478 Journal of Coinparative Neurology end Psyrhology. 



The animal swerves farther toward the aboral side, and this, 

 with the revolution on the long axis, causes it to occupy suc- 

 cessively many different positions. When as a result of these 

 repeated "trials" it comes into such a position that the changes 

 causing the reaction no longer occur, the reaction ceases. Such 

 a position is found only when the axis of the spiral course co- 

 incides with the direction of gravity. It this position the body 

 of the animal, maintaining a constant angle with the axis of 

 the spiral, maintains also a constant angle with the direction of 

 gravity ; changes in the relation of its swerving to the direction 

 of gravity, therefore, no longer occur. To constant conditions 

 Paramecium quickly becomes acclimated, so now reaction no 

 longer takes place. 



Whether the anterior end is directed upward or downward 

 depends upon whether the plus or minus change in resistance 

 induces the reaction. If the minus change — the change from 

 the greater resistance of the upward swing to the less resistance 

 of the downward swing — is the effective stimulus, then the ani- 

 mal will become oriented with the anterior end upward, for 

 every time it swerves downward the reaction is induced, causing 

 it to "try" many new positions, while when it swerves upward 

 no reaction is induced, and it retains the position reached. This 

 is apparently the usual condition of affairs. On the other hand, 

 if it is the plus change — the change from less resistance to greater 

 resistance — that causes the reaction, the animal will become 

 oriented with anterior end downward. To both cases we could 

 apply the detailed analysis given in the account of the reactions 

 to water currents, above. 



Thus as to the nature of the effective stimulus in gravita- 

 tion, our analysis leads to results agreeing with the conclusions 

 ot Davenport (1897). This author holds that the reaction to 

 gravity is due to the fact that the organism "experiences greater 

 resistance (friction + weight) in going upward even to the 

 slightest extent than in going downward (friction - weight)" 

 (/. c, p. 122). What I have set forth above is the way in 

 which this difference in resistance acts in orienting the or- 

 ganism. 



