476 Journal of Comparative Neurology and Psychology. 



pressure. When we consider the large threshold differential re- 

 quired for the perception of differences in pressure in known 

 cases — for example, about -^ in man — we can hardly believe 

 that a differential of ., ^q,^ ,,0, 3 is perceptible by infusoria. Jensen 

 did not calculate this threshold differential, but said in general 

 that the great sensitiveness here shown agreed well with 

 the great sensitiveness to chemical, thermal and other stim- 

 uli. But the great sensitiveness assumed to exist for chem- 

 cals and heat was based on the theory that the reaction was 

 due to the difference in intensity of the agent in question on 

 the two sides or ends of the animals. This I have shown in 

 previous papers not to be the cause of the reactions in question ; 

 they are due to changes in intensity brought about by the 

 movements of the Paramecia from one region to another. The 

 degree of sensitiveness required is therefore much less than 

 would be necessary on Jensen's theory, and does not approach 

 remotely such a minuteness of threshold differential as Jensen's 

 view requires for the reaction to pressure. 



Further, Jensen assumes that the reaction is brought about 

 when there is a difference of a similar order of magnitude to 

 that above mentioned, between the anterior and posterior ends 

 of Paramecium. Now, we know that the anterior end is much 

 more sensitive than the posterior ; this has been shown pre- 

 cisely for mechanical pressure. A Paramecium touched with a 

 glass hair at the anterior end reacts violently, while the same 

 towch or a stronger one on the posterior half of the body pro- 

 duces no reaction. Thus it may be considered practically cer- 

 tain than an increase of pressure on the posterior end such as 

 Jensen's theory assumes to be the effective agent would cause 

 no reaction whatever ; any reaction to the existing pressure 

 which might occur would be due to that at the anterior end. 



Jensen makes one attempt to differentiate experimentally 

 between the direction of gravity and the direction of decrease 

 of pressure, and to show that the Paramecia follow the latter 

 instead of the former. He placed Paramecia in a tube inclined 

 to the perpendicular, and observed that, while often the Para- 



