THE BEHAVIOR OF INFUSORIA; PARAMECIUM 77 



the fact that the resistance in moving upward is greater than the resist- 

 ance in moving downward, owing to the fact that the animal is heavier 

 than water. To the changes in resistance as it swims up or down, the 

 animal reacts. This view was accepted and elaborated by the author 

 of the present work (Jennings, 1904 h). But to this can be made an 

 objection analogous to that which is fatal to the corresponding view for 

 the reaction to water currents. Under the uniform action of gravity, 

 as Radl (1903, p. 139) has pointed out, it is not apparent how any such 

 difference of resistance could be perceived by the organism. The ani- 

 mal would, with the same action of the cilia, and overcoming the same 

 resistance, move somewhat more rapidly downward than upward. But 

 it is very questionable if this slight comparative difference in rate could 

 be perceived by the organism, though this is of course not impossible. 

 In any case, the fact that resting individuals may react to gravity 

 appears fatal to the view at present under consideration. 



The view having the greatest probability is perhaps that suggested 

 by Lyon (1905). The animal contains substances of differing specific 

 gravity; this Lyon has demonstrated. The distribution of these sub- 

 stances must change with the various positions taken by the animal. 

 When the anterior end is directed downward the redistribution of inter- 

 nal substances thus induced acts as a stimulus, causing the usual re- 

 action. The animal " tries" new positions till it reaches one with 

 anterior end upward ; then the reaction ceases and the animal remains 

 in the position so reached. 



Whatever the cause for the reaction to gravity, the stimulation it 

 induces is evidently very slight, and its effect is easily annulled by the 

 action of other agents. As we have seen, the contact reaction usually 

 prevents the reaction to gravity. The same is true of most other stimu- 

 lating agents. Almost any other stimulus that may be present produces 

 its usual effect without interference from gravity, so that the reaction to 

 gravity is seen clearly only in the absence of most other stimuli. Thus, 

 if the walls of the vessel containing the animals are not clean, or if the 

 water contains many solid particles in suspension, often no reaction to 

 gravity can be observed. 



Furthermore, the reaction to gravity becomes reversed under cer- 

 tain conditions. Sometimes nearly all the individuals in a given cul- 

 ture swim downward instead of upward, This result may be produced 

 in cultures having originally the more usual upward tendency, in a 

 number of different ways (Sosnowski, 1899; Moore, 1903). These 

 will be mentioned in our section on reactions to two or more stimuli. 



