iron, its responses are modified by the neighborhood of a 



